Pleiotropic patterning response to activation of Shh signaling in the limb apical ectodermal ridge

Sonic hedgehog (Shh) signaling in the limb plays a central role in coordination of limb patterning and outgrowth. Shh expression in the limb is limited to the cells of the zone of polarizing activity (ZPA), located in posterior limb bud mesoderm. Shh is not expressed by limb ectoderm or apical ectodermal ridge (AER), but recent studies suggest a role for AER-Shh signaling in limb patterning. Here, we have examined the effects of activation of Shh signaling in the AER. We find that targeted expression of Shh in the AER activates constitutive Shh signaling throughout the AER and subjacent limb mesoderm, and causes a range of limb patterning defects with progressive severity from mild polydactyly, to polysyndactyly with proximal defects, to severe oligodactyly with phocomelia and partial limb ventralization. Our studies emphasize the importance of control of the timing, level and location of Shh pathway signaling for limb anterior-posterior, proximal-distal, and dorsal-ventral patterning.

Direct and progressive differentiation of human embryonic stem cells into the chondrogenic lineage

Treatment of common and debilitating degenerative cartilage diseases particularly osteoarthritis is a clinical challenge because of the limited capacity of the tissue for self-repair. Because of their unlimited capacity for self-renewal and ability to differentiate into multiple lineages, human embryonic stem cells (hESCs) are a potentially powerful tool for repair of cartilage defects. The primary objective of the present study was to develop culture systems and conditions that enable hESCs to directly and uniformly differentiate into the chondrogenic lineage without prior embryoid body (EB) formation, since the inherent cellular heterogeneity of EBs hinders obtaining homogeneous populations of chondrogenic cells that can be used for cartilage repair. To this end, we have subjected undifferentiated pluripotent hESCs to the high density micromass culture conditions we have extensively used to direct the differentiation of embryonic limb bud mesenchymal cells into chondrocytes. We report that micromass cultures of pluripotent hESCs undergo direct, rapid, progressive, and substantially uniform chondrogenic differentiation in the presence of BMP2 or a combination of BMP2 and TGF-beta1, signaling molecules that act in concert to regulate chondrogenesis in the developing limb. The gene expression profiles of hESC-derived cultures harvested at various times during the progression of their differentiation has enabled us to identify cultures comprising cells in different phases of the chondrogenic lineage ranging from cultures just entering the lineage to well differentiated chondrocytes. Thus, we are poised to compare the abilities of hESC-derived progenitors in different phases of the chondrogenic lineage for cartilage repair.

Conditional inactivation of Has2 reveals a crucial role for hyaluronan in skeletal growth, patterning, chondrocyte maturation and joint formation in the developing limb

The glycosaminoglycan hyaluronan (HA) is a structural component of extracellular matrices and also interacts with cell surface receptors to directly influence cell behavior. To explore functions of HA in limb skeletal development, we conditionally inactivated the gene for HA synthase 2, Has2, in limb bud mesoderm using mice that harbor a floxed allele of Has2 and mice carrying a limb mesoderm-specific Prx1-Cre transgene. The skeletal elements of Has2-deficient limbs are severely shortened, indicating that HA is essential for normal longitudinal growth of all limb skeletal elements. Proximal phalanges are duplicated in Has2 mutant limbs indicating an involvement of HA in patterning specific portions of the digits. The growth plates of Has2-deficient skeletal elements are severely abnormal and disorganized, with a decrease in the deposition of aggrecan in the matrix and a disruption in normal columnar cellular relationships. Furthermore, there is a striking reduction in the number of hypertrophic chondrocytes and in the expression domains of markers of hypertrophic differentiation in the mutant growth plates, indicating that HA is necessary for the normal progression of chondrocyte maturation. In addition, secondary ossification centers do not form in the central regions of Has2 mutant growth plates owing to a failure of hypertrophic differentiation. In addition to skeletal defects, the formation of synovial joint cavities is defective in Has2-deficient limbs. Taken together, our results demonstrate that HA has a crucial role in skeletal growth, patterning, chondrocyte maturation and synovial joint formation in the developing limb.

Studies on the role of Dlx5 in regulation of chondrocyte differentiation during endochondral ossification in the developing mouse limb

The homeodomain transcription factor Dlx5 has been implicated in the regulation of chondrocyte and osteoblast differentiation during endochondral ossification in the developing limb. In a gain-of-function approach to directly investigate the role of Dlx5 in chondrocyte maturation, we have used cartilage-specific Col2a1-Dlx5 promoter/enhancer constructs to target overexpression of Dlx5 to the differentiating cartilage models of the limbs of transgenic mice. Targeted overexpression of Dlx5 in cartilage rudiments results in the formation of shortened skeletal elements containing excessive numbers of hypertrophic chondrocytes and expanded domains of expression of Ihh and type X collagen, molecular markers of hypertrophic maturation. This suggests that hypertrophic differentiation is enhanced in response to Dlx5 misexpression. Skeletal elements overexpressing Dlx5 also exhibit a marked reduction in the zone of proliferation, indicating that overexpression of Dlx5 reduces chondrocyte proliferation concomitant with promoting hypertrophic maturation. Taken together these results indicate that Dlx5 is a positive regulator of chondrocyte maturation during endochondral ossification, and suggest that it regulates the process at least in part by promoting the conversion of immature proliferating chondrocytes into hypertrophying chondrocytes; a critical step in the maturation process.

Expression of Dlx5 and Dlx6 during specification of the elbow joint

The onset of elbow joint formation in the developing limb is characterized morphologically by the conversion of differentiated chondrocytes at the site of incipient joint formation into the densely packed flattened cells of the joint interzone. However, experimental studies have indicated that the elbow joint is specified well before joint interzone formation by a distinctive population of precursor cells located at the site in the developing limb bud at which the elbow joint will subsequently form. Here we show that during specification of the elbow joint in the chick limb bud, the homeodomain transcription factors Dlx5 and Dlx6 are highly expressed by a discrete group of cells that encompass the prospective elbow joint. The Dlx5- and Dlx6-expressing cells at the prospective elbow joint are located where the differentiating humerus branches into the radius and ulna. Thus, Dlx5 and Dlx6 are the earliest molecular markers of the presumptive elbow joint yet described. The onset of Dlx5 expression in the region of the presumptive elbow joint is shortly followed by the initiation of expression amongst the Dlx5-expressing cells of Gdf5, which encodes a secreted signaling molecule that is involved in regulating the onset of joint formation. These results suggest that Dlx genes may be involved in specification of the elbow joint and/or in providing positional information that specifies the site at which the elbow joint will form.

Hyaluronan in limb morphogenesis

Hyaluronan (HA) is a large glycosaminoglycan that is not only a structural component of extracellular matrices, but also interacts with cell surface receptors to promote cell proliferation, migration, and intracellular signaling. HA is a major component of the extracellular matrix of the distal subapical mesenchymal cells of the developing limb bud that are undergoing proliferation, directed migration, and patterning in response to the apical ectodermal ridge (AER), and has the functional potential to be involved in these processes. Here we show that the HA synthase Has2 is abundantly expressed by the distal subridge mesodermal cells of the chick limb bud and also by the AER itself. Has2 expression and HA production are downregulated in the proximal central core of the limb bud during the formation of the precartilage condensations of the skeletal elements, suggesting that downregulation of HA may be necessary for the close juxtaposition of cells and the resulting cell-cell interactions that trigger cartilage differentiation during condensation. Overexpression of Has2 in the mesoderm of the chick limb bud in vivo results in the formation of shortened and severely malformed limbs that lack one or more skeletal elements. Skeletal elements that do form in limbs overexpressing Has2 are reduced in length, exhibit abnormal morphology, and are positioned inappropriately. We also demonstrate that sustained HA production in micromass cultures of limb mesenchymal cells inhibits formation of precartilage condensations and subsequent chondrogenesis, indicating that downregulation of HA is indeed necessary for formation of the precartilage condensations that trigger cartilage differentiation. Taken together these results suggest involvement of HA in various aspects of limb morphogenesis.

Heparan sulfate proteoglycans including syndecan-3 modulate BMP activity during limb cartilage differentiation

Bone morphogenetic proteins (BMPs) are involved in multiple aspects of limb development including regulation of cartilage differentiation. Several BMPs bind strongly to heparin, and heparan sulfate proteoglycans (HSPGs) at the cell surface or in the extracellular matrix have recently been implicated as modulators of BMP signaling in some developing systems. Here we have explored the role of HSPGs in regulating BMP activity during limb chondrogenesis by evaluating the effects of exogenous heparan sulfate (HS), heparitinase treatment, and overexpression of the HSPG syndecan-3 on the ability of BMP2 to modulate the chondrogenic differentiation of limb mesenchymal cells in micromass culture. Exogenous HS dramatically enhances the ability of BMP2 to stimulate chondrogenesis and cartilage specific gene expression, and reduces the concentration of BMP2 needed to stimulate chondrogenesis. Furthermore, HS stimulates BMP2-mediated phosphorylation of Smad1, Smad5, and Smad8, transcriptional mediators of BMP2 signaling, indicating that HS enhances the interaction of BMP2 with its receptors. Pretreatment of micromass cultures with heparitinase to degrade endogenous HSPGs also enhances the chondrogenic activity of BMP2, and reduces the concentration of BMP2 needed to promote chondrogenesis. Taken together these results indicate that exogenous HS or heparitinase enhance the chondrogenic activity of BMP2 by interfering with its interaction with endogenous HSPGs that would normally restrict its interaction with its receptors. Consistent with the possibility that HSPGs are negative modulators of BMP signaling during chondrogenesis, we have found that overexpression of syndecan-3, which is one of the major HSPGs normally expressed during chondrogenesis, greatly impairs the ability of BMP2 to promote cartilage differentiation. Furthermore, retroviral overexpression of syndecan-3 inhibits BMP2-mediated Smad phosphorylation in the regions of the cultures in which chondrogenesis is inhibited and in which ectopic syndecan-3 protein is highly expressed. These results indicate that syndecan-3 interferes with the interaction of BMP2 with its receptors, and that this interference results in an inhibition of chondrogenesis. Taken together these results indicate that HSPGs including syndecan-3 normally modulate the strength of BMP signaling during limb cartilage differentiation by limiting the effective concentration of BMP available for signaling.

Function of BMPs in the apical ectoderm of the developing mouse limb

Several bone morphogenetic proteins (BMPs) are expressed in the apical ectodermal ridge (AER), a critical signaling center that directs the outgrowth and patterning of limb mesoderm, but little is known about their function. To study the functions of apical ectodermal BMPs, an AER-specific promoter element from the Msx2 gene was used to target expression of the potent BMP antagonist noggin to the apical ectoderm of the limbs of transgenic mice. Msx2-noggin mutant mice have severely malformed limbs characterized by syndactyly, postaxial polydactyly, and dorsal transformations of ventral structures indicated by absence of ventral footpads and presence of supernumerary ventral nails. Mutant limb buds exhibit a dorsoventral (DV) and anteroposterior (AP) expansion in the extent of the AER. AER activity persists longer than normal and is maintained in regions of the apical ectoderm where its activity normally ceases. Mutant limbs possess a broad band of mesodermal tissue along the distal periphery that is absent from normal limbs and which fails to undergo the apoptosis that normally occurs in the subectodermal mesoderm. Taken together, our results suggest that apical ectodermal BMPs may delimit the boundaries of the AER by preventing adjacent nonridge ectodermal cells from becoming AER cells; negatively modulate AER activity and thus fine-tune the strength of AER signaling; and regulate the apoptosis of the distal subectodermal mesoderm that occurs as AER activity attenuates, an event that is essential for normal limb development. Our results also confirm that ectodermal BMP signaling regulates DV patterning.

Distribution and possible function of an adrenomedullin-like peptide in the developing chick limb bud

Adrenomedullin (AM) is a multifunctional peptide that exhibits discrete domains of expression during mouse embryogenesis consistent with a role in regulating growth and differentiation during morphogenesis. Here we report that AM immunoreactivity is present at high levels throughout the apical ectodermal ridge (AER) of the chick limb bud as the AER is directing the outgrowth and patterning of underlying limb mesoderm. Immunostaining is particularly strong along the surfaces of the contiguous cells of the AER. AM immunoreactivity attenuates as the AER regresses and is absent from the distal apical ectoderm of stage 20 limbless mutant limb buds which fail to develop an AER. To explore the possible role of AM in AER activity, we examined the effect of exogenous AM and an AM inhibitor on the in vitro morphogenesis of limb mesoderm, cultured in the presence and absence of the AER. Although exogenous AM cannot substitute for the AER in promoting outgrowth of limb mesoderm in vitro, a specific AM antagonist, AM(22-52), impairs the outgrowth and proliferation of limb mesoderm cultured in the presence of the AER. This is consistent with the possibility that inhibition of endogenous AM activity in the AER impairs the ability of the AER to promote limb morphogenesis. Taken together, these studies suggest that an AM-like molecule may function in an autocrine fashion to regulate some aspect of AER activity.

Dlx5 is a positive regulator of chondrocyte differentiation during endochondral ossification

The process of endochondral ossification in which the bones of the limb are formed after generation of cartilage models is dependent on a precisely regulated program of chondrocyte maturation. Here, we show that the homeobox-containing gene Dlx5 is expressed at the onset of chondrocyte maturation during the conversion of immature proliferating chondrocytes into postmitotic hypertrophying chondrocytes, a critical step in the maturation process. Moreover, retroviral misexpression of Dlx5 during differentiation of the skeletal elements of the chick limb in vivo results in the formation of severely shortened skeletal elements that contain excessive numbers of hypertrophying chondrocytes which extend into ectopic regions, including sites normally occupied by immature chondrocytes. The expansion in the extent of hypertrophic maturation detectable histologically is accompanied by expanded and upregulated domains of expression of molecular markers of chondrocyte maturation, particularly type X collagen and osteopontin, and by expansion of mineralized cartilage matrix, which is characteristic of terminal hypertrophic differentiation. Furthermore, Dlx5 misexpression markedly reduces chondrocyte proliferation concomitant with promoting hypertrophic maturation. Taken together, these results indicate that Dlx5 is a positive regulator of chondrocyte maturation and suggest that it regulates the process at least in part by promoting conversion of immature proliferating chondrocytes into hypertrophying chondrocytes. Retroviral misexpression of Dlx5 also enhances formation of periosteal bone, which is derived from the Dlx5-expressing perichondrium that surrounds the diaphyses of the cartilage models. This suggests that Dlx5 may be involved in regulating osteoblast differentiation, as well as chondrocyte maturation, during endochondral ossification.

Overexpression of Dlx5 in chicken calvarial cells accelerates osteoblastic differentiation

Our laboratory and others have shown that a homeodomain protein binding site plays an important role in transcription of the Collal gene in osteoblasts. This suggests that homeodomain proteins have an important role in osteoblast differentiation. We have investigated the role of Dlx5 in osteoblastic differentiation. In situ hybridization studies indicated that Dlx5 is expressed in chick calvarial osteoblasts (cCOB) in vivo. Northern blot analysis indicated that Dlx5 expression in cultured cCOBs is induced concurrently with osteoblastic markers. To study the effect of overexpression of Dlx5 on osteoblast differentiation, we infected primary osteoblast cultures from 15-day-old embryonal chicken calvaria with replication competent retroviral vectors [RCASBP(A)] expressing Dlx5 or control replication competent avian splice acceptor brianhightiter polymerase subtype A [RCASBP(A)]. Expression of Collal, osteopontin, alkaline phosphatase, and osteocalcin messenger RNA (mRNA) occurred sooner and at higher levels in cultures infected with RCASBP(A)DLX5 than in RCASBP(A)-infected cultures. Mineralization of Dlx5-expressing cultures was evident by days 12-14, and RCAS-infected control osteoblasts did not begin to mineralize until day 17. Dlx5 also stimulated osteoblastic differentiation of calvarial cells that do not normally undergo osteoblastic differentiation in vitro. Our results suggest that Dlx5 plays an important role in inducing calvarial osteoblast differentiation.

Studies on the role of Cux1 in regulation of the onset of joint formation in the developing limb

Joint formation, the onset of which is characterized by the segmentation of continuous skeletal rudiments into two or more separate elements, is a fundamental aspect of limb pattern formation, playing a critical role in determining the size, shape, and number of individual skeletal elements. Joint formation is initiated by conversion of differentiated chondrocytes at sites of presumptive joints into densely packed nonchondrogenic cells of the joint interzone. This conversion is accompanied by loss of Alcian blue-staining cartilage matrix and downregulation of cartilage-specific gene expression. Here, we report that Cux1, which encodes a transcription factor containing a homeodomain and other DNA-binding motifs, is highly expressed at all of the discrete sites of incipient joint formation in the developing limb concomitant with conversion of differentiated chondrocytes into interzone tissue. Moreover, differentiated limb chondrocytes in micromass cultures infected with a Cux1 retroviral expression vector are converted into nonchondrogenic cells which exhibit loss of Alcian blue cartilage matrix and downregulation of cartilage-specific gene expression as occurs at the onset of normal joint formation. These results suggest that Cux1 is involved in regulating the onset of joint formation by facilitating conversion of chondrocytes into nonchondrogenic cells of the interzone.

FGFR2 signaling in normal and limbless chick limb buds

FGF10 and FGF8, which are reciprocally expressed by the mesoderm and AER of the developing limb bud, have been implicated in limb initiation, outgrowth, and patterning. FGF10 and FGF8 signal through the FGFR2b and FGFR2c alternative splice isoforms, respectively [Ornitz DM, et al. 1996. J Biol Chem 271:15292-15297; Igarashi M, et al. 1998. J Biol Chem 273:13230-13235]. A paracrine signaling loop model has been proposed whereby FGF10 expressed by limb mesoderm signals via ectodermally restricted FGFR2b to regulate FGF8 expression by the apical ectoderm; in turn, FGF8 signals via mesodermally restricted FGFR2c to maintain FGF10 expression [Ohuchi H, et al. 1997. Development 124:2235-2244; Xu X, et al. 1998. Development 125:753-765]. To explore this model, we have examined FGFR2b and FGFR2c mRNA expression, using isoform-specific probes during the early stages of development of the chick limb when limb initiation, AER induction, and outgrowth are occurring. We have found that FGFR2b is expressed by limb ectoderm, including the AER, consistent with paracrine signaling of FGF10. By contrast, FGFR2c is expressed by both mesoderm and ectoderm, indicating that FGF8 has the potential to function in an autocrine as well as paracrine fashion. Indeed, as the limb grows out in response to the AER, FGFR2c expression attenuates in the mesoderm of the progress zone, but is maintained in the AER itself, arguing against exclusive paracrine signaling of FGF8 during limb outgrowth. We also report that transcripts for FGF10, FGFR2b, and FGFR2c are expressed normally in the limb buds of limbless mutant embryos, which fail to form an AER and do not express FGF8. Furthermore, we detect no mutations in exons specific for the FGFR2c or FGFR2b isoforms in limbless embryos. Since gene targeting has shown that expression of FGF8 in limb ectoderm depends on FGF10 [Min H, et al. 1998. Genes Dev 12:3156-3161; Sekine K, et al. 1999. Nature Genet 21:138-141], these results indicate that the product of the limbless gene is required for FGF10 to induce expression of FGF8.

Dlx-5 in limb initiation in the chick embryo

Dlx-5 is a vertebrate homolog of the Drosophila Distal-less gene, one of the first genetic signals for limb formation in the fly. In the present study we have explored the possible role of Dlx-5 in limb initiation in the chick embryo. At stage 14 which is well before the initial formation of limb buds Dlx-5 is highly and specifically expressed in the ectoderm of the presumptive wing and leg forming regions of the lateral plate, but not in the intervening non-limb forming prospective flank. Thus, Dlx-5 expression distinguishes the limb-forming territories prior to limb budding, and is one of the first molecular markers of vertebrate limb initiation. Furthermore, Dlx-5 expression is induced in the non-limb-forming flank within 12 hours after implantation of an FGF2-soaked bead, a procedure that results in the induction of an ectopic limb. The rapid induction of Dlx-5 expression in response to a signal which ultimately leads to supernumerary limb formation is consistent with a role for Dlx-5 in limb initiation. We have also examined the expression of Dlx-5 in the limb buds of amelic limbless mutant chick embryos, which undergo normal limb formation but do not form an AER and thus fail to undergo further outgrowth. Dlx-5 is transiently expressed by the ectoderm of emergent limbless limb buds, consistent with a role for Dlx-5 in limb initiation. Together, our results suggest that Dlx-5 may be involved in the specification of the limb territories of the lateral plate, and in the initial formation of the limb bud from these regions. Dev Dyn 1999;216:10-15.

Syndecan-3 in limb skeletal development

Syndecan-3 is a member of a family of heparan sulfate proteoglycans that function as extracellular matrix receptors and as co-receptors for growth factors and signalling molecules. A variety of studies indicate that syndecan-3 is involved in several aspects of limb morphogenesis and skeletal development. Syndecan-3 participates in limb outgrowth and proliferation in response to the apical ectodermal ridge; mediates cell-matrix and/or cell-cell interactions involved in regulating the onset of chondrogenesis; may be involved in regulating the onset of osteogenesis and joint formation and, plays a role in regulating the proliferation of epiphyseal chondrocytes during endochondral ossification.

Ectopic expression of Msx-2 in posterior limb bud mesoderm impairs limb morphogenesis while inducing BMP-4 expression, inhibiting cell proliferation, and promoting apoptosis

During early stages of chick limb development, the homeobox-containing gene Msx-2 is expressed in the mesoderm at the anterior margin of the limb bud and in a discrete group of mesodermal cells at the midproximal posterior margin. These domains of Msx-2 expression roughly demarcate the anterior and posterior boundaries of the progress zone, the highly proliferating posterior mesodermal cells underneath the apical ectodermal ridge (AER) that give rise to the skeletal elements of the limb and associated structures. Later in development as the AER loses its activity, Msx-2 expression expands into the distal mesoderm and subsequently into the interdigital mesenchyme which demarcates the developing digits. The domains of Msx-2 expression exhibit considerably less proliferation than the cells of the progress zone and also encompass several regions of programmed cell death including the anterior and posterior necrotic zones and interdigital mesenchyme. We have thus suggested that Msx-2 may be in a regulatory network that delimits the progress zone by suppressing the morphogenesis of the regions of the limb mesoderm in which it is highly expressed. In the present study we show that ectopic expression of Msx-2 via a retroviral expression vector in the posterior mesoderm of the progress zone from the time of initial formation of the limb bud severely impairs limb morphogenesis. Msx-2-infected limbs are typically very narrow along the anteroposterior axis, are occasionally truncated, and exhibit alterations in the pattern of formation of skeletal elements, indicating that as a consequence of ectopic Msx-2 expression the morphogenesis of large portions of the posterior mesoderm has been suppressed. We further show that Msx-2 impairs limb morphogenesis by reducing cell proliferation and promoting apoptosis in the regions of the posterior mesoderm in which it is ectopically expressed. The domains of ectopic Msx-2 expression in the posterior mesoderm also exhibit ectopic expression of BMP-4, a secreted signaling molecule that is coexpressed with Msx-2 during normal limb development in the anterior limb mesoderm, the posterior necrotic zone, and interdigital mesenchyme. This indicates that Msx-2 regulates BMP-4 expression and that the suppressive effects of Msx-2 on limb morphogenesis might be mediated in part by BMP-4. These studies indicate that during normal limb development Msx-2 is a key component of a regulatory network that delimits the boundaries of the progress zone by suppressing the morphogenesis of the regions of the limb mesoderm in which it is highly expressed, thus restricting the outgrowth and formation of skeletal elements and associated structures to the progress zone. We also report that rather large numbers of apoptotic cells as well as proliferating cells are present throughout the AER during all stages of normal limb development we have examined, indicating that many of the cells of the AER are continuously undergoing programmed cell death at the same time that new AER cells are being generated by cell proliferation. Thus, a balance between cell proliferation and programmed cell death may play a very important role in maintaining the activity of the AER.

FGF-stimulated outgrowth and proliferation of limb mesoderm is dependent on syndecan-3

The outgrowth of the mesoderm of the developing limb bud in response to the apical ectodermal ridge (AER) is mediated at least in part by members of the FGF family. Recent studies have indicated that FGFs need to interact with heparan sulfate proteoglycans in order to bind to and activate their specific cell surface receptors. Syndecan-3 is an integral membrane heparan sulfate proteoglycan that is highly expressed by the distal mesodermal cells of the chick limb bud that are undergoing proliferation and outgrowth in response to the AER. Here we report that maintenance of high-level syndecan-3 expression by the subridge mesoderm of the chick limb bud is directly or indirectly dependent on the AER, since its expression is severely impaired in the distal mesoderm of the limb buds of limbless and wingless mutant embryos which lack functional AERs capable of directing the outgrowth of limb mesoderm. We have also found that exogenous FGF-2 maintains a domain of high-level syndecan-3 expression in the outgrowing mesodermal cells of explants of the posterior mesoderm of normal limb buds cultured in the absence of the AER and in the outgrowing subapical mesoderm of explants of limbless mutant limb buds which lack a functional AER. These results suggest that the domain of high-level syndecan-3 expression in the subridge mesoderm of normal limb buds is maintained by FGFs produced by the AER. Finally, we report that polyclonal antibodies against a syndecan-3 fusion protein inhibit the ability of FGF-2 to promote the proliferation and outgrowth of the posterior subridge mesoderm of limb buds cultured in the absence of the AER. These results suggest that syndecan-3 plays an essential role in limb outgrowth by mediating the interaction of FGFs produced by the AER with the underlying mesoderm of the limb bud.

Syndecan-3 and the control of chondrocyte proliferation during endochondral ossification

During endochondral ossification, chondrocytes progress through several stages of maturation before they are replaced by bone cells. Chondrocyte proliferation, the first step in this complex multistage process, is strictly controlled both spatially and temporally but its underlying mechanisms of regulation remain unclear. In this study we asked whether chondrocytes produce syndecan-3, a cell surface receptor for growth factors such as fibroblast growth factor 2 (FGF-2), and whether syndecan-3 may play a role in proliferation during chondrocyte maturation. We found that proliferating immature cartilage from chick embryo tibia and sternum contained significant amounts of syndecan-3 mRNA, whereas mature hypertrophic cartilage contained markedly lower transcript levels. Immunohistochemical analyses on sections of Day 18 chick embryo tibia revealed that syndecan-3 was spatially restricted and indeed detectable only in immature proliferating chondrocytes in the top zone of growth plate. These syndecan-3-rich proliferating chondrocytes lay beneath developing articular chondrocytes rich in their typical matrix protein tenascin-C, resulting in a striking boundary between these two populations of chondrocytes. Immature proliferating chondrocyte populations reared in growth-promoting culture conditions displayed strong continuous syndecan-3 gene expression; upon induction of maturation by vitamin C treatment, syndecan-3 gene expression was markedly down-regulated. Treatment with FGF-2 for 24 h stimulated both syndecan-3 gene expression and chondrocyte proliferation; this growth stimulation was counteracted by cotreatment with heparinase I or III. The results of the study indicate that syndecan-3 participates in the maturation of chondrocytes during endochondral ossification and represents a regulator of the proliferative phase of this multistage process.

Inhibition of in vitro limb cartilage differentiation by syndecan-3 antibodies

The transmembrane heparan sulfate proteoglycan syndecan-3 is transiently expressed in high amounts during the cellular condensation process that characterizes the onset of limb cartilage differentiation. During condensation, limb mesenchymal cells become closely juxtaposed and undergo cell-cell and cell-matrix interactions that are necessary to trigger cartilage differentiation and cartilage-specific gene expression. To test directly the possible involvement of syndecan-3 in regulating the onset of limb chondrogenesis, we examined the effect of polyclonal antibodies against a syndecan-3 fusion protein on the chondrogenic differentiation of chick limb mesenchymal cells in micromass culture. Syndecan-3 antiserum elicits a dose-dependent inhibition of the accumulation of Alcian blue-stainable cartilage matrix by high density limb mesenchymal cell micromass cultures (2 x 10(5) cells/10 microliters) and a corresponding reduction in steady-state levels of mRNAs for cartilage-characteristic type II collagen and the core protein of the cartilage proteoglycan aggrecan. In preimmune serum-treated control cultures proliferating cells are limited to the periphery of areas of cartilage matrix deposition, whereas large numbers of proliferating cells are uniformly distributed throughout the undifferentiated cultures supplemented with syndecan-3 antiserum. Limb mesenchymal cells cultured at lower densities (1 x 10(5) cells/10 microliters) in the presence of preimmune serum form extensive precartilage condensations characterized by the close juxtaposition of rounded cells by day 2 of culture. In contrast, in the presence of syndecan-3 antiserum, the cells fail to aggregate but rather remain flattened and spatially separated from one another, suggeting that syndecan-3 antibodies impair the formation of precartilage condensations. These results indicate that syndecan-3 plays an important role in regulating the onset of limb chondrogenesis, perhaps by mediating the cell-cell and cell-matrix interactions required for condensation and subsequent cartilage differentiation.

IGF-I and insulin in the acquisition of limb-forming ability by the embryonic lateral plate

Acquisition of limb-forming ability by discrete regions of the lateral plate of the chick embryo is dependent on a medial-lateral inductive signaling cascade moving sequentially from the area of Hensen's node to the somitic mesoderm, the intermediate mesoderm, and then to the prospective limb-forming regions of the lateral plate. IGF-I and insulin are expressed by medial tissues as they are influencing the prospective limb-forming regions of the lateral plate. Here we report that IGF-I and insulin, but not FGF-2 or FGF-4, induce the formation of limb bud-like structures in vitro from prospective limb regions before they have acquired the ability to form limbs independent of medial tissues, and also induce the formation of limb bud-like structures from the prospective flank. The limb bud-like structures induced by IGF-I and insulin possess a thickened cap of ectoderm along their distal tips that resembles the apical ectodermal ridge (AER) and this thickened distal apical ectoderm expresses the AER-characteristic homeobox-containing gene Msx-2. Like in normal limb buds, a population of highly proliferating cells which express the homeobox-containing gene Msx-1 are localized in the mesoderm directly subjacent to the thickened AER-like structures induced by IGF-I and insulin. However, the limb bud-like structures induced by IGF-I and insulin do not express sonic hedgehog, which encodes a secreted signaling molecule that has been implicated in regulating the anteroposterior patterning of the developing limb bud. IGF-I- and insulin-treated prospective limb explants give rise to rudimentary limbs containing identifiable skeletal elements when grafted into the coelom or to somites of host embryos. Overall, these results suggest that IGF-I and insulin may be endogenous signals produced by medial tissues that are involved in conferring limb-forming ability to the lateral plate and may promote the initial outgrowth of limb buds and possibly induce the AER. However, other signals are necessary to promote the expression of genes such as sonic hedgehog that regulate the patterning of the developing limb.

IGF-I, insulin and FGFs induce outgrowth of the limb buds of amelic mutant chick embryos

IGF-I, insulin, FGF-2 and FGF-4 have been implicated in the reciprocal interactions between the apical ectodermal ridge (AER) and underlying mesoderm required for outgrowth and patterning of the developing limb. To study further the roles of these growth factors in limb outgrowth, we have examined their effects on the in vitro morphogenesis of limb buds of the amelic mutant chick embryos wingless (wl) and limbless (ll). Limb buds of wl and ll mutant embryos form at the proper time in development, but fail to undergo further outgrowth and subsequently degenerate. Wl and ll limb buds lack thickened AERs capable of promoting limb outgrowth, and their thin apical ectoderms fail to express the homeobox-containing gene Msx-2, which is highly expressed by normal AERs and has been implicated in regulating AER activity. Here we report that exogenous IGF-I and insulin, and, to a lesser extent, FGF-2 and FGF-4 induce the proliferation and directed outgrowth of explanted wl and ll mutant limb buds, which in vitro, like in vivo, normally fail to undergo outgrowth and degenerate. IGF-I and insulin, but not FGFs, also cause the thin apical ectoderms of wl and ll limb buds to thicken and form structures that grossly resemble normal AERs and, moreover, induce high level expression of Msx-2 in these thickened AER-like structures. Neither IGF-I, insulin nor FGFs induce expression of the homeobox-containing gene Msx-1 in the subapical mesoderm of wl or ll limb buds, although FGFs, but not IGF-I or insulin, maintain Msx-1 expression in normal (non-mutant) limb bud explants lacking an AER. The implications of these results to the relationships among the wl and ll genes, IGF-I/insulin, FGFs, Msx-2 and Msx-1 in the regulation of limb outgrowth is discussed.

Differential regulation of COL2A1 expression in developing and mature chondrocytes

To investigate the regulation of type II collagen gene expression in cells undergoing chondrogenic differentiation, we have employed a 5-kbp genomic fragment of the human type II collagen gene which contains 1.8kbp of upstream sequences, the transcription start site, the first exon and 3 kbp of intronic sequences, fused to either lac Z or chloramphenicol acetyl transferase-reporter gene. Transient expression studies revealed a parallel increase in transgene activity and endogenous type II collagen mRNA levels during the onset of the cartilage differentiation of limb mesenchymal cells in high-density micromass cultures. At later periods in culture, however, the transgene activity declines, although steady-state levels of type II collagen mRNA are reported to continue to increase (Kosher et al.: J. Cell. Biol. 102: 1151-1156, 1986; Kravis and Upholt. Dev. Biol. 108: 164-172, 1985). In addition, the activity of the transgene is seven-fold higher at the onset of chondrogenic differentiation in micromass cultures that in well differentiated sternal chondrocytes, although similar levels of type II collagen transcripts are found in these cells. Furthermore, deletions of intronic segments resulted in greater drop in activity of the constructs in differentiating chondrocytes in micromass cultures than in mature sternal chondrocytes. The expression of the construct in transgenic mice is higher at the onset of chondrogenic differentiation and in newly differentiated chondrocytes than in more mature differentiated chondrocytes. Based on these observations, it appears that the mechanisms involved in the regulation of the type II collagen gene at the onset of chondrocyte differentiation are different from those resulting in the maintenance of its expression in fully differentiated chondrocytes.

The expression pattern of the Distal-less homeobox-containing gene Dlx-5 in the developing chick limb bud suggests its involvement in apical ectodermal ridge activity, pattern formation, and cartilage differentiation

Here we report the isolation from a chick limb bud cDNA library of a cDNA that contains the full coding sequence of chicken Dlx-5, a member of the Distal-less (Dlx) family of homeobox-containing genes that encode homeodomains highly similar to that of the Drosophila Distal-less gene, a gene that is required for limb development in the Drosophila embryo. The expression pattern of Dlx-5 in the developing chick limb bud suggests that it may be involved in several aspects of limb morphogenesis. Dlx-5 is expressed in the apical ectodermal ridge (AER) which directs the outgrowth and patterning of underlying limb mesoderm. During early limb development Dlx-5 is also expressed in the mesoderm at the anterior margin of the limb bud and in a discrete group of mesodermal cells at the mid-proximal posterior margin that corresponds to the posterior necrotic zone. These mesodermal domains of Dlx-5 expression roughly correspond to the anterior and posterior boundaries of the progress zone, the group of highly proliferating undifferentiated mesodermal cells underneath the AER that will give rise to the skeletal elements of the limb and associated structures. The AER and anterior and posterior mesodermal domains of Dlx-5 expression are regions in which the homeobox-containing gene Msx-2 is also highly expressed, suggesting that Dlx-5 and Msx-2 might be involved in regulatory networks that control AER activity and demarcate the progress zone. In addition, Dlx-5 is expressed in high amounts by the differentiating cartilaginous skeletal elements of the limb, suggesting it may be involved in regulating the onset of limb cartilage differentiation.

Identification of a spatially specific enhancer element in the chicken Msx-2 gene that regulates its expression in the apical ectodermal ridge of the developing limb buds of transgenic mice

Msx-2 is a member of the Msx family of homeobox-containing genes expressed in a variety of embryonic tissues involved in epithelial-mesenchymal interactions and pattern formation. In the developing chick limb bud, Msx-2 is expressed in the apical ectodermal ridge, which plays a crucial role in directing the growth and patterning of limb mesoderm. In addition, Msx-2 is expressed in the anterior nonskeletal-forming mesoderm of the limb bud, in the posterior necrotic zone, and in the interdigital mesenchyme. Studies of the altered expression patterns of Msx-2 in amelic and polydactylous mutant chick limbs have suggested that the apical ectodermal ridge and mesodermal domains of Msx-2 expression are independently regulated and that there might be separate cis-regulatory elements in the Msx-2 gene controlling its spatially distinct domains of expression. To test this hypothesis, we have isolated the chicken Msx-2 gene and have tested the ability of various regions of the gene to target expression of LacZ reporter gene to specific regions of the limbs of transgenic mice. A variety of these constructs are consistently expressed only in the apical ectodermal ridge and the ectoderm of the genital tubercle and are not expressed in the mesoderm of the limb bud or in other regions of the embryo where the endogenous Msx-2 gene is expressed. These results suggest the presence of spatially specific cis-regulatory elements in the Msx-2 gene. We identified a 348-bp region in the 5' flanking region of the Msx-2 gene which can act as an apical ectodermal ridge enhancer element when placed in reverse orientation in front of the reporter gene with transcription initiation directed by the minimal hsp68 promoter.

Characterization of chicken syndecan-3 as a heparan sulfate proteoglycan and its expression during embryogenesis

Syndecan-3 is one of four identified members of a family of transmembrane proteoglycans (the syndecans) that possess highly similar cytoplasmic and transmembrane domains and may function as extracellular matrix receptors and/or low affinity receptors for signaling molecules such as FGF. We previously reported the cloning of a partial cDNA for chicken syndecan-3. Here we report the isolation of a syndecan-3 cDNA containing additional 5' sequence which includes a potential methionine start codon and putative signal sequence. In vitro translation of syndecan-3 cDNA in the presence and absence of microsomes suggests that the putative signal sequence is functional, suggesting that the cDNA may encompass the full coding sequence. We also identify syndecan-3 as a heparan sulfate proteoglycan and report its expression pattern during chicken embryogenesis using polyclonal antibodies raised against a recombinant fusion protein. We detect abundant syndecan-3 expression in the developing brain and neural tube, including a striking expression in the floor plate of the neural tube. During limb development, syndecan-3 is expressed in the distal mesenchymal cells of the limb bud which are undergoing outgrowth in response to the apical ectodermal ridge. Syndecan-3 is also transiently expressed during the formation of the precartilage condensations of the skeletal elements of the limb and subsequently in association with the differentiating osteoblasts of the periosteum. Expression is also observed in several areas of tissue interactions including the developing lens, otic vesicle, genital ridge, sclerotome, and feather buds.

Studies on insulin-like growth factor-I and insulin in chick limb morphogenesis

The apical ectodermal ridge (AER) promotes the proliferation and directed outgrowth of the subridge mesodermal cells of the developing limb bud, while suppressing their differentiation. Insulin-like growth factor-I (IGF-I) and its receptor are expressed by the subridge mesodermal cells of the chick limb bud growing out in response to the AER, and specific insulin receptors are present in the limb bud during its outgrowth. To study the possible roles of IGF-I and insulin in limb outgrowth, we have examined their effects on the morphogenesis of posterior and anterior portions of the distal tip of stage 25 embryonic chick wing buds subjected to organ culture in serum-free medium in the presence or absence of the AER and limb ectoderm. The distal mesoderm of control posterior explants lacking an AER or all limb ectoderm ceases expressing IGF-I mRNA, exhibits little or no proliferation, fails to undergo outgrowth, and rapidly differentiates. Exogenous IGF-I and insulin promote the outgrowth and proliferation and suppress the differentiation of distal mesodermal cells in posterior explants lacking an AER or limb ectoderm, thus mimicking at least to some extent the outgrowth promoting and anti-differentiative effects normally elicited on the subridge mesoderm by the AER. Furthermore, IGF-I and insulin-treated posterior explants exhibit high IGF-I mRNA expression, indicating that IGF-I and insulin maintain the expression of endogenous IGF-I by the subridge mesoderm. We have also found IGF-I and insulin can affect the morphology and activity of the AER. When the posterior portion of the wing bud tip is cultured with the AER intact in control medium, on day 4-5 the AER flattens, ceases expressing high amounts of the AER-characteristic homeobox-containing gene Msx2, and concomitantly an elongated cartilaginous element differentiates in the subridge mesoderm. In contrast, in the presence of exogenous IGF-I or insulin the AER of such explants does not flatten, continues expressing high amounts of Msx2, and the subridge mesoderm remains undifferentiated and proliferative. Thus, exogenous IGF-I and insulin maintain the thickness of the AER and sustain its expression of Msx2, while sustaining the anti-differentiative effect normally elicited on the subridge mesoderm by a thickened functional AER. Notably, we have also found that exogenous IGF-I and insulin induce the formation of a thickened ridge-like structure that expresses high amounts of Msx2 from the normally thin distal anterior ectoderm of the limb bud, while promoting dramatic outgrowth and proliferation of the anterior mesoderm, which normally undergoes little outgrowth or proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

Limb mesenchymal cells inhibited from undergoing cartilage differentiation by a tumor promoting phorbol ester maintain expression of the homeobox-containing gene Msx1 and fail to exhibit gap junctional communication

Tumor promoting phorbol esters are potent inhibitors of the chondrogenic differentiation of limb mesenchymal cells, but the mechanism by which these agents elicit their antichondrogenic effect is unknown. Here we report that limb mesenchymal cells inhibited from undergoing chondrogenesis by a tumor promoting phorbol ester exhibit deregulated expression of the homeobox-containing gene Msx1, a gene implicated in suppressing differentiation of limb mesenchymal cells, and fail to exhibit the extensive gap junctional intercellular communication that normally occurs at the onset of chondrogenesis. These results suggest that tumor promoting activity of phorbol esters may relate to their ability to modulate the expression of regulatory genes involved in controlling terminal differentiation, as well as to their ability to disrupt the intercellular communication involved in initiating the differentiated phenotype of cells.

Expression patterns of mRNAs for the gap junction proteins connexin43 and connexin42 suggest their involvement in chick limb morphogenesis and specification of the arterial vasculature

Gap junctions which comprise a family of proteins called connexins have been implicated in the morphogenesis of the chick limb bud. We have examined the expression patterns of two members of the connexin family, connexin43 (Cx43) and connexin42 (Cx42), during the early development of the chick limb bud and embryo by in situ hybridization. Cx43 mRNA is expressed in high amounts in the apical ectodermal ridge (AER), which promotes the outgrowth of the mesodermal cells of the limb bud, and in the ectopic AER of the limb buds of polydactylous diplopodia-5 mutant embryos. In contrast, little Cx43 expression is detectable in nonridge limb ectoderm at early stages of limb development. These results suggest that Cx43 gap junctions may integrate the activity of the cells comprising the AER and compartmentalize them into a functionally distinct entity capable of directing limb outgrowth. In addition, Cx43 exhibits high expression in the posterior subridge mesoderm of the early limb bud that is growing out in response to the AER, but little expression in the anterior mesoderm. This graded distribution of Cx43 transcripts correlates with a functional gradient of gap junctional communication along the anteroposterior (AP) axis, and suggests that Cx43 gap junctions may be involved in pattern formation across the AP axis. At later stages of development, Cx43 is transiently expressed in high amounts in the precartilage condensations of the carpals and metacarpals, at a time when critical cell-cell interactions are occurring that trigger cartilage differentiation. In contrast, in the developing limb, Cx42 is expressed exclusively by the central artery. In the remainder of the chick embryo, Cx42 is expressed in high amounts by the vessels comprising the arterial vasculature, but is not expressed by the venous vasculature. Thus, Cx42 gap junctions may be involved in specification of the arterial vasculature of the limb and embryo. Cx42, but not Cx43, is expressed in the ventricle of the heart, and by cells along the intrasclerotomal fissure that separates the rostral and caudal halves of the sclerotome of somites into distinct communication compartments.

Wnt-5a and Wnt-7a are expressed in the developing chick limb bud in a manner suggesting roles in pattern formation along the proximodistal and dorsoventral axes

The Wnt gene family encodes a group of secreted signalling molecules that have been implicated in the regulation of cell fate and pattern formation during embryogenesis. We have examined the patterns of expression of two members of the chicken Wnt family, Wnt-5a and Wnt-7a, during development of the chick limb bud. Wnt-5a is expressed in the apical ectodermal ridge which directs outgrowth of limb mesoderm. Wnt-5a also exhibits three quantitatively distinct domains of expression along the proximodistal (PD) axis of the limb mesoderm that may correspond to the regions which will give rise to the three distinct PD segments of the limb, the autopod, zeugopod, and stylopod. In contrast, Wnt-7a expression in the limb bud is specifically limited to the dorsal ectoderm. These observations suggest possible roles for Wnt-5a and Wnt-7a in pattern formation along the PD and dorsoventral axes of the developing chick limb bud. In addition, Wnt-5a and Wnt-7a exhibit spatially discrete domains of expression in several other regions of the chick embryo consistent with developmental roles for these genes in a variety of other tissues.

Ectoderm from various regions of the developing chick limb bud differentially regulates the expression of the chicken homeobox-containing genes GHox-7 and GHox-8 by limb mesenchymal cells

The apical ectodermal ridge expresses high amounts of the homeobox gene GHox-8 when placed upon dissociated limb mesenchymal cells in culture and induces high expression of GHox-7, but only low expression of GHox-8, in the underlying mesenchymal cells. Ectoderm from the proximal anterior border of the limb induces high expression of both GHox-7 and GHox-8, while ectoderm from the proximal posterior border does not induce expression of either gene. Thus, ectoderm in various regions of the limb bud has distinct regulatory activities and may be involved in controlling the regionally specific expression of GHox-7 and GHox-8 in the mesoderm.

The expression pattern of the chicken homeobox-containing gene GHox-7 in developing polydactylous limb buds suggests its involvement in apical ectodermal ridge-directed outgrowth of limb mesoderm and in programmed cell death

The limb buds of the polydactylous mutant embryos, talpid2 and diplopodia-5, possess expanded distal apexes surmounted by prolongated thickened apical ectodermal ridges that promote the outgrowth and formation of digits from both the anterior and posterior mesoderm of the mutant limb buds. The chicken homeobox-containing gene GHox-7 exhibits an expanded domain of expression throughout the expanded subridge mesoderm of the mutant limb buds, providing support for the hypothesis that GHox-7 expression by subridge mesenchymal cells is involved in the outgrowth-promoting effect of the apical ectodermal ridge. During normal limb development GHox-7 is also expressed by the mesoderm in the proximal anterior nonchondrogenic periphery of the limb bud, which includes, but is not limited to the anterior necrotic zone. GHox-7 is also expressed in the posterior necrotic zone at the mid-proximal posterior edge of the limb bud. In contrast, GHox-7 is not expressed in either the proximal anterior or posterior peripheral mesoderm of talpid2 and diplopodia-5 limb buds which lack proximal anterior and posterior necrotic zones. Furthermore, retinoic acid-coated bead implants, which diminish cell death in the anterior necrotic zone, elicit a local inhibition of GHox-7 expression in the proximal anterior peripheral mesoderm. These results support the suggestion that GHox-7 may be involved in defining regions of programmed cell death during limb development. Furthermore, these studies indicate that the distal subridge and proximal anterior nonchondrogenic mesodermal domains of GHox-7 expression are independently regulated.

Apical ridge dependent and independent mesodermal domains of GHox-7 and GHox-8 expression in chick limb buds

The homeobox-containing genes GHox-7 and GHox-8 have been proposed to play fundamental roles in limb development. The expression of GHox-8, by the apical ridge cells, and GHox-7, in the subridge mesoderm, suggests the involvement of these two genes in limb outgrowth and proximo-distal pattern formation. A straightforward way to test this is to remove the apical ridge. Here we report the relationship between the mesodermal expression of GHox-7 and GHox-8 and the apical ectodermal ridge in the chick limb bud. The data from ridge removal experiments indicate that there are at least two domains of GHox-7 expression in the apical limb bud mesoderm. The posterior subridge GHox-7 domain in the progress zone requires the influence of the apical ridge for continued expression, while the anterior GHox-7 domain continues expression after ridge removal. Posterior subridge mesoderm is exquisitely sensitive to the loss of the ridge in that GHox-7 expression by these cells is reduced in only two hours and undetectable by three hours after ridge removal. It would appear that one of the ways progress zone cells respond to the apical ridge signal is by expressing GHox-7. The loss of ridge influence whether by growth at the apex or by ridge removal is followed by an unusually rapid decline in detectable GHox-7 transcripts. Maintenance of GHox-8 expression by the anterior mesoderm appears to be independent of the presence of the apical ridge.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of the chicken homeobox-containing genes GHox-4.6 and GHox-8 in the specification of positional identities during the development of normal and polydactylous chick limb buds

During early stages of normal chick limb development, the homeobox-containing (HOX) gene GHox-4.6 is expressed throughout the posterior mesoderm of the wing bud from which most of the skeletal elements including the digits will develop, whereas GHox-8 is expressed in the anterior limb bud mesoderm which will not give rise to skeletal elements. In the present study, we have examined the expression of GHox-4.6 and GHox-8 in the wing buds of two polydactylous mutant chick embryos, diplopodia-5 and talpid2, from which supernumerary digits develop from anterior limb mesoderm, and have also examined the expression of these genes in response to polarizing zone grafts and retinoic acid-coated bead implants which induce the formation of supernumerary digits from anterior limb mesoderm. We have found that the formation of supernumerary digits from the anterior mesoderm in mutant and experimentally induced polydactylous limb buds is preceded by the ectopic expression of GHox-4.6 in the anterior mesoderm and the coincident suppression of GHox-8 expression in the anterior mesoderm. These observations suggest that the anterior mesoderm of the polydactylous limb buds is “posteriorized” and support the suggestion that GHox-8 and GHox-4.6, respectively, are involved in specifying the anterior non-skeletal and posterior digit-forming regions of the limb bud. Although the anterior mesodermal domain of GHox-8 expression is severely impaired in the mutant and experimentally induced polydactylous limb buds, this gene is expressed by the prolonged, thickened apical ectodermal ridges of the polydactylous limb buds that extend along the distal anterior as well as the distal posterior mesoderm.(ABSTRACT TRUNCATED AT 250 WORDS)

Syndecan 3: a member of the syndecan family of membrane-intercalated proteoglycans that is expressed in high amounts at the onset of chicken limb cartilage differentiation

A partial cDNA that encodes a newly discovered member of the syndecan family of integral membrane proteoglycans, which we have termed syndecan 3, has been isolated from an embryonic chicken limb bud cDNA library. Syndecan 3 is distinct from but structurally related to syndecan and fibroglycan, two previously characterized members of this family of membrane-intercalated proteoglycans. Syndecan 3 contains a cytoplasmic domain potentially associated with the cytoskeleton that is 85% identical in amino acid sequence to the cytoplasmic domain of syndecan. Syndecan 3 also possesses a hydrophobic transmembrane domain and an extracellular domain containing several clustered potential glycosaminoglycan attachment sites. Like syndecan, the ectodomain of syndecan 3 has a single dibasic protease-susceptible site adjacent to the transmembrane domain, which might be involved in shedding the ectodomain from the cell surface. A striking feature of syndecan 3 is an extensive (182 amino acid) threonine, serine, and proline (T+S+P)-rich domain that closely resembles T+S+P-rich regions in several mucin-like proteins in which O-linked oligosaccharides are bound to the threonine and serine residues. Syndecan 3 is expressed in high amounts during a critical phase of chicken limb chondrogenesis in which limb mesenchymal cells condense, round up, and interact with one another before depositing a cartilage matrix. The multiple functional domains of syndecan 3 provide potential sites for mediating the adhesive cell-matrix interactions and cytoskeletal reorganization involved in this critical condensation process.

The pattern of expression of the chicken homolog of HOX1I in the developing limb suggests a possible role in the ectodermal inhibition of chondrogenesis

Homeobox-containing genes have been implicated in a variety of patterning events during vertebrate limb development. In an attempt to isolate cDNAs corresponding to 5' members of the chicken HOX 4 cluster of homeobox-containing genes, a cDNA library constructed from mRNAs expressed during early stages of chick limb development was screened with probes generated by the polymerase chain reaction (PCR) using oligonucleotide primers corresponding to sequences in the homeoboxes of the human HOX4C and HOX4F genes, the human homologs of Hox-4.4 and Hox-4.6. This screening resulted in the isolation of full length cDNAs for the chicken homolog of HOX4F (cognate of mouse Hox-4.6), which we have termed GHox-4.6, and the chicken homolog of human HOX1I, which we have named GHox-1i, a paralog of Hox-4.6 in the HOX 1 cluster. The homeodomains encoded by GHox-4.6 and GHox-1i differ by only three amino acids, and the two proteins show extensive similarity along their entire lengths. Despite their sequence similarity, in situ hybridization analysis has revealed that GHox-4.6 and GHox-1i exhibit strikingly different spatial patterns of expression during embryonic chick limb development. At early stages of limb development (stages 20-22), GHox-4.6 transcripts are present in high amounts throughout the posterior half of the limb mesoderm and are absent from the anterior half of the mesoderm, an expression pattern consistent with the possible involvement of GHox-4.6 in the specification of posterior positional identity. In contrast, GHox-1i exhibits no distinct anterior-posterior polarity of expression at stage 22, but rather is expressed in high amounts throughout the mesenchyme of the limb bud. At later stages of development (stage 25), GHox-1i continues to be expressed in high amounts throughout the undifferentiated mesenchyme subjacent to the apical ectodermal ridge, and, in addition, is expressed in the mesodermal cells in the proximal peripheral regions of the limb bud subjacent to the ectoderm which are differentiating into nonchondrogenic lineages. Conversely, little or no expression of GHox-1i is detectable in the proximal central core of the limb bud where chondrogenic differentiation is occurring. Thus, GHox-1i is expressed by the undifferentiated subridge mesenchymal cells and proximal peripheral mesenchymal cells of the limb bud that are being inhibited from undergoing chondrogenesis by the apical ectodermal ridge and nonridge ectoderm.(ABSTRACT TRUNCATED AT 400 WORDS)

GHox-7: A chicken homeobox-containing gene expressed in a fashion consistent with a role in patterning events during embryonic chick limb development

Homeobox-containing (HOX) genes are thought to be involved in the regulation of pattern formation and specification of positional information during vertebrate limb development. We report the isolation from a chick limb bud cDNA library of several overlapping chicken HOX cDNAs, which on the basis of their nucleotide and deduced amino acid sequences have been identified as corresponding to the chicken cognate of mouse Hox-7.1. The gene encoding these chicken (Gallus) HOX cDNAs has been designated GHox-7, and is a member of a family of vertebrate HOX genes that are highly similar in sequence to the Drosophila msh gene. GHox-7 encodes an mRNA transcripts of about 1.8-2.0 kb that is expressed at early stages of chick limb development. In situ hybridization analysis has revealed that GHox-7 is expressed in limb bud mesoderm in a temporal and spatial fashion. This is consistent with its involvement in specifying anterior positional identity and/or in the response of limb mesenchymal cells to the apical ectodermal ridge (AER), which directs polarized proximodistal limb outgrowth. At early stages (stages 20-21) of chick limb development when positional values along the anterior-posterior (A-P) axis are being specified, GHox-7 exhibits an asymmetric arc of expression extending from the anterior border of the limb bud to the mesenchymal cells directly subjacent to the AER.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered expression of the chicken homeobox-containing genes GHox-7 and GHox-8 in the limb buds of limbless mutant chick embryos

It has been suggested that the reciprocal expression of the chicken homeobox-containing genes GHox-8 and GHox-7 by the apical ectodermal ridge and subjacent limb mesoderm might be involved in regulating the proximodistal outgrowth of the developing chick limb bud. In the present study the expression of GHox-7 and GHox-8 has been examined by in situ and dot blot hybridization in the developing limb buds of limbless mutant chick embryos. The limb buds of homozygous mutant limbless embryos form at the proper time in development (stage 17/18), but never develop an apical ectodermal ridge, fail to undergo normal elongation, and eventually degenerate. At stage 18, which is shortly following the formation of the limb bud, the expression of GHox-7 is considerably reduced (about 3-fold lower) in the mesoderm of limbless mutant limb buds compared to normal limb bud mesoderm. By stages 20 and 21, as the limb buds of limbless embryos cease outgrowth, GHox-7 expression in limbless mesoderm declines to very low levels, whereas GHox-7 expression increases in the mesoderm of normal limb buds which are undergoing outgrowth. In contrast to GHox-7, expression of GHox-8 in limbless mesoderm at stage 18 is quantitatively similar to its expression in normal limb bud mesoderm, and in limbless and normal mesoderm GHox-8 expression is highly localized in the anterior mesoderm of the limb bud. In normal limb buds, GHox-8 is also expressed in high amounts by the apical ectodermal ridge.(ABSTRACT TRUNCATED AT 250 WORDS)

A gradient of gap junctional communication along the anterior-posterior axis of the developing chick limb bud

A modification of the scrape-loading/dye transfer technique was used to study gap junctional communication along the anterior-posterior (A-P) axis of embryonic chick wing buds at an early stage of development (stage 20/21) when positional values along the A-P axis are being specified. Extensive intercellular transfer of the gap junction-permeable dye, lucifer yellow, from scrape-loaded mesenchymal cells to contiguous cells occurs in the posterior mesenchymal tissue of the wing bud adjacent to the zone of polarizing activity, which is thought to be the source of a diffusible morphogen that specifies A-P positional identity according to its local concentration. Considerably less transfer of lucifer yellow dye occurs in scrape-loaded mesenchymal tissue in the middle of the limb bud compared to posterior mesenchymal tissue, and little or no transfer of lucifer yellow is observed in the mesenchymal tissue in the anterior portion of the limb bud. No intercellular transfer of the gap junction-impermeable dye, rhodamine dextran, occurs in any region of the limb bud. These results indicate that there is a gradient of gap junctional communication along the A-P axis of the developing chick wing bud. This gradient of gap junctional communication along the A-P axis might generate a graded distribution of a relatively low molecular weight intracellular regulatory molecule involved in specifying A-P positional identities.

Type IX collagen gene expression during limb cartilage differentiation

Changes in the steady-state levels of mRNAs for the alpha 1(IX) and alpha 2(IX) polypeptide chains of cartilage-characteristic type IX collagen were examined during the course of chick limb chondrogenesis in vitro and in vivo. Cytoplasmic type IX collagen mRNAs begin to accumulate at the onset of overt chondrogenesis in high density micromass culture coincident with the crucial condensation phase of the process, in which prechondrogenic mesenchymal cells become closely juxtaposed prior to depositing a cartilage matrix. The initiation of type IX collagen mRNA accumulation at condensation coincides with the initiation of accumulation of cartilage proteoglycan core protein mRNA and with a striking increase in type II collagen mRNA accumulation. Following condensation in vitro, there is a concomitant progressive increase in cytoplasmic type IX collagen, core protein, and type II collagen mRNA levels which parallels the progressive accumulation of cartilage matrix. Type IX collagen mRNAs also begin to accumulate at the initiation of overt chondrogenesis in vivo in the chondrogenic central core of the developing limb bud. In contrast, little, or no type IX collagen mRNAs are detectable in the nonchondrogenic peripheral regions of the developing limb bud.

Expression of the chicken homeobox-containing gene GHox-8 during embryonic chick limb development

Homeobox-containing genes are thought to be involved in the regulation of pattern formation and specification of positional information during vertebrate limb development. Because of its accessibility to microsurgical manipulation, the developing chick limb bud provides a powerful system for investigating the role of homeobox-containing genes in patterning events. We report the isolation from a chick limb bud cDNA library of a chicken homeobox-containing cDNA, which on the basis of its nucleotide and deduced amino acid sequences has been identified as the chicken cognate of mouse Hox-8. The gene encoding this chicken (Gallus) homeobox-containing cDNA has been designated GHox-8, and is a member of a family of vertebrate homeobox-containing genes that are highly similar in sequence to the Drosophila msh gene. GHox-8 encodes an mRNA transcript of about 3 kb that is expressed at several early stages of chick limb development. In situ and dot-blot hybridization analyses have revealed that GHox-8 is expressed in limb bud mesoderm in a temporal and spatial fashion consistent with its involvement in specifying anterior positional identity. At early stages (stages 20-21) of chick limb development when positional values along the anterior-posterior (A-P) axis are being specified, GHox-8 is expressed in high amounts in the anterior mesoderm of the wing bud. Little expression of the gene is detectable in the middle region of the wing bud mesoderm or in the posterior mesoderm that contains the zone of polarizing activity, which is thought to be the source of a diffusible morphogen, possibly retinoic acid, that specifies the A-P positional values of the skeletal elements of the limb according to its local concentration. Similarly, at later stages of development (stages 23-25), high expression of GHox-8 is localized to the proximal anterior periphery of the wing bud, with no detectable expression in the proximal dorsal and ventral (myogenic) regions, or in the chondrogenic central core. In the proximal posterior periphery of the wing bud at these later stages of development, expression of GHox-8 is limited to a small region in the mid-proximal periphery corresponding to the posterior necrotic zone in which programmed cell death is occurring. The possible involvement of GHox-8 in programmed cell death during limb development is also suggested by the fact that it is expressed in the necrotic interdigital mesenchyme in 6-7 day (stage 31-32) wing buds.(ABSTRACT TRUNCATED AT 400 WORDS)

Gap junctional communication during limb cartilage differentiation

The onset of cartilage differentiation in the developing limb bud is characterized by a transient cellular condensation process in which prechondrogenic mesenchymal cells become closely apposed to one another prior to initiating cartilage matrix deposition. During this condensation process intimate cell-cell interactions occur which are necessary to trigger chondrogenic differentiation. In the present study, we demonstrate that extensive cell-cell communication via gap junctions as assayed by the intercellular transfer of lucifer yellow dye occurs during condensation and the onset of overt chondrogenesis in high density micromass cultures prepared from the homogeneous population of chondrogenic precursor cells comprising the distal subridge region of stage 25 embryonic chick wing buds. Furthermore, in heterogeneous micromass cultures prepared from the mesodermal cells of whole stage 23/24 limb buds, extensive gap junctional communication is limited to differentiating cartilage cells, while the nonchondrogenic cells of the cultures that are differentiating into the connective tissue lineage exhibit little or no intercellular communication via gap junctions. These results provide a strong incentive for considering and further investigating the possible involvement of cell-cell communication via gap junctions in the regulation of limb cartilage differentiation.

Stimulation of limb cartilage differentiation by cyclic AMP is dependent on cell density

Cyclic AMP (cAMP) has been implicated in the regulation of limb cartilage differentiation. This study represents an attempt to clarify potential mechanisms by which cAMP might regulate chondrogenesis. We have found that the ability of cAMP to stimulate limb cartilage differentiation in vitro is dependent on cell density. Dibutyryl cAMP (dbcAMP) elicits a striking increase in the accumulation of Alcian blue, pH 1.0-positive cartilage matrix, and a corresponding three- to fourfold increase in the accumulation of 35S-labeled glycosaminoglycans (GAG) by limb mesenchymal cells cultured in low serum medium at densities greater than confluence (i.e. micromass cultures established with 1-2 x 10(5) cells in 10 microliters of medium). Moreover, dbcAMP causes a striking (two- to fourfold) increase in the steady-state cytoplasmic levels of mRNAs for cartilage-characteristic type II collagen and the core protein of cartilage-specific sulfated proteoglycan in these high density, supraconfluent cultures. In contrast, cAMP does not promote the chondrogenesis of limb mesenchymal cells cultured at subconfluent densities (i.e. cultures initiated with 2.5-5 x 10(4) cells in 10 microliters of medium). In these low density cultures, dbcAMP does not promote the formation of cartilage matrix, sulfated GAG accumulation or the accumulation of cartilage-specific mRNAs. These observations suggest that cAMP may exert its regulatory effect in part by facilitating cell-cell communication during the critical condensation phase of chondrogenesis.

Promotion of embryonic chick limb cartilage differentiation by transforming growth factor-beta

This study represents a first step in investigating the possible involvement of transforming growth factor-beta (TGF-beta) in the regulation of embryonic chick limb cartilage differentiation. TGF-beta 1 and 2 (1-10 ng/ml) elicit a striking increase in the accumulation of Alcian blue, pH 1-positive cartilage matrix, and a corresponding twofold to threefold increase in the accumulation of 35S-sulfate- or 3H-glucosamine-labeled sulfated glycosaminoglycans (GAG) by high density micromass cultures prepared from the cells of whole stage 23/24 limb buds or the homogeneous population of chondrogenic precursor cells comprising the distal subridge mesenchyme of stage 25 wing buds. Moreover, TGF-beta causes a striking (threefold to sixfold) increase in the steady-state cytoplasmic levels of mRNAs for cartilage-characteristic type II collagen and the core protein of cartilage-specific proteoglycan. Only a brief (2 hr) exposure to TGF-beta at the initiation of culture is sufficient to stimulate chondrogenesis, indicating that the growth factor is acting at an early step in the process. Furthermore, TGF-beta promotes the formation of cartilage matrix and cartilage-specific gene expression in low density subconfluent spot cultures of limb mesenchymal cells, which are situations in which little, or no chondrogenic differentiation normally occurs. These results provide strong incentive for considering and further investigating the role of TGF-beta in the control of limb cartilage differentiation.

Fibronectin gene expression during limb cartilage differentiation

A critical event in limb cartilage differentiation is a transient cellular condensation process in which prechondrogenic mesenchymal cells become closely juxtaposed and interact with one another prior to initiating cartilage matrix deposition. Fibronectin (FN) has been suggested to be involved in regulating the onset of condensation and chondrogenesis by actively promoting prechondrogenic aggregate formation during the process. We have performed a systematic quantitative study of the expression of the FN gene during the progression of chondrogenesis in vitro and in vivo. In high-density micromass cultures of limb mesenchymal cells, FN mRNA levels increase about 5-fold coincident with the crucial condensation process, and remain relatively high during the initial deposition of cartilage matrix by the cells. Thereafter, FN mRNA levels progressively decline to relatively low levels as the cultures form a virtually uniform mass of cartilage. The changes in FN mRNA levels in vitro are paralleled closely by changes in the relative rate of FN synthesis as determined by pulse-labeling and immunoprecipitation analysis. The relative rate of FN synthesis increases 4- to 5-fold at condensation and the onset of chondrogenesis, after which it progressively declines to low levels as cartilage matrix accumulates. High levels of FN gene expression also occur at the onset of chondrogenesis in vivo. In the proximal central core regions of the limb bud in which condensation and cartilage matrix deposition are being initiated, FN mRNA levels and the relative rates of FN synthesis become progressively about 4-fold higher than in the distal subridge region, which consists of undifferentiated mesenchymal cells that have not yet initiated condensation.(ABSTRACT TRUNCATED AT 250 WORDS)

Widespread distribution of type II collagen during embryonic chick development

Type II collagen is a major component of hyaline cartilage, and has been suggested to be causally involved in promoting chondrogenesis during embryonic development. In the present study we have performed an immunohistochemical analysis of the distribution of type II collagen during several early stages of embryonic chick development. Unexpectedly, we have found that type II collagen is widely distributed in a temporally and spatially regulated fashion in basement membranes throughout the trunk of the embryo at stages 14 through 19, including regions with no apparent relationship to chondrogenesis. Immunohistochemical staining with two different monoclonal antibodies against type II collagen, as well as with an affinity-purified polyclonal antibody, is detectable in the basement membranes of the neural tube, notochord, auditory vesicle, dorsal/lateral surface ectoderm, lateral/ventral gut endoderm, mesonephric duct, and basal surface of the splanchnic mesoderm subjacent to the dorsal aorta, and at the interface between the epimyocardium and endocardium of the developing heart. In contrast, immunoreactive type IX collagen is detectable only in the perinotochordal sheath in the trunk of the embryo at these stages of development. Thus type II collagen is much more widely distributed during early development than previously thought, and may be fulfilling some as yet undefined function, unrelated to chondrogenesis, during early embryogenesis.

In situ hybridization analysis of the expression of the type II collagen gene in the developing chicken limb bud

In situ hybridization with [32P]- or [35S]-labeled double-stranded DNA or single-stranded RNA probes was used to investigate the temporal and spatial distribution of cartilage-characteristic type II collagen mRNA during embryonic chick limb development and cartilage differentiation in vivo. When the type II collagen probes were hybridized to sections through embryonic limb buds at the earliest stages of their development (stages 18-25), an accumulation of silver grains representing type II collagen mRNA first became detectable in the proximal central core of the limb coincident with the prechondrogenic condensation of mesenchymal cells that characterizes the onset of cartilage differentiation. At later stages of development (stage 32; 7 days) intense hybridization signals with the type II collagen probes were localized over the well differentiated cartilage rudiments, whereas few or no silver grains above background were observed over the non-chondrogenic tissues. In contrast, sections hybridized with a probe complementary to mRNA for the alpha 1 chain of type I collagen exhibited an intense hybridization signal over the perichondrium and little or no signal over the cartilage primordia. At all stages of development examined, [32P]-labeled double-stranded DNA probes or single-stranded RNA probes labeled with either [32P] or [35S] provided adequate hybridization signals. Several experimental protocols were employed to control for the potential cross-hybridization and non-specific hybridization of the type II collagen probes. These included the utilization of labeled noncomplementary "sense-strand" type II collagen RNA as a control probe for nonspecific background, and prehybridization with a large excess of appropriate unlabeled RNA to block sequences in heterologous collagen RNAs that might cross-hybridize to the specific labeled probe.

Temporal and spatial analysis of cartilage proteoglycan core protein gene expression during limb development by in situ hybridization

As limb mesenchymal cells differentiate into chondrocytes they initiate the synthesis of a cartilage-specific sulfated proteoglycan, cartilage-characteristic type II collagen, and other cartilage-specific proteins. In the present study, in situ hybridization with a 32P-labeled cloned cDNA probe complementary to mRNA encoding the core protein of cartilage proteoglycan has been used to visualize and localize the accumulation of cartilage proteoglycan core protein mRNA sequences during development of the chick limb bud in vivo. When the probe was hybridized to sections through 7-day (stage 32) limbs, an intense hybridization signal was observed over the well-differentiated cartilage rudiments of the limb, while no signal above background was observed over nonchondrogenic tissues including muscle, loose connective tissue, and epidermis. At early stages of limb development, an accumulation of silver grains representing hybridizable core protein mRNA first became detectable in the proximal central core of the limb where the prechondrogenic condensation of mesenchymal cells that characterizes the onset of cartilage differentiation was occurring. In fact, the pattern of silver grain accumulation closely followed the pattern of mesenchymal cell condensation, and no hybridizable core protein mRNA sequences were detectable in the limb bud prior to condensation. Cartilage-characteristic type II collagen mRNA was colocalized with core protein mRNA in the condensing central core of the limb suggesting that the genes for these two major constituents of cartilage matrix are coordinately regulated at the onset of chondrogenesis. Furthermore, the appearance of hybridizable core protein mRNA was closely followed by the appearance of the protein for which it codes as detected by immunohistochemical staining with monospecific antibody. These observations support the hypothesis that at the initial stages of limb chondrogenesis core protein gene expression is controlled primarily at the transcriptional level.

Separation of the myogenic and chondrogenic progenitor cells of undifferentiated limb mesenchyme

Undifferentiated limb bud mesenchyme consists of at least two separate, possibly predetermined, populations of progenitor cells, one derived from somitic mesoderm that gives rise exclusively to skeletal muscle and one derived from somatopleural mesoderm that gives rise to the cartilage and connective tissue of the limb. In the present study, we demonstrate that the inherent migratory capacity of myogenic precursor cells can be used to physically separate the myogenic and chondrogenic progenitor cells of the undifferentiated limb mesenchyme at the earliest stages of limb development. When the undifferentiated mesenchyme of stage 18/19 chick embryo wing buds or from the distal subridge region of stage 22 wing buds is placed intact upon the surface of fibronectin (FN)-coated petri dishes, a large population of cells emigrates out of the explants onto the FN substrates and differentiates into an extensive interlacing network of bipolar spindle-shaped myoblasts and multinucleated myotubes that stain with monoclonal antibody against muscle-specific fast myosin light chain. In contrast, the cells of the explants that remain in place and do not migrate away undergo extensive cartilage differentiation. Significantly, there is no emigration of myogenic cells out of explants of stage 25 distal subridge mesenchyme, which lacks myogenic progenitor cells. Myogenic precursor cells stream out of mesenchyme explants in one or occasionally two discrete locations, suggesting they are spatially segregated in discrete regions of tissue at the time of its explantation. There are subtle overall differences in the morphologies of the myogenic cells that form in stage 18/19 and stage 22 distal subridge mesenchyme explants. Finally, groups of nonmyogenic nonfibroblastic cells which are fusiform-shaped and oriented in distinct parallel arrays characteristically are found along the periphery of stage 18/19 wing mesenchyme explants. Our observations provide support for the concept that undifferentiated limb mesenchyme consists of independent subpopulations of committed precursor cells and provides a system for studying the early determinative and regulatory events involved in myogenesis or chondrogenesis.

Temporal and spatial analysis of hyaluronidase activity during development of the embryonic chick limb bud

The glycosaminoglycan hyaluronate (HA) appears to play an important role in limb cartilage differentiation. The large amount of extracellular HA accumulated by prechondrogenic mesenchymal cells may prevent the cell-cell and/or cell-matrix interactions necessary to trigger chondrogenesis, and the removal of extracellular HA may be essential to initiate the crucial cellular condensation process that triggers cartilage differentiation. It has generally been assumed that HA turnover during chondrogenesis is controlled by the activity of the enzyme hyaluronidase (HAase). In the present study we have performed a temporal and spatial analysis of HAase activity during the progression of limb development and cartilage differentiation in vivo. We have separated embryonic chick wing buds at several stages of development into well-defined regions along the proximodistal axis in which cells are in different phases of differentiation, and we have examined HAase activity in each region. We have found that HAase activity is clearly detectable in undifferentiated wing buds at stage 18/19, which is shortly following the formation of a morphologically distinct limb bud rudiment, and remains relatively constant throughout subsequent stages of development through stage 27/28, at which time well-differentiated cartilage rudiments are present. Moreover, HAase activity in the prechondrogenic distal subridge regions of the limb at stages 22/23 and 25 is just as high as, or even slightly higher than, it is in proximal central core regions where condensation and cartilage differentiation are progressing. We have also found that limb bud HAase is active between pH 2.2 and 4.5 and is inactive above pH 5.0. This suggests that limb HAase is a lysosomal enzyme and that extracellular HA would have to be internalized to be degraded. These results indicate that the onset of chondrogenesis is not associated with the appearance or increase in activity of HAase. We suggest that possibility that HA turnover may be regulated by the binding and endocytosis of extracellular HA in preparation for its intracellular degradation by lysosomal HAase. Finally, we have found that the apical ectodermal ridge (AER)-containing distal limb bud ectoderm possesses a relatively high HAase activity. We suggest the possibility that a high HAase activity in the AER may ensure a rapid turnover and remodeling of the disorganized HA-rich basal lamina of the AER that might be essential for limb outgrowth.

Cartilage proteoglycan core protein gene expression during limb cartilage differentiation

Changes in the steady-state cytoplasmic levels of mRNA for the core protein of the major sulfated proteoglycan of cartilage were examined during the course of limb chondrogenesis in vitro using cloned cDNA probes. Cytoplasmic core protein mRNA begins to accumulate at the onset of overt chondrogenesis in micromass culture coincident with the crucial condensation phase of the process, in which prechondrogenic mesenchymal cells become closely juxtaposed prior to depositing a cartilage matrix. The initiation of core protein mRNA accumulation coincides with a dramatic increase in the accumulation of mRNA for type II collagen, the other major constituent of hyaline cartilage matrix. Following condensation, there is a concomitant progressive increase in cytoplasmic core protein and type II collagen mRNA accumulation which parallels the progressive accumulation of cartilage matrix by the cells. The relative rate of accumulation of cytoplasmic type II collagen mRNA is greater than twice that of core protein mRNA during chondrogenesis in micromass culture. Cyclic AMP, an agent implicated in the regulation of chondrogenesis elicits a concomitant two- to fourfold increase in both cartilage core protein and type II collagen mRNA levels by limb mesenchymal cells. Core protein gene expression is more sensitive to cAMP than type II collagen gene expression. These results suggest that the cartilage proteoglycan core protein and type II collagen genes are coordinately regulated during the course of limb cartilage differentiation, although there are quantitative differences in the extent of expression of the two genes.