Developmentally regulated synthesis of a low molecular weight protein (Ch 21) by differentiating chondrocytes

The anti-bacterial iron-restriction defence mechanisms of egg white; the potential role of three lipocalin-like proteins in resistance against Salmonella

Salmonella enterica serovar Enteritidis (SE) is the most frequently-detected Salmonella in foodborne outbreaks in the European Union. Among such outbreaks, egg and egg products were identified as the most common vehicles of infection. Possibly, the major antibacterial property of egg white is iron restriction, which results from the presence of the iron-binding protein, ovotransferrin. To circumvent iron restriction, SE synthesise catecholate siderophores (i.e. enterobactin and salmochelin) that can chelate iron from host iron-binding proteins. Here, we highlight the role of lipocalin-like proteins found in egg white that could enhance egg-white iron restriction through sequestration of certain siderophores, including enterobactin. Indeed, it is now apparent that the egg-white lipocalin, Ex-FABP, can inhibit bacterial growth via its siderophore-binding capacity in vitro. However, it remains unclear whether Ex-FABP performs such a function in egg white or during bird infection. Regarding the two other lipocalins of egg white (Cal-γ and α-1-glycoprotein), there is currently no evidence to indicate that they sequester siderophores.

Siderocalins: Siderophore binding proteins evolved for primary pathogen host defense

Bacterial pathogens use siderophores to obtain iron from the host in order to survive and grow. The host defends against siderophore-mediated iron acquisition by producing siderocalins. Siderocalins are a siderophore binding subset of the lipocalin family of proteins. The design of the siderophore binding pocket gives siderocalins the ability to bind a wide variety of siderophores and protect the host against several pathogens. Siderocalins have been identified in humans, chickens, and quail, among other animals. The differences in the respective siderocalins suggest that each was developed in response to the most serious pathogens encountered by that animal. Additionally, siderocalins have been observed in many roles unrelated to pathogen defense including differentiation, embryogenesis, inflammation, and cancer.

The v-myc-induced Q83 lipocalin is a siderocalin

Siderocalins are atypical lipocalins able to capture siderophores with high affinity. They contribute to the innate immune response by interfering with bacterial siderophore-mediated iron uptake but are also involved in numerous physiological processes such as inflammation, iron delivery, tissue differentiation, and cancer progression. The Q83 lipocalin was originally identified based on its overexpression in quail embryo fibroblasts transformed by the v-myc oncogene. We show here that Q83 is a siderocalin, binding the siderophore enterobactin with an affinity and mode of binding nearly identical to that of neutrophil gelatinase-associated lipocalin (NGAL), the prototypical siderocalin. This strengthens the role of siderocalins in cancer progression and inflammation. In addition, we also present the solution structure of Q83 in complex with intact enterobactin and a detailed analysis of the Q83 binding mode, including mutagenesis of the critical residues involved in enterobactin binding. These data provide a first insight into the molecular details of siderophore binding and delineate the common molecular properties defining the siderocalin protein family.

Prawn lipocalin: characteristics and expressional pattern in subepidermal adipose tissue during reproductive molting cycle

In crustaceans, the fascinating processes of maturation, reproductive molting and carapace coloration are regulated by hydrophobic molecules. Interestingly, most of the molecules are ligands of lipocalin. To understand the role of lipocalin in the aforementioned processes at molecular level, we isolated a cDNA that belongs to the lipocalin family, from a central nervous system cDNA library of Macrobrachium rosenbergii. We monitored the spatial and temporal distributions of the mRNA by using Northern Blotting analysis. Our results demonstrated that this gene expresses abundantly in the subepidermal adipose tissue, while faintly in the hepatopancreas and central nervous system. However, no signal was detected in other tissues including muscle, gill and ovary. Its expression levels in subepidermal adipose tissue during various stages of maturation as well as through the whole molting cycle showed that prawn lipocalin is involved in sexual maturation, as the maximal level was observed just after molt.

Acute phase lipocalin Ex-FABP is involved in heart development and cell survival

Ex-FABP is an extracellular fatty acid binding protein, expressed during chicken embryo development in cartilage, muscle fibers, and blood granulocytes. Transfection of chondrocytes and myoblasts with anti-sense Ex-FABP cDNA results in inhibition of cell proliferation and apoptosis induction. Ex-FABP expression is dramatically enhanced by inflammatory stimuli and in pathological conditions. In this paper, by in situ whole mount and immunohistochemistry analysis we show that, at early developmental stage, Ex-FABP is diffuse in all tissues of chick embryos. Particularly high level of transcript and protein are expressed in the heart. During acute phase response (APR) induced by endotoxin LPS injection, a marked increase of Ex-FABP mRNA was observed in embryos, highest Ex-FABP expression being in heart and liver. To investigate in vivo the biological role of Ex-FABP, we have directly microinjected chicken embryos with antibody against Ex-FABP. Almost 70% of chicken embryos died and the target tissue was the heart. We detected in heart of the treated embryos a significant increase of apoptotic cells and high level of fatty acids. We propose that the accumulation of fatty acid, specific ligand of Ex-FABP, in the cell microenvironment is responsible of heart cell death, and we suggest that Ex-FABP may act as a survival protein by playing a role as scavenger for fatty acids.

Expression of serum amyloid A in chondrocytes and myoblasts differentiation and inflammation: possible role in cholesterol homeostasis

Serum amyloid A (SAA) is synthesized by the liver during the acute phase. Local expression of SAA mRNA has been reported also in non-liver cells, a potential local source of SAA protein not related to the systemic acute phase response. SAA function has not been established yet. In the present study, we identified SAA as a protein expressed by chondrocytes and myoblasts in response to inflammatory stimula. In both cell systems, SAA mRNA and protein expression is strongly stimulated by bacterial lipopolysaccharide treatment. SAA mRNA expression is also enhanced during terminal differentiation of cells of the chondrogenic and myogenic lineage; mRNA is barely detectable in prechondrogenic cells and is highly expressed in differentiated hyperthrophic chondrocytes. An increased level of SAA mRNA was also observed in vivo when we compared mRNA extracted from tibiae of 10 day embryos, still fully cartilaginous, with tibiae from 18 day embryos, a stage when the endochondral ossification process has already started. p38 activation, a well-known event of the chondrogenesis signaling cascade, controls expression of SAA in cartilage following inflammatory stimuli. SAA secreted by stimulated chondrocytes is associated with cholesterol. Cholesterol is synthesized by the same chondrocytes and is also increased in inflammatory conditions. A role of SAA in cholesterol homeostasis in chondrocytes is proposed.

Phylogeny and regulation of four lipocalin genes clustered in the chicken genome: evidence of a functional diversification after gene duplication

A novel lipocalin gene is here reported that represents the fourth member of a cluster we have identified in the chicken genome. This cluster also includes Chondrogenesis-Associated Lipocalins beta and gamma (CAL beta, CAL gamma) and Extracellular Fatty Acid Binding Protein (Ex-FABP). The new gene codes for a 22-kDa secreted protein with three cysteine residues and a series of sequence features well conserved in the lipocalin family. All the genes in the cluster are structurally similar presenting comparable exon/intron boundary positions and exon sizes. A phylogenetic analysis indicates the monophyletic grouping of these genes, and their relationship with the lipocalins alpha-1-microglobulin (A1mg), complement factor 8 gamma chain (C8GC), prostaglandin D synthase (PGDS), and neutrophil-gelatinase-associated lipocalin (NGAL). The new cluster gene appears to be the ortholog of the mammalian C8GC and was thus named Ggal-C8GC. This orthology also suggests that this lipocalin was present in the ancestor common to reptiles and mammals. In addition to other expressing tissues, Ex-FABP, CAL beta and CAL gamma genes are highly transcribed in chondrocytes at late stages of chondrogenesis during endochondral bone formation and/or upon inflammatory stimulation. Here, we show that they are also transcriptionally induced when chondrocytes are subjected to various biological events as cell quiescence, cell shape transition, and hormonal stimulation. By contrast, Ggal-C8GC transcripts are only barely detectable in chondrocytes, but are more abundant in liver, kidney, brain, heart, skeletal muscle and particularly in skin. Moreover, no expression induction was observed neither during chondrocyte differentiation, nor upon any of the stimulations mentioned above. This indicates that the Ggal-C8GC gene was co-opted for a novel function after the duplication events that gave rise to the cluster. The peculiar coordinated regulation of Ex-FABP, CAL beta and CAL gamma, and the apparent divergent role of Ggal-C8GC suggest that these gene duplications may have been maintained during evolution by a sub-functionalization mechanism where some common function(s) are shared by several members of the cluster and some other specialized function(s) are unique to other members.

Simultaneous stimulation of EP2 and EP4 is essential to the effect of prostaglandin E2 in chondrocyte differentiation

OBJECTIVE

Prostaglandin E(2)(PGE(2)) has been reported to stimulate chondrocyte differentiation. However, the precise actions and signal transduction pathways of PGE(2)in cartilage are largely unknown. Our purpose is to identify which of the four PGE(2)receptor subtype(s), EP1-4, mediates the action of PGE(2)on chondrocyte differentiation.

DESIGN

We used primary chondrocytes derived from the resting zone of rat rib cartilage. The effects on chondrocyte differentiation were assessed by measuring the Alcian blue-stainable proteoglycan content and the expression levels of type II collagen mRNA by Northern blot analysis. The expression of the four PGE(2)receptor subtypes in rat primary chondrocytes was examined by reverse transcription-polymerase chain reaction.

RESULTS

PGE(2)stimulated the accumulation of proteoglycan and up-regulated the expression of type II collagen mRNA in primary chondrocytes. Dibutyryl cAMP, a cell-permeable analog of cAMP, an important intracellular mediator of PGE(2)signaling, also enhanced the expression of type II collagen mRNA and proteoglycan accumulation in chondrocytes. No EP agonist alone induced the expression of type II collagen mRNA. However, simultaneous administration of EP2 and EP4 agonists at high concentrations cooperatively induced the expression of type II collagen mRNA, mimicking the PGE(2)effect. The simultaneous stimulation of EP2 and EP4 also cooperatively enhanced proteoglycan accumulation and intracellular cAMP production. Moreover, an EP4 antagonist partially blocked the stimulatory actions of PGE(2)on the expression of type II collagen mRNA.

CONCLUSION

These results suggest that simultaneous stimulation of EP2 and EP4 is necessary and sufficient to elicit the effect of PGE(2)on rat primary chondrocyte differentiation.

Inhibition of cell proliferation and induction of apoptosis by ExFABP gene targeting

Ex-FABP, an extracellular fatty acid binding lipocalin, is physiologically expressed by differentiating chicken chondrocytes and myoblasts. Its expression is enhanced after cell treatment with inflammatory stimuli and repressed by anti-inflammatory agents, behaving as an acute phase protein. Chicken liver fragments in culture show enhanced protein expression after bacterial endotoxin treatment. To investigate the biological role of Ex-FABP, we stably transfected proliferating chondrocytes with an expression vector carrying antisense oriented Ex-FABP cDNA. We observed a dramatic loss of cell viability and a strong inhibition of cell proliferation and differentiation. When chondrocytes were transfected with the antisense oriented Ex-FABP cDNA we observed that Ex-FABP down-modulation increased apoptotic cell number. Myoblasts transfected with the same expression vector showed extensive cell death and impaired myotube formation. We suggest that Ex-FABP acts as a constitutive survival protein and that its expression and activation are fundamental to protect chondrocytes from cell death.

Ex-FABP, extracellular fatty acid binding protein, is a stress lipocalin expressed during chicken embryo development

Extracellular Fatty Acid Binding Protein (Ex-FABP) is a 21 kDa lipocalin, expressed during chicken embryo development in hypertrophic cartilage, in muscle fibres and in blood granulocyte. The protein selectively binds with high affinity fatty acids, preferably long chain unsaturated fatty acids in chondrocyte and myoblast cultures Ex-FABP expression is increased by inflammatory-agents and repressed by anti-inflammatory-agents. In adult cartilage, Ex-FABP is expressed only in pathological conditions such as in dyschondroplastic and osteoarthritic chicken cartilage. We propose that lipocalin Ex-FABP represents a stress protein physiologically expressed in tissues where active remodelling is taking place during development and also present in tissues characterized by a stress response due to pathological conditions.

Integrins alpha(6A)beta 1 and alpha(6B)beta 1 promote different stages of chondrogenic cell differentiation

The differentiation of chondrocytes and of several other cell types is associated with a switch from the alpha(6B) to the alpha(6A) isoform of the laminin alpha(6)beta(1) integrin receptor. To define whether this event plays a functional role in cell differentiation, we used an in vitro model system that allows chick chondrogenic cells to remain undifferentiated when cultured in monolayer and to differentiate into chondrocytes when grown in suspension culture. We report that: (i) upon over-expression of the human alpha(6B), adherent chondrogenic cells differentiate to stage I chondrocytes (i.e. increased type II collagen, reduced type I collagen, fibronectin, alpha(5)beta(1) and growth rate, loss of fibroblast morphology); (ii) the expression of type II collagen requires the activation of p38 MAP kinase; (iii) the over-expression of alpha(6A) induces an incomplete differentiation to stage I chondrocytes, whereas no differentiation was observed in alpha(5) and mock-transfected control cells; (iv) a prevalence of the alpha(6A) subunit is necessary to stabilize the differentiated phenotype when cells are transferred to suspension culture. Altogether, these results indicate a functional role for the alpha(6B) to alpha(6A) switch in chondrocyte differentiation; the former promotes chondrocyte differentiation, and the latter is necessary in stabilizing the differentiated phenotype.

CALbeta, a novel lipocalin associated with chondrogenesis and inflammation

We have previously demonstrated the association of the chicken lipocalin Ex-FABP with cartilage formation and inflammatory responses as a marker of these processes (Descalzi Cancedda et al., Biochim. Biophys. Acta 1482, 127-135, 2000). Here we report the isolation and characterisation of a new lipocalin gene laying upstream the Ex-FABP, thus representing the second member of a possible genomic cluster. This gene contains an open reading frame coding for a polypeptide of about 19 kDa. The amino-acid sequence revealed a conserved lipocalin secondary structure. Tissue distribution of the protein in developing embryos showed a preferential expression in the heart although mRNA transcripts could be detected also in muscle, lung and liver. The lowest expression was observed in the stomach, brain and skin. During endochondral formation of long bones, the protein is differentially distributed, as the transcripts, evidenced in the tibia by in situ hybridisation, are present in the hypertrophic cone of the cartilage and mostly absent in the area of the proliferating chondrocytes. Such developmental regulation was observed also in vitro in cultured chondrocytes where the transcripts were barely detectable in dedifferentiated cells but highly expressed in hypertrophic chondrocytes. The protein was also significantly induced by lipopolysaccharide stimulation of chondrocytes, indicating a possible involvement in acute phase response. Raising specific antibodies in a rabbit allowed validating, at the protein level, all the transcriptional data. Moreover, we gained evidence that the protein is actively secreted in the extracellular matrix surrounding the chondrocytes. Because of its peculiar expression in cartilage, this new protein was named chondrogenesis-associated lipocalin beta (thereafter referred to as CAL beta). The close similarity between Ex-FABP and CAL beta expression patterns supports the hypothesis of a genomic organisation in a cluster where both genes could be co-ordinately regulated.

Antiangiogenic treatment delays chondrocyte maturation and bone formation during limb skeletogenesis

Hypertrophic chondrocytes have important roles in promoting invasion of cartilage by blood vessels and its replacement with bone. However, it is unclear whether blood vessels exert reciprocal positive influences on chondrocyte maturation and function. Therefore, we implanted beads containing the antiangiogenic molecule squalamine around humeral anlagen in chick embryo wing buds and monitored the effects over time. Fluorescence microscopy showed that the drug diffused from the beads and accumulated in humeral perichondrial tissues, indicating that these tissues were the predominant targets of drug action. Diaphyseal chondrocyte maturation was indeed delayed in squalamine-treated humeri, as indicated by reduced cell hypertrophy and expression of type X collagen, transferrin, and Indian hedgehog (Ihh). Although reduced in amount, Ihh maintained a striking distribution in treated and control humeri, being associated with diaphyseal chondrocytes as well as inner perichondrial layer. These decreases were accompanied by lack of cartilage invasion and tartrate-resistant acid phosphatase-positive (TRAP+) cells and a significant longitudinal growth retardation. Recovery occurred at later developmental times, when in fact expression in treated humeri of markers such as matrix metalloproteinase 9 (MMP-9) and connective tissue growth factor (CTGF) appeared to exceed that in controls. Treating primary cultures of hypertrophic chondrocytes and osteoblasts with squalamine revealed no obvious changes in cell phenotype. These data provide evidence that perichondrial tissues and blood vessels in particular influence chondrocyte maturation in a positive manner and may cooperate with hypertrophic chondrocytes in dictating the normal pace and location of the transition from cartilage to bone.

Avidin expression during chick chondrocyte and myoblast development in vitro and in vivo: regulation of cell proliferation

Avidin is a major [(35)S]methionine-labeled protein induced by bacterial lipopolysaccharide (LPS) and interleukin 6 (IL-6) in cultured chick embryo myoblasts and chondrocytes. It was identified by N-terminal sequencing of the protein purified from conditioned culture medium of LPS-stimulated myoblasts. In addition, avidin was secreted by unstimulated myoblasts and chondrocytes during in vitro differentiation; maximal expression being observed in differentiated myofibers and hypertrophic chondrocytes. In developing chick embryos, immunohistochemistry revealed avidin in skeletal muscles and growth plate hypertrophic cartilage. Avidin was secreted into culture as a biologically active tetramer. Exogenous avidin added to the medium of proliferating chondrocytes progressively inhibited cell proliferation, whereas addition of avidin to differentiating chondrocytes in suspension allowed full cell differentiation. No toxic effects for the cells were observed in both culture conditions. Western blots of samples from cytosolic extracts using alkaline-phosphatase-conjugated streptavidin showed three biotin-containing proteins. Acetyl-CoA carboxylase was identified by specific antibodies. Based on these data, we propose that avidin binds extracellular biotin and regulates cell proliferation by interfering with fatty acid biosynthesis during terminal cell differentiation and/or in response to inflammatory stimuli.

Ex-FABP: a fatty acid binding lipocalin developmentally regulated in chicken endochondral bone formation and myogenesis

Extracellular fatty acid binding protein (Ex-FABP) is a 21 kDa lipocalin specifically binding fatty acids, expressed during chicken embryo development in hypertrophic cartilage, in muscle fibers and in blood granulocytes. In chondrocyte and myoblast cultures Ex-FABP expression is increased by inflammatory agents and repressed by anti-inflammatory agents. In adult cartilage Ex-FABP is expressed only in pathological conditions such as in dyschondroplastic and osteoarthritic chickens. The possible mammalian counterpart is the Neu-related lipocalin (NRL), a lipocalin overexpressed in rat mammary cancer; NRL is homologous to the human neutrophil gelatinase associated lipocalin (NGAL) expressed in granulocytes and in epithelial cells in inflammation and malignancy and to the Sip24 (super-inducible protein 24), an acute phase lipocalin expressed in mouse after turpentine injection. Immunolocalization and in situ hybridization showed that NRL/NGAL is expressed in hypertrophic cartilage, in forming skeletal muscle fibers and in developing heart. In adult cartilage NRL/NGAL was expressed in articular cartilage from osteoarthritic patients and in chondrosarcoma. Moreover, NRL was induced in chondrocyte and myoblast cultures by an inflammatory agent. We propose that these lipocalins (Ex-FABP, NRL/NGAL, Sip24) represent stress proteins physiologically expressed in tissues where active remodeling is taking place during development and also present in tissues characterized by an acute phase response due to pathological conditions.

Vis-à-vis cells and the priming of bone formation

Bone formation throughout skeletal growth and remodeling always entails deposition of new bone onto a pre-existing mineralized surface. In contrast, the initial deposition of bone in development requires the formation, ex novo, of the first mineralized structure in a nonmineralized tissue. We investigated the cellular events associated with this initial bone formation, with specific reference to the respective role of cartilage and bone cells in bones which form via a cartilage model. The cellular architecture of initial osteogenic sites was investigated by light, confocal, and electron microscopy (EM) in the membranous ossification of fetal calvarial bones (not forming via a cartilage model) and in the membranous ossification of the bony collars of endochondral bones. Bone sialoprotein (BSP), which is expressed during early phases of bone deposition and has been proposed to be involved in the control of both mineral formation and bone cell-matrix interactions, was used as a marker of initial bone formation. We found that at all sites, BSP-producing cells (as identified by intracellular immunoreactivity) are arranged in a characteristic vis-à-vis (face to face) pattern prior to the appearance of the first mineralizing BSP-immunoreactive extracellular matrix. In perichondral osteogenesis, the vis-à-vis pattern comprises osteoblasts differentiating from the perichondrium/periosteum and early hypertrophic chondrocytes located at the lateral aspects of the rudiment. By EM, the first mineral and the first BSP-immunoreactive sites coincide temporally and spatially in the extracellular matrix at the boundary between cartilage and periosteum. We further showed that in an in vitro avian model of chondrocyte differentiation in vitro to osteoblast-like cells, early hypertrophic chondrocytes replated as adherent cells turned on the expression of high levels of BSP in conjunction with the switch to collagen type I synthesis and matrix mineralization. We propose a model for the priming of bone deposition, i.e., the formation of the first bone structure, in which the architectural layout of cells competent to deposit a mineralizing matrix (the vis-à-vis pattern) determines the polarized deposition of bone. For bones forming via a cartilage model, the priming of bone deposition involves and requires cells that differentiate from early hypertrophic chondrocytes.

Expression of the extracellular fatty acid binding protein (Ex-FABP) during muscle fiber formation in vivo and in vitro

We report that Ex-FABP, an extracellular protein belonging to the lipocalin family and involved in the extracellular transport of long-chain fatty acids, is expressed in the forming myotubes both in vivo and in vitro. The presence of the protein and of the mRNA was observed in newly formed myotubes at early stages of chick embryo development by immunohistochemistry and by in situ hybridization. At later stages of development myofibers still expressed both the mRNA and the protein. Ex-FABP expression was observed also in the developing myocardium and the muscular layer of large blood vessels. In agreement with these findings, an initial expression of the mRNA and protein secretion by cultured chicken myoblasts were observed only after the onset of myoblast fusion. Double-immunofluorescence staining of these cultured cells revealed that multinucleate myotubes were stained by antibodies directed against both the Ex-FABP and the sarcomeric myosin, whereas immature myotubes and single myoblasts were not. When added to cultured myoblasts, antibodies against the Ex-FABP induced a strong enhancement of the production of the same protein. In all experiments some cell sufferance and a transient impairment of myotube formation were also observed. The finding that the continuous removal of the Ex-FABP from the culture medium of myoblasts, due to the formation of immune complexes, resulted in an overproduction of the protein suggests a feedback (autocrine) control during myotube differentiation and maturation. We propose that the requirement for increased transport and metabolism of free fatty acid released from the membrane phospholipids and storage lipids, mediated by Ex-FABP, may be essential during differentiation of multinucleated myotubes or that an increased local demand of fatty acids and metabolites may act as a local hormone in tissues differentiating and undergoing morphogenesis.

In vitro differentiation of chick embryo bone marrow stromal cells into cartilaginous and bone-like tissues

Bone marrow stromal cells, progenitor cells involved in repair of bone and cartilage, can potentially provide a source for autologous skeletal tissue engineering. We investigated which factors were required to induce in vitro differentiation of avian bone marrow stromal cells into three-dimensional cartilaginous and bone-like tissues. Bone marrow stromal cells from embryonic chicks were expanded in monolayers, seeded onto biodegradable polyglycolic acid scaffolds, and cultured for 4 weeks in orbitally mixed Petri dishes. Cell-polymer constructs developed an organized extracellular matrix containing glycosaminoglycans and collagen, whereas control bone marrow stromal cell pellet cultures were smaller and consisted predominantly of fibrous tissue. Bone marrow stromal cells expanded with fibroblast growth factor-2 and seeded onto polymer scaffolds formed highly homogeneous three-dimensional tissues that contained cartilage-specific molecular markers and had biochemical compositions comparable with avian epiphyseal cartilage. When cell-polymer constructs were cultured in the presence of beta-glycerophosphate and dexamethasone, the extracellular matrix mineralized and bone-specific proteins were expressed. Our work shows that cell expansion in the presence of fibroblast growth factor-2 and cultivation on a three-dimensional polymer scaffold allows differentiation of chick bone marrow stromal cells into three-dimensional cartilaginous tissues. In the in vitro system studied, the same population could be selectively induced to regenerate either cartilaginous or bone-like tissue.

The developmentally regulated avian Ch21 lipocalin is an extracellular fatty acid-binding protein

Ch21, a developmentally regulated extracellular protein expressed in chick embryos and in cultured chondrocytes, was expressed in the baculovirus system, and the recombinant protein was purified to homogeneity by gel-filtration chromatography. Separation of two isoforms was achieved on an ion-exchange column. Previous work had shown that Ch21 belongs to the superfamily of lipocalins, which are transport proteins for small hydrophobic molecules. Studies were performed to identify the Ch21 ligand. By analysis of recombinant Ch21 on native polyacrylamide gel electrophoresis and by Lipidex assay, the binding of fatty acid to the protein was shown and a preferential binding of long-chain unsaturated fatty acids was observed. Both isoforms had the same behavior. The binding was saturable. Stoichiometry was about 0.7 mol of ligand/mol of protein. The protein binds the ligand in its monomeric form. Calculated dissociation constants were 2 X 10(-7) M for unsaturated fatty acids and 5 X 10(-7) M for stearic acid. The binding was specific; other hydrophobic molecules, as retinoic acid, progesterone, prostaglandins, and long-chain alcohols and aldehydes did not bind to the protein. Short-chain fatty acids did not bind to the protein. Ch21, also present in chicken serum, represents the first extracellular protein able to selectively bind and transport fatty acid in extracellular fluids and serum. We propose to rename the Ch21 protein as extracellular fatty acid-binding protein (Ex-FABP).

The lipocalin protein family: structure and function

The lipocalin protein family is a large group of small extracellular proteins. The family demonstrates great diversity at the sequence level; however, most lipocalins share three characteristic conserved sequence motifs, the kernel lipocalins, while a group of more divergent family members, the outlier lipocalins, share only one. Belying this sequence dissimilarity, lipocalin crystal structures are highly conserved and comprise a single eight-stranded continuously hydrogen-bonded antiparallel beta-barrel, which encloses an internal ligand-binding site. Together with two other families of ligand-binding proteins, the fatty-acid-binding proteins (FABPs) and the avidins, the lipocalins form part of an overall structural superfamily: the calycins. Members of the lipocalin family are characterized by several common molecular-recognition properties: the ability to bind a range of small hydrophobic molecules, binding to specific cell-surface receptors and the formation of complexes with soluble macromolecules. The varied biological functions of the lipocalins are mediated by one or more of these properties. In the past, the lipocalins have been classified as transport proteins; however, it is now clear that the lipocalins exhibit great functional diversity, with roles in retinol transport, invertebrate cryptic coloration, olfaction and pheromone transport, and prostaglandin synthesis. The lipocalins have also been implicated in the regulation of cell homoeostasis and the modulation of the immune response, and, as carrier proteins, to act in the general clearance of endogenous and exogenous compounds.

Morphological and biochemical characterization of mineralizing primary cultures of avian growth plate chondrocytes: evidence for cellular processing of Ca2+ and Pi prior to matrix mineralization

Advances in the culture of mineralizing growth plate chondrocytes provided an opportunity to study endochondral calcification under controlled conditions. Here we report that these cultures synthesize large amounts of proteins characteristically associated with mineralization: type II and X collagens, sulfated proteoglycans, alkaline phosphatase, and the bone-related proteins, osteonectin and osteopontin. Certain chondrocytes appeared to accumulate large amounts of Ca2+ and Pi during the mineralization process: laser confocal imaging revealed high levels of intracellular Ca2+ in their periphery and X-ray microanalytical mapping revealed the presence of many Ca(2+)- and Pi-rich cell surface structures ranging from filamentous processes 0.14 +/- 0.02 microns by 0.5-2.0 microns, to spherical globules 0.70 +/- 0.27 microns in diameter. Removal of organic matter with alkaline sodium hypochlorite revealed numerous deposits of globular (0.77 +/- 0.19 micron) mineral (calcospherites) in the lacunae around these cells. The size and spatial distribution of these mineral deposits closely corresponded to the Ca(2+)-rich cell surface blebs. The globular mineral progressively transformed into clusters of crystallites. Taken with earlier studies, these findings indicate that cellular uptake of Ca2+ and Pi leads to formation of complexes of amorphous calcium phosphate, membrane lipids, and proteins that are released as cell surface blebs analogous to matrix vesicles. These structures initiate development of crystalline mineral. Thus, the current findings support the concept that the peripheral intracellular accumulation of Ca2+ and Pi is directly involved in endochondral calcification.

Chondrocyte differentiation

Data obtained while investigating growth plate chondrocyte differentiation during endochondral bone formation both in vivo and in vitro indicate that initial chondrogenesis depends on positional signaling mediated by selected homeobox-containing genes and soluble mediators. Continuation of the process strongly relies on interactions of the differentiating cells with the microenvironment, that is, other cells and extracellular matrix. Production of and response to different hormones and growth factors are observed at all times and autocrine and paracrine cell stimulations are key elements of the process. Particularly relevant is the role of the TGF-beta superfamily, and more specifically of the BMP subfamily. Other factors include retinoids, FGFs, GH, and IGFs, and perhaps transferrin. The influence of local microenvironment might also offer an acceptable settlement to the debate about whether hypertrophic chondrocytes convert to bone cells and live, or remain chondrocytes and die. We suggest that the ultimate fate of hypertrophic chondrocytes may be different at different microanatomical sites.

Avian tibial dyschondroplasia: a morphological and biochemical review of the growth plate lesion and its causes

Avian tibial dyschondroplasia is a disease found in fast growing strains of chickens, ducks, and turkeys worldwide in which growth plate cartilage accumulates in the metaphyseal region of the tibiotarsus; it is similar to mammalian osteochondrosis. Several biochemical and pathologic studies have shown that the growth plate chondrocytes do not reach their expected size in the hypertrophic zone and necroses prematurely. The chondrocytes also produce decreased amounts of extracellular proteins, such as collagen X and fibroblast growth factor-beta, that are necessary for cartilage maturation. This immature cartilage becomes highly cross-linked in the collagen molecules and apparently resistant to resorption and vascularization by the metaphyseal vessels. The dyschondroplastic cartilage remains in the metaphysis for several weeks. Not until the growth rate of the birds slows down is the cartilage able to be resorbed and replaced by trabecular bone. Many conditions have been found to induce tibial dyschondroplasia, including copper deficiency; fusarochromanone, thiram, and antabuse intoxication; excessive dietary levels of cysteine and homocysteine; metabolic acidosis; and bird rearing environment. However, the mechanism(s) by which these various methods induce tibial dyschondroplasia is presently not known. Current research is focusing on understanding the development of the disease and whether or not all these methods work by the same physiological chain of events. Recent biochemical evidence suggests that a copper deficiency might be caused by a different mechanism than genetically and thiram-induced tibial dyschondroplasia.

Ovotransferrin and ovotransferrin receptor expression during chondrogenesis and endochondral bone formation in developing chick embryo

Ovotransferrin expression during chick embryo tibia development has been investigated in vivo by immunocytochemistry and in situ hybridization. Ovotransferrin was first observed in the 7 day cartilaginous rudiment. At later stages, the factor was localized in the articular zone of the bone epiphysis and in the bone diaphysis where it was concentrated in hypertrophic cartilage, in zones of cartilage erosion and in the osteoid at the chondro-bone junction. When the localization of the ovotransferrin receptors was investigated, it was observed that chondrocytes at all stages of differentiation express a low level of the oviduct (tissue) specific receptor. Interestingly, high levels of the receptor were detectable in the 13-d old tibia in the diaphysis collar of stacked-osteoprogenitor cells and in the layer of derived osteoblasts. High levels of oviduct receptor were also observed in the primordia of the menisci. Metabolic labeling of proteins secreted by cultured chondrocytes and osteoblasts and Northern blot analysis of RNA extracted from the same cells confirmed and completed the above information. Ovotransferrin was expressed by in vitro differentiating chondrocytes in the early phase of the culture and, at least when culture conditions allowed extracellular matrix assembly, also by hypertrophic chondrocytes and derived osteoblast-like cells. Osteoblasts directly obtained from bone chips produced ovotransferrin only at the time of culture mineralization. By Western blot analysis, oviduct receptor proteins were detected at a very low level in extract from differentiating and hypertrophic chondrocytes and at a higher level in extract from hypertrophic chondrocytes undergoing differentiation to osteoblast-like cells and from mineralizing osteoblasts. Based on these results, the existence of autocrine and paracrine loops involving ovotransferrin and its receptor during chondrogenesis and endochondral bone formation is discussed.

Cell proliferation, extracellular matrix mineralization, and ovotransferrin transient expression during in vitro differentiation of chick hypertrophic chondrocytes into osteoblast-like cells

Differentiation of hypertrophic chondrocytes toward an osteoblast-like phenotype occurs in vitro when cells are transferred to anchorage-dependent culture conditions in the presence of ascorbic acid (Descalzi Cancedda, F., C. Gentili, P. Manduca, and R. Cancedda. 1992. J. Cell Biol. 117:427-435). This process is enhanced by retinoic acid addition to the culture medium. Here we compare the growth of hypertrophic chondrocytes undergoing this differentiation process to the growth of hypertrophic chondrocytes maintained in suspension culture as such. The proliferation rate is significantly higher in the adherent hypertrophic chondrocytes differentiating to osteoblast-like cells. In cultures supplemented with retinoic acid the proliferation rate is further increased. In both cases cells stop proliferating when mineralization of the extracellular matrix begins. We also report on the ultrastructural organization of the osteoblast-like cell cultures and we show virtual identity with cultures of osteoblasts grown from bone chips. Cells are embedded in a dense meshwork of type I collagen fibers and mineral is observed in the extracellular matrix associated with collagen fibrils. Differentiating hypertrophic chondrocytes secrete large amounts of an 82-kD glycoprotein. The protein has been purified from conditioned medium and identified as ovotransferrin. It is transiently expressed during the in vitro differentiation of hypertrophic chondrocytes into osteoblast-like cells. In cultured hypertrophic chondrocytes treated with 500 nM retinoic acid, ovotransferrin is maximally expressed 3 d after retinoic acid addition, when the cartilage-bone-specific collagen shift occurs, and decays between the 5th and the 10th day, when cells have fully acquired the osteoblast-like phenotype. Similar results were obtained when retinoic acid was added to the culture at the 50 nM "physiological" concentration. Cells expressing ovotransferrin also coexpress ovotransferrin receptors. This suggests an autocrine mechanism in the control of chondrocyte differentiation to osteoblast-like cells.

A monoclonal antibody distinguishes growth cartilage from other types of cartilage: a new probe for osteogenic cartilage

Monoclonal antibodies (mAbs) were raised by injection of a homogenate of cultured growth cartilage (GC) cells from young rabbit ribs. These mAbs were examined by immunohistochemical staining for their reactivity to paraffin sections of rabbit tissues. The results showed that an mAb reacted preferentially with late hypertrophic and calcified costal GC zones. The mAb also reacted with hypertrophic GC adjacent to bone that existed in sternum and femur, but not to other cartilages, including resting cartilage, articular cartilage, auricular cartilage, nasal cartilage, tracheal cartilage and meniscus cartilage, or with other tissues, including tendon, skin, muscles, lung, liver, heart, thymus, spleen, eye and gut. It reacted with a wider area of the GC zone when the sections were decalcified, although its reactivity with the extended area was much less intensive than that with late hypertrophic and calcified GC zones. On treatment of the sections with bacterial collagenase, neither the reactive area nor its intensity were changed, while when treated with trypsin the reactivity was lost. These results suggest the existence of a certain molecule which distinguishes GC (osteogenic cartilage) from other (non-osteogenic) cartilage. This mAb is a useful probe for distinguishing osteogenic cartilage from non-osteogenic cartilage, and for studying differentiation steps of cartilage cells in endochondral bone formation. The mAb can also be used as a probe for clinical and stored specimens because it reacts with decalcified and paraffin-embedded human specimens.

Hypertrophic chondrocytes undergo further differentiation in culture

Conditions have been defined for promoting growth and differentiation of hypertrophic chondrocytes obtained in culture starting from chick embryo tibiae. Hypertrophic chondrocytes, grown in suspension culture as described (Castagnola P., G. Moro, F. Descalzi Cancedda, and R. Cancedda. 1986. J. Cell Biol. 102:2310-2317), when they reached the stage of single cells, were transferred to substrate-dependent culture conditions in the presence of ascorbic acid. Cells showed a change in morphology, became more elongated and flattened, expressed alkaline phosphatase, and eventually mineralized. Type II and X collagen synthesis was halted and replaced by type I collagen synthesis. In addition the cells started to produce and to secrete in large amount a protein with an apparent molecular mass of 82 KD in reducing conditions and 63 KD in unreducing conditions. This protein is soluble in acidic solutions, does not contain collagenous domains, and is glycosylated. The Ch21 protein, a marker of hypertrophic chondrocytes and bone cells, was synthesized throughout the culture. We have defined this additional differentiation stage as an osteoblast-like stage. Calcium deposition in the extracellular matrix occurred regardless of the addition of beta glycerophosphate to the culture medium. Comparable results were obtained both when the cells were plated at low density and when they were already at confluence and maintained in culture without passaging up to 50 d. When retinoic acid was added to the hypertrophic chondrocyte culture between day 1 and day 5 the maturation of the cells to the osteoblast-like stage was highly accelerated. The switch in the collagen secretion was already observed after 2 d and the production of the 63-kD protein after 3 d. Mineralization was observed after 15-20 d.

Constitutive myc expression impairs hypertrophy and calcification in cartilage

The myc oncogene is expressed by proliferating quail embryo chondrocytes (QEC) grown as adherent cells and is repressed in QEC maintained in suspension culture. To investigate the interference of myc expression during chondrocyte differentiation, QEC were infected with a retrovirus carrying the v-myc oncogene (QEC-v-myc). Uninfected or helper virus-infected QEC were used as control. In adherent culture, QEC-v-myc displayed a chondrocytic phenotype and synthesized type II collagen and Ch21 protein, while control chondrocytes synthesized type I and type II collagen with no Ch21 protein detected as long as the attachment to the plastic was kept. In suspension culture, QEC-v-myc readily aggregated and within 1 week the cell aggregates released small single cells; still they secreted only type II collagen and Ch21 protein. In the same conditions control cell aggregates released hypertrophic chondrocytes producing type II and type X collagens and Ch21 protein. In the appropriate culture conditions, QEC-v-myc reconstituted a tissue defined as nonhypertrophic, noncalcifying cartilage by the high cellularity, the low levels of alkaline phosphatase enzymatic activity, and the absence of type X collagen synthesis and of calcium deposition. We conclude that the constitutive expression of the v-myc oncogene keeps chondrocytes in stage I (active proliferation and synthesis of type II collagen) and prevents these cells from reconstituting hypertrophic calcifying cartilage.

The amino terminal sequence of the developmentally regulated Ch21 protein shows homology with amino terminal sequences of low molecular weight proteins binding hydrophobic molecules

Ch21 protein, a developmentally regulated chick embryo protein of 21,000 apparent molecular weight, was purified from culture medium of hypertrophic chondrocytes. The purification method included a DEAE cellulose chromatography column, a CM cellulose chromatography column and a HPLC molecular sieve column. The amino acid sequence of the amino terminal end of the protein was determined. Computer assisted analysis showed significant homology between this sequence and the amino terminal sequences of proteins that belong to the superfamily of the low molecular weight binding proteins sharing a basic framework for the binding and transport of small hydrophobic molecules. Determination of the amino terminal sequence of the chicken retinol binding protein excluded identity between this protein and the Ch21.

In vitro development of hypertrophic chondrocytes starting from selected clones of dedifferentiated cells

Single cells from enzymatically dissociated chick embryo tibiae have been cloned and expanded in fresh or conditioned culture media. A cloning efficiency of approximately 13% was obtained using medium conditioned by dedifferentiated chondrocytes. A cloning efficiency of only 1.4% was obtained when conditioned medium from hypertrophic chondrocytes was used, and efficiencies of essentially 0 were found with fresh medium or medium conditioned by J2-3T3 mouse fibroblasts. Cell clones were selected by morphological criteria and clones showing a dedifferentiated phenotype (fibroblast-like) were further characterized. Out of 38 clones analyzed, 17 were able to differentiate to the hypertrophic chondrocyte stage and reconstitute hypertrophic cartilage when placed in the appropriate culture conditions. Cells from these clones expressed the typical markers of chondrocyte differentiation, i.e., type II and type X collagens. Clones not undergoing differentiation continued to express only type I collagen. Hypertrophic chondrocytes from differentiating clones were analyzed at the single cell level by immunofluorescence; all the cells were positive for type X collagen, while approximately 50% of them showed positivity for type II collagen.