Characterization of the neural crest defect in Splotch (Sp1H) mutant mice using a lacZ transgene

Muscular loading affects the 3D structure of both the mineralized rudiment and growth plate at early stages of bone formation

Fetal immobilization affects skeletal development and can lead to severe malformations. Still, how mechanical load affects embryonic bone formation is not fully elucidated. This study combines mechanobiology, image analysis and developmental biology, to investigate the structural effects of muscular loading on embryonic long bones. We present a novel approach involving a semi-automatic workflow, to study the spatial and temporal evolutions of both hard and soft tissues in 3D without any contrast agent at micrometrical resolution. Using high-resolution phase-contrast-enhanced X-ray synchrotron microtomography, we compare the humeri of Splotch-delayed embryonic mice lacking skeletal muscles with healthy littermates. The effects of skeletal muscles on bone formation was studied from the first stages of mineral deposition (Theiler Stages 23 and 24) to just before birth (Theiler Stage 27). The results show that muscle activity affects both growth plate and mineralized regions, especially during early embryonic development. When skeletal muscles were absent, there was reduced mineralization, altered tuberosity size and location, and, at early embryonic stages, decreased chondrocyte density, size and elongation compared to littermate controls. The proposed workflow enhances our understanding of mechanobiology of early bone formation and could be implemented for the study of other complex biological tissues.

Pax3 inhibits Neuro-2a cells proliferation and neurite outgrowth

Pax3 and Pax7 are closely related transcription factors that are widely expressed in the developing nervous system and somites. During the normal development in the central nervous system (CNS), Pax3 and Pax7 are mainly expressed in the dorsal part of the neural tube. Further analysis revealed that Pax3 and Pax7 shared redundant functions in the spinal cord development. However, it is still unknown whether Pax3 and Pax7 play a role in neuronal differentiation. In this study, Pax3 and Pax7 genes were overexpressed in Neuro‐2a, the mouse neuroblastoma cell line. CCK‐8 and EdU assay results showed that overexpression of Pax3 inhibited cell viability and proliferation of Neuro‐2a cells, whereas the overexpression of Pax7 had no significant difference on their cell viability and proliferation. Overexpression of Pax3 not only increased the percentage of cells in the S phase and G0/G1 phase, but also decreased that in the G2 phase. Moreover, the total neurite lengths of Neuro‐2a cells were significantly shorter in Pax3 overexpressed group than those in negative control group and showed no significant difference between Pax7 overexpressed group and negative control group. These results suggested that Pax3 not only inhibited the cell viability and proliferation but also affected the cell cycle and the neurite outgrowth of Neuro‐2a cells. RNA sequencing analysis showed up‐regulated genes in Pax3 overexpressed group were involved in cell cycle machinery, which may reveal the potential mechanism of Neuro‐2a cells proliferation.

PAX transcription factors in neural crest development

The nine vertebrate PAX transcription factors (PAX1-PAX9) play essential roles during early development and organogenesis. Pax genes were identified in vertebrates using their homology with the Drosophila melanogaster paired gene DNA-binding domain. PAX1-9 functions are largely conserved throughout vertebrate evolution, in particular during central nervous system and neural crest development. The neural crest is a vertebrate invention, which gives rise to numerous derivatives during organogenesis, including neurons and glia of the peripheral nervous system, craniofacial skeleton and mesenchyme, the heart outflow tract, endocrine and pigment cells. Human and mouse spontaneous mutations as well as experimental analyses have evidenced the critical and diverse functions of PAX factors during neural crest development. Recent studies have highlighted the role of PAX3 and PAX7 in neural crest induction. Additionally, several PAX proteins - PAX1, 3, 7, 9 - regulate cell proliferation, migration and determination in multiple neural crest-derived lineages, such as cardiac, sensory, and enteric neural crest, pigment cells, glia, craniofacial skeleton and teeth, or in organs developing in close relationship with the neural crest such as the thymus and parathyroids. The diverse PAX molecular functions during neural crest formation rely on fine-tuned modulations of their transcriptional transactivation properties. These modulations are generated by multiple means, such as different roles for the various isoforms (formed by alternative splicing), or posttranslational modifications which alter protein-DNA binding, or carefully orchestrated protein-protein interactions with various co-factors which control PAX proteins activity. Understanding these regulations is the key to decipher the versatile roles of PAX transcription factors in neural crest development, differentiation and disease.

Gene regulatory evolution and the origin of macroevolutionary novelties: insights from the neural crest

The appearance of novel anatomic structures during evolution is driven by changes to the networks of transcription factors, signaling pathways, and downstream effector genes controlling development. The nature of the changes to these developmental gene regulatory networks (GRNs) is poorly understood. A striking test case is the evolution of the GRN controlling development of the neural crest (NC). NC cells emerge from the neural plate border (NPB) and contribute to multiple adult structures. While all chordates have a NPB, only in vertebrates do NPB cells express all the genes constituting the neural crest GRN (NC-GRN). Interestingly, invertebrate chordates express orthologs of NC-GRN components in other tissues, revealing that during vertebrate evolution new regulatory connections emerged between transcription factors primitively expressed in the NPB and genes primitively expressed in other tissues. Such interactions could have evolved by two mechanisms. First, transcription factors primitively expressed in the NPB may have evolved new DNA and/or cofactor binding properties (protein neofunctionalization). Alternately, cis-regulatory elements driving NPB expression may have evolved near genes primitively expressed in other tissues (cis-regulatory neofunctionalization). Here we discuss how gene duplication can, in principle, promote either form of neofunctionalization. We review recent published examples of interspecies gene-swap, or regulatory-element-swap, experiments that test both models. Such experiments have yielded little evidence to support the importance of protein neofunctionalization in the emergence of the NC-GRN, but do support the importance of novel cis-regulatory elements in this process. The NC-GRN is an excellent model for the study of gene regulatory and macroevolutionary innovation.

Neural crest cell lineage restricts skeletal muscle progenitor cell differentiation through Neuregulin1-ErbB3 signaling

Coordinating the balance between progenitor self-renewal and myogenic differentiation is required for a regulated expansion of the developing muscles. Previous observation that neural crest cells (NCCs) migrate throughout the somite regions, where trunk skeletal muscles first emerge, suggests a potential role for these cells in influencing early muscle formation. However, specific signaling interactions between NCCs and skeletal muscle cells remain unknown. Here we show that mice with specific NCC and peripheral nervous system defects display impaired survival of skeletal muscle and show skeletal muscle progenitor cell (MPC) depletion due to precocious commitment to differentiation. We show that reduced NCC-derived Neuregulin1 (Nrg1) in the somite region perturbs ErbB3 signaling in uncommitted MPCs. Using a combination of explant culture experiments and genetic ablation in the mouse, we demonstrate that Nrg1 signals provided by the NCC lineage play a critical role in sustainable myogenesis, by restraining MPCs from precocious differentiation.

Cell cycle regulation during proliferation and differentiation of mammalian muscle precursor cells

Proliferation and differentiation of muscle precursor cells are intensively studied not only in the developing mouse embryo but also using models of skeletal muscle regeneration or analyzing in vitro cultured cells. These analyses allowed to show the universality of the cell cycle regulation and also uncovered tissue-specific interplay between major cell cycle regulators and factors crucial for the myogenic differentiation. Examination of the events accompanying proliferation and differentiation leading to the formation of functional skeletal muscle fibers allows understanding the molecular basis not only of myogenesis but also of skeletal muscle regeneration. This chapter presents the basis of the cell cycle regulation in proliferating and differentiating muscle precursor cells during development and after muscle injury. It focuses at major cell cycle regulators, myogenic factors, and extracellular environment impacting on the skeletal muscle.

The Pax3 and Pax7 paralogs cooperate in neural and neural crest patterning using distinct molecular mechanisms, in Xenopus laevis embryos

Pax3 and Pax7 paralogous genes have functionally diverged in vertebrate evolution, creating opportunity for a new distribution of roles between the two genes and the evolution of novel functions. Here we focus on the regulation and function of Pax7 in the brain and neural crest of amphibian embryos, which display a different pax7 expression pattern, compared to the other vertebrates already described. Pax7 expression is restricted to the midbrain, hindbrain and anterior spinal cord, and Pax7 activity is important for maintaining the fates of these regions, by restricting otx2 expression anteriorly. In contrast, pax3 displays broader expression along the entire neuraxis and Pax3 function is important for posterior brain patterning without acting on otx2 expression. Moreover, while both genes are essential for neural crest patterning, we show that they do so using two distinct mechanisms: Pax3 acts within the ectoderm which will be induced into neural crest, while Pax7 is essential for the inducing activity of the paraxial mesoderm towards the prospective neural crest.

Identification and characterization of subpopulations of Pax3 and Pax7 expressing cells in developing chick somites and limb buds

Pax3 and Pax7 are closely related paired-boxed family transcription factors that are known to play important roles in embryonic and adult myogenesis. Previous reports describing the expression of Pax3 and Pax7 transcripts reveal expression in many overlapping domains. In this manuscript, we extend these studies by examining the protein expression profiles for Pax3 and Pax7 in developing chick somites and limbs with cellular resolution. Our studies show the existence of distinct subpopulations of cells in the somite and developing limb that are defined by the relative expression levels of Pax3 and Pax7. We also show that Pax3 and Pax7 negatively regulate each other's expression in the dermomyotome, thus providing a possible mechanism for the maintenance of observed expression patterns in the dermomyotome. Further characterization of Pax3- and/or Pax7-positive cells in the dermomyotome and myotome with respect to proliferation and differentiation reveals subpopulations of cells with distinct properties.

Subnuclear localization and mobility are key indicators of PAX3 dysfunction in Waardenburg syndrome

Mutations in the transcription factor PAX3 cause Waardenburg syndrome (WS) in humans and the mouse Splotch mutant, which display similar neural crest-derived defects. Previous characterization of disease-causing mutations revealed pleiotropic effects on PAX3 DNA binding and transcriptional activity. In this study, we evaluated the impact of disease alleles on PAX3 localization and mobility. Immunofluorescence analyses indicated that the majority of PAX3 occupies the interchromatin space, with only sporadic colocalization with sites of transcription. Interestingly, PAX3 disease alleles fell into two distinct categories when localization and dynamics in fluorescence recovery after photobleaching (FRAP) were assessed. The first group (class I), comprising N47H, G81A and V265F exhibit a diffuse distribution and markedly increased mobility when compared with wild-type PAX3. In contrast, the G42R, F45L, S84F, Y90H and R271G mutants (class II) display evidence of subnuclear compartmentalization and mobility intermediate between wild-type PAX3 and class I proteins. However, unlike class I mutants, which retain DNA binding, class II proteins are deficient for this activity, indicating that DNA binding is not a primary determinant of PAX3 distribution and movement. Importantly, class I properties prevail when combined with a class II mutation, which taken with the proximity of the two mutant classes within the PAX3 protein, suggests class I mutants act by perturbing PAX3 conformation. Together, these results establish that altered localization and dynamics play a key role in PAX3 dysfunction and that loss of the underlying determinants represents the principal defect for a subset of Waardenburg mutations.

Linkage to chromosome 2q36.1 in autosomal dominant Dandy-Walker malformation with occipital cephalocele and evidence for genetic heterogeneity

We previously reported a Vietnamese-American family with isolated autosomal dominant occipital cephalocele. Upon further neuroimaging studies, we have recharacterized this condition as autosomal dominant Dandy-Walker with occipital cephalocele (ADDWOC). A similar ADDWOC family from Brazil was also recently described. To determine the genetic etiology of ADDWOC, we performed genome-wide linkage analysis on members of the Vietnamese-American and Brazilian pedigrees. Linkage analysis of the Vietnamese-American family identified the ADDWOC causative locus on chromosome 2q36.1 with a multipoint parametric LOD score of 3.3, while haplotype analysis refined the locus to 1.1 Mb. Sequencing of the five known genes in this locus did not identify any protein-altering mutations. However, a terminal deletion of chromosome 2 in a patient with an isolated case of Dandy-Walker malformation also encompassed the 2q36.1 chromosomal region. The Brazilian pedigree did not show linkage to this 2q36.1 region. Taken together, these results demonstrate a locus for ADDWOC on 2q36.1 and also suggest locus heterogeneity for ADDWOC.

Expression of Connexin 43 in the hearts of rat embryos exposed to nitrofen and effects of vitamin A on it

Rats with experimental congenital diaphragmatic hernia (CDH) have heart hypoplasia and conotruncal and great vessel malformations that are likely related to disturbed neural crest developmental control. Neural crest cells communicate through intercellular gap junctions whose main protein is Connexin 43 (Cx43). The migration and participation of neural crest cells in heart development is likely influenced by this protein which might be also directly involved in myocardial development. Vitamin A is beneficial for heart hypoplasia in CDH rats. The aims of this study were to examine the status of Cx43 in the heart of embryonal rats exposed to nitrofen and to assess if vitamin A reverts these effects. Pregnant rats received either 100 mg nitrofen or olive oil on E9.5. Each group was divided into two subgroups according to the subsequent treatment with intragastric vitamin A (15,000 i.u.) or vehicle on E10.5 and E11.5. The pups were recovered on E13, E15, and E21 and the hearts were dissected out and pooled. Cx43 mRNA expression was determined by quantitative real-time PCR. Comparisons among groups were made with ANOVA and Bonferroni post hoc tests with a threshold of significance of P<0.05. In control rats Cx43 mRNA was minimally expressed on E13 and E15 and fully expressed on E21. Nitrofen significantly increased Cx43 mRNA on E15. Additional treatment with vitamin A tended to moderate this increase on E15. Cx43 was overexpressed in the hearts of nitrofen-exposed embryonal rats on day E15 of gestation. Vitamin A tended to normalize this expression. The mechanism of action of Cx43 deserves further investigation.

Direct regulation of myelin protein zero expression by the Egr2 transactivator

During myelination of the peripheral nervous system, the myelin protein zero (Mpz) gene is induced to produce the most abundant protein component (P(0)) of mature myelin. Although the basal embryonic expression of Mpz in Schwann cells has been attributed to regulation by Sox10, the molecular mechanism for the profound up-regulation of this gene during myelination has not been established. In this study, we have identified a highly conserved element within the first intron of the Mpz gene, which contains binding sites for the early growth response 2 (Egr2/Krox20) transcription factor, a critical regulator of peripheral nerve myelination. Egr2 can transactivate the intron element, and the induction is blocked by two known repressors of Egr2 activity. Using chromatin immunoprecipitation assays, we find that Egr2 binds in vivo to the intron element, but not to the Mpz promoter. Known inducers of Mpz expression such as forskolin and insulin-like growth factor-1 also activate the element in an Egr2-dependent manner. In addition, we found that Egr2 can act synergistically with Sox10 to activate this intron element, suggesting a model in which cooperative interactions between Egr2 and Sox10 mediate a large increase in Mpz expression to the high levels found in myelinating Schwann cells.

The class III POU domain protein Brn-1 can fully replace the related Oct-6 during schwann cell development and myelination

For differentiation, Schwann cells rely on the class III POU domain transcription factor Oct-6, which is expressed transiently when Schwann cells have established a one-to-one relation with axons but have not yet started to myelinate. Loss of Oct-6 leads to a transient arrest in this promyelinating stage and a delay in myelination. Although the closely related POU domain protein Brn-2 is coexpressed with Oct-6 in Schwann cells, its loss has only mild consequences. Combined loss of both POU domain proteins, in contrast, dramatically increases the myelination delay, raising the question of how related POU domain proteins compare to each other in their activities. Here, we have replaced Oct-6 expression in the mouse with expression of the class III POU domain protein Brn-1. Although this protein is not normally expressed in Schwann cells, Brn-1 was capable of fully replacing Oct-6. Brn-1 efficiently induced Krox-20 expression as a prerequisite for myelination. Onset and extent of myelination were also indistinguishable from that of the wild type in mice that carried only Brn-1 instead of Oct-6 alleles. Similar to Oct-6, Brn-1 down-regulated its own expression at later stages of myelination. Thus, class III POU domain proteins can fully replace each other in Schwann cell development.

Heart and head defects in mice lacking pairs of connexins

Gene ablation studies in mice have revealed roles for gap junction proteins (connexins) in heart development. Of the 20 connexins in vertebrates, four are expressed in developing heart: connexin37 (Cx37), connexin40 (Cx40), connexin43 (Cx43), and connexin45 (Cx45). Although each cardiac connexin has a different pattern of expression, some heart cells coexpress multiple connexins during cardiac morphogenesis. Since different connexins could have overlapping functions, some developmental phenotypes may only become evident when more than one connexin is ablated. In this study, we interbred Cx40(-/-) and Cx43(-/-) mice to generate mice lacking both Cx40 and Cx43. Cx40(-/-)Cx43(-/-) mice die around embryonic day 12.5 (E12.5), much earlier than either Cx40(-/-) or Cx43(-/-) mice, and they exhibit malformed hearts with ventricles that are abnormally rotated, suggesting a looping defect. Some Cx40(-/-)Cx43(-/-) animals also develop head defects characteristic of exencephaly. In addition, we examined mice lacking both Cx40 and Cx37 and found a high incidence of atrial and ventricular septal defects at birth. These results provide further evidence for the importance of gap junctions in embryonic development. Moreover, ablating different pairs of cardiac connexins results in distinct heart defects, suggesting both common and unique functions for Cx40, Cx43, and Cx37 during cardiac morphogenesis.

Neural and orofacial defects in Folp1 knockout mice [corrected]

BACKGROUND

Folic acid is essential for the development of the nervous system and other associated structures. Mice deficient in the folic acid-binding protein one (Folbp1) gene display multiple developmental abnormalities, including neural and craniofacial defects. To better understand potential interactions between Folbp1 gene and selected genes involved in neural and craniofacial morphogenesis, we evaluated the expression patterns of a panel of crucial differentiation markers (Pax-3, En-2, Hox-a1, Shh, Bmp-4, Wnt-1, and Pax-1).

METHODS

Folbp1 mice were supplemented with low dosages of folinic add to rescue nullizygotes from dying in utero before gestational day 10. The gene marker analyses were carried out by in situ hybridization.

RESULTS

In nullizygote embryos with open cranial neural tube defects, the downregulation of Pax-3 and En-2 in the impaired midbrain, along with an observed upregulation of the ventralizing marker Shh in the expanded floor plate, suggested an important regulatory interaction among these three genes. Moreover, the nullizygotes also exhibit craniofacial abnormalities, such as cleft lip and palate. Pax-3 signals in the impaired medial nasal primordia were significantly increased, whereas Pax-1 showed no expression in the undeveloped lateral nasal processes. Although Shh was downregulated, Bmp-4 was strongly expressed in the medial and lateral nasal processes, highlighting the antagonistic activities of these molecules.

CONCLUSIONS

Impairment of Folbp1 gene function adversely impacts the expression of several critical signaling molecules. Mis-expression of these molecules, perhaps mediated by Shh, may potentially contribute to the observed failure of neural tube closure and the development of craniofacial defects in the mutant mice.

Pax3acts cell autonomously in the neural tube and somites by controlling cell surface properties

Pax3 is a member of the paired-box-containing transcription factors. It is expressed in the developing somites, dorsal spinal cord, mesencephalon and neural crest derivatives. Several loss-of-function mutations are correlated with the Splotch phenotype in mice and Waardenburg syndrome in humans. Malformations include a lack of muscle in the limb, a failure of neural tube closure and dysgenesis of numerous neural crest derivatives. In this study we have used embryonic stem (ES) cells to generate a lacZ knock-in into the Pax3 locus. The Pax3 knock-in Splotch allele (Sp2G) was used to generate Pax3-deficient ES cells in order to investigate whether, in chimeric embryos, Pax3 is acting cell autonomously in the somites and the neural tube. We found that while Pax3 function is essential for the neuroepithelium and somites, a wild-type environment rescues mutant neural crest cells. In the two affected embryonic tissues, mutant and wild-type cells undergo segregation and do not intermingle.

The contribution of mutant cells to the neural tube and the somites displayed temporal differences. All chimeric embryos showed a remarkable contribution of blue cells to the neural tube at all stages analyzed, indicating that the Pax3-deficient cells are not excluded from the neural epithelium while development proceeds. In contrast, this is not true for the paraxial mesoderm. The somite contribution of Pax3−/− ES cells becomes less frequent in older embryos as compared to controls with Pax3+/− ES cells. We propose that although Pax3 function is related to cell surface properties, its role may differ in various tissues. In fact, apoptosis was found in Pax3-deficient cells of the lateral dermomyotome but not in the neural tube.

The transcription factor Sox10 is a key regulator of peripheral glial development

The molecular mechanisms that determine glial cell fate in the vertebrate nervous system have not been elucidated. Peripheral glial cells differentiate from pluripotent neural crest cells. We show here that the transcription factor Sox10 is a key regulator in differentiation of peripheral glial cells. In mice that carry a spontaneous or a targeted mutation of Sox10, neuronal cells form in dorsal root ganglia, but Schwann cells or satellite cells are not generated. At later developmental stages, this lack of peripheral glial cells results in a severe degeneration of sensory and motor neurons. Moreover, we show that Sox10 controls expression of ErbB3 in neural crest cells. ErbB3 encodes a Neuregulin receptor, and down-regulation of ErbB3 accounts for many changes in development of neural crest cells observed in Sox10 mutant mice. Sox10 also has functions not mediated by ErbB3, for instance in the melanocyte lineage. Phenotypes observed in heterozygous mice that carry a targeted Sox10 null allele reproduce those observed in heterozygous Sox10(Dom) mice. Haploinsufficiency of Sox10 can thus cause pigmentation and megacolon defects, which are also observed in Sox10(Dom)/+ mice and in patients with Waardenburg-Hirschsprung disease caused by heterozygous SOX10 mutations.

The expression of the homeobox gene Msx1 reveals two populations of dermal progenitor cells originating from the somites

Experimental manipulation in birds has shown that trunk dermis has a double origin: dorsally, it derives from the somite dermomyotome, while ventrally, it is formed by the somatopleure. Taking advantage of an nlacZ reporter gene integrated into the mouse Msx1 locus (Msx1(nlacZ) allele), we detected segmental expression of the Msx1 gene in cells of the dorsal mesenchyme of the trunk between embryonic days 11 and 14. Replacing somites from a chick host embryo by murine Msx1(nlacZ)somites allowed us to demonstrate that these Msx1-(beta)-galactosidase positive cells are of somitic origin. We propose that these cells are dermal progenitor cells that migrate from the somites and subsequently contribute to the dorsalmost dermis. By analysing Msx1(nlacZ) expression in a Splotch mutant, we observed that migration of these cells does not depend on Pax3, in contrast to other migratory populations such as limb muscle progenitor cells and neural crest cells. Msx1 expression was never detected in cells overlying the dermomyotome, although these cells are also of somitic origin. Therefore, we propose that two somite-derived populations of dermis progenitor cells can be distinguished. Cells expressing the Msx1 gene would migrate from the somite and contribute to the dermis of the dorsalmost trunk region. A second population of cells would disaggregate from the somite and contribute to the dermis overlying the dermomyotome. This population never expresses Msx1. Msx1 expression was investigated in the context of the onset of dermis formation monitored by the Dermo1 gene expression. The gene is downregulated prior to the onset of dermis differentiation, suggesting a role for Msx1 in the control of this process.

alyron, an insertional mutation affecting early neural crest development in zebrafish

alyronz12 (aln) is a recessive lethal mutation that affects early stages of neural crest development in the zebrafish. alyron appears to be an insertional mutation as the mutation was generated following microinjection of plasmid DNA into one-cell embryos and the stably integrated transgenic sequences are closely linked to the mutation. The insertion site harbors multiple copies of the plasmid sequence that have experienced complex rearrangements. Host-insert junction fragments have been molecularly cloned and host sequences adjacent to the transgene have been used to map the mutation to the distal arm of linkage group 15. alyron function is required cell-autonomously in the neural crest lineage. alyron mutants have a severe but not complete deficit of premigratory neural crest as judged by reduced expression of several markers associated with early stages of neural crest development. Lack of premigratory neural crest is likely to account for the two most conspicuous characteristics of alyron mutants: the absence of body pigmentation and the inability to affect blood circulation. The neural crest phenotype of alyron mutants resembles that observed in mouse mutants that lack Pax-3 or both Wnt-1 and Wnt-3a function, and expression of the zebrafish homologues of these genes is greatly reduced in the dorsal neural keels of alyron mutants. In contrast, ventral neural keel identity appears unaffected. Given our findings that the mutation is unlinked to pax or wnt genes that have been described in the zebrafish, we propose that alyron is a novel gene function required for the specification and/or proliferative expansion of neural crest progenitors.

Pax-3 regulates neurogenesis in neural crest-derived precursor cells

The peripheral nervous system consists of multiple neural lineages derived from the neural crest (NC). Pax-3 is expressed in the NC and when mutated in the splotch mouse (Sp) results in the loss of derivatives from this precursor cell population. We have investigated the role of Pax-3 in regulating the generation of neurons from NC-derived precursor cells in vitro. Pax-3 mRNA in NC cultures is initially expressed in all NC but is subsequently only retained in neurons, suggesting a role in their generation. To determine whether Pax-3 is involved in neuron development, we first examined the generation of sensory-like neurons in NC cultures from Sp mice. Fivefold less sensory-like neurons were generated in NC cultures from Sp homozygous mice as compared to wild-type littermates. The role of Pax-3 in sensory neuron generation was then directly examined in dorsal root ganglia cultures by down-regulating the expression of Pax-3 protein with antisense oligonucleotides. It was found that antisense oligonucleotides inhibited 80-90% of newly generated sensory neurons; however, there was no significant effect on the survival of sensory neurons or the precursor population. These results suggest that Pax-3 has a role in regulating the differentiation of peripheral neurons.

Transgenic rescue of congenital heart disease and spina bifida in Splotch mice

Pax3-deficient Splotch mice display neural tube defects and an array of neural crest related abnormalities including defects in the cardiac outflow tract, dorsal root ganglia and pigmentation. Pax3 is expressed in neural crest cells that emerge from the dorsal neural tube. Pax3 is also expressed in the somites, through which neural crest cells migrate, where it is required for hypaxial muscle development. Homozygous mutant Splotch embryos die by embryonic day 14. We have utilized the proximal 1.6 kb Pax3 promoter and upstream regulatory elements to engineer transgenic mice reproducing endogenous Pax3 expression in neural tube and neural crest, but not the somite. Over expression of Pax3 in these tissues reveals no discernible phenotype. Breeding of transgenic mice onto a Splotch background demonstrates that neural tube and neural crest expression of Pax3 is sufficient to rescue neural tube closure, cardiac development and other neural crest related defects. Transgenic Splotch mice survive until birth at which time they succumb to respiratory failure secondary to absence of a muscular diaphragm. Limb muscles are also absent. These results indicate that regulatory elements sufficient for functional expression of Pax3 required for cardiac development and neural tube closure are contained within the region 1.6 kb upstream of the Pax3 transcriptional start site. In addition, the single Pax3 isoform used for this transgene is sufficient to execute these developmental processes. Although the extracellular matrix and the environment of the somites through which neural crest migrates is known to influence neural crest behavior, our results indicate that Pax3-deficient somites are capable of supporting proper neural crest migration and function suggesting a cell autonomous role for Pax3 in neural crest.

Role of cardiac neural crest cells in cardiovascular development

The discovery in the chick embryo that a specific region of the neural crest, termed the cardiac neural crest, is essential for septation of the cardiac outflow tract and for aortic arch artery development has led to the classification of a whole series of human cardiac defects as neural crest-associated. Recently, several mouse genetic models have been effectively employed to yield new insights into the relationship between cardiac neural crest and structural heart development. In all the animal models of neural crest-related heart defects, prenatal mortality is too high to be attributed to structural defects of the heart alone, and there are obvious signs of severe cardiac dysfunction. The evidence indicates that poor viability is from impaired cardiac excitation-contraction coupling and contractile function at the myocyte level. The continued study of experimental and genetically defined models with neural crest-associated heart defects will prove useful in identifying the common pathways by which the neural crest contributes to normal heart development.

Heart and neural tube defects in transgenic mice overexpressing the Cx43 gap junction gene

Transgenic mice were generated containing a cytomegaloviral promoter driven construct (CMV43) expressing the gap junction polylpeptide connexin 43. RNA and protein analysis confirmed that the transgene was being expressed. In situ hybridization analysis of embryo sections revealed that transgene expression was targeted to the dorsal neural tube and in subpopulations of neural crest cells. This expression pattern was identical to that seen in transgenic mice harboring other constructs driven by the cytomegaloviral promoter (Kothary, R., Barton, S. C., Franz, T., Norris, M. L., Hettle, S. and Surani, M. A. H. (1991) Mech. Develop. 35, 25–31; Koedood, M., Fitchel, A., Meier, P. and Mitchell, P. (1995) J. Virol. 69, 2194–2207), and corresponded to a subset of the endogenous Cx43 expression domains. Significantly, dye injection studies showed that transgene expression resulted in an increase in gap junctional communication. Though viable and fertile, these transgenic mice exhibited reduced postnatal viability. Examination of embryos at various stages of development revealed developmental perturbations consisting of cranial neural tube defects (NTD) and heart malformations. Interestingly, breeding of the CMV43 transgene into the Cx43 knockout mice extended postnatal viability of mice homozygote for the Cx43 knockout allele, indicating that the CMV43 trangsene may partially complement the Cx43 deletion. Both the Cx43 knockout and the CMV43 transgenic mice exhibit heart defects associated with malformations in the conotruncus, a region of the heart in which neural crest derivatives are known to have important roles during development. Together with our results indicating neural-crest-specific expression of the transgene in our CMV-based constructs, these observations strongly suggest a role for Cx43-mediated gap junctional communication in neural crest development. Furthermore, these observations indicate that the precise level of Cx43 function may be of critical importance in downstream events involving these migratory cell populations. As such, the CMV43 mouse may represent a powerful new model system for examining the role of extracardiac cell populations in cardiac morphogenesis and other developmental processes.

Pax genes and their roles in cell differentiation and development

Members of the Pax gene family are expressed in various tissues during ontogenesis. Evidence for their crucial role in morphogenesis, organogenesis, cell differentiation and oncogenesis is provided by rodent mutants and human diseases. Additionally, recent experimental in vivo and in vitro approaches have led to the identification of molecules that interact with Pax proteins.

Oct-6 (SCIP/Tst-1) is expressed in Schwann cell precursors, embryonic Schwann cells, and postnatal myelinating Schwann cells: comparison with Oct-1, Krox-20, and Pax-3

The POU domain transcription factor Oct-6 (SCIP/Tst-1) is likely to control important stages of Schwann cell development, including the initiation of myelination around birth. Here, we use immunocytochemical and reverse transcriptase-polymerase chain reaction techniques to examine Oct-6 earlier in nerve development, to test the idea that Oct-6 has an additional role in Schwann cell precursors or early embryonic Schwann cells, a possibility raised by previous studies on transgenic mice. Consistent with this, we find low but unambiguous levels of Oct-6 mRNA and protein in Schwann cell precursors of mouse and rat (nerves from 12- and 14-day-old embryos, respectively), with expression levels gradually increasing during early Schwann cell development and towards birth. Unexpectedly, Oct-6 immunoreactivity is clearly present in nuclei of most myelinating cells at least as late as postnatal day 12. Furthermore, many nonmyelinating Schwann cells express Oct-6 in adult life. A comparison of Oct-6 mRNA with other Schwann cell transcription factors-namely, Oct-1, Krox-20, and Pax-3-reveals that each factor exhibits strong developmental regulation and a unique expression pattern in embryonic nerves. Therefore, they are likely to play distinct regulatory roles in early development of the Schwann cell lineage.

Pax: gene regulators in the developing nervous system

In recent years, the discovery of Pax genes in mouse has played an invaluable role in furthering our understanding in mouse developmental processes and disorders. To date, eight murine paired box-containing genes have been cloned. Seven of these exhibit a distinct spatiotemporal expression pattern in the developing nervous system implying a role in the regional specification of the developing spinal cord and brain. The Pax genes encode for sequence-specific DNA binding transcription factors that play a key role in embryonic development. Three of these developmental control genes are altered in mutant mice and two are associated with human diseases. Disruption of these Pax genes leads to abnormalities in neural crest derivatives, neuroectoderm, sclerotome or myotome-derived tissues. Disruption of the Pax-3 gene causes the Splotch phenotype in mice and Waardenburg syndrome in humans. Pax-6 mutations result in Small eye mice and the human genetic disorder aniridia. The Pax-1 gene is mutated in undulated mice. Pax proteins can transform cells in culture which then form tumours following injection in nude mice. Consistent with this activity, PAX3 has been recently implicated in the generation of the tumour alveolar rhabdomyosarcoma.