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Skeletal muscle is generated by the successive incorporation of primary (embryonic), secondary (fetal), and tertiary (adult) fibers into muscle. Conditional excision of Pitx2 function by an MCK(Cre) driver resulted in animals with histological and ultrastructural defects in P30 muscles and fibers, respectively. Mutant muscle showed severe reduction in mitochondria and FoxO3-mediated mitophagy. Both oxidative and glycolytic energy metabolism were reduced. Conditional excision was limited to fetal muscle fibers after the G1–G0 transition and resulted in altered MHC, Rac1, MEF2a, and alpha-tubulin expression within these fibers. The onset of excision, monitored by a nuclear reporter gene, was observed as early as E16. Muscle at this stage was already severely malformed, but appeared to recover by P30 by the expansion of adjoining larger fibers. Our studies demonstrate that the homeodomain transcription factor Pitx2 has a postmitotic role in maintaining skeletal muscle integrity and energy homeostasis in fetal muscle fibers.

Phenotypic screening enables the discovery of new drug leads with novel targets. ES cells differentiate into different lineages by successively making use of different subsets of the genome's possible macromolecular interactions. If a compound effectively targets just one of these interactions, it derails the developmental pathway to produce a phenotypical change. The OTRADI microsource spectrum library of 2000 approved drug components, natural products, and bioactive components was screened for compounds that can induce phenotypic changes in ES cell cultures at 10 µM after 3 days. Twenty-one compounds that induced specific morphologies also induced unique changes to an expression profile of a dozen markers of early embryonic development, indicating that each compound has derailed the molecular developmental process in a characteristic way. Phenotypic screens conducted with ES cultures differentiating along different lineages can be used to efficiently prescreen compounds able to regulate cell differentiation lineage.

Bcl11b is a transcription factor that, within the hematopoietic system, is expressed specifically in T cells. Although Bcl11b is required for T-cell differentiation in newborn Bcl11b-null mice, and for positive selection in the adult thymus of mice bearing a T-cell-targeted deletion, the gene network regulated by Bcl11b in T cells is unclear. We report herein that Bcl11b is a bifunctional transcriptional regulator, which is required for the correct expression of approximately 1000 genes in CD4(+)CD8(+)CD3(lo) double-positive (DP) thymocytes. Bcl11b-deficient DP cells displayed a gene expression program associated with mature CD4(+)CD8(-) and CD4(-)CD8(+) single-positive (SP) thymocytes, including upregulation of key transcriptional regulators, such as Zbtb7b and Runx3. Bcl11b interacted with regulatory regions of many dysregulated genes, suggesting a direct role in the transcriptional regulation of these genes. However, inappropriate expression of lineage-associated genes did not result in enhanced differentiation, as deletion of Bcl11b in DP cells prevented development of SP thymocytes, and that of canonical NKT cells. These data establish Bcl11b as a crucial transcriptional regulator in thymocytes, in which Bcl11b functions to prevent the premature expression of genes fundamental to the SP and NKT cell differentiation programs.

Late-stage Pitx2(+/LacZ) mouse embryos stained with x-gal appeared to have blue muscles, suggesting that Pitx2 expression specifically marks some phase of the myogenic progression or muscle anlagen formation. Detailed temporal and spatial analyses were undertaken to determine the extent and onset of Pitx2 expression in muscle. Pitx2 was specifically expressed in the vast majority of muscles of the head and trunk in late embryos and adults. Early Pitx2 expression in the cephalic mesoderm, first branchial arch and somatopleure preceded specification of head muscle. In contrast, Pitx2 expression appeared to follow muscle specification events in the trunk. However, Pitx2 expression was rapidly upregulated in these myogenic structures by E10.5. Upregulation correlated tightly with the apposition of a non-myogenic, Pitx2-expressing, cell cluster lateral to the dermomyotome. This cluster first appeared at the forelimb level at E10.25, gradually elongated in the posterior direction, appeared to aggregate from delaminated cells emanating from the ventrally located somatopleure, and was named the dorsal somatopleure. Immunohistochemistry on appendicular sections after E10.5 demonstrated that Pitx2 neatly marked the areas of muscle anlagen, that Pax3, Lbx1, and the muscle regulatory factors (MRFs) stained only subsets of Pitx2(+) cells within these areas, and that virtually all Pitx2(+) cells in these areas express at least one of these known myogenic markers. Taken together, the results demonstrate that, within muscle anlagen, Pitx2 marks the muscle lineage more completely that any of the known markers, and are consistent with a role for Pitx2 in muscle anlagen formation or maintenance.

The mouse homeodomain protein Hoxa-7, expressed under the control of an inducible promoter, was able to inducibly activate reporter genes containing multimerized Hoxa-7 binding sites in Saccharomyces cerevisiae . This tight regulation was exploited in an attempt to screen for HOXa-7 responsive elements. A reporter library consisting of a randomised 10bp element inserted into the minimal ga1 promoter was constructed. In a surprisingly small screen, 24 reporters were isolated which had all of the transactivation characteristics expected for a Hoxa-7 binding site insertion. However, further characterisation revealed that the selected elements lacked homeo-domain (HD) binding core motifs and were not bound by a purified Hoxa-7/β-galactosidase fusion protein capable of binding known sites. The minimal promoter context contains 16 HD core motifs in 410 bp. Careful re-examination of basal levels revealed a low residual response of the gal1 minimal promoter to Hoxa-7. The 11 characterised 10 bp inserts amplified Hoxa-7 responsiveness in a manner correlated to increases in basal reporter activity. Thus, a quantitative range of Hox-responsiveness was produced by slight sequence alterations that did not change HD binding sites of their relative spacing in the promoter. These data suggest how, without altering resident HD base contact zones, mouse promoters could be optimised by natural selection to give appropriate quantitative outputs in each anatomical region defined by an assortment of Hox proteins. The selected elements were pyrimidine rich on the sense strand, containing (T) n C motifs, strikingly similar to sequences which enhance Hoxa-7 binding activation from outside the HD contact zone. A search of defined sequence databases demonstrated that these elements were over-represented in promoters. Two elements altered the mobility shift patterns produced by cell extracts on minimal promoter fragments.

The ventrolateral dermomyotome gives rise to all muscles of the limbs through the delamination and migration of cells into the limb buds. These cells proliferate and form myoblasts, withdraw from the cell cycle and become terminally differentiated. The myogenic lineage colonizes pre-patterned regions to form muscle anlagen as muscle fibers are assembled. The regulatory mechanisms that control the later steps of this myogenic program are not well understood. The homeodomain transcription factor Pitx2 is expressed in the muscle lineage from the migration of precursors to adult muscle. Ablation of Pitx2 results in distortion, rather than loss, of limb muscle anlagen, suggesting that its function becomes critical during the colonization of, and/or fiber assembly in, the anlagen. Gene expression arrays were used to identify changes in gene expression in flow-sorted migratory muscle precursors, labeled by Lbx1EGFP, which resulted from the loss of Pitx2. Target genes of Pitx2 were clustered using the “David Bioinformatics Functional Annotation Tool” to bin genes according to enrichment of gene ontology keywords. This provided a way to both narrow the target genes and identify potential gene families regulated by Pitx2. Representative target genes in the most enriched bins were analyzed for the presence and evolutionary conservation of Pitx2 consensus binding sequence, TAATCY, on the − 20 kb, intronic, and coding regions of the genes. Fifteen Pitx2 target genes were selected based on the above analysis and were identified as having functions involving cytoskeleton organization, tissue specification, and transcription factors. Data from these studies suggest that Pitx2 acts to regulate cell motility and expression of muscle specific genes in the muscle precursors during forelimb muscle development. This work provides a framework to develop the gene network leading to skeletal muscle development, growth and regeneration.

Cells of the ventrolateral dermomyotome delaminate and migrate into the limb buds where they give rise to all muscles of the limbs. The migratory cells proliferate and form myoblasts, which withdraw from the cell cycle to become terminally differentiated myocytes. The myogenic lineage colonizes pre-patterned regions to form muscle anlagen as muscle fibers are assembled. The regulatory mechanisms that control the later steps of this myogenic program are not well understood. The homeodomain transcription factor Pitx2 is expressed specifically in the muscle lineage from the migration of precursors to adult muscle. Ablation of Pitx2 results in distortion, rather than loss, of limb muscle anlagen, suggesting that its function becomes critical during the colonization of, and/or fiber assembly in, the anlagen. Microarrays were used to identify changes in gene expression in flow-sorted migratory muscle precursors, labeled by Lbx1(EGFP/+), which resulted from the loss of Pitx2. Very few genes showed changes in expression. Many small-fold, yet significant, changes were observed in genes encoding cytoskeletal and adhesion proteins which play a role in cell motility. Myogenic cells from genetically-tagged mice were cultured and subjected to live cell-tracking analysis using time-lapse imaging. Myogenic cells lacking Pitx2 were smaller, more symmetrical, and had more actin bundling. They also migrated about half of the total distance and velocity. Decreased motility may prevent myogenic cells from filling pre-patterned regions of the limb bud in a timely manner. Altered shape may prevent proper assembly of higher-order fibers within anlagen. Pitx2 therefore appears to regulate muscle anlagen development by appropriately balancing expression of cytoskeletal and adhesion molecules.

The murine developmental control gene product, Hoxa-7, was shown to function as a DNA-binding transactivator in Saccharomyces cerevisiae. The importance of the ATTA core, the preference for antp class flanking nucleotides, the importance of Asn-51 of the homeodomain (HD), and the synergism of multiple binding sites all reflect properties that have previously been described for HOM or Hox proteins in tissue culture systems. A comparison of contact positions among genes of paralog groups and classes of mammalian HDs points to a lack of diversity in positions that make base contact, suggesting that besides the combination of HD amino acid-base pair contacts, another means of recognizing differences between targets must exist if Hox genes select different targets. The HD of antennapedia is identical to the Hoxa-7 HD. The interaction of Hoxa-7 with the exact sequence used in the nuclear magnetic resonance three-dimensional structural analysis on the antennapedia HD was studied. Hoxa-7 binding and transactivation was influenced by sequences outside of the known base contact zone of this site. We conclude that Hoxa-7 protein has a second means to interact with DNA or/and that the sequences flanking the base contact zone influence HD interactions by distorting DNA within the contact zone (base or backbone). This result is discussed in terms of DNA flexure and two modes of transcription used in S. cerevisiae.

The transcription factor Ctip2/Bcl11b plays essential roles in developmental processes of the immune and central nervous systems and skin. Here we show that Ctip2 also plays a key role in tooth development. Ctip2 is highly expressed in the ectodermal components of the developing tooth, including inner and outer enamel epithelia, stellate reticulum, stratum intermedium, and the ameloblast cell lineage. In Ctip2 −/− mice, tooth morphogenesis appeared to proceed normally through the cap stage but developed multiple defects at the bell stage. Mutant incisors and molars were reduced in size and exhibited hypoplasticity of the stellate reticulum. An ameloblast-like cell population developed ectopically on the lingual aspect of mutant lower incisors, and the morphology, polarization, and adhesion properties of ameloblasts on the labial side of these teeth were severely disrupted. Perturbations of gene expression were also observed in the mandible of Ctip2 −/− mice: expression of the ameloblast markers amelogenin , ameloblastin , and enamelin was down-regulated, as was expression of Msx2 and epiprofin , transcription factors implicated in the tooth development and ameloblast differentiation. These results suggest that Ctip2 functions as a critical regulator of epithelial cell fate and differentiation during tooth morphogenesis.

Lbx1is a transcription factor that determines neuronal cell fate and identity in the developing medulla and spinal cord. NewbornLbx1mutant mice die of respiratory distress during the early postnatal period. Usingin vitrobrainstem–spinal cord preparations we tested the hypothesis thatLbx1is necessary for the inception, development and modulation of central respiratory rhythmogenesis. The inception of respiratory rhythmogenesis at embryonic day 15 (E15) was not perturbed inLbx1mutant mice. However, the typical age-dependent increase in respiratory frequency observed in wild-type from E15 to P0 was not observed inLbx1mutant mice. The slow respiratory rhythms in E18.5Lbx1mutant preparations were increased to wild-type frequencies by application of substance P, thyrotropin releasing hormone, serotonin, noradrenaline, or the ampakine drug 1-(1,4-benzodioxan-6-yl-carbonyl) piperidine. Those data suggest that respiratory rhythm generation within the pre-Bötzinger complex (preBötC) is presumably functional inLbx1mutant mice with additional neurochemical drive. This was supported by anatomical data showing that the gross structure of the preBötC was normal, although there were major defects in neuronal populations that provide important modulatory drive to the preBötC including the retrotrapezoid nucleus, catecholaminergic brainstem nuclei, nucleus of the solitary tract, and populations of inhibitory neurons in the ventrolateral and dorsomedial medullary nuclei. Finally, we determined that those defects were caused by abnormalities of neuronal specification early in development or subsequent neuronal migration.

Pitx2 expression is observed during all states of the myogenic progression in embryonic muscle anlagen and persists in adult muscle. Pitx2 mutant mice form all but a few muscle anlagen. Loss or degeneration in muscle anlagen could generally be attributed to the loss of a muscle attachment site induced by some other aspect of the Pitx2 phenotype. Muscles derived from the first branchial arch were absent, whereas muscles derived from the second branchial arch were merely distorted in Pitx2 mutants at midgestation. Pitx2 was expressed well before, and was required for, initiation of the myogenic progression in the first, but not second, branchial arch mesoderm. Pitx2 was also required for expression of premyoblast specification markers Tbx1, Tcf21, and Msc in the first, but not second, branchial arch. First, but not second, arch mesoderm of Pitx2 mutants failed to enlarge after embryonic day 9.5, well before the onset of the myogenic progression. Thus, Pitx2 contributes to specification of first, but not second, arch mesoderm. The jaw of Pitx2 mutants was vestigial by midgestation, but significant size reductions were observed as early as embryonic day 10.5. The diminutive first branchial arch of mutants could not be explained by loss of mesoderm alone, suggesting that Pitx2 contributes to the earliest specification of jaw itself.

A transcriptional network governs patterning in the developing spinal cord. As the developmental program runs, the levels of sequence-specific DNA-binding transcription factors (SSTFs) in each progenitor cell type change to ultimately define a set of postmitotic populations with combinatorial codes of expressed SSTFs. A network description of the neural tube (NT) transcriptional patterning process will require definition of nodes (SSTFs and target enhancers) and edges (interactions between nodes). There are 1,600 SSTF nodes in a given mammalian genome. To limit the complexity of a network description, it will be useful to discriminate between active and passive SSTF nodes. We define active SSTF nodes as those that are differentially expressed within the system. Our system, the developing NT, was partitioned into two pools of genetically defined populations by using flow sorting. Microarray comparisons across the partition led to an estimate of 500–700 active SSTF nodes in the transcriptional network of the developing NT. These included most of the 66 known SSTFs assembled from review articles and recent reports on NT patterning. Empirical cutoffs based on the performance of knowns were used to identify 188 further active SSTFs nodes that performed similarly. The general utility and limitations of the population-partitioning paradigm are discussed.

The neural crest is a migratory cell population that gives rise to multiple cell types in the vertebrate embryo. The intrinsic determinants that segregate neural crest cells from multipotential dorsal progenitors within the neural tube are poorly defined. In this study, we show that the winged helix transcription factor Foxd3 is expressed in both premigratory and migratory neural crest cells. Foxd3 is genetically downstream of Pax3 and is not expressed in regions of Pax3 mutant mice that lack neural crest, implying that Foxd3 may regulate aspects of the neural crest differentiation program. We show that misexpression of Foxd3 in the chick neural tube promotes a neural crest-like phenotype and suppresses interneuron differentiation. Cells that ectopically express Foxd3 upregulate HNK1 and Cad7, delaminate and emigrate from the neural tube at multiple dorsoventral levels. Foxd3 does not induce Slug and RhoB, nor is its ability to promote a neural crest-like phenotype enhanced by co-expression of Slug. Together these results suggest Foxd3 can function independently of Slug and RhoB to promote the development of neural crest cells from neural tube progenitors.

The POU transcription factor Oct-4 is expressed specifically in the germ line, pluripotent cells of the pregastrulation embryo and stem cell lines derived from the early embryo. Osteopontin (OPN) is a protein secreted by cells of the preimplantation embryo and contains a GRGDS motif that can bind to specific integrin subtypes and modulate cell adhesion/migration. We show thatOct-4 and OPN are coexpressed in the preimplantation mouse embryo and during differentiation of embryonal cell lines. Immunoprecipitation of the first intron of OPN (i-opn) from covalently fixed chromatin of embryonal stem cells by Oct-4-specific antibodies indicates that Oct-4 binds to this fragment in vivo. The i-opn fragment functions as an enhancer in cell lines that resemble cells of the preimplantation embryo. Furthermore, it contains a novel palindromic Oct factor recognition element (PORE) that is composed of an inverted pair of homeodomain-binding sites separated by exactly 5 bp (ATTTG +5 CAAAT). POU proteins can homo- and heterodimerize on the PORE in a configuration that has not been described previously. Strong transcriptional activation of the OPN element requires an intact PORE. In contrast, the canonical octamer overlapping with the downstream half of the PORE is not essential. Sox-2 is a transcription factor that contains an HMG box and is coexpressed with Oct-4 in the early mouse embryo. Sox-2 represses Oct-4 mediated activation of i-opn by way of a canonical Sox element that is located close to the PORE. Repression depends on a carboxy-terminal region of Sox-2 that is outside of the HMG box. Expression, DNA binding, and transactivation data are consistent with the hypothesis that OPN expression is regulated by Oct-4 and Sox-2 in preimplantation development.

Pax3 RNA is expressed in neural crest when Schwann cell (SC) precursors migrate to the PNS. Pax3 RNA and SC markers were monitored in sciatic nerves of mice during development and nerve repair. An inverse correlation was observed between expression of Pax3 RNA and myelin basic protein (MBP). Inverse correlation was also observed in SC primary cultures. Treating cultures with forskolin, an adenylate cyclase agonist, repressed Pax3 RNA, GFAP, NGFR, N-CAM, and L1 and elevated MBP. Subsequent microinjection with Pax3 expression vector elevated Pax3 RNA, GFAP, NGFR, N-CAM, and L1 and repressed MBP. Thus, Pax3 is likely involved in the differentiation pathway to myelinating SCs. Pax3 repressed a 1.3 kb MBP promoter fragment in cotransfection assays, suggesting that it represses MBP transcription.

Chmp2b is closely related to Vps2, a key component of the yeast protein complex that creates the intralumenal vesicles of multivesicular bodies. Dominant negative mutations in Chmp2b cause autophagosome accumulation and neurodegenerative disease. Loss of Chmp2b causes failure of dendritic spine maturation in cultured neurons. The homeobox gene Lbx1 plays an essential role in specifying postmitotic dorsal interneuron populations during late pattern formation in the neural tube. We have discovered that Chmp2b is one of the most highly regulated cell-autonomous targets of Lbx1 in the embryonic mouse neural tube. Chmp2b was expressed and depended on Lbx1 in only two of the five nascent, Lbx1-expressing, postmitotic, dorsal interneuron populations. It was also expressed in neural tube cell populations that lacked Lbx1 protein. The observed population-specific expression of Chmp2b indicated that only certain population-specific combinations of sequence specific transcription factors allow Chmp2b expression. The cell populations that expressed Chmp2b corresponded, in time and location, to neurons that make the first synapses of the spinal cord. Chmp2b protein was transported into neurites within the motor- and association-neuropils, where the first synapses are known to form between E11.5 and E12.5 in mouse neural tubes. Selective, developmentally-specified gene expression of Chmp2b may therefore be used to endow particular neuronal populations with the ability to mature dendritic spines. Such a mechanism could explain how mammalian embryos reproducibly establish the disynaptic cutaneous reflex only between particular cell populations.

Sequence specific transcription factors (SSTFs) combinatorially define cell types during development by forming recursively linked network kernels. Pitx2 expression begins during gastrulation, together with Hox genes, and becomes localized to the abdominal lateral plate mesoderm (LPM) before the onset of myogenesis in somites. The somatopleure of Pitx2 null embryos begins to grow abnormally outward before muscle regulatory factors (MRFs) or Pitx2 begin expression in the dermomyotome/myotome. Abdominal somites become deformed and stunted as they elongate into the mutant body wall, but maintain normal MRF expression domains. Subsequent loss of abdominal muscles is therefore not due to defects in specification, determination, or commitment of the myogenic lineage. Microarray analysis was used to identify SSTF families whose expression levels change in E10.5 interlimb body wall biopsies. All Hox9-11 paralogs had lower RNA levels in mutants, whereas genes expressed selectively in the hypaxial dermomyotome/myotome and sclerotome had higher RNA levels in mutants. In situ hybridization analyses indicate that Hox gene expression was reduced in parts of the LPM and intermediate mesoderm of mutants. Chromatin occupancy studies conducted on E10.5 interlimb body wall biopsies showed that Pitx2 protein occupied chromatin sites containing conserved bicoid core motifs in the vicinity of Hox 9-11 and MRF genes. Taken together, the data indicate that Pitx2 protein in LPM cells acts, presumably in combination with other SSTFs, to repress gene expression, that are normally expressed in physically adjoining cell types. Pitx2 thereby prevents cells in the interlimb LPM from adopting the stable network kernels that define sclerotomal, dermomyotomal, or myotomal mesenchymal cell types. This mechanism may be viewed either as lineage restriction or specification.

Thymidine kinase (TK) is representative of a class of enzymes involved in DNA precursor biosynthesis that declines as cells withdraw from the cell cycle. If TK activity is regulated exclusively by the availability of messenger RNA, changes in enzyme activity levels should not precede or excede changes in TK mRNA levels. This prediction was tested in several tissues during chicken embryogenesis and in differentiating muscle cells in culture. A sensitive method of determining absolute TK mRNA levels was developed. A synthetic complimentary RNA probe spanning an intron acceptor site in the chicken TK gene was hybridized with cellular RNA or synthetic colinear TK RNA of known concentration. After RNase digestion and gel electrophoresis, the intensity of the protected fragment was used to calculate absolute TK mRNA levels. As few as 0.02 molecules of TK mRNA per cell could be measured accurately. Depending on the tissue type, 8-day embryos contained between 3 and 12 TK mRNAs per cell. Proliferating mouse muscle cells transformed with the chicken TK gene contained between 30 and 150 TK mRNAs per cell. Both in vivo and in vitro , TK mRNA levels declined as cells withdrew from the cell cycle during differentiation. In vivo , the decline in TK activity never preceded or exceded observed changes in TK mRNA. However, in the cell culture system, TK activity consistently declined to a greater extent than TK mRNA. Thus, a translational or a post-translational mechanism must also be operative in controlling TK activity levels. Estimation of transcription rates in nuclei isolated from proliferating and differentiated muscle cell transformants indicated that the TK gene was transcriptionally repressed in postreplicative cells.

The molecular basis for replication-dependent expression of thymidine kinase (TK) activity (EC 2.7.1.21) was investigated in mouse skeletal muscle cells transformed with multiple copies of the chicken TK gene. When shifted to mitogen-depleted medium, proliferating myoblasts irreversibly withdraw from the cell cycle and commit to terminal differentiation. Early after commitment, postreplicative myocytes maintain nearly proliferative levels of TK mRNA but have greatly reduced levels of TK activity. Metabolic labeling studies with [35S]methionine indicated that the decrease in TK activity was associated with a 10-fold reduction in the rate of TK protein synthesis. Commitment had little effect on the stability or catalytic efficiency of TK protein. The decrease in TK synthetic rate in the continued presence of TK mRNA indicated that translation of TK mRNA was repressed in committed cells. The distribution of TK mRNA between ribonucleoprotein particles and polysomes was determined. In both proliferative cells and committed cells, TK mRNA levels were maximal in polysomes containing five to seven ribosomes. Thus, the synthesis of TK protein in nonreplicating muscle cells was inhibited by a translational mechanism that did not alter the average number of ribosomes engaged by TK mRNA.

Replication-dependent changes in levels of enzymes involved in DNA precursor biosynthesis are accompanied frequently by changes in levels of cognate mRNA. We tested the common assumption that changes in mRNA levels are responsible for growth-dependent expression of these enzymes using a line of mouse muscle cells that irreversibly withdraws from the cell cycle as part of its terminal differentiation program. Thymidine kinase (TK) mRNA, activity, and protein levels were quantitated in cells transformed with multiple copies of the chicken TK gene. The decline in TK mRNA (both whole cell and cytoplasmic) during myogenesis was poor (2-fold average) and variable (1.2 to 8-fold). In contrast, TK activity always was regulated efficiently (20-fold), even in cells which regulated TK mRNA very poorly. Thus, regulation of TK activity was independent of TK mRNA regulation as myoblasts withdrew from the cell cycle. A TK/beta-galactosidase fusion protein was used to derive an antibody against chicken TK. Immunoblot and immunoprecipitation analyses demonstrated TK protein levels, like TK activity levels, declined to a greater extent than TK mRNA levels. Thus, TK activity likely was regulated by a mechanism involving either decreased translation of TK mRNA or increased degradation of TK protein in committed muscle cells.

Interneurons in the ventral spinal cord are essential for coordinated locomotion in vertebrates. During embryogenesis, the V0 and V1 classes of ventral interneurons are defined by expression of the homeodomain transcription factors Evx1/2 and En1, respectively. In this study, we show that Evx1 V0 interneurons are locally projecting intersegmental commissural neurons. In Evx1 mutant embryos, the majority of V0 interneurons fail to extend commissural axons. Instead, they adopt an En1-like ipsilateral axonal projection and ectopically express En1, indicating that V0 interneurons are transfated to a V1 identity. Conversely, misexpression of Evx1 represses En1, suggesting that Evx1 may suppress the V1 interneuron differentiation program. Our findings demonstrate that Evx1 is a postmitotic determinant of V0 interneuron identity and reveal a critical postmitotic phase for neuronal determination in the developing spinal cord.

Association and relay neurons that are generated in the dorsal spinal cord play essential roles in transducing somatosensory information. During development, these two major neuronal classes are delineated by the expression of the homeodomain transcription factor Lbx1. Lbx1 is expressed in and required for the correct specification of three early dorsal interneuron populations and late-born neurons that form the substantia gelatinosa. In mice lacking Lbx1, cells types that arise in the ventral alar plate acquire more dorsal identities. This results in the loss of dorsal horn association interneurons, excess production of commissural neurons, and disrupted sensory afferent innervation of the dorsal horn. Lbx1, therefore, plays a critical role in the development of sensory pathways in the spinal cord that relay pain and touch.

TK mRNA levels were determined in mouse L cells transformed with intron deletion mutations of the chicken TK gene. Whether normalized per cell, per integrated gene, or per internal control signal, intron deletion did not diminish the efficiency of TK mRNA formation in transformed L cells. The results demonstrated that introns are not required for efficient biogenesis of cellular mRNA in transformed mouse L cells.

The totipotent stem cells of the pregastrulation mouse embryo which give rise to all embryonic somatic tissues and germ cells express Oct-4. The expression is downregulated during gastrulation and is thereafter only maintained in the germline lineage. Oct-4/lacZ transgenes were used to determine how this pattern of expression was achieved, and resulted in the identification of two separate regulatory elements. The distal element drives Oct-4 expression in preimplantation embryos, in migratory and postmigratory primordial germ cells but is inactive in cells of the epiblast. In cell lines this element is specifically active in embryonic stem and embryonic germ cells. The proximal element directs the epiblast-specific expression pattern, including downregulation during gastrulation; in cell lines its activity is restricted to epiblast-derived cells. Thus, Oct-4 expression in the germline is regulated separately from epiblast expression. This provides the first marker for the identification of totipotent cells in the embryo, and suggests that expression of Oct-4 in the totipotent cycle is dependent on a set of factors unique to the germline.

In mammalian embryos, myogenic precursor cells emigrate from the ventral lip of the dermomyotome and colonize the limbs, tongue and diaphragm where they differentiate and form skeletal muscle. Previous studies have shown that Pax3, together with the c-Met receptor tyrosine kinase and its ligand Scatter Factor (SF) are necessary for the migration of hypaxial muscle precursors in mice. Lbx1 and Pax3 are co-expressed in all migrating hypaxial muscle precursors, raising the possibility that Lbx1 regulates their migration. To examine the function of Lbx1 in muscle development, we inactivated the Lbx1 gene by homologous recombination. Mice lacking Lbx1 exhibit an extensive loss of limb muscles, although some forelimb and hindlimb muscles are still present. The pattern of muscle loss suggests that Lbx1 is not required for the specification of particular limb muscles, and the muscle defects that occur in Lbx1−/− mice can be solely attributed to changes in muscle precursor migration. c-Met is expressed in Lbx1 mutant mice and limb muscle precursors delaminate from the ventral dermomyotome but fail to migrate laterally into the limb. Muscle precursors still migrate ventrally and give rise to tongue, diaphragm and some limb muscles, demonstrating Lbx1 is necessary for the lateral, but not ventral, migration of hypaxial muscle precursors. These results suggest that Lbx1 regulates responsiveness to a lateral migration signal which emanates from the developing limb.