The origin of GnRH-1 cells and olfactory ensheathing cells (OECs) has been controversial. Genetic Cre-lox lineage tracing of the neural crest (NC) versus ectodermal contribution to the developing nasal placode was performed using two complementary mouse models, the NC specific Wnt1Cre mouse line and an ectodermal specific Crect mouse line. Using these lines we prove that the NC give rise to the olfactory ensheathing cells and subpopulations of GnRH-1 neurons, olfactory and vomeronasal cells. These data demonstrate that Schwann cells and olfactory ensheathing cells share a common developmental origin. Furthermore, the results indicate that certain conditions that impact olfaction and sexual development, such as Kallmann Syndrome, may be in part neurocristopathies.

We previously reported that the chromatin high-mobility group protein 1 (HMG-1) enhances the sequencespecific DNA binding activity of progesterone receptor (PR) in vitro, thus providing the first evidence that HMG-1 may have a coregulatory role in steroid receptor-mediated gene transcription. Here we show that HMG-1 and the highly related HMG-2 stimulate DNA binding by other steroid receptors, including estrogen, androgen, and glucocorticoid receptors, but have no effect on DNA binding by several nonsteroid nuclear receptors, including retinoid acid receptor (RAR), retinoic X receptor (RXR), and vitamin D receptor (VDR). As highly purified recombinant full-length proteins, all steroid receptors tested exhibited weak binding affinity for their optimal palindromic hormone response elements (HREs), and the addition of purified HMG-1 or -2 substantially increased their affinity for HREs. Purified RAR, RXR, and VDR also exhibited little to no detectable binding to their cognate direct repeat HREs but, in contrast to results with steroid receptors, the addition of HMG-1 or HMG-2 had no stimulatory effect. Instead, the addition of purified RXR enhanced RAR and VDR DNA binding through a heterodimerization mechanism and HMG-1 or HMG-2 had no further effect on DNA binding by RXR-RAR or RXR-VDR heterodimers. HMG-1 and HMG-2 (HMG-1/-2) themselves do not bind to progesterone response elements, but in the presence of PR they were detected as part of an HMG-PR-DNA ternary complex. HMG-1/-2 can also interact transiently in vitro with PR in the absence of DNA; however, no direct protein interaction was detected with VDR. These results, taken together with the fact that PR can bend its target DNA and that HMG-1/-2 are non-sequence-specific DNA binding proteins that recognize DNA structure, suggest that HMG-1/-2 are recruited to the PR-DNA complex by the combined effect of transient protein interaction and DNA bending. In transient-transfection assays, coexpression of HMG-1 or HMG-2 increased PR-mediated transcription in mammalian cells by as much as 7-to 10-fold without altering the basal promoter activity of target reporter genes. This increase in PR-mediated gene activation by coexpression of HMG-1/-2 was observed in different cell types and with different target promoters, suggesting a generality to the functional interaction between HMG-1/-2 and PR in vivo. Cotransfection of HMG-1 also increased reporter gene activation mediated by other steroid receptors, including glucocorticoid and androgen receptors, but it had a minimal influence on VDR-dependent transcription in vivo. These results support the conclusion that HMG-1/-2 are coregulatory proteins that increase the DNA binding and transcriptional activity of the steroid hormone class of receptors but that do not functionally interact with certain nonsteroid classes of nuclear receptors.Steroid hormone receptors are members of a superfamily of ligand-dependent transcriptional activators which direct the expression of specific gene networks involved in regulating the differen...

The canonical Wnt/β-catenin pathway is an essential component of multiple developmental processes. To investigate the role of this pathway in the ectoderm during facial morphogenesis, we generated conditional β-catenin mouse mutants using a novel ectoderm-specific Cre recombinase transgenic line. Our results demonstrate that ablating or stabilizing β-catenin in the embryonic ectoderm causes dramatic changes in facial morphology. There are accompanying alterations in the expression of Fgf8 and Shh, key molecules that establish a signaling center critical for facial patterning, the frontonasal ectodermal zone (FEZ). These data indicate that Wnt/β-catenin signaling within the ectoderm is critical for facial development and further suggest that this pathway is an important mechanism for generating the diverse facial shapes of vertebrates during evolution.

HMGB-1/-2 are coregulatory proteins that facilitate the DNA binding and transcriptional activity of steroid receptor members of the nuclear receptor family of transcription factors. We investigated the influence and mechanism of action of HMGB-1/-2 (formerly known as HMG-1/-2) on estrogen receptor ␣ (ER␣) and ER␤. Both ER subtypes were responsive to HMGB-1/-2 with respect to enhancement of receptor DNA binding affinity and transcriptional activity in cells. Responsiveness to HMGB-1/-2 was dependent on the C-terminal extension (CTE) region of the ER DNA binding domain (DBD) and correlated with a direct protein interaction between HMGB-1/-2 and the CTE. Thus the previously reported higher DNA binding affinity and transcription activity of ER␣ as compared with ER␤ is not due to a lack of ER␤ interaction with HMGB-1/-2. Using chimeric receptor DBDs, the higher intrinsic DNA binding affinity of ER␣ than ER␤ was shown to be due to a unique property of the ER␣ CTE, independent of HMGB-1/-2. The CTE of both ER subtypes was also shown to be required for interaction with ERE half-sites. These studies reveal the importance of the CTE and HMGB-1/-2 for ER␣ and ER␤ interaction with their cognate target DNAs.Nuclear hormone receptors comprise a superfamily of liganddependent transcription factors that regulate gene expression through interaction with specific hormone response elements (HREs) 1 in target genes. The superfamily can be subdivided into: 1) classical steroid hormone receptors that typically interact with palindromic hexameric HREs as homodimers, 2) nonsteroidal or class II nuclear receptors for ligands such as thyroid hormone, retinoic acid, vitamin D, and fatty acids, that function primarily as heterodimers with RXR (retinoid X receptor) bound to direct repeat HREs, and 3) orphan receptors without known ligands that interact with HREs in various dimer and monomer configurations. The nuclear receptors are related through a common domain structure including conserved C-terminal ligand binding (LBD) and centrally located DNA binding domains (DBD), and a variable N-terminal domain that is required in many nuclear receptors for maximal transcription activity (Refs. 1 and 2, reviews). The DBD consists of a highly conserved core with two asymmetric zinc fingers and an ϳ30 amino acid segment, termed the C-terminal extension (CTE) (Fig. 1A). Within the core DBD, ␣-helix 1 extends between the two zinc fingers and makes base specific contacts in the major groove of the HRE DNA. The second ␣-helix (helix 2) does not contact DNA but is important for the overall folding of the core DBD (3-5). The CTE is not conserved and adopts different structural motifs dependent on the class of nuclear receptor (6, 7). Nonetheless, the CTE of different receptors does appear to share a functional role to stabilize the receptor-DNA complex by extending the protein-DNA interface beyond that of base-specific contacts made by the core DBD. The CTE of class II receptors (TR and VDR) forms an ␣-helix (helix 3) that projects across the minor groove...

BACKGROUND The regulatory mechanisms underpinning facial development are conserved between diverse species. Therefore, results from model systems provide insight into the genetic causes of human craniofacial defects. Previously, we generated a comprehensive dataset examining gene expression during development and fusion of the mouse facial prominences. Here, we used this resource to identify genes that have dynamic expression patterns in the facial prominences, but for which only limited information exists concerning developmental function. RESULTS This set of ~80 genes was used for a high throughput functional analysis in the zebrafish system using Morpholino gene knockdown technology. This screen revealed three classes of cranial cartilage phenotypes depending upon whether knockdown of the gene affected the neurocranium, viscerocranium, or both. The targeted genes that produced consistent phenotypes encoded proteins linked to transcription (meis1, meis2a, tshz2, vgll4l), signaling (pkdcc, vlk, macc1, wu:fb16h09), and extracellular matrix function (smoc2). The majority of these phenotypes were not altered by reduction of p53 levels, demonstrating that both p53 dependent and independent mechanisms were involved in the craniofacial abnormalities. CONCLUSIONS This Morpholino-based screen highlights new genes involved in development of the zebrafish craniofacial skeleton with wider relevance to formation of the face in other species, particularly mouse and human.

Previously, we and others reported that the high mobility group proteins, HMGB-1/-2, enhance DNA binding in vitro and transactivation in situ by the steroid hormone subgroup of nuclear receptors but did not influence these functions of class II receptors. We show here that the DNA binding domain (DBD) is sufficient to account for the selective influence of HMGB-1/-2 on the steroid class of receptors. Furthermore, the use of chimeric DBDs reveals that this selectivity is dependent on the C-terminal extension (CTE), amino acid sequences adjacent to the zinc finger core DBD. HMGB-1/-2 interact directly with the DBDs of steroid but not class II receptors, and this interaction requires the CTE. This in vitro interaction correlates with a requirement of the CTE for maximal HMGB-1/-2 enhancement of DNA binding in vitro and transcriptional activation in cells. Finally, class II receptor DBDs have a much higher intrinsic affinity for DNA than steroid receptor DBDs, and this affinity difference is also dependent on the CTE. These results reveal the importance of the steroid receptor CTE for DNA binding affinity and functional response to HMGB-1/-2.Nuclear hormone receptors comprise a superfamily of transcription factors that regulates diverse metabolic processes by binding to response elements in the enhancer regions of specific genes. This superfamily consists of three receptor subclasses: 1) the steroid hormone receptors for progesterone (PR) 1 , estrogen (ER), glucocorticoids (GR), androgens (AR), and mineralocorticoids (MR); 2) class II receptors for thyroid hormone (TR), retinoids (RAR and RXR), vitamin D3 (VDR), prostaglandins (PPAR), oxysterols, and bile acids; and 3) orphan receptors for which no endogenous ligand has been identified (1-4). Each of the receptor subclasses is characterized by a unique mechanism of action with respect to dimerization and DNA sequence recognition. Steroid receptors form homodimers that optimally recognize hexameric DNA elements arranged as inverted repeats separated by three unspecified base pairs. PR, GR, AR, and MR bind to the core hexamer AGAACA, whereas ER recognizes AGGTCA (2). Class II receptors preferentially function as heterodimers with RXR and recognize the AGGTCA hexamer arranged as direct repeats. Variable spacing between the direct repeats determines the RXR heterodimer binding specificity. Class II receptors, particularly TR, can also recognize an inverted repeat as homodimers, or half-sites as monomers. Orphan receptors can bind to the AGGTCA hexamer arranged either as a direct repeat, palindrome, or half-site as heterodimers with RXR, homodimers, or monomers (1, 5-7).DNA-bound nuclear receptors activate transcription through assembly of a coactivator protein complex (8 -10). Some of these coactivators possess enzyme activities that are thought to facilitate access of general transcription factors to chromatin templates (11,12). Additionally, the coactivator complex may serve as a protein bridge to facilitate assembly of the basal transcription apparatus (13, 14). We...

Invertebrate and vertebrate vestigial (vg) and vestigial-like (vgl) genes are involved in embryonic patterning and cell fate determination. These genes encode cofactors that interact with members of the TEAD/Scalloped family of transcription factors and modulate their activity. We have previously shown that, in mice, Vgll2 is differentially expressed in the developing facial prominences. In this study, we show that the zebrafish ortholog vgll2a is expressed in the pharyngeal endoderm and ectoderm surrounding the neural crest derived mesenchyme of the pharyngeal arches. Moreover, both the FGF and retinoic acid (RA) signaling pathways, which are critical components of the hierarchy controlling craniofacial patterning, regulate this domain of vgll2a expression. Consistent with these observations, vgll2a is required within the pharyngeal endoderm for NCC survival and pharyngeal cartilage development. Specifically, knockdown of Vgll2a in zebrafish embryos using Morpholino injection results in increased cell death within the pharyngeal arches, aberrant endodermal pouch morphogenesis, and hypoplastic cranial cartilages. Overall, our data reveal a novel non-cell autonomous role for Vgll2a in development of the NCC-derived vertebrate craniofacial skeleton.

The bones of the cranial vault are formed directly from mesenchymal cells through intramembranous ossification rather than via a cartilage intermediate. Formation and growth of the skull bones involves the interaction of multiple cell-cell signaling pathways, with fibroblast growth factors (FGFs) and their receptors exerting a prominent influence. Mutations within the FGF signaling pathway are the most frequent cause of craniosynostosis, which is a common human craniofacial developmental abnormality characterized by the premature fusion of the cranial sutures. Here, we have developed new mouse models to investigate how different levels of increased FGF signaling can affect the formation of the calvarial bones and associated sutures. Whereas moderate Fgf8 overexpression resulted in delayed ossification followed by craniosynostosis of the coronal suture, higher Fgf8 levels promoted a loss of ossification and favored cartilage over bone formation across the skull. By contrast, endochondral bones were still able to form and ossify in the presence of increased levels of Fgf8, although the growth and mineralization of these bones were affected to varying extents. Expression analysis demonstrated that abnormal skull chondrogenesis was accompanied by changes in the genes required for Wnt signaling. Moreover, further analysis indicated that the pathology was associated with decreased Wnt signaling, as the reduction in ossification could be partially rescued by halving Axin2 gene dosage. Taken together, these findings indicate that mesenchymal cells of the skull are not fated to form bone, but can be forced into a chondrogenic fate through the manipulation of FGF8 signaling. These results have implications for evolution of the different methods of ossification as well as for therapeutic intervention in craniosynostosis.