Transcriptional Regulation of Jaw Osteoblasts: Development to Pathology.

Craniofacial and jaw bones have unique physiological specificities when compared to axial and appendicular bones. However, the molecular profile of the jaw osteoblast (OB) remains incomplete. The present study aimed to decipher the bone site-specific profiles of transcription factors (TFs) expressed in OBs in vivo. Using RNA sequencing analysis, we mapped the transcriptome of confirmed OBs from 2 different skeletal sites: mandible (Md) and tibia (Tb). The OB transcriptome contains 709 TF genes: 608 are similarly expressed in Md-OB and Tb-OB, referred to as "OB-core"; 54 TF genes are upregulated in Md-OB, referred to as "Md-set"; and 18 TF genes are upregulated in Tb-OB, referred to as "Tb-set." Notably, the expression of 29 additional TF genes depends on their RNA transcript variants. TF genes with no previously known role in OBs and bone were identified. Bioinformatics analysis combined with review of genetic disease databases and a comprehensive literature search showed a significant contribution of anatomical origin to the OB signatures. Md-set and Tb-set are enriched with site-specific TF genes associated with development and morphogenesis (neural crest vs. mesoderm), and this developmental imprint persists during growth and homeostasis. Jaw and tibia site-specific OB signatures are associated with craniofacial and appendicular skeletal disorders as well as neurocristopathies, dental disorders, and digit malformations. The present study demonstrates the feasibility of a new method to isolate pure OB populations and map their gene expression signature in the context of OB physiological environment, avoiding in vitro culture and its associated biases. Our results provide insights into the site-specific developmental pathways governing OBs and identify new major OB regulators of bone physiology. We also established the importance of the OB transcriptome as a prognostic tool for human rare bone diseases to explore the hidden pathophysiology of craniofacial malformations, among the most prevalent congenital defects in humans.

The homeoprotein DLX3 and tumor suppressor p53 co-regulate cell cycle progression and squamous tumor growth.

Epidermal homeostasis depends on the coordinated control of keratinocyte cell cycle. Differentiation and the alteration of this balance can result in neoplastic development. Here we report on a novel DLX3-dependent network that constrains epidermal hyperplasia and squamous tumorigenesis. By integrating genetic and transcriptomic approaches, we demonstrate that DLX3 operates through a p53-regulated network. DLX3 and p53 physically interact on the p21 promoter to enhance p21 expression. Elevating DLX3 in keratinocytes produces a G1-S blockade associated with p53 signature transcriptional profiles. In contrast, DLX3 loss promotes a mitogenic phenotype associated with constitutive activation of ERK. DLX3 expression is lost in human skin cancers and is extinguished during progression of experimentally induced mouse squamous cell carcinoma (SCC). Reinstatement of DLX3 function is sufficient to attenuate the migration of SCC cells, leading to decreased wound closure. Our data establish the DLX3-p53 interplay as a major regulatory axis in epidermal differentiation and suggest that DLX3 is a modulator of skin carcinogenesis.

DLX3 regulates bone mass by targeting genes supporting osteoblast differentiation and mineral homeostasis in vivo.

Human mutations and in vitro studies indicate that DLX3 has a crucial function in bone development, however, the in vivo role of DLX3 in endochondral ossification has not been established. Here, we identify DLX3 as a central attenuator of adult bone mass in the appendicular skeleton. Dynamic bone formation, histologic and micro-computed tomography analyses demonstrate that in vivo DLX3 conditional loss of function in mesenchymal cells (Prx1-Cre) and osteoblasts (OCN-Cre) results in increased bone mass accrual observed as early as 2 weeks that remains elevated throughout the lifespan owing to increased osteoblast activity and increased expression of bone matrix genes. Dlx3OCN-conditional knockout mice have more trabeculae that extend deeper in the medullary cavity and thicker cortical bone with an increased mineral apposition rate, decreased bone mineral density and increased cortical porosity. Trabecular TRAP staining and site-specific Q-PCR demonstrated that osteoclastic resorption remained normal on trabecular bone, whereas cortical bone exhibited altered osteoclast patterning on the periosteal surface associated with high Opg/Rankl ratios. Using RNA sequencing and chromatin immunoprecipitation-Seq analyses, we demonstrate that DLX3 regulates transcription factors crucial for bone formation such as Dlx5, Dlx6, Runx2 and Sp7 as well as genes important to mineral deposition (Ibsp, Enpp1, Mepe) and bone turnover (Opg). Furthermore, with the removal of DLX3, we observe increased occupancy of DLX5, as well as increased and earlier occupancy of RUNX2 on the bone-specific osteocalcin promoter. Together, these findings provide novel insight into mechanisms by which DLX3 attenuates bone mass accrual to support bone homeostasis by osteogenic gene pathway regulation.

Developmental expression of the homeobox protein Distal-less 3 and its relationship to progesterone production in mouse placenta.

Distal-less 3 (Dlx3) is a homeobox factor that functions as a placental-specific transcriptional regulator. Dlx3 null mice (-/-) have compromised placental development and do not survive in utero past embryonic day (E) 9.5. The current studies were undertaken to examine the expression of Dlx3 in mouse placenta during gestation, and to determine whether Dlx3 was involved in placental progesterone production. Dlx3 was not detectable at E8.5 but was detected in E9.5 placenta with continuing but diminished expression through E15.5. Dlx3 immuno-localization was restricted to the labyrinth, was nuclear and was found in cytokeratin-positive cells. Previous studies in choriocarcinoma cell lines support the conclusion that Dlx3 is required for expression of 3'-hydroxysteroid dehydrogenase VI (3betaHSD VI), an obligate enzyme in the production of progesterone by trophoblast giant cells. In a rat trophoblast stem cell line (Rcho-1), Dlx3 expression was non-detectable in Rcho-1 cells induced to differ-entiate using mitogen withdrawal. In vitro progesterone production in placental cultures and 3betaHSD VI mRNA from Dlx3 (+/+), (+/-) and (-/-) mice were equivalent. In situ hybridization for 3betaHSD VI revealed mRNA expression restricted to trophoblast giants cells with no detectable expression in the labyrinth suggesting that Dlx3 and 3betaHSD VI were not colocalized within the placenta. These studies support the conclusion that Dlx3 protein expression is restricted to the labyrinth region of the murine placenta into late gestation and that Dlx3 does not appear to be expressed in trophoblast giant cells. Further, loss of Dlx3 was not correlated with synthesis of progesterone from E9.5 mouse placentas.

The temporal and spatial expression of the novel Ca++-binding proteins, Scarf and Scarf2, during development and epidermal differentiation.

During the process of epidermal differentiation, intracellular and extracellular calcium (Ca++) concentrations induce an array of signaling pathways . Keratinocytes follow a complex Ca++-dependent program of differentiation moving from the basal proliferative layer, through the spinous and granular differentiated layers to ultimately culminate in the formation of the cornified layer of the epidermis. Members of the Ca++-binding proteins play a central role in the transduction of Ca++ signals. Utilizing a suppressive subtractive hybridization screen comparing basal and differentiated keratinocytes, we identified the novel Ca++-binding protein genes, Scarf (skin Calmodulin-related factor) and Scarf2, which have homology to calmodulin (CaM). In this study, we present a comprehensive analysis of the expression pattern for Scarf and Scarf2 transcripts and proteins in the developing mouse. To examine Scarf2 expression during embryogenesis, we performed in situ hybridization, and detected expression in the hair follicle, skin and nasal epithelium. These results showed substantial overlap with the previously reported Scarf gene expression [Hwang, M., Morasso, M.I., 2003. The novel murine Ca2+-binding protein, Scarf, is differentially expressed during epidermal differentiation. J. Biol. Chem. 278, 47827-47833]. Comparing the expression patterns of Scarf and Scarf2 proteins in neonatal and adult mouse skin with several structural epidermal proteins, i.e. keratin 14 (K14), keratin 1 (K1), loricrin (LOR) and filaggrin (FIL) showed that their expression overlaps K1, an early marker of keratinocyte differentiation. Interestingly, Scarf and Scarf2 were also detected in the tongue and oral epithelia, rib bone undergoing ossification and in the medullar region of thymus.

Phosphorylation of murine homeodomain protein Dlx3 by protein kinase C.

The Dlx3 homeodomain gene is expressed in terminally differentiated murine epidermal cells. As demonstrated for differentiation-specific granular markers, Dlx3 is activated in primary mouse keratinocytes cultured in vitro by increasing the level of the extracellular Ca(2+). This activation is mediated through a protein kinase C-dependent (PKC) pathway. In this study, we investigated whether PKC can modulate the activity of murine Dlx3 protein. Using in vitro kinase assays, we show that PKC enzymes phosphorylate the Dlx3 protein. Using keratinocyte nuclear extracts for the kinase reaction, we determined that Dlx3 protein is phosphorylated, and the phosphorylation is inhibited by the PKC-specific inhibitor GF109203X, suggesting that Dlx3 is phosphorylated by PKC in vivo. Of the PKC isoforms present in the epidermis, we tested alpha, delta, epsilon and zeta. Dlx3 is primarily phosphorylated by PKC alpha. By deletion and mutational analysis, we show that the serine residue S(138), located in the homeodomain of Dlx3 protein, was specifically phosphorylated by PKC. The phosphorylation of purified Dlx3 proteins by PKC partially inhibited formation of complexes between Dlx3 protein and DNA. These results suggest that Dlx3 protein can be directly phosphorylated by PKC and this affects the DNA binding activity of Dlx3.

A role for the homeobox protein Distal-less 3 in the activation of the glycoprotein hormone alpha subunit gene in choriocarcinoma cells.

Synthesis and secretion of chorionic gonadotropin in trophoblast cells of the placenta is required for establishment of early pregnancy in primates. Chorionic gonadotropin is a heterodimeric glycoprotein hormone consisting of alpha and beta subunits. Regulation of the alpha subunit gene within the placenta requires an array of cis elements within the 5'-flanking region of the promoter. Within this array of elements, the junctional regulatory element (JRE) putatively binds a placental-specific transcription factor. The aim of our studies was to determine the identity and role of the transcriptional regulator that binds to the JRE in choriocarcinoma cells (JEG3 cells). Mutations within the JRE resulted in reduction in basal expression of an alpha subunit reporter gene, suggesting that the JRE binding factor was necessary for full basal activity. Using electrophoretic mobility shift assays, we determined that the JRE was capable of serving as a homeobox factor-binding site. The homeobox factor, Distal-less 3 (Dlx 3) was found to be expressed in JEG3 cells and in the trophoblast layer of human chorionic villus but not in a gonadotrope cell line that also expresses the alpha subunit gene. Electrophoretic mobility shift assays revealed that recombinant Dlx 3 could bind specifically to the JRE and endogenous Dlx 3 was present in JRE/JEG3 nuclear protein complexes. Overexpression of Dlx 3 resulted in activation of an alpha subunit reporter gene. A JRE mutation resulted in attenuated activation of the alpha subunit reporter via an adjacent cis element, suggesting that JRE/Dlx 3 interactions may facilitate regulation of the alpha subunit gene at sites immediately upstream of the JRE. Our studies support the conclusion that Dlx 3 is a placental-specific transcriptional regulator that binds to the JRE and contributes to expression of the alpha subunit gene in cells of trophoblast origin.

Residues in the 1A rod domain segment and the linker L2 are required for stabilizing the A11 molecular alignment mode in keratin intermediate filaments.

Both analyses of x-ray diffraction patterns of well oriented specimens of trichocyte keratin intermediate filaments (IF) and in vitro cross-linking experiments on several types of IF have documented that there are three modes of alignment of pairs of antiparallel molecules in all IF: A11, A22 and A12, based on which parts of the major rod domain segments are overlapped. Here we have examined which residues may be important for stabilizing the A11 mode. Using the K5/K14 system, we have made point mutations of charged residues along the chains and examined the propensities of equimolar mixtures of wild type and mutant chains to reassemble using as criteria: the formation (or not) of IF in vitro or in vivo; and stabilities of one- and two-molecule assemblies. We identified that the conserved residue Arg10 of the 1A rod domain, and the conserved residues Glu4 and Glu6 of the linker L2, were essential for stability. Additionally, conserved residues Lys31 of 1A and Asp1 of 2A and non-conserved residues Asp/Asn9 of 1A, Asp/Asn3 of 2A, and Asp7 of L2 are important for stability. Notably, these groups of residues lie close to each other when two antiparallel molecules are aligned in the A11 mode, and are located toward the ends of the overlap region. Although other sets of residues might theoretically also contribute, we conclude that these residues in particular engage in favorable intermolecular ionic and/or H-bonding interactions and thereby may play a role in stabilizing the A11 mode of alignment in keratin IF.

The Dlx3 protein harbors basic residues required for nuclear localization, transcriptional activity and binding to Msx1.

The murine Dlx3 protein is a putative transcriptional activator that has been implicated during development and differentiation of epithelial tissue. Dlx3 contains a homeodomain and mutational analysis has revealed two regions, one N-terminal and one C-terminal to the homeodomain, that act as transcriptional activators in a yeast one-hybrid assay. In addition to transactivation, data are presented to demonstrate specific DNA binding and an association between Dlx3 and the Msx1 protein in vitro. Immunohistochemical analysis confirmed coexpression of Dlx3 and Msx1 proteins in the differentiated layers of murine epidermal tissues. Transcription factor function requires nuclear localization. In this study, the intracellular localization of the green fluorescent protein fused to Dlx3 was examined in keratinocytes induced to differentiate by calcium and is shown to localize to the nucleus. A bipartite nuclear localization signal (NLS) was identified by mutational analysis and shown to be sufficient for nuclear localization. This was demonstrated by insertion of the Dlx3 bipartite NLS sequence into a cytoplasmic fusion protein, GFP-keratin 14, which functionally redirected GFP-keratin 14 expression to the nucleus. Further analysis of Dlx3 NLS mutants revealed that the Dlx3 NLS sequences are required for specific DNA binding, transactivation potential and interactions with the Msx1 protein.

Coiled-coil trigger motifs in the 1B and 2B rod domain segments are required for the stability of keratin intermediate filaments.

Many alpha-helical proteins that form two-chain coiled coils possess a 13-residue trigger motif that seems to be required for the stability of the coiled coil. However, as currently defined, the motif is absent from intermediate filament (IF) protein chains, which nevertheless form segmented two-chain coiled coils. In the present work, we have searched for and identified two regions in IF chains that are essential for the stability necessary for the formation of coiled-coil molecules and thus may function as trigger motifs. We made a series of point substitutions with the keratin 5/keratin 14 IF system. Combinations of the wild-type and mutant chains were assembled in vitro and in vivo, and the stabilities of two-chain (one-molecule) and two-molecule assemblies were examined with use of a urea disassembly assay. Our new data document that there is a region located between residues 100 and 113 of the 2B rod domain segment that is absolutely required for molecular stability and IF assembly. This potential trigger motif differs slightly from the consensus in having an Asp residue at position 4 (instead of a Glu) and a Thr residue at position 9 (instead of a charged residue), but there is an absolute requirement for a Glu residue at position 6. Because these 13 residues are highly conserved, it seems possible that this motif functions in all IF chains. Likewise, by testing keratin IF with substitutions in both chains, we identified a second potential trigger motif between residues 79 and 91 of the 1B rod domain segment, which may also be conserved in all IF chains. However, we were unable to find a trigger motif in the 1A rod domain segment. In addition, many other point substitutions had little detectable effect on IF assembly, except for the conserved Lys-23 residue of the 2B rod domain segment. Cross-linking and modeling studies revealed that Lys-23 may lie very close to Glu-106 when two molecules are aligned in the A(22) mode. Thus, the Glu-106 residue may have a dual role in IF structure: it may participate in trigger formation to afford special stability to the two-chain coiled-coil molecule, and it may participate in stabilization of the two-molecule hierarchical stage of IF structure.

Complex interactions between epidermal POU domain and activator protein 1 transcription factors regulate the expression of the profilaggrin gene in normal human epidermal keratinocytes.

The human profilaggrin gene is expressed in the granular layer during the late stages of the epidermal differentiation. The proximal promoter region of the gene confers high levels of keratinocyte-specific transcription via interactions with c-Jun/c-Fos heterodimers. Here we provide evidence for another level of complexity in the regulation of the profilaggrin promoter activity. The POU domain proteins Oct1, Skn1a/i, and Oct6, which are abundantly expressed in the epidermal cells, act to both stimulate and repress transcription in a general and a cell type-specific mode. While binding to specific recognition elements within the promoter region, they exert their effects by either stimulating or antagonizing the c-Jun-dependent activity of the promoter. The response of the promoter to forced expression of the POU domain proteins reflects the effect of these transcription factors on the endogenous profilaggrin mRNA synthesis and suggests that the latter requires a fine balance in the amounts and the activities of the individual activator protein 1 and POU domain proteins.

Regulation of the Dlx3 homeobox gene upon differentiation of mouse keratinocytes.

The Distal-less Dlx3 homeodomain gene is expressed in terminally differentiated murine epidermal cells, and there is evidence to support an essential role as a transcriptional regulator of the terminal differentiation process in these cells. In an attempt to determine the factors that induce Dlx3 gene expression, we have cloned the 1.2-kilobase pair proximal region of murine gene and analyzed its cis-regulatory elements and potential trans-acting factors. The proximal region of the Dlx3 gene has a canonical TATA box and CCAAT box, and the transcription start site was located 205 base pairs upstream from the initiation of translation site. Serial deletion analysis showed that the region between -84 and -34 confers the maximal promoter activity both in undifferentiated and differentiated primary mouse keratinocytes. Gel retardation assays and mutational analysis demonstrated that the transcriptional regulator NF-Y (also referred to as CBF) binds to a CCAAT box motif within this region and is responsible for the majority of the Dlx3 promoter activity. In addition, an Sp1-binding site was located immediately upstream of transcription start site that acts as a positive regulatory element of the Dlx3 promoter, independent of the CCAAT box motif. Importantly, elements residing between +30 to +60 of the Dlx3 gene are responsible for the Ca(2+)-dependent induction of Dlx3 during keratinocyte differentiation.

Transcriptional activation by the homeodomain protein distal-less 3.

PCR-based methods and mobility shift competition assays were used to determine the basic biochemical features of the homeodomain transcription factor Distal-less 3 (Dlx3), including an optimal DNA binding site, the binding constant and dissociation rates of this protein. Expression of Dlx3 protein in either HeLa cells or Xenopus embryos resulted in strong activation of a model target gene construct containing three tandem copies of the Dlx3 binding site upstream from the TATA element. In addition, deletion analysis revealed that transcriptional activation by Dlx3 depends on two subdomains located on either side of the homeobox: removal of either subdomain resulted in complete loss of Dlx3 function. These observations provide new insight regarding the function of Dlx3 in vertebrate development and tissue differentiation and also suggest a mechanism for the dominant inheritance pattern of a hereditary disease resulting from mutation of the DLX3 gene in human.

Placental failure in mice lacking the homeobox gene Dlx3.

Dlx3 is a homeodomain transcription factor and a member of the vertebrate Distal-less family. Targeted deletion of the mouse Dlx3 gene results in embryonic death between day 9.5 and day 10 because of placental defects that alter the development of the labyrinthine layer. In situ hybridization reveals that the Dlx3 gene is initially expressed in ectoplacental cone cells and chorionic plate, and later in the labyrinthine trophoblast of the chorioallantoic placenta, where major defects are observed in the Dlx3 -/- embryos. The expression of structural genes, such as 4311 and PL-1, which were used as markers to follow the fate of different derivatives of the placenta, was not affected in the Dlx3-null embryos. However, by day 10.5 of development, expression of the paired-like homeodomain gene Esx1 was strongly down-regulated in affected placenta tissue, suggesting that Dlx3 is required for the maintenance of Esx1 expression, normal placental morphogenesis, and embryonic survival.

Regulation of epidermal differentiation by a Distal-less homeodomain gene.

The Distal-less-related homeodomain gene Dlx3 is expressed in terminally differentiated murine epidermal cells. Ectopic expression of this gene in the basal cell layer of transgenic skin results in a severely abnormal epidermal phenotype and leads to perinatal lethality. The basal cells of affected mice ceased to proliferate, and expressed the profilaggrin and loricrin genes which are normally transcribed only in the latest stages of epidermal differentiation. All suprabasal cell types were diminished and the stratum corneum was reduced to a single layer. These data indicate that Dlx3 misexpression results in transformation of basal cells into more differentiated keratinocytes, suggesting that this homeoprotein is an important regulator of epidermal differentiation.

A Xenopus distal-less gene in transgenic mice: conserved regulation in distal limb epidermis and other sites of epithelial-mesenchymal interaction.

In this paper, we show the conserved regulation of the homeodomain gene Distal-less-3 (Dlx-3) by analyzing the expression of a promoter from the Xenopus ortholog, Xdll-2, in transgenic mice. A 470-bp frog regulatory sequence confers appropriate expression on a lacZ reporter gene in the ectodermal component of structures derived from epithelial-mesenchymal interactions. Remarkably, this includes structures absent in Xenopus, such as the hair follicle and mammary gland, suggesting that conserved regulatory elements can be used to control the formation of structures peculiar to individual species. In addition, expression of Dlx-3 in developing limbs is highest at the most distal portion. This pattern is duplicated by the Xenopus promoter, indicating that this DNA may include sequences responsive to conserved proximodistal patterning signals in the vertebrate limb.

Differential expression of a Distal-less homeobox gene Xdll-2 in ectodermal cell lineages.

Neural induction in Xenopus requires the activation of new sets of genes that are necessary for cellular and regional specification of the neural tube. It has been reported earlier that members of the Distal-less homeobox gene family are specifically activated in distinct regions of the central nervous system (CNS) of Xenopus embryos (Dirksen et al., 1993; Papalopulu and Kintner, 1993). In this paper we describe in detail a Xenopus homeobox containing gene Xdll-2, which belongs to the Distal-less gene family. In contrast to other previously described Xenopus family members, Xdll-2 is expressed in the embryonic ectoderm and is specifically repressed in the CNS. This repression can be mimicked in isolated animal caps by treatment with activin. Expression of Xdll-2 persists in the epidermis and some neural crest cells. Because of its spatial and temporal expression pattern this gene is a good candidate to have a regulatory function in the initial formation of the epidermis. Its high level of expression in adult skin indicates that its function is continuously required in this tissue.

The homeodomain gene Xenopus Distal-less-like-2 (Xdll-2) is regulated by a conserved mechanism in amphibian and mammalian epidermis.

Xenopus Distal-less-like-2 (Xdll-2) is a gene encoding a homeodomain protein expressed predominantly in the epidermis of frog embryos. We report here that this epidermal expression is specified by a regulatory 5' flanking DNA region located within 933 bp of the start of transcription. This regulatory DNA also confers upon a globin reporter gene calcium-inducible expression in cultured murine keratinocytes and induction-dependent repression in frog ectodermal cells treated in vitro with activin A. These results reveal a new example of phylogenetically conserved, tissue-specific transcriptional regulation of a homeodomain gene.

The MyoD family of myogenic factors is regulated by electrical activity: isolation and characterization of a mouse Myf-5 cDNA.

A full-length cDNA coding for a homolog of the human Myf-5 was isolated from a BC3H-1 mouse library and characterized. The clone codes for a protein of 255 amino acids that is 89%, 88% and 68% identical to the human, bovine and Xenopus myf-5, respectively. The mouse Myf-5 cDNA (mmyf-5), as well as sequences coding for MyoD, myogenin and Mrf-4, were used to probe Northern blots to analyze the effects of innervation on the expression of the MyoD family of myogenic factors. Mouse myf-5, MyoD and myogenin mRNAs levels were found to decline in hind limb muscles of mice between embryonic day 15 (E15) and the first postnatal week, a period that coincides with innervation. In contrast, Mrf-4 transcripts increase during this period and reach steady-state levels by 1-week after birth. To distinguish if the changes in myogenic factor expression are due to a developmental program or to innervation, mRNA levels were analyzed at different times after muscle denervation. Mmyf-5 transcripts begin to accumulate 2 days postdenervation; after 1 week levels are 7-fold higher than in innervated muscle. Mrf-4, MyoD and myogenin transcripts begin to accumulate as soon as 8h after denervation, and attain levels that are 8-, 15- and 40-fold higher than found in innervated skeletal muscle, respectively. The accumulation of these three mRNAs precedes the increase of nicotinic acetylcholine receptor alpha subunit transcripts, a gene that is transcriptionally regulated by MyoD-related factors in vitro. Using extracellular electrodes to directly stimulate in situ the soleus muscle of rats, we found that 'electrical activity' per se, in absence of the nerve, represses the increases of myogenic factor mRNAs associated with denervation.