Muscle differentiation is antagonized by SOX15, a new member of the SOX protein family

Evolutionary Landscape of SOX Genes to Inform Genotype-to-Phenotype Relationships

The SOX transcription factor family is pivotal in controlling aspects of development. To identify genotype-phenotype relationships of SOX proteins, we performed a non-biased study of SOX using 1890 open-reading frame and 6667 amino acid sequences in combination with structural dynamics to interpret 3999 gnomAD, 485 ClinVar, 1174 Geno2MP, and 4313 COSMIC human variants. We identified, within the HMG (High Mobility Group)- box, twenty-seven amino acids with changes in multiple SOX proteins annotated to clinical pathologies. These sites were screened through Geno2MP medical phenotypes, revealing novel SOX15 R104G associated with musculature abnormality and SOX8 R159G with intellectual disability. Within gnomAD, SOX18 E137K (rs201931544), found within the HMG box of ~0.8% of Latinx individuals, is associated with seizures and neurological complications, potentially through blood-brain barrier alterations. A total of 56 highly conserved variants were found at sites outside the HMG-box, including several within the SOX2 HMG-box-flanking region with neurological associations, several in the SOX9 dimerization region associated with Campomelic Dysplasia, SOX14 K88R (rs199932938) flanking the HMG box associated with cardiovascular complications within European populations, and SOX7 A379V (rs143587868) within an SOXF conserved far C-terminal domain heterozygous in 0.716% of African individuals with associated eye phenotypes. This SOX data compilation builds a robust genotype-to-phenotype association for a gene family through more robust ortholog data integration.

Single-cell Transcriptomic Analysis Reveals the Cellular Heterogeneity of Mesenchymal Stem Cells

Ex vivo-expanded mesenchymal stem cells (MSCs) have been demonstrated to be a heterogeneous mixture of cells exhibiting varying proliferative, multipotential, and immunomodulatory capacities. However, the exact characteristics of MSCs remain largely unknown. By single-cell RNA sequencing of 61,296 MSCs derived from bone marrow and Wharton’s jelly, we revealed five distinct subpopulations. The developmental trajectory of these five MSC subpopulations was mapped, revealing a differentiation path from stem-like active proliferative cells (APCs) to multipotent progenitor cells, followed by branching into two paths: 1) unipotent preadipocytes or 2) bipotent prechondro-osteoblasts that were subsequently differentiated into unipotent prechondrocytes. The stem-like APCs, expressing the perivascular mesodermal progenitor markers CSPG4/MCAM/NES, uniquely exhibited strong proliferation and stemness signatures. Remarkably, the prechondrocyte subpopulation specifically expressed immunomodulatory genes and was able to suppress activated CD3⁺ T cell proliferation in vitro, supporting the role of this population in immunoregulation. In summary, our analysis mapped the heterogeneous subpopulations of MSCs and identified two subpopulations with potential functions in self-renewal and immunoregulation. Our findings advance the definition of MSCs by identifying the specific functions of their heterogeneous cellular composition, allowing for more specific and effective MSC application through the purification of their functional subpopulations.

Infantile Rhabdomyosarcomas With VGLL2 Rearrangement Are Not Always an Indolent Disease: A Study of 4 Aggressive Cases With Clinical, Pathologic, Molecular, and Radiologic Findings

VGLL2-rearranged rhabdomyosarcomas (RMS) are rare low-grade tumors with only favorable outcomes reported to date. We describe 4 patients with VGLL2-rearranged RMS confirmed by molecular studies, who experienced local progression and distant metastases, including 2 with fatal outcomes. Tumors were diagnosed at birth (n=3) or at 12 months of age (n=1), and were all localized at initial diagnosis, but unresectable and therefore managed with chemotherapy and surveillance. Metastatic progression occurred from 1 to 8 years from diagnosis (median, 3.5 y). Three patients experienced multimetastatic spread and one showed an isolated adrenal metastasis. At initial diagnosis, 3 tumors displaying bland morphology were misdiagnosed as fibromatosis or infantile fibrosarcoma and initially managed as such, while 1 was a high-grade sarcoma. At relapse, 3 tumors showed high-grade morphology, while 1 retained a low-grade phenotype. Low-grade primary tumors showed only very focal positivity for desmin, myogenin, and/or MyoD1, while high-grade tumors were heterogenously or diffusely positive. Whole-exome sequencing, performed on primary and relapse samples for 3 patients, showed increased genomic instability and additional genomic alterations (eg, TP53, CDKN2A/B, FGFR4) at relapse, but no recurrent events. RNA sequencing confirmed that high-grade tumors retained VGLL2 fusion transcripts and transcriptomic profiles consistent with VGLL2-rearranged RMS. High-grade samples showed a high expression of genes encoding cell cycle proteins, desmin, and some developmental factors. These 4 cases with distinct medical history imply the importance of complete surgical resection, and suggest that RMS-type chemotherapy should be considered in unresectable cases, given the risk of high-grade transformation. They also emphasize the importance of correct initial diagnosis.

Inhibition of SOX15 Sensitizes Esophageal Squamous Carcinoma Cells to Paclitaxel

BACKGROUND

SOX15 is a crucial transcription factor involved in the regulation of embryonic development and in the cell fate determination. It is also an important mediator of tumorigenesis in cancer.

METHODS

Here, we sought to explore the expression patterns and biological functions of SOX15 in esophageal squamous cell carcinomas (ESCC). SOX15 was found aberrantly overexpressed in ESCC tumors.

RESULTS

Experimentally, inhibition of SOX15 through RNAi suppressed cell proliferation in ESCC cells and sensitized cancer cells to paclitaxel, but not to Cisplatin. Moreover, inhibition of SOX15 significantly repressed the expression of genes associated with WNT and NOTCH signaling pathways, which may contribute to the increased sensitivity to paclitaxel.

CONCLUSION

In conclusion, the current study revealed that inhibition of SOX15 in ESCC cells sensitizes the ESCC cells to paclitaxel, suggesting that the SOX15 expression level may predict the therapeutic outcomes for paclitaxel treatment for ESCC.

SOX15 exerts antitumor function in glioma by inhibiting cell proliferation and invasion via downregulation of Wnt/β-catenin signaling

AIMS

Sex-determining region of Y chromosome-related high-mobility-group box 15 (SOX15) has recently emerged as a candidate tumor-inhibitor in multiple types of human tumors. To date, the involvement of SOX15 in glioma is undetermined. The purpose of this study was to investigate the expression, function and potential molecular mechanism of SOX15 in glioma.

MAIN METHODS

Relative mRNA expression was analyzed by real-time quantitative PCR. Protein expression was determined by Western blot. Cell proliferation was assessed by cell counting kit-8 and colony formation assay. Cell invasion was evaluated by Matrigel invasion assay. Wnt/β-catenin activation was monitored by luciferase reporter assay.

KEY FINDINGS

SOX15 expression was decreased in glioma tissues and cell lines compared with normal controls. Kaplan-Meier analysis revealed that patients with low expression of SOX15 had shorter survival than those who had high expression of SOX15. The upregulation of SOX15 markedly repressed the proliferation and invasion of glioma cells, whereas its depletion enhanced glioma cell proliferation and invasion. Research into the mechanism revealed that SOX15 exerted an inhibitory effect on Wnt/β-catenin signaling in glioma cells. Notably, overexpression of β-catenin partially reversed the SOX15 overexpression-mediated tumor-suppressive effect. In addition, SOX15 overexpression significantly impeded tumor formation by glioma cells in vivo in a mouse xenograft model associated with downregulation of active β-catenin expression.

SIGNIFICANCE

These data demonstrate that SOX15 functions as a potential tumor-suppressor in glioma by inhibiting cell proliferation and invasion via the downregulation of Wnt/β-catenin signaling.

Downregulation of microR-147b represses the proliferation and invasion of thyroid carcinoma cells by inhibiting Wnt/β-catenin signaling via targeting SOX15

microRNA-147b (miR-147b) is a newly identified tumor-related miRNA that is dysregulated in multiple cancer types. Yet, the role of miR-147b in thyroid carcinoma remains unknown. Herein, we found that miR-147b expression was upregulated in thyroid carcinoma tissues and cell lines. miR-147b inhibition decreased the proliferation, colony formation, and invasion of thyroid carcinoma cells. The tumor suppressive gene SRY-related high-mobility-group box gene 15 (SOX15) was predicted as a miR-147b target gene. SOX15 expression was markedly decreased in thyroid carcinoma tissues and inversely correlated with the miR-147b expression. SOX15 overexpression repressed the proliferation and invasion of thyroid carcinoma cells associated with downregulation of Wnt/β-catenin signaling. SOX15 knockdown abolished the miR-147b-inhibition-mediated antitumor effect. miR-147b inhibition or SOX15 overexpression retarded the tumor growth of thyroid carcinoma cells in vivo. Overall, our study suggests that miR-147b inhibition restrains the proliferation and invasion of thyroid carcinoma cells through upregulation of SOX15 and inhibition of Wnt/β-catenin signaling.

Testing the role of SOX15 in human primordial germ cell fate

Background: Potentially novel regulators of early human germline development have been identified recently, including SOX15 and SOX17, both of which show specific expression in human primordial germ cells. SOX17 is now known to be a critical specifier of human germ cell identity. There have been suggestions, as yet without evidence, that SOX15 might also play a prominent role. The early human germline is inaccessible for direct study, but an in vitro model of human primordial germ cell-like cell (hPGCLC) specification from human embryonic stem cells (hESCs) has been developed. This enables mechanistic study of human germ cell specification using genetic tools to manipulate the levels of SOX15 and SOX17 proteins to explore their roles in hPGCLC specification. Methods: SOX15 and SOX17 proteins were depleted during hPGCLC specification from hESCs using the auxin-inducible degron system, combined with a fluorescent reporter for tracking protein levels. Additionally, SOX15 protein was overexpressed using the ProteoTuner system. Protein-level expression changes were confirmed by immunofluorescence. The impact on hPGCLC specification efficiency was determined by flow cytometry at various time points. qPCR experiments were performed to determine some transcriptional effects of SOX15 perturbations. Results: We observed specific SOX15 expression in hPGCLCs by using immunofluorescence and flow cytometry analysis. Depletion of SOX15 had no significant effect on hPGCLC specification efficiency on day 4 after induction, but there was a significant and progressive decrease in hPGCLCs on days 6 and 8. By contrast, depletion of SOX17 completely abrogated hPGCLC specification. Furthermore, SOX15 overexpression resulted in a significant increase in hPGCLC fraction on day 8. qPCR analysis revealed a possible role for the germ cell and pluripotency regulator PRDM14 in compensating for changes to SOX15 protein levels. Conclusions: SOX17 is essential for hPGCLC specification, yet SOX15 is dispensable. However, SOX15 may have a role in maintaining germ cell identity.

Testing the role of SOX15 in human primordial germ cell fate

Background: Potentially novel regulators of early human germline development have been identified recently, including SOX15 and SOX17, both of which show specific expression in human primordial germ cells. SOX17 is now known to be a critical specifier of human germ cell identity. There have been suggestions, as yet without evidence, that SOX15 might also play a prominent role. The early human germline is inaccessible for direct study, but an in vitro model of human primordial germ cell-like cell (hPGCLC) specification from human embryonic stem cells (hESCs) has been developed. This enables mechanistic study of human germ cell specification using genetic tools to manipulate the levels of SOX15 and SOX17 proteins to explore their roles in hPGCLC specification. Methods: SOX15 and SOX17 proteins were depleted during hPGCLC specification from hESCs using the auxin-inducible degron system, combined with a fluorescent reporter for tracking protein levels. Additionally, SOX15 protein was overexpressed using the ProteoTuner system. Protein-level expression changes were confirmed by immunofluorescence. The impact on hPGCLC specification efficiency was determined by flow cytometry at various time points. qPCR experiments were performed to determine some transcriptional effects of SOX15 perturbations. Results: We observed specific SOX15 expression in hPGCLCs by using immunofluorescence and flow cytometry analysis. Depletion of SOX15 had no significant effect on hPGCLC specification efficiency on day 4 after induction, but there was a significant and progressive decrease in hPGCLCs on days 6 and 8. By contrast, depletion of SOX17 completely abrogated hPGCLC specification. Furthermore, SOX15 overexpression resulted in a significant increase in hPGCLC fraction on day 8. qPCR analysis revealed a possible role for the germ cell and pluripotency regulator PRDM14 in compensating for changes to SOX15 protein levels. Conclusions: SOX17 is essential for hPGCLC specification, yet SOX15 is dispensable. However, SOX15 may have a role in maintaining germ cell identity.

Xenopus SOX5 enhances myogenic transcription indirectly through transrepression

In anamniotes, somite compartimentalization in the lateral somitic domain leads simultaneously to myotome and dermomyotome formation. In the myotome, Xenopus Sox5 is co-expressed with Myod1 in the course of myogenic differentiation. Here, we studied the function of Sox5 using a Myod1-induced myogenic transcription assay in pluripotent cells of animal caps. We found that Sox5 enhances myogenic transcription of muscle markers Des, Actc1, Ckm and MyhE3. The use of chimeric transactivating or transrepressive Sox5 proteins indicates that Sox5 acts as a transrepressor and indirectly stimulates myogenic transcription except for the slow muscle-specific genes Myh7L, Myh7S, Myl2 and Tnnc1. We showed that this role is shared by Sox6, which is structurally similar to Sox5, both belonging to the SoxD subfamily of transcription factors. Moreover, Sox5 can antagonize the inhibitory function of Meox2 on myogenic differentiation. Meox2 which is a dermomyotome marker, represses myogenic transcription in Myod-induced myogenic transcription assay and in Nodal5-induced mesoderm from animal cap assay. The inhibitory function of Meox2 and the pro-myogenic function of Sox5 were confirmed during Xenopus normal development by the use of translation-blocking oligomorpholinos and dexamethasone inducible chimeric Sox5 and Meox2 proteins. We have therefore identified a new function for SoxD proteins in muscle cells, which can indirectly enhance myogenic transcription through transrepression, in addition to the previously identified function as a direct repressor of slow muscle-specific genes.

SOX15 regulates proliferation and migration of endometrial cancer cells

The study aimed to investigate the effects of Sry-like high mobility group box 15 (SOX15) on proliferation and migration of endometrial cancer (EC) cells. Immunohistochemistry (IHC) was applied to determine the expression of SOX15 in EC tissues and adjacent tissues. We used cell transfection method to construct the HEC-1-A and Ishikawa cell lines with stable overexpression and low expression SOX15. Reverse-transcription quantitative real-time PCR (RT-qPCR) and Western blot were performed to examine expression of SOX15 mRNA and SOX15 protein, respectively. By conducting a series of cell proliferation assay and migration assay, we analyzed the influence of SOX15 overexpression or low expression on EC cell proliferation and migration. The expression of SOX15 mRNA and protein in EC tissues was significantly lower than that in adjacent tissues. After lentivirus-transfecting SOX15, the expression level of SOX15 mRNA and protein was significantly increased in cells of SOX15 group, and decreased in sh-SOX15 group. Overexpression of SOX15 could suppress cell proliferation, while down-regulation of SOX15 increased cell proliferation. Flow cytometry results indicated that overexpression of SOX15 induced the ratio of cell-cycle arrest in G₁ stage. In addition, Transwell migration assay results showed that SOX15 overexpression significantly inhibited cell migration, and also down-regulation of SOX15 promoted the migration. As a whole, SOX15 could regulate the proliferation and migration of EC cells and up- regulation of SOX15 could be valuable for EC treatment.

SOX15 and other SOX family members are important mediators of tumorigenesis in multiple cancer types

SOX genes are transcription factors with important roles in embryonic development and carcinogenesis. The SOX family of 20 genes is responsible for regulating lineage and tissue specific gene expression patterns, controlling numerous developmental processes including cell differentiation, sex determination, and organogenesis. As is the case with many genes involved in regulating development, SOX genes are frequently deregulated in cancer. In this perspective we provide a brief overview of how SOX proteins can promote or suppress cancer growth. We also present a pan-cancer analysis of aberrant SOX gene expression and highlight potential molecular mechanisms responsible for their disruption in cancer. Our analyses indicate the prominence of SOX deregulation in different cancer types and reveal potential roles for SOX genes not previously described in cancer. Finally, we summarize our recent identification of SOX15 as a candidate tumor suppressor in pancreatic cancer and propose several research avenues to pursue to further delineate the emerging role of SOX15 in development and carcinogenesis.

Transcriptional targets of Foxd3 in murine ES cells

Understanding gene regulatory networks controlling properties of pluripotent stem cells will facilitate development of stem cell-based therapies. The transcription factor Foxd3 is critical for maintenance of self-renewal, survival, and pluripotency in murine embryonic stem cells (ESCs). Using a conditional deletion of Foxd3 followed by gene expression analyses, we demonstrate that genes required for several developmental processes including embryonic organ development, epithelium development, and epithelial differentiation were misregulated in the absence of Foxd3. Additionally, we identified 6 novel targets of Foxd3 (Sox4, Safb, Sox15, Fosb, Pmaip1 and Smarcd3). Finally, we present data suggesting that Foxd3 functions upstream of genes required for skeletal muscle development. Together, this work provides further evidence that Foxd3 is a critical regulator of murine development through the regulation of lineage specific differentiation.

SOX15 is a candidate tumor suppressor in pancreatic cancer with a potential role in Wnt/β-catenin signaling

Pancreatic cancer is among the top five deadliest cancers in developed countries. Better knowledge of the molecular mechanisms contributing to its tumorigenesis is imperative to improve patient prognosis. Identification of novel tumor suppressor genes (TSGs) in pancreatic cancer will reveal new mechanisms of pathway deregulation and will ultimately help improve our understanding of this aggressive disease. According to Knudson's two-hit model, TSGs are classically disrupted by two concerted genetic events. In this study, we combined DNA methylation profiling with copy number and mRNA expression profiling to identify novel TSGs in a set of 20 pancreatic cancer cell lines. These data sets were integrated for each of ∼12 000 genes in each cell line enabling the elucidation of those genes that undergo DNA hypermethylation, copy-number loss and mRNA downregulation simultaneously in multiple cell lines. Using this integrative genomics strategy, we identified SOX15 (sex determining region Y-box 15) as a candidate TSG in pancreatic cancer. Expression of SOX15 in pancreatic cancer cell lines with undetectable expression resulted in reduced viability of cancer cells both in vitro and in vivo demonstrating its tumor suppressive capability. We also found reduced expression, homozygous deletion and aberrant DNA methylation of SOX15 in clinical pancreatic tumor data sets. Furthermore, we deduced a novel role for SOX15 in suppressing the Wnt/β-catenin signaling pathway, which we hypothesize is a pathway through which SOX15 may exert its tumor suppressive effects in pancreatic cancer.

Histidine-rich calcium binding protein: the new regulator of sarcoplasmic reticulum calcium cycling

The histidine-rich calcium binding protein (HRC) is a novel regulator of sarcoplasmic reticulum (SR) Ca(2+)-uptake, storage and release. Residing in the SR lumen, HRC binds Ca(2+) with high capacity but low affinity. In vitro phosphorylation of HRC affects ryanodine affinity of the ryanodine receptor (RyR), suggesting a functional role of HRC on SR Ca(2+)-release. Indeed, acute HRC overexpression in isolated rodent cardiomyocytes decreases Ca(2+)-induced Ca(2+)-release, increases SR Ca(2+)-load, and impairs contractility. The HRC effects on RyR may be regulated by the Ca(2+)-sensitivity of its interaction with triadin. However, HRC also affects the SR Ca(2+)-ATPase, as shown by HRC overexpression in transgenic mouse hearts, which resulted in reduced SR Ca(2+)-uptake rates, cardiac remodeling and hypertrophy. In fact, in vitro generated evidence suggests that HRC directly interacts with SR Ca(2+)-ATPase2, supporting a dual role of HRC in Ca(2+)-homeostasis: regulation of both SR Ca(2+)-uptake and Ca(2+)-release. Furthermore, HRC plays an important role in myocyte differentiation and in antiapoptotic cardioprotection against ischemia/reperfusion induced cardiac injury. Interestingly, HRC has been linked with familiar cardiac conduction disease and an HRC polymorphism was shown to associate with malignant ventricular arrhythmias in the background of idiopathic dilated cardiomyopathy. This review summarizes studies, which have established the critical role of HRC in Ca(2+)-homeostasis, suggesting its importance in cardiac physiology and pathophysiology.

Sox9 represses alpha-sarcoglycan gene expression in early myogenic differentiation

Alpha sarcoglycan (alpha-SG) is highly expressed in differentiated striated muscle, and its disruption causes limb-girdle muscular dystrophy. Accordingly, the myogenic master regulator MyoD finely modulates its expression. However, the mechanisms preventing alpha-SG gene expression at early stages of myogenic differentiation remain unknown. In this study, we uncovered Sox9, which was not previously known to directly bind muscle gene promoters, as a negative regulator of alpha-SG gene expression. Reporter gene and chromatin immunoprecipitation assays revealed three functional Sox-binding sites that mediate alpha-SG promoter activity repression during early myogenic differentiation. In addition, we show that Sox9-mediated inhibition of alpha-SG gene expression is independent of MyoD. Moreover, we provide evidence suggesting that Smad3 enhances the repressive activity of Sox9 over alpha-SG gene expression in a transforming growth factor-beta-dependent manner. On the basis of these results, we propose that Sox9 and Smad3 are responsible for preventing precocious activation of alpha-SG gene expression during myogenic differentiation.

SoxB1 transcription factors and Notch signaling use distinct mechanisms to regulate proneural gene function and neural progenitor differentiation

The preservation of a pool of neural precursors is a prerequisite for proper establishment and maintenance of a functional central nervous system(CNS). Both Notch signaling and SoxB1 transcription factors have been ascribed key roles during this process, but whether these factors use common or distinct mechanisms to control progenitor maintenance is unsettled. Here, we report that the capacity of Notch to maintain neural cells in an undifferentiated state requires the activity of SoxB1 proteins, whereas the mechanism by which SoxB1 block neurogenesis is independent of Notch signaling. A common feature of Notch signaling and SoxB1 proteins is their ability to inhibit the activity of proneural bHLH proteins. Notch represses the transcription of proneural bHLH genes, while SoxB1 proteins block their neurogenic capacity. Moreover, E-proteins act as functional partners of proneural proteins and the suppression of E-protein expression is an important mechanism by which Notch counteracts neurogenesis. Interestingly, in contrast to the Hes-dependent repression of proneural genes, suppression of E-protein occurs in a Hes-independent fashion. Together, these data reveal that Notch signaling and SoxB1 transcription factors use distinct regulatory mechanisms to control proneural protein function and to preserve neural cells as undifferentiated precursors.

The chromosome 11 region from strain 129 provides protection from sex reversal in XYPOS mice

C57BL/6J (B6) mice containing the Mus domesticus poschiavinus Y chromosome, YPOS, develop ovarian tissue, whereas testicular tissue develops in DBA/2J or 129S1/SvImJ (129) mice containing the YPOS chromosome. To identify genes involved in sex determination, we used a congenic strain approach to determine which chromosomal regions from 129Sl/SvImJ provide protection against sex reversal in XYPOS mice of the C57BL/6J.129-YPOS strain. Genome scans using microsatellite and SNP markers identified a chromosome 11 region of 129 origin in C57BL/6J.129-YPOS mice. To determine if this region influenced testis development in XYPOS mice, two strains of C57BL/6J-YPOS mice were produced and used in genetic experiments. XYPOS adults homozygous for the 129 region had a lower incidence of sex reversal than XYPOS adults homozygous for the B6 region. In addition, many homozygous 129 XYPOS fetuses developed normal-appearing testes, an occurrence never observed in XYPOS mice of the C57BL/6J-YPOS strain. Finally, the amount of testicular tissue observed in ovotestes of heterozygous 129/B6 XYPOS fetuses was greater than the amount observed in ovotestes of homozygous B6 XYPOS fetuses. We conclude that a chromosome 11 locus derived from 129Sl/SvImJ essentially protects against sex reversal in XYPOS mice. A number of genes located in this chromosome 11 region are discussed as potential candidates.

Control of cell fate and differentiation by Sry-related high-mobility-group box (Sox) transcription factors

Maintain stemness, commit to a specific lineage, differentiate, proliferate, or die. These are essential decisions that every cell is constantly challenged to make in multi-cellular organisms to ensure proper development, adult maintenance, and adaptability. SRY-related high-mobility-group box (Sox) transcription factors have emerged in the animal kingdom to help cells effect such decisions. They are encoded by 20 genes in humans and mice. They share a highly conserved high-mobility-group box domain that was originally identified in SRY, the sex-determining gene on the Y chromosome, and that has derived from a canonical high-mobility-group domain characteristic of chromatin-associated proteins. The high-mobility-group box domain binds DNA in the minor groove and increases its DNA binding affinity and specificity by interacting with many types of transcription factors. It also bends DNA and may thereby confer on Sox proteins a unique and critical role in the assembly of transcriptional enhanceosomes. Sox proteins fall into eight groups. Most feature a transactivation or transrepression domain and thereby also act as typical transcription factors. Each gene has distinct expression pattern and molecular properties, often redundant with those in the same group and overlapping with those in other groups. As a whole the Sox family controls cell fate and differentiation in a multitude of processes, such as male differentiation, stemness, neurogenesis, and skeletogenesis. We review their specific molecular properties and in vivo roles, stress recent advances in the field, and suggest directions for future investigations.

Sox15 and Fhl3 transcriptionally coactivate Foxk1 and regulate myogenic progenitor cells

The regulation of myogenic progenitor cells during muscle regeneration is not clearly understood. We have previously shown that the Foxk1 gene, a member of the forkhead/winged helix family of transcription factors, is expressed in myogenic progenitor cells in adult skeletal muscle. In the present study, we utilize transgenic technology and demonstrate that the 4.6 kb upstream fragment of the Foxk1 gene directs beta-galactosidase expression to the myogenic progenitor cell population. We further establish that Sox15 directs Foxk1 expression to the myogenic progenitor cell population, as it binds to an evolutionarily conserved site and recruits Fhl3 to transcriptionally coactivate Foxk1 gene expression. Knockdown of endogenous Sox15 results in perturbed cell cycle kinetics and decreased Foxk1 expression. Furthermore, Sox15 mutant mice display perturbed skeletal muscle regeneration, due in part to decreased numbers of satellite cells and decreased Foxk1 expression. These studies demonstrate that Sox15, Fhl3 and Foxk1 function to coordinately regulate the myogenic progenitor cell population and skeletal muscle regeneration.

Sox15 enhances trophoblast giant cell differentiation induced by Hand1 in mouse placenta

Some members of the Sry-type HMG box (Sox) protein family play important roles in embryogenesis as transcription factors. Here, we report that Sox15 transcripts were much more abundant in mouse placenta than in the fetus, the yolk sac, or several adult tissues. In situ hybridization analysis of the mouse E8.0 conceptus indicated that Sox15 mRNA was predominantly expressed in the trophoblast giant cells of the placenta. We also observed that the amount of Sox15 mRNA dramatically increased during the differentiation of mouse trophoblast stem cells. Ectopic expression of Sox15 in Rat choriocarcinoma cells enhanced the giant cell differentiation induced by a bHLH transcription factor, Hand1. Binding experiments in cotransfected 293 T cells and in vitro revealed that Sox15 interacted with Hand1. We next examined the effects of this interaction on the transcriptional activity of Hand1 and Sox15 using the luciferase reporter assay. Overexpression of Hand1 repressed the Sox15-driven reporter expression, but Sox15 enhanced the Hand1-driven transcription. This enhancement required both the Hand1-binding region and the transactivation domain of Sox15. These results may suggest that the increased transcriptional activity of Hand1 caused by Sox15 might promote the transcription of the target gene resulting in the trophoblast giant cell differentiation in the mouse placenta.

Muscle stem cells in development, regeneration, and disease

Somatic stem cell populations participate in the development and regeneration of their host tissues. Skeletal muscle is capable of complete regeneration due to stem cells that reside in skeletal muscle and nonmuscle stem cell populations. However, in severe myopathic diseases such as Duchenne Muscular Dystrophy, this regenerative capacity is exhausted. In the present review, studies will be examined that focus on the origin, gene expression, and coordinated regulation of stem cell populations to highlight the regenerative capacity of skeletal muscle and emphasize the challenges for this field. Intense interest has focused on cell-based therapies for chronic, debilitating myopathic diseases. Future studies that enhance our understanding of stem cell biology and repair mechanisms will provide a platform for therapeutic applications directed toward these chronic, life-threatening diseases.

Sox21 promotes the progression of vertebrate neurogenesis

The generation of neurons constitutes the foundation of nervous system development, yet the mechanisms underlying neurogenesis are not well established. The HMG-box transcription factors Sox1, Sox2 and Sox3 (Sox1-3) have previously been shown to suppress neurogenesis by maintaining neural cells in an undifferentiated state. Here we report that another HMG-box protein, Sox21, has the opposite activity and promotes neuronal differentiation. Using genetic studies in the chick embryo, we found that Sox21 mediates this function by counteracting the activity of Sox1-3. Accordingly, the balance of Sox21 and Sox1-3 activities determines whether neural cells remain as progenitors or commit to differentiation. Proneural basic helix-loop-helix proteins are essential for the establishment of neuronal fates. We now show that proneural proteins promote neurogenesis by upregulating Sox21 expression. These data establish a key role for Sox21 in the progression of neuronal differentiation and indicate that an important role of proneural proteins is their capacity to upregulate the expression of Sox21.

Differential Roles for Sox15 and Sox2 in Transcriptional Control in Mouse Embryonic Stem Cells

Sox family transcription factors play essential roles in cell differentiation, development, and sex determination. Sox2 was previously thought to be the sole Sox protein expressed in mouse embryonic stem (ES) cells. Sox2 associates with Oct3/4 to maintain self-renewal of ES cells. In the current study, digital differential display identified transcripts for an additional Sox family member, Sox15, enriched in mouse ES cells. Reverse transcription-PCR confirmed that Sox15 expression is highest in undifferentiated ES cells and repressed upon differentiation. Sox15 is expressed at low levels in several tissues, including testis and muscle. In vitro studies showed that Sox15, like Sox2, associated with Oct3/4 on DNA sequences containing the octamer motif and Sox-binding site. Gel mobility shift assays and SELEX analyses showed that Sox15 binds similar DNA sequences as Sox2 but with weaker affinity. In contrast to the early embryonic lethality observed in Sox2-null mice, Sox15-null ES cells and mice were grossly normal. DNA microarray analyses revealed that Otx2, Ctgf, Ebaf, and Hrc are dysregulated in Sox15-null ES cells, however. Chromatin immunoprecipitation showed that Sox15, but not Sox2, bound to a Sox consensus binding site within the Hrc gene. Taken together, these data demonstrate differential roles for Sox15 and Sox2 in transcriptional control in mouse ES cells.

Sox15 is required for skeletal muscle regeneration

The Sox genes define a family of transcription factors that play a key role in the determination of cell fate during development. The preferential expression of the Sox15 in the myogenic precursor cells led us to suggest that the Sox15 is involved in the specification of myogenic cell lineages or in the regulation of the fusion of myoblasts to form myotubes during the development and regeneration of skeletal muscle. To identify the physiological function of Sox15 in mice, we disrupted the Sox15 by homologous recombination in mice. Sox15-deficient mice were born at expected ratios, were healthy and fertile, and displayed normal long-term survival rates. Histological analysis revealed the normal ultrastructure of myofibers and the presence of comparable amounts of satellite cells in the skeletal muscles of Sox15(-/-) animals compared to wild-type animals. These results exclude the role of Sox15 in the development of satellite cells. However, cultured Sox15(-/-) myoblasts displayed a marked delay in differentiation potential in vitro. Moreover, skeletal muscle regeneration in Sox15(-/-) mice was attenuated after application of a crush injury. These results suggest a requirement for Sox15 in the myogenic program. Expression analyses of the early myogenic regulated factors MyoD and Myf5 showed the downregulation of the MyoD and upregulation of the Myf5 in Sox15(-/-) myoblasts. These results show an increased proportion of the Myf5-positive cells and suggest a role for Sox15 in determining the early myogenic cell lineages during skeletal muscle development.

Identification of genes contributing to the obese yellow Avy phenotype: caloric restriction, genotype, diet x genotype interactions

The incidence and severity of obesity and type 2 diabetes are increasing in Western societies. The progression of obesity to type 2 diabetes is gradual with overlapping symptoms of insulin resistance, hyperinsulinemia, hyperglycemia, dyslipidemias, ion imbalance, and inflammation; this complex syndrome has been called diabesity. We describe here comparisons of gene expression in livers of A/a (agouti) vs. A(vy)/A (obese yellow) segregants (i.e., littermates) from BALB/cStCrlfC3H/Nctr x VYWffC3Hf/Nctr-A(vy)/a matings in response to 70% and 100% of ad libitum caloric intakes of a reproducible diet. Twenty-eight (28) genes regulated by diet, genotype, or diet x genotype interactions mapped to diabesity quantitative trait loci. A subset of the identified genes is linked to abnormal physiological signs observed in obesity and diabetes.

Detection of functional DNA motifs via statistical over-representation

The interaction of proteins with DNA recognition motifs regulates a number of fundamental biological processes, including transcription. To understand these processes, we need to know which motifs are present in a sequence and which factors bind to them. We describe a method to screen a set of DNA sequences against a precompiled library of motifs, and assess which, if any, of the motifs are statistically over- or under-represented in the sequences. Over-represented motifs are good candidates for playing a functional role in the sequences, while under-representation hints that if the motif were present, it would have a harmful dysregulatory effect. We apply our method (implemented as a computer program called Clover) to dopamine-responsive promoters, sequences flanking binding sites for the transcription factor LSF, sequences that direct transcription in muscle and liver, and Drosophila segmentation enhancers. In each case Clover successfully detects motifs known to function in the sequences, and intriguing and testable hypotheses are made concerning additional motifs. Clover compares favorably with an ab initio motif discovery algorithm based on sequence alignment, when the motif library includes only a homolog of the factor that actually regulates the sequences. It also demonstrates superior performance over two contingency table based over-representation methods. In conclusion, Clover has the potential to greatly accelerate characterization of signals that regulate transcription.

SNCF, a SoxNeuro interacting protein, defines a novel protein family in Drosophila melanogaster

The involvement of the Sox family of transcription factors in the development of the central nervous system (CNS) appears to be conserved in invertebrates and vertebrates. In Drosophila, SoxNeuro (SoxN) was recently shown to be involved in the formation of neuroblasts [Development 129 (2002) 4193; Development 129 (2002) 4219]. Through a yeast two-hybrid assay searching for proteins interacting with SoxN, we have isolated a novel protein in Drosophila, SoxNeuro Co-Factor (SNCF). The expression of the SNCF gene was detected during early embryogenesis at the blastoderm stages, and stopped just at the beginning of gastrulation. In transfected cells, the protein localised to nuclei, and strongly accumulated in nucleoli. SNCF was able to enhance SoxN mediated transcriptional activity in transfected cells, suggesting that SNCF might act as a SoxN co-activator. Finally, data are presented showing the existence in Drosophila of several proteins with a domain of homology to SNCF, which are all expressed early in embryogenesis at the blastoderm stage.

Origin and possible roles of the SOX8 transcription factor gene during sexual development

SOX8 is a member of the SOX family of developmental transcription factor genes and is closely related to SOX9, a critical gene involved in mammalian sex determination and differentiation. Both genes encode proteins with the ability to bind similar DNA target sequences, and to activate transcription in in vitro assays. Expression studies indicate that the two genes have largely overlapping patterns of activity during mammalian embryonic development. A knockout of SOX8 in mice has no obvious developmental phenotype, suggesting that the two genes are able to act redundantly in a variety of developmental contexts. In particular, both genes are expressed in the developing Sertoli cell lineage of the developing testes in mice, and both proteins are able to activate transcription of the gene encoding anti-Müllerian hormone (AMH), through synergistic action with steroidogenic factor 1 (SF1). We have hypothesized that SOX8 may substitute for SOX9 in species where SOX9 is expressed too late to be involved in sex determination or regulation of AMH expression. However, our studies involving the red-eared slider turtle indicate that SOX8 is expressed at similar levels in males and females throughout the sex-determining period, suggesting that SOX8 is neither a transcriptional regulator for AMH, nor responsible for sex determination or gonad differentiation in that species. Similarly, SOX8 is not expressed in a sexually dimorphic pattern during gonadogenesis in the chicken. Since a functional role(s) for SOX8 is implied by its conservation during evolution, the significance of SOX8 for sexual and other aspects of development will need to be uncovered through more directed lines of experimentation. Copyright 2003 S. Karger AG, Basel

Sox8 is a specific marker for muscle satellite cells and inhibits myogenesis

Sox8 belongs to a family of transcription regulators characterized by a unique DNA-binding domain known as the high mobility group box. Many Sox proteins play fundamental roles in vertebrate development and differentiation processes. Expression of Sox8 is strong during embryonic muscle development and gradually declines postnatally. In this study, we report that in adult skeletal muscle Sox8 is confined to satellite cells. Down-regulation during myogenic differentiation was also detected in cell culture systems and occurred in parallel with down-regulation of the related Sox9. Overexpression of Sox8 or Sox9 on the other hand disrupted myoblasts in their ability to form myotubes. Concomitantly, expression of MyoD and myogenin decreased and basal as well as MyoD-induced activities of the myogenin promoter were strongly reduced in a Sox8-dependent manner. Our data suggest that Sox8 acts as a specific negative regulator of skeletal muscle differentiation, possibly by interfering with the function of myogenic basic helix-loop-helix proteins.

Development and differentiation of the intestinal epithelium

The gastrointestinal tract develops from a simple tube to a complex organ with patterns of differentiation along four axes of asymmetry. The organ is composed of all three germ layers signaling to each other during development to form the adult structure. The gut epithelium is a constitutively developing tissue, constantly differentiating from a stem cell in a progenitor pool throughout the life of the organism. Signals from the adjacent mesoderm and between epithelial cells are required for normal orderly development/differentiation, homeostasis, and apoptosis. Embryonically important patterning factors are used during adult stages for these processes. Such critical pathways as the hedgehog, bone morphogenetic protein, Notch, Sox, and Wnt systems are used both in embryologic and adult times of gut development. We focus on and review the roles of these factors in gut epithelial cell development and differentiation.

Transactivation of capn2 by myogenic regulatory factors during myogenesis

The calcium-activated cysteine protease m-calpain plays a pivotal role during the earlier stages of myogenesis, particularly during fusion. The enzyme is a heterodimer, encoded by the genes capn2, for the large subunit, and capn4, for the small subunit. To study the regulation of m-calpain, the DNA sequence upstream of capn2 was analyzed for promoter elements, revealing the existence of five consensus-binding sites (E-box) for several myogenic regulatory factors and one binding site for myocyte enhancer factor-2 (MEF-2). Transient transfections with reporter gene constructs containing the E-box revealed that MyoD presents a high level of transactivation of reporter constructs containing this region, in particular the sequences including the MEF-2/E4-box. In addition, over-expression of various myogenic factors demonstrated that MyoD and myogenin with much less efficiency, can up-regulate capn2, both singly and synergistically, while Myf5 has no effect on synthesis of the protease. Experiments with antisense oligonucleotides directed against each myogenic factor revealed that MyoD plays a specific and pivotal role during capn2 regulation, and cannot be replaced wholly by myogenin and Myf5.

Myostatin inhibits myoblast differentiation by down-regulating MyoD expression

Myostatin, a negative regulator of myogenesis, is shown to function by controlling the proliferation of myoblasts. In this study we show that myostatin is an inhibitor of myoblast differentiation and that this inhibition is mediated through Smad 3. In vitro, increasing concentrations of recombinant mature myostatin reversibly blocked the myogenic differentiation of myoblasts, cultured in low serum media. Western and Northern blot analysis indicated that addition of myostatin to the low serum culture media repressed the levels of MyoD, Myf5, myogenin, and p21 leading to the inhibition of myogenic differentiation. The transient transfection of C(2)C(12) myoblasts with MyoD expressing constructs did not rescue myostatin-inhibited myogenic differentiation. Myostatin signaling specifically induced Smad 3 phosphorylation and increased Smad 3.MyoD association, suggesting that Smad 3 may mediate the myostatin signal by interfering with MyoD activity and expression. Consistent with this, the expression of dominant-negative Smad3 rescued the activity of a MyoD promoter-reporter in C(2)C(12) myoblasts treated with myostatin. Taken together, these results suggest that myostatin inhibits MyoD activity and expression via Smad 3 resulting in the failure of the myoblasts to differentiate into myotubes. Thus we propose that myostatin plays a critical role in myogenic differentiation and that the muscular hyperplasia and hypertrophy seen in animals that lack functional myostatin is because of deregulated proliferation and differentiation of myoblasts.

Idiopathic weight reduction in mice deficient in the high-mobility-group transcription factor Sox8

Sox8, Sox9, and Sox10 constitute subgroup E within the Sox family of transcription factors. Many Sox proteins are essential regulators of development. Sox9, for instance, is required for chondrogenesis and male sex determination; Sox10 plays key roles in neural crest development and peripheral gliogenesis. The function of Sox8 has not been studied so far. Here, we generated mice deficient in this third member of subgroup E. In analogy to the case for the related Sox9 and Sox10, we expected severe developmental defects in these mice. Despite strong expression of Sox8 in many tissues, including neural crest, nervous system, muscle, cartilage, adrenal gland, kidney, and testis, homozygous mice developed normally in utero, were born at Mendelian frequencies, and were viable. A substantial reduction in weight was observed in these mice; however, this reduction was not attributable to significant structural deficits in any of the Sox8-expressing tissues. Because of frequent coexpression with either Sox9 or Sox10, the mild phenotype of Sox8-deficient mice might at least in part be due to functional redundancy between group E Sox proteins.

Phylogeny of the SOX family of developmental transcription factors based on sequence and structural indicators

Members of the SOX family of transcription factors are found throughout the animal kingdom, are characterized by the presence of a DNA-binding HMG domain, and are involved in a diverse range of developmental processes. Previous attempts to group SOX genes and deduce their structural, functional, and evolutionary relationships have relied largely on complete or partial HMG box sequence of a limited number of genes. In this study, we have used complete HMG domain sequence, full-length protein structure, and gene organization data to study the pattern of evolution within the family. For the first time, a substantial number of invertebrate SOX sequences have been included in the analysis. We find support for subdivision of the family into groups A-H, as has been suggested in some previous studies, and for the assignment of two new groups, I and J. For vertebrate genes, it appears that relatedness as suggested by HMG domain sequence is congruent with relatedness as indicated by overall structure of the full-length protein and intron-exon structure of the genes. Most of the SOX groups identified in vertebrates were represented by a single SOX sequence in each invertebrate species studied. We have named anonymous sequences and, where appropriate, have suggested systematic names for some previously identified sequences. In addition, we identify an HMG domain signature motif which may be considered representative of the SOX family. Based on our data, we propose a robust phylogeny of SOX genes that reflects their evolutionary history in metazoans.