Regulatory role of Six1 in the development of taste papillae

The role of homeobox gene Six1 in cancer progression and its potential as a therapeutic target: A review

The sine oculis homeobox gene 1 (Six1), a member of the Six transcription factor family, specifically binds to defined DNA regions, regulates target gene expression, and plays a crucial role in various tissue and organ development processes. Moreover, Six1 is a critical factor in cancer progression and prognosis making it a central focus in cancer research. Consequently, a comprehensive review of involvement of the Six1 gene in cancer research has a high relevance. This review synthesizes findings from other researches, examines the gene structure and protein functionality of Six1, summarizes its relationship with various cancers, elucidates its mechanisms in promoting tumor progression and development, explores potential possibilities for targeting Six1 as a therapeutic approach for cancer treatment. Six1 is correlated with tumor malignancy and poor prognosis, plays a critical role in promoting tumor cell proliferation, invasion, metastasis, and energy metabolism. Targeting Six1 degradation or expression can potentially suppress tumor progression. This review aims to enhance our understanding of the function and significance of Six1 in cancers while providing a valuable reference for Six1-based cancer diagnosis, prognosis, and therapeutic interventions. This knowledge will facilitate more in-depth oncology research related to Six1, particularly in identifying drug resistance mechanisms and developing precision-targeted therapies.

Development of ectodermal and endodermal taste buds

The sense of taste is mediated primarily by taste buds on the tongue. These multicellular sensory organs are induced, patterned and become innervated during embryogenesis such that a functional taste system is present at birth when animals begin to feed. While taste buds have been considered ectodermal appendages, this is only partly accurate as only fungiform taste buds in the anterior tongue arise from the ectoderm. Taste buds found in the posterior tongue actually derive from endoderm. Nonetheless, both anterior and posterior buds are functionally similar, despite their disparate embryonic origins. In this review, I compare the development of ectodermal vs endodermal taste buds, highlighting the many differences in the cellular and molecular genetic mechanisms governing their formation.

The role of Eya1 and Eya2 in the taste system of mice from embryonic stage to adulthood

Members of the Eya family, which are a class of transcription factors with phosphatase activity, are widely expressed in cranial sensory organs during development. However, it is unclear whether these genes are expressed in the taste system during development and whether they play any role in specifying taste cell fate. In this study, we report that Eya1 is not expressed during embryonic tongue development but that Eya1-expressing progenitors in somites or pharyngeal endoderm give rise to tongue musculature or taste organs, respectively. In the Eya1-deficient tongues, these progenitors do not proliferate properly, resulting in a smaller tongue at birth, impaired growth of taste papillae, and disrupted expression of Six1 in the papillary epithelium. On the other hand, Eya2 is specifically expressed in endoderm-derived circumvallate and foliate papillae located on the posterior tongue during development. In adult tongues, Eya1 is predominantly expressed in IP3R3-positive taste cells in the taste buds of the circumvallate and foliate papillae, while Eya2 is persistently expressed in these papillae at higher levels in some epithelial progenitors and at lower levels in some taste cells. We found that conditional knockout of Eya1 in the third week or Eya2 knockout reduced Pou2f3+, Six1+ and IP3R3+ taste cells. Our data define for the first time the expression patterns of Eya1 and Eya2 during the development and maintenance of the mouse taste system and suggest that Eya1 and Eya2 may act together to promote lineage commitment of taste cell subtypes.

Role of Transient Receptor Potential Vanilloid 1 in Sonic Hedgehog-Dependent Taste Bud Differentiation

Taste bud cell differentiation is extremely important for taste sensation. Immature taste bud cells cannot function during taste perception transmission to the nerve. In this study, we investigated whether hedgehog signaling affected taste bud cell differentiation and whether transient receptor potential vanilloid 1 (TRPV1) played a key role in dry mouth. The induction of dry mouth due to salivary gland resection (SGR) was confirmed on the basis of reduced salivation and disrupted fungiform papillae. The expression of keratin 8 (K8) of taste bud cells, neurofilament (NF), sonic hedgehog (Shh), and glioma-associated oncogene homolog 1 (Gli1) around taste bud cells was downregulated; however, the expression of TRPV1, P2X purinoceptor 3 (P2X3), and hematopoietic stem cell factor (c-Kit) was upregulated at the NF ends in the dry mouth group. To investigate the effect of TRPV1 defect on dry mouth, we induced dry mouth in the TRPV-/- group. The K8, NF, and P2X3 expression patterns were the same in the TRPV1 wild-type and TRPV1-/- dry mouth groups. However, Shh and c-Kit expression decreased regardless of dry mouth in the case of TRPV1 deficiency. These results indicated that TRPV1 positively regulated proliferation during taste bud cell injury by blocking the Shh/Gli1 pathway. In addition, not only cell proliferation but also differentiation of taste bud cells could not be regulated under TRPV1-deficiency conditions. Thus, TRPV1 positively regulates taste bud cell innervation and differentiation; this finding could be valuable in the clinical treatment of dry mouth-related taste dysfunction.

Nkx2-2 expressing taste cells in endoderm-derived taste papillae are committed to the type III lineage

In mammals, multiple cell-signaling pathways and transcription factors regulate development of the embryonic taste system and turnover of taste cells in the adult stage. Using single-cell RNA-Seq of mouse taste cells, we found that the homeobox-containing transcription factor Nkx2-2, a target of the Sonic Hedgehog pathway and a key regulator of the development and regeneration of multiple cell types in the body, is highly expressed in type III taste cells but not in type II or taste stem cells. Using in situ hybridization and immunostaining, we confirmed that Nkx2-2 is expressed specifically in type III taste cells in the endoderm-derived circumvallate and foliate taste papillae but not in the ectoderm-derived fungiform papillae. Lineage tracing revealed that Nkx2-2-expressing cells differentiate into type III, but not type II or type I cells in circumvallate and foliate papillae. Neonatal Nkx2-2-knockout mice did not express key type III taste cell marker genes, while the expression of type II and type I taste cell marker genes were unaffected in these mice. Our findings indicate that Nkx2-2-expressing cells are committed to the type III lineage and that Nkx2-2 may be critical for the development of type III taste cells in the posterior tongue, thus illustrating a key difference in the mechanism of type III cell lineage specification between ectoderm- and endoderm-derived taste fields.

Expression of Eya1 in mouse taste buds

Taste substances are detected by taste receptor cells in the taste buds in the oral epithelium. Individual taste receptor cells contribute to evoking one of the five taste qualities: sweet, umami, bitter, sour, and salty (sodium). They are continuously replaced every few weeks by new ones generated from local epithelial stem cells. A POU transcription factor, Pou2f3 (also known as Skn-1a), regulates the generation and differentiation of sweet, umami, and bitter cells. However, the molecular mechanisms underlying terminal differentiation into these Pou2f3-dependent taste receptor cells remain unknown. To identify the candidate molecules that regulate the differentiation of these taste receptor cells, we searched for taste receptor type-specific transcription factors using RNA-sequence data of sweet and bitter cells. No transcription factor gene showing higher expression in sweet cells than in bitter cells was found. Eyes absent 1 (Eya1) was identified as the only transcription factor gene showing higher expression in bitter cells than in sweet cells. In situ hybridization revealed that Eya1 was predominantly expressed in bitter cells and also in the putative immature/differentiating taste bud cells in circumvallate and fungiform papillae and soft palate. Eya1 is a candidate molecule that regulates the generation and differentiation of bitter cells.

SIX1 is upregulated in gastric cancer and regulates proliferation and invasion by targeting the ERK pathway and promoting epithelial-mesenchymal transition

Sine oculis homeobox homologue 1 (SIX1) is a Six class homeobox gene conserved throughout many species. It has been reported to act as an oncogene and is overexpressed in many cancers. However, the function and regulatory mechanism of SIX1 in gastric cancer (GC) remains unclear. In our study, we detected protein levels of SIX1 via immunohistochemistry (IHC) and its proliferation and invasion effects via CCK8 and transwell assays. Additionally, expression of cyclin D1, MMP2, p-ERK, and EMT-related proteins was measured by western blotting. We found that SIX1 had significantly higher expression in GC tissues and that it could promote GC cell proliferation and invasion. Also, overexpression of SIX1 increased the expression of cyclin D1, MMP2, p-ERK, and EMT-related proteins, which could all be inhibited by knocking down SIX1. In conclusion, SIX1 is upregulated in GC tissues. It can promote GC cell proliferation by targeting cyclin D1, invasion via ERK signalling, and EMT pathways by targeting MMP2 and E-cadherin. SIGNIFICANCE OF THE STUDY: Our study showed that SIX1 was upregulated in GC tissues, and promoted GC cell proliferation by targeting cyclin D1, invasion via ERK signalling, and EMT pathways by targeting MMP2 and E-cadherin. These results suggested the potential regulatory mechanism of SIX1 in proliferation and invasion of gastric cancer.

Early steps in inner ear development: induction and morphogenesis of the otic placode

Various cellular replacement therapies using in vitro generated cells to replace damaged tissue have been proposed as strategies to alleviate hearing loss. All such therapies must involve a complete understanding of the earliest steps in inner ear development; its induction as a thickened plate of cells in the non-neural, surface ectoderm of the embryo, to its internalization as an otocyst embedded in the head mesenchyme of the embryo. Such knowledge informs researchers addressing the feasibility of the proposed strategy and present alternatives if needed. In this review we describe the mechanisms of inner ear induction, concentrating on the factors that steer the fate of ectoderm into precursors of the inner ear. Induction then leads to inner ear morphogenesis and we describe the cellular changes that occur as the inner ear is converted from a superficial placode to an internalized otocyst, and how they are coordinated with a particular emphasis on how the signaling environment surrounding the inner ear influences these processes.

Regulation of Six1 expression by evolutionarily conserved enhancers in tetrapods

The Six1 homeobox gene plays critical roles in vertebrate organogenesis. Mice deficient for Six1 show severe defects in organs such as skeletal muscle, kidney, thymus, sensory organs and ganglia derived from cranial placodes, and mutations in human SIX1 cause branchio-oto-renal syndrome, an autosomal dominant developmental disorder characterized by hearing loss and branchial defects. The present study was designed to identify enhancers responsible for the dynamic expression pattern of Six1 during mouse embryogenesis. The results showed distinct enhancer activities of seven conserved non-coding sequences (CNSs) retained in tetrapod Six1 loci. The activities were detected in all cranial placodes (excluding the lens placode), dorsal root ganglia, somites, nephrogenic cord, notochord and cranial mesoderm. The major Six1-expression domains during development were covered by the sum of activities of these enhancers, together with the previously identified enhancer for the pre-placodal region and foregut endoderm. Thus, the eight CNSs identified in a series of our study represent major evolutionarily conserved enhancers responsible for the expression of Six1 in tetrapods. The results also confirmed that chick electroporation is a robust means to decipher regulatory information stored in vertebrate genomes. Mutational analysis of the most conserved placode-specific enhancer, Six1-21, indicated that the enhancer integrates a variety of inputs from Sox, Pax, Fox, Six, Wnt/Lef1 and basic helix-loop-helix proteins. Positive autoregulation of Six1 is achieved through the regulation of Six protein-binding sites. The identified Six1 enhancers provide valuable tools to understand the mechanism of Six1 regulation and to manipulate gene expression in the developing embryo, particularly in the sensory organs.

Patterns of immunoreactivity specific for gustducin and for NCAM differ in developing rat circumvallate papillae and their taste buds

α-Gustducin and neural cell adhesion molecule (NCAM) are molecules previously found to be expressed in different cell types of mammalian taste buds. We examined the expression of α-gustducin and NCAM during the morphogenesis of circumvallate papillae and the formation of their taste buds by immunofluorescence staining and laser-scanning microscopy of semi-ultrathin sections of fetal and juvenile rat tongues. Images obtained by confocal laser scanning microscopy in transmission mode were also examined to provide outlines of histology and cell morphology. Morphogenesis of circumvallate papillae had already started on embryonic day 13 (E13) and was evident as the formation of placode. By contrast, taste buds in the circumvallate papillae started to appear between postnatal day 0 (P0) and P7. Although no cells with immunoreactivity specific for α-gustducin were detected in fetuses from E13 to E19, cells with NCAM-specific immunoreactivity were clearly apparent in the entire epithelium of the circumvallate papillary placode, the rudiment of each circumvallate papilla and the developing circumvallate papilla itself from E13 to E19. However, postnatally, both α-gustducin and NCAM became concentrated within taste cells as the formation of taste buds advanced. After P14, neither NCAM nor α-gustducin was detectable in the epithelium around the taste buds. In conclusion, α-gustducin appeared in the cytoplasm of taste cells during their formation after birth, while NCAM appeared in the epithelium of the circumvallate papilla-forming area. However, these two markers of taste cells were similarly distributed within mature taste cells.

Development of gustatory papillae in the absence of Six1 and Six4

Six family genes encode homeobox transcription factors, and a deficiency in them leads to abnormal structures of the sensory organs. In a previous paper, Six1 was reported to be expressed in the taste bud-bearing lingual papillae of mice, and loss of Six1 affected the development of these gustatory papillae. We show here that embryos lacking both Six1 and Six4 revealed more severe abnormalities than those lacking Six1 alone during morphogenesis of their gustatory papillae. By in situ hybridization, Six4 was shown to be broadly distributed in the epithelium of the lateral lingual swellings at embryonic day (E) 11.5, and in the tongue epithelium, mesenchyme, and muscles at E12.5. From E14, Six4 was similar in expression pattern to Six1, as previously reported. In the fungiform papillae, Six4 was expressed in the epithelium at E14-E16.5. In the circumvallate and foliate papillae, Six4 expression was observed in the trench wall of these papillae at E15.5-P0. Although Six4-deficient mice had no abnormalities, Six1/Six4-deficient mice showed distinct morphological changes: fusion of the lateral lingual swellings was delayed, and the tongue was poorly developed. The primordia of fungiform papillae appeared earlier than those in the wild-type or Six1-deficient mice, and the papillae rapidly increased in size; thus fusion of each papilla was evident. The circumvallate papillae showed severe defects; for example, invagination of the trenches started asymmetrically, which resulted in longer and shorter trenches. The foliate papillae elevated initially, and showed stunted trenches. Therefore, Six1 and Six4 function synergistically to form gustatory papillae during development of the tongue.

A high-resolution molecular atlas of the fetal mouse lower urogenital tract

Epithelial-stromal interactions in the lower urogenital tract (LUT) are integral to prostatic and seminal vesicle development in males, vaginal and uterine development in females, and urethral development in both sexes. Gene expression profiling of isolated LUT stroma and epithelium has unraveled mechanisms of LUT development, but such studies are confounded by heterogeneous and ill-defined cell sub-populations contained within each tissue compartment. We used in situ hybridization to synthesize a high-resolution molecular atlas of 17-day post-coitus fetal mouse LUT. We identified mRNAs that mark selective cell populations of the seminal vesicle, ejaculatory duct, prostate, urethra, and vagina, subdividing these tissues into 16 stromal and 8 epithelial sub-compartments. These results provide a powerful tool for mapping LUT gene expression patterns and also reveal previously uncharacterized sub-compartments that may play mechanistic roles in LUT development of which we were previously unaware.

Six1 transcription factor is critical for coordination of epithelial, mesenchymal and vascular morphogenesis in the mammalian lung

Six1 is a member of the six-homeodomain family of transcription factors. Six1 is expressed in multiple embryonic cell types and plays important roles in proliferation, differentiation and survival of precursor cells of different organs, yet its function during lung development was hitherto unknown. Herein we show that Six1(-/-) lungs are severely hypoplastic with greatly reduced epithelial branching and increased mesenchymal cellularity. Six1 is expressed at the distal epithelial tips of branching tubules as well as in the surrounding distal mesenchyme. Six1(-/-) lung epithelial cells show increased expression of differentiation markers, but loss of progenitor cell markers. Six1 overexpression in MLE15 lung epithelial cells in vitro inhibited cell differentiation, but increases the expression of progenitor cell markers. In addition, Six1(-/-) embryos and newborn mice exhibit mesenchymal overproliferation, decreased Fgf10 expression and severe defects in the smooth muscle component of the bronchi and major pulmonary vessels. These defects lead to rupture of major vessels in mutant lungs after birth. Treatment of Six1(-/-) epithelial explants in culture with recombinant Fgf10 protein restores epithelial branching. As Shh expression is abnormally increased in Six1(-/-) lungs, we also treated mutant mesenchymal explants with recombinant Shh protein and found that these explants were competent to respond to Shh and continued to grow in culture. Furthermore, inhibition of Shh signaling with cyclopamine stimulated Six1(-/-) lungs to grow and branch in culture. This study provides the first evidence for the requirement of Six1 in coordinating Shh-Fgf10 signaling in embryonic lung to ensure proper levels of proliferation and differentiation along the proximodistal axis of epithelial, mesenchymal and endothelial cells. These findings uncover novel and essential functions for Six1 as a critical coordinator of Shh-Fgf10 signaling during embryonic lung development. We propose that Six1 is hence critical for coordination of proper lung epithelial, mesenchymal and vascular development.

Expression of Six1 and Six4 in mouse taste buds

Members of the Six gene family are expressed in various tissues including sensory organs, such as the inner ear and olfactory epithelium. We examined the expression of Six1 and Six4 mRNAs in mouse taste buds by using in situ hybridization. Six1 was detected immunohistochemically in the nuclei of taste bud cells, in a subset of type-II cells, as shown by double-immunolabeling with anti-Six1 together with anti-PLCβ2 or anti-IP(3)R3 antibodies. Six1-immunoreactive (IR) nuclei appeared at embryonic day 17.5 in the dorsal epithelium, and in the trench wall epithelium of circumvallate papillae at postnatal day 5. At this stage, Six1-IR nuclei were observed in all newly-formed type-II cells. During postnatal development, type-II cells increased in number, but those with Six1-IR nuclei showed no apparent increase. After transection of the bilateral glossopharyngeal nerve, type-II cells gradually disappeared; but some of them remained in the epithelium even at 11-17 days post-transection. The remaining type-II cells showed Six1-immunoreactivity. At 24 days after nerve transection, regenerating type-II cells appeared; and strong Six1-immunoreactivity was observed in them. Also, enhanced green fluorescent protein-immunoreactivity and β-galactosidase-immunoreactivity, which were indicators for Six1 transcripts and Six4 transcripts, respectively, overlapped. These results suggest that Six1 and Six4 genes are expressed in the taste bud cells, in newly formed or surviving type-II cells.