All-trans retinoic acid suppresses exocrine differentiation and branching morphogenesis in the embryonic pancreas

From iPSCs to Pancreatic β Cells: Unveiling Molecular Pathways and Enhancements with Vitamin C and Retinoic Acid in Diabetes Research

Diabetes mellitus, a chronic and non-transmissible disease, triggers a wide range of micro- and macrovascular complications. The differentiation of pancreatic β-like cells (PβLCs) from induced pluripotent stem cells (iPSCs) offers a promising avenue for regenerative medicine aimed at treating diabetes. Current differentiation protocols strive to emulate pancreatic embryonic development by utilizing cytokines and small molecules at specific doses to activate and inhibit distinct molecular signaling pathways, directing the differentiation of iPSCs into pancreatic β cells. Despite significant progress and improved protocols, the full spectrum of molecular signaling pathways governing pancreatic development and the physiological characteristics of the differentiated cells are not yet fully understood. Here, we report a specific combination of cofactors and small molecules that successfully differentiate iPSCs into PβLCs. Our protocol has shown to be effective, with the resulting cells exhibiting key functional properties of pancreatic β cells, including the expression of crucial molecular markers (pdx1, nkx6.1, ngn3) and the capability to secrete insulin in response to glucose. Furthermore, the addition of vitamin C and retinoic acid in the final stages of differentiation led to the overexpression of specific β cell genes.

Pancreas morphogenesis: Branching in and then out

The pancreas of adult mammals displays a branched structure which transports digestive enzymes produced in the distal acini through a tree-like network of ducts into the duodenum. In contrast to several other branched organs, its branching patterns are not stereotypic. Moreover, the branches do not grow from dichotomic splitting of an initial stem but rather from the formation of microlumen in a mass of cells. These lumen progressively assemble into a hyperconnected network that refines into a tree by the time of birth. We review the cell remodeling events and the molecular mechanisms governing pancreas branching, as well as the role of the surrounding tissues in this process. Furthermore, we draw parallels with other branched organs such as the salivary and mammary gland.

Vav1 Sustains the In Vitro Differentiation of Normal and Tumor Precursors to Insulin Producing Cells Induced by all-Trans Retinoic Acid (ATRA)

All-trans retinoic acid (ATRA) promotes the development and the function of insulin producing cells and induces partial differentiation of pancreatic tumor cells. A number of evidences clearly indicate that the ATRA mediated signaling may have a substantial role in therapeutic approaches based on restoration of functional β-cells. Among the proteins up-regulated by ATRA, Vav1 is involved in maturation and function of haematopoietic cells and is essential for retinoids induced differentiation of tumor promyelocytes. The presence of Vav1 in solid tissues, including pancreas, is considered ectopic and no role in the differentiation of human epithelial cells has so far been described. We demonstrated here that Vav1 sustains the maturation to β-cells of the normal precursors human Biliary Tree Stem/progenitor Cells (hBTSCs) induced by a differentiation medium containing ATRA and that, in the mature normal pancreas, insulin-producing cells express variable levels of Vav1. Using pancreatic ductal adenocarcinoma (PDAC)-derived cells, we also revealed that the ATRA induced up-modulation of Vav1 is essential for the retinoid-induced trans-differentiation of neoplastic cells into insulin producing cells. The results of this study identify Vav1 as crucial molecule in ATRA induced maturation of insulin producing cells and suggest this protein as a marker for new strategies ended to restore functional β-cells.

Differentiation Potential of Mesenchymal Stem Cells into Pancreatic β-Cells

Stem cells having the capability to differentiate into other type of cells and renewing themselves, gained so much importance in recent years. Investigations in stem cells revealed that mesenchymal stem cells can successfully differentiate into other type of cells like adipocytes, hepatocytes, osteocytes, neurocytes and chondrocytes. In addition, these cells can also differentiate into insulin-producing beta cells. Insulin is a crucial hormone for glucose balance of the body. Insufficiency or unavailability of insulin is called diabetes. External insulin intake, as well as pancreas or islet transplantation, is the most basic treatment of diabetes. In vivo and in vitro studies demonstrate that stem cell therapy is also used in the cure of diabetes. Differentiation process of stem cells into beta cells releasing insulin is quite complicated. There are many different reports for the differentiation of stem cells in the literature. The success of differentiation of stem cells into beta cells depends on several factors like the source of stem cells, chemicals added into the differentiation medium and the duration of differentiation protocol. Distinct studies for the differentiation of stem cells into insulin-secreting cells are available in the literature. Moreover, thanks to the superior differentiation capacity of stem cells, they are being preferred in clinical studies. Stem cells were clinically used to heal diabetic ulcer, to increase c-peptide level and insulin secretion in both type 1 and type 2 diabetes. Mesenchymal stem cells having high differentiation potential to insulin-secreting cells are encouraging vehicles for both in vivo and in vitro studies together with clinical trials for diabetes mellitus.

Retinoic acid signaling regulates Krt5 and Krt14 independently of stem cell markers in submandibular salivary gland epithelium

BACKGROUND

Retinoic acid (RA), the active metabolite of vitamin A, has been demonstrated to be important for growth and branching morphogenesis of mammalian embryonic salivary gland epithelium. However, it is not known whether RA functions directly within epithelial cells or in associated tissues that influence morphogenesis of salivary epithelium. Moreover, downstream targets of RA regulation have not been identified.

RESULTS

Here, we show that canonical RA signaling occurs in multiple tissues of embryonic mouse salivary glands, including epithelium, associated parasympathetic ganglion neurons, and nonneuronal mesenchyme. By culturing epithelium explants in isolation from other tissues, we demonstrate that RA influences epithelium morphogenesis by direct action in that tissue. Moreover, we demonstrate that inhibition of RA signaling represses cell proliferation and expression of FGF10 signaling targets, and upregulates expression of basal epithelial keratins Krt5 and Krt14. Importantly, we show that the stem cell gene Kit is regulated inversely from Krt5/Krt14 by RA signaling.

CONCLUSIONS

RA regulates Krt5 and Krt14 expression independently of stem cell character in developing salivary epithelium. RA, or chemical inhibitors of RA signaling, could potentially be used for modulating growth and differentiation of epithelial stem cells for the purpose of re-populating damaged glands or generating bioengineered organs. Developmental Dynamics 246:135-147, 2017. © 2016 Wiley Periodicals, Inc.

Retinoids in the pancreas

Retinoids (vitamin A and its natural and synthetic analogs) are required by most tissues for maintaining the normal health of the tissue. This is certainly true for the pancreas. The recent literature is convincing that retinoids are needed by the adult to assure normal pancreatic endocrine functions, especially those of the α- and β-cells. It is also well established that retinoids are required to insure normal pancreas development in utero, including the development of the endocrine pancreas. The actions of retinoids for maintaining normal pancreatic islet functions has drawn considerable research interest from investigators interested in understanding and treating metabolic disease. Pancreatic retinoids are also of interest to investigators studying the origins of pancreatic disease, including the development of pancreatic fibrosis and its sequelae. This research interest is focused on pancreatic stellate cells (PSCs) which store retinoids and possess the metabolic machinery needed to metabolize retinoids. The literature on pancreatic disease and retinoids suggests that there is an association between impairments in pancreatic retinoid storage and metabolism and the development of pancreatic disease. These topics will be considered in this review.

Acinar cell reprogramming: a clinically important target in pancreatic disease

Acinar cells of the pancreas produce the majority of enzymes required for digestion and make up >90% of the cells within the pancreas. Due to a common developmental origin and the plastic nature of the acinar cell phenotype, these cells have been identified as a possible source of β cells as a therapeutic option for Type I diabetes. However, recent evidence indicates that acinar cells are the main source of pancreatic intraepithelial neoplasias (PanINs), the predecessor of pancreatic ductal adenocarcinoma (PDAC). The conversion of acinar cells to either β cells or precursors to PDAC is dependent on reprogramming of the cells to a more primitive, progenitor-like phenotype, which involves changes in transcription factor expression and activity, and changes in their epigenetic program. This review will focus on the mechanisms that promote acinar cell reprogramming, as well as the factors that may affect these mechanisms.

Ectopic Ptf1a expression in murine ESCs potentiates endocrine differentiation and models pancreas development in vitro

Besides its role in exocrine differentiation, pancreas-specific transcription factor 1a (PTF1a) is required for pancreas specification from the foregut endoderm and ultimately for endocrine cell formation. Examining the early role of PTF1a in pancreas development has been challenging due to limiting amounts of embryonic tissue material for study. Embryonic stem cells (ESCs) which can be differentiated in vitro, and without limit to the amount of experimental material, can serve as a model system to study these early developmental events. To this end, we derived and characterized a mouse ESC line with tetracycline-inducible expression of PTF1a (tet-Ptf1a mESCs). We found that transient ectopic expression of PTF1a initiated the pancreatic program in differentiating ESCs causing cells to activate PDX1 expression in bud-like structures resembling pancreatic primordia in vivo. These bud-like structures also expressed progenitor markers characteristic of a developing pancreatic epithelium. The epithelium differentiated to generate a wave of NGN3+ endocrine progenitors, and further formed cells of all three pancreatic lineages. Notably, the insulin+ cells in the cultures were monohormonal, and expressed PDX1 and NKX6.1. PTF1a-induced cultures differentiated into significantly more endocrine and exocrine cells and the ratio of endocrine-to-exocrine cell differentiation could be regulated by retinoic acid (RA) and nicotinamide (Nic) signaling. Moreover, induced cultures treated with RA and Nic exhibited a modest glucose response. Thus, this tet-Ptf1a ESC-based in vitro system is a valuable new tool for interrogating the role of PTF1a in pancreas development and in directing differentiation of ESCs to endocrine cells.

Aldehyde dehydrogenase 1 activity in the developing human pancreas modulates retinoic acid signalling in mediating islet differentiation and survival

AIMS/HYPOTHESIS

Aldehyde dehydrogenase 1 (ALDH1), a human stem-cell marker, is an enzyme responsible for converting retinaldehydes to retinoic acids (RAs) to modulate cell differentiation. However, data on expression levels and functional roles of ALDH1 during human fetal pancreatic development are limited. The focus of this study was to characterise ALDH1 expression patterns and to determine its functional role in islet cell differentiation.

METHODS

The presence of ALDH1 in the human fetal pancreas (8-22 weeks) was characterised by microarray, quantitative RT-PCR, western blotting and immunohistological approaches. Isolated human fetal islet-epithelial cell clusters were treated with ALDH1 inhibitors, retinoic acid receptor (RAR) agonists and ALDH1A1 small interfering (si)RNA.

RESULTS

In the developing human pancreatic cells, high ALDH1 activity frequently co-localised with key stem-cell markers as well as endocrine transcription factors. A high level of ALDH1 was expressed in newly differentiated insulin(+) cells and this decreased as development progressed. Pharmacological inhibition of ALDH1 activity in human fetal islet-epithelial cell clusters resulted in reduced endocrine cell differentiation and increased cell apoptosis, and was reversed with co-treatment of RAR/RXR agonists. Furthermore, siRNA knockdown of ALDH1A1 significantly decreased RAR expression and induced cell apoptosis via suppression of the phosphoinositide 3-kinase (PI3K) pathway and activation of caspase signals.

CONCLUSIONS/INTERPRETATION

Our findings indicate that ALDH1(+) cells represent a pool of endocrine precursors in the developing human pancreas and that ALDH1 activity is required during endocrine cell differentiation. Inhibition of ALDH1-mediated retinoid signalling impairs human fetal islet cell differentiation and survival.

Directed differentiation of embryonic stem cells allows exploration of novel transcription factor genes for pancreas development

Embryonic stem cells (ESCs) have been promised as a renewable source for regenerative medicine, including providing a replacement therapy in type 1 diabetes. However, they have not yet been differentiated into functional insulin-secreting β cells. This is due partially to the knowledge gap regarding the transcription factors (TFs) required for pancreas development. We hypothesize that, if directed differentiation in vitro recapitulates the developmental process in vivo, ESCs provide a powerful model to discover novel pancreatic TF genes. Guided by knowledge of their normal development and using RT-PCR and immunochemical analyses, we have established protocols for directed differentiation of mouse ESCs into pancreatic progenitors. Microarray analyses of these differentiating ESC cells at days 0, 4, 8 and 15 confirmed their sequential differentiation. By day 15, we found up-regulation of a group of pancreatic progenitor marker genes including Pdx1, Ptf1a, Nkx6.1, Pax4 and Pax6. Consistently, Pdx1-immunoreactive cells were detected on day 15. Most of these Pdx1(+) cells also expressed Nkx6.1. Bioinformatic analyses of sequential datasets allowed identification of over 20 novel TF genes potentially important for pancreas development. The dynamic expression of representative known and novel genes was confirmed by quantitative real time RT-PCR analysis. This strategy may be modified to study novel regulatory molecules for development of other tissue and organ systems.

Retinoic acid synthesis and functions in early embryonic development

Retinoic acid (RA) is a morphogen derived from retinol (vitamin A) that plays important roles in cell growth, differentiation, and organogenesis. The production of RA from retinol requires two consecutive enzymatic reactions catalyzed by different sets of dehydrogenases. The retinol is first oxidized into retinal, which is then oxidized into RA. The RA interacts with retinoic acid receptor (RAR) and retinoic acid X receptor (RXR) which then regulate the target gene expression. In this review, we have discussed the metabolism of RA and the important components of RA signaling pathway, and highlighted current understanding of the functions of RA during early embryonic development.

Retinoic acid-induced pancreatic stellate cell quiescence reduces paracrine Wnt-β-catenin signaling to slow tumor progression

BACKGROUND & AIMS

Patients with pancreatic ductal adenocarcinoma are deficient in vitamin A, resulting in activation of pancreatic stellate cells (PSCs). We investigated whether restoration of retinol to PSCs restores their quiescence and affects adjacent cancer cells.

METHODS

PSCs and cancer cell lines (AsPc1 and Capan1) were exposed to doses and isoforms of retinoic acid (RA) in 2-dimensional and 3-dimensional culture conditions (physiomimetic organotypic culture). The effects of all-trans retinoic acid (ATRA) were studied in LSL-KrasG12D/+;LSL-Trp53R172H/+;Pdx-1-Cre mice, a model of human pancreatic ductal adenocarcinoma.

RESULTS

After incubation with ATRA, PSCs were quiescent and had altered expression of genes that regulate proliferation, morphology, and motility; genes that encode cytoskeletal proteins and cytokines; and genes that control other functions, irrespective of culture conditions or dosage. In the organotypic model, and in mice, ATRA induced quiescence of PSCs and thereby reduced cancer cell proliferation and translocation of β-catenin to the nucleus, increased cancer cell apoptosis, and altered tumor morphology. ATRA reduced the motility of PSCs, so these cells created a "wall" at the junction between the tumor and the matrix that prevented cancer cell invasion. Restoring secreted frizzled-related protein 4 (sFRP4) secretion to quiescent PSCs reduced Wnt-β-catenin signaling in cancer cells and their invasive ability. Human primary and metastatic pancreatic tumor tissues stained strongly for cancer cell nuclear β-catenin but had low levels of sFRP4 (in cancer cells and PSCs).

CONCLUSIONS

RA induces quiescence and reduces motility of PSCs, leading to reduced proliferation and increased apoptosis of surrounding pancreatic cancer cells. RA isoforms might be developed as therapeutic reagents for pancreatic cancer.

Isolation and culture of embryonic pancreas and liver

Culturing embryonic tissue in an in vitro setting offers the unique ability to manipulate the external medium and therefore to investigate the pathways involved in regulating normal organogenesis as well as providing models for developmental disorders. Here we describe a system for the in vitro culture of the dorsal pancreatic buds and liver buds from mouse embryos. The tissues are dissected from day 9.0 or 11.5 mouse embryos. The tissues are placed on fibronectin-coated coverslips in serum-containing medium and allowed to attach. Over the next few days, the buds grow as flattened structures which are thin enough to allow the use of wholemount immunostaining methods.

E2F-1-deficient NOD/SCID mice developed showing decreased saliva production

The non-obese diabetic mouse (NOD) is the most characterized model used to study insulin-dependent type 1 diabetes mellitus (IDDM) and Sjoögren's syndrome (SS). In a previous report, we found NOD.E2f1(-/-) mice show a greater progressive development to IDDM and SS compared to NOD mice. Our previous data indicated a progressive decrease in regulatory T cells (CD4(+)CD25(+)) and a decrease in the systemic secretion systems for insulin, and saliva was associated with the progression of IDDM and SS. Therefore, to define the mechanism of early-onset IDDM SS in E2F-1 deficient NOD mice required further investigation by producing E2F-1 deficient NOD/SCID mice in which the T and B cells do not develop. The purpose here was to analyze the essential function of the E2F-1 molecule in the development of IDDM and SS; and the dysfunction of the pancreas islet and salivary gland in the NOD background using NOD/SCID mice. We produced NOD/SCID.E2f1(-/-) mice using homologous recombination; determined diabetes development; measured saliva and insulin production; and performed a histological analysis. The deficient mice showed a decreasing volume of saliva; no infiltration of lymphocytes into salivary glands; no development of diabetes; and no protein localization of FGFR-2b in the ducts of the salivary gland that regulates submandibular gland proliferation and morphogenesis. Therefore, we considered a deficiency in E2F-1 induces a decrease in regulatory T cells and an increase in auto-reactive T cells; however, the E2F-1 deficiency is not associated with T and B cells-independent dysfunction of pancreatic beta cell in insulin secretion. Further, the E2F-1 deficiency is associated with T and B cells-independent dysfunction of the salivary gland exhibits a decrease in saliva production volume. We suggest E2F-1 may be also associated with the differentiation of exocrine cells in the duct where FGFR-2b is expressed in the salivary gland. The E2F-1 deficient NOD/SCID mouse model is useful for showing the development of the salivary gland; and is also useful for various experiments in humanized mice.

Exdpf is a key regulator of exocrine pancreas development controlled by retinoic acid and ptf1a in zebrafish

Both endocrine and exocrine pancreatic cells arise from pancreatic-duodenal homeobox 1 (pdx1)-positive progenitors. The molecular mechanisms controlling cell fate determination and subsequent proliferation, however, are poorly understood. Unlike endocrine cells, less is known about exocrine cell specification. We report here the identification and characterization of a novel exocrine cell determinant gene, exocrine differentiation and proliferation factor (exdpf), which is highly expressed in the exocrine cell progenitors and differentiated cells of the developing pancreas in zebrafish. Knockdown of exdpf by antisense morpholino caused loss or significant reduction of exocrine cells due to lineage-specific cell cycle arrest but not apoptosis, whereas the endocrine cell mass appeared normal. Real-time PCR results demonstrated that the cell cycle arrest is mediated by up-regulation of cell cycle inhibitor genes p21(Cip), p27(Kip), and cyclin G1 in the exdpf morphants. Conversely, overexpression of exdpf resulted in an overgrowth of the exocrine pancreas and a severe reduction of the endocrine cell mass, suggesting an inhibitory role for exdpf in endocrine cell progenitors. We show that exdpf is a direct target gene of pancreas-specific transcription factor 1a (Ptf1a), a transcription factor critical for exocrine formation. Three consensus Ptf1a binding sites have been identified in the exdpf promoter region. Luciferase assay demonstrated that Ptf1a promotes transcription of the exdpf promoter. Furthermore, exdpf expression in the exocrine pancreas was lost in ptf1a morphants, and overexpression of exdpf successfully rescued exocrine formation in ptf1a-deficient embryos. Genetic evidence places expdf downstream of retinoic acid (RA), an instructive signal for pancreas development. Knocking down exdpf by morpholino abolished ectopic carboxypeptidase A (cpa) expression induced by RA. On the other hand, exdpf mRNA injection rescued endogenous cpa expression in embryos treated with diethylaminobenzaldehyde, an inhibitor of RA signaling. Moreover, exogenous RA treatment induced anterior ectopic expression of exdpf and trypsin in a similar pattern. Our study provides a new understanding of the molecular mechanisms controlling exocrine cell specification and proliferation by a novel gene, exdpf. Highly conserved in mammals, the expression level of exdpf appears elevated in several human tumors, suggesting a possible role in tumor pathogenesis.

Production of beta-cells from human embryonic stem cells

The making of functional pancreatic islets in renewable numbers has been a goal of stem cell biologists since early 2000. Since that time, many studies have reported successful creation of glucose-responsive pancreatic beta-cells. Not until the more recent systematic application of developmental principles to stem cell biology systems were breakthroughs achieved on directed specification of the required early developmental intermediates. The most important first step is the formation of the definitive endoderm (DE) lineage which is compulsory for production of the epithelium of the pancreas and the other important endoderm-derived organs such as the liver, intestine and lung. The formation of DE from embryonic stem cells made possible additional experimentation aimed at directing the endoderm to further specified foregut and pancreatic endoderm lineages. With these discoveries came the first production of immature pancreatic endocrine cells. Most recently, the production in vivo of glucose-responsive insulin-producing cells with the capacity to correct Steptozotocin-induced hyperglycaemia in mice has been achieved. The work leading up to this achievement, in relation to the other principle human stem cell studies conducted in this area, will be briefly described. The necessary steps and ideal characteristics of embryonic stem cell-based differentiation to pancreatic beta-cells capable of glucose stimulated insulin secretion will be underscored.

Developmental biology of the pancreas: a comprehensive review

Pancreatic development represents a fascinating process in which two morphologically distinct tissue types must derive from one simple epithelium. These two tissue types, exocrine (including acinar cells, centro-acinar cells, and ducts) and endocrine cells serve disparate functions, and have entirely different morphology. In addition, the endocrine tissue must become disconnected from the epithelial lining during its development. The pancreatic development field has exploded in recent years, and numerous published reviews have dealt specifically with only recent findings, or specifically with certain aspects of pancreatic development. Here I wish to present a more comprehensive review of all aspects of pancreatic development, though still there is not a room for discussion of stem cell differentiation to pancreas, nor for discussion of post-natal regeneration phenomena, two important fields closely related to pancreatic development.

Retinoic acid promotes the generation of pancreatic endocrine progenitor cells and their further differentiation into beta-cells

The identification of secreted factors that can selectively stimulate the generation of insulin producing beta-cells from stem and/or progenitor cells represent a significant step in the development of stem cell-based beta-cell replacement therapy. By elucidating the molecular mechanisms that regulate the generation of beta-cells during normal pancreatic development such putative factors may be identified. In the mouse, beta-cells increase markedly in numbers from embryonic day (e) 14.5 and onwards, but the extra-cellular signal(s) that promotes the selective generation of beta-cells at these stages remains to be identified. Here we show that the retinoic acid (RA) synthesizing enzyme Raldh1 is expressed in developing mouse and human pancreas at stages when beta-cells are generated. We also provide evidence that RA induces the generation of Ngn3(+) endocrine progenitor cells and stimulates their further differentiation into beta-cells by activating a program of cell differentiation that recapitulates the normal temporal program of beta-cell differentiation.

Retinoic acid-induced inner ear teratogenesis caused by defective Fgf3/Fgf10-dependent Dlx5 signaling

BACKGROUND

Retinoic acid (RA) is essential for inner ear development. However, exposure to excess RA at a critical period leads to inner ear defects. These defects are associated with disruption in epithelial-mesenchymal interactions.

METHODS

This study investigates the role of Dlx5 in the epithelial-mesenchymal interactions that guide otic capsule chondrogenesis, as well as the effect of excess in utero RA exposure on Dlx5 expression in the developing mouse inner ear. Control of Dlx5 by Fgf3 and Fgf10 under excess RA conditions is investigated by examining the developmental window during which Fgf3 and Fgf10 are altered by in utero RA exposure and by testing the ability of Fgf3 and Fgf10 to mitigate the reduction in chondrogenesis and Dlx5 expression mediated by RA in high-density cultures of periotic mesenchyme containing otic epithelium, a model of epithelial-mesenchymal interactions in which chondrogenic differentiation of periotic mesenchyme ensues in response to induction by otic epithelium.

RESULTS

Dlx5 deletion alters expression of TGFbeta(1), important for otic capsule chondrogenesis, in the developing inner ear and compromises the ability of cultured periotic mesenchyme containing otic epithelium, harvested from Dlx5 null embryos, to differentiate into cartilage when compared with control cultures. Downregulation in Dlx5 ensues as a consequence of in utero RA exposure in association with inner ear dysmorphogenesis. This change in Dlx5 is noted at embryonic day 10.5 (E10.5), but not at E9.5, suggesting that Dlx5 is not a direct RA target. Before Dlx5 downregulation, Fgf3 and Fgf10 expression is modified in the inner ear by excess RA, with the ability of exogenous Fgf3 and Fgf10 to rescue chondrogenesis and Dlx5 expression in RA-treated cultures of periotic mesenchyme containing otic epithelium supporting these fibroblast growth factors (FGFs) as intermediary genes by which RA mediates its effects.

CONCLUSIONS

Disruption in an Fgf3, -10/Dlx5 signaling cascade is operant in molecular mechanisms of inner ear teratogenesis by excess RA.