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

Comparative assessment of Fgf's diverse roles in inner ear development: A zebrafish perspective

Progress in understanding mechanisms of inner ear development has been remarkably rapid in recent years. The research community has benefited from the availability of several diverse model organisms, including zebrafish, chick, and mouse. The complexity of the inner ear has proven to be a challenge, and the complexity of the mammalian cochlea in particular has been the subject of intense scrutiny. Zebrafish lack a cochlea and exhibit a number of other differences from amniote species, hence they are sometimes seen as less relevant for inner ear studies. However, accumulating evidence shows that underlying cellular and molecular mechanisms are often highly conserved. As a case in point, consideration of the diverse functions of Fgf and its downstream effectors reveals many similarities between vertebrate species, allowing meaningful comparisons the can benefit the entire research community. In this review, I will discuss mechanisms by which Fgf controls key events in early otic development in zebrafish and provide direct comparisons with chick and mouse.

Cyp1B1 expression patterns in the developing chick inner ear

BACKGROUND

Retinoic acid (RA) plays an important role in organogenesis as a paracrine signal through transcriptional regulation of an increasing number of known downstream target genes, regulating cell proliferation, and differentiation. During the development of the inner ear, RA directly governs the morphogenesis and specification processes mainly by means of RA-synthesizing retinaldehyde dehydrogenase (RALDH) enzymes. Interestingly, CYP1B1, a cytochrome P450 enzyme, is able to mediate the oxidative metabolisms also leading to RA generation, its expression patterns being associated with many known sites of RA activity.

RESULTS

This study describes for the first time the presence of CYP1B1 in the developing chick inner ear as a RALDH-independent RA-signaling mechanism. In our in situ hybridization analysis, Cyp1B1 expression was first observed in a domain located in the ventromedial wall of the otic anlagen, being included within the rostralmost aspect of an Fgf10-positive pan-sensory domain. As development proceeds, all identified Fgf10-positive areas were Cyp1B1 stained, with all sensory patches being Cyp1B1 positive at stage HH34, except the macula neglecta.

CONCLUSIONS

Cyp1B1 expression suggested a possible contribution of CYP1B1 action in the specification of the lateral-to-medial and dorsal-to-ventral axes of the developing chick inner ear.

Fgf3 and Fgf16 expression patterns define spatial and temporal domains in the developing chick inner ear

The inner ear is a morphologically complex sensory structure with auditory and vestibular functions. The developing otic epithelium gives rise to neurosensory and non-sensory elements of the adult membranous labyrinth. Extrinsic and intrinsic signals manage the patterning and cell specification of the developing otic epithelium by establishing lineage-restricted compartments defined in turn by differential expression of regulatory genes. FGF3 and FGF16 are excellent candidates to govern these developmental events. Using the chick inner ear, we show that Fgf3 expression is present in the borders of all developing cristae. Strong Fgf16 expression was detected in a portion of the developing vertical and horizontal pouches, whereas the cristae show weaker or undetected Fgf16 expression at different developmental stages. Concerning the rest of the vestibular sensory elements, both the utricular and saccular maculae were Fgf3 positive. Interestingly, strong Fgf16 expression delimited these Fgf16-negative sensory patches. The Fgf3-negative macula neglecta and the Fgf3-positive macula lagena were included within weakly Fgf16-expressing areas. Therefore, different FGF-mediated mechanisms might regulate the specification of the anterior (utricular and saccular) and posterior (neglecta and lagena) maculae. In the developing cochlear duct, dynamic Fgf3 and Fgf16 expression suggests their cooperation in the early specification and later cell differentiation in the hearing system. The requirement of Fgf3 and Fgf16 genes in endolymphatic apparatus development and neurogenesis are discussed. Based on these observations, FGF3 and FGF16 seem to be key signaling pathways that control the inner ear plan by defining epithelial identities within the developing otic epithelium.

Gestational vitamin A deficiency: a novel cause of sensorineural hearing loss in the developing world?

Hearing loss is a substantial public health problem with profound social and economic consequences in the developing world. The World Health Organization (WHO) estimates that there are 360 million people living with disabling hearing loss globally, and 80% of these individuals are from low- and middle-income countries. The epidemiology of hearing impairment remains poorly defined in most impoverished societies. Middle ear infections in childhood are a key determinant; however, congenital anomalies may also comprise an important etiology and may arise from gestational malnutrition. While evidence exists that preventable vitamin A deficiency exacerbates the severity of ear infections and, consequently, hearing loss, antenatal vitamin A deficiency during sensitive periods of fetal development may represent an etiologically distinct and virtually unexplored causal pathway. Evidence from multiple animal systems clearly shows that fetal inner ear development requires adequate vitamin A nutriture to proceed normally. Inner ear malformations occur in experimentally imposed maternal vitamin A deficiency in multiple species in a dose-response manner. These anomalies are likely due to the loss of retinoic acid-dependent regulation of both hindbrain development and otic morphogenic processes. Based on in vivo evidence in experimental animals, we hypothesize that preventable gestational vitamin A deficiency, especially during early stages of fetal development, may predispose offspring to inner ear malformations and sensorineural hearing loss. As vitamin A deficiency affects an estimated 20 million pregnant women globally, we hypothesize that, in undernourished settings, routine provision of supplemental vitamin A at the recommended allowance throughout pregnancy may promote normal inner ear development and reduce risk of an as yet unknown fraction of sensorineural hearing loss. If our hypothesis proves correct, gestational vitamin A deficiency would represent a potentially preventable etiology of sensorineural hearing loss of substantial public health significance.

Fgf10 expression patterns in the developing chick inner ear

The inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. It originates from the otic placode, which invaginates, forming the otic vesicle; the latter gives rise to neurosensory and nonsensory elements of the adult membranous labyrinth. A hypothesis based on descriptive and experimental evidence suggests that the acquisition of discrete sensory patches during evolution of this primordium may be related to subdivision of an early pansensory domain. In order to gain insight into this developmental mechanism, we carried out a detailed analysis of the spatial and temporal expression pattern of the gene Fgf10, by comparing different markers of otic patterning and hair cell differentiation. Fgf10 expression labels a sensory-competent domain included in a Serrate-positive territory from which most of the sensory epithelia arise. Our data show that Fgf10 transcripts are present initially in a narrow ventromedial band of the rudimentary otocyst, extending between its rostral and caudal poles. During development, this Fgf10-expressing area splits repetitively into several separate subareas, creating six of the eight sensory organs present in birds. Only the lateral crista and the macula neglecta were initially Fgf10 negative, although they activated Fgf10 expression after their specification as sensory elements. These results allowed us to determine a timetable of sensory specification in the developing chick inner ear. The comparison of the expression pattern of Fgf10 with those of other markers of sensory differentiation contributes to our understanding of the mechanism by which vertebrate inner ear prosensory domains have arisen during evolution.

ISL1 directly regulates FGF10 transcription during human cardiac outflow formation

The LIM homeodomain gene Islet-1 (ISL1) encodes a transcription factor that has been associated with the multipotency of human cardiac progenitors, and in mice enables the correct deployment of second heart field (SHF) cells to become the myocardium of atria, right ventricle and outflow tract. Other markers have been identified that characterize subdomains of the SHF, such as the fibroblast growth factor Fgf10 in its anterior region. While functional evidence of its essential contribution has been demonstrated in many vertebrate species, SHF expression of Isl1 has been shown in only some models. We examined the relationship between human ISL1 and FGF10 within the embryonic time window during which the linear heart tube remodels into four chambers. ISL1 transcription demarcated an anatomical region supporting the conserved existence of a SHF in humans, and transcription factors of the GATA family were co-expressed therein. In conjunction, we identified a novel enhancer containing a highly conserved ISL1 consensus binding site within the FGF10 first intron. ChIP and EMSA demonstrated its direct occupation by ISL1. Transcription mediated by ISL1 from this FGF10 intronic element was enhanced by the presence of GATA4 and TBX20 cardiac transcription factors. Finally, transgenic mice confirmed that endogenous factors bound the human FGF10 intronic enhancer to drive reporter expression in the developing cardiac outflow tract. These findings highlight the interest of examining developmental regulatory networks directly in human tissues, when possible, to assess candidate non-coding regions that may be responsible for congenital malformations.

Retinoid signaling in inner ear development: A "Goldilocks" phenomenon

Retinoic acid (RA) is a biologically active derivative of vitamin A that is indispensable for inner ear development. The normal function of RA is achieved only at optimal homeostatic concentrations, with an excess or deficiency in RA leading to inner ear dysmorphogenesis. We present an overview of the role of RA in the developing mammalian inner ear, discussing both how and when RA may act to critically control a program of inner ear development. Molecular mechanisms of otic teratogenicity involving two members of the fibroblast growth factor family, FGF3 and FGF10, and their downstream targets, Dlx5 and Dlx6, are examined under conditions of both RA excess and deficiency. We term the effect of too little or too much RA on FGF/Dlx signaling a Goldilocks phenomenon. We demonstrate that in each case (RA excess, RA deficiency), RA can directly affect FGF3/FGF10 signaling within the otic epithelium, leading to downregulated expression of these essential signaling molecules, which in turn, leads to diminution in Dlx5/Dlx6 expression. Non-cell autonomous affects of the otic epithelium subsequently occur, altering transforming growth factor-beta (TGFβ) expression in the neighboring periotic mesenchyme and serving as a putative explanation for RA-mediated otic capsule defects. We conclude that RA coordinates inner ear morphogenesis by controlling an FGF/Dlx signaling cascade, whose perturbation by deviations in local retinoid concentrations can lead to inner ear dysmorphogenesis.