Sox2 is required for maintenance and regeneration, but not initial development, of hair cells in the zebrafish inner ear

Transdifferentiation is uncoupled from progenitor pool expansion during hair cell regeneration in the zebrafish inner ear

Death of mechanosensory hair cells in the inner ear is a common cause of auditory and vestibular impairment in mammals, which have a limited ability to regrow these cells after damage. In contrast, non-mammalian vertebrates including zebrafish can robustly regenerate hair cells following severe organ damage. The zebrafish inner ear provides an understudied model system for understanding hair cell regeneration in organs that are highly conserved with their mammalian counterparts. Here we quantitatively examine hair cell addition during growth and regeneration of the larval zebrafish inner ear. We used a genetically encoded ablation method to induce hair cell death and observed gradual regeneration with correct spatial patterning over two weeks following ablation. Supporting cells, which surround and are a source of new hair cells, divide in response to hair cell ablation, expanding the possible progenitor pool. In parallel, nascent hair cells arise from direct transdifferentiation of progenitor pool cells uncoupled from progenitor division. These findings reveal a previously unrecognized mechanism of hair cell regeneration with implications for how hair cells may be encouraged to regenerate in the mammalian ear.

Hair Cell Regeneration: From Animals to Humans

Cochlear hair cells convert sound into electrical signals that are relayed via the spiral ganglion neurons to the central auditory pathway. Hair cells are vulnerable to damage caused by excessive noise, aging, and ototoxic agents. Non-mammals can regenerate lost hair cells by mitotic regeneration and direct transdifferentiation of surrounding supporting cells. However, in mature mammals, damaged hair cells are not replaced, resulting in permanent hearing loss. Recent studies have uncovered mechanisms by which sensory organs in non-mammals and the neonatal mammalian cochlea regenerate hair cells, and outlined possible mechanisms why this ability declines rapidly with age in mammals. Here, we review similarities and differences between avian, zebrafish, and mammalian hair cell regeneration. Moreover, we discuss advances and limitations of hair cell regeneration in the mature cochlea and their potential applications to human hearing loss.

Fgf, Hh, and pax2a differentially regulate expression of pax5 and pou3f3b in vestibular and auditory maculae in the zebrafish otic vesicle

BACKGROUND

The vertebrate inner ear contains distinct sensory epithelia specialized for auditory or vestibular function. In zebrafish, the first sensory epithelia form at opposite ends of the otic vesicle and are functionally distinct: the anterior utricular macula is essential for vestibular function whereas the posterior saccular macula is critical for hearing. Mechanisms distinguishing these maculae are not clear. Here, we examined the effects of manipulating Fgf or Hh on expression of pax5 and pou3f3b, unique markers of utricular and saccular identity. We also examined the roles of pax2a and atoh1a/b, early regulators of sensory specification.

RESULTS

fgf3 and fgf8a were uniquely required for pax5 and pou3f3b, respectively. Elevating Fgf or blocking Hh expanded expression of pax5 but repressed pou3f3b, while blocking Fgf had the opposite effect. Blocking sensory specification did not affect pax5 or pou3f3b, but both markers were lost in pax2a-/- mutants. Maintenance of pax2a expression requires Fgf, Hh and Pax2a itself.

CONCLUSION

Specification of utricular identity requires high Fgf and is repressed by Hh, whereas saccular identity requires Hh plus low Fgf. pax2a acts downstream of Fgf and Hh to maintain both fates. Comparison with mouse suggests this may reflect a broadly conserved developmental mechanism.

Spinal cord repair is modulated by the neurogenic factor Hb-egf under direction of a regeneration-associated enhancer

Unlike adult mammals, zebrafish regenerate spinal cord tissue and recover locomotor ability after a paralyzing injury. Here, we find that ependymal cells in zebrafish spinal cords produce the neurogenic factor Hb-egfa upon transection injury. Animals with hb-egfa mutations display defective swim capacity, axon crossing, and tissue bridging after spinal cord transection, associated with disrupted indicators of neuron production. Local recombinant human HB-EGF delivery alters ependymal cell cycling and tissue bridging, enhancing functional regeneration. Epigenetic profiling reveals a tissue regeneration enhancer element (TREE) linked to hb-egfa that directs gene expression in spinal cord injuries. Systemically delivered recombinant AAVs containing this zebrafish TREE target gene expression to crush injuries of neonatal, but not adult, murine spinal cords. Moreover, enhancer-based HB-EGF delivery by AAV administration improves axon densities after crush injury in neonatal cords. Our results identify Hb-egf as a neurogenic factor necessary for innate spinal cord regeneration and suggest strategies to improve spinal cord repair in mammals.

Loss of ndrg2 Function Is Involved in Notch Activation in Neuromast Hair Cell Regeneration in Zebrafish

The regeneration of hair cells in zebrafish is a complex process involving the precise regulation of multiple signaling pathways, but this complicated regulatory network is not fully understood. Current research has primarily focused on finding molecules and pathways that can regulate hair cell regeneration and restore hair cell functions. Here, we show the role of N-Myc downstream regulated gene 2 (ndrg2) in zebrafish hair cell regeneration. We first found that ndrg2 was dynamically expressed in neuromasts of the developing zebrafish, and this expression was increased after neomycin-induced hair cell damage. Then, ndrg2 loss-of-function larvae showed reduced numbers of regenerated hair cells but increased numbers of supporting cells after neomycin exposure. By in situ hybridization, we further observed that ndrg2 loss of function resulted in the activation of Notch signaling and downregulation of atoh1a during hair cell regeneration in vivo. Additionally, blocking Notch signaling rescued the number of regenerated hair cells in ndrg2-deficient larvae. Together, this study provides evidence for the role of ndrg2 in regulating hair cell regeneration in zebrafish neuromasts.

Exposure to Tetrabutylammonium Bromide Impairs Cranial Neural Crest Specification, Neurogenic Program, and Brain Morphogenesis

Tetrabutylammonium bromide (TBAB) is a widely used industrial reagent and is commonly found in our aquatic ecosystem as an industrial byproduct. In humans, the ingestion of TBAB causes severe neurological impairments and disorders such as vertigo, hallucinations, and delirium. Yet, the extent of environmental risk and TBAB toxicity to human health is poorly understood. In this study, we aim to determine the developmental toxicity of TBAB using zebrafish embryos as a model and provide novel insights into the mechanism of action of such chemicals on neurodevelopment and the overall embryonic program. Our results show that exposure to TBAB results in impaired development of the brain, inner ear, and pharyngeal skeletal elements in the zebrafish embryo. TBAB treatment resulted in aberrations in the specification of the neural crest precursors, hindbrain segmentation, and otic neurogenesis. TBAB treatment also induced a surge in apoptosis in the head, tail, and trunk regions of the developing embryo. Long-term TBAB exposure resulted in cardiac edema and craniofacial defects. Further, in silico molecular docking analysis indicated that TBAB binds to AMPA receptors and modulates neural developmental genes such as olfactomedin and acetylcholinesterase in the embryonic brain. To summarize, our study highlights the novel effects of TBAB on embryonic brain formation and segmentation, ear morphogenesis, and craniofacial skeletal development.

Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration in adult mice

Sox2 is a transcription factor that is necessary in the mammalian inner ear for development of sensory hair cells and supporting cells. Sox2 is expressed in supporting cells of adult mammals, but its function in this context is poorly understood. Given its role in the developing inner ear, we hypothesized that Sox2 is required in vestibular supporting cells for regeneration of type II hair cells after damage. Using adult mice, we deleted Sox2 from Sox9-CreER-expressing supporting cells prior to diphtheria toxin-mediated hair cell destruction and used fate-mapping to assess regeneration. In utricles of control mice with normal Sox2 expression, supporting cells regenerated nearly 200 hair cells by 3 weeks post-damage, which doubled by 12 weeks. In contrast, mice with Sox2 deletion from supporting cells had approximately 20 fate-mapped hair cells at 3 weeks post-damage, and this number did not change significantly by 12 weeks, indicating regeneration was dramatically curtailed. We made similar observations for saccules and ampullae. We found no evidence that supporting cells lacking Sox2 had altered cellular density, morphology, or ultrastructure. However, some Sox2-negative supporting cell nuclei appeared to migrate apically but did not turn on hair cell markers, and type I hair cell survival was higher. Sox2 heterozygotes also had reduced regeneration in utricles, but more hair cells were replaced than mice with Sox2 deletion. Our study determined that Sox2 is required in supporting cells for normal levels of vestibular hair cell regeneration but found no other major requirements for Sox2 in adult supporting cells.

Kölliker’s organ-supporting cells and cochlear auditory development

The Kölliker’s organ is a transient cellular cluster structure in the development of the mammalian cochlea. It gradually degenerates from embryonic columnar cells to cuboidal cells in the internal sulcus at postnatal day 12 (P12)–P14, with the cochlea maturing when the degeneration of supporting cells in the Kölliker’s organ is complete, which is distinct from humans because it disappears at birth already. The supporting cells in the Kölliker’s organ play a key role during this critical period of auditory development. Spontaneous release of ATP induces an increase in intracellular Ca²⁺ levels in inner hair cells in a paracrine form via intercellular gap junction protein hemichannels. The Ca²⁺ further induces the release of the neurotransmitter glutamate from the synaptic vesicles of the inner hair cells, which subsequently excite afferent nerve fibers. In this way, the supporting cells in the Kölliker’s organ transmit temporal and spatial information relevant to cochlear development to the hair cells, promoting fine-tuned connections at the synapses in the auditory pathway, thus facilitating cochlear maturation and auditory acquisition. The Kölliker’s organ plays a crucial role in such a scenario. In this article, we review the morphological changes, biological functions, degeneration, possible trans-differentiation of cochlear hair cells, and potential molecular mechanisms of supporting cells in the Kölliker’s organ during the auditory development in mammals, as well as future research perspectives.

Identification of an evolutionarily conserved domain in Neurod1 favouring enteroendocrine versus goblet cell fate

ARP/ASCL transcription factors are key determinants of cell fate specification in a wide variety of tissues, coordinating the acquisition of generic cell fates and of specific subtype identities. How these factors, recognizing highly similar DNA motifs, display specific activities, is not yet fully understood. To address this issue, we overexpressed different ARP/ASCL factors in zebrafish ascl1a-/- mutant embryos to determine which ones are able to rescue the intestinal secretory lineage. We found that Ascl1a/b, Atoh1a/b and Neurod1 factors are all able to trigger the first step of the secretory regulatory cascade but distinct secretory cells are induced by these factors. Indeed, Neurod1 rescues the enteroendocrine lineage while Ascl1a/b and Atoh1a/b rescue the goblet cells. Gain-of-function experiments with Ascl1a/Neurod1 chimeric proteins revealed that the functional divergence is encoded by a 19-aa ultra-conserved element (UCE), present in all Neurod members but absent in the other ARP/ASCL proteins. Importantly, inserting the UCE into the Ascl1a protein reverses the rescuing capacity of this Ascl1a chimeric protein that cannot rescue the goblet cells anymore but can efficiently rescue the enteroendocrine cells. This novel domain acts indeed as a goblet cell fate repressor that inhibits gfi1aa expression, known to be important for goblet cell differentiation. Deleting the UCE domain of the endogenous Neurod1 protein leads to an increase in the number of goblet cells concomitant with a reduction of the enteroendocrine cells, phenotype also observed in the neurod1 null mutant. This highlights the crucial function of the UCE domain for NeuroD1 activity in the intestine. As Gfi1 acts as a binary cell fate switch in several tissues where Neurod1 is also expressed, we can envision a similar role of the UCE in other tissues, allowing Neurod1 to repress Gfi1 to influence the balance between cell fates.

It is not yet clear how highly related factors like the ARP/Ascl factors display specific activities even though they recognize the same consensus DNA motif. This specificity could be provided by their cellular environment or by intrinsic properties of the factors themselves. To distinguish between these two possibilities, we have expressed several ARP/Ascl factors in the ascl1a-/- mutant to determine which ones are able to rescue the intestinal secretory defects. We found that Ascl1a/b and Atoh1a/b are able to rescue the goblet cells while Neurod1 rescues the enteroendocrine lineage. Furthermore, we show that the specific Neurod1 activity is conferred by the presence of a 19-aa ultra-conserved element (UCE), present in all vertebrate Neurod members but absent in all the other ARP/ASCL proteins. This UCE domain, so far uncharacterized, acts as a goblet cell fate repressor and inhibits gfi1aa expression, known to be important for goblet cell differentiation. Inserting the UCE into Ascl1a protein reverses the rescuing capacity of this chimeric protein that cannot rescue the goblet cells anymore but can efficiently rescue the enteroendocrine cells. This study therefore highlights an unique intrinsic property of Neurod1 allowing it to repress Gfi1 to influence the balance between cell fates. As Gfi1 acts as a binary cell fate switch in several tissues where Neurod1 is also expressed, we can envision a similar role of the UCE in other tissues, allowing Neurod1 to repress Gfi1 to influence the balance between cell fates.

Sox2 overexpression alleviates noise-induced hearing loss by inhibiting inflammation-related hair cell apoptosis

Background

The transcription factor Sox2 plays important roles in the developmental processes of multiple organs and tissues. However, whether Sox2 can protect mature or terminally differentiated cells against injury is still unknown.

Methods

We investigated the roles of Sox2 in cochlear hair cells, which are terminally differentiated cells, using conditional transgenic mice and several hearing loss models.

Results

Sox2 overexpression dramatically mitigated the degree of cochlear hair cell loss when exposed to ototoxic drugs. Noise-induced apoptosis of cochlear hair cells and hearing loss were also significantly alleviated by Sox2 overexpression. Notably, noise-induced upregulation of pro-inflammatory factors such as TNF-α and IL6 was inhibited by Sox2 overexpression. Then we used lipopolysaccharide to clarify the effect of Sox2 on cochlear inflammation, and Sox2 overexpression significantly inhibited lipopolysaccharide-induced upregulation of pro-inflammatory factors and alleviated inflammation-related cochlear hair cell death.

Conclusions

These results demonstrate a novel protective role of Sox2 in mature and terminally differentiated cochlear hair cells by inhibiting inflammation.

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.

Otic Neurogenesis in Xenopus laevis: Proliferation, Differentiation, and the Role of Eya1

Using immunostaining and confocal microscopy, we here provide the first detailed description of otic neurogenesis in Xenopus laevis. We show that the otic vesicle comprises a pseudostratified epithelium with apicobasal polarity (apical enrichment of Par3, aPKC, phosphorylated Myosin light chain, N-cadherin) and interkinetic nuclear migration (apical localization of mitotic, pH3-positive cells). A Sox3-immunopositive neurosensory area in the ventromedial otic vesicle gives rise to neuroblasts, which delaminate through breaches in the basal lamina between stages 26/27 and 39. Delaminated cells congregate to form the vestibulocochlear ganglion, whose peripheral cells continue to proliferate (as judged by EdU incorporation), while central cells differentiate into Islet1/2-immunopositive neurons from stage 29 on and send out neurites at stage 31. The central part of the neurosensory area retains Sox3 but stops proliferating from stage 33, forming the first sensory areas (utricular/saccular maculae). The phosphatase and transcriptional coactivator Eya1 has previously been shown to play a central role for otic neurogenesis but the underlying mechanism is poorly understood. Using an antibody specifically raised against Xenopus Eya1, we characterize the subcellular localization of Eya1 proteins, their levels of expression as well as their distribution in relation to progenitor and neuronal differentiation markers during otic neurogenesis. We show that Eya1 protein localizes to both nuclei and cytoplasm in the otic epithelium, with levels of nuclear Eya1 declining in differentiating (Islet1/2+) vestibulocochlear ganglion neurons and in the developing sensory areas. Morpholino-based knockdown of Eya1 leads to reduction of proliferating, Sox3- and Islet1/2-immunopositive cells, redistribution of cell polarity proteins and loss of N-cadherin suggesting that Eya1 is required for maintenance of epithelial cells with apicobasal polarity, progenitor proliferation and neuronal differentiation during otic neurogenesis.

An Integrated Perspective of Evolution and Development: From Genes to Function to Ear, Lateral Line and Electroreception

Four sensory systems (vestibular, lateral line, electroreception, auditory) are unique and project exclusively to the brainstem of vertebrates. All sensory neurons depend on a common set of genes (Eya1, Sox2, Neurog1, Neurod1) that project to a dorsal nucleus and an intermediate nucleus, which differentiate into the vestibular ear, lateral line and electroreception in vertebrates. In tetrapods, a loss of two sensory systems (lateral line, electroreception) leads to the development of a unique ear and auditory system in amniotes. Lmx1a/b, Gdf7, Wnt1/3a, BMP4/7 and Atoh1 define the lateral line, electroreception and auditory nuclei. In contrast, vestibular nuclei depend on Neurog1/2, Ascl1, Ptf1a and Olig3, among others, to develop an independent origin of the vestibular nuclei. A common origin of hair cells depends on Eya1, Sox2 and Atoh1, which generate the mechanosensory cells. Several proteins define the polarity of hair cells in the ear and lateral line. A unique connection of stereocilia requires CDH23 and PCDH15 for connections and TMC1/2 proteins to perceive mechanosensory input. Electroreception has no polarity, and a different system is used to drive electroreceptors. All hair cells function by excitation via ribbons to activate neurons that innervate the distinct target areas. An integrated perspective is presented to understand the gain and loss of different sensory systems.

Gonadotropin Releasing Hormone (Gnrh) Triggers Neurogenesis in the Hypothalamus of Adult Zebrafish

Recently, it has been shown in adult mammals that the hypothalamus can generate new cells in response to metabolic changes, and tanycytes, putative descendants of radial glia, can give rise to neurons. Previously we have shown in vitro that neurospheres generated from the hypothalamus of adult zebrafish show increased neurogenesis in response to exogenously applied hormones. To determine whether adult zebrafish have a hormone-responsive tanycyte-like population in the hypothalamus, we characterized proliferative domains within this region. Here we show that the parvocellular nucleus of the preoptic region (POA) labels with neurogenic/tanycyte markers vimentin, GFAP/Zrf1, and Sox2, but these cells are generally non-proliferative. In contrast, Sox2+ proliferative cells in the ventral POA did not express vimentin and GFAP/Zrf1. A subset of the Sox2+ cells co-localized with Fezf2:GFP, a transcription factor important for neuroendocrine cell specification. Exogenous treatments of GnRH and testosterone were assayed in vivo. While the testosterone-treated animals showed no significant changes in proliferation, the GnRH-treated animals showed significant increases in the number of BrdU-labeled cells and Sox2+ cells. Thus, cells in the proliferative domains of the zebrafish POA do not express radial glia (tanycyte) markers vimentin and GFAP/Zrf1, and yet, are responsive to exogenously applied GnRH treatment.

Transcription Factor Reprogramming in the Inner Ear: Turning on Cell Fate Switches to Regenerate Sensory Hair Cells

Non-mammalian vertebrates can restore their auditory and vestibular hair cells naturally by triggering the regeneration of adjacent supporting cells. The transcription factor ATOH1 is a key regulator of hair cell development and regeneration in the inner ear. Following the death of hair cells, supporting cells upregulate ATOH1 and give rise to new hair cells. However, in the mature mammalian cochlea, such natural regeneration of hair cells is largely absent. Transcription factor reprogramming has been used in many tissues to convert one cell type into another, with the long-term hope of achieving tissue regeneration. Reprogramming transcription factors work by altering the transcriptomic and epigenetic landscapes in a target cell, resulting in a fate change to the desired cell type. Several studies have shown that ATOH1 is capable of reprogramming cochlear non-sensory tissue into cells resembling hair cells in young animals. However, the reprogramming ability of ATOH1 is lost with age, implying that the potency of individual hair cell-specific transcription factors may be reduced or lost over time by mechanisms that are still not clear. To circumvent this, combinations of key hair cell transcription factors have been used to promote hair cell regeneration in older animals. In this review, we summarize recent findings that have identified and studied these reprogramming factor combinations for hair cell regeneration. Finally, we discuss the important questions that emerge from these findings, particularly the feasibility of therapeutic strategies using reprogramming factors to restore human hearing in the future.

Yap-lin28a axis targets let7-Wnt pathway to restore progenitors for initiating regeneration

The sox2 expressing (sox2⁺) progenitors in adult mammalian inner ear lose the capacity to regenerate while progenitors in the zebrafish lateral line are able to proliferate and regenerate damaged HCs throughout lifetime. To mimic the HC damage in mammals, we have established a zebrafish severe injury model to eliminate both progenitors and HCs. The atoh1a expressing (atoh1a⁺) HC precursors were the main population that survived post severe injury, and gained sox2 expression to initiate progenitor regeneration. In response to severe injury, yap was activated to upregulate lin28a transcription. Severe-injury-induced progenitor regeneration was disabled in lin28a or yap mutants. In contrary, overexpression of lin28a initiated the recovery of sox2⁺ progenitors. Mechanistically, microRNA let7 acted downstream of lin28a to activate Wnt pathway for promoting regeneration. Our findings that lin28a is necessary and sufficient to regenerate the exhausted sox2⁺ progenitors shed light on restoration of progenitors to initiate HC regeneration in mammals.

The Warburg Effect and lactate signaling augment Fgf-MAPK to promote sensory-neural development in the otic vesicle

Recent studies indicate that many developing tissues modify glycolysis to favor lactate synthesis (Agathocleous et al., 2012; Bulusu et al., 2017; Gu et al., 2016; Oginuma et al., 2017; Sá et al., 2017; Wang et al., 2014; Zheng et al., 2016), but how this promotes development is unclear. Using forward and reverse genetics in zebrafish, we show that disrupting the glycolytic gene phosphoglycerate kinase-1 (pgk1) impairs Fgf-dependent development of hair cells and neurons in the otic vesicle and other neurons in the CNS/PNS. Fgf-MAPK signaling underperforms in pgk1- / - mutants even when Fgf is transiently overexpressed. Wild-type embryos treated with drugs that block synthesis or secretion of lactate mimic the pgk1- / - phenotype, whereas pgk1- / - mutants are rescued by treatment with exogenous lactate. Lactate treatment of wild-type embryos elevates expression of Etv5b/Erm even when Fgf signaling is blocked. However, lactate’s ability to stimulate neurogenesis is reversed by blocking MAPK. Thus, lactate raises basal levels of MAPK and Etv5b (a critical effector of the Fgf pathway), rendering cells more responsive to dynamic changes in Fgf signaling required by many developing tissues.

The Requirement of Sox2 for the Spinal Cord Motor Neuron Development of Zebrafish

Sex-determining region Y box 2 (Sox2), expressed in neural tissues, plays an important role as a transcription factor not only in the pluripotency and proliferation of neuronal cells but also in the opposite function of cell differentiation. Nevertheless, how Sox2 is linked to motor neuron development remains unknown. Here, we showed that Sox2 was localized in the motor neurons of spinal cord by in situ hybridization and cell separation, which acted as a positive regulator of motor neuron development. The deficiency of Sox2 in zebrafish larvae resulted in abnormal PMN development, including truncated but excessively branched CaP axons, loss of MiP, and increase of undifferentiated neuron cells. Importantly, transcriptome analysis showed that Sox2-depleted embryos caused many neurogenesis, axonogenesis, axon guidance, and differentiation-related gene expression changes, which further support the vital function of Sox2 in motor neuron development. Taken together, these data indicate that Sox2 plays a crucial role in the motor neuron development by regulating neuron differentiation and morphology of neuron axons.

Increased Type I and Decreased Type II Hair Cells after Deletion of Sox2 in the Developing Mouse Utricle

The vestibular system of the inner ear contains Type I and Type II hair cells (HCs) generated from sensory progenitor cells; however, little is known about how the HC subtypes are formed. Sox2 (encoding SRY-box 2) is expressed in Type II, but not in Type I, HCs. The present study aimed to investigate the role of SOX2 in cell fate determination in Type I vs. Type II HCs. First, we confirmed that Type I HCs developed from Sox2-expressing cells through lineage tracing of Sox2-positive cells using a CAG-tdTomato reporter mouse crossed with a Sox2-CreER mouse. Then, Sox2 loss of function was induced in HCs, using Sox2flox transgenic mice crossed with a Gfi1-Cre driver mouse. Knockout of Sox2 in HCs increased the number of Type I HCs and decreased the number of Type II HCs, while the total number of HCs and Sox2-positive supporting cells did not change. In addition, the effect of Sox2-knockout persisted into adulthood, resulting in an increased number of Type I HCs. These results demonstrate that SOX2 plays a critical role in the determination of Type II vs. Type I HC fate. The results suggested that Sox2 is a potential target for generating Type I HCs, which may be important for regenerative strategies for balance disorders.

Reproductive-state dependent changes in saccular hair cell density of the vocal male plainfin midshipman fish

The plainfin midshipman fish (Porichthys notatus) is a nocturnal, seasonally breeding, intertidal-nesting teleost fish that produces social acoustic signals for intraspecific communication. Type I or "nesting" males produce agonistic and reproductive-related acoustic signals including a multiharmonic advertisement call during the summer breeding season. Previous work showed that type I male auditory sensitivity of the saccule, the primary midshipman auditory end organ, changes seasonally with reproductive state such that reproductive males become more sensitive and better suited than nonreproductive males to detect the dominant frequencies contained within type I vocalizations. Here, we examine whether reproductive type I males also exhibit reproductive-state dependent changes in hair cell (HC) density in the three putative auditory end organs (saccule, lagena, and utricle). We show that saccular HC density was greater in reproductive type I males compared to nonreproductive type I males, and that the increase in HC density occurs throughout the saccular epithelium in both the central and marginal epithelia regions. We also show as saccular HC density increases there is a concurrent decrease in saccular support cell (SC) density in reproductive type I males with no overall change in total cell density (i.e., HC + SC). In contrast, we did not observe any seasonal changes in HC density in the utricle or lagena between nonreproductive and reproductive type I males. In addition, we compare the saccular HC densities in reproductive type I males with that of reproductive females and show that females have greater saccular HC densities, which suggest a sexually dimorphic difference in HC receptor density between the two sexual phenotypes, at least during the summer breeding season.

Expression of Protein-Coding Gene Orthologs in Zebrafish and Mouse Inner Ear Non-sensory Supporting Cells

Non-mammalian vertebrates, including zebrafish, retain the ability to regenerate hair cells (HCs) due to unknown molecular mechanisms that regulate proliferation and conversion of non-sensory supporting cells (nsSCs) to HCs. This regenerative capacity is not conserved in mammals. Identification of uniquely expressed orthologous genes in zebrafish nsSCs may reveal gene candidates involved in the proliferation and transdifferentiation of zebrafish nsSCs to HCs in the inner ear. A list of orthologous protein-coding genes was generated based on an Ensembl Biomart comparison of the zebrafish and mouse genomes. Our previously published RNA-seq-based transcriptome datasets of isolated inner ear zebrafish nsSCs and HCs, and mouse non-sensory supporting pillar and Deiters’ cells, and HCs, were merged to analyze gene expression patterns between the two species. Out of 17,498 total orthologs, 11,752 were expressed in zebrafish nsSCs and over 10,000 orthologs were expressed in mouse pillar and Deiters’ cells. Differentially expressed genes common among the zebrafish nsSCs and mouse pillar and Deiters’ cells, compared to species-specific HCs, included 306 downregulated and 314 upregulated genes; however, over 1,500 genes were uniquely upregulated in zebrafish nsSCs. Functional analysis of genes uniquely expressed in nsSCs identified several transcription factors associated with cell fate determination, cell differentiation and nervous system development, indicating inherent molecular properties of nsSCs that promote self-renewal and transdifferentiation into new HCs. Our study provides a means of characterizing these orthologous genes, involved in proliferation and transdifferentiation of nsSCs to HCs in zebrafish, which may lead to identification of potential targets for HC regeneration in mammals.

Wnt/β-catenin interacts with the FGF pathway to promote proliferation and regenerative cell proliferation in the zebrafish lateral line neuromast

Wnt and FGF are highly conserved signaling pathways found in various organs and have been identified as important regulators of auditory organ development. In this study, we used the zebrafish lateral line system to study the cooperative roles of the Wnt and FGF pathways in regulating progenitor cell proliferation and regenerative cell proliferation. We found that activation of Wnt signaling induced cell proliferation and increased the number of hair cells in both developing and regenerating neuromasts. We further demonstrated that FGF signaling was critically involved in Wnt-regulated proliferation, and inhibition of FGF abolished the Wnt stimulation-mediated effects on cell proliferation, while activating FGF signaling with basic fibroblast growth factor (bFGF) led to a partial rescue of the proliferative failure and hair cell defects in the absence of Wnt activity. Whole-mount in situ hybridization analysis showed that the expression of several FGF pathway genes, including pea3 and fgfr1, was increased in neuromasts after treatment with the Wnt pathway inducer BIO. Interestingly, when SU5402 was used to inhibit FGF signaling, neuromast cells expressed much lower levels of the FGF receptor gene, fgfr1, but produced increased levels of Wnt target genes, including ctnnb1, ctnnb2, and tcf7l2, while bFGF treatment produced no alterations in the expression of those genes, suggesting that fgfr1 might restrict Wnt signaling in neuromasts during proliferation. In summary, our analysis demonstrates that both the Wnt and FGF pathways are tightly integrated to modulate the proliferation of progenitor cells during early neuromast development and regenerative cell proliferation after neomycin-induced injury in the zebrafish neuromast.

Studying sensory organs on the skin of zebrafish is revealing details of molecular signaling pathways that may be relevant to our own sensory systems, especially the hair cells of the ear. These cells have fine hair-like structures that move in response to sound waves and help generate electrical signals to the brain that result in perception of sound. Huawei Li and colleagues at Fudan University, Shanghai, China, studied the roles of two well-known cellular signaling pathways in regulating the proliferation of similar sensory hair cells in zebrafish, a commonly used model organism. These pathways involve cell surface proteins that interact with small extracellular molecules to stimulate molecular changes within cells. Learning how the pathways control hair cell generation and multiplication may assist modification of similar systems in humans to study and treat hearing loss.

Anteroposterior patterning of the zebrafish ear through Fgf- and Hh-dependent regulation of hmx3a expression

In the zebrafish, Fgf and Hh signalling assign anterior and posterior identity, respectively, to the poles of the developing ear. Mis-expression of fgf3 or inhibition of Hh signalling results in double-anterior ears, including ectopic expression of hmx3a. To understand how this double-anterior pattern is established, we characterised transcriptional responses in Fgf gain-of-signalling or Hh loss-of-signalling backgrounds. Mis-expression of fgf3 resulted in rapid expansion of anterior otic markers, refining over time to give the duplicated pattern. Response to Hh inhibition was very different: initial anteroposterior asymmetry was retained, with de novo duplicate expression domains appearing later. We show that Hmx3a is required for normal anterior otic patterning, and that otic patterning defects in hmx3a^-/- mutants are a close phenocopy to those seen in fgf3^-/- mutants. However, neither loss nor gain of hmx3a function was sufficient to generate full ear duplications. Using our data to infer a transcriptional regulatory network required for acquisition of otic anterior identity, we can recapitulate both the wild-type and the double-anterior pattern in a mathematical model.

Understanding how signalling molecules impart information to developing organ systems, and how this is interpreted through networks of gene activity, is a key goal of developmental genetic analysis. In the developing zebrafish inner ear, differences in gene expression arise between the anterior and posterior poles of the ear placode, ensuring that sensory structures in the ear develop in their correct positions. If signalling pathways are disrupted, a mirror-image ear can result, developing with two anterior poles. We have used genetic, pharmacological and mathematical modelling approaches to decipher the pathway of gene action required to specify anterior structures in the zebrafish ear. Patterns of gene expression are dynamic and plastic, with two different routes leading to the formation of duplicate anterior structures. Expression of the hmx3a gene is an early response to the anterior signalling molecule Fgf3, but is not sufficient to drive the formation of ectopic anterior structures at the posterior of the ear. The hmx3a gene codes for a protein that regulates other genes, and in humans, mutation of HMX genes results in diseases affecting inner ear function. Our work provides a framework for understanding the dynamics of early patterning events in the developing inner ear.

Is Alzheimer's Also a Stem Cell Disease? - The Zebrafish Perspective

Alzheimer's disease (AD) is the most common neurodegenerative disease and is the leading form of dementia. AD entails chronic inflammation, impaired synaptic integrity and reduced neurogenesis. The clinical and molecular onsets of the disease do not temporally overlap and the initiation phase of the cellular changes might start with a complex causativeness between chronic inflammation, reduced neural stem cell plasticity and neurogenesis. Although the immune and neuronal aspects in AD are well studied, the neural stem cell-related features are far less investigated. An intriguing question is, therefore, whether a stem cell can ever be made proliferative and neurogenic during the prevalent AD in the brain. Recent findings affirm this hypothesis and thus a plausible way to circumvent the AD phenotypes could be to mobilize the endogenous stem cells by enhancing their proliferative and neurogenic capacity as well as to provide the newborn neurons the potential to survive and integrate into the existing circuitry. To address these questions, zebrafish offers unprecedented information and tools, which can be effectively translated into mammalian experimental systems.

Modulating Cell Fate as a Therapeutic Strategy

In injured tissues, regeneration is often associated with cell fate plasticity, in that cells deviate from their normal lineage paths. It is becoming increasingly clear that this plasticity often creates alternative strategies to restore damaged or lost cells. Alternatively, cell fate plasticity is also part and parcel of pathologic tissue transformations that accompany disease. In this Perspective, we summarize a few illustrative examples of physiologic and aberrant cellular plasticity. Then, we speculate on how one could enhance endogenous plasticity to promote regeneration and reverse pathologic plasticity, perhaps inspiring interest in a new class of therapies targeting cell fate modulation.

NvERTx: a gene expression database to compare embryogenesis and regeneration in the sea anemone Nematostella vectensis

For over a century, researchers have been comparing embryogenesis and regeneration hoping that lessons learned from embryonic development will unlock hidden regenerative potential. This problem has historically been a difficult one to investigate because the best regenerative model systems are poor embryonic models and vice versa. Recently, however, there has been renewed interest in this question, as emerging models have allowed researchers to investigate these processes in the same organism. This interest has been further fueled by the advent of high-throughput transcriptomic analyses that provide virtual mountains of data. Here, we present Nematostella vectensis Embryogenesis and Regeneration Transcriptomics (NvERTx), a platform for comparing gene expression during embryogenesis and regeneration. NvERTx consists of close to 50 transcriptomic data sets spanning embryogenesis and regeneration in Nematostella These data were used to perform a robust de novo transcriptome assembly, with which users can search, conduct BLAST analyses, and plot the expression of multiple genes during these two developmental processes. The site is also home to the results of gene clustering analyses, to further mine the data and identify groups of co-expressed genes. The site can be accessed at http://nvertx.kahikai.org.

Inhibition of SoxB2 or HDACs suppresses Hydractinia head regeneration by affecting blastema formation

Regeneration has long been known to occur in the cnidarian Hydractinia. This process refers to its ability to regrow structures, i.e a head, lost by injury, a phenomenon that depends on the migration of proliferative cells to the site of injury, and the formation of a blastema, a mass of undifferentiated cells that will restore the missing head tissues. In our study, we showed that members of SoxB transcription factors and HDACs are involved in the regulation of Hydractinia neurogenesis in tissue homeostasis and regeneration. Particularly, we revealed that knockdown of SoxB2 or Hdac2 (a class I HDAC) knockdown, or inhibition of HDAC activity, suppress head regeneration. Here, we show that SoxB2 knockdown, or the inhibition of HDACs activity by TSA, a HDAC Class I and II inhibitor, interfere with head regeneration by affecting the migration of proliferative cells and the formation of a proliferative blastema.

Sox2 haploinsufficiency primes regeneration and Wnt responsiveness in the mouse cochlea

During development, Sox2 is indispensable for cell division and differentiation, yet its roles in regenerating tissues are less clear. Here, we used combinations of transgenic mouse models to reveal that Sox2 haploinsufficiency (Sox2haplo) increases rather than impairs cochlear regeneration in vivo. Sox2haplo cochleae had delayed terminal mitosis and ectopic sensory cells, yet normal auditory function. Sox2haplo amplified and expanded domains of damage-induced Atoh1+ transitional cell formation in neonatal cochlea. Wnt activation via β-catenin stabilization (β-cateninGOF) alone failed to induce proliferation or transitional cell formation. By contrast, β-cateninGOF caused proliferation when either Sox2haplo or damage was present, and transitional cell formation when both were present in neonatal, but not mature, cochlea. Mechanistically, Sox2haplo or damaged neonatal cochleae showed lower levels of Sox2 and Hes5, but not of Wnt target genes. Together, our study unveils an interplay between Sox2 and damage in directing tissue regeneration and Wnt responsiveness and thus provides a foundation for potential combinatorial therapies aimed at stimulating mammalian cochlear regeneration to reverse hearing loss in humans.

Loss of ZNF32 augments the regeneration of nervous lateral line system through negative regulation of SOX2 transcription

Human zinc finger protein 32 (ZNF32) is a Cys2-His2 zinc-finger transcription factor that plays an important role in cell fate, yet much of its function remains unknown. Here, we reveal that the zebrafish ZNF32 homologue zfZNF32 is expressed in the nervous system, particularly in the lateral line system. ZfZNF32 knock-out zebrafish (zfZNF^−/−) were generated using the CRISPR-associated protein 9 system. We found that the regenerative capacity of the lateral line system was increased in zfZNF^−/− upon hair cell damage compared with the wild type. Moreover, SOX2 was essential for the zfZNF32-dependent modulation of lateral line system regeneration. Mechanistic studies showed that ZNF32 suppressed SOX2 transcription by directly binding to a consensus sequence (5′-gcattt-32) in the SOX2 promoter. In addition, ZNF32 localizes to the nucleus, and we have identified that amino acids 1-169 (Aa 1-169) and each of three independent nuclear localization signals (NLSs) in ZNF32 are indispensable for ZNF32 nuclear trafficking. Mutating the NLSs disrupted the inhibitory effect of ZNF32 in SOX2 expression, highlighting the critical role of the NLSs in ZNF32 function. Our findings reveal a pivotal role for ZNF32 function in SOX2 expression and regeneration regulation.

sox2 and sox3 cooperate to regulate otic/epibranchial placode induction in zebrafish

Expression of sox3 is one of the earliest markers of Fgf-dependent otic/epibranchial placode induction. We report here that sox2 is also expressed in the early otic/epibranchial placode in zebrafish. To address functions of sox2 and sox3, we generated knockouts and heat shock-inducible transgenes. Mutant analysis, and low-level misexpression, showed that sox2 and sox3 act redundantly to establish a full complement of otic/epibranchial cells. Disruption of pax8, another early regulator, caused similar placodal deficiencies to sox3 mutants or pax8-sox3 double mutants, suggesting that sox3 and pax8 operate in the same pathway. High-level misexpression of sox2 or sox3 during early stages cell-autonomously blocked placode induction, whereas misexpression several hours later could not reverse placodal differentiation. In an assay for ectopic placode-induction, we previously showed that misexpression of fgf8 induces a high level of ectopic sox3, but not pax8. Partial knockdown of sox3 significantly enhanced ectopic induction of pax8, whereas full knockdown of sox3 inhibited this process. Together these findings show that sox2 and sox3 are together required for proper otic induction, but the level of expression must be tightly regulated to avoid suppression of differentiation and maintenance of pluripotency.

sox2 and sox3 Play unique roles in development of hair cells and neurons in the zebrafish inner ear

Formation of neural and sensory progenitors in the inner ear requires Sox2 in mammals, and in other species is thought to rely on both Sox2 and Sox3. How Sox2 and/or Sox3 promote different fates is poorly understood. Our mutant analysis in zebrafish showed that sox2 is uniquely required for sensory development while sox3 is uniquely required for neurogenesis. Moderate misexpression of sox2 during placodal stages led to development of otic vesicles with expanded sensory and reduced neurogenic domains. However, high-level misexpression of sox2 or sox3 expanded both sensory and neurogenic domains to fill the medial and lateral halves of the otic vesicle, respectively. Disruption of medial factor pax2a eliminated the ability of sox2/3 misexpression to expand sensory but not neurogenic domains. Additionally, mild misexpression of fgf8 during placodal development was sufficient to specifically expand the zone of prosensory competence. Later, cross-repression between atoh1a and neurog1 helps maintain the sensory-neural boundary, but unlike mouse this does not require Notch activity. Together, these data show that sox2 and sox3 exhibit intrinsic differences in promoting sensory vs. neural competence, but at high levels these factors can mimic each other to enhance both states. Regional cofactors like pax2a and fgf8 also modify sox2/3 functions.

Recovery of shape and size in a developing organ pair

BACKGROUND

Paired organs in animals are largely bilaterally symmetric despite inherent noise in most biological processes. How is precise organ shape and size achieved during development despite this noise? Examining paired organ development is a challenge because it requires repeated quantification of two structures in parallel within living embryos. Here we combine bilateral quantification of morphology through time with asymmetric perturbations to study regulation of organ shape, size, and symmetry in developing organ pairs.

RESULTS

We present quantitative live imaging tools to measure the shape and size of the developing inner ears on both the left and right side simultaneously over time. By quantifying variation between the left and right inner ear (intrinsic noise) and between different individuals (extrinsic noise), we find that initial variability decreases over time in normal development to achieve symmetry. Early asymmetry is increased by environmental stress, but symmetry is still recovered over subsequent developmental time. Using multiple unilateral perturbations including Fgf signaling and ultraviolet light, we find that growth can be adjusted to compensate for a range of initial size and shape differences.

CONCLUSIONS

We propose that symmetry in developmental systems does not emerge through precise deterministic bilateral development, but rather through feedback mechanisms that adjust morphogenesis rates to account for variation. Developmental Dynamics 246:451-465, 2016. © 2017 Wiley Periodicals, Inc.

Sox2 regulates Müller glia reprogramming and proliferation in the regenerating zebrafish retina via Lin28 and Ascl1a

Sox2 is a well-established neuronal stem cell-associated transcription factor that regulates neural development and adult neurogenesis in vertebrates, and is one of the critical genes used to reprogram differentiated cells into induced pluripotent stem cells. We examined if Sox2 was involved in the early reprogramming-like events that Müller glia undergo as they upregulate many pluripotency- and neural stem cell-associated genes required for proliferation in light-damaged adult zebrafish retinas. In the undamaged adult zebrafish retina, Sox2 is expressed in Müller glia and a subset of amacrine cells, similar to other vertebrates. Following 31 h of light damage, Sox2 expression significantly increased in proliferating Müller glia. Morpholino-mediated knockdown of Sox2 expression resulted in decreased numbers of proliferating Müller glia, while induced overexpression of Sox2 stimulated Müller glia proliferation in the absence of retinal damage. Thus, Sox2 is necessary and sufficient for Müller glia proliferation. We investigated the role of Wnt/β-catenin signaling, which is a known regulator of sox2 expression during vertebrate retinal development. While β-catenin 2, but not β-catenin 1, was necessary for Müller glia proliferation, neither β-catenin paralog was required for sox2 expression following retinal damage. Sox2 expression was also necessary for ascl1a (neurogenic) and lin28a (reprogramming) expression, but not stat3 expression following retinal damage. Furthermore, Sox2 was required for Müller glial-derived neuronal progenitor cell amplification and expression of the pro-neural marker Tg(atoh7:EGFP). Finally, loss of Sox2 expression prevented complete regeneration of cone photoreceptors. This study is the first to identify a functional role for Sox2 during Müller glial-based regeneration of the vertebrate retina.

Rapid, Dynamic Activation of Müller Glial Stem Cell Responses in Zebrafish

Purpose

Zebrafish neurons regenerate from Müller glia following retinal lesions. Genes and signaling pathways important for retinal regeneration in zebrafish have been described, but our understanding of how Müller glial stem cell properties are regulated is incomplete. Mammalian Müller glia possess a latent neurogenic capacity that might be enhanced in regenerative therapies to treat degenerative retinal diseases.

Methods

To identify transcriptional changes associated with stem cell properties in zebrafish Müller glia, we performed a comparative transcriptome analysis from isolated cells at 8 and 16 hours following an acute photic lesion, prior to the asymmetric division that produces retinal progenitors.

Results

We report a rapid, dynamic response of zebrafish Müller glia, characterized by activation of pathways related to stress, nuclear factor–κB (NF-κB) signaling, cytokine signaling, immunity, prostaglandin metabolism, circadian rhythm, and pluripotency, and an initial repression of Wnt signaling. When we compared publicly available transcriptomes of isolated mouse Müller glia from two retinal degeneration models, we found that mouse Müller glia showed evidence of oxidative stress, variable responses associated with immune regulation, and repression of pathways associated with pluripotency, development, and proliferation.

Conclusions

Categories of biological processes/pathways activated following photoreceptor loss in regeneration-competent zebrafish Müller glia, which distinguished them from mouse Müller glia in retinal degeneration models, included cytokine signaling (notably NF-κB), prostaglandin E2 synthesis, expression of core clock genes, and pathways/metabolic states associated with pluripotency. These regulatory mechanisms are relatively unexplored as potential mediators of stem cell properties likely to be important in Müller glial cells for successful retinal regeneration.

Proliferation-independent regulation of organ size by Fgf/Notch signaling

Organ morphogenesis depends on the precise orchestration of cell migration, cell shape changes and cell adhesion. We demonstrate that Notch signaling is an integral part of the Wnt and Fgf signaling feedback loop coordinating cell migration and the self-organization of rosette-shaped sensory organs in the zebrafish lateral line system. We show that Notch signaling acts downstream of Fgf signaling to not only inhibit hair cell differentiation but also to induce and maintain stable epithelial rosettes. Ectopic Notch expression causes a significant increase in organ size independently of proliferation and the Hippo pathway. Transplantation and RNASeq analyses revealed that Notch signaling induces apical junctional complex genes that regulate cell adhesion and apical constriction. Our analysis also demonstrates that in the absence of patterning cues normally provided by a Wnt/Fgf signaling system, rosettes still self-organize in the presence of Notch signaling.

DOI:http://dx.doi.org/10.7554/eLife.21049.001

FGFR1 is an adverse outcome indicator for luminal A breast cancers

Fibroblast growth factor receptor 1 (FGFR1) has been suggested to be the candidate gene for 8p11-12 amplification in breast cancer and its therapeutic/ prognostic value is explored. Most previous studies focused on FGFR1 gene amplification, which may not necessarily lead to protein expression. Therefore, analysis of protein level may have more clinical relevance. We evaluated FGFR1 expression in a large cohort of breast cancer by immunohistochemistry, correlated with the tumor clinic-pathologic features, biomarkers expression, and patient's survival. FGFR1 expression was associated mainly with luminal cancers, particularly luminal B subtype (23.5%; p < 0.001), and it also showed adverse prognostic impact on luminal A cancers. FGFR1 expression was associated with higher pN (p = 0.023), pT (p = 0.003) stages, lymphovascular invasion (p = 0.010), p-cadherin (p = 0.028), synaptophysin (p = 0.009) and SOX2 expression (p = 0.034) in luminal A cancers. FGFR1 expressing luminal A cancers showed a similar outcome as luminal B cancers. Multivariate Cox regression analysis demonstrated FGFR1 positive luminal A cancers to be an independently poor prognosticator for disease free survival in luminal cancers (hazard ratio = 3.341, p = 0.008). Thus FGFR1 could be useful in identifying the aggressive cases amongst heterogeneous luminal A cancers. Given the relevance of FGFR pathway in treatment resistance in luminal cancers, FGFR1 could be an important tumor biomarker and adverse prognostic factor potentially exploitable in the clinical management of luminal cancers.

Spemann organizer gene Goosecoid promotes delamination of neuroblasts from the otic vesicle

Neurons of the Statoacoustic Ganglion (SAG), which innervate the inner ear, originate as neuroblasts in the floor of the otic vesicle and subsequently delaminate and migrate toward the hindbrain before completing differentiation. In all vertebrates, locally expressed Fgf initiates SAG development by inducing expression of Neurogenin1 (Ngn1) in the floor of the otic vesicle. However, not all Ngn1-positive cells undergo delamination, nor has the mechanism controlling SAG delamination been elucidated. Here we report that Goosecoid (Gsc), best known for regulating cellular dynamics in the Spemann organizer, regulates delamination of neuroblasts in the otic vesicle. In zebrafish, Fgf coregulates expression of Gsc and Ngn1 in partially overlapping domains, with delamination occurring primarily in the zone of overlap. Loss of Gsc severely inhibits delamination, whereas overexpression of Gsc greatly increases delamination. Comisexpression of Ngn1 and Gsc induces ectopic delamination of some cells from the medial wall of the otic vesicle but with a low incidence, suggesting the action of a local inhibitor. The medial marker Pax2a is required to restrict the domain of gsc expression, and misexpression of Pax2a is sufficient to block delamination and fully suppress the effects of Gsc The opposing activities of Gsc and Pax2a correlate with repression or up-regulation, respectively, of E-cadherin (cdh1). These data resolve a genetic mechanism controlling delamination of otic neuroblasts. The data also elucidate a developmental role for Gsc consistent with a general function in promoting epithelial-to-mesenchymal transition (EMT).

Dual role for Sox2 in specification of sensory competence and regulation of Atoh1 function

The formation of inner ear sensory epithelia is believed to occur in two steps, initial specification of sensory competent (prosensory) regions followed by determination of specific cell-types, such as hair cells (HCs) and supporting cells. However, studies in which the HC determination factor Atoh1 was ectopically expressed in nonprosensory regions indicated that expression of Atoh1 alone is sufficient to induce HC formation suggesting that prosensory formation may not be a prerequisite for HC development. To test this hypothesis, interactions between Sox2 and Atoh1, which are required for prosensory and HC formation respectively, were examined. Forced expression of Atoh1 in nonprosensory cells resulted in transient expression of Sox2 prior to HC formation, suggesting that expression of Sox2 is required for formation of ectopic HCs. Moreover, Atoh1 overexpression failed to induce HC formation in Sox2 mutants, confirming that Sox2 is required for prosensory competence. To determine whether expression of Sox2 alone is sufficient to induce prosensory identity, Sox2 was transiently activated in a manner that mimicked endogenous expression. Following transient Sox2 activation, nonprosensory cells developed as HCs, a result that was never observed in response to persistent expression of Sox2. These results, suggest a dual role for Sox2 in inner ear formation. Initially, Sox2 is required to specify prosensory competence, but subsequent down-regulation of Sox2 must occur to allow Atoh1 expression, most likely through a direct interaction with the Atoh1 promoter. These results implicate Sox2-mediated changes in prosensory cells as an essential step in their ability to develop as HCs. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 3-13, 2017.

Insights into sensory hair cell regeneration from the zebrafish lateral line

Loss of mechanosensory hair cells in the inner ear leads to loss of hearing. In humans this results in permanent deafness, as mammals are largely unable to regenerate hair cells. In contrast, zebrafish robustly regenerate hair cells in the sensory lateral line and ear and recent gene expression and time-lapse analyses of cell behaviors at the single cell level have greatly advanced our understanding of the mechanisms responsible for hair cell regeneration. In the lateral line, hair cell regeneration is controlled via dynamic interactions between Notch and Wnt/β-catenin signaling, and likely also between Fgf and the retinoic acid signaling pathways. Less is known about what initiates regeneration and we discuss potential pathways that may trigger proliferation after hair cell damage.

Causes and Consequences of Sensory Hair Cell Damage and Recovery in Fishes

Sensory hair cells are the mechanotransductive receptors that detect gravity, sound, and vibration in all vertebrates. Damage to these sensitive receptors often results in deficits in vestibular function and hearing. There are currently two main reasons for studying the process of hair cell loss in fishes. First, fishes, like other non-mammalian vertebrates, have the ability to regenerate hair cells that have been damaged or lost via exposure to ototoxic chemicals or acoustic overstimulation. Thus, they are used as a biomedical model to understand the process of hair cell death and regeneration and find therapeutics that treat or prevent human hearing loss. Secondly, scientists and governmental natural resource managers are concerned about the potential effects of intense anthropogenic sounds on aquatic organisms, including fishes. Dr. Arthur N. Popper and his students, postdocs and research associates have performed pioneering experiments in both of these lines of fish hearing research. This review will discuss the current knowledge regarding the causes and consequences of both lateral line and inner ear hair cell damage in teleost fishes.

Retinoic Acid Signaling Mediates Hair Cell Regeneration by Repressing p27kip and sox2 in Supporting Cells

During development, otic sensory progenitors give rise to hair cells and supporting cells. In mammalian adults, differentiated and quiescent sensory cells are unable to generate new hair cells when these are lost due to various insults, leading to irreversible hearing loss. Retinoic acid (RA) has strong regenerative capacity in several organs, but its role in hair cell regeneration is unknown. Here, we use genetic and pharmacological inhibition to show that the RA pathway is required for hair cell regeneration in zebrafish. When regeneration is induced by laser ablation in the inner ear or by neomycin treatment in the lateral line, we observe rapid activation of several components of the RA pathway, with dynamics that position RA signaling upstream of other signaling pathways. We demonstrate that blockade of the RA pathway impairs cell proliferation of supporting cells in the inner ear and lateral line. Moreover, in neuromast, RA pathway regulates the transcription of p27(kip) and sox2 in supporting cells but not fgf3. Finally, genetic cell-lineage tracing using Kaede photoconversion demonstrates that de novo hair cells derive from FGF-active supporting cells. Our findings reveal that RA has a pivotal role in zebrafish hair cell regeneration by inducing supporting cell proliferation, and shed light on the underlying transcriptional mechanisms involved. This signaling pathway might be a promising approach for hearing recovery.

SIGNIFICANCE STATEMENT

Hair cells are the specialized mechanosensory cells of the inner ear that capture auditory and balance sensory input. Hair cells die after acoustic trauma, ototoxic drugs or aging diseases, leading to progressive hearing loss. Mammals, in contrast to zebrafish, lack the ability to regenerate hair cells. Here, we find that retinoic acid (RA) pathway is required for hair cell regeneration in vivo in the zebrafish inner ear and lateral line. RA pathway is activated very early upon hair cell loss, promotes cell proliferation of progenitor cells, and regulates two key genes, p27(kip) and sox2. Our results position RA as an essential signal for hair cell regeneration with relevance in future regenerative strategies in mammals.

Regeneration, Plasticity, and Induced Molecular Programs in Adult Zebrafish Brain

Regenerative capacity of the brain is a variable trait within animals. Aquatic vertebrates such as zebrafish have widespread ability to renew their brains upon damage, while mammals have—if not none—very limited overall regenerative competence. Underlying cause of such a disparity is not fully evident; however, one of the reasons could be activation of peculiar molecular programs, which might have specific roles after injury or damage, by the organisms that regenerate. If this hypothesis is correct, then there must be genes and pathways that (a) are expressed only after injury or damage in tissues, (b) are biologically and functionally relevant to restoration of neural tissue, and (c) are not detected in regenerating organisms. Presence of such programs might circumvent the initial detrimental effects of the damage and subsequently set up the stage for tissue redevelopment to take place by modulating the plasticity of the neural stem/progenitor cells. Additionally, if transferable, those “molecular mechanisms of regeneration” could open up new avenues for regenerative therapies of humans in clinical settings. This review focuses on the recent studies addressing injury/damage-induced molecular programs in zebrafish brain, underscoring the possibility of the presence of genes that could be used as biomarkers of neural plasticity and regeneration.

Methods for studying the zebrafish brain: past, present and future

The zebrafish (Danio rerio) is one of the most promising new model organisms. The increasing popularity of this amazing small vertebrate is evident from the exponentially growing numbers of research articles, funded projects and new discoveries associated with the use of zebrafish for studying development, brain function, human diseases and screening for new drugs. Thanks to the development of novel technologies, the range of zebrafish research is constantly expanding with new tools synergistically enhancing traditional techniques. In this review we will highlight the past and present techniques which have made, and continue to make, zebrafish an attractive model organism for various fields of biology, with a specific focus on neuroscience.

Inexhaustible hair-cell regeneration in young and aged zebrafish

Animals have evolved two general strategies to counter injury and maintain physiological function. The most prevalent is protection by isolating vital organs into body cavities. However, protection is not optimal for sensory systems because their external components need to be exposed to the environment to fulfill their receptive function. Thus, a common strategy to maintain sensory abilities against persistent environmental insult involves repair and regeneration. However, whether age or frequent injuries affect the regenerative capacity of sensory organs remains unknown. We have found that neuromasts of the zebrafish lateral line regenerate mechanosensory hair cells after recurrent severe injuries and in adulthood. Moreover, neuromasts can reverse transient imbalances of Notch signaling that result in defective organ proportions during repair. Our results reveal inextinguishable hair-cell regeneration in the lateral line, and suggest that the neuromast epithelium is formed by plastic territories that are maintained by continuous intercellular communication.

Sensory hair cell death and regeneration in fishes

Sensory hair cells are specialized mechanotransductive receptors required for hearing and vestibular function. Loss of hair cells in humans and other mammals is permanent and causes reduced hearing and balance. In the early 1980’s, it was shown that hair cells continue to be added to the inner ear sensory epithelia in cartilaginous and bony fishes. Soon thereafter, hair cell regeneration was documented in the chick cochlea following acoustic trauma. Since then, research using chick and other avian models has led to great insights into hair cell death and regeneration. However, with the rise of the zebrafish as a model organism for studying disease and developmental processes, there has been an increased interest in studying sensory hair cell death and regeneration in its lateral line and inner ears. Advances derived from studies in zebrafish and other fish species include understanding the effect of ototoxins on hair cells and finding otoprotectants to mitigate ototoxin damage, the role of cellular proliferation vs. direct transdifferentiation during hair cell regeneration, and elucidating cellular pathways involved in the regeneration process. This review will summarize research on hair cell death and regeneration using fish models, indicate the potential strengths and weaknesses of these models, and discuss several emerging areas of future studies.

Using zebrafish as the model organism to understand organ regeneration

The limited regenerative capacity of several organs, such as central nervous system (CNS), heart and limb in mammals makes related major diseases quite difficult to recover. Therefore, dissection of the cellular and molecular mechanisms underlying organ regeneration is of great scientific and clinical interests. Tremendous progression has already been made after extensive investigations using several model organisms for decades. Unfortunately, distance to the final achievement of the goal still remains. Recently, zebrafish became a popular model organism for the deep understanding of regeneration based on its powerful regenerative capacity, in particular the organs that are limitedly regenerated in mammals. Additionally, zebrafish are endowed with other advantages good for the study of organ regeneration. This review summarizes the recent progress in the study of zebrafish organ regeneration, in particular regeneration of fin, heart, CNS, and liver as the representatives. We also discuss reasons of the reduced regenerative capacity in higher vertebrate, the roles of inflammation during regeneration, and the difference between organogenesis and regeneration.

Tfap2a promotes specification and maturation of neurons in the inner ear through modulation of Bmp, Fgf and notch signaling

Neurons of the statoacoustic ganglion (SAG) transmit auditory and vestibular information from the inner ear to the hindbrain. SAG neuroblasts originate in the floor of the otic vesicle. New neuroblasts soon delaminate and migrate towards the hindbrain while continuing to proliferate, a phase known as transit amplification. SAG cells eventually come to rest between the ear and hindbrain before terminally differentiating. Regulation of these events is only partially understood. Fgf initiates neuroblast specification within the ear. Subsequently, Fgf secreted by mature SAG neurons exceeds a maximum threshold, serving to terminate specification and delay maturation of transit-amplifying cells. Notch signaling also limits SAG development, but how it is coordinated with Fgf is unknown. Here we show that transcription factor Tfap2a coordinates multiple signaling pathways to promote neurogenesis in the zebrafish inner ear. In both zebrafish and chick, Tfap2a is expressed in a ventrolateral domain of the otic vesicle that includes neurogenic precursors. Functional studies were conducted in zebrafish. Loss of Tfap2a elevated Fgf and Notch signaling, thereby inhibiting SAG specification and slowing maturation of transit-amplifying cells. Conversely, overexpression of Tfap2a inhibited Fgf and Notch signaling, leading to excess and accelerated SAG production. However, most SAG neurons produced by Tfap2a overexpression died soon after maturation. Directly blocking either Fgf or Notch caused less dramatic acceleration of SAG development without neuronal death, whereas blocking both pathways mimicked all observed effects of Tfap2a overexpression, including apoptosis of mature neurons. Analysis of genetic mosaics showed that Tfap2a acts non-autonomously to inhibit Fgf. This led to the discovery that Tfap2a activates expression of Bmp7a, which in turn inhibits both Fgf and Notch signaling. Blocking Bmp signaling reversed the effects of overexpressing Tfap2a. Together, these data support a model in which Tfap2a, acting through Bmp7a, modulates Fgf and Notch signaling to control the duration, amount and speed of SAG neural development.

Neurons of the statoacoustic ganglion (SAG) transmit impulses from the inner ear necessary for hearing and balance. SAG cells exhibit a complex pattern of development, regulation of which remains poorly understood. Here we show that transcription factor Tfap2a coordinates multiple cell signaling pathways needed to regulate the quantity and pace of SAG neuron production. SAG progenitors originate within the developing inner ear and then migrate out of the ear towards the hindbrain before forming mature neurons. We showed previously that Fgf initiates formation of SAG progenitors in the inner ear, but rising levels of Fgf signaling eventually terminate this process. Elevated Fgf also stimulates proliferation of SAG progenitors outside the ear and delays their maturation. Notch signaling is also known to limit SAG development. Tfap2a governs the strength of Fgf and Notch signaling by activating expression of Bmp7a, which inhibits Fgf and Notch. Together these signals stabilize the pool of SAG progenitors outside the ear by equalizing rates of maturation and proliferation. This balance is critical for sustained accumulation of SAG neurons during larval growth as well as regeneration following neural damage. These findings could inform development of stem cell therapies to correct auditory neuropathies in humans.