Regeneration in zebrafish lateral line neuromasts: Expression of the neural progenitor cell marker sox2 and proliferation-dependent and-independent mechanisms of hair cell renewal

The role of cilia in the development, survival, and regeneration of hair cells

Mutations impacting cilia genes lead to a class of human diseases known as ciliopathies. This is due to the role of cilia in the development, survival, and regeneration of many cell types. We investigated the extent to which disrupting cilia impacted these processes in lateral line hair cells of zebrafish. We found that mutations in two intraflagellar transport (IFT) genes, ift88 and dync2h1, which lead to the loss of kinocilia, caused increased hair cell apoptosis. IFT gene mutants also have a decreased mitochondrial membrane potential, and blocking the mitochondrial uniporter causes a loss of hair cells in wild-type zebrafish but not mutants, suggesting mitochondria dysfunction may contribute to the apoptosis seen in these mutants. These mutants also showed decreased proliferation during hair cell regeneration but did not show consistent changes in support cell number or proliferation during hair cell development. These results show that the loss of hair cells seen following disruption of cilia through either mutations in anterograde or retrograde IFT genes appears to be due to impacts on hair cell survival but not necessarily development in the zebrafish lateral line.

The TET-Sall4-BMP regulatory axis controls craniofacial cartilage development

Craniofacial microsomia (CFM) is a congenital defect that usually results from aberrant development of embryonic pharyngeal arches. However, the molecular basis of CFM pathogenesis is largely unknown. Here, we employ the zebrafish model to investigate mechanisms of CFM pathogenesis. In early embryos, tet2 and tet3 are essential for pharyngeal cartilage development. Single-cell RNA sequencing reveals that loss of Tet2/3 impairs chondrocyte differentiation due to insufficient BMP signaling. Moreover, biochemical and genetic evidence reveals that the sequence-specific 5mC/5hmC-binding protein, Sall4, binds the promoter of bmp4 to activate bmp4 expression and control pharyngeal cartilage development. Mechanistically, Sall4 directs co-phase separation of Tet2/3 with Sall4 to form condensates that mediate 5mC oxidation on the bmp4 promoter, thereby promoting bmp4 expression and enabling sufficient BMP signaling. These findings suggest the TET-BMP-Sall4 regulatory axis is critical for pharyngeal cartilage development. Collectively, our study provides insights into understanding craniofacial development and CFM pathogenesis.

HIF signaling overactivation inhibits lateral line neuromast development through Wnt in zebrafish

The lateral line is critical for prey detection, predator avoidance, schooling, and rheotaxis behavior in fish. As similar to hair cells in the mammalian inner ear, the lateral line sensory organ called neuromasts is a popular model for hair cell regeneration. However, the mechanism of lateral line development has not been fully understood. In this study, we showed for the first time that hypoxia-inducible factor (HIF) signaling is involved in lateral line development in zebrafish. hif1ab and epas1b were highly expressed in neuromasts during lateral line development. Hypoxia response induced by a prolyl hydroxylase domain-containing proteins (PHD) inhibitor treatment or vhl gene knockout significantly reduced hair cells and support cells in neuromast during lateral line development. In addition, inhibition of Hif-1α or Epas1 could partially rescue hair cells in the larvae with increased HIF activity, respectively. Moreover, the support cell proliferation and the expression of Wnt target genes decreased in vhl mutants which suggests that Wnt signaling mediated the role of HIF signaling in lateral line development. Collectively, our results demonstrate that HIF signaling overactivation inhibits lateral line development in zebrafish and suggest that inhibition of HIF signaling might be a potential therapeutic method for hair cell death.

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.

Neuromasts and Olfactory Organs of Zebrafish Larvae Represent Possible Sites of SARS-CoV-2 Pseudovirus Host Cell Entry

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the acute respiratory disease coronavirus disease 2019 (COVID-19), which has resulted in millions of deaths globally. Here, we explored the mechanism of host cell entry of a luciferase-ZsGreen spike (SARS-CoV-2)-pseudotyped lentivirus using zebrafish embryos/larvae as an in vivo model. Successful pseudovirus entry was demonstrated via the expression of the luciferase (luc) gene, which was validated by reverse transcription-PCR (RT-PCR). Treatment of larvae with chloroquine (a broad-spectrum viral inhibitor that blocks membrane fusion) or bafilomycin A1 (a specific inhibitor of vacuolar proton ATPases, which blocks endolysosomal trafficking) significantly reduced luc expression, indicating the possible involvement of the endolysosomal system in the viral entry mechanism. The pharmacological inhibition of two-pore channel (TPC) activity or use of the tpcn2^dhkz1a mutant zebrafish line also led to diminished luc expression. The localized expression of ACE2 and TPC2 in the anterior neuromasts and the forming olfactory organs was demonstrated, and the occurrence of endocytosis in both locations was confirmed. Together, our data indicate that zebrafish embryos/larvae are a viable and tractable model to explore the mechanism of SARS-CoV-2 host cell entry, that the peripheral sense organs are a likely site for viral host cell entry, and that TPC2 plays a key role in the translocation of the virus through the endolysosomal system. IMPORTANCE Despite the development of effective vaccines to combat the COVID-19 pandemic, which help prevent the most life-threatening symptoms, full protection cannot be guaranteed, especially with the emergence of new viral variants. Moreover, some resistance to vaccination remains in certain age groups and cultures. As such, there is an urgent need for the development of new strategies and therapies to help combat this deadly disease. Here, we provide compelling evidence that the peripheral sensory organs of zebrafish possess several key components required for SARS-CoV-2 host cell entry. The nearly transparent larvae provide a most amenable complementary platform to investigate the key steps of viral entry into host cells, as well as its spread through the tissues and organs. This will help in the identification of key viral entry steps for therapeutic intervention, provide an inexpensive model for screening novel antiviral compounds, and assist in the development of new and more effective vaccines.

Comparison of Aminoglycoside Antibiotics and Cobalt Chloride for Ablation of the Lateral Line System in Giant Danios

The mechanoreceptive lateral line system in fish is composed of neuromasts containing hair cells, which can be temporarily ablated by aminoglycoside antibiotics and heavy metal ions. These chemicals have been used for some time in studies exploring the functional role of the lateral line system in many fish species. However, little information on the relative effectiveness and rate of action of these chemicals for ablation is available. In particular, aminoglycoside antibiotics are thought to affect canal neuromasts, which sit in bony or trunk canals, differently from superficial neuromasts, which sit directly on the skin. This assumed ablation pattern has not been fully quantified for commonly used lateral line ablation agents. This study provides a detailed characterization of the effects of two aminoglycoside antibiotics, streptomycin sulfate and neomycin sulfate, and a heavy metal salt, cobalt (II) chloride hexahydrate (CoCl2), on the ablation of hair cells in canal and superficial neuromasts in the giant danio (Devario aequipinnatus) lateral line system, as a model for adult teleost fishes. We also quantified the regeneration of hair cells after ablation using CoCl2 and gentamycin sulfate to verify the time course to full recovery, and whether the ablation method affects the recovery time. Using a fluorescence stain, 4-Di-2-ASP, we verified the effectiveness of each chemical by counting the number of fluorescing canal and superficial neuromasts present throughout the time course of ablation and regeneration of hair cells. We found that streptomycin and neomycin were comparably effective at ablating all neuromasts in less than 12 h using a 250 μM dosage and in less than 8 h using a 500 μM dosage. The 500 μM dosage of either streptomycin or neomycin can ablate hair cells in superficial neuromasts within 2–4 h, while leaving those in canal neuromasts mostly intact. CoCl2 (0.1 mM) worked the fastest, ablating all of the hair cells in less than 6 h. Complete regeneration of the neuromasts in the lateral line system took 7 days regardless of chemicals used to ablate the hair cells. This study adds to the growing knowledge in hearing research about how effective specific chemicals are at ablating hair cells in the acoustic system of vertebrates.

Single-cell transcriptome analysis reveals three sequential phases of gene expression during zebrafish sensory hair cell regeneration

Loss of sensory hair cells (HCs) in the mammalian inner ear leads to permanent hearing and vestibular defects, whereas loss of HCs in zebrafish results in their regeneration. We used single-cell RNA sequencing (scRNA-seq) to characterize the transcriptional dynamics of HC regeneration in zebrafish at unprecedented spatiotemporal resolution. We uncovered three sequentially activated modules: first, an injury/inflammatory response and downregulation of progenitor cell maintenance genes within minutes after HC loss; second, the transient activation of regeneration-specific genes; and third, a robust re-activation of developmental gene programs, including HC specification, cell-cycle activation, ribosome biogenesis, and a metabolic switch to oxidative phosphorylation. The results are relevant not only for our understanding of HC regeneration and how we might be able to trigger it in mammals but also for regenerative processes in general. The data are searchable and publicly accessible via a web-based interface.

Evaluating the Death and Recovery of Lateral Line Hair Cells Following Repeated Neomycin Treatments

Acute chemical ablation of lateral line hair cells is an important tool to understand lateral line-mediated behaviors in free-swimming fish larvae and adults. However, lateral line-mediated behaviors have not been described in fish larvae prior to swim bladder inflation, possibly because single doses of ototoxin do not effectively silence lateral line function at early developmental stages. To determine whether ototoxins can disrupt lateral line hair cells during early development, we repeatedly exposed zebrafish larvae to the ototoxin neomycin during a 36 h period from 3 to 4 days post-fertilization (dpf). We use simultaneous transgenic and vital dye labeling of hair cells to compare 6-h and 12-h repeated treatment timelines and neomycin concentrations between 0 and 400 µM in terms of larval survival, hair cell death, regeneration, and functional recovery. Following exposure to neomycin, we find that the emergence of newly functional hair cells outpaces cellular regeneration, likely due to the maturation of ototoxin-resistant hair cells that survive treatment. Furthermore, hair cells of 4 dpf larvae exhibit faster recovery compared to 3 dpf larvae. Our data suggest that the rapid functional maturation of ototoxin-resistant hair cells limits the effectiveness of chemical-based methods to disrupt lateral line function. Furthermore, we show that repeated neomycin treatments can continually ablate functional lateral line hair cells between 3 and 4 dpf in larval zebrafish.

Evaluation of the hair cell regeneration and claudin b and phoenix gene expression during exposure to low concentrations of cadmium and zinc in early developing zebrafish larvae

Zebrafish possess hair cells on the body surface similar to that of mammals inner hear, in particular in the neuromasts, and due to its ability in regenerating damaged hair cells, is regularly used as a powerful animal model to study in vivo cytotoxicity. Among the factors leading to hair cell disruption, metal ions are of particular concern since they are important environmental pollutants. To date, several studies on zebrafish hair cell regeneration after metal exposure exist, while no data on regeneration during continuous metal exposure are available. In the present study, neuromast hair cell disruption and regeneration were assessed in zebrafish larvae for the first time during zinc (Zn) and cadmium (Cd) continuous exposure and a visual and molecular approach was adopted. Fluorescent vital dye DASPEI was used to assess hair cell regeneration and the gene expression of claudin b (cldnb) and phoenix (pho), was analyzed. Metallotionein-2 (mt2) gene expression was used as standard molecular marker of metal toxicity and confirmed the higher toxicity of Cd compared to Zn. In addition, Cd caused a delay in hair cell regeneration compared to Zn. Molecular analysis showed cldnb gene expression increased in relation to the metal concentrations used, confirming the involvement of this gene in hair cell regeneration. On the contrary, a dramatic decrease of pho gene expression was observed in Cd exposed groups, suggesting a negative impact of Cd on pho expression, thus negatively interfering with hair cell regeneration in zebrafish larvae exposed to this metal.

Imaging cytoplasmic lipid droplets in vivo with fluorescent perilipin 2 and perilipin 3 knock-in zebrafish

Cytoplasmic lipid droplets are highly dynamic storage organelles that are critical for cellular lipid homeostasis. While the molecular details of lipid droplet dynamics are a very active area of investigation, this work has been primarily performed in cultured cells. Taking advantage of the powerful transgenic and in vivo imaging opportunities available in zebrafish, we built a suite of tools to study lipid droplets in real time from the subcellular to the whole organism level. Fluorescently tagging the lipid droplet-associated proteins, perilipin 2 and perilipin 3, in the endogenous loci permits visualization of lipid droplets in the intestine, liver, and adipose tissue. Using these tools, we found that perilipin 3 is rapidly loaded on intestinal lipid droplets following a high-fat meal and later replaced by perilipin 2. These powerful new tools will facilitate studies on the role of lipid droplets in different tissues, under different genetic and physiological manipulations, and in a variety of human disease models.

Functional development and regeneration of hair cells in the zebrafish lateral line

Hair cells are mechanosensory receptors responsible for transducing auditory and vestibular information into electrical signals, which are then transmitted with remarkable precision to afferent neurons. Different from mammals, the hair cells of lower vertebrates, including those present in the neuromasts of the zebrafish lateral line, regenerate following environmental or chemical insults. Here we investigate the time-course of regeneration of hair cells in vivo using electrophysiology, 2-photon imaging and immunostaining applied to wild-type and genetically-encoded fluorescent indicator zebrafish lines. Functional hair cells drive spontaneous action potentials in the posterior lateral line afferent fibres, the frequency of which progressively increases over the first 10-days post-fertilization (dpf). Higher firing-rate fibres are only observed from ~6 dpf. Following copper treatment, newly formed hair cells become functional and are able to drive APs in the afferent fibres within 48 hours in both early-larval (≤8 dpf) and late-larval (12-17 dpf) zebrafish. However, the complete functional regeneration of the entire neuromast is delayed in late-larval compared to early-larval zebrafish. We propose that while individual regenerating hair cells can rapidly become active, the acquisition of fully functional neuromasts progresses faster at early-larval stages, a time when hair cells are still under development. At both ages, the afferent terminals in the regenerating neuromast appear to make initial contact with supporting cells. The ablation of the lateral line afferent neurons prevents the timely regeneration of supporting cells and hair cells. These findings indicate that the afferent system is likely to facilitate or promote the neuromast regeneration process.

Loss of vacuolar-type H+-ATPase induces caspase-independent necrosis-like death of hair cells in zebrafish neuromasts

The vacuolar-type H+-ATPase (V-ATPase) is a multi-subunit proton pump that regulates cellular pH. V-ATPase activity modulates several cellular processes, but cell-type-specific functions remain poorly understood. Patients with mutations in specific V-ATPase subunits can develop sensorineural deafness, but the underlying mechanisms are unclear. Here, we show that V-ATPase mutations disrupt the formation of zebrafish neuromasts, which serve as a model to investigate hearing loss. V-ATPase mutant neuromasts are small and contain pyknotic nuclei that denote dying cells. Molecular markers and live imaging show that loss of V-ATPase induces mechanosensory hair cells in neuromasts, but not neighboring support cells, to undergo caspase-independent necrosis-like cell death. This is the first demonstration that loss of V-ATPase can lead to necrosis-like cell death in a specific cell type in vivo. Mechanistically, loss of V-ATPase reduces mitochondrial membrane potential in hair cells. Modulating the mitochondrial permeability transition pore, which regulates mitochondrial membrane potential, improves hair cell survival. These results have implications for understanding the causes of sensorineural deafness, and more broadly, reveal functions for V-ATPase in promoting survival of a specific cell type in vivo.

Neurogenesis Potential Evaluation and Transcriptome Analysis of Fetal Hypothalamic Neural Stem/Progenitor Cells With Prenatal High Estradiol Exposure

High maternal estradiol is reported to induce metabolic disorders by modulating hypothalamic gene expression in offspring. Since neurogenesis plays a crucial role during hypothalamus development, we explored whether prenatal high estradiol exposure (HE) affects proliferation and differentiation of fetal hypothalamic neural stem/progenitor cells (NSC/NPCs) in mice and performed RNA sequencing to identify the critical genes involved. NSC/NPCs in HE mice presented attenuated cell proliferation but increased neuronal differentiation in vitro compared with control (NC) cells. Gene set enrichment analysis of mRNA profiles indicated that genes downregulated in HE NSC/NPCs were enriched in neurogenesis-related Gene Ontology (GO) terms, while genes upregulated in HE NSC/NPCs were enriched in response to estradiol. Protein-protein interaction analysis of genes with core enrichment in GO terms of neurogenesis and response to estradiol identified 10 Hub mRNAs, among which three were potentially correlated with six differentially expressed (DE) lncRNAs based on lncRNA profiling and co-expression analysis. These findings offer important insights into developmental modifications in hypothalamic NSC/NPCs and may provide new clues for further investigation on maternal environment programmed neural development disorders.

Emx2 regulates hair cell rearrangement but not positional identity within neuromasts

Each hair cell (HC) precursor of zebrafish neuromasts divides to form two daughter HCs of opposite hair bundle orientations. Previously, we showed that transcription factor Emx2, expressed in only one of the daughter HCs, generates this bidirectional HC pattern (Jiang et al., 2017). Here, we asked whether Emx2 mediates this effect by changing location of hair bundle establishment or positions of HCs since daughter HCs are known to switch positions with each other. We showed this HC rearrangement, redefined as two processes named Rock and Roll, is required for positional acquisition of HCs. Apical protrusion formation of nascent HCs and planar polarity signaling are both important for the Rock and Roll. Emx2 facilitates Rock and Roll by delaying apical protrusion of its nascent HCs but it does not determine HCs' ultimate positions, indicating that Emx2 mediates bidirectional HC pattern by changing the location where hair bundle is established in HCs.

Multimodal mechanosensing enables treefrog embryos to escape egg-predators

Mechanosensory-cued hatching (MCH) is widespread, diverse and important for survival in many animals. From flatworms and insects to frogs and turtles, embryos use mechanosensory cues and signals to inform hatching timing, yet mechanisms mediating mechanosensing in ovo are largely unknown. The arboreal embryos of red-eyed treefrogs, Agalychnis callidryas, hatch prematurely to escape predation, cued by physical disturbance in snake attacks. When otoconial organs in the developing vestibular system become functional, this response strengthens, but its earlier occurrence indicates another sensor must contribute. Post-hatching, tadpoles use lateral line neuromasts to detect water motion. We ablated neuromast function with gentamicin to assess their role in A. callidryas' hatching response to disturbance. Prior to vestibular function, this nearly eliminated the hatching response to a complex simulated attack cue, egg jiggling, revealing that neuromasts mediate early MCH. Vestibular function onset increased hatching, independent of neuromast function, indicating young embryos use multiple mechanosensory systems. MCH increased developmentally. All older embryos hatched in response to egg jiggling, but neuromast function reduced response latency. In contrast, neuromast ablation had no effect on the timing or level of hatching in motion-only vibration playbacks. It appears only a subset of egg-disturbance cues stimulate neuromasts; thus, embryos in attacked clutches may receive unimodal or multimodal stimuli. Agalychnis callidryas embryos have more neuromasts than described for any other species at hatching, suggesting precocious sensory development may facilitate MCH. Our findings provide insight into the behavioral roles of two mechanosensory systems in ovo and open possibilities for exploring sensory perception across taxa in early life stages.

Cisplatin-induced hair cell loss in zebrafish neuromasts is accompanied by protein nitration and Lmo4 degradation

Generation of reactive oxygen species, a critical factor in cisplatin-induced ototoxicity, leads to the formation of peroxynitrite, which in turn results in the nitration of susceptible proteins. Previous studies indicated that LMO4, a transcriptional regulator, is the most abundantly nitrated cochlear protein after cisplatin treatment and that LMO4 nitration facilitates ototoxicity in rodents. However, the role of this mechanism in regulating cisplatin-induced hair cell loss in non-mammalian models is unknown. As the mechanosensory hair cells in the neuromasts of zebrafish share many features with mammalian inner ear and is a good model for studying ototoxicity, we hypothesized that cisplatin treatment induces protein nitration and Lmo4 degradation in zebrafish hair cells, thereby facilitating hair cell loss. Immunostaining with anti-parvalbumin revealed a significant decrease in the number of hair cells in the neuromast of cisplatin treated larvae. In addition, cisplatin treatment induced a significant decrease in the expression of Lmo4 protein and a significant increase in nitrotyrosine levels, in the hair cells. The cisplatin-induced changes in Lmo4 and nitrotyrosine levels strongly correlated with hair cell loss, implying a potential link. Furthermore, a significant increase in the expression of activated Caspase-3 in zebrafish hair cells, post cisplatin treatment, suggested that cisplatin-induced decrease in Lmo4 levels is accompanied by apoptosis. These findings suggest that nitrative stress and Lmo4 degradation are important factors in cisplatin-induced hair cell loss in zebrafish neuromasts and that zebrafish could be used as a model to screen the otoprotective efficacy of compounds that inhibit protein nitration.

An image-based data-driven analysis of cellular architecture in a developing tissue

Quantitative microscopy is becoming increasingly crucial in efforts to disentangle the complexity of organogenesis, yet adoption of the potent new toolbox provided by modern data science has been slow, primarily because it is often not directly applicable to developmental imaging data. We tackle this issue with a newly developed algorithm that uses point cloud-based morphometry to unpack the rich information encoded in 3D image data into a straightforward numerical representation. This enabled us to employ data science tools, including machine learning, to analyze and integrate cell morphology, intracellular organization, gene expression and annotated contextual knowledge. We apply these techniques to construct and explore a quantitative atlas of cellular architecture for the zebrafish posterior lateral line primordium, an experimentally tractable model of complex self-organized organogenesis. In doing so, we are able to retrieve both previously established and novel biologically relevant patterns, demonstrating the potential of our data-driven approach.

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.

A model for reticular dysgenesis shows impaired sensory organ development and hair cell regeneration linked to cellular stress

Mutations in the gene AK2 are responsible for reticular dysgenesis (RD), a rare and severe form of primary immunodeficiency in children. RD patients have a severely shortened life expectancy and without treatment die, generally from sepsis soon after birth. The only available therapeutic option for RD is hematopoietic stem cell transplantation (HSCT). To gain insight into the pathophysiology of RD, we previously created zebrafish models for Ak2 deficiencies. One of the clinical features of RD is hearing loss, but its pathophysiology and causes have not been determined. In adult mammals, sensory hair cells of the inner ear do not regenerate; however, their regeneration has been observed in several non-mammalian vertebrates, including zebrafish. Therefore, we used our RD zebrafish models to determine whether Ak2 deficiency affects sensory organ development and/or hair cell regeneration. Our studies indicated that Ak2 is required for the correct development, survival and regeneration of sensory hair cells. Interestingly, Ak2 deficiency induces the expression of several oxidative stress markers and it triggers an increased level of cell death in the hair cells. Finally, we show that glutathione treatment can partially rescue hair cell development in the sensory organs in our RD models, pointing to the potential use of antioxidants as a therapeutic treatment supplementing HSCT to prevent or ameliorate sensorineural hearing deficits in RD patients.

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.

Yap1 promotes proliferation of transiently amplifying stress erythroid progenitors during erythroid regeneration

In contrast to steady-state erythropoiesis, which generates new erythrocytes at a constant rate, stress erythropoiesis rapidly produces a large bolus of new erythrocytes in response to anemic stress. In this study, we illustrate that Yes-associated protein (Yap1) promotes the rapid expansion of a transit-amplifying population of stress erythroid progenitors in vivo and in vitro. Yap1-mutated erythroid progenitors failed to proliferate in the spleen after transplantation into lethally irradiated recipient mice. Additionally, loss of Yap1 impaired the growth of actively proliferating erythroid progenitors in vitro. This role in proliferation is supported by gene expression profiles showing that transiently amplifying stress erythroid progenitors express high levels of genes associated with Yap1 activity and genes induced by Yap1. Furthermore, Yap1 promotes the proliferation of stress erythroid progenitors in part by regulating the expression of key glutamine-metabolizing enzymes. Thus, Yap1 acts as an erythroid regulator that coordinates the metabolic status with the proliferation of erythroid progenitors to promote stress erythropoiesis.

Wnt/β-catenin signaling was activated in supporting cells during exposure of the zebrafish lateral line to cisplatin

Zebrafish lateral line neuromasts are composed of central hair cells surrounded by supporting cells. Cisplatin is a common anticancer drug, with hair cell disruption being a frequent side effect of this drug. In our study, we observed complete functional hair cell loss after six hours of cisplatin insult in neuromasts, as demonstrated by anti-parvalbumin 3 immunofluorescence staining or YO-PRO1 vital dye staining. Time course analysis of neuromast hair cell regeneration showed that regenerated hair cells first appeared between 12 and 24h after damage, and the abundance of these cells increased stepwise with recovery time. After 72h, 90% of the hair cells were regenerated, and after 84h, the number of regenerated hair cells was comparable to the number of neuromast hair cells before treatment. The expression pattern of slc17a8 also showed that hair cells were regenerated after cisplatin exposure. Meanwhile, peripheral supporting cells moved toward the center of the neuromasts, as shown by the in situ expression pattern of sox21a. Increased hair cell progenitor formation was also observed, as demonstrated by the in situ expression pattern of atoh1a. Furthermore, we detected increased expression of wnt2, wnt3a, and ctnnb1 in sorted supporting cells from the sqet10 transgenic line, which labels neuromast supporting cells specifically. In situ hybridization analysis also showed decreased expression of dkk1a and dkk2. Regenerated hair cells were inhibited by early inhibition of Wnt/β-catenin signaling. Taken together, the results presented here showed that Wnt/β-catenin signaling was activated in supporting cells during cisplatin exposure earlier than expected. Our results also indicated that supporting cells enabled hair cell regeneration via Wnt/β-catenin signaling during cisplatin exposure.

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.

Distinct progenitor populations mediate regeneration in the zebrafish lateral line

Mechanosensory hair cells of the zebrafish lateral line regenerate rapidly following damage. These renewed hair cells arise from the proliferation of surrounding support cells, which undergo symmetric division to produce two hair cell daughters. Given the continued regenerative capacity of the lateral line, support cells presumably have the ability to replenish themselves. Utilizing novel transgenic lines, we identified support cell populations with distinct progenitor identities. These populations show differences in their ability to generate new hair cells during homeostasis and regeneration. Targeted ablation of support cells reduced the number of regenerated hair cells. Furthermore, progenitors regenerated after targeted support cell ablation in the absence of hair cell damage. We also determined that distinct support cell populations are independently regulated by Notch signaling. The existence of independent progenitor populations could provide flexibility for the continued generation of new hair cells under a variety of conditions throughout the life of the animal.

Lateral line placodes of aquatic vertebrates are evolutionarily conserved in mammals

Placodes are focal thickenings of the surface ectoderm which, together with neural crest, generate the peripheral nervous system of the vertebrate head. Here we examine how, in embryonic mice, apoptosis contributes to the remodelling of the primordial posterior placodal area (PPA) into physically separated otic and epibranchial placodes. Using pharmacological inhibition of apoptosis-associated caspases, we find evidence that apoptosis eliminates hitherto undiscovered rudiments of the lateral line sensory system which, in fish and aquatic amphibia, serves to detect movements, pressure changes or electric fields in the surrounding water. Our results refute the evolutionary theory, valid for more than a century that the whole lateral line was completely lost in amniotes. Instead, those parts of the PPA which, under experimental conditions, escape apoptosis have retained the developmental potential to produce lateral line placodes and the primordia of neuromasts that represent the major functional units of the mechanosensory lateral line system.

Summary: Inhibition of apoptosis in mouse embryos reveals rudiments of the lateral line system, a sensory system common to fish and aquatic amphibia, but hypothesized to be completely lost in amniotes.

Effects of copper on olfactory, behavioral, and other sublethal responses of saltwater organisms: Are estimated chronic limits using the biotic ligand model protective?

There is concern over whether regulatory criteria for copper (Cu) are protective against chemosensory and behavioral impairment in aquatic organisms. We compiled Cu toxicity data for these and other sublethal endpoints in 35 tests with saltwater organisms and compared the Cu toxicity thresholds with biotic ligand model (BLM)-based estimated chronic limits (ECL values, which are 20% effect concentrations [EC20s] for the embryo-larval life stage of the blue mussel [Mytilus edulis], a saltwater species sensitive to Cu that has historically been used to derive saltwater Cu criteria). Only 8 of the 35 tests had sufficient toxicity and chemistry data to support unequivocal conclusions (i.e., a Cu EC20 or no-observed-effect concentration could be derived, and Cu and dissolved organic carbon [DOC] concentrations were measured [or DOC concentrations could be inferred from the test-water source]). The BLM-based ECL values would have been protective (i.e., the ECL was lower than the toxicity threshold) in 7 of those 8 tests. In the remaining 27 tests, this meta-analysis was limited by several factors, including 1) the Cu toxicity threshold was a "less than" value in 19 tests because only a lowest-observed-effect concentration could be calculated and 2) Cu and/or DOC concentrations often were not measured. In 2 of those 27 tests, the ECL would not have been protective if based only on a conservatively high upper-bound DOC estimate. To facilitate future evaluations of the protectiveness of aquatic life criteria for metals, we urge researchers to measure and report exposure-water chemistry and test-metal concentrations that bracket regulatory criteria. Environ Toxicol Chem 2018;37:1515-1522. © 2018 SETAC.

Protectiveness of Cu water quality criteria against impairment of behavior and chemo/mechanosensory responses: An update

A meta-analysis was conducted of studies that reported behavior and chemo/mechanosensory responses by fish, amphibians, and aquatic invertebrates in Cu-containing waters and also reported sufficient water chemistry for calculation of hardness-based and biotic ligand model (BLM)-based water quality criteria (WQC) for Cu. The calculated WQC concentrations were then compared with the corresponding 20% impairment concentrations (IC20) of Cu for those behavior and chemo/mechanosensory responses. The hardness-based acute and chronic WQC for Cu would not have been protective (i.e., the IC20 would have been lower than the WQC) in 33.6 and 26.2%, respectively, of the 107 combined behavior- and chemo/mechanosensory-response cases that also had adequate water chemistry data for BLM-based WQC calculations (32.7% inconclusive). In comparison, the BLM-based acute and chronic WQC for Cu would not have been protective in only 10.3 and 4.7%, respectively, of the same 107 cases (29.9% inconclusive). To improve evaluations of regulatory effectiveness, researchers conducting aquatic Cu toxicity tests should measure and report complete BLM-input water chemistry and bracket the hardness-based and BLM-based WQC concentrations for Cu that would be applicable in their exposure waters. This meta-analysis demonstrates that, overall, the BLM-based WQC for Cu were considerably more protective than the hardness-based WQC for Cu against impairment of behavior and chemo/mechanosensory responses. Environ Toxicol Chem 2018;37:1260-1279. © 2018 SETAC.

Neuromast hair cells retain the capacity of regeneration during heavy metal exposure

The neuromast is the morphological unit of the lateral line of fishes and is composed of a cluster of central sensory cells (hair cells) surrounded by support and mantle cells. Heavy metals exposure leads to disruption of hair cells within the neuromast. It is well known that the zebrafish has the ability to regenerate the hair cells after damage caused by toxicants. The process of regeneration depends on proliferation, differentiation and cellular migration of sensory and non-sensory progenitor cells. Therefore, our study was made in order to identify which cellular types are involved in the complex process of regeneration during heavy metals exposure. For this purpose, adult zebrafish were exposed to various heavy metals (Arsenic, cadmium and zinc) for 72h. After acute (24h) exposure, immunohistochemical localization of S100 (a specific marker for hair cells) in the neuromasts highlighted the hair cells loss. The immunoreaction for Sox2 (a specific marker for stem cells), at the same time, was observed in the support and mantle cells, after exposure to arsenic and cadmium, while only in the support cells after exposure to zinc. After chronic (72h) exposure the hair cells were regenerated, showing an immunoreaction for S100 protein. At the same exposure time to the three metals, a Sox2 immunoreaction was expressed in support and mantle cells. Our results showed for the first time the regenerative capacity of hair cells, not only after, but also during exposure to heavy metals, demonstrated by the presence of different stem cells that can diversify in hair cells.

Fine structure of the canal neuromasts of the lateral line system in the adult zebrafish

The mechanosensory lateral line system of fish is responsible for several functions such as balance, hearing, and orientation in water flow and is formed by neuromast receptor organs distributed on head, trunk and tail. Superficial and canal neuromasts can be distinguished for localization and morphological differences. Several information is present regarding the superficial neuromasts of zebrafish and other teleosts especially during larval and juvenile stages, while not as numerous data are so far available about the ultrastructural characteristics of the canal neuromasts in adult zebrafish. Therefore, the aim of this study was to investigate by transmission electron microscopy the ultrastructural aspects of cells present in the canal neuromasts. Besides the typical cellular aspects of the neuromast, different cellular types of hair cells were observed that could be identified as developing hair cells during the physiological turnover. The knowledge of the observed cellular types of the canal neuromasts and their origin could give a contribution to studies carried out on adult zebrafish used as model in neurological and non-neurological damages, such as deafness and vestibular disorders.

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.

Effect of JNK inhibitor SP600125 on hair cell regeneration in zebrafish (Danio rerio) larvae

The c-Jun amino-terminal kinase (JNK) proteins are a subgroup of the mitogen-activated protein kinase family. They play a complex role in cell proliferation, survival, and apoptosis. Here, we report a novel role of JNK signalling in hair cell regeneration. We eliminated hair cells of 5-day post-fertilization zebrafish larvae using neomycin followed by JNK inhibition with SP600125. JNK inhibition strongly decreased the number of regenerated hair cells in response to neomycin damage. These changes were associated with reduced proliferation. JNK inhibition also increased cleaved caspase-3 activity and induced apoptosis in regenerating neuromasts. Finally, JNK inhibition with SP600125 decreased the expression of genes related to Wnt. Over-activation of the Wnt signalling pathway partly rescued the hair cell regeneration defects induced by JNK inhibition. Together, our findings provide novel insights into the function of JNK and show that JNK inhibition blocks hair cell regeneration by controlling the Wnt signalling pathway.

The Effect of Traditional Chinese Medicine on Neural Stem Cell Proliferation and Differentiation

Neural stem cells (NSCs) are special types of cells with the potential for self-renewal and multi-directional differentiation. NSCs are regulated by multiple pathways and pathway related transcription factors during the process of proliferation and differentiation. Numerous studies have shown that the compound medicinal preparations, single herbs, and herb extracts in traditional Chinese medicine (TCM) have specific roles in regulating the proliferation and differentiation of NSCs. In this study, we investigate the markers of NSCs in various stages of differentiation, the related pathways regulating the proliferation and differentiation, and the corresponding transcription factors in the pathways. We also review the influence of TCM on NSC proliferation and differentiation, to facilitate the development of TCM in neural regeneration and neurodegenerative diseases.

Efficient gene knockin in axolotl and its use to test the role of satellite cells in limb regeneration

Salamanders exhibit extensive regenerative capacities and serve as a unique model in regeneration research. However, due to the lack of targeted gene knockin approaches, it has been difficult to label and manipulate some of the cell populations that are crucial for understanding the mechanisms underlying regeneration. Here we have established highly efficient gene knockin approaches in the axolotl (Ambystoma mexicanum) based on the CRISPR/Cas9 technology. Using a homology-independent method, we successfully inserted both the Cherry reporter gene and a larger membrane-tagged Cherry-ERT2-Cre-ERT2 (∼5-kb) cassette into axolotl Sox2 and Pax7 genomic loci. Depending on the size of the DNA fragments for integration, 5-15% of the F0 transgenic axolotl are positive for the transgene. Using these techniques, we have labeled and traced the PAX7-positive satellite cells as a major source contributing to myogenesis during axolotl limb regeneration. Our work brings a key genetic tool to molecular and cellular studies of axolotl regeneration.

Notch and Fgf signaling during electrosensory versus mechanosensory lateral line organ development in a non-teleost ray-finned fish

The lateral line system is a useful model for studying the embryonic and evolutionary diversification of different organs and cell types. In jawed vertebrates, this ancestrally comprises lines of mechanosensory neuromasts over the head and trunk, flanked on the head by fields of electrosensory ampullary organs, all innervated by lateral line neurons in cranial lateral line ganglia. Both types of sense organs, and their afferent neurons, develop from cranial lateral line placodes. Current research primarily focuses on the posterior lateral line primordium in zebrafish, which migrates as a cell collective along the trunk; epithelial rosettes form in the trailing zone and are deposited as a line of neuromasts, within which hair cells and supporting cells differentiate. However, in at least some other teleosts (e.g. catfishes) and all non-teleosts, lines of cranial neuromasts are formed by placodes that elongate to form a sensory ridge, which subsequently fragments, with neuromasts differentiating in a line along the crest of the ridge. Furthermore, in many non-teleost species, electrosensory ampullary organs develop from the flanks of the sensory ridge. It is unknown to what extent the molecular mechanisms underlying neuromast formation from the zebrafish migrating posterior lateral line primordium are conserved with the as-yet unexplored molecular mechanisms underlying neuromast and ampullary organ formation from elongating lateral line placodes. Here, we report experiments in an electroreceptive non-teleost ray-finned fish, the Mississippi paddlefish Polyodon spathula, that suggest a conserved role for Notch signaling in regulating lateral line organ receptor cell number, but potentially divergent roles for the fibroblast growth factor signaling pathway, both between neuromasts and ampullary organs, and between paddlefish and zebrafish.

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Notch and Fgf pathway genes are expressed during paddlefish lateral line development.

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Fgf ligand genes are differentially expressed in neuromasts and ampullary organs.

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DAPT treatment results in irregular organ spacing and supernumerary receptor cells.

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SU5402 treatment yields fewer neuromasts, but ampullary organs form precociously.

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SU5402 treatment also results in supernumerary receptor cells.

Neural stem cells induce the formation of their physical niche during organogenesis

Most organs rely on stem cells to maintain homeostasis during post-embryonic life. Typically, stem cells of independent lineages work coordinately within mature organs to ensure proper ratios of cell types. Little is known, however, on how these different stem cells locate to forming organs during development. Here we show that neuromasts of the posterior lateral line in medaka are composed of two independent life-long lineages with different embryonic origins. Clonal analysis and 4D imaging revealed a hierarchical organisation with instructing and responding roles: an inner, neural lineage induces the formation of an outer, border cell lineage (nBC) from the skin epithelium. Our results demonstrate that the neural lineage is necessary and sufficient to generate nBCs highlighting self-organisation principles at the level of the entire embryo. We hypothesise that induction of surrounding tissues plays a major role during the establishment of vertebrate stem cell niches.

Inhibition of H3K27me3 Histone Demethylase Activity Prevents the Proliferative Regeneration of Zebrafish Lateral Line Neuromasts

The H3K27 demethylases are involved in a variety of biological processes, including cell differentiation, proliferation, and cell death by regulating transcriptional activity. However, the function of H3K27 demethylation in the field of hearing research is poorly understood. Here, we investigated the role of H3K27me3 histone demethylase activity in hair cell regeneration using an in vivo animal model. Our data showed that pharmacologic inhibition of H3K27 demethylase activity with the specific small-molecule inhibitor GSK-J4 decreased the number of regenerated hair cells in response to neomycin damage. Furthermore, inhibition of H3K27me3 histone demethylase activity dramatically suppressed cell proliferation and activated caspase-3 levels in the regenerating neuromasts of the zebrafish lateral line. GSK-J4 administration also increased the expression of p21 and p27 in neuromast cells and inhibited the ERK signaling pathway. Collectively, our findings indicate that H3K27me3 demethylation is a key epigenetic regulator in the process of hair cell regeneration in zebrafish and suggest that H3K27me3 histone demethylase activity might be a novel therapeutic target for the treatment of hearing loss.

An optimized method to assess ototoxic effects in the lateral line of zebrafish (Danio rerio) embryos

In order to clarify the suitability of the lateral line of zebrafish (Danio rerio) embryos as a model for the screening of ototoxic (neurotoxic) effects, existing neuromast assays were adapted, improved and validated with a series of chemicals known or unknown for their ototoxic potential (caffeine copper sulfate, dichlorvos, 2.4-dinitrotoluene, neomycin, 4-nonylphenol, perfluorooctanesulfonic acid). Present methods were improved by (1) the introduction of a 4-step scoring system, (2) the selection of neuromasts from both the anterior and posterior lateral line systems, (3) a combined DASPEI/DAPI staining applied after both a continuous and pulse exposure scenario, and (4) an additional screening for nuclear fragmentation. Acute toxicities of the model substances were determined by means of the fish embryo test as specified in OECD TG 236, and EC₁₀ concentrations were used as the highest test concentration in the neuromast assay. The enhanced neuromast assay identified known ototoxic substances such as neomycin and copper sulfate as ototoxic at sensitivities similar to those of established methods, with pulse exposure leading to stronger effects than continuous exposure. Except for caffeine, all substances tested (dichlorvos, 2.4-dinitrotoluene, 4-nonylphenol, perfluorooctanesulfonic acid) produced significant toxic effects in neuromasts at EC₁₀ concentrations. Depending on the test substances and their location along the lateral line, specific neuromasts differed in sensitivity. Generally, neuromasts proved more sensitive in the pulse exposure scenario. Whereas for neomycin and copper sulfate neuromasts located along the anterior lateral line were more sensitive, posterior lateral line neuromasts proved more sensitive for the other test substances. Nuclear fragmentation could not only be associated with all test substances, but, albeit at lower frequencies, also with negative controls, and could, therefore, not be assigned specifically to chemical damage. The study thus documented that for a comprehensive evaluation of lateral line damage both neuromasts from the anterior and the posterior lateral line have to be considered. Given the apparently rapid regeneration of hair cells, pulse exposure seems more appropriate for the identification of lateral line neurotoxicity than continuous exposure.

Mammalian Cochlear Hair Cell Regeneration and Ribbon Synapse Reformation

Hair cells (HCs) are the sensory preceptor cells in the inner ear, which play an important role in hearing and balance. The HCs of organ of Corti are susceptible to noise, ototoxic drugs, and infections, thus resulting in permanent hearing loss. Recent approaches of HCs regeneration provide new directions for finding the treatment of sensor neural deafness. To have normal hearing function, the regenerated HCs must be reinnervated by nerve fibers and reform ribbon synapse with the dendrite of spiral ganglion neuron through nerve regeneration. In this review, we discuss the research progress in HC regeneration, the synaptic plasticity, and the reinnervation of new regenerated HCs in mammalian inner ear.

Inhibition of H3K9me2 Reduces Hair Cell Regeneration after Hair Cell Loss in the Zebrafish Lateral Line by Down-Regulating the Wnt and Fgf Signaling Pathways

The activation of neuromast (NM) supporting cell (SC) proliferation leads to hair cell (HC) regeneration in the zebrafish lateral line. Epigenetic mechanisms have been reported that regulate HC regeneration in the zebrafish lateral line, but the role of H3K9me2 in HC regeneration after HC loss remains poorly understood. In this study, we focused on the role of H3K9me2 in HC regeneration following neomycin-induced HC loss. To investigate the effects of H3K9me2 in HC regeneration, we took advantage of the G9a/GLP-specific inhibitor BIX01294 that significantly reduces the dimethylation of H3K9. We found that BIX01294 significantly reduced HC regeneration after neomycin-induced HC loss in the zebrafish lateral line. BIX01294 also significantly reduced the proliferation of NM cells and led to fewer SCs in the lateral line. In situ hybridization showed that BIX01294 significantly down-regulated the Wnt and Fgf signaling pathways, which resulted in reduced SC proliferation and HC regeneration in the NMs of the lateral line. Altogether, our results suggest that down-regulation of H3K9me2 significantly decreases HC regeneration after neomycin-induced HC loss through inactivation of the Wnt/β-catenin and Fgf signaling pathways. Thus H3K9me2 plays a critical role in HC regeneration.

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.

Mechanosensory organ regeneration in zebrafish depends on a population of multipotent progenitor cells kept latent by Schwann cells

BACKGROUND

Regenerating damaged tissue is a complex process, requiring progenitor cells that must be stimulated to undergo proliferation, differentiation and, often, migratory behaviors and morphological changes. Multiple cell types, both resident within the damaged tissue and recruited to the lesion site, have been shown to participate. However, the cellular and molecular mechanisms involved in the activation of progenitor cell proliferation and differentiation after injury, and their regulation by different cells types, are not fully understood. The zebrafish lateral line is a suitable system to study regeneration because most of its components are fully restored after damage. The posterior lateral line (PLL) is a mechanosensory system that develops embryonically and is initially composed of seven to eight neuromasts distributed along the trunk and tail, connected by a continuous stripe of interneuromastic cells (INCs). The INCs remain in a quiescent state owing to the presence of underlying Schwann cells. They become activated during development to form intercalary neuromasts. However, no studies have described if INCs can participate in a regenerative event, for example, after the total loss of a neuromast.

RESULTS

We used electroablation in transgenic larvae expressing fluorescent proteins in PLL components to completely ablate single neuromasts in larvae and adult fish. This injury results in discontinuity of the INCs, Schwann cells, and the PLL nerve. In vivo imaging showed that the INCs fill the gap left after the injury and can regenerate a new neuromast in the injury zone. Further, a single INC is able to divide and form all cell types in a regenerated neuromast and, during this process, it transiently expresses the sox2 gene, a neural progenitor cell marker. We demonstrate a critical role for Schwann cells as negative regulators of INC proliferation and neuromast regeneration, and that this inhibitory property is completely dependent on active ErbB signaling.

CONCLUSIONS

The potential to regenerate a neuromast after damage requires that progenitor cells (INCs) be temporarily released from an inhibitory signal produced by nearby Schwann cells. This simple yet highly effective two-component niche offers the animal robust mechanisms for organ growth and regeneration, which can be sustained throughout life.

Imaging collective cell migration and hair cell regeneration in the sensory lateral line

The accessibility of the lateral line system and its amenability to long-term in vivo imaging transformed the developing lateral line into a powerful model system to study fundamental morphogenetic events, such as guided migration, proliferation, cell shape changes, organ formation, organ deposition, cell specification and differentiation. In addition, the lateral line is not only amenable to live imaging during migration stages but also during postembryonic events such as sensory organ tissue homeostasis and regeneration. The robust regenerative capabilities of the mature, mechanosensory lateral line hair cells, which are homologous to inner ear hair cells and the ease with which they can be imaged, have brought zebrafish into the spotlight as a model to develop tools to treat human deafness. In this chapter, we describe protocols for long-term in vivo confocal imaging of the developing and regenerating lateral line.

Macrophage Recruitment Contributes to Regeneration of Mechanosensory Hair Cells in the Zebrafish Lateral Line

In vertebrates, damage to mechanosensory hair cells elicits an inflammatory response, including rapid recruitment of macrophages and neutrophils. While hair cells in amniotes usually become permanently lost, they readily regenerate in lower vertebrates such as fish. Damage to hair cells of the fish lateral line is followed by inflammation and rapid regeneration; however the role of immune cells in this process remains unknown. Here, we show that recruited macrophages are required for normal regeneration of lateral line hair cells after copper damage. We found that genetic ablation or local ablation using clodronate liposomes of macrophages recruited to the site of injury, significantly delays hair cell regeneration. Neutrophils, on the other hand, are not needed for this process. We anticipate our results to be a starting point for a more detailed description of extrinsic signals important for regeneration of mechanosensory cells in vertebrates. J. Cell. Biochem. 117: 1880-1889, 2016. © 2016 Wiley Periodicals, Inc.

Chemical Ototoxicity of the Fish Inner Ear and Lateral Line

Hair cell-driven mechanosensory systems are crucial for successful execution of a number of behaviors in fishes, and have emerged as good models for exploring questions relevant to human hearing. This review focuses on ototoxic effects in the inner ear and lateral line system of fishes. We specifically examine studies where chemical ototoxins such as aminoglycoside antibiotics have been employed as tools to disable the lateral line. Lateral line ablation results in alterations to feeding behavior and orientation to water current in a variety of species. However, neither behavior is abolished in the presence of additional sensory cues, supporting the hypothesis that many fish behaviors are driven by multisensory integration. Within biomedical research, the larval zebrafish lateral line has become an important model system for understanding signaling mechanisms that contribute to hair cell death and for developing novel pharmacological therapies that protect hair cells from ototoxic damage. Furthermore, given that fishes robustly regenerate damaged hair cells, ototoxin studies in fishes have broadened our understanding of the molecular and genetic events in an innately regenerative system, offering potential targets for mammalian hair cell regeneration. Collectively, studies of fish mechanosensory systems have yielded insight into fish behavior and in mechanisms of hair cell death, protection, and regeneration.