Loss of Slc4a1b chloride/bicarbonate exchanger function protects mechanosensory hair cells from aminoglycoside damage in the zebrafish mutant persephone.

Mechanosensory hair cell death is a leading cause of hearing and balance disorders in the human population. Hair cells are remarkably sensitive to environmental insults such as excessive noise and exposure to some otherwise therapeutic drugs. However, individual responses to damaging agents can vary, in part due to genetic differences. We previously carried out a forward genetic screen using the zebrafish lateral line system to identify mutations that alter the response of larval hair cells to the antibiotic neomycin, one of a class of aminoglycoside compounds that cause hair cell death in humans. The persephone mutation confers resistance to aminoglycosides. 5 dpf homozygous persephone mutants are indistinguishable from wild-type siblings, but differ in their retention of lateral line hair cells upon exposure to neomycin. The mutation in persephone maps to the chloride/bicarbonate exchanger slc4a1b and introduces a single Ser-to-Phe substitution in zSlc4a1b. This mutation prevents delivery of the exchanger to the cell surface and abolishes the ability of the protein to import chloride across the plasma membrane. Loss of function of zSlc4a1b reduces hair cell death caused by exposure to the aminoglycosides neomycin, kanamycin, and gentamicin, and the chemotherapeutic drug cisplatin. Pharmacological block of anion transport with the disulfonic stilbene derivatives DIDS and SITS, or exposure to exogenous bicarbonate, also protects hair cells against damage. Both persephone mutant and DIDS-treated wild-type larvae show reduced uptake of labeled aminoglycosides. persephone mutants also show reduced FM1-43 uptake, indicating a potential impact on mechanotransduction-coupled activity in the mutant. We propose that tight regulation of the ionic environment of sensory hair cells, mediated by zSlc4a1b activity, is critical for their sensitivity to aminoglycoside antibiotics.

Identification of modulators of hair cell regeneration in the zebrafish lateral line.

The external location of the zebrafish lateral line makes it a powerful model for studying mechanosensory hair cell regeneration. We have developed a chemical screen to identify FDA-approved drugs and biologically active compounds that modulate hair cell regeneration in zebrafish. Of the 1680 compounds evaluated, we identified two enhancers and six inhibitors of regeneration. The two enhancers, dexamethasone and prednisolone, are synthetic glucocorticoids that potentiated hair cell numbers during regeneration and also induced hair cell addition in the absence of damage. BrdU analysis confirmed that the extra hair cells arose from mitotic activity. We found that dexamethasone and prednisolone, like other glucocorticoids, suppress zebrafish caudal fin regeneration, indicating that hair cell regeneration occurs by a distinctly different process. Further analyses of the regeneration inhibitors revealed that two of the six, flubendazole and topotecan, significantly suppress hair cell regeneration by preventing proliferation of hair cell precursors. Flubendazole halted support cell division in M-phase, possibly by interfering with normal microtubule activity. Topotecan, a topoisomerase inhibitor, killed both hair cells and proliferating hair cell precursors. A third inhibitor, fulvestrant, moderately delayed hair cell regeneration by reducing support cell proliferation. Our observation that hair cells do not regenerate when support cell proliferation is impeded confirms previous observations that cell division is the primary route for hair cell regeneration after neomycin treatment in zebrafish.

Experiences of genetic counselors in referring young and metastatic breast cancer patients to support services: A needs assessment.

OBJECTIVE: Conduct a needs assessment to explore the experiences, barriers, and needs of genetic counselors (GCs), who counsel and refer young and metastatic breast cancer (BC) patients to support services, in order to develop resources to address any noticeable gaps. METHODS: GCs providing care to BC patients were eligible to complete the survey. Support services were defined as resources to address patient-centered healthcare, emotional, and quality-of-life needs. RESULTS: Most participants (n = 117) reported familiarity with cancer prevention services (93%); fewer were familiar with services secondary to a BC diagnosis (e.g., fatigue=16% and sexual health=24%). The volume of GCs indicating familiarity with support services increased significantly as work experience increased for seven services. Many (>50%) never referred patients to most (9/12) support services, excluding cancer prevention, mental health, and financial issues. Open-ended responses highlighted that GCs considered referrals to be outside their scope of practice or that healthcare systems prevent GCs from making referrals. CONCLUSION: GCs may benefit from curated resources and materials, especially for support services secondary to a BC diagnosis, to better support their patients. PRACTICAL IMPLICATIONS: Collaboration of GCs with other health professionals through integrative care programs may decrease burdens to accessing support services.

CC2D2A is mutated in Joubert syndrome and interacts with the ciliopathy-associated basal body protein CEP290.

Joubert syndrome and related disorders (JSRD) are primarily autosomal-recessive conditions characterized by hypotonia, ataxia, abnormal eye movements, and intellectual disability with a distinctive mid-hindbrain malformation. Variable features include retinal dystrophy, cystic kidney disease, and liver fibrosis. JSRD are included in the rapidly expanding group of disorders called ciliopathies, because all six gene products implicated in JSRD (NPHP1, AHI1, CEP290, RPGRIP1L, TMEM67, and ARL13B) function in the primary cilium/basal body organelle. By using homozygosity mapping in consanguineous families, we identify loss-of-function mutations in CC2D2A in JSRD patients with and without retinal, kidney, and liver disease. CC2D2A is expressed in all fetal and adult tissues tested. In ciliated cells, we observe localization of recombinant CC2D2A at the basal body and colocalization with CEP290, whose cognate gene is mutated in multiple hereditary ciliopathies. In addition, the proteins can physically interact in vitro, as shown by yeast two-hybrid and GST pull-down experiments. A nonsense mutation in the zebrafish CC2D2A ortholog (sentinel) results in pronephric cysts, a hallmark of ciliary dysfunction analogous to human cystic kidney disease. Knockdown of cep290 function in sentinel fish results in a synergistic pronephric cyst phenotype, revealing a genetic interaction between CC2D2A and CEP290 and implicating CC2D2A in cilium/basal body function. These observations extend the genetic spectrum of JSRD and provide a model system for studying extragenic modifiers in JSRD and other ciliopathies.

Identification of genetic and chemical modulators of zebrafish mechanosensory hair cell death.

Inner ear sensory hair cell death is observed in the majority of hearing and balance disorders, affecting the health of more than 600 million people worldwide. While normal aging is the single greatest contributor, exposure to environmental toxins and therapeutic drugs such as aminoglycoside antibiotics and antineoplastic agents are significant contributors. Genetic variation contributes markedly to differences in normal disease progression during aging and in susceptibility to ototoxic agents. Using the lateral line system of larval zebrafish, we developed an in vivo drug toxicity interaction screen to uncover genetic modulators of antibiotic-induced hair cell death and to identify compounds that confer protection. We have identified 5 mutations that modulate aminoglycoside susceptibility. Further characterization and identification of one protective mutant, sentinel (snl), revealed a novel conserved vertebrate gene. A similar screen identified a new class of drug-like small molecules, benzothiophene carboxamides, that prevent aminoglycoside-induced hair cell death in zebrafish and in mammals. Testing for interaction with the sentinel mutation suggests that the gene and compounds may operate in different pathways. The combination of chemical screening with traditional genetic approaches is a new strategy for identifying drugs and drug targets to attenuate hearing and balance disorders.

Phenotypic Optimization of Urea-Thiophene Carboxamides To Yield Potent, Well Tolerated, and Orally Active Protective Agents against Aminoglycoside-Induced Hearing Loss.

Hearing loss is a major public health concern with no pharmaceutical intervention for hearing protection or restoration. Using zebrafish neuromast hair cells, a robust model for mammalian auditory and vestibular hair cells, we identified a urea-thiophene carboxamide, 1 (ORC-001), as protective against aminoglycoside antibiotic (AGA)-induced hair cell death. The 50% protection (HC50) concentration conferred by 1 is 3.2 µM with protection against 200 µM neomycin approaching 100%. Compound 1 was sufficiently safe and drug-like to validate otoprotection in an in vivo rat hearing loss model. We explored the structure-activity relationship (SAR) of this compound series to improve otoprotective potency, improve pharmacokinetic properties and eliminate off-target activity. We present the optimization of 1 to yield 90 (ORC-13661). Compound 90 protects mechanosensory hair cells with HC50 of 120 nM and demonstrates 100% protection in the zebrafish assay and superior physiochemical, pharmacokinetic, and toxicologic properties, as well as complete in vivo protection in rats.

Ultrastructural analysis of aminoglycoside-induced hair cell death in the zebrafish lateral line reveals an early mitochondrial response.

Loss of the mechanosensory hair cells in the auditory and vestibular organs leads to hearing and balance deficits. To investigate initial, in vivo events in aminoglycoside-induced hair cell damage, we examined hair cells from the lateral line of the zebrafish, Danio rerio. The mechanosensory lateral line is located externally on the animal and therefore allows direct manipulation and observation of hair cells. Labeling with vital dyes revealed a rapid response of hair cells to the aminoglycoside neomycin. Similarly, ultrastructural analysis revealed structural alteration among hair cells within 15 minutes of neomycin exposure. Animals exposed to a low, 25-microM concentration of neomycin exhibited hair cells with swollen mitochondria, but little other damage. Animals treated with higher concentrations of neomycin (50-200 microM) had more severe and heterogeneous cellular changes, as well as fewer hair cells. Both necrotic-like and apoptotic-like cellular damage were observed. Quantitation of the types of alterations observed indicated that mitochondrial defects appear earlier and more predominantly than other structural alterations. In vivo monitoring demonstrated that mitochondrial potential decreased following neomycin treatment. These results indicate that perturbation of the mitochondrion is an early, central event in aminoglycoside-induced damage.

Cilia-Associated Genes Play Differing Roles in Aminoglycoside-Induced Hair Cell Death in Zebrafish.

Hair cells possess a single primary cilium, called the kinocilium, early in development. While the kinocilium is lost in auditory hair cells of most species it is maintained in vestibular hair cells. It has generally been believed that the primary role of the kinocilium and cilia-associated genes in hair cells is in the establishment of the polarity of actin-based stereocilia, the hair cell mechanotransduction apparatus. Through genetic screening and testing of candidate genes in zebrafish (Danio rerio) we have found that mutations in multiple cilia genes implicated in intraflagellar transport (dync2h1, wdr35, ift88, and traf3ip), and the ciliary transition zone (cc2d2a, mks1, and cep290) lead to resistance to aminoglycoside-induced hair cell death. These genes appear to have differing roles in hair cells, as mutations in intraflagellar transport genes, but not transition zone genes, lead to defects in kinocilia formation and processes dependent upon hair cell mechanotransduction activity. These mutants highlight a novel role of cilia-associated genes in hair cells, and provide powerful tools for further study.

The zebrafish merovingian mutant reveals a role for pH regulation in hair cell toxicity and function.

Control of the extracellular environment of inner ear hair cells by ionic transporters is crucial for hair cell function. In addition to inner ear hair cells, aquatic vertebrates have hair cells on the surface of their body in the lateral line system. The ionic environment of these cells also appears to be regulated, although the mechanisms of this regulation are less understood than those of the mammalian inner ear. We identified the merovingian mutant through genetic screening in zebrafish for genes involved in drug-induced hair cell death. Mutants show complete resistance to neomycin-induced hair cell death and partial resistance to cisplatin-induced hair cell death. This resistance is probably due to impaired drug uptake as a result of reduced mechanotransduction ability, suggesting that the mutants have defects in hair cell function independent of drug treatment. Through genetic mapping we found that merovingian mutants contain a mutation in the transcription factor gcm2. This gene is important for the production of ionocytes, which are cells crucial for whole body pH regulation in fish. We found that merovingian mutants showed an acidified extracellular environment in the vicinity of both inner ear and lateral line hair cells. We believe that this acidified extracellular environment is responsible for the defects seen in hair cells of merovingian mutants, and that these mutants would serve as a valuable model for further study of the role of pH in hair cell function.

Screen of FDA-approved drug library reveals compounds that protect hair cells from aminoglycosides and cisplatin.

Loss of mechanosensory hair cells in the inner ear accounts for many hearing loss and balance disorders. Several beneficial pharmaceutical drugs cause hair cell death as a side effect. These include aminoglycoside antibiotics, such as neomycin, kanamycin and gentamicin, and several cancer chemotherapy drugs, such as cisplatin. Discovering new compounds that protect mammalian hair cells from toxic insults is experimentally difficult because of the inaccessibility of the inner ear. We used the zebrafish lateral line sensory system as an in vivo screening platform to survey a library of FDA-approved pharmaceuticals for compounds that protect hair cells from neomycin, gentamicin, kanamycin and cisplatin. Ten compounds were identified that provide protection from at least two of the four toxins. The resulting compounds fall into several drug classes, including serotonin and dopamine-modulating drugs, adrenergic receptor ligands, and estrogen receptor modulators. The protective compounds show different effects against the different toxins, supporting the idea that each toxin causes hair cell death by distinct, but partially overlapping, mechanisms. Furthermore, some compounds from the same drug classes had different protective properties, suggesting that they might not prevent hair cell death by their known target mechanisms. Some protective compounds blocked gentamicin uptake into hair cells, suggesting that they may block mechanotransduction or other routes of entry. The protective compounds identified in our screen will provide a starting point for studies in mammals as well as further research discovering the cellular signaling pathways that trigger hair cell death.

Chemical screening for hair cell loss and protection in the zebrafish lateral line.

In humans, most hearing loss results from death of hair cells, the mechanosensory receptors of the inner ear. Two goals of current hearing research are to protect hair cells from degeneration and to regenerate new hair cells, replacing those that are lost due to aging, disease, or environmental challenges. One limitation of research in the auditory field has been the relative inaccessibility of the mechanosensory systems in the inner ear. Zebrafish possess hair cells in both their inner ear and their lateral line system that are morphologically and functionally similar to human hair cells. The external location of the mechanosensory hair cells in the lateral line and the ease of in vivo labeling and imaging make the zebrafish lateral line a unique system for the study of hair cell toxicity, protection, and regeneration. This review focuses on the lateral line system as a model for understanding loss and protection of mechanosensory hair cells. We discuss chemical screens to identify compounds that induce hair cell loss and others that protect hair cells from known toxins and the potential application of these screens to human medicine.

Extracellular divalent cations modulate aminoglycoside-induced hair cell death in the zebrafish lateral line.

Aminoglycoside antibiotics cause death of sensory hair cells. Research over the past decade has identified several key players in the intracellular cascade. However, the role of the extracellular environment in aminoglycoside ototoxicity has received comparatively little attention. The present study uses the zebrafish lateral line to demonstrate that extracellular calcium and magnesium ions modulate hair cell death from neomycin and gentamicin in vivo, with high levels of either divalent cation providing significant protection. Imaging experiments with fluorescently-tagged gentamicin show that drug uptake is reduced under high calcium conditions. Treating fish with the hair cell transduction blocker amiloride also reduces aminoglycoside uptake, preventing the toxicity, and experiments with variable calcium and amiloride concentrations suggest complementary effects between the two protectants. Elevated magnesium, in contrast, does not appear to significantly attenuate drug uptake, suggesting that the two divalent cations may protect hair cells from aminoglycoside damage through different mechanisms. These results provide additional evidence for calcium- and transduction-dependent aminoglycoside uptake. Divalent cations provided differential protection from neomycin and gentamicin, with high cation concentrations almost completely protecting hair cells from neomycin and acute gentamicin toxicity, but offering reduced protection from continuous (6 h) gentamicin exposure. These experiments lend further support to the hypothesis that aminoglycoside toxicity occurs via multiple pathways in a both a drug and time course-specific manner.

Response of mechanosensory hair cells of the zebrafish lateral line to aminoglycosides reveals distinct cell death pathways.

We report a series of experiments investigating the kinetics of hair cell loss in lateral line neuromasts of zebrafish larvae following exposure to aminoglycoside antibiotics. Comparisons of the rate of hair cell loss and the differential effects of acute versus chronic exposure to gentamicin and neomycin revealed markedly different results. Neomycin induced rapid and dramatic concentration-dependent hair cell loss that is essentially complete within 90 min, regardless of concentration or exposure time. Gentamicin-induced loss of half of the hair cells within 90 min and substantial additional loss, which was prolonged and cumulative over exposure times up to at least 24h. Small molecules and genetic mutations that inhibit neomycin-induced hair cell loss were ineffective against prolonged gentamicin exposure supporting the hypothesis that these two drugs are revealing at least two cellular pathways. The mechanosensory channel blocker amiloride blocked both neomycin and gentamicin-induced hair cell death acutely and chronically indicating that these aminoglycosides share a common entry route. Further tests with additional aminoglycosides revealed a spectrum of differential responses to acute and chronic exposure. The distinctions between the times of action of these aminoglycosides indicate that these drugs induce multiple cell death pathways.