Dale E. Cunningham

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‐μM concentration of neomycin exhibited hair cells with swollen mitochondria, but little other damage. Animals treated with higher concentrations of neomycin (50–200 μM) 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. J. Comp. Neurol. 502:522–543, 2007.

Ultrastructural analysis of [3H]thymidine-labeled cells in the rat utricular macula

Ototoxic drugs stimulate cell proliferation in adult rat vestibular sensory epithelia, as does the infusion of transforming growth factor alpha (TGFα) plus insulin. We sought to determine whether new hair cells can be regenerated by means of a mitotic pathway. Previously, studies have shown that the nuclei of some newly generated cells are located in the lumenal half of the sensory epithelium, suggesting that some may be newly generated sensory hair cells. The aim of this study was to examine the ultrastructural characteristics of newly proliferated cells after TGFα stimulation and/or aminoglycoside damage in the utricular sensory epithelium of the adult rat. The cell proliferation marker tritiated‐thymidine was infused, with or without TGFα plus insulin, into the inner ears of normal or aminoglycoside‐damaged rats for 3 or 7 days by means of osmotic pumps. Autoradiographic techniques and light microscopy were used to identify cells synthesizing DNA. Sections with labeled cells were re‐embedded, processed for transmission electron microscopy, and the ultrastructural characteristics of the labeled cells were examined. The following five classes of tritiated‐thymidine labeled cells were identified in the sensory epithelium: (1) labeled cells with synaptic specializations that appeared to be newly generated hair cells, (2) labeled supporting cells, (3) labeled leukocytes, (4) labeled cells that we have classified as “active cells” in that they are relatively nondescript but contain massive numbers of polyribosomes, and (5) labeled degenerating hair cells. These findings suggest that new hair cells can be generated in situ by means of a mitotic mechanism in the vestibular sensory epithelium of adult mammals. J. Comp. Neurol. 463:177–195, 2003.

Deafness and retinal degeneration in a novel USH1C knock-in mouse model.

Usher syndrome is the leading cause of combined deaf–blindness, but the molecular mechanisms underlying the auditory and visual impairment are poorly understood. Usher I is characterized by profound congenital hearing loss, vestibular dysfunction, and progressive retinitis pigmentosa beginning in early adolescence. Using the c.216G>A cryptic splice site mutation in Exon 3 of the USH1C gene found in Acadian Usher I patients in Louisiana, we constructed the first mouse model that develops both deafness and retinal degeneration. The same truncated mRNA transcript found in Usher 1C patients is found in the cochleae and retinas of these knock‐in mice. Absent auditory‐evoked brainstem responses indicated that the mutant mice are deaf at 1 month of age. Cochlear histology showed disorganized hair cell rows, abnormal bundles, and loss of both inner and outer hair cells in the middle turns and at the base. Retinal dysfunction as evident by an abnormal electroretinogram was seen as early as 1 month of age, with progressive loss of rod photoreceptors between 6 and 12 months of age. This knock‐in mouse reproduces the dual sensory loss of human Usher I, providing a novel resource to study the disease mechanism and the development of therapies.

Electron Microscopy of Degenerative Changes in the Chick Basilar Papilla after Gentamicin Exposure

We present a sequential study of the substructural alterations in the chick basilar papilla at the earliest signs of hair cell degeneration. Three‐day posthatch chicks received a single injection of gentamicin (300 mg/kg) and were killed at 6, 8, 12, 15, 18, 21, and 24 hours after the injection. The basilar papillae were studied by conventional transmission electron microscopy. Examination was limited to the basal region, where all hair cells are eliminated by this treatment. As early as 8 hours and clearly by 12 hours, altered fine structure was seen in hair cells. Changes included rounding and swelling of the hair cells, condensation of nuclear chromatin, dissolution of ribosomes, dilatation of the mitochondria, and accumulation of inclusion bodies and lysosomes. By 15–18 hours, lysosomes increased and became denser, afferent terminals appeared swollen, and the first cell extrusion was seen. Efferents were unaffected, and supporting cells, though having inclusion bodies now, retained normal intercellular junctions. By 21–24 hours, large regions of complete hair cell loss were composed of expanded supporting cell processes with normal‐appearing intercellular junctions and portions of extruded hair cells, partially attached to the supporting cell surface. These observations demonstrate that auditory hair cells undergo a rapid and controlled process of hair cell extrusion that allows preservation of the reticular lamina and minimal contamination of surrounding structures by intracytoplasmic contents of the damaged hair cells. J. Comp. Neurol. 470:164–180, 2004.

Regenerated hair cells in the European starling: are they more resistant to kanamycin ototoxicity than original hair cells?

Previous work from our laboratory [Marean et al. (1993) Hear. Res. 71, 125-136] has shown that a 10 day dose of 200 mg/kg/day kanamycin produced damage to the basal 34% of the starling basilar papilla. We also observed that repeating the dosing schedule following a 4 month survival period resulted in significantly less damage to the regenerated auditory epithelium. The present study investigated whether or not this apparent resistance was the result of a tendency for regenerated hair cells to be less susceptible to kanamycin ototoxicity, or if other, systemic factors may be involved. Eight European starlings were given subcutaneous injections of 200 mg/kg/day kanamycin for 10 days. Serum levels of kanamycin were measured at the time of sacrifice for all birds, and the basilar papillae of all birds were examined by scanning electron microscopy (SEM). Two of these birds (Group 1) were sacrificed immediately following the dosing period. Two of the birds were allowed to survive for 60 days (Group 2). Two of the birds were redosed with 200 mg/kg/day for 10 days after 60 days survival (Group 3). Finally, two birds were redosed with 250 mg/kg/day until serum levels of kanamycin were the same as Group 1 when sacrificed (> 9 micrograms/ml). The SEM results showed that the regenerated auditory epithelium of the birds dosed a second time sustained less damage compared to previously untreated ears, even though the dosing regimen was the same (Group 3 versus Group 1). The regenerated auditory epithelium of birds dosed a second time sustained the same damage as previously untreated animals when the dosage was increased to attain similar serum levels (Group 4 versus Group 1). These results suggest metabolic changes occur in the starling in response to the initial dose of kanamycin which do not necessarily involve changes in hair cell resistance to ototoxicity.

Compartment-specific regulation of plasma membrane calcium ATPase type 2 in the chick auditory brainstem

Calcium signaling plays a role in synaptic regulation of dendritic structure, usually on the time scale of hours or days. Here we use immunocytochemistry to examine changes in expression of plasma membrane calcium ATPase type 2 (PMCA2), a high‐affinity calcium efflux protein, in the chick nucleus laminaris (NL) following manipulations of synaptic inputs. Dendrites of NL neurons segregate into dorsal and ventral domains, receiving excitatory input from the ipsilateral and contralateral ears, respectively, via nucleus magnocellularis (NM). Deprivation of the contralateral projection from NM to NL leads to rapid retraction of ventral, but not the dorsal, dendrites of NL neurons. Immunocytochemistry revealed symmetric distribution of PMCA2 in two neuropil regions of normally innervated NL. Electron microscopy confirmed that PMCA2 localizes in both NM terminals and NL dendrites. As early as 30 minutes after transection of the contralateral projection from NM to NL or unilateral cochlea removal, significant decreases in PMCA2 immunoreactivity were seen in the deprived neuropil of NL compared with the other neuropil that continued to receive normal input. The rapid decrease correlated with reductions in the immunoreactivity for microtubule‐associated protein 2, which affects cytoskeleton stabilization. These results suggest that PMCA2 is regulated independently in ventral and dorsal NL dendrites and/or their inputs from NM in a way that is correlated with presynaptic activity. This provides a potential mechanism by which deprivation can change calcium transport that, in turn, may be important for rapid, compartment‐specific dendritic remodeling. J. Comp. Neurol. 514:624–640, 2009.

Innervation regulates synaptic ribbons in lateral line mechanosensory hair cells

ABSTRACT Failure to form proper synapses in mechanosensory hair cells, the sensory cells responsible for hearing and balance, leads to deafness and balance disorders. Ribbons are electron-dense structures that tether synaptic vesicles to the presynaptic zone of mechanosensory hair cells where they are juxtaposed with the post-synaptic endings of afferent fibers. They are initially formed throughout the cytoplasm, and, as cells mature, ribbons translocate to the basolateral membrane of hair cells to form functional synapses. We have examined the effect of post-synaptic elements on ribbon formation and maintenance in the zebrafish lateral line system by observing mutants that lack hair cell innervation, wild-type larvae whose nerves have been transected and ribbons in regenerating hair cells. Our results demonstrate that innervation is not required for initial ribbon formation but suggest that it is crucial for regulating the number, size and localization of ribbons in maturing hair cells, and for ribbon maintenance at the mature synapse. Summary: Hearing and balance rely on mechanosensory hair cell synaptogenesis. Our data shows that innervation is crucial for formation of ribbons, the scaffolding structures that tether synaptic vesicles at the presynaptic site of hair cells.