Functional expression of exogenous proteins in mammalian sensory hair cells infected with adenoviral vectors

Toward the Optical Cochlear Implant

When hearing fails, cochlear implants (CIs) provide open speech perception to most of the currently half a million CI users. CIs bypass the defective sensory organ and stimulate the auditory nerve electrically. The major bottleneck of current CIs is the poor coding of spectral information, which results from wide current spread from each electrode contact. As light can be more conveniently confined, optical stimulation of the auditory nerve presents a promising perspective for a fundamental advance of CIs. Moreover, given the improved frequency resolution of optical excitation and its versatility for arbitrary stimulation patterns the approach also bears potential for auditory research. Here, we review the current state of the art focusing on the emerging concept of optogenetic stimulation of the auditory pathway. Developing optogenetic stimulation for auditory research and future CIs requires efforts toward viral gene transfer to the neurons, design and characterization of appropriate optogenetic actuators, as well as engineering of multichannel optical implants.

Direct cellular reprogramming and inner ear regeneration

INTRODUCTION

Sound is integral to communication and connects us to the world through speech and music. Cochlear hair cells are essential for converting sounds into neural impulses. However, these cells are highly susceptible to damage from an array of factors, resulting in degeneration and ultimately irreversible hearing loss in humans. Since the discovery of hair cell regeneration in birds, there have been tremendous efforts to identify therapies that could promote hair cell regeneration in mammals.

AREAS COVERED

Here, we will review recent studies describing spontaneous hair cell regeneration and direct cellular reprograming as well as other factors that mediate mammalian hair cell regeneration.

EXPERT OPINION

Numerous combinatorial approaches have successfully reprogrammed non-sensory supporting cells to form hair cells, albeit with limited efficacy and maturation. Studies on epigenetic regulation and transcriptional network of hair cell progenitors may accelerate discovery of more promising reprogramming regimens.

Diagnosis, Intervention, and Prevention of Genetic Hearing Loss

It is estimated that at least 50% of congenital or childhood hearing loss is attributable to genetic causes. In non-syndromic hearing loss, which accounts for 70% of genetic hearing loss, approximately 80% of cases are autosomal recessive, 15% autosomal dominant, and 1-2% mitochondrial or X-linked. In addition, 30% of genetic hearing loss is syndromic. The genetic causes of hearing loss are highly heterogeneous. So far, more than 140 deafness-related genes have been discovered. Studies on those genes tremendously increased our understanding of the inner ear functions at the molecular level. It also offers important information for the patients and allows personalized and accurate genetic counseling. In many cases, genetic diagnosis of hearing loss can help to avoid unnecessary and costly clinical testing, offer prognostic information, and guide future medical management. On the other hand, a variety of gene therapeutic approaches have been developed aiming to relieve or converse the hearing loss due to genetic causes. Prevention of genetic hearing loss is feasible through prepregnancy and prenatal genetic diagnosis and counseling.

Modifying inter-cistronic sequence significantly enhances IRES dependent second gene expression in bicistronic vector: Construction of optimised cassette for gene therapy of familial hypercholesterolemia

Internal ribosome entry site (IRES) sequences have become a valuable tool in the construction of gene transfer and therapeutic vectors for multi-cistronic gene expression from a single mRNA transcript. The optimal conditions for effective use of this sequence to construct a functional expression vector are not precisely defined but it is generally assumed that the internal ribosome entry site dependent expression of the second gene in such as cassette is less efficient than the cap-dependent expression of the first gene. Mainly tailoring inter-cistronic sequence significantly enhances IRES dependent second gene expression in bicistronic vector further in construction of optimised cassette for gene therapy of familial hypercholesterolemia. We tailored the size of the inter-cistronic spacer sequence at the 5′ region of the internal ribosome entry site sequence using sequential deletions and demonstrated that the expression of the 3′ gene can be significantly increased to similar levels as the cap-dependent expression of the 5’ gene. Maximum expression efficiency of the downstream gene was obtained when the spacer is composed of 18–141 base pairs. In this case a single mRNA transcriptional unit containing both the first and the second Cistron was detected. Whilst constructs with spacer sequences of 216 bp or longer generate a single transcriptional unit containing only the first Cistron. This suggests that long spacers may affect transcription termination. When the spacer is 188 bp, both transcripts were produced simultaneously in most transfected cells, while a fraction of them expressed only the first but not the second gene. Expression analyses of vectors containing optimised cassettes clearly confirm that efficiency of gene transfer and biological activity of the expressed transgenic proteins in the transduced cells can be achieved. Furthermore, Computational analysis was carried out by molecular dynamics (MD) simulation to determine the most emerges as viable containing specific binding site and bridging of 5′ and 3′ ends involving direct RNA-RNA contacts and RNA-protein interactions. These results provide a mechanistic basis for translation stimulation and RNA resembling for the synergistic stimulation of cap-dependent translation.

TMEM16A regulates portal vein smooth muscle cell proliferation in portal hypertension

The aim of the present study was to elucidate the effect of transmembrane protein 16A (TMEM16A) on portal vein smooth muscle cell (PVSMC) proliferation associated with portal vein remodeling in portal hypertension (PHT). Sprague-Dawley rats were subjected to bile duct ligation to establish a rat model of liver cirrhosis and PHT. Sham-operated animals served as controls. At 8 weeks after bile duct ligation, the extent of liver fibrosis and the portal vein wall thickness were assessed using hematoxylin-eosin staining. The protein expression levels of TMEM16A, extracellular signal-regulated kinase 1 and 2 (ERK1/2) and phosphorylated ERK1/2 (p-ERK1/2) in the portal vein were detected by immunohistochemistry and western blotting. In vitro, the lentivirus vectors were constructed and transfected into PVSMCs to upregulate the expression of TMEM16A. Isolated rat primary PVSMCs were treated with a small molecule inhibitor of TMEM16A, T16A-inhA01. Cell cycle was detected by flow cytometry. The activity of TMEM16A in the portal vein isolated from bile duct ligated rats was decreased, while the expression level of p-ERK1/2 was increased. However, in vitro, upregulation of TMEM16A promoted the proliferation PVSMCs, while inhibition of TMEM16A channels inhibited the proliferation of PVSMCs. The results indicated that TMEM16A contributes to PVSMCs proliferation in vitro, but in vivo, it may be a negative regulator of cell proliferation influenced by numerous factors.

Viral-mediated Ntf3 overexpression disrupts innervation and hearing in nondeafened guinea pig cochleae

Synaptopathy in the cochlea occurs when the connection between inner hair cells and the auditory nerve is disrupted, leading to impaired hearing and nerve degeneration. Experiments using transgenic mice have shown that overexpression of NT3 by supporting cells repairs synaptopathy caused by overstimulation. To accomplish such therapy in the clinical setting, it would be necessary to activate the neurotrophin receptor on auditory neurons by other means. Here we test the outcome of NT3 overexpression using viral-mediated gene transfer into the perilymph versus the endolymph of the normal guinea pig cochlea. We inoculated two different Ntf3 viral vectors, adenovirus (Adv) or adeno-associated virus (AAV) into the perilymph, to facilitate transgene expression in the mesothelial cells and cochlear duct epithelium, respectively. We assessed outcomes by comparing Auditory brainstem response (ABR) thresholds prior to that at baseline to thresholds at 1 and 3 weeks after inoculation, and then performed histologic evaluation of hair cells, nerve endings, and synaptic ribbons. We observed hearing threshold shifts as well as disorganization of peripheral nerve endings and disruption of synaptic connections between inner hair cells and peripheral nerve endings with both vectors. The data suggest that elevation of NT3 levels in the cochlear fluids can disrupt innervation and degrade hearing.

On the Distant Horizon--Medical Therapy for Sensorineural Hearing Loss

Hearing loss is the most common sensory deficit in developed societies. Hearing impairment in children, particularly of prelingual onset, has been shown to negatively affect educational achievement, future employment and earnings, and even life expectancy. Sensorineural hearing loss (SNHL), which refers to defects within the cochlea or auditory nerve itself, far outweighs conductive causes for permanent hearing loss in both children and adults. The causes of SNHL in children are heterogeneous, including both congenital and acquired causes. This article identifies potential mechanisms of intervention both at the level of the hair cell and the spiral ganglion neurons.

Targeted delivery of brain-derived neurotrophic factor for the treatment of blindness and deafness

Neurodegenerative causes of blindness and deafness possess a major challenge in their clinical management as proper treatment guidelines have not yet been found. Brain-derived neurotrophic factor (BDNF) has been established as a promising therapy against neurodegenerative disorders including hearing and visual loss. Unfortunately, the blood–retinal barrier and blood–cochlear barrier, which have a comparable structure to the blood–brain barrier prevent molecules of larger sizes (such as BDNF) from exiting the circulation and reaching the targeted cells. Anatomical features of the eye and ear allow use of local administration, bypassing histo-hematic barriers. This paper focuses on highlighting a variety of strategies proposed for the local administration of the BDNF, like direct delivery, viral gene therapy, and cell-based therapy, which have been shown to successfully improve development, survival, and function of spiral and retinal ganglion cells. The similarities and controversies for BDNF treatment of posterior eye diseases and inner ear diseases have been analyzed and compared. In this review, we also focus on the possibility of translation of this knowledge into clinical practice. And finally, we suggest that using nanoparticulate drug-delivery systems may substantially contribute to the development of clinically viable techniques for BDNF delivery into the cochlea or posterior eye segment, which, ultimately, can lead to a long-term or permanent rescue of auditory and optic neurons from degeneration.

Using injectoporation to deliver genes to mechanosensory hair cells

Mechanosensation, the transduction of mechanical force into electrochemical signals, allows organisms to detect touch and sound, to register movement and gravity, and to sense changes in cell volume and shape. The hair cells of the mammalian inner ear are the mechanosensors for the detection of sound and head movement. The analysis of gene function in hair cells has been hampered by the lack of an efficient gene transfer method. Here we describe a method termed injectoporation that combines tissue microinjection with electroporation to express cDNAs and shRNAs in mouse cochlear hair cells. Injectoporation allows for gene transfer into dozens of hair cells, and it is compatible with the analysis of hair cell function using imaging approaches and electrophysiology. Tissue dissection and injectoporation can be carried out within a few hours, and the tissue can be cultured for days for subsequent functional analyses.

Electroacoustic stimulation: now and into the future

Cochlear implants have provided hearing to hundreds of thousands of profoundly deaf people around the world. Recently, the eligibility criteria for cochlear implantation have been relaxed to include individuals who have some useful residual hearing. These recipients receive inputs from both electric and acoustic stimulation (EAS). Implant recipients who can combine these hearing modalities demonstrate pronounced benefit in speech perception, listening in background noise, and music appreciation over implant recipients that rely on electrical stimulation alone. The mechanisms bestowing this benefit are unknown, but it is likely that interaction of the electric and acoustic signals in the auditory pathway plays a role. Protection of residual hearing both during and following cochlear implantation is critical for EAS. A number of surgical refinements have been implemented to protect residual hearing, and the development of hearing-protective drug and gene therapies is promising for EAS recipients. This review outlines the current field of EAS, with a focus on interactions that are observed between these modalities in animal models. It also outlines current trends in EAS surgery and gives an overview of the drug and gene therapies that are clinically translatable and may one day provide protection of residual hearing for cochlear implant recipients.

Vestibular regeneration--experimental models and clinical implications

Therapies aimed at the protection and/or regeneration of inner ear hair cells are of great interest, given the significant monetary and quality of life impact of balance disorders. Different viral vectors have been shown to transfect various cell types in the inner ear. The past decade has provided tremendous advances in the use of adenoviral vectors to achieve targeted treatment delivery. Several routes of delivery have been identified to introduce vectors into the inner ear while minimizing injury to surrounding structures. Recently, the transcription factor Atoh1 was determined to play a critical role in hair cell differentiation. Adenoviral-mediated overexpression of Atoh1 in culture and in vivo has demonstrated the ability to regenerate vestibular hair cells by causing transdifferentiation of neighbouring epithelial-supporting cells. Functional recovery of the vestibular system has also been documented following adenoviral-induced Atoh1 overexpression. Experiments demonstrating gene transfer in human vestibular epithelial cells reveal that the human inner ear is a suitable target for gene therapy.

MYC gene delivery to adult mouse utricles stimulates proliferation of postmitotic supporting cells in vitro

The inner ears of adult humans and other mammals possess a limited capacity for regenerating sensory hair cells, which can lead to permanent auditory and vestibular deficits. During development and regeneration, undifferentiated supporting cells within inner ear sensory epithelia can self-renew and give rise to new hair cells; however, these otic progenitors become depleted postnatally. Therefore, reprogramming differentiated supporting cells into otic progenitors is a potential strategy for restoring regenerative potential to the ear. Transient expression of the induced pluripotency transcription factors, Oct3/4, Klf4, Sox2, and c-Myc reprograms fibroblasts into neural progenitors under neural-promoting culture conditions, so as a first step, we explored whether ectopic expression of these factors can reverse supporting cell quiescence in whole organ cultures of adult mouse utricles. Co-infection of utricles with adenoviral vectors separately encoding Oct3/4, Klf4, Sox2, and the degradation-resistant T58A mutant of c-Myc (c-MycT58A) triggered significant levels of supporting cell S-phase entry as assessed by continuous BrdU labeling. Of the four factors, c-MycT58A alone was both necessary and sufficient for the proliferative response. The number of BrdU-labeled cells plateaued between 5–7 days after infection, and then decreased ∼60% by 3 weeks, as many cycling cells appeared to enter apoptosis. Switching to differentiation-promoting culture medium at 5 days after ectopic expression of c-MycT58A temporarily attenuated the loss of BrdU-labeled cells and accompanied a very modest but significant expansion of the sensory epithelium. A small number of the proliferating cells in these cultures labeled for the hair cell marker, myosin VIIA, suggesting they had begun differentiating towards a hair cell fate. The results indicate that ectopic expression of c-MycT58A in combination with methods for promoting cell survival and differentiation may restore regenerative potential to supporting cells within the adult mammalian inner ear.

Gene transfer in inner ear cells: a challenging race

Recent advances in human genomics led to the identification of numerous defective genes causing deafness, which represent novel putative therapeutic targets. Future gene-based treatment of deafness resulting from genetic or acquired sensorineural hearing loss may include strategies ranging from gene therapy to antisense delivery. For successful development of gene therapies, a minimal requirement involves the engineering of appropriate gene carrier systems. Transfer of exogenous genetic material into the mammalian inner ear using viral or non-viral vectors has been characterized over the last decade. The nature of inner ear cells targeted, as well as the transgene expression level and duration, are highly dependent on the vector type, the route of administration and the strength of the promoter driving expression. This review summarizes and discusses recent advances in inner ear gene-transfer technologies aimed at examining gene function or identifying new treatment for inner ear disorders.

The function and molecular identity of inward rectifier channels in vestibular hair cells of the mouse inner ear

Inner ear hair cells respond to mechanical stimuli with graded receptor potentials. These graded responses are modulated by a host of voltage-dependent currents that flow across the basolateral membrane. Here, we examine the molecular identity and the function of a class of voltage-dependent ion channels that carries the potassium-selective inward rectifier current known as I(K1). I(K1) has been identified in vestibular hair cells of various species, but its molecular composition and functional contributions remain obscure. We used quantitative RT-PCR to show that the inward rectifier gene, Kir2.1, is highly expressed in mouse utricle between embryonic day 15 and adulthood. We confirmed Kir2.1 protein expression in hair cells by immunolocalization. To examine the molecular composition of I(K1), we recorded voltage-dependent currents from type II hair cells in response to 50-ms steps from -124 to -54 in 10-mV increments. Wild-type cells had rapidly activating inward currents with reversal potentials close to the K(+) equilibrium potential and a whole-cell conductance of 4.8 ± 1.5 nS (n = 46). In utricle hair cells from Kir2.1-deficient (Kir2.1(-/-)) mice, I(K1) was absent at all stages examined. To identify the functional contribution of Kir2.1, we recorded membrane responses in current-clamp mode. Hair cells from Kir2.1(-/-) mice had significantly (P < 0.001) more depolarized resting potentials and larger, slower membrane responses than those of wild-type cells. These data suggest that Kir2.1 is required for I(K1) in type II utricle hair cells and contributes to hyperpolarized resting potentials and fast, small amplitude receptor potentials in response to current inputs, such as those evoked by hair bundle deflections.

Inhibition of Notch activity promotes nonmitotic regeneration of hair cells in the adult mouse utricles

The capacity of adult mammals to regenerate sensory hair cells is not well defined. To explore early steps in this process, we examined reactivation of a transiently expressed developmental gene, Atoh1, in adult mouse utricles after neomycin-induced hair cell death in culture. Using an adenoviral reporter for Atoh1 enhancer, we found that Atoh1 transcription is activated in some hair cell progenitors (supporting cells) 3 d after neomycin treatment. By 18 d after neomycin, the number of cells with Atoh1 transcriptional activity increased significantly, but few cells acquired hair cell features (i.e., accumulated ATOH1 or myosin VIIa protein or developed stereocilia). Treatment with DAPT, an inhibitor of γ-secretase, reduced notch pathway activity, enhanced Atoh1 transcriptional activity, and dramatically increased the number of Atoh1-expressing cells with hair cell features, but only in the striolar/juxtastriolar region. Similar effects were seen with TAPI-1, an inhibitor of another enzyme required for notch activity (TACE). Division of supporting cells was rare in any control or DAPT-treated utricles. This study shows that mature mammals have a natural capacity to initiate vestibular hair cell regeneration and suggests that regional notch activity is a significant inhibitor of direct transdifferentiation of supporting cells into hair cells following damage.

Towards gene therapy for deafness

Many hearing disorders are associated with the damage or loss of sensory hair cells (HC) which can produce a profound and irreversible deafness. Apoptosis pathway is reported to play an important role leading to rapid expansion of the HC lesion after exposure to intense noise. Furthermore, progress made over the last year in understanding molecular mechanisms involved in the proliferative and regenerative capacity of sensory cells in the mammalian inner ear has raised the possibility that targeted therapies might prevent the loss of these cells and preserve the patient's hearing. A first step towards the successful therapeutic exploitation is a better understanding of the different pathways that control survival and proliferation of sensory cells. In this review, we provide an overview of recent findings concerning the possibility to prevent apoptosis in auditory cells. We also show the current knowledge on the molecular mechanisms involved in the potential regenerative behavior of these cells and the progress of gene therapy to prevent deafness noise-induced.

A review of gene delivery and stem cell based therapies for regenerating inner ear hair cells

Sensory neural hearing loss and vestibular dysfunction have become the most common forms of sensory defects, affecting millions of people worldwide. Developing effective therapies to restore hearing loss is challenging, owing to the limited regenerative capacity of the inner ear hair cells. With recent advances in understanding the developmental biology of mammalian and non-mammalian hair cells a variety of strategies have emerged to restore lost hair cells are being developed. Two predominant strategies have developed to restore hair cells: transfer of genes responsible for hair cell genesis and replacement of missing cells via transfer of stem cells. In this review article, we evaluate the use of several genes involved in hair cell regeneration, the advantages and disadvantages of the different viral vectors employed in inner ear gene delivery and the insights gained from the use of embryonic, adult and induced pluripotent stem cells in generating inner ear hair cells. Understanding the role of genes, vectors and stem cells in therapeutic strategies led us to explore potential solutions to overcome the limitations associated with their use in hair cell regeneration.

Viral vector tropism for supporting cells in the developing murine cochlea

Gene-based therapeutics are being developed as novel treatments for genetic hearing loss. One roadblock to effective gene therapy is the identification of vectors which will safely deliver therapeutics to targeted cells. The cellular heterogeneity that exists within the cochlea makes viral tropism a vital consideration for effective inner ear gene therapy. There are compelling reasons to identify a viral vector with tropism for organ of Corti supporting cells. Supporting cells are the primary expression site of connexin 26 gap junction proteins that are mutated in the most common form of congenital genetic deafness (DFNB1). Supporting cells are also primary targets for inducing hair cell regeneration. Since many genetic forms of deafness are congenital it is necessary to administer gene transfer-based therapeutics prior to the onset of significant hearing loss. We have used transuterine microinjection of the fetal murine otocyst to investigate viral tropism in the developing inner ear. For the first time we have characterized viral tropism for supporting cells following in utero delivery to their progenitors. We report the inner ear tropism and potential ototoxicity of three previously untested vectors: early-generation adenovirus (Ad5.CMV.GFP), advanced-generation adenovirus (Adf.11D) and bovine adeno-associated virus (BAAV.CMV.GFP). Adenovirus showed robust tropism for organ of Corti supporting cells throughout the cochlea but induced increased ABR thresholds indicating ototoxicity. BAAV also showed tropism for organ of Corti supporting cells, with preferential transduction toward the cochlear apex. Additionally, BAAV readily transduced spiral ganglion neurons. Importantly, the BAAV-injected ears exhibited normal hearing at 5 weeks of age when compared to non-injected ears. Our results support the use of BAAV for safe and efficient targeting of supporting cell progenitors in the developing murine inner ear.

Postsynaptic spiking homeostatically induces cell-autonomous regulation of inhibitory inputs via retrograde signaling

Developing neural circuits face the dual challenge of growing in an activity-induced fashion and maintaining stability through homeostatic mechanisms. Compared to our understanding of homeostatic regulation of excitatory synapses, relatively little is known about the mechanism mediating homeostatic plasticity of inhibitory synapses, especially that following activity elevation. Here, we found that elevating neuronal activity in cultured hippocampal neurons for 4 h significantly increased the frequency and amplitude of mIPSCs, before detectable change at excitatory synapses. Consistently, we observed increases in presynaptic and postsynaptic proteins of GABAergic synapses, including GAD65, vGAT, and GABA(A)Rα1. By suppressing activity-induced increase of neuronal firing with expression of the inward rectifier potassium channel Kir2.1 in individual neurons, we showed that elevation in postsynaptic spiking activity is required for activity-dependent increase in the frequency and amplitude of mIPSCs. Importantly, directly elevating spiking in individual postsynaptic neurons, by capsaicin activation of overexpressed TRPV1 channels, was sufficient to induce increased mIPSC amplitude and frequency, mimicking the effect of elevated neuronal activity. Downregulating BDNF expression in the postsynaptic neuron or its extracellular scavenging prevented activity-induced increase in mIPSC frequency, consistent with a role of BDNF-dependent retrograde signaling in this process. Finally, elevating activity in vivo by kainate injection increased both mIPSC amplitude and frequency in CA1 pyramidal neurons. Thus, spiking-induced, cell-autonomous upregulation of GABAergic synaptic inputs, through retrograde BDNF signaling, represents an early adaptive response of neural circuits to elevated network activity.

Adeno-associated virus-mediated gene delivery into the scala media of the normal and deafened adult mouse ear

Murine models are ideal for studying cochlear gene transfer, as many hearing loss-related mutations have been discovered and mapped within the mouse genome. However, because of the small size and delicate nature, the membranous labyrinth of the mouse is a challenging target for the delivery of viral vectors. To minimize injection trauma, we developed a procedure for the controlled release of adeno-associated viruses (AAVs) into the scala media of adult mice. This procedure poses minimal risk of injury to structures of the cochlea and middle ear, and allows for near-complete preservation of low and middle frequency hearing. In this study, transduction efficiency and cellular specificity of AAV vectors (serotypes 1, 2, 5, 6 and 8) were investigated in normal and drug-deafened ears. Using the cytomegalovirus promoter to drive gene expression, a variety of cell types were transduced successfully, including sensory hair cells and supporting cells, as well as cells in the auditory nerve and spiral ligament. Among all five serotypes, inner hair cells were the most effectively transduced cochlear cell type. All five serotypes of AAV vectors transduced cells of the auditory nerve, though serotype 8 was the most efficient vector for transduction. Our findings indicate that efficient AAV inoculation (via the scala media) can be performed in adult mouse ears, with hearing preservation a realistic goal. The procedure we describe may also have applications for intra-endolymphatic drug delivery in many mouse models of human deafness.

Effects of localized neurotrophin gene expression on spiral ganglion neuron resprouting in the deafened cochlea

A cochlear implant may be used to electrically stimulate spiral ganglion neurons (SGNs) in people with severe sensorineural hearing loss (SNHL). However, these neurons progressively degenerate after SNHL due to loss of neurotrophins normally supplied by sensory hair cells (HCs). Experimentally, exogenous neurotrophin administration prevents SGN degeneration but can also result in abnormal resprouting of their peripheral fibers. This study aimed to create a target-derived neurotrophin source to increase neuron survival and redirect fiber resprouting following SNHL. Adenoviral (Ad) vectors expressing green fluorescent protein (GFP) alone or in combination with brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT3) were injected into the cochlear scala tympani or scala media of guinea-pigs (GPs) deafened via aminoglycosides for 1 week. After 3 weeks, cochleae were examined for gene expression, neuron survival, and the projection of peripheral fibers in response to gene expression. Injection of vectors into the scala media resulted in more localized gene expression than scala tympani injection with gene expression consistently observed within the partially degenerated organ of Corti. There was also greater neuron survival and evidence of localized fiber responses to neurotrophin-expressing cells within the organ of Corti from scala media injections (P < 0.05), a first step in promoting organized resprouting of auditory peripheral fibers via gene therapy.

Histone deacetylase inhibition enhances adenoviral vector transduction in inner ear tissue

Adenovirus vectors (AdVs) are efficient tools for gene therapy in many tissues. Several studies have demonstrated successful transgene transduction with AdVs in the inner ear of rodents [Kawamoto K, Ishimoto SI, Minoda R, Brough DE, Raphael Y (2003) J Neurosci 23:4395-4400]. However, toxicity of AdVs [Morral N, O'Neal WK, Rice K, Leland MM, Piedra PA, Aguilar-Cordova E, Carey KD, Beaudet AL, Langston C (2002) Hum Gene Ther 13:143-154.] or lack of tropism to important cell types such as hair cells [Shou J, Zheng JL, Gao WQ (2003) Mol Cell Neurosci 23:169-179] appears to limit their experimental and potential clinical utility. Histone deacetylase inhibitors (HDIs) are known to enhance AdV-mediated transgene expression in various organs [Dion LD, Goldsmith KT, Tang DC, Engler JA, Yoshida M, Garver RI Jr (1997) Virology 231:201-209], but their effects in the inner ear have not been documented. We investigated the ability of one HDI, trichostatin A (TSA), to enhance AdV-mediated transgene expression in inner ear tissue. We cultured neonatal rat macular and cochlear explants, and transduced them with an AdV encoding green fluorescent protein (Ad-GFP) under the control of a constitutive promoter for 24 h. In the absence of TSA, GFP expression was limited, and very few hair cells were transduced. TSA did not enhance transduction when applied at the onset of Ad-GFP transduction. However, administration of TSA during or just after Ad-GFP application increased GFP expression in supporting cells approximately fourfold. Moreover, vestibular hair cell transduction was enhanced approximately sixfold, and that of inner hair cells by more than 17-fold. These results suggest that TSA increases AdV-mediated transgene expression in the inner ear, including the successful transduction of hair cells. HDIs, some of which are currently under clinical trials (Sandor et al., 2002), could be useful tools in overcoming current limitations of gene therapy in the inner ear using Ad-GFP.

Gene therapy in the inner ear using adenovirus vectors

Therapies for the protection and regeneration of auditory hair cells are of great interest given the significant monetary and lifestyle impact of hearing loss. The past decade has seen tremendous advances in the use of adenoviral vectors to achieve these aims. Preliminary data demonstrated the functional capacity of this technique as adenoviral-induced expression of neurotrophic and growth factors protected hair cells and spiral ganglion neurons from ototoxic insults. Subsequent efforts confirmed the feasibility of adenoviral transfection of cells in the auditory neuroepithelium via cochleostomy into the scala media. Most recently, efforts have focused on regeneration of depleted hair cells. Mammalian hearing loss is generally considered a permanent insult as the auditory epithelium lacks a basal layer capable of producing new hair cells. Recently, the transcription factor Atoh1 has been found to play a critical role in hair cell differentiation. Adenoviral-mediated overexpression of Atoh1 in culture and in vivo have shown the ability to regenerate auditory and vestibular hair cells by causing transdifferentiation of neighboring epithelial-supporting cells. Functional recovery of both the auditory and vestibular systems has been documented following adenoviral induced Atoh1 overexpression.

New ectopic vestibular hair cell-like cells induced by Math1 gene transfer in postnatal rats

Overexpression of Math1 has been demonstrated to induce robust ectopic new hair cells in greater epithelial ridge (GER) and lesser epithelial ridge (LER) of the postnatal rats' cochlear in vitro. In spite of the similarities between cochlear and vestibular epithelia in origin and structure, no similar results have been reported in the non-sensory region of vestibular epithelia in vitro. In the study, the adenoviral vectors inserted with Math1 gene were constructed to examine their effect on vestibular epithelia in postnatal rats. In vivo, the adenovirus vectors administered in vestibular perilymphatic or endolymphatic space, their transduction efficiency and other indexes are different. We set up a culture construction to simulate and show the differences. In the study, we also developed a new dissection protocol to be quick to harvest the whole maculae and cristae. The new ectopic vestibular hair cell-like cells induced by overexpression of Math1, were found to appear in the non-sensory region of the postnatal rats' vestibular epithelia as observed in the cochlear, and the number of the new cells was different when a different virus administration was simulated in vestibular perilymphatic or endolymphatic space, suggesting that the cells found could have the capability to differentiate into new hair cells. Our study might pave the way for further in vivo studies on vestibular Math1 adenoviral vector gene transfer.

Gene transfer in human vestibular epithelia and the prospects for inner ear gene therapy

Transfer of exogenous genetic material into the mammalian inner ear using viral vectors has been characterized over the last decade. A number of different viral vectors have been shown to transfect the varying cell types of the nonprimate mammalian inner ear. Several routes of delivery have been identified for introduction of vectors into the inner ear while minimizing injury to existing structures and at the same time ensuring widespread distribution of the agent throughout the cochlea and the rest of the inner ear. These studies raise the possibility that gene transfer may be developed as a potential strategy for treating inner ear dysfunction in humans. Furthermore, a recent report showing successful transfection of excised human vestibular epithelia offers proof of principle that viral gene transfer is a viable strategy for introduction and expression of exogenous genetic material to restore function to the inner ear. Human vestibular epithelia were harvested from patients undergoing labyrinthectomy, either for intractable Ménière's disease or vestibular schwannoma resection, and cultured for as long as 5 days. In those experiments, recombinant, multiply-deleted, replication-deficient adenoviral vectors were used to transfect and express a reporter gene as well as the functionally relevant gene, wild-type KCNQ4, a potassium channel gene that when mutated causes the autosomal dominant HL DFNA2.Here, we review the current state of viral-mediated gene transfer in the inner ear and discuss different viral vectors, routes of delivery, and potential applications of gene therapy. Emphasis is placed on experiments demonstrating viral transfection of human inner ear tissue and implications of these findings and for the future of gene therapy in the human inner ear.

Helper-dependent adenovirus-mediated gene transfer into the adult mouse cochlea

BACKGROUND

Gene therapy may provide a way to restore cochlear function to deaf patients. The most successful techniques for cochlear gene therapy have been injection of early-generation adenoviral vectors into scala media in guinea pigs. However, it is important to be able to perform gene therapy research in mice because there is wide availability of transgenic strains with hereditary hearing loss.

PURPOSE

We demonstrate our technique for delivery of a third-generation adenoviral vector, helper-dependent adenovirus (HDAd), to the adult mouse cochlea.

METHODS

Mice were injected with an HDAd that contained a reporter gene for either beta-galactosidase or green fluorescent protein into scala media. After 4 days, the cochleae were harvested for analyses. Auditory brainstem response monitoring of cochlear function was performed before making a cochleostomy, after making a cochleostomy, and before killing the animal.

RESULTS

Beta-galactosidase was identified in the spiral ligament, the organ of Corti, and spiral ganglion cells by light microscopy. Green fluorescent protein epifluorescence was assessed in whole-mount organ of Corti preparations using confocal microscopy. This demonstrated transduction of inner hair cells, outer hair cells, and supporting cells. Paraffin-embedded cross sections similarly revealed gene transduction within the organ of Corti. Threshold shifts of 39.8 +/- 5.4 and 37.7 +/- 5.5 dB were observed in mice injected with HDAd or control buffer, respectively.

CONCLUSION

The technique of scala media HDAd injection reliably infects the adult mouse cochlea, including cells within the organ of Corti, although the procedure itself adversely affects hearing.

Dominant-negative inhibition of M-like potassium conductances in hair cells of the mouse inner ear

Sensory hair cells of the inner ear express multiple physiologically defined conductances, including mechanotransduction, Ca(2+), Na(+), and several distinct K(+) conductances, all of which are critical for normal hearing and balance function. Yet, the molecular underpinnings and their specific contributions to sensory signaling in the inner ear remain obscure. We sought to identify hair-cell conductances mediated by KCNQ4, which, when mutated, causes the dominant progressive hearing loss DFNA2. We used the dominant-negative pore mutation G285S and packaged the coding sequence of KCNQ4 into adenoviral vectors. We transfected auditory and vestibular hair cells of organotypic cultures generated from the postnatal mouse inner ear. Cochlear outer hair cells and vestibular type I cells that expressed the transfection marker, green fluorescent protein, and the dominant-negative KCNQ4 construct lacked the M-like conductances that typify nontransfected control hair cells. As such, we conclude that the M-like conductances in mouse auditory and vestibular hair cells can include KCNQ4 subunits and may also include KCNQ4 coassembly partners. To examine the function of M-like conductances in hair cells, we recorded from cells transfected with mutant KCNQ4 and injected transduction current waveforms in current-clamp mode. Because the M-like conductances were active at rest, they contributed to the very low potassium-selective input resistance, which in turn hyperpolarized the resting potential and significantly attenuated the amplitude of the receptor potential. Modulation of M-like conductances may allow hair cells the ability to control the amplitude of their response to sensory stimuli.

An in vitro model system to study gene therapy in the human inner ear

The confined fluid-filled labyrinth of the human inner ear presents an opportunity for introduction of gene therapy reagents designed to treat hearing and balance dysfunction. Here we present a novel model system derived from the sensory epithelia of human vestibular organs and show that the tissue can survive up to 5 days in vitro. We generated organotypic cultures from 26 human sensory epithelia excised at the time of labyrinthectomy for intractable Meniere's disease or vestibular schwannoma. We applied multiply deleted adenoviral vectors at titers between 10(5) and 10(8) viral particles/ml directly to the cultures for 4-24 h and examined the tissue 12-96 h post-transfection. We noted robust expression of the exogenous transgene, green fluorescent protein (GFP), in hair cells and supporting cells suggesting both were targets of adenoviral transfection. We also transfected cultures with a vector that carried the genes for GFP and KCNQ4, a potassium channel subunit that causes dominant-progressive hearing loss when mutated. We noted a positive correlation between GFP fluorescence and KCNQ4 immunolocalization. We conclude that our in vitro model system presents a novel and effective experimental paradigm for evaluation of gene therapy reagents designed to restore cellular function in patients who suffer from inner ear disorders.

In vivo delivery of recombinant viruses to the fetal murine cochlea: transduction characteristics and long-term effects on auditory function

Congenital hearing deficits can be caused by a variety of genetic and acquired conditions. Complete reversal of deficits in the peripheral auditory system may require delivery of corrective genes to cochlear progenitor cells. We tested delivery of lentivirus and an array of recombinant adeno-associated viral (AAV) serotypes for efficiency and cellular specificity of transgene expression after in utero delivery to the developing mouse otocyst. Stability of expression and safety with respect to auditory function were then tested in those vectors that had the most favorable in utero cochlear transduction characteristics (AAV2/1, AAV2/8, and lentivirus). AAV2/1 was found to be the optimal vector for in utero cochlear gene transfer. It efficiently transduced progenitors giving rise to both inner and outer hair cells and supporting cells and had no adverse effect on cochlear cell differentiation. Further, it had no pathological effect on differentiated hair cells or the integrity of the auditory nerve or brain-stem nuclei as measured by auditory brain-stem response testing. AAV2/1 promises to be useful in further studies evaluating differentiation pathways of cochlear cells in health and disease and for developing gene-based therapies for congenital and acquired forms of peripheral hearing loss.

Specific and efficient transduction of cochlear inner hair cells with recombinant adeno-associated virus type 3 vector

Recombinant adeno-associated virus (AAV) vectors are of interest for cochlear gene therapy because of their ability to mediate the efficient transfer and long-term stable expression of therapeutic genes in a wide variety of postmitotic tissues with minimal vector-related cytotoxicity. In the present study, seven AAV serotypes (AAV1-5, 7, 8) were used to construct vectors. The expression of EGFP by the chicken beta-actin promoter associated with the cytomegalovirus immediate-early enhancer in cochlear cells showed that each of these serotypes successfully targets distinct cochlear cell types. In contrast to the other serotypes, the AAV3 vector specifically transduced cochlear inner hair cells with high efficiency in vivo, while the AAV1, 2, 5, 7, and 8 vectors also transduced these and other cell types, including spiral ganglion and spiral ligament cells. There was no loss of cochlear function with respect to evoked auditory brain-stem responses over the range of frequencies tested after the injection of AAV vectors. These findings are of value for further molecular studies of cochlear inner hair cells and for gene replacement strategies to correct recessive genetic hearing loss due to monogenic mutations in these cells.

Therapeutics of hearing loss: expectations vs reality

With the completion of the sequencing of the human genome, the field of medicine is undergoing a dramatic and fundamental change. The identification of our genes and the proteins they encode and the mechanisms of mutations that are pathogenic will allow us to devise revolutionary new ways to diagnose, treat and prevent the thousands of disorders that affect us. Certainly, disorders of the auditory system are no exception. Revealing the molecular mechanisms of hearing and understanding the role of each player in the intricate auditory network could enable us to employ gene- or cell-based therapy to cure or prevent hearing loss. To this end, much emphasis has been placed on the identification and characterization of genes involved in human deafness, as well as research on mouse models for deafness. Ultimately, the effect of genomics on medicine will be dramatic, providing us with the ability to cure sensory defects, a tangible goal that is now within our reach.

A Novel Bovine Virus Efficiently Transduces Inner Ear Neuroepithelial Cells

Disruption of the cellular composition or arrangement of the sensory epithelia due to hair cell or supporting cell damage leads to hearing loss and vestibular dysfunctions. These peripheral hearing disorders make good targets for gene therapy; however, development requires efficient gene transfer methods for the inner ear. Here we characterized the cellular tropism of a novel adeno-associated bovine virus vector (BAAV) in cultured rat inner ear epithelia. To help identify transduced cells, we used beta-actin-GFP as a reporter gene. We found that BAAV efficiently transduced auditory and vestibular hair cells as well as all types of supporting cells with no apparent pathological effects. The number of transduced hair cells significantly increased in both a dose- and a time-dependent manner. Transduction was independent of the cells' maturation state and was observed in both P2 and P10 cultures. Interestingly, even after several days of incubation with BAAV, hair cells demonstrated varying progression of beta-actin-GFP incorporation into the stereocilia. This suggests that the onset of viral transduction can occur throughout the course of the experiment. Of the other tested AAVs, AAV2 and AAV5 transduced only a small percentage of inner and vestibular hair cells, respectively, whereas no transduction was detected with AAV4.

Reverse engineering the L-type Ca2+ channel alpha1c subunit in adult cardiac myocytes using novel adenoviral vectors

The alpha(1c) subunit of the cardiac L-type Ca(2+) channel, which contains the channel pore, voltage- and Ca(2+)-dependent gating structures, and drug binding sites, has been well studied in heterologous expression systems, but many aspects of L-type Ca(2+) channel behavior in intact cardiomyocytes remain poorly characterized. Here, we develop adenoviral constructs with E1, E3 and fiber gene deletions, to allow incorporation of full-length alpha(1c) gene cassettes into the adenovirus backbone. Wild-type (alpha(1c-wt)) and mutant (alpha(1c-D-)) Ca(2+) channel adenoviruses were constructed. The alpha(1c-D-) contained four point substitutions at amino acid residues known to be critical for dihydropyridine binding. Both alpha(1c-wt) and alpha(1c-D-) expressed robustly in A549 cells (peak L-type Ca(2+) current (I(CaL)) at 0 mV: alpha(1c-wt) -9.94+/-1.00pA/pF, n=9; alpha(1c-D-) -10.30pA/pF, n=12). I(CaL) carried by alpha(1c-D-) was markedly less sensitive to nitrendipine (IC(50) 17.1 microM) than alpha(1c-wt) (IC(50) 88 nM); a feature exploited to discriminate between engineered and native currents in transduced guinea-pig myocytes. 10 microM nitrendipine blocked only 51+/-5% (n=9) of I(CaL) in alpha(1c-D-)-expressing myocytes, in comparison to 86+/-8% (n=9) of I(CaL) in control myocytes. Moreover, in 20 microM nitrendipine, calcium transients could still be evoked in alpha(1c-D-)-transduced cells, but were largely blocked in control myocytes, indicating that the engineered channels were coupled to sarcoplasmic reticular Ca(2+) release. These alpha(1c) adenoviruses provide an unprecedented tool for structure-function studies of cardiac excitation-contraction coupling and L-type Ca(2+) channel regulation in the native myocyte background.

Proliferation of Functional Hair Cells in Vivo in the Absence of the Retinoblastoma Protein

In mammals, hair cell loss causes irreversible hearing and balance impairment because hair cells are terminally differentiated and do not regenerate spontaneously. By profiling gene expression in developing mouse vestibular organs, we identified the retinoblastoma protein (pRb) as a candidate regulator of cell cycle exit in hair cells. Differentiated and functional mouse hair cells with a targeted deletion of Rb1 undergo mitosis, divide, and cycle, yet continue to become highly differentiated and functional. Moreover, acute loss of Rb1 in postnatal hair cells caused cell cycle reentry. Manipulation of the pRb pathway may ultimately lead to mammalian hair cell regeneration.

Peptidyl-Glycine α-Amidating Monooxygenase Targeting and Shaping of Atrial Secretory Vesicles

ANP (atrial natriuretic peptide) is widely recognized as an important vasorelaxant, diuretic, and cardioprotective hormone. Little is known, however, about how ANP-secretory vesicles form within the atrial myocytes. Secretory vesicles were visualized by fluorescence microscope imaging in live rat atrial myocytes expressing proANP–enhanced green fluorescent protein (EGFP), or N-terminal–mutated fusion proteins thought to suppress the calcium-dependent aggregation of proANP. Results showed the following: (1) aggregates of proANP and coexpressed proANP-EGFP recruited peptidylglycine α-amidating monooxygenase (PAM)-1, an abundant atrial integral vesicle membrane protein; (2) coexpressed N-terminal–mutated (Glu23,24→Gln23,24) and N-terminal–deleted proANP-EGFP inhibited recruitment of PAM-1 by up to 60%; (3) 4-phenyl-3-butenoic acid (PBA) (10 μmol/L), a pharmacological inhibitor of the lumenal peptidylglycine α-hydroxylating monooxygenase domain of PAM proteins, inhibited recruitment of endogenous PAM-1 and of coexpressed pro-EGFP–PAM-1; (4) PBA had no effect on exocytosis of the potassium inward rectifier KIR2.1; (5) PBA induced a deformation of the secretory vesicles but did not inhibit docking. These findings suggest that recruitment of PAM-1 to secretory vesicles depends on intact N-terminal proANP and on the lumenal domain of PAM-1. Conversely, PAM-1 participates in shaping the proANP-secretory vesicles. The full text of this article is available online at http://circres.ahajournals.org.

TRPA1 is a candidate for the mechanosensitive transduction channel of vertebrate hair cells

Mechanical deflection of the sensory hair bundles of receptor cells in the inner ear causes ion channels located at the tips of the bundle to open, thereby initiating the perception of sound. Although some protein constituents of the transduction apparatus are known, the mechanically gated transduction channels have not been identified in higher vertebrates. Here, we investigate TRP (transient receptor potential) ion channels as candidates and find one, TRPA1 (also known as ANKTM1), that meets criteria for the transduction channel. The appearance of TRPA1 messenger RNA expression in hair cell epithelia coincides developmentally with the onset of mechanosensitivity. Antibodies to TRPA1 label hair bundles, especially at their tips, and tip labelling disappears when the transduction apparatus is chemically disrupted. Inhibition of TRPA1 protein expression in zebrafish and mouse inner ears inhibits receptor cell function, as assessed with electrical recording and with accumulation of a channel-permeant fluorescent dye. TRPA1 is probably a component of the transduction channel itself.

Sensory hair cell death and regeneration: two halves of the same equation

PURPOSE OF REVIEW

Sensory hair cells are susceptible to ototoxic damage from a variety of sources, including antibiotic treatment. Unfortunately, this often results in permanent hearing and/or balance problems in humans. By understanding how sensory hair cells die in response to aminoglycoside treatment, preventive strategies may be developed. This review will discuss some of the key recent findings in sensory hair cell death and regeneration.

RECENT FINDINGS

Aminoglycosides induce hair cell death through the initiation of apoptosis. Early and late stages of hair cell apoptosis have been defined, and several of the key molecules involved in the cascade have been identified. Moreover, specific inhibitors of apoptosis rescue hair cells from death and preserve function. Hair cell death has been shown to induce regeneration through supporting cell transdifferentiation, proliferation, and new hair cell differentiation in birds and lower vertebrates. Regeneration in the mammalian cochlea does not occur spontaneously, but genetic manipulation of cell cycle genes, induction of new hair cells through gene therapy, and introduction of stem cells into damaged cochleas suggest that repair and replacement of lost hair cells in the organ of Corti may be possible. Finally, continuing investigations of the mouse, zebrafish, and human genomes may one day enable manipulation of the cochlea so that functional regeneration is readily available as a therapeutic intervention.

SUMMARY

The discovery that hair cells can regenerate in birds and other nonmammalian vertebrates has fueled a wide range of studies to find ways to restore hearing and balance in mammals. The demonstration that apoptosis and proliferation are coupled as controlling factors in regeneration and the advent of new approaches such as gene therapy, stem cell transplantation, and genomics may lead to methods for inducing hair cell regeneration and repair in the mammalian cochlear and vestibular systems.

Differential roles of fibroblast growth factor-2 during development and maintenance of auditory sensory epithelia

Fibroblast growth factor-2 (FGF2) has been postulated to be a key regulator involved in the proliferation, differentiation, and regeneration of sensory hair cells. Here we have addressed the potential functions of FGF2 during the formation and regeneration of the auditory epithelium in chicken and mice. By using viral gene transfer, based on herpes simplex type 1 virus (HSV-1), we show that ectopically applied FGF2 drastically increases the number of cells expressing early hair cell markers during embryonic development in avians. Intriguingly, FGF2 does not stimulate cell division during this process. These data suggest that FGF2 plays a role during differentiation of sensory hair cells in avians. To address the potential functions of FGF2 during murine inner ear development, we analyzed FGF2 mouse mutants. Mice lacking FGF2 showed normal formation of the inner ear, and no abnormalities were observed at the adult stage. Moreover, FGF2 mouse mutants showed similar hearing thresholds compared with those observed in control mice before and after noise damage. Therefore, endogenous FGF2 appears not to be essential for the development or functional maintenance of the auditory organ in mammals. In light of these results, the differential roles of FGF2 in the vertebrate inner ear are discussed with respect to its previously postulated functions.

Robust generation of new hair cells in the mature mammalian inner ear by adenoviral expression of Hath1

Although hair cells regenerate spontaneously in birds and lower vertebrates following injury, there is yet no effective way to stimulate hair cell regeneration in mature mammalian inner ears. Here we report that a large number of hair cells are produced in the sensory epithelium of cultured adult rat utricular maculae, via adenovirus-mediated overexpression of Hath1, a human atonal homolog. The generation of new hair cells via Hath1 expression does not involve cell proliferation based on bromodeoxyuridine immunocytochemistry. Furthermore, using a similar approach, hair cells are regenerated following aminoglycoside injury in these cultures. These data show conclusively that mature mammalian inner ears have the competence to produce a large number of new hair cells. Local adenoviral gene therapy in the inner ear may be a potential approach to treatment of hearing and balance disorders.

GFP-tagged CFTR transgene is functional in the G551D cystic fibrosis mouse colon

Trafficking of the cystic fibrosis transmembrane conductance regulator (CFTR) is central to its function, with the most common mutation, deltaF508, resulting in abnormal processing and trafficking. Therefore, there is a significant need to develop tools, which enable the trafficking of CFTR to be studied in vitro and in vivo. In previous studies it has been demonstrated that fusion of the green fluorescent protein (GFP) to the N-terminus of CFTR does lead to functional expression of CFTR chloride channels in epithelial cell lines. The aim of the present study was to examine whether it is possible to express GFP-tagged CFTR as a transgene in colonic and airway epithelial cells of cystic fibrosis (CF) mice and to correct the CF defect. Using the epithelial-specific human cytokeratin promoter K18, we generated bitransgenic mice cftr(G551D/G551) K18-GFP-CFTR(+/-), designated GFP mice. Transcripts for GFP-CFTR could be detected in bitransgenic mice by use of RT-PCR techniques. Expression of GFP-CFTR protein was detected specifically in the colonic epithelium by both direct GFP fluorescence and the use of an anti-GFP antibody. Ussing chamber studies showed that the ion transport defect in colon and airways observed in cftr(G551D/G551D) mice was partially corrected in the bitransgenic animals. Thus, K18-GFP-CFTR is functionally expressed in transgenic mice, which will be a valuable tool in studies on CFTR synthesis, processing and ion transport in native epithelial tissues.

Multiple forms of synaptic plasticity triggered by selective suppression of activity in individual neurons

The rules by which neuronal activity causes long-term modification of synapses in the central nervous system are not fully understood. Whereas competitive or correlation-based rules result in local modification of synapses, homeostatic modifications allow neuron-wide changes in synaptic strength, promoting stability. Experimental investigations of these rules at central nervous system synapses have relied generally on manipulating activity in populations of neurons. Here, we investigated the effect of suppressing excitability in single neurons within a network of active hippocampal neurons by overexpressing an inward-rectifier potassium channel. Reducing activity in a neuron before synapse formation leads to a reduction in functional synaptic inputs to that neuron; no such reduction was observed when activity of all neurons was uniformly suppressed. In contrast, suppressing activity in a single neuron after synapses are established results in a homeostatic increase in synaptic input, which restores the activity of the neuron to control levels. Our results highlight the differences between global and selective suppression of activity, as well as those between early and late manipulation of activity.

Gene transfer into supporting cells of the organ of Corti

To utilize the rapidly accumulating genetic information for developing new therapeutic technologies for inner ear disease, it is necessary to design technologies for expressing transgenes in the inner ear, especially in the organ of Corti. We examined the outcome of an adenovirus gene transfer into the organ of Corti via the scala media in guinea pigs. The transgene insert is the bacterial lacZ gene driven by a cytomegalovirus promoter. We demonstrate that the inoculation is detrimental to the hair cells that surround the site of inoculation, but the supporting cells in the organ of Corti survive and retain the ability to express the reporter transgene beta-gal. The ability to deliver transgenes that are expressed in the supporting cells is an important step in the development of clinically applicable treatments that involve hair cell regeneration.

Transgene expression in neonatal mouse inner ear explants mediated by first and advanced generation adenovirus vectors

The mouse serves as a valuable model for treatment leading to the prevention and therapy of inner ear disease. Transgenic correction of genetic inner ear disease in mice may help develop treatment for human genetic inner ear disease. In mutations involving hair cells (HCs) or supporting cells (SCs), it is necessary to insert the wild-type transgenes directly into these cells. We used inner ear explants to characterize the transgenic expression using adenovirus-mediated reporter genes (bacterial lacZ). The variable parameters were the age of the explants (P1-P5), the type of vector (first and advanced generation adenovirus) and the genotype of the mouse (wild-type versus shaker-2 mutant). Transduction of cochlear HCs was detected at P1 and in some of the P3 cochleae. Low efficiency transduction of SCs was observed in P1 explants, but the efficiency increased with age and reached high levels at P5. The pattern of transduction was similar regardless of the genotype and the type of vector used. The data demonstrate that differentiating HCs and SCs in mouse explants can be transduced by adenovirus vectors, suggesting that cultures of mouse ears are a valuable model for developing inner ear gene therapy protocols.

Application of new biological approaches to stimulate sensory repair and protection

The inner ear governs hearing and balance via six sense organs, each composed of a few thousand mechanosensory hair cells. Most inner ear disorders involve irreversible loss of hair cells and their associated nerves. They are a function of age, genetic abnormalities and environmental factors such as noise and the use of ototoxic drugs. The genetics and cell biology of the inner ear have revealed some key molecular mechanisms of development and sensory degeneration that raise hopes for new therapeutic approaches to the regeneration of sensory function. This review highlights these advances and the approaches that might be taken to effect protection and repair. It concludes with the suggestion that we can expect tangible, practical progress towards the clinic over the next 5-10 years and that, to provide the training and skills required to take full advantage of emerging technologies, we should forge much closer links between specialist clinicians and basic scientists.

Transfection of young guinea pig vestibular cells in vitro with an adenovirus vector

This study shows distributions of lacZ-positive cells in the vestibular labyrinthine explants of young guinea pigs with mature ears. When adenovirus lacZ vectors were administered to the vestibular labyrinth following removal of the otoconial membrane, lacZ-positive cells were observed in the mesothelial cells in the perilymphatic space, in the sensory and supporting cells in the utricle and ampulla, and in the transitional and dark cells in the ampulla. When the otoconial membrane was preserved, lacZ-positive cells were not distributed in the utricular sensory epithelium. These findings suggest that adenovirus vectors can transform a variety of vestibular epithelial cells, but that it is difficult for adenovirus vectors to pass through the otoconial membrane.

Molecular genetic approaches to the targeted suppression of neuronal activity

Understanding how the diverse cells of the nervous system generate sensations, memories and behaviors is a profound challenge. This is because the activity of most neurons cannot easily be monitored or individually manipulated in vivo. As a result, it has been difficult to determine how different neurons contribute to nervous system function, even in simple organisms like Drosophila. Recent advances promise to change this situation by supplying molecular genetic tools for modulating neuronal activity that can be deployed in a spatially and temporally restricted fashion. In some cases, targeted groups of neurons can be 'switched off' and back 'on' at will in living, behaving animals.

Hepatitis C viral proteins affect cell viability and membrane permeability

To determine the effect of hepatitis C virus (HCV) proteins on cell growth, Huh-7 cells were transfected with a full-length HCV cDNA (pMO9.6-T7 Rz) clone and HCV proteins were expressed using a replication-defective adenovirus that encodes the gene for the T7 RNA polymerase. Expression of HCV proteins from this full-length clone resulted in reduction in viability of transfected cells as measured by trypan blue viability assay. For identification and separation of cells expressing hepatitis C virus proteins by fluorescence microscopy and flow cytometry, GFP was cloned in the HCV full-length clone. Cells transfected with the HCV-GFP chimera clone produced high levels of accurately processed structural and nonstructural proteins similar to those of the HCV full-length clone, which could be detected by Western blot analysis. Cells expressing all HCV proteins lost membrane permeability and underwent apoptotic cell death, indicated by the appearance of a sub-G0 peak in cell cycle analysis, DNA fragmentation in a TUNEL assay, and microscopic detection of nuclear condensation. Using double-channel flow analysis we confirmed that high-level expression of HCV proteins affected membrane permeability and cell survival. These results suggest that expression of all structural and nonstructural proteins from HCV cDNA in hepatic cells induces apoptotic cell death, which might be an important event in chronic hepatitis infection in humans.

A modified adenovirus can transfect cochlear hair cells in vivo without compromising cochlear function

The loss of cochlear hair cells, or the loss of their capacity to transduce acoustic signals, is believed to be the underlying mechanism in many forms of hearing loss. To develop viral vectors that allow for the introduction of genes directly into the cochleae of adult animals, replication-deficient (E1(-), E3(-)) and replication-defective (E1(-), E3(-), pol(-)) adenovirus vectors were used to transduce the bacterial beta-galactosidase gene into the hair cells of the guinea pig cochlea in vivo. Distortion product otoacoustic emissions, which monitor the functional status of outer hair cells, were measured throughout the viral infection periods to identify hair cell ototoxicity. The results demonstrated that the use of the (E1(-), E3(-)) adenovirus vectors containing CMV-driven LacZ, compromised cochlear function when gradually introduced into scala tympani via an osmotic pump. However, when (E1(-), E3(-), pol(-)) adenoviral vectors containing CMV-driven LacZ were used to transduce cochlear hair cells, there was no loss of cochlear function over the frequency regions tested, and beta-galactosidase (beta-gal) was detected in over 80% of all hair cells. Development of a viral vector that infects cochlear hair cells without virus-induced ototoxic effects is crucial for gene replacement strategies to treat certain forms of inherited deafness and for otoprotective strategies to prevent hair cell losses to treat progressive hearing disorders. Moreover, in vivo (E1(-), E3(-), pol(-)) adenovirus mediated gene-transfer techniques applied to adult guinea pig cochleae may be useful in testing several hypotheses concerning what roles specific genes play in normal cochlear function.