The p75 neurotrophin receptor protects primary auditory neurons against acoustic trauma in mice

Increased susceptibility to acoustic trauma in a mouse model of non-syndromic sensorineural deafness, DFNB91

Inactivating mutations of SERPINB6 in humans result in progressive hearing loss starting in early adulthood (DFNB91). We have previously shown that C57BL/6J mice lacking the orthologous gene, Serpinb6a, exhibit progressive hearing loss, which is associated with progressive loss of distinct cell types in the organ of Corti beginning with outer hair cells (OHCs). However, deafness in these animals occurs much earlier than expected, possibly because C57BL/6J mice also carry an age-related hearing loss mutation in the cadherin 23 gene (Cdh23^ahl ) that causes late onset hearing loss. The CBA/CaH strain of mice does not carry Cdh23^ah/ahl and may represent a better model of the human DFNB91 patients. Here, we show that transfer of the mutant Serpinb6a allele onto the Cdh23 normal CBA/CaH background markedly delays onset of hearing loss, more closely phenocopying DFNB91, without altering the pattern of cellular loss. Young, pre-symptomatic mice of this genotype exposed to acoustic trauma exhibit permanent hearing loss, compared to controls, associated with the disappearance of OHCs. We conclude that Serpinb6 helps to maintain hearing by protecting hair cells from stress.

Small molecule modulation of the p75 neurotrophin receptor suppresses age- and genotype-associated neurodegeneration in HIV gp120 transgenic mice

The persistence of HIV in the central nervous system leads to cognitive deficits in up to 50% of people living with HIV even with systemic suppression by antiretroviral treatment. The interaction of chronic inflammation with age-associated degeneration places these individuals at increased risk of accelerated aging and other neurodegenerative diseases and no treatments are available that effectively halt these processes. The adverse effects of aging and inflammation may be mediated, in part, by an increase in the expression of the p75 neurotrophin receptor (p75^NTR) which shifts the balance of neurotrophin signaling toward less protective pathways. To determine if modulation of p75^NTR could modify the disease process, we treated HIV gp120 transgenic mice with a small molecule ligand designed to engage p75^NTR and downregulate degenerative signaling. Daily treatment with 50 mg/kg LM11A-31 for 4 months suppressed age- and genotype-dependent activation of microglia, increased microtubule associated protein-2 (MAP-2), reduced dendritic varicosities and slowed the loss of parvalbumin immunoreactive neurons in the hippocampus. An age related accumulation of microtubule associated protein Tau was identified in the hippocampus in extracellular clusters that co-expressed p75^NTR suggesting a link between Tau and p75^NTR. Although the significance of the relationship between p75^NTR and Tau is unclear, a decrease in Tau-1 immunoreactivity as gp120 mice entered old age (>16 months) suggests that the Tau may transition to more pathological modifications; a process blocked by LM11A-31. Overall, the effects of LM11A-31 are consistent with strong neuroprotective and anti-inflammatory actions that have significant therapeutic potential.

Myelin-induced inhibition in a spiral ganglion organ culture - Approaching a natural environment in vitro

The performance of a cochlear implant depends on the defined interaction between afferent neurons of the spiral ganglion and the inserted electrode. Neurite outgrowth can be induced by neurotrophins such as brain-derived neurotrophic factor (BDNF) via tropomyosin kinase receptor B (TrkB). However, neurotrophin signaling through the p75 neurotrophin receptor (p75) inhibits neurite outgrowth in the presence of myelin. Organotypic cultures derived from postnatal (P3-5) mice were used to study myelin-induced inhibition in the cochlear spiral ganglion. Neurite outgrowth was analyzed and quantified utilizing an adapted Sholl analysis. Stimulation of neurite outgrowth was quantified after application of BDNF, the selective TrkB agonist 7,8-dihydroxyflavone (7,8-DHF) and a selective inhibitor of the Rho-associated kinase (Y27632), which inhibits the p75 pathway. Myelin-induced inhibition was assessed by application of myelin-associated glycoprotein (MAG-Fc) to stimulate the inhibitory p75 pathway. Inhibition of neurite outgrowth was achieved by the selective TrkB inhibitor K252a. Stimulation of neurite outgrowth was observed after treatment with BDNF, 7,8 DHF and a combination of BDNF and Y27632. The 7,8-DHF-induced growth effects could be inhibited by K252a. Furthermore, inhibition of neurite outgrowth was observed after supplementation with MAG-Fc. Myelin-induced inhibition could be overcome by 7,8-DHF and the combination of BDNF and Y27632. In this study, myelin-induced inhibition of neurite outgrowth was established in a spiral ganglion model. We reveal that 7,8-DHF is a viable novel compound for the stimulation of neurite outgrowth in a myelin-induced inhibitory environment. The combination of TrkB stimulation and ROCK inhibition can be used to overcome myelin inhibition.

Neurog1 can partially substitute for Atoh1 function in hair cell differentiation and maintenance during organ of Corti development

Atoh1, a basic helix-loop-helix (bHLH) transcription factor (TF), is essential for the differentiation of hair cells (HCs), mechanotransducers that convert sound into auditory signals in the mammalian organ of Corti (OC). Previous work demonstrated that replacing mouse Atoh1 with the fly ortholog atonal rescues HC differentiation, indicating functional replacement by other bHLH genes. However, replacing Atoh1 with Neurog1 resulted in reduced HC differentiation compared with transient Atoh1 expression in a 'self-terminating' Atoh1 conditional null mouse (Atoh1-Cre; Atoh1(f/f)). We now show that combining Neurog1 in one allele with removal of floxed Atoh1 in a self-terminating conditional mutant (Atoh1-Cre; Atoh1(f/kiNeurog1)) mouse results in significantly more differentiated inner HCs and outer HCs that have a prolonged longevity of 9 months compared with Atoh1 self-terminating littermates. Stereocilia bundles are partially disorganized, disoriented and not HC type specific. Replacement of Atoh1 with Neurog1 maintains limited expression of Pou4f3 and Barhl1 and rescues HCs quantitatively, but not qualitatively. OC patterning and supporting cell differentiation are also partially disrupted. Diffusible factors involved in patterning are reduced (Fgf8) and factors involved in cell-cell interactions are affected (Jag1, Hes5). Despite the presence of many HCs with stereocilia these mice are deaf, possibly owing to HC and OC patterning defects. This study provides a novel approach to disrupt OC development through modulating the HC-specific intracellular TF network. The resulting disorganized OC indicates that normally differentiated HCs act as 'self-organizers' for OC development and that Atoh1 plays a crucial role to initiate HC stereocilia differentiation independently of HC viability.

The biological functions and signaling mechanisms of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including programmed cell death, axonal growth and degeneration, cell proliferation, myelination, and synaptic plasticity. The multiplicity of cellular functions governed by the receptor arises from the variety of ligands and co-receptors which associate with p75(NTR) and regulate its signaling. P75(NTR) promotes survival through interactions with Trk receptors, inhibits axonal regeneration via partnerships with Nogo receptor (Nogo-R) and Lingo-1, and promotes apoptosis through association with Sortilin. Signals downstream of these interactions are further modulated through regulated intramembrane proteolysis (RIP) of p75(NTR) and by interactions with numerous cytosolic partners. In this chapter, we discuss the intricate signaling mechanisms of p75(NTR), emphasizing how these signals are differentially regulated to mediate these diverse cellular functions.

Absence of SERPINB6A causes sensorineural hearing loss with multiple histopathologies in the mouse inner ear

A homozygous mutation of SERPINB6, a gene encoding an intracellular protease inhibitor, has recently been associated with post-lingual, autosomal-recessive, nonsyndromic hearing loss in humans (DFNB91). Herein, we describe the physiological changes underlying SERPINB6 deficiency by analyzing mutant mice in which the orthologous gene is replaced by enhanced green fluorescent protein. SERPINB6A is present in the neurosensory epithelium, lateral wall, and spiral limbus of the cochlea, with highest levels in the inner and outer hair cells of the organ of Corti, cells lining the inner sulcus, and supporting cells distributed along the epithelial gap junction layer to the outer sulcus. Measurements of hearing thresholds in these mice demonstrated age-related hearing loss in all homozygous-null, but not heterozygous, mice. Hearing impairment was first detected at 3 weeks of age, affecting only high frequencies before spreading to other frequencies as the mice aged. The defect is associated with progressive cellular degeneration within the cochlea. This begins with the hair cells, then involves the primary auditory neurons, and, finally, the fibrocytes in the lateral wall. These findings establish these mutant mice as a suitable model system to elucidate how SERPINB6 deficiency causes deafness in humans.

Lack of brain-derived neurotrophic factor hampers inner hair cell synapse physiology, but protects against noise-induced hearing loss

The precision of sound information transmitted to the brain depends on the transfer characteristics of the inner hair cell (IHC) ribbon synapse and its multiple contacting auditory fibers. We found that brain derived neurotrophic factor (BDNF) differentially influences IHC characteristics in the intact and injured cochlea. Using conditional knock-out mice (BDNF(Pax2) KO) we found that resting membrane potentials, membrane capacitance and resting linear leak conductance of adult BDNF(Pax2) KO IHCs showed a normal maturation. Likewise, in BDNF(Pax2) KO membrane capacitance (ΔC(m)) as a function of inward calcium current (I(Ca)) follows the linear relationship typical for normal adult IHCs. In contrast the maximal ΔC(m), but not the maximal size of the calcium current, was significantly reduced by 45% in basal but not in apical cochlear turns in BDNF(Pax2) KO IHCs. Maximal ΔC(m) correlated with a loss of IHC ribbons in these cochlear turns and a reduced activity of the auditory nerve (auditory brainstem response wave I). Remarkably, a noise-induced loss of IHC ribbons, followed by reduced activity of the auditory nerve and reduced centrally generated wave II and III observed in control mice, was prevented in equally noise-exposed BDNF(Pax2) KO mice. Data suggest that BDNF expressed in the cochlea is essential for maintenance of adult IHC transmitter release sites and that BDNF upholds opposing afferents in high-frequency turns and scales them down following noise exposure.

Role of PGE-type receptor 4 in auditory function and noise-induced hearing loss in mice

This study explored the physiological roles of PGE-type receptor 4 (EP4) in auditory function. EP4-deficient mice exhibited slight hearing loss and a reduction of distortion-product otoacoustic emissions (DPOAEs) with loss of outer hair cells (OHCs) in cochleae. After exposure to intense noise, these mice showed significantly larger threshold shifts of auditory brain-stem responses (ABRs) and greater reductions of DPOAEs than wild-type mice. A significant increase of OHC loss was confirmed morphologically in the cochleae of EP4-deficient mice. Pharmacological inhibition of EP4 had a similar effect to genetic deletion, causing loss of both hearing and OHCs in C57BL/6 mice, indicating a critical role for EP4 signaling in the maintenance of auditory function. Pharmacological activation of EP4 significantly protected OHCs against noise trauma, and attenuated noise-induced hearing loss in C57BL/6 mice. These findings suggest that EP4 signaling is necessary for the maintenance of cochlear physiological function and for cochlear protection against noise-induced damage, in particular OHCs. EP4 might therefore be an effective target for cochlear disease therapeutics.

Purification and transfection of cochlear Schwann cells

Schwann cells line nerve fibers in the peripheral nervous system (PNS) and synthesize myelin. In addition, they support neuronal survival, neurite growth and regeneration. In dissociated cultures of postnatal mouse spiral ganglia, regenerating neurites spontaneously associate with Schwann cells. However, the mechanisms and consequences of interactions between cochlear Schwann cells and spiral ganglion neurites have not been examined. Further, the similarities and differences between cochlear Schwann cells and other PNS Schwann cells have not been studied. Experiments to examine these questions will rely on the ability to purify and characterize cochlear Schwann cells. Here we present methods for purifying Schwann cells from postnatal mouse cochleas and for transfecting them with expression plasmids. Dissociated spiral ganglia were plated on poly-D-lysine/laminin in medium containing neurotrophins, leukemia inhibitory factor (LIF), N2 supplement and serum and maintained for 5 days. Cells were harvested with trypsin/EDTA and subjected to an immuno-magnetic purification procedure. After 24 h in vitro, cultures were >85% Schwann cells. Nucleofection of purified Schwann cells with pMax-green fluorescent protein (pMax-GFP) plasmid, or with pEGFP-C-vimentin plasmid returned >45% transfection efficiency. These methods will allow the in-depth characterization of cochlear Schwann cells and an evaluation of their biochemical, functional, and genetic mechanisms that may promote neurite growth from the spiral ganglion.