Genetic, cellular, and functional evidence for Ca2+ inflow through Cav1.2 and Cav1.3 channels in murine spiral ganglion neurons

The role of GABAB receptors in the subcortical pathways of the mammalian auditory system

GABAB receptors are G-protein coupled receptors for the inhibitory neurotransmitter GABA. Functional GABAB receptors are formed as heteromers of GABAB1 and GABAB2 subunits, which further associate with various regulatory and signaling proteins to provide receptor complexes with distinct pharmacological and physiological properties. GABAB receptors are widely distributed in nervous tissue, where they are involved in a number of processes and in turn are subject to a number of regulatory mechanisms. In this review, we summarize current knowledge of the cellular distribution and function of the receptors in the inner ear and auditory pathway of the mammalian brainstem and midbrain. The findings suggest that in these regions, GABAB receptors are involved in processes essential for proper auditory function, such as cochlear amplifier modulation, regulation of spontaneous activity, binaural and temporal information processing, and predictive coding. Since impaired GABAergic inhibition has been found to be associated with various forms of hearing loss, GABAB dysfunction could also play a role in some pathologies of the auditory system.

CACHD1-deficient mice exhibit hearing and balance deficits associated with a disruption of calcium homeostasis in the inner ear

CACHD1 recently was shown to be an α2δ-like subunit that can modulate the activity of some types of voltage-gated calcium channels, including the low-voltage activated, T-type Ca_V3 channels. CACHD1 is widely expressed in the central nervous system but its biological functions and relationship to disease states are unknown. Here, we report that mice with deleterious Cachd1 mutations are hearing impaired and have balance defects, demonstrating that CACHD1 is functionally important in the peripheral auditory and vestibular organs of the inner ear. The vestibular dysfunction of Cachd1 mutant mice, exhibited by leaning and head tilting behaviors, is related to a deficiency of calcium carbonate crystals (otoconia) in the saccule and utricle. The auditory dysfunction, shown by ABR threshold elevations and reduced DPOAEs, is associated with reduced endocochlear potentials and increased endolymph calcium concentrations. Paint-fills of mutant inner ears from prenatal and newborn mice revealed dilation of the membranous labyrinth caused by an enlarged volume of endolymph. These pathologies all can be related to a disturbance of calcium homeostasis in the endolymph of the inner ear, presumably caused by the loss of CACHD1 regulatory effects on voltage-gated calcium channel activity. Cachd1 expression in the cochlea appears stronger in late embryonic stages than in adults, suggesting an early role in establishing endolymph calcium concentrations. Our findings provide new insights into CACHD1 function and suggest the involvement of voltage-gated calcium channels in endolymph homeostasis, essential for normal auditory and vestibular function.

Single-Cell RNA Analysis of Type I Spiral Ganglion Neurons Reveals a Lmx1a Population in the Cochlea

In the mature cochlea, each inner hair cell (IHC) is innervated by multiple spiral ganglion neurons of type I (SGNI). SGNIs are morphologically and electro-physiologically diverse. Also, they differ in their susceptibility to noise insult. However, the molecular underpinnings of their identity and physiological differences remain poorly understood. In this study, we developed a novel triple transgenic mouse, which enabled the isolation of pure populations of SGNIs and the analysis of a 96-gene panel via single-cell qPCR. We found three distinct populations of Type I SGNs, which were marked by their exclusive expression of Lmx1a, Slc4a4, or Mfap4/Fzd2, respectively, at postnatal days P3, P8, and P12. Our data suggest that afferent SGN subtypes are established genetically before the onset of hearing and that the expression of key physiological markers, such as ion channels, is heterogeneous and may be underlying the heterogeneous firing proprieties of SGNIs.

Early functional alterations in membrane properties and neuronal degeneration are hallmarks of progressive hearing loss in NOD mice

Presbycusis or age-related hearing loss (ARHL) is the most common sensory deficit in the human population. A substantial component of the etiology stems from pathological changes in sensory and non-sensory cells in the cochlea. Using a non-obese diabetic (NOD) mouse model, we have characterized changes in both hair cells and spiral ganglion neurons that may be relevant for early signs of age-related hearing loss (ARHL). We demonstrate that hair cell loss is preceded by, or in parallel with altered primary auditory neuron functions, and latent neurite retraction at the hair cell-auditory neuron synapse. The results were observed first in afferent inner hair cell synapse of type I neurites, followed by type II neuronal cell-body degeneration. Reduced membrane excitability and loss of postsynaptic densities were some of the inaugural events before any outward manifestation of hair bundle disarray and hair cell loss. We have identified profound alterations in type I neuronal membrane properties, including a reduction in membrane input resistance, prolonged action potential latency, and a decrease in membrane excitability. The resting membrane potential of aging type I neurons in the NOD, ARHL model, was significantly hyperpolarized, and analyses of the underlying membrane conductance showed a significant increase in K⁺ currents. We propose that attempts to alleviate some forms of ARHL should include early targeted primary latent neural degeneration for effective positive outcomes.

Deletion of the Ca2+ Channel Subunit α2δ3 Differentially Affects Cav2.1 and Cav2.2 Currents in Cultured Spiral Ganglion Neurons Before and After the Onset of Hearing

Voltage-gated Ca²⁺ channels are composed of a pore-forming α₁ subunit and auxiliary β and α₂δ subunits, which modulate Ca²⁺ current properties and channel trafficking. So far, the partial redundancy and specificity of α₁ for α₂δ subunits in the CNS have remained largely elusive. Mature spiral ganglion (SG) neurons express α₂δ subunit isoforms 1, 2, and 3 and multiple Ca²⁺ channel subtypes. Differentiation and in vivo functions of their endbulb of Held synapses, which rely on presynaptic P/Q channels (Lin et al., 2011), require the α₂δ3 subunit (Pirone et al., 2014). This led us to hypothesize that P/Q channels may preferentially co-assemble with α₂δ3. Using a dissociated primary culture, we analyzed the effects of α₂δ3 deletion on somatic Ca²⁺ currents (I_Ca) of SG neurons isolated at postnatal day 20 (P20), when the cochlea is regarded to be mature. P/Q currents were the dominating steady-state Ca²⁺ currents (54% of total) followed by T-type, L-type, N-type, and R-type currents. Deletion of α₂δ3 reduced P/Q- and R-type currents by 60 and 38%, respectively, whereas L-type, N-type, and T-type currents were not altered. A subset of I_Ca types was also analyzed in SG neurons isolated at P5, i.e., before the onset of hearing (P12). Both L-type and N-type current amplitudes of wildtype SG neurons were larger at P5 compared with P20. Deletion of α₂δ3 reduced L-type and N-type currents by 23 and 44%, respectively. In contrast, small P/Q currents, which were just being up-regulated at P5, were unaffected by the lack of α₂δ3. In summary, α₂δ3 regulates amplitudes of L- and N-type currents of immature cultured SG neurons, whereas it regulates P/Q- and R-type currents at P20. Our data indicate a developmental switch from dominating somatic N- to P/Q-type currents in cultured SG neurons. A switch from N- to P/Q-type channels, which has been observed at several central synapses, may also occur at developing endbulbs of Held. In this case, reduction of both neonatal N- (P5) and more mature P/Q-type currents (around/after hearing onset) may contribute to the impaired morphology and function of endbulb synapses in α₂δ3-deficient mice.

Semaphorin 3A induces acute changes in membrane excitability in spiral ganglion neurons in vitro

The development and survival of spiral ganglion neurons (SGNs) are dependent on multiple trophic factors as well as membrane electrical activity. Semaphorins (Sema) constitute a family of membrane-associated and secreted proteins that have garnered significant attention as a potential SGN "navigator" during cochlea development. Previous studies using mutant mice demonstrated that Sema3A plays a role in the SGN pathfinding. The mechanisms, however, by which Sema3A shapes SGNs firing behavior are not known. In these studies, we found that Sema3A plays a novel role in regulating SGN resting membrane potential and excitability. Using dissociated SGN from pre-hearing (P3-P5) and post-hearing mice (P12-P15), we recorded membrane potentials using whole-cell patch clamp recording techniques in apical and basal SGN populations. Recombinant Sema3A was applied to examine the effects on intrinsic membrane properties and action potentials evoked by current injections. Apical and basal SGNs from newborn mice treated with recombinant Sema3A (100 ng/ml) displayed a higher resting membrane potential, higher threshold, decreased amplitude, and prolonged latency and duration of spikes. Although a similar phenomenon was observed in SGNs from post-hearing mice, the resting membrane potential was essentially indistinguishable before and after Sema3A exposure. Sema3A-mediated changes in membrane excitability were associated with a significant decrease in K⁺ and Ca²⁺ currents. Sema3A acts through linopirdine-sensitive K⁺ channels in apical, but not in the basal SGNs. Therefore, Sema3A induces differential effects in SGN membrane excitability that are dependent on age and location, and constitutes an additional early and novel effect of Sema3A SGNs in vitro.

Otoprotective Effects of Stephania tetrandra S. Moore Herb Isolate against Acoustic Trauma

Noise is the most common occupational and environmental hazard, and noise-induced hearing loss (NIHL) is the second most common form of sensorineural hearing deficit. Although therapeutics that target the free-radical pathway have shown promise, none of these compounds is currently approved against NIHL by the United States Food and Drug Administration. The present study has demonstrated that tetrandrine (TET), a traditional Chinese medicinal alkaloid and the main chemical isolate of the Stephania tetrandra S. Moore herb, significantly attenuated NIHL in CBA/CaJ mice. TET is known to exert antihypertensive and antiarrhythmic effects through the blocking of calcium channels. Whole-cell patch-clamp recording from adult spiral ganglion neurons showed that TET blocked the transient Ca2+ current in a dose-dependent manner and the half-blocking concentration was 0.6 + 0.1 μM. Consistent with previous findings that modulations of calcium-based signaling pathways have both prophylactic and therapeutic effects against neural trauma, NIHL was significantly diminished by TET administration. Importantly, TET has a long-lasting protective effect after noise exposure (48 weeks) in comparison to 2 weeks after noise exposure. The otoprotective effects of TET were achieved mainly by preventing outer hair cell damage and synapse loss between inner hair cells and spiral ganglion neurons. Thus, our data indicate that TET has great potential in the prevention and treatment of NIHL.

Functions of CaBP1 and CaBP2 in the peripheral auditory system

CaBPs are a family of Ca²⁺ binding proteins related to calmodulin. Two CaBP family members, CaBP1 and CaBP2, are highly expressed in the cochlea. Here, we investigated the significance of CaBP1 and CaBP2 for hearing in mice lacking expression of these proteins (CaBP1 KO and CaBP2 KO) using auditory brain responses (ABRs) and distortion product otoacoustic emissions (DPOAEs). In CaBP1 KO mice, ABR wave I was larger in amplitude, and shorter in latency and faster in decay, suggestive of enhanced synchrony of auditory nerve fibers. This interpretation was supported by the greater excitability of CaBP1 KO than WT neurons in whole-cell patch clamp recordings of spiral ganglion neurons in culture, and normal presynaptic function of CaBP1 KO IHCs. DPOAEs and ABR thresholds were normal in 4-week old CaBP1 KO mice, but elevated ABR thresholds became evident at 32 kHz at 9 weeks, and at 8 and 16 kHz by 6 months of age. In contrast, CaBP2 KO mice exhibited significant ABR threshold elevations at 4 weeks of age that became more severe in the mid-frequency range by 9 weeks. Though normal at 4 weeks, DPOAEs in CaBP2 KO mice were significantly reduced in the mid-frequency range by 9 weeks. Our results reveal requirements for CaBP1 and CaBP2 in the peripheral auditory system and highlight the diverse modes by which CaBPs influence sensory processing.

CaBP1 regulates Cav1 L-type Ca2+ channels and their coupling to neurite growth and gene transcription in mouse spiral ganglion neurons

CaBP1 is a Ca²⁺ binding protein that is widely expressed in neurons in the brain, retina, and cochlea. In heterologous expression systems, CaBP1 interacts with and regulates voltage-gated Ca_v Ca²⁺ channels but whether this is the case in neurons is unknown. Here, we investigated the cellular functions of CaBP1 in cochlear spiral ganglion neurons (SGNs), which express high levels of CaBP1. Consistent with the role of CaBP1 as a suppressor of Ca²⁺-dependent inactivation (CDI) of Ca_v1 (L-type) channels, Ca_v1 currents underwent greater CDI in SGNs from mice lacking CaBP1 (C-KO) than in wild-type (WT) SGNs. The coupling of Ca_v1 channels to downstream signaling pathways was also disrupted in C-KO SGNs. Activity-dependent repression of neurite growth was significantly blunted and unresponsive to Ca_v1 antagonists in C-KO SGNs in contrast to WT SGNs. Moreover, Ca_v1-mediated Ca²⁺ signals and phosphorylation of cAMP-response element binding protein were reduced in C-KO SGNs compared to WT SGNs. Our findings establish a role for CaBP1 as an essential regulator of Ca_v1 channels in SGNs and their coupling to downstream pathways controlling activity-dependent transcription and neurite growth.

The extracellular matrix molecule brevican is an integral component of the machinery mediating fast synaptic transmission at the calyx of Held

KEY POINTS

The proteoglycan brevican is a major component of the extracellular matrix of perineuronal nets and is highly enriched in the perisynaptic space suggesting a role for synaptic transmission. We have introduced the calyx of Held in the auditory brainstem as a model system to study the impact of brevican on dynamics and reliability of synaptic transmission. In vivo extracellular single-unit recordings at the calyx of Held in brevican-deficient mice yielded a significant increase in the action potential (AP) transmission delay and a prolongation of pre- and postsynaptic APs. The changes in dynamics of signal transmission were accompanied by the reduction of presynaptic vGlut1 and ultrastructural changes in the perisynaptic space. These data show that brevican is an important mediator of fast synaptic transmission at the calyx of Held.

ABSTRACT

The extracellular matrix is an integral part of the neural tissue. Its most conspicuous manifestation in the brain are the perineuronal nets (PNs) which surround somata and proximal dendrites of distinct neuron types. The chondroitin sulfate proteoglycan brevican is a major component of PNs. In contrast to other PN-comprising proteoglycans (e.g. aggrecan and neurocan), brevican is mainly expressed in the perisynaptic space closely associated with both the pre- and postsynaptic membrane. This specific localization prompted the hypothesis that brevican might play a role in synaptic transmission. In the present study we specifically investigated the role of brevican in synaptic transmission at a central synapse, the calyx of Held in the medial nucleus of the trapezoid body, by the use of in vivo electrophysiology, immunohistochemistry, biochemistry and electron microscopy. In vivo extracellular single-unit recordings were acquired in brevican-deficient mice and the dynamics and reliability of synaptic transmission were compared to wild-type littermates. In knockout mice, the speed of pre-to-postsynaptic action potential (AP) transmission was reduced and the duration of the respective pre- and postsynaptic APs increased. The reliability of signal transmission, however, was not affected by the lack of brevican. The changes in dynamics of signal transmission were accompanied by the reduction of (i) presynaptic vGlut1 and (ii) the size of subsynaptic cavities. The present results suggest an essential role of brevican for the functionality of high-speed synaptic transmission at the calyx of Held.

L-type Calcium Channel Cav1.2 Is Required for Maintenance of Auditory Brainstem Nuclei

Cav1.2 and Cav1.3 are the major L-type voltage-gated Ca(2+) channels in the CNS. Yet, their individual in vivo functions are largely unknown. Both channel subunits are expressed in the auditory brainstem, where Cav1.3 is essential for proper maturation. Here, we investigated the role of Cav1.2 by targeted deletion in the mouse embryonic auditory brainstem. Similar to Cav1.3, loss of Cav1.2 resulted in a significant decrease in the volume and cell number of auditory nuclei. Contrary to the deletion of Cav1.3, the action potentials of lateral superior olive (LSO) neurons were narrower compared with controls, whereas the firing behavior and neurotransmission appeared unchanged. Furthermore, auditory brainstem responses were nearly normal in mice lacking Cav1.2. Perineuronal nets were also unaffected. The medial nucleus of the trapezoid body underwent a rapid cell loss between postnatal days P0 and P4, shortly after circuit formation. Phosphorylated cAMP response element-binding protein (CREB), nuclear NFATc4, and the expression levels of p75NTR, Fas, and FasL did not correlate with cell death. These data demonstrate for the first time that both Cav1.2 and Cav1.3 are necessary for neuronal survival but are differentially required for the biophysical properties of neurons. Thus, they perform common as well as distinct functions in the same tissue.

Considering optogenetic stimulation for cochlear implants

Electrical cochlear implants are by far the most successful neuroprostheses and have been implanted in over 300,000 people worldwide. Cochlear implants enable open speech comprehension in most patients but are limited in providing music appreciation and speech understanding in noisy environments. This is generally considered to be due to low frequency resolution as a consequence of wide current spread from stimulation contacts. Accordingly, the number of independently usable stimulation channels is limited to less than a dozen. As light can be conveniently focused, optical stimulation might provide an alternative approach to cochlear implants with increased number of independent stimulation channels. Here, we focus on summarizing recent work on optogenetic stimulation as one way to develop optical cochlear implants. We conclude that proof of principle has been presented for optogenetic stimulation of the cochlea and central auditory neurons in rodents as well as for the technical realization of flexible μLED-based multichannel cochlear implants. Still, much remains to be done in order to advance the technique for auditory research and even more for eventual clinical translation. This article is part of a Special Issue entitled <Lasker Award>.

Dynamic firing properties of type I spiral ganglion neurons

Spiral ganglion neurons, the first neural element in the auditory system, possess complex intrinsic properties, possibly required to process frequency-specific sensory input that is integrated with extensive efferent regulation. Together with their tonotopically-graded sizes, the somata of these neurons reveal a sophisticated electrophysiological profile. Type I neurons, which make up ~95 % of the ganglion, have myriad voltage-gated ion channels that not only vary along the frequency contour of the cochlea, but also can be modulated by regulators such as voltage, calcium, and second messengers. The resultant developmentally- and tonotopically-regulated neuronal firing patterns conform to three distinct response modes (unitary, rapid, and slow) based on threshold and accommodation. This phenotype, however, is not static for any individual type I neuron. Recent observations have shown that, as neurons become less excitable with age, they demonstrate enhanced plasticity enabling them to change from one response mode to another depending upon resting membrane potential and the presence of neurotrophin-3. Thus, the primary auditory afferents utilized to encode dynamic acoustic stimuli possess the intrinsic specializations that allow them dynamically to alter their firing pattern.