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

CaV 2.1 α1 subunit motifs that control presynaptic CaV 2.1 subtype abundance are distinct from CaV 2.1 preference

Presynaptic voltage-gated Ca2+ channel (CaV ) subtype abundance at mammalian synapses regulates synaptic transmission in health and disease. In the mammalian central nervous system (CNS), most presynaptic terminals are CaV 2.1 dominant with a developmental reduction in CaV 2.2 and CaV 2.3 levels, and CaV 2 subtype levels are altered in various diseases. However, the molecular mechanisms controlling presynaptic CaV 2 subtype levels are largely unsolved. Because the CaV 2 α1  subunit cytoplasmic regions contain varying levels of sequence conservation, these regions are proposed to control presynaptic CaV 2 subtype preference and abundance. To investigate the potential role of these regions, we expressed chimeric CaV 2.1 α1  subunits containing swapped motifs with the CaV 2.2 and CaV 2.3 α1  subunit on a CaV 2.1/CaV 2.2 null background at the calyx of Held presynaptic terminals. We found that expression of CaV 2.1 α1  subunit chimeras containing the CaV 2.3 loop II-III region or cytoplasmic C-terminus (CT) resulted in a large reduction of presynaptic Ca2+ currents compared to the CaV 2.1 α1  subunit. However, the Ca2+ current sensitivity to the CaV 2.1 blocker agatoxin-IVA was the same between the chimeras and the CaV 2.1 α1  subunit. Additionally, we found no reduction in presynaptic Ca2+ currents with CaV 2.1/2.2 cytoplasmic CT chimeras. We conclude that the motifs in the CaV 2.1 loop II-III and CT do not individually regulate CaV 2.1 preference, although these motifs control CaV 2.1 levels and the CaV 2.3 CT contains motifs that negatively regulate presynaptic CaV 2.3 levels. We propose that the motifs controlling presynaptic CaV 2.1 preference are distinct from those regulating CaV 2.1 levels and may act synergistically to impact pathways regulating CaV 2.1 preference and abundance. KEY POINTS: Presynaptic CaV 2 subtype abundance regulates neuronal circuit properties, although the mechanisms regulating presynaptic CaV 2 subtype abundance and preference remain enigmatic. The CaV α1  subunit determines subtype and contains multiple motifs implicated in regulating presynaptic subtype abundance and preference. The CaV 2.1 α1  subunit domain II-III loop and cytoplasmic C-terminus are positive regulators of presynaptic CaV 2.1 abundance but do not regulate preference. The CaV 2.3 α1  subunit cytoplasmic C-terminus negatively regulates presynaptic CaV 2 subtype abundance but not preference, whereas the CaV 2.2 α1  subunit cytoplasmic C-terminus is not a key regulator of presynaptic CaV 2 subtype abundance or preference. The CaV 2 α1  subunit motifs determining the presynaptic CaV 2 preference are distinct from abundance.

CaV2.1 α1 subunit motifs that control presynaptic CaV2.1 subtype abundance are distinct from CaV2.1 preference

Presynaptic voltage-gated Ca2+ channels (CaV) subtype abundance at mammalian synapses regulates synaptic transmission in health and disease. In the mammalian central nervous system, most presynaptic terminals are CaV2.1 dominant with a developmental reduction in CaV2.2 and CaV2.3 levels, and CaV2 subtype levels are altered in various diseases. However, the molecular mechanisms controlling presynaptic CaV2 subtype levels are largely unsolved. Since the CaV2 α1 subunit cytoplasmic regions contain varying levels of sequence conservation, these regions are proposed to control presynaptic CaV2 subtype preference and abundance. To investigate the potential role of these regions, we expressed chimeric CaV2.1 α1subunits containing swapped motifs with the CaV2.2 and CaV2.3 α1 subunit on a CaV2.1/CaV2.2 null background at the calyx of Held presynaptic terminal. We found that expression of CaV2.1 α1 subunit chimeras containing the CaV2.3 loop II-III region or cytoplasmic C-terminus (CT) resulted in a large reduction of presynaptic Ca2+ currents compared to the CaV2.1 α1 subunit. However, the Ca2+ current sensitivity to the CaV2.1 blocker Agatoxin-IVA, was the same between the chimeras and the CaV2.1 α1 subunit. Additionally, we found no reduction in presynaptic Ca2+ currents with CaV2.1/2.2 cytoplasmic CT chimeras. We conclude that the motifs in the CaV2.1 loop II-III and CT do not individually regulate CaV2.1 preference, but these motifs control CaV2.1 levels and the CaV2.3 CT contains motifs that negatively regulate presynaptic CaV2.3 levels. We propose that the motifs controlling presynaptic CaV2.1 preference are distinct from those regulating CaV2.1 levels and may act synergistically to impact pathways regulating CaV2.1 preference and abundance.

Deletions of Cacna2d3 in parvalbumin-expressing neurons leads to autistic-like phenotypes in mice

Autism spectrum disorder (ASD) is a series of highly inherited neurodevelopmental disorders. Loss-of-function (LOF) mutations in the CACNA2D3 gene are associated with ASD. However, the underlying mechanism is unknown. Dysfunction of cortical interneurons (INs) is strongly implicated in ASD. Parvalbumin-expressing (PV) INs and somatostatin-expressing (SOM) INs are the two most subtypes. Here, we characterized a mouse knockout of the Cacna2d3 gene in PV-expressing neurons (PVCre;Cacna2d3f/f mice) or in SOM-expressing neurons (SOMCre;Cacna2d3f/f mice), respectively. PVCre;Cacna2d3f/f mice showed deficits in the core ASD behavioral domains (including impaired sociability and increased repetitive behavior), as well as anxiety-like behavior and improved spatial memory. Furthermore, loss of Cacna2d3 from a subset of PV neurons results in a reduction of GAD67 and PV expression in the medial prefrontal cortex (mPFC). These may underlie the increased neuronal excitability in the mPFC, which contribute to the abnormal social behavior in PVCre;Cacna2d3f/f mice. Whereas, SOMCre;Cacna2d3f/f mice showed no obvious deficits in social, cognitive, or emotional phenotypes. Our findings provide the first evidence suggesting the causal role of Cacna2d3 insufficiency in PV neurons in autism.

Altered population activity and local tuning heterogeneity in auditory cortex of Cacna2d3-deficient mice

The α2δ3 auxiliary subunit of voltage-activated calcium channels is required for normal synaptic transmission and precise temporal processing of sounds in the auditory brainstem. In mice its loss additionally leads to an inability to distinguish amplitude-modulated tones. Furthermore, loss of function of α2δ3 has been associated with autism spectrum disorder in humans. To investigate possible alterations of network activity in the higher-order auditory system in α2δ3 knockout mice, we analyzed neuronal activity patterns and topography of frequency tuning within networks of the auditory cortex (AC) using two-photon Ca2+ imaging. Compared to wild-type mice we found distinct subfield-specific alterations in the primary auditory cortex, expressed in overall lower correlations between the network activity patterns in response to different sounds as well as lower reliability of these patterns upon repetitions of the same sound. Higher AC subfields did not display these alterations but showed a higher amount of well-tuned neurons along with lower local heterogeneity of the neurons' frequency tuning. Our results provide new insight into AC network activity alterations in an autism spectrum disorder-associated mouse model.

The circadian clock in the piriform cortex intrinsically tunes daily changes of odor-evoked neural activity

The daily activity in the brain is typically fine-tuned by the circadian clock in the local neurons as well as by the master circadian clock in the suprachiasmatic nucleus (SCN) of the hypothalamus. In the olfactory response, odor-evoked activity in the piriform cortex (PC) and olfactory behavior retain circadian rhythmicity in the absence of the SCN, yet how the circadian rhythm in the PC is achieved independently of the SCN remains elusive. Here, to define neurons regulating the circadian rhythm of the odor-evoked activity in the PC, we knocked out the clock gene Bmal1 in a host of specific neurons along the olfactory circuit. We discovered that Bmal1 knockout in the PC largely abolishes the circadian rhythm of the odor-evoked activity. We further showed that isolated PC exhibits sustained circadian rhythms of the clock gene Per2 expression. Quantitative PCR analysis revealed that expression patterns of multiple genes involved in neural activity and synaptic transmission exhibit circadian rhythm in the PC in a BMAL1-dependent manner. Our findings indicate that BMAL1 acts intrinsically in the PC to control the circadian rhythm of the odor-evoked activity in the PC, possibly through regulating expression patterns of multiple genes involved in neural activity and transmission.

Impaired Subcortical Processing of Amplitude-Modulated Tones in Mice Deficient for Cacna2d3, a Risk Gene for Autism Spectrum Disorders in Humans

Temporal processing of complex sounds is a fundamental and complex task in hearing and a prerequisite for processing and understanding vocalization, speech, and prosody. Here, we studied response properties of neurons in the inferior colliculus (IC) in mice lacking Cacna2d3, a risk gene for autism spectrum disorders (ASDs). The α₂δ3 auxiliary Ca²⁺ channel subunit encoded by Cacna2d3 is essential for proper function of glutamatergic synapses in the auditory brainstem. Recent evidence has shown that much of auditory feature extraction is performed in the auditory brainstem and IC, including processing of amplitude modulation (AM). We determined both spectral and temporal properties of single- and multi-unit responses in the IC of anesthetized mice. IC units of α₂δ3^−/− mice showed normal tuning properties yet increased spontaneous rates compared with α₂δ3^+/+. When stimulated with AM tones, α₂δ3^−/− units exhibited less precise temporal coding and reduced evoked rates to higher modulation frequencies (f_m). Whereas first spike latencies (FSLs) were increased for only few modulation frequencies, population peak latencies were increased for f_m ranging from 20 to 100 Hz in α₂δ3^−/− IC units. The loss of precision of temporal coding with increasing f_m from 70 to 160 Hz was characterized using a normalized offset-corrected (Pearson-like) correlation coefficient, which appeared more appropriate than the metrics of vector strength. The processing deficits of AM sounds analyzed at the level of the IC indicate that α₂δ3^−/− mice exhibit a subcortical auditory processing disorder (APD). Similar deficits may be present in other mouse models for ASDs.

Aging Affects KV7 Channels and Perivascular Adipose Tissue-Mediated Vascular Tone

Aging is an independent risk factor for hypertension, cardiovascular morbidity, and mortality. However, detailed mechanisms linking aging to cardiovascular disease are unclear. We studied the aging effects on the role of perivascular adipose tissue and downstream vasoconstriction targets, voltage-dependent K_V7 channels, and their pharmacological modulators (flupirtine, retigabine, QO58, and QO58-lysine) in a murine model. We assessed vascular function of young and old mesenteric arteries in vitro using wire myography and membrane potential measurements with sharp electrodes. We also performed bulk RNA sequencing and quantitative reverse transcription-polymerase chain reaction tests in mesenteric arteries and perivascular adipose tissue to elucidate molecular underpinnings of age-related phenotypes. Results revealed impaired perivascular adipose tissue-mediated control of vascular tone particularly via K_V7.3–5 channels with increased age through metabolic and inflammatory processes and release of perivascular adipose tissue-derived relaxation factors. Moreover, QO58 was identified as novel pharmacological vasodilator to activate XE991-sensitive KCNQ channels in old mesenteric arteries. Our data suggest that targeting inflammation and metabolism in perivascular adipose tissue could represent novel approaches to restore vascular function during aging. Furthermore, K_V7.3–5 channels represent a promising target in cardiovascular aging.

Presynaptic voltage-gated calcium channels in the auditory brainstem

Sound information encoding within the initial synapses in the auditory brainstem requires reliable and precise synaptic transmission in response to rapid and large fluctuations in action potential (AP) firing rates. The magnitude and location of Ca²⁺ entry through voltage-gated Ca²⁺ channels (Ca_V) in the presynaptic terminal are key determinants in triggering AP-mediated release. In the mammalian central nervous system (CNS), the Ca_V2.1 subtype is the critical subtype for CNS function, since it is the most efficient Ca_V2 subtype in triggering AP-mediated synaptic vesicle (SV) release. Auditory brainstem synapses utilize Ca_V2.1 to sustain fast and repetitive SV release to encode sound information. Therefore, understanding the presynaptic mechanisms that control Ca_V2.1 localization, organization and biophysical properties are integral to understanding auditory processing. Here, we review our current knowledge about the control of presynaptic Ca_V2 abundance and organization in the auditory brainstem and impact on the regulation of auditory processing.

Phenotypic Characterization and Brain Structure Analysis of Calcium Channel Subunit α2δ-2 Mutant (Ducky) and α2δ Double Knockout Mice

Auxiliary α2δ subunits of voltage-gated calcium channels modulate channel trafficking, current properties, and synapse formation. Three of the four isoforms (α2δ-1, α2δ-2, and α2δ-3) are abundantly expressed in the brain; however, of the available knockout models, only α2δ-2 knockout or mutant mice display an obvious abnormal neurological phenotype. Thus, we hypothesize that the neuronal α2δ isoforms may have partially specific as well as redundant functions. To address this, we generated three distinct α2δ double knockout mouse models by crossbreeding single knockout (α2δ-1 and -3) or mutant (α2δ-2/ducky) mice. Here, we provide a first phenotypic description and brain structure analysis. We found that genotypic distribution of neonatal litters in distinct α2δ-1/-2, α2δ-1/-3, and α2δ-2/-3 breeding combinations did not conform to Mendel's law, suggesting premature lethality of single and double knockout mice. Notably, high occurrences of infant mortality correlated with the absence of specific α2δ isoforms (α2Δ-2 > α2δ-1 > α2δ-3), and was particularly observed in cages with behaviorally abnormal parenting animals of α2δ-2/-3 cross-breedings. Juvenile α2δ-1/-2 and α2δ-2/-3 double knockout mice displayed a waddling gate similar to ducky mice. However, in contrast to ducky and α2δ-1/-3 double knockout animals, α2δ-1/-2 and α2δ-2/-3 double knockout mice showed a more severe disease progression and highly impaired development. The observed phenotypes within the individual mouse lines may be linked to differences in the volume of specific brain regions. Reduced cortical volume in ducky mice, for example, was associated with a progressively decreased space between neurons, suggesting a reduction of total synaptic connections. Taken together, our findings show that α2δ subunits differentially regulate premature survival, postnatal growth, brain development, and behavior, suggesting specific neuronal functions in health and disease.

Hsp70/Bmi1-FoxO1-SOD Signaling Pathway Contributes to the Protective Effect of Sound Conditioning against Acute Acoustic Trauma in a Rat Model

Sound conditioning (SC) is defined as “toughening” to lower levels of sound over time, which reduces a subsequent noise-induced threshold shift. Although the protective effect of SC in mammals is generally understood, the exact mechanisms involved have not yet been elucidated. To confirm the protective effect of SC against noise exposure (NE) and the stress-related signaling pathway of its rescue, we observed target molecule changes caused by SC of low frequency prior to NE as well as histology analysis in vivo and verified the suggested mechanisms in SGNs in vitro. Further, we investigated the potential role of Hsp70 and Bmi1 in SC by targeting SOD1 and SOD2 which are regulated by the FoxO1 signaling pathway based on mitochondrial function and reactive oxygen species (ROS) levels. Finally, we sought to identify the possible molecular mechanisms associated with the beneficial effects of SC against noise-induced trauma. Data from the rat model were evaluated by western blot, immunofluorescence, and RT-PCR. The results revealed that SC upregulated Hsp70, Bmi1, FoxO1, SOD1, and SOD2 expression in spiral ganglion neurons (SGNs). Moreover, the auditory brainstem responses (ABRs) and electron microscopy revealed that SC could protect against acute acoustic trauma (AAT) based on a significant reduction of hearing impairment and visible reduction in outer hair cell loss as well as ultrastructural changes in OHCs and SGNs. Collectively, these results suggested that the contribution of Bmi1 toward decreased sensitivity to noise-induced trauma following SC was triggered by Hsp70 induction and associated with enhancement of the antioxidant system and decreased mitochondrial superoxide accumulation. This contribution of Bmi1 was achieved by direct targeting of SOD1 and SOD2, which was regulated by FoxO1. Therefore, the Hsp70/Bmi1-FoxO1-SOD signaling pathway might contribute to the protective effect of SC against AAT in a rat model.

Neuronal α2δ proteins and brain disorders

α2δ proteins are membrane-anchored extracellular glycoproteins which are abundantly expressed in the brain and the peripheral nervous system. They serve as regulatory subunits of voltage-gated calcium channels and, particularly in nerve cells, regulate presynaptic and postsynaptic functions independently from their role as channel subunits. α2δ proteins are the targets of the widely prescribed anti-epileptic and anti-allodynic drugs gabapentin and pregabalin, particularly for the treatment of neuropathic pain conditions. Recently, the human genes (CACNA2D1-4) encoding for the four known α2δ proteins (isoforms α2δ-1 to α2δ-4) have been linked to a large variety of neurological and neuropsychiatric disorders including epilepsy, autism spectrum disorders, bipolar disorders, schizophrenia, and depressive disorders. Here, we provide an overview of the hitherto identified disease associations of all known α2δ genes, hypothesize on the pathophysiological mechanisms considering their known physiological roles, and discuss the most immanent future research questions. Elucidating their specific physiological and pathophysiological mechanisms may open the way for developing entirely novel therapeutic paradigms for treating brain disorders.

Axon growth and synaptic function: A balancing act for axonal regeneration and neuronal circuit formation in CNS trauma and disease

Axons in the adult mammalian central nervous system (CNS) fail to regenerate inside out due to intrinsic and extrinsic neuronal determinants. During CNS development, axon growth, synapse formation, and function are tightly regulated processes allowing immature neurons to effectively grow an axon, navigate toward target areas, form synaptic contacts and become part of information processing networks that control behavior in adulthood. Not only immature neurons are able to precisely control the expression of a plethora of genes necessary for axon extension and pathfinding, synapse formation and function, but also non-neuronal cells such as astrocytes and microglia actively participate in sculpting the nervous system through refinement, consolidation, and elimination of synaptic contacts. Recent evidence indicates that a balancing act between axon regeneration and synaptic function may be crucial for rebuilding functional neuronal circuits after CNS trauma and disease in adulthood. Here, we review the role of classical and new intrinsic and extrinsic neuronal determinants in the context of CNS development, injury, and disease. Moreover, we discuss strategies targeting neuronal and non-neuronal cell behaviors, either alone or in combination, to promote axon regeneration and neuronal circuit formation in adulthood.

Emerging roles for α2δ subunits in calcium channel function and synaptic connectivity

Central nervous system function requires the proper formation and function of synapses. The α2δ auxiliary subunits of voltage-gated calcium channels have emerged as regulators of a number of critical events associated with regulation of synaptic function, including channel trafficking and localization, as well as the initial establishment of synaptic structures. In this review, we will discuss some of these recent studies which have uncovered novel mechanisms for α2δ function at the synapse, including the regulation of calcium channel α1 subunit specificity and the promotion of dendritic spine growth. Moreover, we will cover recent advances that have been made in understanding the consequences of aberrant α2δ signaling in injury and disease.

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.

RBP2 stabilizes slow Cav1.3 Ca2+ channel inactivation properties of cochlear inner hair cells

Cav1.3 L-type Ca²⁺ channels (LTCCs) in cochlear inner hair cells (IHCs) are essential for hearing as they convert sound-induced graded receptor potentials into tonic postsynaptic glutamate release. To enable fast and indefatigable presynaptic Ca²⁺ signaling, IHC Cav1.3 channels exhibit a negative activation voltage range and uniquely slow inactivation kinetics. Interaction with CaM-like Ca²⁺-binding proteins inhibits Ca²⁺-dependent inactivation, while the mechanisms underlying slow voltage-dependent inactivation (VDI) are not completely understood. Here we studied if the complex formation of Cav1.3 LTCCs with the presynaptic active zone proteins RIM2α and RIM-binding protein 2 (RBP2) can stabilize slow VDI. We detected both RIM2α and RBP isoforms in adult mouse IHCs, where they co-localized with Cav1.3 and synaptic ribbons. Using whole-cell patch-clamp recordings (tsA-201 cells), we assessed their effect on the VDI of the C-terminal full-length Cav1.3 (Cav1.3_L) and a short splice variant (Cav1.3_42A) that lacks the C-terminal RBP2 interaction site. When co-expressed with the auxiliary β3 subunit, RIM2α alone (Cav1.3_42A) or RIM2α/RBP2 (Cav1.3_L) reduced Cav1.3 VDI to a similar extent as observed in IHCs. Membrane-anchored β2 variants (β2a, β2e) that inhibit inactivation on their own allowed no further modulation of inactivation kinetics by RIM2α/RBP2. Moreover, association with RIM2α and/or RBP2 consolidated the negative Cav1.3 voltage operating range by shifting the channel's activation threshold toward more hyperpolarized potentials. Taken together, the association with "slow" β subunits (β2a, β2e) or presynaptic scaffolding proteins such as RIM2α and RBP2 stabilizes physiological gating properties of IHC Cav1.3 LTCCs in a splice variant-dependent manner ensuring proper IHC function.