Stabilization of negative activation voltages of Cav1.3 L-Type Ca2+-channels by alternative splicing

Presynaptic Proteins and Their Roles in Visual Processing by the Retina

The sense of vision begins in the retina, where light is detected and processed through a complex series of synaptic connections into meaningful information relayed to the brain via retinal ganglion cells. Light responses begin as tonic and graded signals in photoreceptors, later emerging from the retina as a series of spikes from ganglion cells. Processing by the retina extracts critical features of the visual world, including spatial frequency, temporal frequency, motion direction, color, contrast, and luminance. To achieve this, the retina has evolved specialized and unique synapse types. These include the ribbon synapses of photoreceptors and bipolar cells, the dendritic synapses of amacrine and horizontal cells, and unconventional synaptic feedback from horizontal cells to photoreceptors. We review these unique synapses in the retina with a focus on the presynaptic molecules and physiological properties that shape their capabilities.

Aging differentially alters the transcriptome and landscape of chromatin accessibility in the male and female mouse hippocampus

Aging-related memory impairment and pathological memory disorders such as Alzheimer's disease differ between males and females, and yet little is known about how aging-related changes in the transcriptome and chromatin environment differ between sexes in the hippocampus. To investigate this question, we compared the chromatin accessibility landscape and gene expression/alternative splicing pattern of young adult and aged mouse hippocampus in both males and females using ATAC-seq and RNA-seq. We detected significant aging-dependent changes in the expression of genes involved in immune response and synaptic function and aging-dependent changes in the alternative splicing of myelin sheath genes. We found significant sex-bias in the expression and alternative splicing of hundreds of genes, including aging-dependent female-biased expression of myelin sheath genes and aging-dependent male-biased expression of genes involved in synaptic function. Aging was associated with increased chromatin accessibility in both male and female hippocampus, especially in repetitive elements, and with an increase in LINE-1 transcription. We detected significant sex-bias in chromatin accessibility in both autosomes and the X chromosome, with male-biased accessibility enriched at promoters and CpG-rich regions. Sex differences in gene expression and chromatin accessibility were amplified with aging, findings that may shed light on sex differences in aging-related and pathological memory loss.

Isradipine therapy in Cacna1dIle772Met/+ mice ameliorates primary aldosteronism and neurologic abnormalities

Somatic gain-of-function mutations in the L-type calcium channel CaV1.3 (CACNA1D gene) cause adrenal aldosterone-producing adenomas and micronodules. De novo germline mutations are found in a syndrome of primary aldosteronism, seizures, and neurologic abnormalities (PASNA) as well as in autism spectrum disorder. Using CRISPR/Cas9, we here generated mice with a Cacna1d gain-of-function mutation found in both adenomas and PASNA syndrome (Cacna1dIle772Met/+). These mice show reduced body weight and increased mortality from weaning to approximately 100 days of age. Male mice do not breed, likely due to neuromotor impairment, and the offspring of female mice die perinatally, likely due to lack of maternal care. Mice generated by in vitro fertilization showed elevated intracellular calcium in the aldosterone-producing zona glomerulosa, an elevated aldosterone/renin ratio, and persistently elevated serum aldosterone on a high-salt diet as signs of primary aldosteronism. Anesthesia with ketamine and xylazine induced tonic-clonic seizures. Neurologic abnormalities included hyperlocomotion, impaired performance in the rotarod test, impaired nest building, and slight changes in social behavior. Intracellular calcium in the zona glomerulosa, aldosterone levels, and rotarod performance responded to treatment with the calcium channel blocker isradipine, with implications for the therapy of patients with aldosterone-producing lesions and with PASNA syndrome.

A novel calcium channel Cavβ2 splice variant with unique properties predominates in the retina

Cavβ subunits are essential for surface expression of voltage-gated calcium channel complexes and crucially modulate biophysical properties like voltage-dependent inactivation. Here, we describe the discovery and characterization of a novel Cavβ2 variant with distinct features that predominates in the retina. We determined spliced exons in retinal transcripts of the Cacnb2 gene, coding for Cavβ2, by RNA-Seq data analysis and quantitative PCR. We cloned a novel Cavβ2 splice variant from mouse retina, which we are calling β2i, and investigated biophysical properties of calcium currents with this variant in a heterologous expression system as well as its intrinsic membrane interaction when expressed alone. Our data showed that β2i predominated in the retina with expression in photoreceptors and bipolar cells. Furthermore, we observed that the β2i N-terminus exhibited an extraordinary concentration of hydrophobic residues, a distinct feature not seen in canonical variants. The biophysical properties resembled known membrane-associated variants, and β2i exhibited both a strong membrane association and a propensity for clustering, which depended on hydrophobic residues in its N-terminus. We considered available Cavβ structure data to elucidate potential mechanisms underlying the observed characteristics but resolved N-terminus structures were lacking and thus, precluded clear conclusions. With this description of a novel N-terminus variant of Cavβ2, we expand the scope of functional variation through N-terminal splicing with a distinct form of membrane attachment. Further investigation of the molecular mechanisms underlying the features of β2i could provide new angles on the way Cavβ subunits modulate Ca2+ channels at the plasma membrane.

Germline de novo variant F747S extends the phenotypic spectrum of CACNA1D Ca2+ channelopathies

Germline gain-of-function missense variants in the pore-forming Cav1.3 α1-subunit (CACNA1D gene) confer high risk for a severe neurodevelopmental disorder with or without endocrine symptoms. Here, we report a 4-week-old new-born with the novel de novo missense variant F747S with a so far not described prominent jittering phenotype in addition to symptoms previously reported for CACNA1D mutations including developmental delay, elevated aldosterone level and transient hypoglycemia. We confirmed the pathogenicity of this variant in whole-cell patch-clamp experiments with wild-type and F747S mutant channels heterologously expressed together with α2δ1 and cytosolic β3 or membrane-bound β2a subunits. Mutation F747S caused the quantitatively largest shift in the voltage dependence of activation (-28 mV) reported so far for CACNA1D germline mutations. It also shifted inactivation to more negative voltages, slowed the time course of current inactivation and slowed current deactivation upon repolarization with both co-expressed β-subunits. In silico modelling and molecular docking, simulations revealed that this gain-of-function phenotype can be explained by formation of a novel inter-domain hydrogen bond between mutant residues S747 (IIS6) with N1145 (IIIS6) stabilizing selectively the activated open channel state. F747S displayed 2-6-fold increased sensitivity for the L-type Ca2+ channel blocker isradipine compared to wild type. Our data confirm the pathogenicity of the F747S variant with very strong gain-of-function gating changes, which may contribute to the novel jittering phenotype. Increased sensitivity for isradipine suggests this drug for potential symptomatic off-label treatment for carriers of this mutation.

CACNA1D-Related Channelopathies: From Hypertension to Autism

Tightly controlled Ca²⁺ influx through voltage-gated Ca²⁺ channels (Cavs) is indispensable for proper physiological function. Thus, it is not surprising that Cav loss and/or gain of function have been implicated in human pathology. Deficiency of Cav1.3 L-type Ca²⁺ channels (LTCCs) causes deafness and bradycardia, whereas several genetic variants of CACNA1D, the gene encoding the pore-forming α1 subunit of Cav1.3, have been linked to various disease phenotypes, such as hypertension, congenital hypoglycemia, or autism. These variants include not only common polymorphisms associated with an increased disease risk, but also rare de novo missense variants conferring high risk. This review provides a concise summary of disease-associated CACNA1D variants, whereas the main focus lies on de novo germline variants found in individuals with a neurodevelopmental disorder of variable severity. Electrophysiological recordings revealed activity-enhancing gating changes induced by these de novo variants, and tools to predict their pathogenicity and to study the resulting pathophysiological consequences will be discussed. Despite the low number of affected patients, potential phenotype-genotype correlations and factors that could impact the severity of symptoms will be covered. Since increased channel activity is assumed as the disease-underlying mechanism, pharmacological inhibition could be a treatment option. In the absence of Cav1.3-selective blockers, dihydropyridine LTCC inhibitors clinically approved for the treatment of hypertension may be used for personalized off-label trials. Findings from in vitro studies and treatment attempts in some of the patients seem promising as outlined. Taken together, due to advances in diagnostic sequencing techniques the number of reported CACNA1D variants in human diseases is constantly rising. Evidence from in silico, in vitro, and in vivo disease models can help to predict the pathogenic potential of such variants and to guide diagnosis and treatment in the clinical practice when confronted with patients harboring CACNA1D variants.

Pathophysiology of Cav1.3 L-type calcium channels in the heart

Ca²⁺ plays a crucial role in excitation-contraction coupling in cardiac myocytes. Dysfunctional Ca²⁺ regulation alters the force of contraction and causes cardiac arrhythmias. Ca²⁺ entry into cardiomyocytes is mediated mainly through L-type Ca²⁺ channels, leading to the subsequent Ca²⁺ release from the sarcoplasmic reticulum. L-type Ca²⁺ channels are composed of the conventional Ca_v1.2, ubiquitously expressed in all heart chambers, and the developmentally regulated Ca_v1.3, exclusively expressed in the atria, sinoatrial node, and atrioventricular node in the adult heart. As such, Ca_v1.3 is implicated in the pathogenesis of sinoatrial and atrioventricular node dysfunction as well as atrial fibrillation. More recently, Ca_v1.3 de novo expression was suggested in heart failure. Here, we review the functional role, expression levels, and regulation of Ca_v1.3 in the heart, including in the context of cardiac diseases. We believe that the elucidation of the functional and molecular pathways regulating Ca_v1.3 in the heart will assist in developing novel targeted therapeutic interventions for the aforementioned arrhythmias.

The Role of Striatal Cav1.3 Calcium Channels in Therapeutics for Parkinson's Disease

Parkinson's disease (PD) is a relentlessly progressive neurodegenerative disorder with typical motor symptoms that include rigidity, tremor, and akinesia/bradykinesia, in addition to a host of non-motor symptoms. Motor symptoms are caused by progressive and selective degeneration of dopamine (DA) neurons in the SN pars compacta (SNpc) and the accompanying loss of striatal DA innervation from these neurons. With the exception of monogenic forms of PD, the etiology of idiopathic PD remains unknown. While there are a number of symptomatic treatment options available to individuals with PD, these therapies do not work uniformly well in all patients, and eventually most are plagued with waning efficacy and significant side-effect liability with disease progression. The incidence of PD increases with aging, and as such the expected burden of this disease will continue to escalate as our aging population increases (Dorsey et al. Neurology 68:384-386, 2007). The daunting personal and socioeconomic burden has pressed scientists and clinicians to find improved symptomatic treatment options devoid side-effect liability and meaningful disease-modifying therapies. Federal and private sources have supported clinical investigations over the past two-plus decades; however, no trial has yet been successful in finding an effective therapy to slow progression of PD, and there is currently just one FDA approved drug to treat the antiparkinsonian side-effect known as levodopa-induced dyskinesia (LID) that impacts approximately 90% of all individuals with PD. In this review, we present biological rationale and experimental evidence on the potential therapeutic role of the L-type voltage-gated Cav1.3 calcium (Ca2+) channels in two distinct brain regions, with two distinct mechanisms of action, in impacting the lives of individuals with PD. Our primary emphasis will be on the role of Cav1.3 channels in the striatum and the compelling evidence of their involvement in LID side-effect liability. We also briefly discuss the role of these same Ca2+ channels in the SNpc and the longstanding interest in Cav1.3 in this brain region in halting or delaying progression of PD.

Calpain-2 Mediates MBNL2 Degradation and a Developmental RNA Processing Program in Neurodegeneration

Increasing loss of structure and function of neurons and decline in cognitive function is commonly seen during the progression of neurologic diseases, although the causes and initial symptoms of individual diseases are distinct. This observation suggests a convergence of common degenerative features. In myotonic dystrophy type 1 (DM1), the expression of expanded CUG RNA induces neurotransmission dysfunction before axon and dendrite degeneration and reduced MBNL2 expression associated with aberrant alternative splicing. The role of loss of function of MBNL2 in the pathogenesis of neurodegeneration and the causal mechanism of neurodegeneration-reduced expression of MBNL2 remain elusive. Here, we show that increased MBNL2 expression is associated with neuronal maturation and required for neuronal morphogenesis and the fetal to adult developmental transition of RNA processing. Neurodegenerative conditions including NMDA receptor (NMDAR)-mediated excitotoxicity and dysregulated calcium homeostasis triggered nuclear translocation of calpain-2, thus resulting in MBNL2 degradation and reversal of MBNL2-regulated RNA processing to developmental patterns. Nuclear expression of calpain-2 resembled its developmental pattern and was associated with MBNL2 degradation. Knock-down of calpain-2 expression or inhibition of calpain-2 nuclear translocation prevented neurodegeneration-reduced MBNL2 expression and dysregulated RNA processing. Increased calpain-2 nuclear translocation associated with reduced MBNL2 expression and aberrant RNA processing occurred in models for DM1 and Alzheimer's disease (AD) including EpA960/CaMKII-Cre mice of either sex and female APP/PS1 and THY-Tau22 mice. Our results identify a regulatory mechanism for MBNL2 downregulation and suggest that calpain-2-mediated MBNL2 degradation accompanied by re-induction of a developmental RNA processing program may be a converging pathway to neurodegeneration.SIGNIFICANCE STATEMENT Neurologic diseases share many features during disease progression, such as cognitive decline and brain atrophy, which suggests a common pathway for developing degenerative features. Here, we show that the neurodegenerative conditions glutamate-induced excitotoxicity and dysregulated calcium homeostasis induced translocation of the cysteine protease calpain-2 into the nucleus, resulting in MBNL2 degradation and reversal of MBNL2-regulated RNA processing to an embryonic pattern. Knock-down or inhibition of nuclear translocation of calpain-2 prevented MBNL2 degradation and maintained MBNL2-regulated RNA processing in the adult pattern. Models of myotonic dystrophy and Alzheimer's disease (AD) also showed calpain-2-mediated MBNL2 degradation and a developmental RNA processing program. Our studies suggest MBNL2 function disrupted by calpain-2 as a common pathway, thus providing an alternative therapeutic strategy for neurodegeneration.

Divergent Ca2+/calmodulin feedback regulation of CaV1 and CaV2 voltage-gated calcium channels evolved in the common ancestor of Placozoa and Bilateria

Ca_V1 and Ca_V2 voltage-gated calcium channels evolved from an ancestral Ca_V1/2 channel via gene duplication somewhere near the stem animal lineage. The divergence of these channel types led to distinguishing functional properties that are conserved among vertebrates and bilaterian invertebrates and contribute to their unique cellular roles. One key difference pertains to their regulation by calmodulin (CaM), wherein bilaterian Ca_V1 channels are uniquely subject to pronounced, buffer-resistant Ca²⁺/CaM-dependent inactivation, permitting negative feedback regulation of calcium influx in response to local cytoplasmic Ca²⁺ rises. Early diverging, nonbilaterian invertebrates also possess Ca_V1 and Ca_V2 channels, but it is unclear whether they share these conserved functional features. The most divergent animals to possess both Ca_V1 and Ca_V2 channels are placozoans such as Trichoplax adhaerens, which separated from other animals over 600 million years ago shortly after their emergence. Hence, placozoans can provide important insights into the early evolution of Ca_V1 and Ca_V2 channels. Here, we build upon previous characterization of Trichoplax Ca_V channels by determining the cellular expression and ion-conducting properties of the Ca_V1 channel orthologue, TCa_V1. We show that TCa_V1 is expressed in neuroendocrine-like gland cells and contractile dorsal epithelial cells. In vitro, this channel conducts dihydropyridine-insensitive, high-voltage–activated Ca²⁺ currents with kinetics resembling those of rat Ca_V1.2 but with left-shifted voltage sensitivity for activation and inactivation. Interestingly, TCa_V1, but not TCa_V2, exhibits buffer-resistant Ca²⁺/CaM-dependent inactivation, indicating that this functional divergence evolved prior to the emergence of bilaterian animals and may have contributed to their unique adaptation for cytoplasmic Ca²⁺ signaling within various cellular contexts.

DM1 Transgenic Mice Exhibit Abnormal Neurotransmitter Homeostasis and Synaptic Plasticity in Association with RNA Foci and Mis-Splicing in the Hippocampus

Myotonic dystrophy type 1 (DM1) is a severe neuromuscular disease mediated by a toxic gain of function of mutant RNAs. The neuropsychological manifestations affect multiple domains of cognition and behavior, but their etiology remains elusive. Transgenic DMSXL mice carry the DM1 mutation, show behavioral abnormalities, and express low levels of GLT1, a critical regulator of glutamate concentration in the synaptic cleft. However, the impact of glutamate homeostasis on neurotransmission in DM1 remains unknown. We confirmed reduced glutamate uptake in the DMSXL hippocampus. Patch clamp recordings in hippocampal slices revealed increased amplitude of tonic glutamate currents in DMSXL CA1 pyramidal neurons and DG granule cells, likely mediated by higher levels of ambient glutamate. Unexpectedly, extracellular GABA levels and tonic current were also elevated in DMSXL mice. Finally, we found evidence of synaptic dysfunction in DMSXL mice, suggestive of abnormal short-term plasticity, illustrated by an altered LTP time course in DG and in CA1. Synaptic dysfunction was accompanied by RNA foci accumulation in localized areas of the hippocampus and by the mis-splicing of candidate genes with relevant functions in neurotransmission. Molecular and functional changes triggered by toxic RNA may induce synaptic abnormalities in restricted brain areas that favor neuronal dysfunction.