Loss of the calcium channel β4 subunit impairs parallel fibre volley and Purkinje cell firing in cerebellum of adult ataxic mice

Motor learning is regulated by postnatal GDNF levels in Purkinje cells

Purkinje cells (PCs), the sole output neurons of the cerebellar cortex, are crucial for cerebellum-dependent motor learning. In cerebellar ataxia, reduction in motor function and learning associates with decreased spontaneous activity of PCs. Thus, understanding what molecules regulate PCs activity is important. Previously, we demonstrated that a ubiquitous 2-fold increase of endogenous glial cell line-derived neurotrophic factor (GDNF) improves motor function in adult mice and motor learning and coordination in aged mice. However, since GDNF impacts many organ systems the underlying mechanism remained elusive. Here we utilize GDNF Hypermorphic, conditional GDNF Hypermorphic and conditional knock-out mouse models to reveal that up to a 2-fold increase in endogenous GDNF, specifically in PCs postnatally, is sufficient to enhance motor learning. We find that improved motor learning associates with increased glutamatergic input to PCs and with elevated spontaneous firing rate of PCs, opposite to cerebellar ataxia where reduction in motor function and learning associates with decreased spontaneous activity of PCs. Analysis of the human cerebellum revealed that normal interindividual variation in GDNF expression levels falls in the same variation range as studied in the mouse models, suggesting that interindividual variation in PC GDNF levels may contribute to interindividual variation in PC function. Collectively, our findings reveal how a relatively small change in postnatal GDNF expression level within the physiological range in one cell type, the PCs, affects motor learning. Thus, drugs enhancing postnatal GDNF expression in PCs or cerebellar GDNF signaling may have potential in treating cerebellar ataxias, making an interesting topic for future studies.

The awakening of dormant neuronal precursors in the adult and aged brain

Beyond the canonical neurogenic niches, there are dormant neuronal precursors in several regions of the adult mammalian brain. Dormant precursors maintain persisting post-mitotic immaturity from birth to adulthood, followed by staggered awakening, in a process that is still largely unresolved. Strikingly, due to the slow rate of awakening, some precursors remain immature until old age, which led us to question whether their awakening and maturation are affected by aging. To this end, we studied the maturation of dormant precursors in transgenic mice (DCX-CreERT2 /flox-EGFP) in which immature precursors were labelled permanently in vivo at different ages. We found that dormant precursors are capable of awakening at young age, becoming adult-matured neurons (AM), as well as of awakening at old age, becoming late AM. Thus, protracted immaturity does not prevent late awakening and maturation. However, late AM diverged morphologically and functionally from AM. Moreover, AM were functionally most similar to neonatal-matured neurons (NM). Conversely, late AM were endowed with high intrinsic excitability and high input resistance, and received a smaller amount of spontaneous synaptic input, implying their relative immaturity. Thus, late AM awakening still occurs at advanced age, but the maturation process is slow.

AHNAKs roles in physiology and malignant tumors

The AHNAK family currently consists of two members, namely AHNAK and AHNAK2, both of which have a molecular weight exceeding 600 kDa. Homologous sequences account for approximately 90% of their composition, indicating a certain degree of similarity in terms of molecular structure and biological functions. AHNAK family members are involved in the regulation of various biological functions, such as calcium channel modulation and membrane repair. Furthermore, with advancements in biological and bioinformatics technologies, research on the relationship between the AHNAK family and tumors has rapidly increased in recent years, and its regulatory role in tumor progression has gradually been discovered. This article briefly describes the physiological functions of the AHNAK family, and reviews and analyzes the expression and molecular regulatory mechanisms of the AHNAK family in malignant tumors using Pubmed and TCGA databases. In summary, AHNAK participates in various physiological and pathological processes in the human body. In multiple types of cancers, abnormal expression of AHNAK and AHNAK2 is associated with prognosis, and they play a key regulatory role in tumor progression by activating signaling pathways such as ERK, MAPK, Wnt, and MEK, as well as promoting epithelial-mesenchymal transition.

β subunits of voltage-gated calcium channels in cardiovascular diseases

Calcium signaling is required in bodily functions essential for survival, such as muscle contractions and neuronal communications. Of note, the voltage-gated calcium channels (VGCCs) expressed on muscle and neuronal cells, as well as some endocrine cells, are transmembrane protein complexes that allow for the selective entry of calcium ions into the cells. The α1 subunit constitutes the main pore-forming subunit that opens in response to membrane depolarization, and its biophysical functions are regulated by various auxiliary subunits-β, α2δ, and γ subunits. Within the cardiovascular system, the γ-subunit is not expressed and is therefore not discussed in this review. Because the α1 subunit is the pore-forming subunit, it is a prominent druggable target and the focus of many studies investigating potential therapeutic interventions for cardiovascular diseases. While this may be true, it should be noted that the direct inhibition of the α1 subunit may result in limited long-term cardiovascular benefits coupled with undesirable side effects, and that its expression and biophysical properties may depend largely on its auxiliary subunits. Indeed, the α2δ subunit has been reported to be essential for the membrane trafficking and expression of the α1 subunit. Furthermore, the β subunit not only prevents proteasomal degradation of the α1 subunit, but also directly modulates the biophysical properties of the α1 subunit, such as its voltage-dependent activities and open probabilities. More importantly, various isoforms of the β subunit have been found to differentially modulate the α1 subunit, and post-translational modifications of the β subunits further add to this complexity. These data suggest the possibility of the β subunit as a therapeutic target in cardiovascular diseases. However, emerging studies have reported the presence of cardiomyocyte membrane α1 subunit trafficking and expression in a β subunit-independent manner, which would undermine the efficacy of β subunit-targeting drugs. Nevertheless, a better understanding of the auxiliary β subunit would provide a more holistic approach when targeting the calcium channel complexes in treating cardiovascular diseases. Therefore, this review focuses on the post-translational modifications of the β subunit, as well as its role as an auxiliary subunit in modulating the calcium channel complexes.

Functional Characterization of Four Known Cav2.1 Variants Associated with Neurodevelopmental Disorders

Cav2.1 channels are expressed throughout the brain and are the predominant Ca²⁺ channels in the Purkinje cells. These cerebellar neurons fire spontaneously, and Cav2.1 channels are involved in the regular pacemaking activity. The loss of precision of the firing pattern of Purkinje cells leads to ataxia, a disorder characterized by poor balance and difficulties in performing coordinated movements. In this study, we aimed at characterizing functional and structural consequences of four variations (p.A405T in I-II loop and p.R1359W, p.R1667W and p.S1799L in IIIS4, IVS4, and IVS6 helices, respectively) identified in patients exhibiting a wide spectrum of disorders including ataxia symptoms. Functional analysis using two major Cav2.1 splice variants (Cav2.1+e47 and Cav2.1-e47) in Xenopus laevis oocytes, revealed a lack of effect upon A405T substitution and a significant loss-of-function caused by R1359W, whereas R1667W and S1799L caused both channel gain-of-function and loss-of-function, in a splice variant-dependent manner. Structural analysis revealed the loss of interactions with S1, S2, and S3 helices upon R1359W and R1667W substitutions, but a lack of obvious structural changes with S1799L. Computational modeling suggests that biophysical changes induced by Cav2.1 pathogenic mutations might affect action potential frequency in Purkinje cells.

AHNAK: The quiet giant in calcium homeostasis

The phosphoprotein AHNAK is a large, ubiquitously expressed scaffolding protein involved in mediating a host of protein-protein interactions. This enables AHNAK to participate in various multi-protein complexes thereby orchestrating a range of diverse biological processes, including tumour suppression, immune regulation and cell architecture maintenance. A less studied but nonetheless equally important function occurs in calcium homeostasis. It does so by largely interacting with the L-type voltage-gated calcium channel (LVGCC) present in the plasma membrane of excitable cells such as muscles and neurons. Several studies have characterized the underlying basis of AHNAK's functional role in calcium channel modulation, which has led to a greater understanding of this cellular process and its associated pathologies. In this article we review and examine recent advances relating to the physiological aspects of AHNAK in calcium regulation. Specifically, we will provide a broad overview of AHNAK including its structural makeup and its interaction with several isoforms of LVGCC, and how these molecular interactions regulate calcium modulation across various tissues and their implication in muscle and neuronal function.

A homozygous missense variant in CACNB4 encoding the auxiliary calcium channel beta4 subunit causes a severe neurodevelopmental disorder and impairs channel and non-channel functions

P/Q-type channels are the principal presynaptic calcium channels in brain functioning in neurotransmitter release. They are composed of the pore-forming CaV2.1 α1 subunit and the auxiliary α2δ-2 and β4 subunits. β4 is encoded by CACNB4, and its multiple splice variants serve isoform-specific functions as channel subunits and transcriptional regulators in the nucleus. In two siblings with intellectual disability, psychomotor retardation, blindness, epilepsy, movement disorder and cerebellar atrophy we identified rare homozygous variants in the genes LTBP1, EMILIN1, CACNB4, MINAR1, DHX38 and MYO15 by whole-exome sequencing. In silico tools, animal model, clinical, and genetic data suggest the p.(Leu126Pro) CACNB4 variant to be likely pathogenic. To investigate the functional consequences of the CACNB4 variant, we introduced the corresponding mutation L125P into rat β4b cDNA. Heterologously expressed wild-type β4b associated with GFP-CaV1.2 and accumulated in presynaptic boutons of cultured hippocampal neurons. In contrast, the β4b-L125P mutant failed to incorporate into calcium channel complexes and to cluster presynaptically. When co-expressed with CaV2.1 in tsA201 cells, β4b and β4b-L125P augmented the calcium current amplitudes, however, β4b-L125P failed to stably complex with α1 subunits. These results indicate that p.Leu125Pro disrupts the stable association of β4b with native calcium channel complexes, whereas membrane incorporation, modulation of current density and activation properties of heterologously expressed channels remained intact. Wildtype β4b was specifically targeted to the nuclei of quiescent excitatory cells. Importantly, the p.Leu125Pro mutation abolished nuclear targeting of β4b in cultured myotubes and hippocampal neurons. While binding of β4b to the known interaction partner PPP2R5D (B56δ) was not affected by the mutation, complex formation between β4b-L125P and the neuronal TRAF2 and NCK interacting kinase (TNIK) seemed to be disturbed. In summary, our data suggest that the homozygous CACNB4 p.(Leu126Pro) variant underlies the severe neurological phenotype in the two siblings, most likely by impairing both channel and non-channel functions of β4b.

Impact of copy number variation on human neurocognitive deficits and congenital heart defects: A systematic review

Copy number variant (CNV) syndromes are often associated with both neurocognitive deficits (NCDs) and congenital heart defects (CHDs). Children and adults with cardiac developmental defects likely to have NCDs leading to increased risk of hospitalisation and reduced level of independence. To date, the association between these two phenotypes have not been explored in relation to CNV syndromes. In order to address this question, we systematically reviewed the prevalence of CHDs in a range of CNV syndromes associated with NCDs. A meta-analysis showed a relationship with the size of CNV and its association with both NCDs and CHDs, and also inheritance pattern. To our knowledge, this is the first review to establish association between NCD and CHDs in CNV patients, specifically in relation to the severity of NCD. Importantly, we also found specific types of CHDs were associated with severe neurocognitive deficits. Finally, we discuss the implications of these results for patients in the clinical setting which warrants further exploration of this association in order to lead an improvement in the quality of patient's life.

ELKS active zone proteins as multitasking scaffolds for secretion

Synaptic vesicle exocytosis relies on the tethering of release ready vesicles close to voltage-gated Ca²⁺ channels and specific lipids at the future site of fusion. This enables rapid and efficient neurotransmitter secretion during presynaptic depolarization by an action potential. Extensive research has revealed that this tethering is mediated by an active zone, a protein dense structure that is attached to the presynaptic plasma membrane and opposed to postsynaptic receptors. Although roles of individual active zone proteins in exocytosis are in part understood, the molecular mechanisms that hold the protein scaffold at the active zone together and link it to the presynaptic plasma membrane have remained unknown. This is largely due to redundancy within and across scaffolding protein families at the active zone. Recent studies, however, have uncovered that ELKS proteins, also called ERC, Rab6IP2 or CAST, act as active zone scaffolds redundant with RIMs. This redundancy has led to diverse synaptic phenotypes in studies of ELKS knockout mice, perhaps because different synapses rely to a variable extent on scaffolding redundancy. In this review, we first evaluate the need for presynaptic scaffolding, and we then discuss how the diverse synaptic and non-synaptic functional roles of ELKS support the hypothesis that ELKS provides molecular scaffolding for organizing vesicle traffic at the presynaptic active zone and in other cellular compartments.

Protein kinase N1 critically regulates cerebellar development and long-term function

Increasing evidence suggests that synapse dysfunctions are a major determinant of several neurodevelopmental and neurodegenerative diseases. Here we identify protein kinase N1 (PKN1) as a novel key player in fine-tuning the balance between axonal outgrowth and presynaptic differentiation in the parallel fiber-forming (PF-forming) cerebellar granule cells (Cgcs). Postnatal Pkn1-/- animals showed a defective PF-Purkinje cell (PF-PC) synapse formation. In vitro, Pkn1-/- Cgcs exhibited deregulated axonal outgrowth, elevated AKT phosphorylation, and higher levels of neuronal differentiation-2 (NeuroD2), a transcription factor preventing presynaptic maturation. Concomitantly, Pkn1-/- Cgcs had a reduced density of presynaptic sites. By inhibiting AKT with MK-2206 and siRNA-mediated knockdown, we found that AKT hyperactivation is responsible for the elongated axons, higher NeuroD2 levels, and reduced density of presynaptic specifications in Pkn1-/- Cgcs. In line with our in vitro data, Pkn1-/- mice showed AKT hyperactivation, elevated NeuroD2 levels, and reduced expression of PF-PC synaptic markers during stages of PF maturation in vivo. The long-term effect of Pkn1 knockout was further seen in cerebellar atrophy and mild ataxia. In summary, our results demonstrate that PKN1 functions as a developmentally active gatekeeper of AKT activity, thereby fine-tuning axonal outgrowth and presynaptic differentiation of Cgcs and subsequently the correct PF-PC synapse formation.

Motor Deficits and Cerebellar Atrophy in Elovl5 Knock Out Mice

Spino-Cerebellar-Ataxia type 38 (SCA38) is caused by missense mutations in the very long chain fatty acid elongase 5 gene, ELOVL5. The main clinical findings in this disease are ataxia, hyposmia and cerebellar atrophy. Mice in which Elovl5 has been knocked out represent a model of the loss of function hypothesis of SCA38. In agreement with this hypothesis, Elovl5 knock out mice reproduced the main symptoms of patients, motor deficits at the beam balance test and hyposmia. The cerebellar cortex of Elovl5 knock out mice showed a reduction of thickness of the molecular layer, already detectable at 6 months of age, confirmed at 12 and 18 months. The total perimeter length of the Purkinje cell (PC) layer was also reduced in Elovl5 knock out mice. Since Elovl5 transcripts are expressed by PCs, whose dendrites are a major component of the molecular layer, we hypothesized that an alteration of their dendrites might be responsible for the reduced thickness of this layer. Reconstruction of the dendritic tree of biocytin-filled PCs, followed by Sholl analysis, showed that the distribution of distal dendrites was significantly reduced in Elovl5 knock out mice. Dendritic spine density was conserved. These results suggest that Elovl5 knock out mice recapitulate SCA38 symptoms and that their cerebellar atrophy is due, at least in part, to a reduced extension of PC dendritic arborization.

Keeping Our Calcium in Balance to Maintain Our Balance

Calcium is a key signaling molecule and ion involved in a variety of diverse processes in our central nervous system (CNS) which include gene expression, synaptic transmission and plasticity, neuronal excitability and cell maintenance. Proper control of calcium signaling is not only vital for neuronal physiology but also cell survival. Mutations in fundamental channels, transporters and second messenger proteins involved in orchestrating the balance of our calcium homeostasis can lead to severe neurodegenerative disorders, such as Spinocerebellar (SCA) and Episodic (EA) ataxias. Hereditary ataxias make up a remarkably diverse group of neurological disorders clinically characterized by gait ataxia, nystagmus, dysarthria, trunk and limb ataxia and often atrophy of the cerebellum. The largest family of hereditary ataxias is SCAs which consists of a growing family of 42 members. A relatively smaller family of 8 members compose the EAs. The gene mutations responsible for half of the EA members and over 35 of the SCA subtypes have been identified, and several have been found to be responsible for cerebellar atrophy, abnormal intracellular calcium levels, dysregulation of Purkinje cell pacemaking, altered cerebellar synaptic transmission and/or ataxia in mouse models. Although the genetic diversity and affected cellular pathways of hereditary ataxias are broad, one common theme amongst these genes is their effects on maintaining calcium balance in primarily the cerebellum. There is emerging evidence that the pathogenesis of hereditary ataxias may be caused by imbalances in intracellular calcium due to genetic mutations in calcium-mediating proteins. In this review we will discuss the current evidence supporting the role of deranged calcium as the culprit to neurodegenerative diseases with a primary focus on SCAs and EAs.