Rab11-dependent recycling of calcium channels is mediated by auxiliary subunit α2δ-1 but not α2δ-3

The importance of cache domains in α2δ proteins and the basis for their gabapentinoid selectivity

In this hybrid review, we have first collected and reviewed available information on the structure and function of the enigmatic cache domains in α₂δ proteins. These are organized into two double cache (dCache_1) domains, and they are present in all α₂δ proteins. We have also included new data on the key function of these domains with respect to amino acid and gabapentinoid binding to the universal amino acid-binding pocket, which is present in α₂δ-1 and α₂δ-2. We have now identified the reason why α₂δ-3 and α₂δ-4 do not bind gabapentinoid drugs or amino acids with bulky side chains. In relation to this, we have determined that the bulky amino acids Tryptophan and Phenylalanine prevent gabapentin from inhibiting cell surface trafficking of α₂δ-1. Together, these novel data shed further light on the importance of the cache domains in α₂δ proteins.

Active Zone Trafficking of CaV2/UNC-2 Channels Is Independent of β/CCB-1 and α2δ/UNC-36 Subunits

The CaV2 voltage-gated calcium channel is the major conduit of calcium ions necessary for neurotransmitter release at presynaptic active zones (AZs). The CaV2 channel is a multimeric complex that consists of a pore-forming α1 subunit and two auxiliary β and α2δ subunits. Although auxiliary subunits are critical for channel function, whether they are required for α1 trafficking is unresolved. Using endogenously fluorescent protein-tagged CaV2 channel subunits in Caenorhabditis elegans, we show that UNC-2/α1 localizes to AZs even in the absence of CCB-1/β or UNC-36/α2δ, albeit at low levels. When UNC-2 is manipulated to be trapped in the endoplasmic reticulum (ER), CCB-1 and UNC-36 fail to colocalize with UNC-2 in the ER, indicating that they do not coassemble with UNC-2 in the ER. Moreover, blocking ER-associated degradation does not further increase presynaptic UNC-2 channels in ccb-1 or unc-36 mutants, indicating that UNC-2 levels are not regulated in the ER. An unc-2 mutant lacking C-terminal AZ protein interaction sites with intact auxiliary subunit binding sites displays persistent presynaptic UNC-2 localization and a prominent increase of UNC-2 channels in nonsynaptic axonal regions, underscoring a protective role of auxiliary subunits against UNC-2 degradation. In the absence of UNC-2, presynaptic CCB-1 and UNC-36 are profoundly diminished to barely detectable levels, indicating that UNC-2 is required for the presynaptic localization of CCB-1 and UNC-36. Together, our findings demonstrate that although the pore-forming subunit does not require auxiliary subunits for its trafficking and transport to AZs, it recruits auxiliary subunits to stabilize and expand calcium channel signalosomes.SIGNIFICANCE STATEMENT Synaptic transmission in the neuron hinges on the coupling of synaptic vesicle exocytosis with calcium influx. This calcium influx is mediated by CaV2 voltage-gated calcium channels. These channels consist of one pore-forming α1 subunit and two auxiliary β and α2δ subunits. The auxiliary subunits enhance channel function and regulate the overall level of channels at presynaptic terminals. However, it is not settled how these auxiliary subunits regulate the overall channel level. Our study in C. elegans finds that although the auxiliary subunits do not coassemble with α1 and aid trafficking, they are recruited to α1 and stabilize the channel complex at presynaptic terminals. Our study suggests that drugs that target the auxiliary subunits can directly destabilize and have an impact on CaV2 channels.

Structural basis for CaVα2δ:gabapentin binding

Gabapentinoid drugs for pain and anxiety act on the CaVα2δ-1 and CaVα2δ-2 subunits of high-voltage-activated calcium channels (CaV1s and CaV2s). Here we present the cryo-EM structure of the gabapentin-bound brain and cardiac CaV1.2/CaVβ3/CaVα2δ-1 channel. The data reveal a binding pocket in the CaVα2δ-1 dCache1 domain that completely encapsulates gabapentin and define CaVα2δ isoform sequence variations that explain the gabapentin binding selectivity of CaVα2δ-1 and CaVα2δ-2.

Cigarette smoke triggers calcium overload in mouse hippocampal neurons via the ΔFOSB-CACNA2D1 axis to impair cognitive performance

A growing body of evidence shows that cigarette smoking impairs cognitive performance. The 'Calcium Hypothesis' theory of neuronopathies reveals a critical role of aberrant calcium signaling in compromised cognitive functions. However, the underlying implications of abnormalities in calcium signaling in the neurotoxicity induced by cigarette smoke (CS) have not yet been identified. CACNA2D1, an important auxiliary subunit involved in the composition of voltage-gated calcium channels (VGCCs), was reported to affect the calcium signaling in neurons by facilitating VGCCs-mediated Ca²⁺ influx. ΔFOSB, an alternatively-spliced product of the Fosb gene, is an activity-dependent transcription factor induced robustly in the brain in response to environmental stimuli such as CS. Interestingly, our preliminary bioinformatics analysis revealed a significant co-expression between ΔFOSB and CACNA2D1 in brain tissues of patients with neurodegenerative diseases characterized by progressive cognitive decline. Therefore, we hypothesized that the activation of the ΔFOSB-CACNA2D1 axis in response to CS exposure might cause dysregulation of calcium homeostasis in hippocampal neurons via VGCCs-mediated Ca²⁺ influx, thereby contributing to cognitive deficits. To this end, the present study established a CS-induced mouse model of hippocampus-dependent cognitive impairment, in which the activation of the ΔFOSB-CACNA2D1 axis accompanied by severe calcium overload was observed in the mouse hippocampal tissues. More importantly, ΔFOSB knockdown-/overexpression-mediated inactivation/activation of the ΔFOSB-CACNA2D1 axis interdicted/mimicked CS-induced dysregulation of calcium homeostasis followed by severe cellular damage in HT22 mouse hippocampal neurons. Mechanistically speaking, a further ChIP-qPCR assay confirmed the physical interaction between transcription factor ΔFOSB and the Cacna2d1 gene promoter, suggesting a direct transcriptional regulation of the Cacna2d1 gene by ΔFOSB. Overall, our current work aims to deliver a unique insight into the neurotoxic mechanisms induced by CS to explore potential targets for intervention.

Capsaicin-Induced Endocytosis of Endogenous Presynaptic CaV2.2 in DRG-Spinal Cord Co-Cultures Inhibits Presynaptic Function

The N-type calcium channel, CaV2.2 is key to neurotransmission from the primary afferent terminals of dorsal root ganglion (DRG) neurons to their postsynaptic targets in the spinal cord. In this study, we have utilized CaV2.2_HA knock-in mice, because the exofacial epitope tag in CaV2.2_HA enables accurate detection and localization of endogenous CaV2.2. CaV2.2_HA knock-in mice were used as a source of DRGs to exclusively study the presynaptic expression of N-type calcium channels in co-cultures between DRG neurons and wild-type spinal cord neurons. CaV2.2_HA is strongly expressed on the cell surface, particularly in TRPV1-positive small and medium DRG neurons. Super-resolution images of the presynaptic terminals revealed an increase in CaV2.2_HA expression and increased association with the postsynaptic marker Homer over time in vitro. Brief application of the TRPV1 agonist, capsaicin, resulted in a significant down-regulation of cell surface CaV2.2_HA expression in DRG neuron somata. At their presynaptic terminals, capsaicin caused a reduction in CaV2.2_HA proximity to and co-localization with the active zone marker RIM 1/2, as well as a lower contribution of N-type channels to single action potential-mediated Ca2+ influx. The mechanism of this down-regulation of CaV2.2_HA involves a Rab11a-dependent trafficking process, since dominant-negative Rab11a (S25N) occludes the effect of capsaicin on presynaptic CaV2.2_HA expression, and also prevents the effect of capsaicin on action potential-induced Ca2+ influx. Taken together, these data suggest that capsaicin causes a decrease in cell surface CaV2.2_HA expression in DRG terminals via a Rab11a-dependent endosomal trafficking pathway.

Subcellular trafficking and endocytic recycling of KATP channels

Sarcolemmal/plasmalemmal ATP-sensitive K+ (KATP) channels have key roles in many cell types and tissues. Hundreds of studies have described how the KATP channel activity and ATP sensitivity can be regulated by changes in the cellular metabolic state, by receptor signaling pathways and by pharmacological interventions. These alterations in channel activity directly translate to alterations in cell or tissue function, that can range from modulating secretory responses, such as insulin release from pancreatic β-cells or neurotransmitters from neurons, to modulating contractile behavior of smooth muscle or cardiac cells to elicit alterations in blood flow or cardiac contractility. It is increasingly becoming apparent, however, that KATP channels are regulated beyond changes in their activity. Recent studies have highlighted that KATP channel surface expression is a tightly regulated process with similar implications in health and disease. The surface expression of KATP channels is finely balanced by several trafficking steps including synthesis, assembly, anterograde trafficking, membrane anchoring, endocytosis, endocytic recycling, and degradation. This review aims to summarize the physiological and pathophysiological implications of KATP channel trafficking and mechanisms that regulate KATP channel trafficking. A better understanding of this topic has potential to identify new approaches to develop therapeutically useful drugs to treat KATP channel-related diseases.

The Putative Drosophila TMEM184B Ortholog Tmep Ensures Proper Locomotion by Restraining Ectopic Firing at the Neuromuscular Junction

TMEM184B is a putative seven-pass membrane protein that promotes axon degeneration after injury. TMEM184B mutation causes aberrant neuromuscular architecture and sensory and motor behavioral defects in mice. The mechanism through which TMEM184B causes neuromuscular defects is unknown. We employed Drosophila melanogaster to investigate the function of the closely related gene, Tmep (CG12004), at the neuromuscular junction. We show that Tmep is required for full adult viability and efficient larval locomotion. Tmep mutant larvae have a reduced body contraction rate compared to controls, with stronger deficits in females. In recordings from body wall muscles, Tmep mutants show substantial hyperexcitability, with many postsynaptic potentials fired in response to a single stimulation, consistent with a role for Tmep in restraining synaptic excitability. Additional branches and satellite boutons at Tmep mutant neuromuscular junctions are consistent with an activity-dependent synaptic overgrowth. Tmep is expressed in endosomes and synaptic vesicles within motor neurons, suggesting a possible role in synaptic membrane trafficking. Using RNAi knockdown, we show that Tmep is required in motor neurons for proper larval locomotion and excitability, and that its reduction increases levels of presynaptic calcium. Locomotor defects can be rescued by presynaptic knockdown of endoplasmic reticulum calcium channels or by reducing evoked release probability, further suggesting that excess synaptic activity drives behavioral deficiencies. Our work establishes a critical function for Tmep in the regulation of synaptic transmission and locomotor behavior.