The calcium channel alpha2/delta1 subunit is involved in extracellular signalling

Influence of the calcium voltage-gated channel auxiliary subunit (CACNA2D1) absence on intraocular pressure in mice

The etiology of elevated intraocular pressure (IOP), a major risk factor for glaucoma (optic nerve atrophy), is poorly understood despite continued efforts. Although the gene variant of CACNA2D1 (encoding α2δ1), a calcium voltage-gated channel auxiliary subunit, has been reported to be associated with primary open-angle glaucoma, and the pharmacological mitigation of α2δ1 activity by pregabalin lowers IOP, the cellular basis for α2δ1 role in the modulation of IOP remains unclear. Our recent findings reveled readily detectable levels of α2δ1 and its ligand thrombospondin in the cytoskeletome fraction of human trabecular meshwork (TM) cells. To understand the direct role of α2δ1 in the modulation of IOP, we evaluated α2δ1 null mice for changes in IOP and found a moderate (∼10%) but significant decrease in IOP compared to littermate wild type control mice. Additionally, to gain cellular insights into α2δ1 antagonist (pregabalin) induced IOP changes, we assessed pregabalin's effects on human TM cell actin cytoskeletal organization and cell adhesive interactions in comparison with a Rho kinase inhibitor (Y27632), a known ocular hypotensive agent. Unlike Y27632, pregabalin did not have overt effects on cell morphology, actin cytoskeletal organization, or cell adhesion in human TM cells. These results reveal a modest but significant decrease in IOP in α2δ1 deficient mice, and this response appears to be not associated with the contractile and cell adhesive characteristics of TM cells based on the findings of pregabalin effects on isolated TM cells. Therefore, the mechanism by which pregabalin lowers IOP remains elusive.

Loss of the auxiliary α2δ1 voltage-sensitive calcium channel subunit impairs bone formation and anabolic responses to mechanical loading

Voltage-sensitive calcium channels (VSCCs) influence bone structure and function, including anabolic responses to mechanical loading. While the pore-forming (α1) subunit of VSCCs allows Ca2+ influx, auxiliary subunits regulate the biophysical properties of the pore. The α2δ1 subunit influences gating kinetics of the α1 pore and enables mechanically induced signaling in osteocytes; however, the skeletal function of α2δ1 in vivo remains unknown. In this work, we examined the skeletal consequences of deleting Cacna2d1, the gene encoding α2δ1. Dual-energy X-ray absorptiometry and microcomputed tomography imaging demonstrated that deletion of α2δ1 diminished bone mineral content and density in both male and female C57BL/6 mice. Structural differences manifested in both trabecular and cortical bone for males, while the absence of α2δ1 affected only cortical bone in female mice. Deletion of α2δ1 impaired skeletal mechanical properties in both sexes, as measured by three-point bending to failure. While no changes in osteoblast number or activity were found for either sex, male mice displayed a significant increase in osteoclast number, accompanied by increased eroded bone surface and upregulation of genes that regulate osteoclast differentiation. Deletion of α2δ1 also rendered the skeleton insensitive to exogenous mechanical loading in males. While previous work demonstrates that VSCCs are essential for anabolic responses to mechanical loading, the mechanism by which these channels sense and respond to force remained unclear. Our data demonstrate that the α2δ1 auxiliary VSCC subunit functions to maintain baseline bone mass and strength through regulation of osteoclast activity and also provides skeletal mechanotransduction in male mice. These data reveal a molecular player in our understanding of the mechanisms by which VSCCs influence skeletal adaptation.

HIF-1α/MMP-9 Axis Is Required in the Early Phases of Skeletal Myoblast Differentiation under Normoxia Condition In Vitro

Hypoxia-inducible factor (HIF)-1α represents an oxygen-sensitive subunit of HIF transcriptional factor, which is usually degraded in normoxia and stabilized in hypoxia to regulate several target gene expressions. Nevertheless, in the skeletal muscle satellite stem cells (SCs), an oxygen level-independent regulation of HIF-1α has been observed. Although HIF-1α has been highlighted as a SC function regulator, its spatio-temporal expression and role during myogenic progression remain controversial. Herein, using biomolecular, biochemical, morphological and electrophysiological analyses, we analyzed HIF-1α expression, localization and role in differentiating murine C2C12 myoblasts and SCs under normoxia. In addition, we evaluated the role of matrix metalloproteinase (MMP)-9 as an HIF-1α effector, considering that MMP-9 is involved in myogenesis and is an HIF-1α target in different cell types. HIF-1α expression increased after 24/48 h of differentiating culture and tended to decline after 72 h/5 days. Committed and proliferating mononuclear myoblasts exhibited nuclear HIF-1α expression. Differently, the more differentiated elongated and parallel-aligned cells, which are likely ready to fuse with each other, show a mainly cytoplasmic localization of the factor. Multinucleated myotubes displayed both nuclear and cytoplasmic HIF-1α expression. The MMP-9 and MyoD (myogenic activation marker) expression synchronized with that of HIF-1α, increasing after 24 h of differentiation. By means of silencing HIF-1α and MMP-9 by short-interfering RNA and MMP-9 pharmacological inhibition, this study unraveled MMP-9's role as an HIF-1α downstream effector and the fact that the HIF-1α/MMP-9 axis is essential in morpho-functional cell myogenic commitment.

Examining Mechanisms for Voltage-Sensitive Calcium Channel-Mediated Secretion Events in Bone Cells

In addition to their well-described functions in cell excitability, voltage-sensitive calcium channels (VSCCs) serve a critical role in calcium (Ca2+)-mediated secretion of pleiotropic paracrine and endocrine factors, including those produced in bone. Influx of Ca2+ through VSCCs activates intracellular signaling pathways to modulate a variety of cellular processes that include cell proliferation, differentiation, and bone adaptation in response to mechanical stimuli. Less well understood is the role of VSCCs in the control of bone and calcium homeostasis mediated through secreted factors. In this review, we discuss the various functions of VSCCs in skeletal cells as regulators of Ca2+ dynamics and detail how these channels might control the release of bioactive factors from bone cells. Because VSCCs are druggable, a better understanding of the multiple functions of these channels in the skeleton offers the opportunity for developing new therapies to enhance and maintain bone and to improve systemic health.

Recognition mechanism of a novel gabapentinoid drug, mirogabalin, for recombinant human α2δ1, a voltage-gated calcium channel subunit

Mirogabalin is a novel gabapentinoid drug with a hydrophobic bicyclo substituent on the γ-aminobutyric acid moiety that targets the voltage-gated calcium channel subunit α₂δ1. Here, to reveal the mirogabalin recognition mechanisms of α₂δ1, we present structures of recombinant human α₂δ1 with and without mirogabalin analyzed by cryo-electron microscopy. These structures show the binding of mirogabalin to the previously reported gabapentinoid binding site, which is the extracellular dCache_1 domain containing a conserved amino acid binding motif. A slight conformational change occurs around the residues positioned close to the hydrophobic group of mirogabalin. Mutagenesis binding assays identified that residues in the hydrophobic interaction region, in addition to several amino acid binding motif residues around the amino and carboxyl groups of mirogabalin, are critical for mirogabalin binding. The A215L mutation introduced to decrease the hydrophobic pocket volume predictably suppressed mirogabalin binding and promoted the binding of another ligand, L-Leu, with a smaller hydrophobic substituent than mirogabalin. Alterations of residues in the hydrophobic interaction region of α₂δ1 to those of the α₂δ2, α₂δ3, and α₂δ4 isoforms, of which α₂δ3 and α₂δ4 are gabapentin-insensitive, suppressed the binding of mirogabalin. These results support the importance of hydrophobic interactions in α₂δ1 ligand recognition.

Gabapentin Disrupts Binding of Perlecan to the α2δ1 Voltage Sensitive Calcium Channel Subunit and Impairs Skeletal Mechanosensation

Our understanding of how osteocytes, the principal mechanosensors within bone, sense and perceive force remains unclear. Previous work identified "tethering elements" (TEs) spanning the pericellular space of osteocytes and transmitting mechanical information into biochemical signals. While we identified the heparan sulfate proteoglycan perlecan (PLN) as a component of these TEs, PLN must attach to the cell surface to induce biochemical responses. As voltage-sensitive calcium channels (VSCCs) are critical for bone mechanotransduction, we hypothesized that PLN binds the extracellular α₂δ₁ subunit of VSCCs to couple the bone matrix to the osteocyte membrane. Here, we showed co-localization of PLN and α₂δ₁ along osteocyte dendritic processes. Additionally, we quantified the molecular interactions between α₂δ₁ and PLN domains and demonstrated for the first time that α₂δ₁ strongly associates with PLN via its domain III. Furthermore, α₂δ₁ is the binding site for the commonly used pain drug, gabapentin (GBP), which is associated with adverse skeletal effects when used chronically. We found that GBP disrupts PLN::α₂δ₁ binding in vitro, and GBP treatment in vivo results in impaired bone mechanosensation. Our work identified a novel mechanosensory complex within osteocytes composed of PLN and α₂δ₁, necessary for bone force transmission and sensitive to the drug GBP.

Effects of Gabapentin and Pregabalin on Calcium Homeostasis: Implications for Physical Rehabilitation of Musculoskeletal Tissues

PURPOSE OF REVIEW

In this review, we discuss the mechanism of action of gabapentinoids and the potential consequences of long-term treatment with these drugs on the musculoskeletal system.

RECENT FINDINGS

Gabapentinoids, such as gabapentin (GBP) and pregabalin (PGB) were designed as antiepileptic reagents and are now commonly used as first-line treatment for neuropathic pain and increasingly prescribed off-label for other pain disorders such as migraines and back pain. GBP and PGB exert their analgesic actions by selectively binding the α₂δ₁ auxiliary subunit of voltage-sensitive calcium channels, thereby inhibiting channel function. Numerous tissues express the α₂δ₁ subunit where GBP and PGB can alter calcium-mediated signaling events. In tissues such as bone, muscle, and cartilage, α₂δ₁ has important roles in skeletal formation, mechanosensation, and normal tissue function/repair that may be affected by chronic use of gabapentinoids. Long-term use of gabapentinoids is associated with detrimental musculoskeletal outcomes, including increased fracture risk. Therefore, understanding potential complications is essential for clinicians to guide appropriate treatments.

Skeletal Functions of Voltage Sensitive Calcium Channels

Voltage-sensitive calcium channels (VSCCs) are ubiquitous multimeric protein complexes that are necessary for the regulation of numerous physiological processes. VSCCs regulate calcium influx and various intracellular processes including muscle contraction, neurotransmission, hormone secretion, and gene transcription, with function specificity defined by the channel's subunits and tissue location. The functions of VSCCs in bone are often overlooked since bone is not considered an electrically excitable tissue. However, skeletal homeostasis and adaptation relies heavily on VSCCs. Inhibition or deletion of VSCCs decreases osteogenesis, impairs skeletal structure, and impedes anabolic responses to mechanical loading. RECENT FINDINGS: While the functions of VSCCs in osteoclasts are less clear, VSCCs have distinct but complementary functions in osteoblasts and osteocytes. PURPOSE OF REVIEW: This review details the structure, function, and nomenclature of VSCCs, followed by a comprehensive description of the known functions of VSCCs in bone cells and their regulation of bone development, bone formation, and mechanotransduction.

Design of an Imaging Probe to Monitor Real-Time Redistribution of L-type Voltage-Gated Calcium Channels in Astrocytic Glutamate Signaling

PURPOSE

In the brain, astrocytes are non-excitable cells that undergo rapid morphological changes when stimulated by the excitatory neurotransmitter glutamate. We developed a chemical probe to monitor how glutamate affects the density and distribution of astrocytic L-type voltage-gated calcium channels (LTCC).

PROCEDURES

The imaging probe FluoBar1 was created from a barbiturate ligand modified with a fluorescent coumarin moiety. The probe selectivity was examined with colocalization analyses of confocal fluorescence imaging in U118-MG and transfected COS-7 cells. Living cells treated with 50 nM FluoBar1 were imaged in real time to reveal changes in density and distribution of astrocytic LTCCs upon exposure to glutamate.

RESULTS

FluoBar1 was synthesized in ten steps. The selectivity of the probe was demonstrated with immunoblotting and confocal imaging of immunostained cells expressing the Ca_V1.2 isoform of LTCCs proteins. Applying FluoBar1 to astrocyte model cells U118-MG allowed us to measure a fivefold increase in fluorescence density of LTCCs upon glutamate exposure.

CONCLUSIONS

Imaging probe FluoBar1 allows the real-time monitoring of LTCCs in living cells, revealing for first time that glutamate causes a rapid increase of LTCC membranar density in astrocyte model cells. FluoBar1 may help tackle previously intractable questions about LTCC dynamics in cellular events.

Retinal teratogenicity of pregabalin in chick embryo model

BACKGROUND

Pregabalin is a gamma-aminobutyric acid analog that binds to the α2-δ subunits of the pre-synaptic voltage-dependent calcium channels of nerves with a high affinity and selectivity. In this study, the retinal teratogenic potential of pregabalin was investigated in a chick embryo model.

MATERIALS AND METHODS

Fertilised chicken eggs were divided into groups for administration with different doses of pregabalin. All eggs were opened on the 10th day of incubation. The embryos were dissected and the effects of pregabalin on the retina were investigated histopathologically, morphometrically, and immunohistochemically (Caspase-3).

RESULTS

There was no statistically significant difference between the low dose pregabalin, control, or vehicle control groups in terms of the number of retina layers and retinal thickness. Medium and high dose pregabalin caused a statistically significant decrease in the number of retina layers, as well as sensory retinal and pigment epithelium layer thicknesses. The outer nuclear and outer plexiform layer did not form in the group administered a medium dose. Similarly, the outer nuclear, outer plexiform, inner nuclear, and inner plexiform layer did not form in the high-dose group. No statistically significant difference was observed between the groups in terms of cellular damage and Caspase-3 expression.

CONCLUSION

The use of pregabalin during pregnancy compromises retinal development in a dose-dependent manner. The use of pregabalin in pregnancy causes the aforementioned defects in this system and it may have developmental effects that needs to be further evaluated.

Ma2/d promotes myonuclear positioning and association with the sarcoplasmic reticulum

The cytoplasm of striated myofibers contains a large number of membrane organelles, including sarcoplasmic reticulum (SR), T-tubules and the nuclear membrane. These organelles maintain a characteristic juxtaposition that appears to be essential for efficient inter-membranous exchange of RNA, proteins and ions. We found that the membrane-associated Muscle-specific α2/δ (Ma2/d) subunit of the Ca²⁺ channel complex localizes to the SR and T-tubules, and accumulates at the myonuclear surfaces. Furthermore, Ma2/d mutant larval muscles exhibit nuclear positioning defects, disruption of the nuclear-SR juxtapositioning, as well as impaired larval locomotion. Ma2/d localization at the nuclear membrane depends on the proper function of the nesprin ortholog Msp300 and the BAR domain protein Amphiphysin (Amph). Importantly, live imaging of muscle contraction in intact Drosophila larvae indicated altered distribution of Sarco/Endoplamic Reticulum Ca²⁺-ATPase (SERCA) around the myonuclei of Ma2/d mutant larvae. Co-immunoprecipitation analysis supports association between Ma2/d and Amph, and indirectly with Msp300. We therefore suggest that Ma2/d, in association with Msp300 and Amph, mediates interactions between the SR and the nuclear membrane.

Thrombospondin-1 protects against Aβ-induced mitochondrial fragmentation and dysfunction in hippocampal cells

Alzheimer's disease (AD) is often characterized by the impairment of mitochondrial function caused by excessive mitochondrial fragmentation. Thrombospondin-1 (TSP-1), which is primarily secreted from astrocytes in the central nervous system (CNS), has been suggested to play a role in synaptogenesis, spine morphology, and synaptic density of neurons. In this study, we investigate the protective role of TSP-1 in the recovery of mitochondrial morphology and function in amyloid β (Aβ)-treated mouse hippocampal neuroblastoma cells (HT22). We observe that TSP-1 inhibits Aβ-induced mitochondrial fission by maintaining phosphorylated-Drp1 (p-Drp1) levels, which results in reduced Drp1 translocation to the mitochondria. By using gabapentin, a drug that antagonizes the interaction between TSP-1 and its neuronal receptor α2δ1, we observe that α2δ1 acts as one of the target receptors for TSP-1, and blocks the reduction of the p-Drp1 to Drp1 ratio, in the presence of Aβ. Taken together, TSP-1 appears to contribute to maintaining the balance in mitochondrial dynamics and mitochondrial functions, which is crucial for neuronal cell viability. These data suggest that TSP-1 may be a potential therapeutic target for AD.

Calcium channel α2δ1 subunit as a novel biomarker for diagnosis of hepatocellular carcinoma

Objective:

Hepatocellular carcinoma (HCC) is the fifth most common malignancy worldwide. The identification of new simple, inexpensive and highly accurate markers for HCC diagnosis and screening is needed. This case-control study evaluates the role of annexin A2 and voltage-gated calcium channels α2δ1 subunit as serum biomarkers for HCC diagnosis.

Methods:

The study comprised three groups: group 1, 50 patients with an initial diagnosis of HCC associated with chronic hepatitis C virus infection; group 2, 25 patients diagnosed with chronic hepatitis C virus infection and cirrhosis without any evidence of HCC; and group 3, 15 healthy controls. All participants were subjected to clinical and laboratory investigations, and radiological scanning. The serum levels of alpha-fetoprotein (AFP), annexin A2, and the α2δ1 subunit were evaluated by using ELISA technique.

Results:

The serum levels of annexin A2 significantly increased in patients with HCC (10.4±2.5 ng/mL; P<0.001) or with cirrhosis (9.31±1.8 ng/mL;P<0.001) comparing to that of healthy controls (0.296±0.09 ng/mL). However, there was no significant difference in serum annexin A2 levels in patients with HCC comparing to those with cirrhosis. Serum α2δ1 subunit significantly increased in patients with HCC (20.12±3.7 ng/mL) comparing to that in patients with cirrhosis (10.41±3.4 ng/mL,P<0.001) and healthy controls (10.2±2.9 ng/mL,P<0.001).

Conclusions:

The serum α2δ1 subunit may function as a new biomarker for HCC diagnosis. Conversely, serum annexin A2 has low diagnostic value as an HCC marker, especially in patients with underlying cirrhosis.

Temporal expression of calcium channel subunits in satellite cells and bone marrow mesenchymal cells

Bone marrow-derived mesenchymal stem cells (MSC) can be differentiated into myocytes, as well as adipocytes, chondrocytes, and osteocytes in culture. Calcium channels mediate excitation-contraction coupling and are essential for the function of muscle. However, little is known about the expression of calcium channel subunits and calcium handling in stem cells. We examined whether the expression of calcium channel subunits in MSC is similar to that of skeletal muscle satellite cells and if their levels of expression are modified after treatment with bone morphogenetic protein-4 (BMP4). We found that during myogenic differentiation, MSC first express the α2δ1 subunit and the cardiac channel subunit Cav1.2. In contrast to the α2δ1 subunit levels, the Cav1.2 subunit decreases rapidly with time. The skeletal channel subunit Cav1.1 is detected at day 3 but its expression increases considerably, resembling more closely the expression of the subunits in satellite cells. Treatment of MSC with BMP4 caused a significant increase in expression of Cav1.2, a delay in expression of Cav1.1, and a reduction in the duration of calcium transients when extracellular calcium was removed. Calcium currents and transients followed a pattern related to the expression of the cardiac (Cav1.2) or skeletal (Cav1.1) α1subunits. These results indicate that differentiation of untreated MSC resembles differentiation of skeletal muscle and that BMP4 reduces skeletal muscle calcium channel expression and promotes the expression of cardiac calcium channels during myogenic differentiation.

Mechanisms of the gabapentinoids and α 2 δ-1 calcium channel subunit in neuropathic pain

The gabapentinoid drugs gabapentin and pregabalin are key front‐line therapies for various neuropathies of peripheral and central origin. Originally designed as analogs of GABA, the gabapentinoids bind to the α₂δ‐1 and α₂δ‐2 auxiliary subunits of calcium channels, though only the former has been implicated in the development of neuropathy in animal models. Transgenic approaches also identify α₂δ‐1 as key in mediating the analgesic effects of gabapentinoids, however the precise molecular mechanisms remain unclear. Here we review the current understanding of the pathophysiological role of the α₂δ‐1 subunit, the mechanisms of analgesic action of gabapentinoid drugs and implications for efficacy in the clinic. Despite widespread use, the number needed to treat for gabapentin and pregabalin averages from 3 to 8 across neuropathies. The failure to treat large numbers of patients adequately necessitates a novel approach to treatment selection. Stratifying patients by sensory profiles may imply common underlying mechanisms, and a greater understanding of these mechanisms could lead to more direct targeting of gabapentinoids.

Evidence for MOR on cell membrane, sarcoplasmatic reticulum and mitochondria in left ventricular myocardium in rats

Cardiac function is one important determinant to maintain tissue oxygenation and is thus highly regulated. In this context, it is interesting that centrally mediated opioidergic influence on cardiac function has long been known. Only recently, KOR and DOR have been found to be expressed in healthy left ventricular myocardium in rats and colocalized with parts of the excitation-contraction-coupling system. However, several comments in literature exist doubting the existence of MOR in cardiac tissue. We, therefore, aimed to detect MOR in rat left ventricular cardiomyocytes, and to evaluate whether MOR and POMC are regulated during heart failure. After IRB approval, heart failure was induced using a modified infrarenal aortocaval fistula (ACF) in male Wistar rats. All rats of the control and ACF group were characterized by their morphometrics and hemodynamics and the existence of MOR and POMC was investigated by means of radioligand binding, double immunofluorescence confocal analysis, RT-PCR and Western blot. Membrane MOR selective binding sites were detected in the left ventricular myocardium, however, they were lower in abundance than KOR- and DOR-specific binding sites and B max of MOR could not be determined. In left ventricular cardiomyocytes, MOR colocalized with parts of the excitation-coupling mechanism, e.g., Cav1.2 of the cell membrane and invaginated T-tubules as well as the ryanodine receptor of the sarcoplasmatic reticulum. More importantly, MOR strongly colocalized with mitochondria of left ventricular cardiomyocytes. Volume overload was not associated with an altered expression of MOR and POMC on both mRNA and protein level. These findings provide evidence for the existence of MOR on the cell membrane, sarcoplasmatic reticulum and mitochondria in left ventricular cardiomyocytes in rats. However, heart failure does not result in an altered expression of the cardiac MOR-opioid system. Thus, MOR agonist treatment-commonly used in the clinical setting-might directly affect cardiac function, which needs to be evaluated in greater detail in the near future.

Thrombospondins and synaptogenesis

Here, we review research on the mechanisms underlying the ability of thrombospondin to promote synaptogenesis and examine its role in central nervous system diseases and drug actions. Thrombospondin secreted by glial cells plays a critical role in synaptogenesis and maintains synapse stability. Thrombospondin regulates synaptogenesis through receptor α2δ-1 and neuroligin 1, and promotes the proliferation and differentiation of neural progenitor cells. It also participates in synaptic remodeling following injury and in the action of some nervous system drugs.

CACNA2D2 promotes tumorigenesis by stimulating cell proliferation and angiogenesis

In the present study, we have assessed whether a putative calcium channel α2δ2 auxiliary subunit (CACNA2D2 gene) could be involved in prostate cancer (PCA) progression. We therefore carried out experiments to determine whether this protein is expressed in PCA LNCaP cells and in PCA tissues, and whether its expression may be altered during cancer development. In addition, we evaluated the influence on cell proliferation of overexpressing or downregulating this subunit. In vitro experiments show that α2δ2 subunit overexpression is associated with increased cell proliferation, alterations of calcium homeostasis and the recruitment of a nuclear factor of activated T-cells pathway. Furthermore, we carried out in vivo experiments on immuno-deficient nude mice in order to evaluate the tumorigenic potency of the α2δ2 subunit. We show that α2δ2-overexpressing PCA LNCaP cells are more tumorigenic than control LNCaP cells when injected into nude mice. In addition, gabapentin, a ligand of α2δ2, reduces tumor development in LNCaP xenografts. Finally, we show that the action of α2δ2 on tumor development occurs not only through a stimulation of proliferation, but also through a stimulation of angiogenesis, via an increased secretion of vascular endothelial growth factor in cells overexpressing α2δ2.

Ubiquitin-specific protease USP2-45 acts as a molecular switch to promote α2δ-1-induced downregulation of Cav1.2 channels

Availability of voltage-gated calcium channels (Ca_v) at the plasma membrane is paramount to maintaining the calcium homeostasis of the cell. It is proposed that the ubiquitylation/de-ubiquitylation balance regulates the density of ion channels at the cell surface. Voltage-gated calcium channels Ca_v1.2 have been found to be ubiquitylated under basal conditions both in vitro and in vivo. In a previous study, we have shown that Ca_v1.2 channels are ubiquitylated by neuronal precursor cell-expressed developmentally downregulated 4 (Nedd4-1) ubiquitin ligases, but the identity of the counterpart de-ubiquitylating enzyme remained to be elucidated. Regarding sodium and potassium channels, it has been reported that the action of the related isoform Nedd4-2 is counteracted by the ubiquitin-specific protease (USP) 2-45. In this study, we show that USP 2-45 also de-ubiquitylates Ca_v channels. We co-expressed USPs and Ca_v1.2 channels together with the accessory subunits β₂ and α₂δ-1, in tsA-201 and HEK-293 mammalian cell lines. Using whole-cell current recordings and surface biotinylation assays, we show that USP2-45 specifically decreases both the amplitude of Ca_v currents and the amount of Ca_v1.2 subunits inserted at the plasma membrane. Importantly, co-expression of the α₂δ-1 accessory subunit is necessary to support the effect of USP2-45. We further show that USP2-45 promotes the de-ubiquitylation of both Ca_v1.2 and α₂δ-1 subunits. Remarkably, α₂δ-1, but not Ca_v1.2 nor β₂, co-precipitated with USP2-45. These results suggest that USP2-45 binding to α₂δ-1 promotes the de-ubiquitylation of both Ca_v1.2 and α₂δ-1 subunits, in order to regulate the expression of Ca_v1.2 channels at the plasma membrane.

Hypothalamic dysfunction of the thrombospondin receptor α2δ-1 underlies the overeating and obesity triggered by brain-derived neurotrophic factor deficiency

Brain-derived neurotrophic factor (BDNF) and its receptor, TrkB, are critical components of the neural circuitry controlling appetite and body weight. Diminished BDNF signaling in mice results in severe hyperphagia and obesity. In humans, BDNF haploinsufficiency and the functional Bdnf Val66Met polymorphism have been linked to elevated food intake and body weight. The mechanisms underlying this dysfunction are poorly defined. We demonstrate a chief role of α2δ-1, a calcium channel subunit and thrombospondin receptor, in triggering overeating in mice with central BDNF depletion. We show reduced α2δ-1 cell-surface expression in the BDNF mutant ventromedial hypothalamus (VMH), an energy balance-regulating center. This deficit contributes to the hyperphagia exhibited by BDNF mutant mice because selective inhibition of α2δ-1 by gabapentin infusion into wild-type VMH significantly increases feeding and body weight gain. Importantly, viral-mediated α2δ-1 rescue in BDNF mutant VMH significantly mitigates their hyperphagia, obesity, and liver steatosis and normalizes deficits in glucose homeostasis. Whole-cell recordings in BDNF mutant VMH neurons revealed normal calcium currents but reduced frequency of EPSCs. These results suggest calcium channel-independent effects of α2δ-1 on feeding and implicate α2δ-1-thrombospondin interactions known to facilitate excitatory synapse assembly. Our findings identify a central mechanism mediating the inhibitory effects of BDNF on feeding. They also demonstrate a novel and critical role for α2δ-1 in appetite control and suggest a mechanism underlying weight gain in humans treated with gabapentinoid drugs.

Commitment of Satellite Cells Expressing the Calcium Channel α2δ1 Subunit to the Muscle Lineage

Satellite cells can maintain or repair muscle because they possess stem cell properties, making them a valuable option for cell therapy. However, cell transplants into skeletal muscle of patients with muscular dystrophy are limited by donor cell attachment, migration, and survival in the host tissue. Cells used for therapy are selected based on specific markers present in the plasma membrane. Although many markers have been identified, there is a need to find a marker that is expressed at different states in satellite cells, activated, quiescent, or differentiated cell. Furthermore, the marker has to be present in human tissue. Recently we reported that the plasma membrane α2δ1 protein is involved in cell attachment and migration in myoblasts. The α2δ1 subunit forms a part of the L-type voltage-dependent calcium channel in adult skeletal muscle. We found that the α2δ1 subunit is expressed in the majority of newly isolated satellite cells and that it appears earlier than the α1 subunits and at higher levels than the β or γ subunits. We also found that those cells that expressed α2δ1 would differentiate into muscle cells. This evidence indicates that the α2δ1 may be used as a marker of satellite cells that will differentiate into muscle.

Calcium channel auxiliary α2δ and β subunits: trafficking and one step beyond

The voltage-gated calcium channel α(2)δ and β subunits are traditionally considered to be auxiliary subunits that enhance channel trafficking, increase the expression of functional calcium channels at the plasma membrane and influence the channels' biophysical properties. Accumulating evidence indicates that these subunits may also have roles in the nervous system that are not directly linked to calcium channel function. For example, β subunits may act as transcriptional regulators, and certain α(2)δ subunits may function in synaptogenesis. The aim of this Review is to examine both the classic and novel roles for these auxiliary subunits in voltage-gated calcium channel function and beyond.

Kinetic considerations on the development of binding assays in single-addition mode: application to the search for α2δ1 modulators

The development of assays in single-addition mode is of great interest for screening purposes given the multiple advantages of minimizing the number of intervention steps. Binding assays seem to be more prone to this attractive format because no functional biological activity is taking place but instead a biophysical process, whose dynamics seem easier to control without introducing significant alterations, is happening. Therefore, single-addition assays based on the displacement of prebound labeled ligands can be conceived, but careful kinetic considerations must still be taken to maximize the sensitivity of the assay and to avoid jeopardizing the identification of compounds with slow-binding kinetics. This article shows the development of a single-addition, displacement-based binding assay intended to identify modulators that act by binding to the gabapentin site of the ion channel regulatory protein α2δ1. After studying the kinetics of gabapentin binding and the influence they might have on the assay sensitivity, the best conditions were identified, and the sensitivity was compared with that of the more classical two-additions competition-based assay. Although the present study focuses on α2δ1 and its interaction with gabapentin, the rationale and the methodology followed are of broad purpose and can be applied to virtually every binding assay.

Association of the α(2)δ(1) subunit with Ca(v)3.2 enhances membrane expression and regulates mechanically induced ATP release in MLO-Y4 osteocytes

Voltage-sensitive calcium channels (VSCCs) mediate signaling events in bone cells in response to mechanical loading. Osteoblasts predominantly express L-type VSCCs composed of the α(1) pore-forming subunit and several auxiliary subunits. Osteocytes, in contrast, express T-type VSCCs and a relatively small amount of L-type α(1) subunits. Auxiliary VSCC subunits have several functions, including modulating gating kinetics, trafficking of the channel, and phosphorylation events. The influence of the α(2)δ auxiliary subunit on T-type VSCCs and the physiologic consequences of that association are incompletely understood and have yet to be investigated in bone. In this study we postulated that the auxiliary α(2) δ subunit of the VSCC complex modulates mechanically regulated ATP release in osteocytes via its association with the T-type Ca(v) 3.2 (α(1H) ) subunit. We demonstrated by reverse-transcriptase polymerase chain reaction, Western blotting, and immunostaining that MLO-Y4 osteocyte-like cells express the T-type Ca(v)3.2(α(1H)) subunit more abundantly than the L-type Ca(v)1.2 (α(1C)) subunit. We also demonstrated that the α(2) δ(1) subunit, previously described as an L-type auxiliary subunit, complexes with the T-type Ca(v)3.2 (α(1H)) subunit in MLO-Y4 cells. Interestingly, siRNA-mediated knockdown of α(2) δ(1) completely abrogated ATP release in response to membrane stretch in MLO-Y4 cells. Additionally, knockdown of the α(2)δ(1) subunit resulted in reduced ERK1/2 activation. Together these data demonstrate a functional VSCC complex. Immunocytochemistry following α(2)δ(1) knockdown showed decreased membrane localization of Ca(v) 3.2 (α(1H)) at the plasma membrane, suggesting that the diminished ATP release and ERK1/2 activation in response to membrane stretch resulted from a lack of Ca(v) 3.2 (α(1H)) at the cell membrane.

The calcium channel α2/δ1 subunit interacts with ATP5b in the plasma membrane of developing muscle cells

The α2/δ1 and α(1)1.1 subunits are present at a 1:1 ratio in the dihydropyridine receptor (DHPR) from adult skeletal muscle. In contrast, during early myotube development α2/δ1 is present at higher levels than α(1)1.1 and localizes at the ends of the cells, suggesting that α2/δ1 may have a role independent from DHPRs. We sought to identify binding partners of α2/δ1 at a period when levels of α(1)1.1 are low. Analysis of protein complexes in their native configuration established that α2/δ1 may be associating with ATP5b, a subunit of a mitochondrial ATP synthase complex. This interaction was confirmed with fluorescence resonance energy transfer and coimmunoprecipitation. The association of α2/δ1 and ATP5b occurs in intracellular membranes and at the plasma membrane, where they form a functional signaling complex capable of accelerating the rate of decline of calcium transients. The acceleration of decay was more evident when myotubes were stimulated with a train of pulses. Our data indicate that the α2/δ1 subunit is not only part of the DHPR but that it may interact with other cellular components in developing myotubes, such as the ATP5b in its atypical localization in the plasma membrane.

A new look at calcium channel α2δ subunits

The classical roles of α(2)δ proteins are as accessory calcium channel subunits, enhancing channel trafficking. They were thought to have type-I transmembrane topology, but we find that they can form GPI-anchored proteins. Moreover α(2)δ-1 and α(2)δ-3 have been shown to have novel functions in synaptogenesis, independent of their effect on calcium channels. In neurons, the α(2)δ-1 subunits are present mainly in presynaptic terminals. Peripheral sensory nerve injury results in the up-regulation of α(2)δ-1 in dorsal root ganglion (DRG) neurons, and there is a consequent increase in trafficking of α(2)δ-1 to their terminals. Furthermore, gabapentinoid drugs, which bind to α(2)δ-1 and α(2)δ-2, not only impair their trafficking, but also affect α(2)δ-1-dependent synaptogenesis. These drugs may interfere with α(2)δ function at several different levels.

Gabapentin receptor alpha2delta-1 is a neuronal thrombospondin receptor responsible for excitatory CNS synaptogenesis

Synapses are asymmetric cellular adhesions that are critical for nervous system development and function, but the mechanisms that induce their formation are not well understood. We have previously identified thrombospondin as an astrocyte-secreted protein that promotes central nervous system (CNS) synaptogenesis. Here, we identify the neuronal thrombospondin receptor involved in CNS synapse formation as alpha2delta-1, the receptor for the anti-epileptic and analgesic drug gabapentin. We show that the VWF-A domain of alpha2delta-1 interacts with the epidermal growth factor-like repeats common to all thrombospondins. alpha2delta-1 overexpression increases synaptogenesis in vitro and in vivo and is required postsynaptically for thrombospondin- and astrocyte-induced synapse formation in vitro. Gabapentin antagonizes thrombospondin binding to alpha2delta-1 and powerfully inhibits excitatory synapse formation in vitro and in vivo. These findings identify alpha2delta-1 as a receptor involved in excitatory synapse formation and suggest that gabapentin may function therapeutically by blocking new synapse formation.

Effects of inserting fluorescent proteins into the alpha1S II-III loop: insights into excitation-contraction coupling

In skeletal muscle, intermolecular communication between the 1,4-dihydropyridine receptor (DHPR) and RYR1 is bidirectional: orthograde coupling (skeletal excitation-contraction coupling) is observed as depolarization-induced Ca(2+) release via RYR1, and retrograde coupling is manifested by increased L-type Ca(2+) current via DHPR. A critical domain (residues 720-765) of the DHPR alpha(1S) II-III loop plays an important but poorly understood role in bidirectional coupling with RYR1. In this study, we examine the consequences of fluorescent protein insertion into different positions within the alpha(1S) II-III loop. In four constructs, a cyan fluorescent protein (CFP)-yellow fluorescent protein (YFP) tandem was introduced in place of residues 672-685 (the peptide A region). All four constructs supported efficient bidirectional coupling as determined by the measurement of L-type current and myoplasmic Ca(2+) transients. In contrast, insertion of a CFP-YFP tandem within the N-terminal portion of the critical domain (between residues 726 and 727) abolished bidirectional signaling. Bidirectional coupling was partially preserved when only a single YFP was inserted between residues 726 and 727. However, insertion of YFP near the C-terminal boundary of the critical domain (between residues 760 and 761) or in the conserved C-terminal portion of the alpha(1S) II-III loop (between residues 785 and 786) eliminated bidirectional coupling. None of the fluorescent protein insertions, even those that interfered with signaling, significantly altered membrane expression or targeting. Thus, bidirectional signaling is ablated by insertions at two different sites in the C-terminal portion of the alpha(1S) II-III loop. Significantly, our results indicate that the conserved portion of the alpha(1S) II-III loop C terminal to the critical domain plays an important role in bidirectional coupling either by conveying conformational changes to the critical domain from other regions of the DHPR or by serving as a site of interaction with other junctional proteins such as RYR1.

Unexpected roles of the Na-K-ATPase and other ion transporters in cell junctions and tubulogenesis

Recent work shows that transport-independent as well as transport-dependent functions of ion transporters, and in particular the Na-K-ATPase, are required for formation and maintenance of several intercellular junctions. Furthermore, these junctional and other nonjunctional functions of ion transporters contribute to development of epithelial tubes. Here, we consider what has been learned about the roles of ion pumps in formation of junctions and epithelial tubes in mammals, zebrafish, Drosophila, and C. elegans. We propose that asymmetric association of the Na-K-ATPase with cell junctions early in metazoan evolution enabled vectorial transcellular ion transport and control of intraorganismal environment. Ion transport-independent functions of the Na-K-ATPase arose as junctional complexes evolved.

Calcium channel diversity: multiple roles of calcium channel subunits

Until recently we held the simple view that voltage-gated calcium channels consisted of an alpha1 subunit, usually associated with auxiliary beta subunits and alpha(2)delta subunits and that skeletal muscle calcium channels were also associated with a gamma subunit. However, as discussed here, there is now evidence that the auxiliary subunits may also perform other roles unrelated to voltage-gated calcium entry. In the past students were taught the simplistic view that second messenger signaling to voltage-gated calcium channels involved mainly phosphorylation of L-type calcium channels, Ca(2+)-dependent inactivation via calmodulin, and direct G-protein-mediated inhibition of the neuronal N and P/Q channels. However, it is now clear that there are many other means of modulating calcium channel activity, including receptor-mediated internalization, proteolytic cleavage, phosphorylation of beta subunits, and interaction of calcium channels with other proteins, including enzymes masquerading as scaffold proteins.

Targeted disruption of the voltage-dependent calcium channel alpha2/delta-1-subunit

Cardiac L-type voltage-dependent Ca(2+) channels are heteromultimeric polypeptide complexes of alpha(1)-, alpha(2)/delta-, and beta-subunits. The alpha(2)/delta-1-subunit possesses a stereoselective, high-affinity binding site for gabapentin, widely used to treat epilepsy and postherpetic neuralgic pain as well as sleep disorders. Mutations in alpha(2)/delta-subunits of voltage-dependent Ca(2+) channels have been associated with different diseases, including epilepsy. Multiple heterologous coexpression systems have been used to study the effects of the deletion of the alpha(2)/delta-1-subunit, but attempts at a conventional knockout animal model have been ineffective. We report the development of a viable conventional knockout mouse using a construct targeting exon 2 of alpha(2)/delta-1. While the deletion of the subunit is not lethal, these animals lack high-affinity gabapentin binding sites and demonstrate a significantly decreased basal myocardial contractility and relaxation and a decreased L-type Ca(2+) current peak current amplitude. This is a novel model for studying the function of the alpha(2)/delta-1-subunit and will be of importance in the development of new pharmacological therapies.

Proper restoration of excitation-contraction coupling in the dihydropyridine receptor beta1-null zebrafish relaxed is an exclusive function of the beta1a subunit

The paralyzed zebrafish strain relaxed carries a null mutation for the skeletal muscle dihydropyridine receptor (DHPR) beta(1a) subunit. Lack of beta(1a) results in (i) reduced membrane expression of the pore forming DHPR alpha(1S) subunit, (ii) elimination of alpha(1S) charge movement, and (iii) impediment of arrangement of the DHPRs in groups of four (tetrads) opposing the ryanodine receptor (RyR1), a structural prerequisite for skeletal muscle-type excitation-contraction (EC) coupling. In this study we used relaxed larvae and isolated myotubes as expression systems to discriminate specific functions of beta(1a) from rather general functions of beta isoforms. Zebrafish and mammalian beta(1a) subunits quantitatively restored alpha(1S) triad targeting and charge movement as well as intracellular Ca(2+) release, allowed arrangement of DHPRs in tetrads, and most strikingly recovered a fully motile phenotype in relaxed larvae. Interestingly, the cardiac/neuronal beta(2a) as the phylogenetically closest, and the ancestral housefly beta(M) as the most distant isoform to beta(1a) also completely recovered alpha(1S) triad expression and charge movement. However, both revealed drastically impaired intracellular Ca(2+) transients and very limited tetrad formation compared with beta(1a). Consequently, larval motility was either only partially restored (beta(2a)-injected larvae) or not restored at all (beta(M)). Thus, our results indicate that triad expression and facilitation of 1,4-dihydropyridine receptor (DHPR) charge movement are common features of all tested beta subunits, whereas the efficient arrangement of DHPRs in tetrads and thus intact DHPR-RyR1 coupling is only promoted by the beta(1a) isoform. Consequently, we postulate a model that presents beta(1a) as an allosteric modifier of alpha(1S) conformation enabling skeletal muscle-type EC coupling.

The alpha(1S) III-IV loop influences 1,4-dihydropyridine receptor gating but is not directly involved in excitation-contraction coupling interactions with the type 1 ryanodine receptor

In skeletal muscle, coupling between the 1,4-dihydropyridine receptor (DHPR) and the type 1 ryanodine receptor (RyR1) underlies excitation-contraction (EC) coupling. The III-IV loop of the DHPR alpha(1S) subunit binds to a segment of RyR1 in vitro, and mutations in the III-IV loop alter the voltage dependence of EC coupling, raising the possibility that this loop is directly involved in signal transmission from the DHPR to RyR1. To clarify the role of the alpha(1S) III-IV loop in EC coupling, we examined the functional properties of a chimera (GFP-alpha(1S)[III-IVa]) in which the III-IV loop of the divergent alpha(1A) isoform replaced that of alpha(1S). Dysgenic myotubes expressing GFP-alpha(1S)[III-IVa] yielded myoplasmic Ca(2+) transients that activated at approximately 10 mV more hyperpolarized potentials and that were approximately 65% smaller than those of GFP-alpha(1S). A similar reduction was observed in voltage-dependent charge movements for GFP-alpha(1S)[III-IVa], indicating that the chimeric channels trafficked less well to the membrane but that those that were in the membrane functioned as efficiently in EC coupling as GFP-alpha(1S). Relative to GFP-alpha(1S), L-type currents mediated by GFP-alpha(1S)[III-IVa] were approximately 40% smaller and activated at approximately 5 mV more hyperpolarized potentials. The altered gating of GFP-alpha(1S)[III-IVa] was accentuated by exposure to +/-Bay K 8644, which caused a much larger hyperpolarizing shift in activation compared with its effect on GFP-alpha(1S). Taken together, our observations indicate that the alpha(1S) III-IV loop is not directly involved in EC coupling but does influence DHPR gating transitions important both for EC coupling and activation of L-type conductance.