Neuronal α2δ proteins and brain disorders

Abstract

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

Keywords: Autism; CACNA2D1–4; Epilepsy; Neurological disease; Schizophrenia; Synapses.

Fig. 1

Reference maps of predicted human α₂δ-1 and α₂δ-2 neuronal disease mutations and SNPs. Models of CACNA2D1 and CACNA2D2 genes (upper panels in a and b) and protein structures (lower panels in a and b), including exon positions (light blue) and protein domains, are based on Ensembl and UniProt databases (ENST00000356253.9/P54289; ENST00000266039.7/Q9NY47). Previously published potentially disease-associated SNPs and disease mutations are indicated (see text for references). The protein structure of all α₂δ proteins is highly conserved sharing an N-terminal signal peptide (SP, yellow), a von Willebrand factor A domain (VWA, turquois), a cache domain (magenta), and a MIDAS site (green) (see also Fig. 2). ASD (violet), autism spectrum disorders; BPD (green), bipolar disorder; CA (blue), cerebellar atrophy; EE (blue), epileptic encephalopathy; MDD (red), major depressive disorder; NB (gray), night blindness; ND (nude), nicotine dependence; P (bordeaux), pain; SCZ (magenta), schizophrenia

Fig. 2

Reference maps of predicted human α₂δ-3 and α₂δ-4 neuronal disease mutations and SNPs. Models of CACNA2D3 and CACNA2D4 genes (upper panels in a and b) and protein structures (lower panels in a and b), including exon positions (light blue) and protein domains, are based on Ensembl and UniProt databases (ENST00000288197.9/Q8IZS8; ENST00000382722.10/Q7Z3S7). Previously published potentially disease-associated SNPs and disease mutations are indicated (see text for references). Large genomic deletions of CACNA2D4 (b) including an inactive cache domain have been linked to ASD (violet bars) and BPD (green bars). α₂δ-4 protein mutations are so far only known to cause night blindness (NB) and gliomas (not indicated). ASD (violet), autism spectrum disorders; BPD (green), bipolar disorder; CA (blue), cerebellar atrophy; EE (blue), epileptic encephalopathy; MDD (red), major depressive disorder; NB (gray), night blindness; ND (nude), nicotine dependence; P (bordeaux), pain; SCZ (magenta), schizophrenia

Fig. 3

Model summarizing proposed synaptic functions of α₂δ proteins. α₂δ proteins as calcium channel subunits enhance plasma membrane expression and modulate current properties of both presynaptic and postsynaptic α₁ subunits (1). At the presynaptic terminal, α₂δ proteins mediate the accumulation of synaptic vesicles (SV, 2). They regulate active zone architecture (AZ) and bouton morphogenesis (3) either directly by interacting with proteins of the AZ and cytoskeleton (3) or indirectly via the VGCC complex (4). By aligning the presynaptic AZ with the postsynaptic membrane and postsynaptic AMPAR and GABA_AR (receptors in blue; 5 and 6, respectively), they act as trans-synaptic organizers either partly (5) or entirely (6) independent of the VGCC complex. This may be mediated by a direct interaction with postsynaptic receptors (6), by interacting with prototypical cell adhesion molecules such as presynaptic neurexins and postsynaptic neuroligins (7), or by interacting with proteins of the extracellular matrix or secreted proteins (e.g. BDNF, TSP; yellow ellipses) (8). A transmembrane form of α₂δ-1 bound to TSP is suggested to initiate the recruitment and stabilization of NMDAR (receptors in auburn) on the presynaptic (9) and postsynaptic (10) surface, a mechanism which contributes to enhanced synaptic transmission (9) and regulates intracellular signaling pathways as well as dendritic spine maturation via a small Rho GTPase (10). By regulating calcium currents of VGCC, α₂δ proteins are further predicted to modulate neuronal excitability (11) and gene expression (12)

Fig. 4

Putative consequences of α₂δ splicing and selected disease mutations on protein structure. Using homology modeling based on the 2.7 Å resolution structure of α₂δ-1 (PDB code: 6JP8) [6, 135], we tested the potential consequences of alternative splicing of exon 23 on the structure prediction of hα₂δ-2 (a) [40], the potentially autism-causing mutation p.Ala917Thr in hα₂δ-3 (b) [42], and the hα₂δ-1 frameshift mutation p.Val875fs associated with epilepsy (c) [44]. a As previously suggested for mouse α₂δ-2 [40], the inclusion of exon 23 in hα₂δ-2 suggests the formation of an extra loop leading to the disruption of an α-helix present. b Mutation p.Ala917Thr is not predicted to alter hα₂δ-3 protein structure; however, altered electrostatic potential (EP) on the surface (red, negative EP; blue, positive EP) and hence altered surface hydrophobicity may influence protein interactions. c The hα₂δ-1 frameshift mutation p.Val875fs deletes the C-terminal part of α₂ and the entire δ peptide (green). Furthermore, the altered reading frame introduces a random 36-amino acid sequence stretch (red). Molecular modeling was performed with the MOE software [69]. EP calculations were performed using Amber 10:EHT charges and the Poisson-Boltzmann approach as implemented in the software. Prior to the calculations, the structure was prepared and protonated with Protonate3D within MOE

Fig. 5

Model summarizing channel-dependent and channel-independent roles of α₂δ proteins. Mutations of α₂δ genes can affect expression levels, protein structure, and splicing. These alterations have consequences on calcium channel–dependent functions (membrane expression, current modulation, channel subtype–specific functions), and channel-independent functions (extracellular and/or trans-synaptic interactions and signaling pathways). The two mechanisms should not be considered entirely independent as they are likely influencing each other. For example, alterations in synapse differentiation will also affect VGCC expression with further consequences on neuronal excitability and synaptic transmission