Stim and Orai proteins in neuronal Ca(2+) signaling and excitability

Abstract

Stim1 and Orai1 are ubiquitous proteins that have long been known to mediate Ca(2+) release-activated Ca(2+) (CRAC) current (ICRAC) and store-operated Ca(2+) entry (SOCE) only in non-excitable cells. SOCE is activated following the depletion of the endogenous Ca(2+) stores, which are mainly located within the endoplasmic reticulum (ER), to replete the intracellular Ca(2+) reservoir and engage specific Ca(2+)-dependent processes, such as proliferation, migration, cytoskeletal remodeling, and gene expression. Their paralogs, Stim2, Orai2 and Orai3, support SOCE in heterologous expression systems, but their physiological role is still obscure. Ca(2+) inflow in neurons has long been exclusively ascribed to voltage-operated and receptor-operated channels. Nevertheless, recent work has unveiled that Stim1-2 and Orai1-2, but not Orai3, proteins are also expressed and mediate SOCE in neurons. Herein, we survey current knowledge about the neuronal distribution of Stim and Orai proteins in rodent and human brains; we further discuss that Orai2 is the main pore-forming subunit of CRAC channels in central neurons, in which it may be activated by either Stim1 or Stim2 depending on species, brain region and physiological stimuli. We examine the functions regulated by SOCE in neurons, where this pathway is activated under resting conditions to refill the ER, control spinogenesis and regulate gene transcription. Besides, we highlighted the possibility that SOCE also controls neuronal excitation and regulate synaptic plasticity. Finally, we evaluate the involvement of Stim and Orai proteins in severe neurodegenerative and neurological disorders, such as Alzheimer's disease and epilepsy.

Keywords: Ca2+ signaling; Orai1; Orai2; STIM1; STIM2; neurons; store-operated Ca2+ entry.

FIGURE 1

The neuronal Ca²⁺ signalling toolkit. Neuronal Ca²⁺ signals are shaped by the interaction between Ca²⁺ inflow from the outside and Ca²⁺ mobilization from the endoplasmic reticulum (ER), their most abundant endogenous Ca²⁺ pool. At excitatory synapses, the signaling cascade is initiated when glutamate is released into the synaptic cleft. Glutamate binds to receptor-operated channels, such as α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) and N-methyl-D-aspartate receptors (NMDARs), and to metabotropic receptors, such as type 1 metabotropic glutamate receptors (mGluR1). AMPAR gates Na+ entry, thereby causing the excitatory postsynaptic potential (EPSP) that removes the Mg²⁺ block from NMDAR , enabling it to open in response to Glu and to mediate Ca²⁺ inflow. Moreover, the EPSP recruits an additional pathway for Ca²⁺ entry by activating voltage-operated Ca²⁺ channels (VOCCs). Outside the postsynaptic density is located mGluR1, that is coupled to PLCb by a trimeric Gq protein and, therefore, leads to inositol-1,4,5-trisphosphate (InsP₃) synthesis. InsP₃, in turn, induces Ca²⁺ release from ER by binding to and gating the so-called InsP₃ receptors (InsP₃Rs). ER-dependent Ca²⁺ discharge also involves ryanodine receptors (RyRs) which are activated by Ca2+ delivered either by adjoining InsP₃Rs or by plasmalemmal VOCs or NMDARs according to the process of Ca²⁺-induced Ca²⁺ release (CICR). An additional route for Ca²⁺ influx is provided by store-operated Ca²⁺ entry, which is mediated by the interaction between the ER Ca²⁺-sensors, Stim1 and Stim2, and the Ca²⁺-permeable channels, Orai1 and Orai2. As more extensively illustrated in the text, depending on the species (rat, mouse, or human) and on the brain region (cortex, hippocampus, or cerebellum), Stim and Orai isoforms interact to mediate Ca²⁺ entry either in the presence or in the absence of synaptic activity to ensure adequate replenishment of ER Ca²⁺ loading and engage in Ca²⁺-sensitive decoders.

FIGURE 2

Topology and predicted domains of Stim1 and Orai1. (A) Stim1 comprises a signal peptide (Sig), a canonical EF-hand (cEF) domain, a hidden EF (hEF) domain, a sterile alpha motif (SAM), a transmembrane domain (TM), three coiled-coil domains (CC1, CC2, CC3), CAD, SOAR, serine/proline-rich domain (S/P), and lysine-rich domain (K-rich). (B) Each Orai1 monomer consists of four transmembrane domains (TM1TM4) and presents CAD binding domains in the cytosolic NH₂ and COOH termini. E106 is the residue crucial for conferring Ca²⁺-selectivity to the channel pore.

FIGURE 3

Current model of the mechanistic coupling between Stim1 and Orai1. In the absence of extracellular stimulation, Stim1 is uniformly distributed throughout ER membrane. Upon agonist (in this case, glutamate or Glu)-dependent PLCb activation, InsP3 is produced thereby depleting the InsP3-sensitive Ca²⁺ stores. Consequently, Ca²⁺ dissociates from Stim1 NH₂-terminal cEF domain, resulting in SAM-mediate Stim1 oligomerization and translocation into punctate clusters in regions closely apposed to the plasma membrane. Herein, Stim1 binds to and gates Orai1 through physical interaction between, respectively, their CC domains (CC2 and CC3) and CAD binding domains, thereby activating SOCE.