Stromal Interaction Molecule
STIM1 is a key protein in initiation of calcium flux. STIM1 senses and reacts to changes in the levels of stored calcium ions (Ca+2) in a cell. Cells keep tight control on many important intracellular calcium-dependent processes by controlling the availability of free Ca+2, maintaining low levels of free Ca+2 in the cytosol relative to extracellular and stored intracellular Ca+2 levels. In calcium flux, intracellular release of stored Ca+2 ions, primarily stored in the endoplasmic reticulum (ER) of non-muscle cells or in the sarcoplasmic reticulum in muscle cells, can initiate specific calcium-dependent activities in the cell, such as enzyme activation. The release of stored Ca+2 into the cytosol then triggers an influx of extracellular Ca+2 through cell membrane channels to amplify the calcium-dependent activities. This is referred to as SOCE (storage-operated calcium entry). The extracellular Ca+2 ions enter through CRAC channels (calcium release-activated channels) as well as through nonspecific cation channels. An additional part of calcium flux is the subsequent reestablishment of homeostasis with low levels of intracellular free Ca+2 in the cytosol by pumping Ca+2 ions back into storage in the ER using SERCA (sarco/endoplasmic reticulum calcium ATPase) pumps.
STIM1 was identified as a unique gene (initially called GOK) located at p15.5 in chromosome 11 in humans (Parker et al. 1996). This is in a region of the short arm of chromosome 11 named “multiple tumor-associated chromosome region 1” which is suspected of involvement in many different tumors, including pediatric malignancies (Richard et al. 1993). The human gene is approximately 4 kb long and encodes a single protein of an estimated 84 kDa that includes a transmembrane section. The gene is highly conserved with 90% identity between the mouse and human for both the DNA and amino acid sequences (Parker et al. 1996).
The involvement of STIM1 in calcium flux was first proposed by Zhang and colleagues (Zhang et al. 2005) based on their research with STIM1 EF-hand mutants that activated calcium release-activated calcium (CRAC) channels without there being a decrease of stored Ca+2. They also reported movement of wild-type STIM1 to the plasma membrane with depletion of stored Ca+2. This suggested that STIM1 is important in sensing changes in stored Ca+2 levels and in the activation of CRAC channels. Subsequently, STIM2 was identified as a STIM1 homologue with two STIM2 isoforms of 105 kDa and 115 kDa based on differential phosphorylation (Williams et al. 2001). STIM2, like STIM1, contains EF-hand and SAM domains and coprecipitates with STIM1 as oligomeric complexes. Although STIM1 and STIM2 show possible hetero-oligomeric associations, more recently, differences in the roles of STIM1 and STIM2 have emerged (Shalygin et al. 2014). In rat neurons, activation of SOCE to allow rapid influx of Ca+2 appears to be a primary function of STIM1, whereas STIM2 is involved in maintaining levels of intracellular Ca+2 in resting cells (Gruszczynska-Biegala et al. 2011).
Structure, Biosynthesis, and Metabolism
De novo synthesis of STIM1 and ORAI1 is under the control of NF-κB (nuclear factor-kappa B) which binds in the gene promoter regions to stimulate expression (Lang et al. 2012). Further control of STIM1 and ORAI1 is provided by SGK1 (serum and glucocorticoid inducible kinase 1) which suppresses ubiquitination to reduce protein turnover and by AMPK (AMP activated kinase) which downregulates NF-κB. In addition, oligomerization of STIM1 and ORAI1 in CRAC channel activation further reduces their turnover.
The most important posttranslational modifications identified for STIM1 and STIM2 so far are phosphorylation and glycosylation. There are multiple phosphorylation sites on STIM1 and STIM2 that can alter protein-protein interactions. Multiple pathways are suspected for the variations of STIM phosphorylation. For example, phosphorylation of STIM1 at ERK1/ERK2 sites (SER575, Ser608, and Ser621) leads to release of STIM1 from EB1 during Ca+2 store depletion followed by return of STIM1 phosphorylation to basal levels (Pozo-Guisado et al. 2013). This suggests a means by which STIM1 activation of CRAC channels can be turned off following calcium influx. N-linked glycosylation at two asparagine sites within the STIM1 SAM domain, at N131 and N171, establishes STIM1 in the ER and assists in recognition and recycling of STIM1 (Williams et al. 2002). O-linked N-acetylglucosamine (O-GlcNAc) of STIM1 attenuates activation of SOCE in neonatal cardiomyocytes suggesting a role in early development of muscle tissue (Zhu-Mauldin et al. 2012).
Mechanism of Action
STIM in Health and Disease
Calcium dysregulation with STIM and SOCE involvement has been linked to some cancers, immunological diseases, neurological diseases, and numerous other diseases (Mukherjee and Brooks 2014). The key position of STIM molecules in initiating intracellular Ca+2-dependent actions indicates that STIM molecules and their partners are potential therapeutic targets.
Alzheimer’s disease (AD) is a chronic neurodegenerative disorder in which neurons are destroyed resulting in dementia and cognitive disorders due to synapse loss in the hippocampus and cortex. Plaque formation from accumulation of β-amyloid peptide (Aβ) can lead to neuron degeneration in AD. The Aβ occurs from cleavage of β-amyloid precursor protein (APP), a membrane protein. The enzymes that cleave the APP are β-secretase and γ-secretase. Accumulation of Aβ can induce oxidative stress and disrupt intracellular Ca+2 homeostasis in neurons. Mutations in the presenilin-1 (PS1) gene have been associated with some cases of familial AD. PS1, an aspartyl protease that is part of the γ-secretase complex, participates in the cleavage of APP. PS1 can affect ER C1+2 release channels as well as induce increased synthesis of STIM1 and STIM2. The resulting cellular stress can lead to Ca+2 dysregulation (Mukherjee and Brooks 2014).
STIM molecules have shown involvement in cancers (reviewed in: Johnstone et al. 2010; Lang et al. 2012; Mukherjee and Brooks 2014). Disruption of Ca+2 homeostasis can lead to the abnormalities in metastasis, migration, motility, and transcription seen in cancer cells. In colorectal cancer, increased STIM2 expression is suspected of interfering with the tumor suppressor effects of STIM1-mediated apoptosis, leaving tumor cells capable of further progression (Aytes et al. 2012). In hepatocellular carcinomas, STIM1 has higher expression levels compared to the patient’s normal hepatocytes (Yang et al. 2013). And STIM1 overexpression is seen in a majority of early stage cervical cancers leading to Ca+2 dysregulation (Chen et al. 2013). These are just a few examples of the cancers with STIM-related Ca+2 dysregulation.
The immune system has numerous vulnerabilities with regard to Ca+2 regulation from NETosis by neutrophils, as part of the early innate immune response to infections; to motility; and control of T cell and B cell proliferation in the adaptive immune response. SOCE involvement, including STIM1 and STIM2 expression, in different immune processes and immunodeficiencies has been described previously (Notarangela 2013; Robert et al. 2011; Baba and Kurosaki 2011).
Other Diseases and Abnormal States
SOCE with STIM1 involvement can be involved in other diseases and abnormal states such as stroke, infertility, tumor growth, hypertension, obesity, thrombosis, and diabetes. Many of these show abnormalities in SGK1 expression which affects SOCE and turnover rates of STIM1 and ORAI1 molecules.
STIM1 and STIM2 play important roles in calcium flux connecting the release of stored ER Ca+2 ions, triggered by cell surface receptors and intracellular secondary signals, to the opening of Ca+2 entry channels in the cell membrane by SOCE. Abnormalities in STIM expression and actions are involved in many varied diseases. However, we still need to identify and study all the components of calcium flux and homeostasis. There are other means of calcium entry besides the STIM1-ORAI1 CRAC channels, and these alternate routes need further definition. Also there is need to identify other binding partners of STIM1 and STIM2 when they are in their inactive states. The key roles of STIM1 and STIM2 in initiation of SOCE show that STIM molecules are promising targets for new therapeutics that will be effective in a broad variety of diseases. With regard to the STIM structures, further research is needed to determine how loss of the bound Ca+2 ion from the EF-hand domain in the ER lumen leads to conformational changes in the cytosolic portion of STIM as well as further protein-protein interactions during oligomerizations. In addition, the means by which different cell surface receptors using shared secondary signals to initiate release of stored Ca+2 from the ER can trigger different intracellular effects via STIMs and SOCE. Variations in location, duration, strength, and oscillations of the signals appear to have influence on the STIM1 and/or STIM2 responses and how intensely they affect CRAC channels and the subsequent return to Ca+2 homeostasis.
- Aytes A, Mollevi D, Martinez-Iniesta M, Nadal M, Vidal A, Morales A, Salazar R, Capella G, Villaneuva A. Stromal interaction molecule 2 (STIM2) is frequently overexpressed in colorectal tumors and confers a tumor cell growth suppressor phenotype. Mol Carcinog. 2012;51:746–53.PubMedCrossRefGoogle Scholar
- Baba Y, Hayashi K, Fujii Y, Mizushima A, Watarai H, Wakamori M, et al. Coupling of STIM1 to store-operated Ca2+ entry through its constitutive and inducible movement in the endoplasmic reticulum. Proc Natl Acad Sci U S A. 2006;103:16704–9. doi:10.1073/pnas.0608358103.PubMedPubMedCentralCrossRefGoogle Scholar
- Pozo-Guisado E, Casas-Rua V, Tomas-Martin P, Lopez-Guerrero AM, Alvarez-Barrientos A, Martin-Romero FJ. Phosphorylation of STIM1 at ERK1/2 target sites regulates interaction with the microtubule plus-end binding protein EB1. J Cell Sci. 2013;126:3170–80. doi:10.1242/jcs.125054.PubMedCrossRefGoogle Scholar
- Williams RT, Manji SSM, Parker NJ, Hancock MS, van Stekelenburg L, Eidn JP, et al. Identification and characterization of the STIM (stromal interaction molecule) gene family: coding for a novel class of transmembrane proteins. Biochem J. 2001;357:673–85. doi:10.1042/bj3570673.PubMedPubMedCentralCrossRefGoogle Scholar
- Williams RT, Senior PV, Van Stekelenburg L, Layton JE, Smith PJ, Dziadek MA. Stromal interaction molecule 1 (STIM1), a transmembrane protein with growth suppressor activity, contains an extracellular SAM domain modified by N-linked glycosylation. Biochim Biophys Acta. 2002;1596:131–7. doi:10.1016/S0167-4838(02)00211-X.PubMedCrossRefGoogle Scholar