Ageing is an irreversible process involving the gradual decline of cellular functions in all tissues. In male mice, age-related loss of muscle force is accompanied by the formation of tubular aggregates, which are honeycomb-like structures composed of membrane tubules, proteins and Ca2+ deposits. Tubular aggregates are also found in tubular aggregate myopathy (TAM) and Stormorken syndrome (STRMK), two clinically overlapping human disorders affecting skeletal muscle, bones, skin, spleen and platelets. TAM/STRMK is caused by gain-of-function mutations in the ubiquitously expressed Ca2+ sensor STIM1 and results in excessive extracellular Ca2+ entry and the dysregulation of Ca2+ homeostasis. To understand the correlation between ageing, tubular aggregate formation, Ca2+ and STIM1, we conducted comparative analyses of WT and Stim1+/- male mice until 18 months of age. We examined growth, general and specific muscle force, fatigability and muscle structure. Stim1+/- mice were born with the expected Mendelian ratio and showed unremarkable postnatal development with normal body and organ weight. However, at 18 months, Stim1+/- mice manifested delayed muscle contraction (Δ = 28%, p < 0.05) and relaxation (Δ = 40%, p < 0.01) kinetics as well as exacerbated fatigue (Δ = 28%, p < 0.05) compared with age-matched controls. Morphological investigations of Stim1+/- muscle sections by light and electron microscopy uncovered a shift towards slow myofibres and mitochondrial proliferation accompanied by enhanced SDH activity (Δ = 27%, p < 0.0001), an almost twofold increase in ROS production (p < 0.05), and signs of mitophagy-all representing histopathological hallmarks of age-related deterioration of muscle function known as sarcopenia. Strikingly, tubular aggregates-though abundant in WT muscles at 18 months-were absent in Stim1+/- mice. Taken together, STIM1 depletion by 50% had no discernible effect on muscle function in young adult male mice, but compromised muscle performance and resistance to fatigue at later life stages. These findings highlight a critical role of STIM1 and Ca2+ balance in the maintenance of muscle physiology, fibre type composition and mitochondrial bioenergetics. The absence of tubular aggregates in Stim1+/- mice indicates that tubular aggregates possibly play a protective role and may contribute to the prevention of age-related muscle alterations.

Depletion of calcium from ER stores leads to the activation of calcium channels on the plasma membrane. This process is termed store-operated calcium entry (SOCE). The proteins STIM1 and STIM2 function as ER calcium sensors, and upon store depletion, they undergo a conformational change that allows them to bind to and gate Orai calcium channels on the plasma membrane. We have shown that both Orai1 and STIM1 are dynamically S-acylated after store depletion, which is required for SOCE. These results suggest the requirement of a calcium-activated protein S-acyltransferase such as DHHC21. Here, we show that DHHC21 is essential for SOCE in vitro and in vivo. Using the depilated mouse model that expresses DHHC21 but can no longer be activated by calcium, we show that DHHC21 activation is required for STIM1 S-acylation and subsequent calcium entry. Plasma membrane-localized DHHC21 is dynamically recruited into Orai1/STIM1 puncta upon store depletion, where it physically binds to STIM1. Finally, we show that depilated mice phenocopy many aspects of autoimmune lymphoproliferative syndrome (ALPS), including defective Fas-mediated calcium release, T cell death, neutropenia, and increased serum vitamin B12 levels. These results suggest that dynamic S-acylation has underappreciated and expansive roles in second messenger signaling and immune system function. Targeting DHHC21 may be therapeutically beneficial for ALPS and diseases associated with deregulated activation of STIM1, such as tubular aggregate myopathy and Stormorken syndrome.

Tubular aggregates are granular inclusions of unclear function postulated to be massive proliferations from lateral sacs of the sarcoplasmic reticulum. However, immunohistochemical studies suggest a more complex origin. They usually accumulate in type 2 fibers and do not react to the mitochondrial oxidative enzymes. Tubular aggregates are present in diverse acquired and inherited clinical conditions. Myopathies in which the predominant pathological hallmark is tubular aggregates usually manifest in one of three major phenotypes: 1) slowly progressive, proximal predominant weakness; 2) myalgia with or without stiffness and cramps; 3) limb-girdle myasthenic syndrome. Muscle biopsy may play an indispensable role in diagnosing these conditions. Mutations in STIM1 and ORAI1 proteins cause tubular aggregate myopathies (TAM) and Stormorken syndrome (STRMK). These proteins are the main players in store-operated Ca2+ entry (SOCE) mechanism, a major regulator of Ca2+ homeostasis. Other regulators of SOCE include calsequestrin, ALG2, ALG14, DPAGT1, and GFPT1. Their dysfunction impacts Ca2+ homeostasis and causes conditions overlapping with TAM/STRMK at the clinical and histological level.

Store-operated calcium entry (SOCE) is the major mechanism for cellular calcium homeostasis that is ubiquitous across cell types and is responsible for replenishing Ca2+ in the endoplasmic reticulum. The major calcium channel that facilitates this role is Orai1. Orai1 is regulated by proteins that interact with either its N- or C-terminus. Stromal interaction molecule 1 (STIM1) is an activator of Orai1 that binds to Orai1's C-terminus, causing the channel to open and allow for Ca2+ influx. Together, Orai1 and STIM1 constitute a calcium release-activated calcium channel that is critical for SOCE. Alternatively, adenylyl cyclase type 8 (AC8) binds to Orai1's N-terminus, causing the Orai1 channel to close after phosphorylation by protein kinase A. Other proteins also interact with Orai1 to elicit modulatory effects and influence the gating properties of this channel. As SOCE is critical for cellular Ca2+ balance and calcium-sensitive cellular functions, impairment of Orai1 function by restricting its ability to form normal protein-protein interactions (PPIs) can be deleterious and lead to pathologies. It has been discovered that overexpression of Orai1 and AC8 leads to proliferation of triple negative breast cancer cells through mechanisms dependent on Ca2+ signaling. Thus, PPIs involving Orai1 can be approached as therapeutic targets in diseases that arise from aberrant Ca2+ signaling. Orai1 PPIs can serve as targets for diseases that currently lack targeted therapies, such as triple negative breast cancer. This review examines Orai1 PPIs with STIM1 and AC8, discusses the relevance of these PPIs in cancer, and reviews the landscape of Orai1 inhibitors. SIGNIFICANCE STATEMENT: The study of proteins that are involved in cancer progression is important for developing targeted cancer therapies. Store-operated calcium entry-based proteins have been proposed as therapeutic cancer targets because inhibition of these proteins disrupts Ca2+ influx, thereby decreasing cell proliferation in certain cancers. Additionally, store-operated calcium entry-based proteins are implicated in many other disease states such as Stormorken syndrome, tubular aggregate myopathy, and immunodeficiency, highlighting the therapeutic relevance of these proteins.