STIM1/ORAI1 Loss-of-Function and Gain-of-Function Mutations Inversely Impact on SOCE and Calcium Homeostasis and Cause Multi-Systemic Mirror Diseases

Store-operated Ca2+ entry (SOCE) is a ubiquitous and essential mechanism regulating Ca2+ homeostasis in all tissues, and controls a wide range of cellular functions including keratinocyte differentiation, osteoblastogenesis and osteoclastogenesis, T cell proliferation, platelet activation, and muscle contraction. The main SOCE actors are STIM1 and ORAI1. Depletion of the reticular Ca2+ stores induces oligomerization of the luminal Ca2+ sensor STIM1, and the oligomers activate the plasma membrane Ca2+ channel ORAI1 to trigger extracellular Ca2+ entry. Mutations in STIM1 and ORAI1 result in abnormal SOCE and lead to multi-systemic disorders. Recessive loss-of-function mutations are associated with CRAC (Ca2+ release-activated Ca2+) channelopathy, involving immunodeficiency and autoimmunity, muscular hypotonia, ectodermal dysplasia, and mydriasis. In contrast, dominant STIM1 and ORAI1 gain-of-function mutations give rise to tubular aggregate myopathy and Stormorken syndrome (TAM/STRMK), forming a clinical spectrum encompassing muscle weakness, thrombocytopenia, ichthyosis, hyposplenism, short stature, and miosis. Functional studies on patient-derived cells revealed that CRAC channelopathy mutations impair SOCE and extracellular Ca2+ influx, while TAM/STRMK mutations induce excessive Ca2+ entry through SOCE over-activation. In accordance with the opposite pathomechanisms underlying both disorders, CRAC channelopathy and TAM/STRMK patients show mirror phenotypes at the clinical and molecular levels, and the respective animal models recapitulate the skin, bones, immune system, platelet, and muscle anomalies. Here we review and compare the clinical presentations of CRAC channelopathy and TAM/STRMK patients and the histological and molecular findings obtained on human samples and murine models to highlight the mirror phenotypes in different tissues, and to point out potentially undiagnosed anomalies in patients, which may be relevant for disease management and prospective therapeutic approaches.


INTRODUCTION
Calcium (Ca 2+ ) is an elemental factor regulating a multitude of metabolic processes, signaling pathways, and cellular functions in all tissues, and mediates muscle contraction, nerve conduction, hormone release, and blood coagulation. Consistently, normal tissue and organ physiology strictly depends on the precise control of Ca 2+ entry, storage, and release, while abnormal Ca 2+ homeostasis induces various rare and common disorders affecting skeletal muscle, heart, bones, brain, skin, or the immune and hormonal systems (Peacock, 2010;Gattineni, 2014).
The present review aims to provide an update on the current knowledge of CRAC channelopathy and TAM/STRMK, to highlight the molecular and/or clinical mirror phenotypes caused by either LoF or GoF mutations in the SOCE key players, and to provide an overview of the available animal models recapitulating the human disorders. We thoroughly and stepwise compare the eye, skin, bone, enamel, spleen, immune, platelet, and muscle phenotypes in human and mouse, and we detail the inverse mutational effects and pathomechanisms underlying CRAC channelopathy and TAM/STRMK, and their impact on the sequence of events leading to the diverging clinical manifestations and mirror-image anomalies in most affected tissues. We also point to clinical signs that are potentially underdiagnosed in patients, and may be relevant FIGURE 1 | Schematic representation of STIM1, ORAI1, and CASQ1 with position of the CRAC channelopathy and TAM/STRMK mutations. STIM1 is composed of a luminal part with a reticular signal sequence (S), Ca 2+ -binding EF-hands and a SAM domain, a transmembrane domain (TM), and a cytosolic part with coiled-coil (CC) domains 1-3, an inhibitory domain (ID), a proline/serine-rich region (PS), a TRIP domain (T), and a lysine-rich region (K). ORAI1 contains arginine (R), proline (P), and lysine (K)-rich regions, four transmembrane domains (M1-M4), and a coiled-coil domain (CC), and calsequestrin (CASQ1) contains a reticular signal sequence (S), three thioredoxin domains (TR1-3), and an N-terminal stretch of Ca 2+ -binding aspartic acid residues (DDDDD). CRAC channelopathy mutations are depicted in blue and TAM/STRMK mutations in red. Note that an additional STIM1 splice site mutation (c.970-1G>A) generates unstable transcripts and causes CRAC channelopathy.
for diagnosis and disease management, and disclose treatment options. A schematic illustration opposing the clinical pictures of CRAC channelopathy versus TAM/STRMK is shown in Figure 2, and is supported by a detailed description in Table 1.

PHENOTYPIC TRAITS IN CRAC CHANNELOPATHY AND TAM/STRMK PATIENTS AND MICE
CRAC channelopathy and TAM/STRMK are multi-systemic disorders, and patients with either disease can manifest impairments of pupillary function, eye movement, skin, enamel, bones, immune system, spleen, coagulation, and skeletal muscle.
The following chapter provides a comparative overview of the clinical anomalies and the molecular and mechanistic causes, and also refers to the phenotypic traits of diverse CRAC channelopathy and TAM/STRMK mouse models.

Pupillary Dysfunction and Eye Movement Limitations
Vision is primarily a photochemical process, and can be adapted to the lighting conditions through iris constriction/dilatation and eye movement, both governed by Ca 2+ -dependent muscle contraction. Ca 2+ release from the reticulum activates the contractile apparatus, which generates force, causing the shortening of the muscle cells (Ebashi, 1974). The iris acts as a diaphragm controlling the amount of light entering the eye through the pupil, and SOCE substantially sustains the muscle tonus for the steady contraction of the smooth sphincter and dilator muscles for an appropriate view in brightness and obscurity Feldman et al., 2017).

Skin Anomalies and Enamel Defects
Skin forms the first defense barrier to protect from external agents, and also plays a pivotal role in thermoregulation by sweat production. Keratinocytes are the principal components of the outermost skin layer, the epidermis, and their growth, differentiation, and migration is driven by SOCE in both humans and mice (Numaga-Tomita and Putney, 2013; Vandenberghe et al., 2013). SOCE also triggers the opening of the Ca 2+activated chloride channel TMEM16A in the sweat glands, and thereby enables chloride secretion and sweat production (Concepcion et al., 2016). In the absence of SOCE, CRAC channelopathy patients present with thermoregulatory instability and anhidrosis accompanied by heat intolerance, dry skin, and eczema (Feske et al., 2006;Fuchs et al., 2012;Schaballie et al., 2015;Lian et al., 2018). Skin anomalies including eczema and ichthyosis are also commonly seen in TAM/STRMK patients (Misceo et al., 2014;Morin et al., 2014Morin et al., , 2020Bohm et al., 2017;Harris et al., 2017;Claeys et al., 2020), and one patient additionally manifested anhidrosis (Ishitsuka et al., 2019). Histological examinations of the skin biopsy revealed an obstruction of the spiral duct in the eccrine gland, the acrosyringia, resulting in sweat retention. This is different from CRAC channelopathy patients, where the sweat glands display a reduced lumen due the lack of sweat production (Lian et al., 2018). Noteworthy, the ectodermal barrier protein proflaggrin was found to be aggregated in the acrosyringia of the TAM/STRMK patient. Loss of proflaggrin is a major predisposing factor of idiopathic ichthyosis (Palmer et al., 2006), indicating that the skin phenotype in TAM/STRMK patients may be a direct consequence of the abnormal proflaggrin accumulation in the sweat glands.

Reduced platelet activation, reduced thrombus formation
Thrombocytopenia, Platelet pre-activation, aberrant size and morphology, reduced platelet-platelet adhesion Alike skin and sweat glands, teeth derive from the ectoderm, and CRAC channelopathy patients also manifest dental maturation defects including major enamel loss, discoloration and poor mineralization of both deciduous and permanent teeth (Feske et al., 2006;McCarl et al., 2009;Picard et al., 2009;Fuchs et al., 2012;Wang et al., 2014;Schaballie et al., 2015;Lian et al., 2018), highlighting the importance of SOCE in ameloblast formation and mineralization (Wang et al., 2014). In contrast, amelogenesis imperfecta is not a typical feature of TAM/STRMK, and enamel hypocalcification was only noted in a single patient (Noury et al., 2017).
CRAC channelopathy and TAM/STRMK animal models partially recapitulate the human enamel and skin phenotypes. Mice lacking ORAI1 manifest reduced enamel mineralization (Robinson et al., 2012) and thinner skin with elongated keratinocytes and smaller vibrissae follicles (Gwack et al., 2008), and the ectodermal-specific knockout of Orai1 or Stim1/Stim2 impairs SOCE and results in anhidrosis and a reduced sweat gland lumen (Concepcion et al., 2016). The TAM/STRMK mouse harboring the STIM1 R304W mutation shows a thickened dermis and a reduction of the subcutaneous fat layer (Silva-Rojas et al., 2019), and a subset of the animals additionally exhibit subgingival hair growth on the lower incisors (Gamage et al., 2020).

Bone Anomalies
Bones represent 15% of the total body weight and are essential for motion, mineral storage, and hematopoiesis. Bone deposition and resorption are dynamic and balanced processes driven by bone-forming osteoblasts and boneresorbing osteoclasts (Florencio-Silva et al., 2015), and their growth and differentiation is regulated by SOCE-dependent Ca 2+ homeostasis (Eapen et al., 2010;Blair et al., 2011;Chen et al., 2018). Bone resorption by osteoclasts generates a local increment of extracellular Ca 2+ , inducing the activation of the calcineurin/NFAT signaling pathway, and resulting in osteoblastogenesis (Zayzafoon, 2006). Calcineurin/NFAT signaling is also essential for osteoclastogenesis and T cell activation, and the inhibition of this pathway with cyclosporine A to prevent transplant rejection is associated with an increased incidence of bone fractures (Zayzafoon, 2006).
Overt bone anomalies are largely absent in CRAC channelopathy and TAM/STRMK patients with exception of individual cases with facial dysmorphism , fusion of the cervical vertebrae (Klippel-Feil anomaly) (Morin et al., 2020), brachydactyly (Morin et al., 2020), or syndactyly of the second and third toes (Borsani et al., 2018). Bone mineralization was found to be increased in two CRAC channelopathy patients (osteopetrosis) and decreased in a single TAM/STRMK patient (osteopenia), and accordingly, functional studies demonstrated a decreased osteoclastogenesis in bone marrow mononuclear macrophages from the CRAC channelopathy patients, and an increased osteoclastogenesis in cells derived from the TAM/STRMK patient (Huang et al., 2020). Of note, a number of TAM/STRMK patients exhibit a short stature (Misceo et al., 2014;Morin et al., 2014Morin et al., , 2020Noury et al., 2017;Borsani et al., 2018), and other more subtle or late-onset bone disorders might have been overlooked in CRAC channelopathy and TAM/STRMK patients. This is supported by the impaired differentiation and function of osteoblasts and osteoclasts leading to osteopenia with decreased bone density and trabecular bone volume in ORAI1-deficient mice (Hwang et al., 2012;Robinson et al., 2012). Alike patients, TAM/STRMK mice are smaller than their littermates, and micro-CT analyses revealed a decreased cellular density and a reduced bone marrow area in femur and tibia, potentially affecting bone strength and stiffness (Silva-Rojas et al., 2019). Surviving mice carrying the STIM1 R304W mutation at the homozygous state show a more severe skeletal phenotype with thinner and more compact bones, and also feature a reduced number of ribs (Gamage et al., 2020).

Immune System and Spleen Anomalies
The immune system is an essential and complex defense network, and SOCE directs the fate and function of diverse cells of the innate and adaptive immune system, including dendritic cell maturation (Felix et al., 2013), neutrophil activation (Zhang et al., 2014), lymphocyte cytotoxicity and cytokine production (Maul-Pavicic et al., 2011), as well as T cell proliferation, differentiation, and metabolism (Vaeth et al., 2017;Vaeth and Feske, 2018). T cells play a pivotal role in the adaptive immune system and act as effector, memory, suppressor, or helper cells in response to external agents. The antigen recognition by the T cell receptors activates a signaling cascade resulting in the continuous depletion of the reticular Ca 2+ stores and a durable extracellular Ca 2+ entry via SOCE to initiate the Ca 2+ -dependent transcriptional program necessary for T cell function (Feske, 2007).
The spleen is the largest lymphoid organ and functions as a blood filter, and ensures the biogenesis and storage of white and red blood cells, as well as the phagocytosis of circulating microorganisms (de Porto et al., 2010). CRAC channelopathy patients develop hepatosplenomegaly Byun et al., 2010;Schaballie et al., 2015;Lian et al., 2018), while asplenia or hyposplenia is a clinical hallmark of TAM/STRMK (Morin et al., 2020). As an indication of abnormal spleen function, Howell-Jolly bodies have moreover been found on peripheral blood film in several affected individuals (Misceo et al., 2014;Morin et al., 2014;Markello et al., 2015;Harris et al., 2017;Noury et al., 2017), but an increased rate of infections has nevertheless not been reported. Contrasting the patients, the TAM/STRMK mouse models present with splenomegaly, and histological investigations of the spleen revealed megakaryocyte hypoplasia (Grosse et al., 2007;Silva-Rojas et al., 2019). This is possibly related to a physiological difference between both species, as hematopoiesis lowers with age in humans, while it is maintained throughout life in mice (Bronte and Pittet, 2013). Of note, hematological analyses disclosed abnormal B, NK, and Treg counts in the STIM1 R304W mouse (Silva-Rojas et al., 2019), indicating that disturbances of the immune system may also occur in TAM/STRMK patients and potentially contribute to the spleen, platelet, and skin anomalies. This is sustained by the detection of lymphoproliferation and circulating antibodies against platelets in a single patient with STIM1 R304W mutation (Sura et al., 2020).

Coagulation Defects
Hemostasis prevents and stops bleeding through the formation of a thrombus, which is ultimately resolved in the process of wound healing. Platelets play an essential role in thrombus formation, and the activation of platelets is induced by the presence of the subcortical component collagen in the blood flow following vessel wall damage (Bye et al., 2016). The collagen fragments bind to glycoprotein VI (GPVI) at the surface of the platelets and trigger a signaling cascade involving SOCE and leading to the Ca 2+ -dependent exposure of phosphatidylserine (PS) and the secretion of alpha granules containing thrombotic factors (Berna-Erro et al., 2016; van der Meijden and Heemskerk, 2019), which will then prompt the coagulation process and modulate inflammation and angiogenesis in the injured area (Blair and Flaumenhaft, 2009).
As a result of SOCE deficiency, PS exposure and alpha granule secretion is reduced in platelets from CRAC channelopathy patients, impeding platelet aggregation and thrombus formation (Nakamura et al., 2013). In consequence of the overall reduction of Treg cells, high titers of anti-platelet autoantibodies are detectable in the serum of the patients, lead to hemolytic anemia, and contribute to mild or intermittent susceptibility to bleed in several affected individuals Byun et al., 2010;Fuchs et al., 2012;Lian et al., 2018). Bleeding diathesis associated with thrombocytopenia is a major clinical feature of TAM/STRMK (Misceo et al., 2014;Morin et al., 2014Morin et al., , 2020Nesin et al., 2014;Markello et al., 2015;Bohm et al., 2017;Harris et al., 2017;Noury et al., 2017;Alonso-Jimenez et al., 2018;Borsani et al., 2018;Li et al., 2019;Claeys et al., 2020;Sura et al., 2020), and the analysis of blood samples from patients revealed increased platelet activation markers and enhanced secretion of alpha granules in unstimulated platelets (Misceo et al., 2014). Despite this pre-activation state caused by elevated resting Ca 2+ , the platelet-platelet adhesion is impaired, and platelets often appeared with aberrant size and morphology (Markello et al., 2015), suggesting that the coagulation defect in TAM/STRMK patients results from a combination of platelet loss and platelet dysfunction.
In analogy to CRAC channelopathy patients, PS exposure and secretion of alpha granules is diminished in mice with plateletspecific deletion of Stim1 and in chimeric Orai1 R93W /R93W animals (Bergmeier et al., 2009;Ahmad et al., 2011). Chimeric Stim1 −/− and Orai1 −/− mice additionally show impaired platelet aggregation and thrombus formation, leading to a slight increase in bleeding time (Varga-Szabo et al., 2008;Braun et al., 2009;Gilio et al., 2010). The murine TAM/STRMK models similarly recapitulate the coagulation defects seen in the patients, as thrombocytopenia is evident in all three STIM1 D84G, I115F, and R304W models (Grosse et al., 2007;Cordero-Sanchez et al., 2019;Silva-Rojas et al., 2019). Further analyses on the STIM1 D84G mice uncovered that the pre-activation state of the platelets increases platelet clearance, and thereby prevents efficient platelet aggregation (Grosse et al., 2007). If and to what extent the bleeding diathesis is exacerbated by the immune system defects in TAM/STRMK mice and potentially in patients remains to be determined.

Muscle Weakness
Skeletal muscles maintain posture and allow movements under the voluntary control of the somatic nervous system, and also regulate body temperature and nutrition storage. SOCE activation and extracellular Ca 2+ entry is significantly faster in myofibers compared with other cell types, occurring within milliseconds after each action potential (Launikonis et al., 2009;Edwards et al., 2010). This is believed to be related to the presence of the muscle-specific STIM1L isoform forming pre-activated Ca 2+ entry units with ORAI1 at the SR/plasma membrane junction (Darbellay et al., 2011). Refilling of the Ca 2+ stores is mediated by the ATP-dependent SERCA pumps to maintain high Ca 2+ gradients across the SR membrane, thus limiting the SR depletion of Ca 2+ during repetitive tetanic stimulations (Pan et al., 2002;Zhao et al., 2005).
CRAC channelopathy patients manifest neonatal hypotonia and generalized muscle weakness, and show delayed motor milestones and reduced walking distance in infancy, with additional respiratory insufficiency in individual cases (Feske et al., 2006;McCarl et al., 2009;Picard et al., 2009;Fuchs et al., 2012;Chou et al., 2015;Schaballie et al., 2015;Badran et al., 2016;Lian et al., 2018). Histological investigations were performed on muscle biopsies from two patients, and revealed fiber type I fiber predominance and type II atrophy Lian et al., 2018). Muscle weakness and exercise intolerance are primary clinical features of TAM/STRMK, and the onset and severity depend on the causative gene and correlate with the position of the mutation (Morin et al., 2020). In most cases, disease onset is during infancy or childhood, and first and foremost affects the proximal muscles of the lower limbs. Muscle weakness is generally accompanied by elevated serum creatine kinase (CK) levels, indicating moderate fiber degeneration, and myalgia and cramps are often observed as secondary features, but can also occur as isolated signs (Bohm et al., 2013;Bohm et al., 2014Bohm et al., , 2017Hedberg et al., 2014;Misceo et al., 2014;Morin et al., 2014Morin et al., , 2020Nesin et al., 2014;Endo et al., 2015;Markello et al., 2015;Walter et al., 2015;Barone et al., 2017;Garibaldi et al., 2017;Harris et al., 2017;Noury et al., 2017;Alonso-Jimenez et al., 2018;Borsani et al., 2018;Li et al., 2019;Claeys et al., 2020). Noteworthy, Ca 2+ overload in skeletal muscle fibers has been shown to disrupt excitation-contraction coupling (ECC) (Lamb et al., 1995), which possibly contributes to the reduced muscle force in TAM/STRMK patients. Muscle sections from affected individuals typically show tubular aggregates appearing in red on Gomori trichrome staining, and adopting a honeycomb structure of densely packed tubules on electron microscopy (Chevessier et al., 2004(Chevessier et al., , 2005Bohm and Laporte, 2018). The tubular aggregates are highly Ca 2+ -rich, and immunofluorescence studies have shown that they essentially contain SR proteins such as STIM1, calsequestrin, triadin, or RyR1, indicating that they are of reticular origin (Chevessier et al., 2004(Chevessier et al., , 2005Bohm et al., 2013Bohm et al., , 2017Endo et al., 2015). It has been suggested that the abundance of Ca 2+ in muscle fibers may cause SR protein misfolding and aggregation, leading to the formation of membrane stacks as precursors of tubular aggregates (Morin et al., 2020). Alternatively, the occurrence of tubular aggregates may reflect the attempt to regenerate a functional triad, a specialized membrane complex in skeletal muscle hosting the ECC machinery. This is supported by the observation that Ca 2+ excess induces the proteolysis of junctophilins, which tether the SR membrane to deep plasma membrane invaginations known as T-tubules to form the triad (Murphy et al., 2013). Further histopathological signs on TAM/STRMK biopsies encompass fiber size variability, type I fiber predominance, type II fiber atrophy, internalized nuclei, vacuoles, and fibrosis (Bohm et al., 2013(Bohm et al., , 2014Hedberg et al., 2014;Morin et al., 2014Morin et al., , 2020Nesin et al., 2014;Endo et al., 2015;Markello et al., 2015;Walter et al., 2015;Harris et al., 2017;Noury et al., 2017;Borsani et al., 2018;Li et al., 2019;Claeys et al., 2020).
Stim1 KO and Orai1 KO Mice die perinatally (Baba et al., 2008;Gwack et al., 2008;Oh-Hora et al., 2008), and the muscle-specific invalidation of either gene results in diminished cellular Ca 2+ transients following stimulation, and interferes with muscle contractility and the production of force (Stiber et al., 2008;Li et al., 2012;Wei-Lapierre et al., 2013;Carrell et al., 2016). The mice also show an increased susceptibility to fatigue (Stiber et al., 2008;Wei-Lapierre et al., 2013;Carrell et al., 2016), and histological and ultrastructural investigations of muscle samples uncovered a reduction in fiber size and overall muscle mass, and swollen mitochondria (Stiber et al., 2008;Li et al., 2012;Wei-Lapierre et al., 2013;Carrell et al., 2016). The STIM1 I115F and R304W TAM/STRMK mouse models exhibit reduced muscle force (Cordero-Sanchez et al., 2019;Silva-Rojas et al., 2019), and continuous muscle stimulation evidenced a slower force decay compared with WT littermates, presumably reflecting an increased resistance to fatigue (Silva-Rojas et al., 2019). The animals exhibit elevated serum CK levels, and histological examinations of muscle samples revealed an increased proportion of type I fibers, an overall reduction of fiber diameter with signs of muscle fiber degeneration and regeneration, and electron microscopy uncovered swollen mitochondria (Cordero-Sanchez et al., 2019;Silva-Rojas et al., 2019). Most strikingly, tubular aggregates are absent from muscles in both murine TAM/STRMK models, highlighting a major structural difference between human and mouse muscle pathologies despite the concordance of the overall clinical picture. Considering the observation that dystrophic signs are more prominent in TAM/STRMK mice than in patients, the tubular aggregates may protect the human muscle fibers from degeneration by bundling excessive free Ca 2+ . Another STIM1 R304W mouse model does not show functional or structural skeletal muscle aberrations (Gamage et al., 2018), and a potential muscle phenotype of the STIM1 D84G mouse was not assessed (Grosse et al., 2007).

SOCE REGULATORS, ASSOCIATED DISEASES AND ANIMAL MODELS
Ca 2+ controls a multitude of metabolic processes, signaling pathways, and cellular functions including transcription, proliferation, differentiation, and exocytosis. As a major regulator of Ca 2+ homeostasis, SOCE takes a central role in the physiology of all tissues and organs, and needs to be adaptable to the Ca 2+ sensitivity and Ca 2+ balance of the individual cell types forming an organism.
LoF mutations in ITPR1, encoding IP 3 R type 1, cause Gillepsie syndrome (GLSP), characterized by muscular hypotonia, mydriasis, ataxia, and intellectual disability (Gerber et al., 2016), and Itpr1-null mice manifest severe ataxia and epileptic seizures (Matsumoto et al., 1996). Mutations in ITPR2 and ITRP3, respectively, encoding IP 3 R types 2 and 3, are associated with anhidrosis in patients (Klar et al., 2014;Kerkhofs et al., 2018), and the same phenotype is also observed in Itpr2 and Itpr3 double knockout mice (Futatsugi et al., 2005). This is in accordance with the idea that the reduction of SOCE through the loss of the positive effector IP 3 R results in a clinical phenotype resembling CRAC channelopathy. In the same line, STING GoF mutations are found in patients with systemic inflammatory syndrome and autoimmunity (Jeremiah et al., 2014), and a mouse model carrying a patient mutation recapitulates the clinical signs (Bouis et al., 2019). STING is a signaling adaptor residing in the ER, and is retained in an inactive state through direct interaction with STIM1. In response to DNA pathogens, STING translocates to the ER-Golgi intermediate compartment to trigger an interferon response through the STING-TBK1-IRF3 pathway Barber, 2008, 2011). Loss of STIM1 in mouse and human CRAC channelopathy cell lines induces a spontaneous activation of STING and an enhanced expression of type 1 interferons under sterile conditions, thereby stimulating the immune system even in the absence of pathogens (Srikanth et al., 2019).
Calsequestrin (CASQ1) is the major Ca 2+ buffering protein in the sarcoplasmic reticulum in skeletal muscle, and polymerizes with increasing luminal Ca 2+ concentrations (Manno et al., 2017). In turn, Ca 2+ store depletion promotes depolymerization, and the calsequestrin monomers sequester STIM1 and hence negatively regulate SOCE (Wang et al., 2015). Specific missense mutations in CASQ1 interfere with the polymerization and depolymerization dynamics of calsequestrin, lower the Ca 2+ buffer capacities of the reticulum and impair calsequestrin monomerization, leading to an increase in SOCE activity (Barone et al., 2017;. As calsequestrin expression is restricted to skeletal muscle, patients with CASQ1 mutations show a mild form of TAM/STRMK with late-onset muscle weakness, myalgia, and abundant tubular aggregates, but without additional multi-systemic signs (Bohm and Laporte, 2018;. A murine model harboring a CASQ1 mutation found in patients does not exist, and the total loss of calsequestrin generates a malignant hyperthermia phenotype with an increased risk of sudden death in mice (Protasi et al., 2009). Noteworthy, CASQ1 null mice show an increased expression of STIM1 and ORAI1 associated with enhanced SOCE activity, possibly reflecting a compensatory mechanism to ensure the maintenance of contractile force despite the reduction of bound and releasable Ca 2+ in the SR (Michelucci et al., 2020). Tubular aggregates containing STIM1 and calsequestrin are also seen on muscle biopsies from patients with limb-girdle congenital myasthenic syndrome (LG-CMS), marked by fluctuating muscle weakness and fatigability (Evangelista et al., 2015). LG-CMS is caused by the impaired transmission at the neuromuscular junction, the relay between motor neuron and muscle fiber, and results from LoF mutations in ALG2, ALG14, DPAGT1, GFPT1, or GMPP8 (Belaya et al., 2012(Belaya et al., , 2015Guergueltcheva et al., 2012;Cossins et al., 2013). All five genes code for proteins of the glycosylation pathway and procure posttranslational modifications to a wide variety of proteins including STIM1 and ORAI1. Hypoglycosylation of STIM1 and ORAI1 stimulates SOCE and extracellular Ca 2+ influx (Selcen et al., 2014), and the muscle-specific deletion of Gfpt1 in mice causes myasthenia and the occurrence of tubular aggregates in muscle fibers (Issop et al., 2018). These examples show that the dysfunction of proteins directly or indirectly associated with STIM1 and ORAI1 can cause human pathologies overlapping with TAM/STRMK at the clinical and histological level.

CONCLUSION AND PERSPECTIVES
LoF mutations in STIM1 and ORAI1 impair SOCE and cause CRAC channelopathy, while GoF mutations in both genes involve SOCE over-activation and result in TAM/STRMK (Lacruz and Feske, 2015;Bohm and Laporte, 2018). In agreement with the opposite mutational effects and pathomechanisms leading to either CRAC channelopathy or TAM/STRMK, both disorders by and large show clinical mirror phenotypes affecting the eyes, bones, immune system, platelets, and skeletal muscle. While CRAC channelopathy is characterized by mydriasis, increased bone mineralization, immunodeficiency, splenomegaly, impaired platelet activation, and muscle hypotonia, TAM/STRMK patients typically present with miosis, decreased bone mineralization, hyposplenism, platelet pre-activation, and muscle cramping. A single TAM/STRMK patient was additionally diagnosed with lymphoproliferation (Sura et al., 2020), indicating an over-active immune system. Investigations on TAM/STRMK mouse models confirmed a dysregulation of various immune system cells, which may account for the skin phenotype in humans and mice (Silva-Rojas et al., 2019). It is interesting to note that the clinical anomalies of platelets and skeletal muscle are similar in CRAC channelopathy and TAM/STRMK patients despite the inverse pathogenic effect of STIM1 and ORAI1 LoF and GoF mutations at the molecular level, highlighting the importance of strict SOCE regulation for normal tissue physiology. Thrombus formation is impaired in both disorders and enhances the tendency to bleed following injury. This is due to the reduced activation of platelets in CRAC channelopathy (Nakamura et al., 2013), and results from the impaired adhesion between platelets in TAM/STRMK (Markello et al., 2015). Similarly, muscle weakness either arises from the incapacity to sustain a sufficient muscle tonus in the absence of Ca 2+ store refill in CRAC channelopathy, or from cytosolic Ca 2+ excess disrupting excitation-contraction coupling and/or restraining proper muscle relaxation in TAM/STRMK. CRAC channelopathy and TAM/STRMK mouse models recapitulate the main clinical signs of the human disorders, and are valuable and powerful tools to understand the importance of Ca 2+ balance and the impact of Ca 2+ imbalance on eye, bones, enamel, skin, platelets, spleen, immune system, and skeletal muscle physiology. Patients are usually examined by specialized physicians with a major focus on the principal handicap, and additional phenotypic anomalies might be overlooked, especially in the context of multi-systemic disorders with mild to moderate involvement of specific tissues. In contrast, murine models generally undergo unbiased phenotyping and offer the possibility for a detailed analysis of all organs to provide an overview of the disease. As an example, the complete characterization of the STIM1 R304W TAM/STRMK mouse model unveiled anomalies of the glucose metabolism, hepatic function, and the immune system (Silva-Rojas et al., 2019), which have not been described in patients yet, but might be of medical importance. Conversely, psychiatric diseases including confusion (Misceo et al., 2014;Harris et al., 2017), Capgras syndrome (Harris et al., 2017), and manic psychosis (Harris et al., 2017) have only been reported in individual TAM/STRMK cases, and thorough investigations on the mouse model might help to determine if these anomalies are disease-related or unrelated. Lastly, the mouse models faithfully recapitulating CRAC channelopathy and TAM/STRMK can serve for the assessment of therapeutic approaches, which may also be relevant for other Ca 2+ -related disorders affecting the bones, platelets, spleen, immune system, or skeletal muscle.

AUTHOR CONTRIBUTIONS
RS-R and JB wrote the manuscript. All authors contributed to the article and approved the submitted version.