Discovery of new intracellular junctions: The calcium entry units (CEUs): Calcium Signaling and Excitation–Contraction in Cardiac, Skeletal and Smooth Muscle

In 2017, Boncompagni, Michelucci et al. demonstrated that during exercise the sarcotubular system of extensor digitorum longus (EDL) fibers undergoes a profound remodeling that leads to the assembly of new junctions between T-tubule extensions at the I band and sarcoplasmic reticulum (SR) stacks. As these junctions contain colocalized STIM1 and Orai1 and enhance store-operated Ca²⁺ entry (SOCE), they have been named Ca²⁺ entry units (CEUs). In addition, it has been more recently shown that (1) CEUs disassemble following recovery, with T-tubules retraction from the I band faster than SR stacks disassembly, and (2) lack of calsequestrin-1 (CASQ1) induces a constitutive assembly of CEUs, resulting in enhanced SOCE that counteracts the SR Ca²⁺ depletion. We have now analyzed (1) CEUs during postnatal maturation (at 2 wk of age) and (2) whether CEUs form in slow-twitch fibers (soleus). (a) Compared with adult (4 mo) EDL fibers of resting WT mice, at 2 wk of age we found a greater longitudinal disposition of T-tubules associated to SR membranes forming junctions virtually identical to CEUs in adult EDLs of exercised WT mice, which promote increased STIM1/Orai1-mediated SOCE. (b) We also compared structure and function of soleus (which also express the cardiac isoform CASQ2) from WT mice and from mice lacking either CASQ1 (CASQ1-null) or CASQ1/2 (dCASQ-null). In soleus from both genotypes, CEUs are constitutively assembled although they appear structurally smaller than those described previously in exercised WT or CASQ1-null EDLs. A detailed EM quantitative analysis revealed that CEUs were more abundant in dCASQ-null than CASQ1-null mice. The amount of CEUs strictly correlated with the ability of soleus fibers to recover extracellular Ca²⁺ via SOCE to support contractility during high-frequency stimulation. These data were supported by molecular analysis of Western blots, showing that Orai1 expression was enhanced following ablation of CASQ.