Activity-dependent localization and dynamics of STIM1 and STIM2 at ER-PM contacts in hippocampal neurons

Stromal interaction molecules (STIMs) are Ca²⁺ sensors within the endoplasmic reticulum (ER) plasma membrane (PM) that contribute to homeostatic functions in neurons. Upon depletion of Ca²⁺ from the ER, STIMs translocate to ER-PM junctions to contact the inner leaflet of the PM. Using single-particle tracking, we characterize the dynamic properties of endogenous STIM1 and STIM2 proteins in hippocampal neurons. STIMs form clusters in the somato-dendritic compartment but only transiently visit synapses. A substantial fraction of STIM2 proteins define ER-PM contacts under resting conditions and is dependent on the constitutive activity of NMDARs. STIM1 proteins are transiently recruited to ER-PM junctions only during strong activation of NMDARs. Activity-dependent confinement of STIM proteins is not influenced by Ca_V1.2 channel activity. We propose that STIM proteins fulfill a dominant structural function in neurons by regulating the size and frequency of ER-PM contacts to promote ER-PM communication along dendrites.