SIGMA-1R: A KEY MODULATOR OF CA<SUP>2+</SUP> HOMEOSTASIS AND INTER-ORGANELLE COMMUNICATION IN PRECLINICAL MODELS OF AMYOTROPHIC LATERAL SCLEROSIS

Sigma-1R is a molecular chaperone enriched at the mitochondria-associated membranes (MAMs), where it regulates endoplasmic reticulum (ER) Ca²⁺ channels and is involved in the functional crosstalk between intracellular organelles. Pharmacological activation of Sigma-1R by small molecules such as Pridopidine has shown neuroprotective potential and is currently under clinical evaluation for Amyotrophic Lateral Sclerosis (ALS). With the aim to investigate the role of Sigma-1R in organelle crosstalk, we used two different preclinical ALS models: NSC-34 motor neuron–like cells exposed to L-BMAA (beta-methylamino-L-alanine), a widely used ALS/PDC model, and SOD1^G93A transgenic mice at different stages of the disease. Immunofluorescence analysis in control NSC-34 cells revealed that Sigma-1R, expressed on ER, co-localized with lysosomes, indicating a potential role in ER–lysosome communication. Moreover, we detected a significant reduction of Sigma-1R expression in the spinal cord, brainstem and cortex of symptomatic SOD1^G93A mice, three regions critically affected in ALS, as well as in motor neurons exposed to the neurotoxin. In parallel, the expression of the lysosomal Ca²⁺ channel two-pore channel 2 (TPC2) was also downregulated in the same experimental models. These reductions were associated with enhanced ER stress responses and impaired mitochondrial function, suggestive of altered Ca²⁺ homeostasis. Notably, pharmacological activation of Sigma-1R with Pridopidine partially restored cellular homeostasis in vitro. Collectively, these findings indicated alterations of Sigma-1R, ER and lysosomal Ca²⁺ signaling as common features of ALS preclinical models, supporting a crucial role for disrupted inter-organelle Ca²⁺ communication in ALS pathogenesis and highlighting Sigma-1R as a promising therapeutic target.