Sigma receptor (σR)

Sigma receptors are a unique class of non-opioid, non-dopaminergic intracellular proteins located primarily in the endoplasmic reticulum (ER) and its mitochondria-associated membranes (MAM). They are classified into two distinct subtypes: the Sigma-1 receptor, which functions as a molecular chaperone to ensure proper protein folding and calcium signaling, and the Sigma-2 receptor, recently identified as the transmembrane protein 197 (TMEM97), which is involved in cholesterol homeostasis and cell proliferation. These receptors play a critical role in cellular homeostasis, particularly in response to ER stress and oxidative stress, making them vital for neuronal survival and plasticity. In clinical contexts, Sigma-1 agonists are being investigated for neurodegenerative conditions like Alzheimer's and ALS due to their neuroprotective properties, while Sigma-2 ligands are primarily explored as biomarkers and therapeutic targets in oncology for their ability to modulate tumor cell viability. Their wide distribution across the central nervous system and peripheral tissues allows them to influence a variety of physiological processes, including pain perception, mood regulation, and immune response.

Other names

Sigma non-opioid intracellular receptorSigma-1 receptorSigma-2 receptorσ1Rσ2RTMEM97 (Sigma-2)

Sigma-1 receptors act as ligand-operated molecular chaperones that stabilize proteins at the mitochondria-associated ER membrane (MAM), regulating calcium signaling and ER stress responses. Sigma-2 receptors (TMEM97) modulate cholesterol trafficking and cell proliferation, with agonists often inducing apoptosis in tumor cells.