Remodelling of smooth endoplasmic reticulum of Müller cells in aged human retina

The smooth endoplasmic reticulum (SER) is a system of interconnected cytoplasmic tubules with various subdomains, which can remodel in response to certain stimuli, as in the retinal pigment epithelium. Earlier studies reported Müller cells (MC) to contain the SER tubules as the main subdomain. A routine examination of postmortem human retinas (54-94 years; N = 8) revealed diverse SER subdomains in aged MC. Besides the presence of tubules with a distinct lumen, MC possessed tubules that were rather compressed, with partial to fully obliterated lumen. The other SER subdomains detected were lamellar and whorl that assembled near the endfeet plasma membrane. The compressed SER were quite longer, often curved and detected in MC endfeet and outer processes in Henle's fiber layer of aged macula (>80 years). No such features were detected in any cells of the peripheral retina nor in young donor retinas examined. Because lipid peroxidation was prominent in macular MC, it may be involved in the remodelling of SER shape. While the organized lamellar and whorl subdomains can be related to certain physiological demands, the peculiar SER subdomains with obliterated lumen could limit the normal functions of SER, e.g., calcium storage and carbohydrate metabolism.