CD38 expression and NAD⁺-induced intracellular Ca²⁺ mobilization in isolated retinal Müller cells

Müller cells of the vertebrate retina are prominent radial glia that provide essential support to sustain homeostasis of the tissue, including redistribution of external potassium, uptake and metabolism of neurotransmitters, and secretion of factors that stabilize the retina. Meeting this diversity of functional supports requires that Müller cells express numerous receptors, transporters, enzymes, and tissue factors. In this study, we provide evidence that adds to the dimensions of Müller cell function by demonstrating a unique relationship between external NAD⁺ and the mobilization of internal calcium, expressed in the form of calcium waves. The cellular mechanism that supports internal mobilization of calcium appears to depend on a complex multifunctional ectoenzyme, CD38, which converts NAD⁺ into the intracellular Ca²⁺-mobilizing second-messenger cyclic ADP-ribose (cADPR) and could function as a detector for extracellular NAD⁺, thus providing a novel signal detection system for evaluating the extracellular environment. Our results are consistent with a model of intracellular Ca²⁺ mobilization in which membrane-bound CD38 binds extracellular NAD⁺ and triggers intracellular Ca²⁺ waves either by direct conversion of NAD⁺ to cADPR or by activating intracellular cADPR synthesis. Our preliminary results indicate that the Ca²⁺ waves induced by external NAD⁺ propagate through an internal pathway that depends on the activation of ryanodine receptors, which appear to be distributed throughout the Müller cell cytosol. Because NAD⁺ is likely to be enhanced when cells are stress or damaged, CD38 could enable Müller cells to detect NAD⁺ under these circumstances and respond appropriately. Alternatively, NAD⁺ could also represent a novel extracellular, paracrine function that mediates signaling between glial cells and/or other cellular elements of the retina.