Abstract CD38, an ADP‐ribosyl cyclase that generates cyclic ADP‐ribose (cADPR), is essential for Ca 2+ ‐dependent oxytocin release. However, its subcellular localisation and membrane topology within oxytocin neurones have remained unclear. We investigated the distribution and orientation of CD38 in oxytocin‐producing neurones of Japanese macaques ( Macaca fuscata ) using immunoelectron microscopy combined with biochemical isolation of neurosecretory vesicles (NSVs). CD38 immunoreactivity was selectively detected on oxytocin‐containing NSVs in axon terminals in the posterior pituitary and dendrites of the supraoptic nucleus, whereas vasopressin vesicles and the plasma membrane lacked detectable labelling. Cryo‐electron microscopy confirmed the structural integrity of purified NSV fractions, and Western blotting verified the presence of CD38 protein within these fractions. Permeabilisation‐dependent immunogold labelling further demonstrated that the NSV membrane localisation of CD38 and that the N‐terminal region of CD38 is oriented toward the vesicle lumen, consistent with a type III membrane topology in which the catalytic domain faces the cytosol. This arrangement positions the active site near cytosolic NAD + , enabling localised cADPR production adjacent to Ca 2+ ‐mobilising channels that support regulated exocytosis. These findings identify, in primate oxytocin neurones, a previously unrecognised, vesicle‐associated pool of CD38 with a cytosol‐facing catalytic domain and provide a structural framework for understanding how intracellular type III CD38 contributes to neuropeptide release.
Miyamoto et al. (Wed,) studied this question.