Point of NO Return for Nitrergic Nerves in Diabetes: A New Insight into Diabetic Complications

Abstract

Aberrations in nitrergic neurotransmission, due to a decrease in neuronal nitric oxide (NO) synthase (nNOS) protein, play an important role in the pathogenesis of autonomic neuropathy in diabetes. Until recently the mechanism of the decrease in nNOS protein content in nitrergic nerves during diabetes was debated. Two different views were prevailing, one attributing the nNOS decrease to nitrergic nerve degeneration, the other to an alteration in nNOS expression. Our recent study in which we showed that nitrergic nerves undergo a degenerative process in two phases might bring a solution to this debate. Our model suggests that, in the early stages of diabetes, nNOS expression is decreased in the nitrergic axons while nNOS levels are unaffected in the cell bodies, most probably due to a defect in axonal transport. This decrease is reversible with insulin treatment. As the diabetes progresses, nNOS starts to accumulate in the cell bodies since it cannot be transported down to the axons. Increased nNOS protein and NO production coincide with accumulation of advanced glycation endproducts (AGEs) in the blood and tissues. Synergistic action of AGEs and endogenous NO leads to increased oxidative stress within the cell bodies, resulting in apoptosis. This degenerative phase of nitrergic neuropathy is not reversible with insulin treatment. This suggests a point of no return for autonomic nerves after which the degenerative changes become irreversible. Future therapeutic approaches could target the defective axonal transport and prevention of AGEs accumulation before this point of no return. In the later stages, reduction of AGEs, replenishment of lost nitrergic neurons and restoration of function are putative therapeutic targets.