Pathogenesis of diabetic neuropathy: role of altered phosphoinositide metabolism

A unifying metabolic hypothesis completely accounting for the development of one or more of the chronic complications of diabetes on the basis of a single aspect of disturbed glucose metabolism resulting from insulin deficiency and/or hyperglycemia has been sought by clinical and basic scientists for decades. A growing body of loosely related but internally consistent scientific data obtained from cultured cells, incubated tissue preparations, animal models, and man implicate sorbitol- and glucose-induced myo-inositol depletion and altered phosphoinositide metabolism in a series of secondary biochemical, functional, and architectural abnormalities in the PNS in diabetes. These early metabolically based functional and structural changes simulate those that characterize human diabetic neuropathy. Can abnormal phosphoinositide metabolism in diabetic nerve thereby by itself explain the development of chronic diabetic neuropathy with all of its clinical complexity and heterogeneity? Almost certainly not. Even if the entire contribution of hyperglycemia to the development of diabetic neuropathy were mediated by secondary abnormalities in phosphoinositide metabolism, other factors must also play a role. Witness the differences in the histopathological picture of neuropathy in patients with IDDM and NIDDM despite similar durations and severity of diabetes, the apparent influence of age and gender on the appearance of early neuropathy in patients with IDDM, and the association of alcohol consumption with diabetic neuropathy. While early metabolic and functional disturbances in diabetic nerve such as impaired (Na,K)-ATPase function and paranodal swelling are empirically attributable to abnormal myo-inositol and phosphoinositide metabolism, more advanced abnormalities such as axo-glial dysjunction may reflect superimposed independent biochemical and/or hormonal defects (although, as mentioned previously, aldose reductase inhibition decreases axo-glial dysjunction in diabetic humans). The PNS has only a limited repertoire of responses to a variety of insults, so that Wallerian degeneration, axonal atrophy, impaired axonal transport, and dystrophic changes in diabetic neuropathy may represent multiple factors. On the other hand, the increasingly recognized importance of the phosphoinositide cascade in neuromodulation may attribute a progressively wider range of disturbances in the diabetic PNS to myo-inositol depletion and associated defects in phosphoinositide metabolism. Thus, while all effects of aldose reductase inhibitors in the PNS of diabetic rats have been reproduced by myo-inositol supplementation when this alternative intervention has been tested, the exact role of phosphoinositide metabolism in most of these responses is not well understood.(ABSTRACT TRUNCATED AT 400 WORDS)