Impact of diabetic neuropathy in small fiber alterations across sexes

Diabetes is a chronic progressive disorder characterized by hyperglycemia, with a growing global prevalence affecting approximately 10% of adults. Diabetic neuropathy (DN), a common and severe complication that affects one-third of diabetic patients, lacks effective treatments to halt or slow its progression. The onset of DN is primarily linked to microvascular and metabolic alterations in the peripheral nervous system (PNS) and alteration of small C-fibers is clinically relevant for the development of pain symptoms. Yet, which type of small fibers is more affected by diabetes is still unclear. We adopted a streptozotocin (STZ)-induced T1D model in male and female mice expressing the green fluorescent protein Venus in a population of C-low threshold mechanoreceptors (C-LTMRs; tafa4-Venus knock-in), involved in touch hypersensitivity after injury. After a behavioral assessment, mice were sacrificed and three main types of C-fibers were investigated in glabrous and hairy skin, dorsal root ganglia (DRG), and spinal dorsal horn, namely: CGRP+ peptidergic fibers, Isolcectin B4 (IB4)+ non-peptidergic fibers and VGLUT3/TAFA4+ C-LTMR fibers. Interestingly, we found that females required higher STZ doses to develop diabetes, but also that they are more prone to display mechanical sensitivity at the von Frey test. Confocal microscopy analysis of small fiber alterations suggests that behavioral sex differences are accompanied by sexually dimorphic alterations of small fibers, particularly at the spinal level. Preliminary results indicate that females exhibit higher IB4 and VGLUT3 expression levels in controls, but after the STZ-treatment, a general reduction in these markers was observed. Conversely in diabetic males, CGRP modestly decreased, while IB4 remained unchanged, and VGLUT3 increased. These findings emphasize the necessity of including both sexes in diabetes research and suggest that specific types of sensory fibers are more likely to be affected in DN.