Neuro-immune mechanisms of diabetic neuropathy

Abstract

Diabetes and associated co-morbidities are some of the fastest-growing global healthcare concerns afflicting over 500 million people. Diabetic peripheral neuropathy (DPN) is a common peripheral nervous system complication of diabetes, causing sensory loss and debilitating pain. Although some metabolic syndrome co-morbidities like hyperglycemia and hyperlipidemia have been linked to the onset of DPN, whether the immune system is involved in the disease and whether the degeneration in DPN leads to nerve injury-like transcriptional changes in DRG neurons remain elusive. Because of the rise of high-caloric diets worldwide contributing to the obesity and diabetes crises, a high-fat high fructose diet-induced mouse model was employed to model metabolic syndrome co-morbidities, diabetes, and neuropathy. The model developed here results in a persistent reduction of responsiveness to noxious heat (heat hypoalgesia) and transient mechanical hypersensitivity. Intriguingly, only at 24 weeks, these mice show a reduction in epidermal skin innervation, a hallmark of peripheral neuropathy. To determine if early neuroimmune interactions drive or modify the disease, single-cell sequencing of immune cells in the skin and nerves of diabetic mice revealed an increased infiltration of CCR2+ macrophages in the nerve at 12 weeks of diet. These infiltrating macrophages share similarities with nerve crush- responsive macrophages although they are likely responding to excess fatty acids rather than demyelination in the nerve. Inhibiting macrophage recruitment in diabetic mice by genetically or pharmacologically blocking CCR2 signaling, resulted in a more severe heat hypoalgesia and accelerated skin denervation, compared to diabetic mice with intact CCR2 signaling. In addition, even though degeneration in the HFHFD model of DPN is Sarm1-dependent, it does not lead to injury-associated transcriptional reprogramming of DRG neurons thus explaining the lack of regeneration in this disease, unlike nerve injury models. Finally, we independently observed that a lack of sensory neurons in mice leads to poor wound healing via a CGRP-Ramp1 mechanism, providing a neuro-immune explanation for diabetic wounds. The findings described in this thesis highlight multiple discoveries of importance to the pain, immunology, and metabolism fields. First, the recruitment of neuroprotective macrophages into nerves of diabetic mice that delay onset of the terminal axonal degeneration and reduce sensory loss is a potentially novel therapeutic avenue to prevent peripheral neuropathy. Second, previous knowledge of regeneration transcriptional reprogramming of sensory neurons upon nerve injuries does not apply to peripheral neuropathies which explains the lack of regeneration in neuropathy and potential failures of translatability in the clinic. Finally, delayed wound healing in mice lacking sensory neurons suggests that peripheral neuropathy may be a mechanism explaining poor wound healing in diabetic patients opening novel therapeutic avenues and highlighting that sensory neurons are an important player in diabetes complications beyond peripheral neuropathy.