Peripheral neuropathy is an increasingly common disorder that affects up to 25% of diabetics and 35-40% of elderly individuals. The pathological underpinning of peripheral neuropathy from any etiology is the loss of axonal integrity and function. In diabetics and the aged, the axonal loss is accompanied by impaired nerve fiber regeneration after injury; a state that is recapitulated in rodent models. Axonal injury triggers a dramatic reprogramming of the Schwann cells (SCs) surrounding the damaged axon that culminates in the adoption of a `repair SC' phenotype. The repair SC promotes axon/myelin breakdown and disposal, attracts macrophages, and produces neurotrophic factors. Upon contact with the regenerating axon, it transforms back into a differentiated SC to ensure remyelination or Remak bundle formation. The primary regulator of this transition to the repair SC is the transcription factor JUN, which is rapidly activated after injury in SCs surrounding damaged axons. In Jun-deficient mice, nerve regeneration is impaired. In keeping with the impaired axon regeneration in diabetic and aged animals, we find that mice with mutations that alter SC metabolism fail to effectively promote nerve regeneration. Most recently, we characterized OGT-SCKO mice that lack SC expression of O-GlcNAc transferase (OGT), the enzyme that catalyzes addition of O-GlcNAc moieties to proteins at Ser and Thr residues. OGT activity is regulated by the flux of glucose through the hexosamine biosynthetic pathway, thus it serves as a sensor that aggregates information regarding glucose metabolism and transmits it into changes in cell physiology. Notably, abnormal O-GlcNAcylation has been implicated in diabetes, cancer, and neurodegenerative diseases. Mice lacking O- GlcNAcylation in SCs develop a tomaculous demyelinating neuropathy. Expression profiling of OGT-SCKO sciatic nerve revealed high expression of many AP-1 targets. Moreover, we find that JUN phosphorylation and transcriptional activity are regulated by O-GlcNAcylation. In keeping with abnormalities in JUN activity, we find that loss of OGT leads to a substantial decrease in regeneration/remyelination, indicating that the SC injury response is modulated by metabolism. These results lead us to hypothesize that poor nerve regeneration, and potentially the neuropathy itself, in diabetic and aged individuals is caused by the impact of abnormal metabolism on the generation, function, and/or cessation of the SC injury response via direct effects on JUN activity. To pursue this hypothesis we propose three aims: 1) To investigate how metabolism impacts the SC injury response; 2) To investigate the role of O-GlcNAcylation in regulating JUN activity; and 3) To determine the role of AP-1 partners and other regulators in the SC injury response. Through these studies, we hope to show that therapies targeting Schwann cells and their repair functions will be useful in treating peripheral neuropathy and traumatic nerve injury.
Our research is focused on defining how abnormal metabolism impairs Schwann cell axonal support and leads to axon degeneration and peripheral neuropathy. We have identified a molecular link between abnormal metabolism and ineffective nerve regeneration after injury. The identification of drugs to enhance Schwann cell repair function will be helpful in treating diabetic peripheral neuropathy as well as traumatic nerve injury.
|Kim, Sungsu; Maynard, Jason C; Strickland, Amy et al. (2018) Schwann cell O-GlcNAcylation promotes peripheral nerve remyelination via attenuation of the AP-1 transcription factor JUN. Proc Natl Acad Sci U S A 115:8019-8024|