The Kelch-like (KLHL) family of proteins are critical substrate adaptors for E3 ubiquitin ligases and mutations in KLHL genes underlie a range of human diseases, including cancer and neurodegeneration. However, the upstream regulation and downstream effects of most KLHL proteins remain poorly understood. The best-characterized KLHL protein is KEAP1, which binds to the CUL3/RBX1 E3 ligase components to mediate the ubiquitination and destruction of NRF2, a master transcriptional regulator of redox stress signaling. Redox-active compounds disable KEAP1, preventing the ubiquitination of NRF2 by KEAP1/CUL3/RBX1 and allowing NRF2 accumulation and activation. Recently, we demonstrated that KEAP1 is modified by O-GlcNAc, a nutrient-sensitive, reversible form of glycosylation, and this modification is required for efficient NRF2 ubiquitination and degradation. Blockade of KEAP1 O-GlcNAcylation by chemical inhibition or nutrient deprivation stabilizes and activates NRF2, revealing a new regulatory link between nutrient sensing and redox stress signaling. Interestingly, we recently discovered that the KLHL protein gigaxonin is also O-GlcNAcylated in a nutrient-dependent manner, indicating that glycosylation of KLHL proteins may be a general regulatory link between metabolic status and proteostasis. Based on these results, we hypothesize that nutrient-sensitive O-GlcNAcylation of KLHL proteins regulates proteostasis in response to a range of nutrient levels and metabolic stresses. To test this hypothesis, we will first exploit KEAP1 as a model KLHL protein to determine how various stresses impact KEAP1 glycosylation and activity. Next, we will determine how KEAP1 glycosylation affects the KEAP1/CUL3/RBX1 complex and KEAP1-substrate interactions, using both targeted biochemical and unbiased proteomic approaches. We will then determine the functional role of O-GlcNAcylation on gigaxonin, whose mutations cause the fatal human disease giant axon neuropathy (GAN). Finally, we will elucidate the relationship between gigaxonin glycosylation and GAN disease phenotypes using murine neuronal and human GAN patient fibroblast models. The successful completion of our project will provide broad and significant insights into KLHL protein regulation and function, and may point to new therapeutic opportunities in neurodegeneration, cancer and other diseases by manipulating KLHL proteins through O- GlcNAc modulation.
Cells must strictly regulate proteostasis (i.e., the levels of their proteins) to maintain normal physiology, and dysregulation of proteostasis is a characteristic of many diseases. In preliminary work, we discovered that cells respond to changes in nutrients by attaching sugars to the KLHL family of proteins, which are key regulators of proteostasis in multiple human tissues. In this project, we will test the hypothesis that nutrient-sensitive sugar modifications of KLHL proteins govern proteostasis, a concept with important implications for such diseases as cancer and neurodegeneration.
