A conserved role for AMP-activated protein kinase in NGLY1 deficiency

Author summary Thousands of proteins in most organisms harbor a type of sugar modification called N-glycan. Addition of N-glycans to proteins has long been known to affect protein quality control and function. However, recent studies have shown that removing N-glycans from proteins also plays key roles in cellular biology and animal development. Importantly, children with mutations in the enzyme responsible for N-glycan removal (NGLY1) exhibit developmental delay, seizures, movement disorder, and other abnormalities. NGLY1 is responsible for adjusting the activity of proteasome, a cellular machinery involved in protein degradation. NGLY1 has also been linked to the function and homeostasis of mitochondria, the major energy production engine in animal cells. However, the link between these processes and patient symptoms is not clear, and no therapies exist for this disease. We now report that loss of NGLY1 results in reduced levels of a cellular energy sensor called AMPK alpha in fruit flies, mouse cells and patient cells. Restoring AMPK alpha level or its pharmacological activation improves the energy homeostasis defects in all three systems and significantly increases the survival of NGLY1-mutant fruit flies independently of proteasome. Our data suggest that enhancement of AMPK alpha activity can serve as a potential therapeutic approach in NGLY1 deficiency patients. Mutations in human N-glycanase 1 (NGLY1) cause the first known congenital disorder of deglycosylation (CDDG). Patients with this rare disease, which is also known as NGLY1 deficiency, exhibit global developmental delay and other phenotypes including neuropathy, movement disorder, and constipation. NGLY1 is known to regulate proteasomal and mitophagy gene expression through activation of a transcription factor called nuclear factor erythroid 2-like 1 (NFE2L1). Loss of NGLY1 has also been shown to impair energy metabolism, but the molecular basis for this phenotype and its in vivo consequences are not well understood. Using a combination of genetic studies, imaging, and biochemical assays, here we report that loss of NGLY1 in the visceral muscle of the Drosophila larval intestine results in a severe reduction in the level of AMP-activated protein kinase alpha (AMPK alpha), leading to energy metabolism defects, impaired gut peristalsis, failure to empty the gut, and animal lethality. Ngly1(-/-)mouse embryonic fibroblasts and NGLY1 deficiency patient fibroblasts also show reduced AMPK alpha levels. Moreover, pharmacological activation of AMPK signaling significantly suppressed the energy metabolism defects in these cells. Importantly, the reduced AMPK alpha level and impaired energy metabolism observed in NGLY1 deficiency models are not caused by the loss of NFE2L1 activity. Taken together, these observations identify reduced AMPK signaling as a conserved mediator of energy metabolism defects in NGLY1 deficiency and suggest AMPK signaling as a therapeutic target in this disease.