Regulation of the hif-1-dependent hypoxic stress response by C. elegans

Abstract

All aerobic organisms need a way to sense oxygen levels and respond accordingly when in an unfavorable environment. In almost all metazoans, oxygen is both sensed and regulated by the HIF-1 (hypoxia inducible factor) transcription factor that is activated in periods of hypoxia and goes on to regulate hundreds of genes allowing for appropriate adaptations to hypoxia. HIF-1 activation results in changes at the cellular, tissue and whole organism levels such as increases in glycolysis, vascularization and erythropoiesis; HIF-1 is a critical factor in human development as well as progression of numerous diseases including ischemic stroke, COPD and cancer. HIF-1 is negatively regulated by the O2-dependent prolyl hydroxylase EGL-9 (known as EGLN, PHD, or HIF-PH in mammals). In normoxic conditions, EGL-9 uses ambient O2 to hydroxylate HIF-1. Hydroxylated HIF-1 is recognized by the von Hippel-Lindau (VHL-1) tumor suppressor protein, a component of an E3-ubiquitin ligase complex that targets HIF-1 for proteasomal degradation. In hypoxic conditions, EGL-9 is unable to hydroxylate HIF-1; stabilized HIF-1 enters the nucleus to regulate the expression of target genes that coordinate the hypoxia response. Increased activity of HIF-1, produced by either hypoxia or an egl-9(lf) mutation, induces the hypoxic stress response, which coordinates numerous adaptive changes in C. elegans, including retention of eggs in the uterus, decreases in locomotion and defecation rates, and increased resistance to not only hypoxia but also other stresses including oxidative stress and ER stress. By identifying suppressors of the egl-9(lf) mutant phenotype of egg retention, we have identified two independent pathways that regulate aspects of the hypoxic response in C. elegans. First, we discovered that loss of the conserved nonsense-mediated decay (NMD) pathway, an RNA surveillance mechanism that degrades aberrant mRNA transcripts with premature termination codons, suppressed the egl-9(lf)-induced changes in egg laying and defecation and caused increased hypoxia sensitivity. Other aspects of the egl-9(lf) phenotype, such as resistance to oxidative stress and changes in locomotion, were not affected by NMD-pathway mutations, indicating that NMD modulates specific aspects of the hypoxia response. Secondly, we found that loss of the neprilysin metallopeptidase, nep-2, suppressed the egl-9 Egl phenotype through the degradation of multiple neuropeptides including the known NEP-2 target SNET-1. Our findings reveal two different pathways that function downstream of egl-9 to regulate aspects of the hypoxic stress response, both providing a new pathway with which to study the neuromuscular control of egg laying using NEP-2, and critically showing the integration of the evolutionarily conserved hypoxic-stress response and nonsense-mediated decay pathways.