Neuronal HIF-1 alpha in the nucleus tractus solitarius contributes to ventilatory acclimatization to hypoxia

Abstract

We hypothesized that hypoxia inducible factor 1 alpha (HIF-1 alpha) in CNS respiratory centres is necessary for ventilatory acclimatization to hypoxia (VAH); VAH is a time-dependent increase in baseline ventilation and the hypoxic ventilatory response (HVR) occurring over days to weeks of chronic sustained hypoxia (CH). Constitutive deletion of HIF-1 alpha in CNS neurons in transgenic mice tended to blunt the increase in HVR that occurs in wild-type mice with CH. Conditional deletion of HIF-1 alpha in glutamatergic neurons of the nucleus tractus solitarius during CH significantly decreased ventilation in acute hypoxia but not normoxia in CH mice. These effects are not explained by changes in metabolic rate, nor CO2, and there were no changes in the HVR in normoxic mice. HIF-1 alpha mediated changes in gene expression in CNS respiratory centres are necessary in addition to plasticity of arterial chemoreceptors for normal VAH. Chronic hypoxia (CH) produces a time-dependent increase of resting ventilation and the hypoxic ventilatory response (HVR) that is called ventilatory acclimatization to hypoxia (VAH). VAH involves plasticity in arterial chemoreceptors and the CNS [e.g. nucleus tractus solitarius (NTS)], although the signals for this plasticity are not known. We hypothesized that hypoxia inducible factor 1 alpha (HIF-1 alpha), an O-2-sensitive transcription factor, is necessary in the NTS for normal VAH. We tested this in two mouse models using loxP-Cre gene deletion. First, HIF-1 alpha was constitutively deleted in CNS neurons (CNS-HIF-1 alpha(-/-)) by breeding HIF-1 alpha floxed mice with mice expressing Cre-recombinase driven by the calcium/calmodulin-dependent protein kinase II alpha promoter. Second, HIF-1 alpha was deleted in NTS neurons in adult mice (NTS-HIF-1 alpha(-/-)) by microinjecting adeno-associated virus that expressed Cre-recombinase in HIF-1 alpha floxed mice. In normoxic control mice, HIF-1 alpha deletion in the CNS or NTS did not affect ventilation, nor the acute HVR (10-15 min hypoxic exposure). In mice acclimatized to CH for 1 week, ventilation in hypoxia was blunted in CNS-HIF-1 alpha(-/-) and significantly decreased in NTS-HIF-1 alpha(-/-) compared to control mice (P < 0.0001). These changes were not explained by differences in metabolic rate or CO2. Immunofluorescence showed that HIF-1 alpha deletion in NTS-HIF-1 alpha(-/-) was restricted to glutamatergic neurons. The results indicate that HIF-1 alpha is a necessary signal for VAH and the previously described plasticity in glutamatergic neurotransmission in the NTS with CH. HIF-1 alpha deletion had no effect on the increase in normoxic ventilation with acclimatization to CH, indicating this is a distinct mechanism from the increased HVR with VAH.

We hypothesized that hypoxia inducible factor 1 alpha (HIF-1 alpha) in CNS respiratory centres is necessary for ventilatory acclimatization to hypoxia (VAH); VAH is a time-dependent increase in baseline ventilation and the hypoxic ventilatory response (HVR) occurring over days to weeks of chronic sustained hypoxia (CH). Constitutive deletion of HIF-1 alpha in CNS neurons in transgenic mice tended to blunt the increase in HVR that occurs in wild-type mice with CH. Conditional deletion of HIF-1 alpha in glutamatergic neurons of the nucleus tractus solitarius during CH significantly decreased ventilation in acute hypoxia but not normoxia in CH mice. These effects are not explained by changes in metabolic rate, nor CO2, and there were no changes in the HVR in normoxic mice. HIF-1 alpha mediated changes in gene expression in CNS respiratory centres are necessary in addition to plasticity of arterial chemoreceptors for normal VAH. Chronic hypoxia (CH) produces a time-dependent increase of resting ventilation and the hypoxic ventilatory response (HVR) that is called ventilatory acclimatization to hypoxia (VAH). VAH involves plasticity in arterial chemoreceptors and the CNS [e.g. nucleus tractus solitarius (NTS)], although the signals for this plasticity are not known. We hypothesized that hypoxia inducible factor 1 alpha (HIF-1 alpha), an O-2-sensitive transcription factor, is necessary in the NTS for normal VAH. We tested this in two mouse models using loxP-Cre gene deletion. First, HIF-1 alpha was constitutively deleted in CNS neurons (CNS-HIF-1 alpha(-/-)) by breeding HIF-1 alpha floxed mice with mice expressing Cre-recombinase driven by the calcium/calmodulin-dependent protein kinase II alpha promoter. Second, HIF-1 alpha was deleted in NTS neurons in adult mice (NTS-HIF-1 alpha(-/-)) by microinjecting adeno-associated virus that expressed Cre-recombinase in HIF-1 alpha floxed mice. In normoxic control mice, HIF-1 alpha deletion in the CNS or NTS did not affect ventilation, nor the acute HVR (10-15 min hypoxic exposure). In mice acclimatized to CH for 1 week, ventilation in hypoxia was blunted in CNS-HIF-1 alpha(-/-) and significantly decreased in NTS-HIF-1 alpha(-/-) compared to control mice (P < 0.0001). These changes were not explained by differences in metabolic rate or CO2. Immunofluorescence showed that HIF-1 alpha deletion in NTS-HIF-1 alpha(-/-) was restricted to glutamatergic neurons. The results indicate that HIF-1 alpha is a necessary signal for VAH and the previously described plasticity in glutamatergic neurotransmission in the NTS with CH. HIF-1 alpha deletion had no effect on the increase in normoxic ventilation with acclimatization to CH, indicating this is a distinct mechanism from the increased HVR with VAH.