Hypoxia-inducible factors (HIFs) are a highly conserved group of transcriptional regulators responsible for cellular and systemic O2 homeostasis in animals. However, how HIFs are involved in basic adaptive ventilatory and metabolic responses to acute and chronic hypoxia remains incompletely characterized. Naked mole rats are among the most hypoxia tolerant mammals identified. As opposed to the typical hyperventilatory response of most adult mammals, naked mole rats exhibit a unique decline in ventilation, matching their substantial decrease in metabolic rate. Naked mole rats therefore provide an excellent model in which to investigate adaptations to hypoxic ventilatory and metabolic responses (HVR and HMR, respectively). Interestingly, naked mole rats possess a mutation within the von Hippel-Lindau (VHL) binding domain—a protein necessary for proteasomal degradation of HIF subunits in normal O2 concentrations—suggesting they retain elevated baseline expression of HIF and thus an upregulation of downstream gene targets. In designing our experiment, we focused on sustained hypoxia and HIF1, which is typically the first responder subunit upon exposure to low O2 stress. We sought to determine how increased HIF1 expression might contribute to the distinct HVR and HMR of naked mole rats, first by confirming the observed VHL mutation translates into increased HIF1 protein expression via immunoblotting. HIF1 protein expression was found to be 3-fold higher in naked mole rat brain than mouse brain and 4-fold higher than in mouse liver tissue (p < 0.05). We then investigated how elevated HIF1 levels might contribute to the HVR and HMR by treating naked mole rats with two different HIF1 inhibitors (either echinomycin; 0.5 and 1.0 mg kg-1, or PX-478; 80.0 mg kg-1) and subsequently examined changes in ventilatory and metabolic parameters in awake animals exposed to sustained hypoxia (7% O2; 1 hour). In control naked mole rats, minute ventilation (V̇E) reversibly decreased by 32% in hypoxia (1298.3 ± 188.5 to 882.6 ± 117.0 mL min-1 kg-1) because of changes in both breathing frequency (fR) and tidal volume (VT). Conversely, the HVR was not significantly affected in any of our three treatment groups however, normoxic ventilation increased in naked mole rats treated with low dose echinomycin (0.5 mg kg-1) by 72% (from 1298.3 ± 188.5 to 2239.5 ± 221.1 mL min-1 kg-1). Consistent with previous findings, metabolic rate in control naked mole rats decreased 70% (from 40.1 ± 5.0 to 11.9 ± 0.9 mL O2 min-1 kg-1). Again, treatment with our pharmacological agents did not significantly alter this response but did result in a 43% decrease in basal metabolic rate (V̇O2 and V̇CO2) in both high-dose echinomycin and PX-478 treated naked mole rats (40.1 ± 5.0 to 22.5 ± 3.6 and 23.0 ± 1.88 mL O2 min-1 kg-1 respectively, p < 0.05), dulling the magnitude of the HMR. As a result of unmatched changes in V̇E and V̇O2, HIF1 deficient naked mole rats treated with both low-dose echinomycin and PX-478 experienced an atypical increase in their air convection requirement (ACR; V̇E:V̇O2-1) in hypoxia (from 77.4 ± 11.3 to 159.2 ± 34.63 and 123.5 ± 35.5 respectively, p < 0.05), resembling a hyperventilation response closer to that of hypoxia-intolerant mammals.
To further determine how increased HIF1 availability affects the HMR and HVR, we administered hypoxia-intolerant mice with a pharmacological HIF1 agonist (3,4- EDHB; 180 mg kg-1) and used identical experimental design to measure downstream ventilatory and metabolic responses. Mice exhibit similar reductions in metabolic rate during hypoxic exposure (from 60.3 ± 2.4 to 21.8 ± 1.8 mL O2 min-1 kg-1, p < 0.05) but experience a 30% increase in fR (from 157.5 ± 9.5 to 200.4 ± 10.8 breaths min-1, p < 0.05). In contrast, mice treated with EDHB and to exposed 7% O2 exhibited a 20% increase in fR (200.4 ± 10.8 to 236.5 ± 14.1 breaths min-1, p < 0.05) and a 30% reduction in the magnitude of their HMR (from 38.5 ± 2.8 to 27.8 ± 3.6 ΔV̇O2). No other significant trends were observed in any of the other parameters measured. We conclude metabolic and ventilatory control in naked mole rats and mice may partially depend on increased HIF1 expression.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/37908 |
| Date | 23 July 2018 |
| Creators | Borecky, Lisa |
| Contributors | Pamenter, Matthew |
| Publisher | Université d'Ottawa / University of Ottawa |
| Source Sets | Université d’Ottawa |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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