APOE4, Age, and Sex Regulate Respiratory Plasticity Elicited by Acute Intermittent Hypercapnic-Hypoxia

Abstract

Rationale: Acute intermittent hypoxia (AIH) shows promise for enhancing motor recovery in chronic spinal cord injuries and neurodegenerative diseases. However, human trials of AIH have reported significant variability in individual responses.

Objectives: Identify individual factors (eg, genetics, age, and sex) that determine response magnitude of healthy adults to an optimized AIH protocol, acute intermittent hypercapnic-hypoxia (AIHH).

Methods: In 17 healthy individuals (age = 27 ± 5 yr), associations between individual factors and changes in the magnitude of AIHH (15, 1-min O2 = 9.5%, CO2 = 5% episodes) induced changes in diaphragm motor-evoked potential (MEP) amplitude and inspiratory mouth occlusion pressures (P0.1) were evaluated. Single nucleotide polymorphisms (SNPs) in genes linked with mechanisms of AIH induced phrenic motor plasticity (BDNF, HTR2A, TPH2, MAOA, NTRK2) and neuronal plasticity (apolipoprotein E, APOE) were tested. Variations in AIHH induced plasticity with age and sex were also analyzed. Additional experiments in humanized (h)ApoE knock-in rats were performed to test causality.

Results: AIHH-induced changes in diaphragm MEP amplitudes were lower in individuals heterozygous for APOE4 (i.e., APOE3/4) compared to individuals with other APOE genotypes (P = 0.048) and the other tested SNPs. Males exhibited a greater diaphragm MEP enhancement versus females, regardless of age (P = 0.004). Additionally, age was inversely related with change in P0.1 (P = 0.007). In hApoE4 knock-in rats, AIHH-induced phrenic motor plasticity was significantly lower than hApoE3 controls (P < 0.05).

Conclusions: APOE4 genotype, sex, and age are important biological determinants of AIHH-induced respiratory motor plasticity in healthy adults.

Addition to knowledge base: AIH is a novel rehabilitation strategy to induce functional recovery of respiratory and non-respiratory motor systems in people with chronic spinal cord injury and/or neurodegenerative disease. Figure 5 Since most AIH trials report considerable inter-individual variability in AIH outcomes, we investigated factors that potentially undermine the response to an optimized AIH protocol, AIHH, in healthy humans. We demonstrate that genetics (particularly the lipid transporter, APOE), age and sex are important biological determinants of AIHH-induced respiratory motor plasticity.

Keywords: APOE4; age; biomarkers; genetics; intermittent hypercapnic-hypoxia; respiratory neuroplasticity; sex.

Graphical Abstract

Figure 1.

Conceptual diagram depicting cell signaling mechanisms (and candidate biomarker genes) for AIHH-induced respiratory motor plasticity. The panel of SNPs with a population prevalence of >10% were tested for association with reduced AIHH-induced plasticity in humans. These include 6 SNPs in genes involved in AIH cell signaling: (1) raphe chemosensitive cells (PHOX2B), (2) serotonin precursors in the central nervous system (tryptophan hydroxylase-2, TPH-2), (3) serotonin clearance enzyme (monoamine oxidase A, MAOA), (4) serotonin-2A receptors (HTR2A), (5) brain-derived neurotrophic factor (BDNF), and (6) TrkB receptors (NTRK2). A seventh dysfunctional SNP in neuroplasticity related gene, APOE (APOE4), was also tested for association.

Figure 2.

Relative (percentage change from baseline) changes in diaphragm MEP amplitudes and mouth occlusion pressure (P0.1) in individuals with BDNFval/met (panels A and B) and APOE3/4 (panels C and D) SNP. No associations were observed between individuals with BDNFval/met and the change in MEP amplitudes (panel A) or P0.1 (panel B). Individuals with dysfunctional APOE3/4 allele were associated with a significantly lower AIHH-induced change in MEP amplitude (t = −2.28, P = 0.048, panel C). However, no association between APOE3/4 and AIHH-induced P0.1 responses were observed (panel D). Δ = change. *P < 0.05. Results expressed as mean ± SD. = participant (S6) was identified as the most influential point (Cook’s D > 4) in the percentage change in diaphragm MEP amplitudes, therefore, the data were not included in group analyses.

Figure 3.

AIHH elicits phrenic long-term facilitation in hApoE3 but not hApoE4 knock-in rats. Panel A shows average traces of phrenic nerve amplitude for hApoE3 (n = 4; gray) and hApoE4 (n = 3; black) knock-in rats, *P < 0.050 versus baseline. Panel B phrenic burst amplitude (percentage change from baseline) in hApoE3 (gray circles) and hApoE4 (black circles) rats, +P < 0.005 versus hApoE4. Δ = change. Results expressed as mean ± SD.

Figure 4.

Relationship between age and sex on the magnitude (percentage change from baseline) of change in diaphragm MEP amplitudes (panels A and B), and mouth occlusion pressure in 0.1 s (P0.1, panels C and D) following AIHH. No association between age and the magnitude of change in diaphragm MEP amplitudes was observed (panel A). Regardless of age, males (black line, panel B) had significantly greater responses in MEP amplitudes versus females (gray line, panel B). The magnitude of change in P0.1 reduced significantly with age (panel C); however, the decline was more pronounced in males (r = −0.73, P = 0.036, black line, panel D) versus females (r = −0.29, P = 0.480, gray line, panel D). Δ = change. *P < 0.05. Results expressed as mean ± SD. = participant (S6) was identified as the most influential point (Cook’s D > 4) in the percentage change in diaphragm MEP amplitudes, therefore, the data was not included in group analyses.