APOE4, Age & Sex Regulate Respiratory Plasticity Elicited By Acute Intermittent Hypercapnic-Hypoxia

RationaleAcute intermittent hypoxia (AIH) is a promising strategy to induce functional motor recovery following chronic spinal cord injuries and neurodegenerative diseases. Although significant results are obtained, human AIH trials report considerable inter-individual response variability. ObjectivesIdentify individual factors (e.g., genetics, age, and sex) that determine response magnitude of healthy adults to an optimized AIH protocol, acute intermittent hypercapnic-hypoxia (AIHH). MethodsAssociations of individual factors with the magnitude of AIHH (15, 1-min O2=9.5%, CO2=5% episodes) induced changes in diaphragm motor-evoked potential amplitude (MEP) and inspiratory mouth occlusion pressures (P0.1) were evaluated in 17 healthy individuals (age=27{+/-}5 years) compared to Sham. 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. ResultsAIHH-induced changes in diaphragm MEP amplitudes were lower in individuals heterozygous for APOE4 (i.e., APOE3/4) allele versus other APOE genotypes (p=0.048). No significant differences were observed between any other SNPs investigated, notably BDNFval/met (all p>0.05). Males exhibited a greater diaphragm MEP enhancement versus females, regardless of age (p=0.004). Age was inversely related with change in P0.1 within the limited age range studied (p=0.007). In hApoE4 knock-in rats, AIHH-induced phrenic motor plasticity was significantly lower than hApoE3 controls (p<0.05). ConclusionsAPOE4 genotype, sex and age are important biological determinants of AIHH-induced respiratory motor plasticity in healthy adults. ADDITION TO KNOWLEDGE BASEAcute intermittent hypoxia (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 diseases. 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, acute intermittent hypercapnic-hypoxia (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.