Mechanisms of Ventilatory Adaptations to Chronic Hypercapnia

Clement J. Zablocki Va Medical Center, Milwaukee WI

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Abstract

Chronic lung or neuromuscular diseases impair gas exchange leading to chronic hypercapnia (CH). CH is unfortunately common in the Veteran’s Affairs patient population, and is associated with poor long-term prognoses, higher mortality rates and reduced cognitive function. While systemic physiologic adaptations may limit the negative consequences of CH, patients with CH may be predisposed to pathological maladaptive responses to acute-on-chronic exacerbations thereof, especially within CNS networks that control breathing. However, very little is known about these adaptive and/or maladaptive mechanisms elicited by varying degrees of CH. Our proposed studies are focused on testing the overall hypothesis that CH induces compensatory shifts in gene expression within key cell populations controlling breathing and/or cognitive function, where further exacerbation of hypercapnia cause maladaptive changes in gene expression and physiologic function. We recently established the time-dependent physiologic adaptions to mild CH over 30 days (d) of chronic exposure to 6% inspired CO2 (InCO2; PaCO2 ~55 mmHg) in our freely behaving adult goat model. Among others, mild CH induced time-dependent adaptive changes in steady state ventilation, ventilatory CO2/H+ sensitivity, heart rate, blood pressure, renal bicarbonate and potassium reclamation and metabolic rate, but impaired cognitive function. Ventilation dramatically increased within 1-3 hours (h) but by 24h decreased to a steady-state above normal. In contrast, the ventilatory CO2/H+ chemoreflex decreased within 1-2d but normalized by 7d. Steady-state ventilation was greater than predicted throughout the 30d CH, indicative of a yet-to-be-identified “missing stimulus” to breathe which we hypothesize represents a CH-induced respiratory neuroplasticity. To gain insight into mild CH-induced neuroplasticity, we identified time-dependent shifts in select markers of neuroplasticity within brainstem and cortical sites important in respiratory control and cognition. We found transient changes in interleukin 1-ß (IL-1ß), glutamate receptor subunit expression/phosphorylation, and serotonergic system markers. However, these correlative changes in markers of neuroplasticity failed to adequately explain the mechanisms of neuroadaption during CH. Accordingly, we propose to apply bulk tissue (bt) and/or single nuclear (sn)RNA sequencing technologies to query the molecular underpinnings of the physiologic respiratory adaptations and cognitive decline induced by CH in goats. Our team has previously and successfully applied these cutting-edge approaches in rats to identify differentially-expressed genes (DEGs) in brainstem regions (btRNA-Seq) or within specific cell types (snRNA-seq), and are thus poised to apply these technologies to our goat model of CH. Preliminary physiologic studies simulating acute-on-chronic hypercapnia (by further chronically increasing inspired CO2 from 6% to 8% to induce moderate hypercapnia) showed a pathological depression of cardiorespiratory variables during acute and severe hypercapnia. Thus, our published and preliminary data support our overall hypothesis, which will be further tested by applying cutting- edge, established methodologies to fill existing gaps in knowledge regarding the fundamental neurobiological effects of CH. We will achieve our goal through four Specific Aims: Aim 1.1 tests the hypothesis that 3 to 24h of mild CH induces dynamic, adaptive shifts in gene expression/cellular signaling pathways within CNS regions controlling cardiorespiratory and cognitive functions. Aim 1.2 tests the hypothesis that 7d of mild CH induces cell type-specific changes in gene expression/signaling pathways that underlie adaptive CH-induced respiratory neuroplasticity. Aim 2 tests the hypothesis that moderate CH predisposes goats to pathophysiological responses to severe hypercapnia (acute-on-chronic exacerbation) due to maladaptive shifts in gene expression profiles/cellular signaling pathways within CNS regions controlling cardiorespiratory and cognitive functions.