A discrete group of neurons located in the retrotrapezoid nucleus (RTN) that express the transcription factor, Phox2b and the neuropeptide, Neuromedin B (Nmb) provide a crucial excitatory drive to regulate downstream respiratory rhythm/pattern-generating circuits. The activity of these neurons is modulated by changes in CO2 (or H+) and various other sensory and arousal-state inputs. Dysfunction of this neuronal system is implicated in potentially fatal syndromes (e.g., sudden infant death, SIDS; congenital central hypoventilation, CCHS) and re- setting of CO2 threshold/sensitivity can accompany and exacerbate various chronic disorders of breathing (e.g., chronic obstructive pulmonary disease, COPD). Previous work in our group has identified two CO2/H+ sensors in RTN neurons: the proton-activated GPCR, GPR4 and the proton-inactivated potassium channel, TASK-2. Most RTN neurons express both sensors but it is unclear whether the two proteins provide redundancy or underlie different cellular responses to increased H+ concentration. In this respect, RTN neurons are highly enriched in expression of a neuropeptide, PACAP, that has been implicated in SIDS and which our laboratory has shown contributes to CO2-regulated breathing (respiratory chemoreflex): deletion of PACAP from RTN neurons blunts the respiratory chemoreflex, and PACAP injection into RTN-targeted respiratory nuclei enhances respiratory output. Metabotropic signaling, such as that initiated by GPR4 activation, is thought to play a critical role in neuropeptide release from dense core vesicles compared to small transmitter release from synaptic vesicles. Thus, I hypothesize that GPR4-mediated pH-sensitivity and cAMP elevation is crucial for the release of PACAP from RTN neurons to control aspects of the central chemoreflex.
In Specific Aim 1, I use genetically modified mice to test whether the pH sensitivity of GPR4, per se, is required for its cellular and physiological actions, and use electrophysiology in brainstem to examine the role of downstream G?s-coupled signaling, specifically adenylyl cyclase activation, in pH sensitivity of RTN neurons. My preliminary data with a novel line of CRISPR-modified knock-in mice and pharmacological manipulation of cAMP are consistent with this hypothesis.
In Specific Aim 2, I use a viral approach to express a genetically-encoded, photo-activated adenylyl cyclase (bPAC) to test whether this particular form of metabotropic signaling confers the ability to release an excitatory neuropeptide, PACAP, that supports CO2-stimulated breathing by RTN neurons. I have prepared a virus for RTN-selective expression of bPAC, and implemented a cell-based optical system to detect PACAP release from RTN neurons in vitro. Collectively, the proposed studies will provide novel information regarding molecular mechanisms that regulate the pH sensitivity and downstream actions of RTN neurons during breathing regulation and, more generally, the role of cAMP-mediated signaling in neuropeptide release. Identification of these novel molecular mechanisms may provide new therapeutic targets for disorders of breathing.
Breathing is essential to maintain cellular O2 levels for ATP production and remove CO2, the product of respiration; deficits in brainstem homeostatic mechanisms that sense CO2 to regulate breathing are associated with a number of syndromes associated with central hypoventilation. Therefore, determining the mechanism by which this central respiratory chemoreflex functions and identifying possible points of intervention or diagnosis of respiratory disorders is of high priority. This application seeks new information on receptor and signaling mechanisms that support neuropeptide release from a preeminent population of respiratory chemoreceptors for CO2-regulated breathing.
