Pain is an essential defensive response in conditions such as acute cystitis that alert the host of an adverse insult. However, in chronic conditions such as bladder pain syndrome (BPS) or overactive bladder, pathological neuronal and resultant pain is debilitating. Sensory afferents in the bladder emanate from sensory neurons in the dorsal root ganglia (DRG). There is extensive heterogeneity in DRG at several levels including neurochemistry, pharmacology, morphology, and molecular profiles. There is also cross-sensitization of visceral afferents of different organs and between the visceral and somatic sensory nervous systems. The molecular basis for this heterogeneity and diversity, its implication in normal and abnormal genitourinary sensory function and how this heterogeneity arises from undifferentiated sensory neurons is lacking. Delineating gene expression patterns and molecular networks in distinct types of DRG neurons during ontogeny is an essential first step to begin understanding the molecular basis of normal sensation and pain.
The majority of nociceptive sensory neurons and a subset of low threshold mechanoreceptors express the receptor tyrosine kinase RET. RET is the signaling receptor for a class of neurotrophic factors known as glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs). GFLs have known roles in neuronal cell survival, axon growth and maintenance and modulating physiological and pathological sensory responses. Ret is expressed in 60-70% of DRG sensory neurons that respond to innocuous and noxious stimuli. Following bladder injury in animal models, molecular and morphological changes have been observed in Ret-positive DRG neurons and in their central projections to the spinal cord, suggesting that they have a role in bladder injury.
We have made several innovative and unique animal models that allow labeling of distinct Ret-positive neurons in the DRG. Relevant to nGUDMAP, we have generated a Ret reporter mouse strain that expresses EGFP from the Ret locus (Ret-EGFP) and labels all cells that express Ret. We have also generated a Ret conditional reporter mouse strain that enables inducible detection of only a specific population of Ret expressing cells using Cre-lox system (Ret cond reporter). In DRG of Ret-EGFP mice, a subset of myelinated (low threshold mechanoreceptors) and unmyelinated (nociceptors) sensory neurons are EGFP-positive. In DRG of nociceptor-specific Ret-cond reporter mice only nociceptors are EGFP-positive. Our preliminary data show: 1) Ret-EGFP mice label DRG sensory neurons, 2) Ret conditional reporter mice when bred with Nav1.8Cre mice label small diameter sensory neurons (nociceptors), 3) Ret conditional reporter mice label DRG sensory neurons when bred with sensory neuron specific AdvCre mice, 4) Ret-positive neurons innervate the mouse bladder, 5) DiI retrograde tracer injected into the bladder colocalizes with Ret-positive neurons in the DRG, 6) Ret-positive neurons are present in human DRG and Ret-positive nerve fibers are detected in human bladder tissue.
Our objective for the nGUDMAP project is to generate mRNA expression profiles of Ret-positive sensory neurons in DRG neurons and in specific nociceptors at different developmental stages using Ret-EGFP, Ret conditional reporter and cell specific Cre mice.
Aim 1.1. Generate expression profiles of Ret+ DRG neurons at embryonic and perinatal time points using Ret-EGFP mice.
Aim 1.2. Generate expression profiles of an enriched population of Ret+ pelvic afferents (lumbo-sacral DRG sensory neurons) using postnatal Ret-reporter mice.
Aim 1.3. Generate expression profiles of Ret+ nociceptors (generated by breeding Ret-conditional reporter and nociceptor specific Nav1.8Cre mice) during late gestation and postnatal time points.
Grant number: 1U01DK101039