Sensory Cross-Activation in Bowel Dysfunction

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PROJECT SUMMARY It is a common occurrence in human that pain is perceived at a remote location away from the diseased organ. This is also true with bowel dysfunction. Patients with inflammatory bowel diseases (IBD) and/or irritable bowel syndrome (IBS) often experience bladder hyperactivity and somatic pain. Understanding the peripheral mechanisms of pain generation and sensory cross-sensitization helps development of therapeutic approaches with minimum central adverse effects to treat pain comorbidity. Peripheral glial cells are gaining increased recognition for their roles in modulating sensory neuron activity. Satellite glial cells (SGCs) of dorsal root ganglia (DRG) reside around sensory neurons and connect sensory neurons through SGC networks. Using Cre-based expression of hM3Dq to activate glial cells by clozapine-N-oxide (CNO), we show that activation of glial cells leads to an enhanced calcitonin gene-related peptide (CGRP) release to the spinal cord, an essential process in spinal central sensitization. Activation of glial cells also facilitates hypersensitivity of the colon, urinary bladder and hind paw in mice. In DRG, TrkB.T1 is expressed by SGCs but not neurons. Calcium (Ca2+) activity is a common pathway mediated by hM3Dq and TrkB.T1 in SGCs. Importantly, using a unique SGC- sensory neuron co-culture system we show that activation of SGCs mediated by TrkB.T1 leads to activation of adjacent sensory neurons that participate in pain processing. We therefore hypothesize that TrkB.T1 mediates glia-neuron interaction and participates in sensory neuron cross-activation and cross-organ sensitization. To test this hypothesis, we will use mice with inducible conditional TrkB.T1 deletion (TrkB.T1cKO) from peripheral glia mainly from SGCs and mice with hM3Dq or hM4Di expression in peripheral glia. We will perform molecular and functional analysis of colon-bladder sensory neuron cross-activation and cross-organ sensitization in these unique mouse lines (Aim 1). The functional roles of TrkB.T1 in SGCs are to increase the levels of Ca2+ to promote gliotransmitter release. We have identified a number of glial mediators that are regulated by TrkB.T1 in SGCs through proteomic screening and transcriptional analysis. We will characterize TrkB.T1-mediated gliotransmission and gliotransmitter-facilitated sensory neuron activation in the context of cross-organ sensitization (Aim 2). Central sensitization not only contributes to colon-bladder cross-sensitization but also underlies viscero-somatic cross-sensitization. We therefore will examine whether TrkB.T1-mediated SGC- sensory neuron crosstalk contributes to CGRP central release and leads to viscero-somatic cross-sensitization (Aim 3). Throughout the three aims, we will use more than one animal models and apply in vivo and in vitro approaches for functional and mechanistic studies. Interestingly, our preliminary data show that TrkB.T1 is sexually dimorphic. We therefore will perform our experiments in both genders. We anticipate revealing the role of TrkB.T1 in glia-neuron interaction to provide insights in understanding the development of cross-organ sensitization and suggest therapeutic targets.