PDX1, a transcription factor essential for organ differentiation, regulates SERCA-dependent Ca2+ homeostasis in sensory neurons.

Pancreatic and duodenal homeobox 1 (PDX1) is a transcription factor required for the development and differentiation of the pancreas. Previous studies indicated that PDX1 expression was restricted to the gastrointestinal tract. Using a cre-dependent reporter, we observed PDX1-dependent expression of tdtomato (PDX1-tom) in a subpopulation of sensory nerves. Many of these PDX1-tom afferents expressed the neurofilament 200 protein and projected to the skin. Tdtomato-labeled terminals were associated with hair follicles in the form of longitudinal and circumferential lanceolate endings suggesting a role in tactile and proprioceptive perception. To begin to examine the functional significance of PDX1 in afferents, we used Fura-2 imaging to examine calcium (Ca2+) handling under naïve and nerve injury conditions. Neuropathic injury is associated with increased intracellular Ca2+ signaling that in part results from dysregulation of the sarco/endoplasmic reticulum calcium transport ATPase (SERCA). Here we demonstrate that under naïve conditions, PDX1 regulates expression of the SERCA2B isoform in sensory neurons. In response to infraorbital nerve injury, a significant reduction of PDX1 and SERCA2B expression and dysregulation of Ca2+ handling occurs in PDX1-tom trigeminal ganglia neurons. The identification of PDX1 expression in the somatosensory system and its regulation of SERCA2B and Ca2+ handling provide a new mechanism to explain pathological changes in primary afferents that may contribute to pain associated with nerve injury.

Neuronally expressed PDL1, not PD1, suppresses acute nociception.

PDL1 is a protein that induces immunosuppression by binding to PD1 expressed on immune cells. In line with historical studies, we found that membrane-bound PD1 expression was largely restricted to immune cells; PD1 was not detectable at either the mRNA or protein level in peripheral neurons using single neuron qPCR, immunolabeling and flow cytometry. However, we observed widespread expression of PDL1 in both sensory and sympathetic neurons that could have important implications for patients receiving immunotherapies targeting this pathway that include unexpected autonomic and sensory related effects. While signaling pathways downstream of PD1 are well established, little to no information is available regarding the intracellular signaling downstream of membrane-bound PDL1 (also known as reverse signaling). Here, we administered soluble PD1 to engage neuronally expressed PDL1 and found that PD1 significantly reduced nocifensive behaviors evoked by algogenic capsaicin. We used calcium imaging to examine the underlying neural mechanism of this reduction and found that exogenous PD1 diminished TRPV1-dependent calcium transients in dissociated sensory neurons. Furthermore, we observed a reduction in membrane expression of TRPV1 following administration of PD1. Exogenous PD1 had no effect on pain-related behaviors in sensory neuron specific PDL1 knockout mice. These data indicate that neuronal PDL1 activation is sufficient to modulate sensitivity to noxious stimuli and as such, may be an important homeostatic mechanism for regulating acute nociception.

Optogenetic inhibition of the colon epithelium reduces hypersensitivity in a mouse model of inflammatory bowel disease.

ABSTRACT: Visceral pain is a prevalent symptom of inflammatory bowel disease that can be difficult to treat. Pain and hypersensitivity are mediated by extrinsic primary afferent neurons (ExPANs) that innervate the colon. Recent studies indicate that the colon epithelium contributes to initiating ExPAN firing and nociceptive responses. Based on these findings, we hypothesized that the epithelium contributes to inflammation-induced hypersensitivity. A key prediction of this hypothesis is that inhibition of the epithelium would attenuate nociceptive signaling and inflammatory hypersensitivity. To test this hypothesis, the inhibitory yellow light-activated protein archaerhodopsin was targeted to the intestinal epithelium (villin-Arch) or the ExPANs (TRPV1-Arch) that innervate the colon. Visceral sensitivity was assessed by measuring the visceromotor response (VMR) to colorectal distension (CRD), with and without yellow light illumination of the colon lumen. Inhibition of the colon epithelium in healthy villin-Arch mice significantly diminished the CRD-induced VMR. Direct inhibition of ExPANs during CRD using TRPV1-Arch mice showed that ExPAN and epithelial inhibition were similarly effective in reducing the VMR to CRD. We then investigated the effect of epithelial and ExPAN inhibition in the dextran sulfate sodium model of inflammatory bowel disease. Inhibition of the colon epithelium significantly decreased dextran sulfate sodium-induced hypersensitivity and was comparable with the inhibition of ExPANs. Together, these results reveal the potential of targeting the colon epithelium for the treatment of pain.

Animal Models: Challenges and Opportunities to Determine Optimal Experimental Models of Pancreatitis and Pancreatic Cancer.

At the 2018 PancreasFest meeting, experts participating in basic research met to discuss the plethora of available animal models for studying exocrine pancreatic disease. In particular, the discussion focused on the challenges currently facing the field and potential solutions. That meeting culminated in this review, which describes the advantages and limitations of both common and infrequently used models of exocrine pancreatic disease, namely, pancreatitis and exocrine pancreatic cancer. The objective is to provide a comprehensive description of the available models but also to provide investigators with guidance in the application of these models to investigate both environmental and genetic contributions to exocrine pancreatic disease. The content covers both nongenic and genetically engineered models across multiple species (large and small). Recommendations for choosing the appropriate model as well as how to conduct and present results are provided.