A Toolbox for Observing and Modulating the Gut-Brain Axis

An estimated 10% of people worldwide have an enteric nervous system (ENS) related illness including irritable bowel syndrome (IBS), diabetes, colorectal cancer, fecal incontinence, and chronic constipation or diarrhea. Current drug treatments have severe side effects and often do not adequately address symptoms; a new approach is needed. ENS stimulation is a promising therapy for these patients, but a major limitation to this approach is our lack of knowledge. The human ENS is comprised of 5 million neurons and drives the digestive system, but its normal function and connections to the central nervous system (CNS) remain poorly understood. One of the major canonical signaling pathways between the ENS and the CNS is the vagus nerve, but the neural circuits involved are still under investigation. Better understanding of these circuits would provide a potential method of treatment for ENS related illness, with neurostimulation serving as an alternative to pharmaceutical treatments. Herein I describe a project which addresses these needs via development of new imaging tools to better understand the gut-brain axis, as well as demonstrating its utility as a target for treatment of gastrointestinal (GI) illness, specifically cancer-associated cachexia. Leaders in enteric neuroscience note that the continued inconsistencies in GI electrotherapies are driven by a fundamental lack of understanding of gut innervation and circuitry. New tools to directly observe colonic innervation and neuronal response, as well as a map of the whole peripheral nervous system, will reveal crucial targets for stimulation and enable more efficient targeting selection for neurostimulation or other local interventions, which will reduce off target effects and improve efficacy. To address these issues, I have developed an intravital window for direct imaging of the colon, enabling observation of colonic ENS response to stimulation in vivo for the first time. Additionally, I have developed an embryonic window, allowing visualization of embryonic GI development from E9.5 through birth. Finally, I have generated a mouse peripheral nerve map based on Diffusion Tensor Magnetic Resonance Imaging (DT MRI). Using novel scan parameters and post-processing algorithms, I identified nerve fibers throughout the body and generated quantitative tractography which specifically highlights GI innervation via the vagus nerve. Cachexia is a multi-systemic syndrome which produces weight loss, muscle atrophy, adipose wasting, fatigue, and anorexia. Affecting an estimated 1% of the global population and up to 80% of all cancer patients, cachexia is fatal in roughly 30% of cases and is incurable. Cancer-associated cachexia (CAC) is particularly devastating as in addition to resulting in decreased quality of life, CAC reduces tolerance and efficacy of cancer treatments and higher overall mortality. As many as half of all cancer deaths are attributed to CAC. There are currently no clinically meaningful treatments for CAC, despite attempts to employ dietary support, physical therapy, anti-inflammatory medication, appetite stimulants, and other supportive therapies. Herein I describe potential therapeutic approach for treatment of CAC via vagal perturbation – either by vagotomy or ultra-low frequency vagal block with an implanted stimulator. This intervention significantly attenuates weight loss, skeletal muscle atrophy, anorexia, urea cycle dysregulation, and circulating inflammatory cytokine elevation. Most importantly, it increases survival time in mice injected with tumor cells, suggesting this could be a clinically meaningful approach for treatment of CAC.