Five Subtypes of Colon Sensory Colon Neurons Identified
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The gut–brain connection is a crucial component of healthy digestive function. Conditions of the gastrointestinal (GI) tract like inflammatory bowel disease (IBD) can be painful and debilitating for patients. However, little is known about the sensory (or afferent) neurons that innervate the gut and respond to different stimuli, limiting our ability to understand their role in health and disease.
Professor David Ginty’s lab at Harvard Medical School previously developed genetic tools in mice that enable the labeling of sensory neuronal subtypes, helping neuroscientists explore their structure, organization and function, particularly in the skin.
In a new study, Dr. Rachel Wolfson, research fellow at Harvard Medical School and a gastroenterology fellow at Massachusetts General Hospital, repurposed these genetic tools to study colon neurons.
The research – published in Cell – describes five distinct subtypes of sensory neurons present in the colon that are also found in the skin. To analyze the function of these subsets, Wolfson and colleagues used a balloon to stretch the colon and recorded the resulting neuronal activity. Two types of neurons were found to respond to gentle forces, while two other types responded to intense forces such as extreme stretching; activation of the latter neurons resulted in mice demonstrating a pain response, which was alleviated when Wolfson removed the neurons.
She also conducted experiments triggering inflammation in the colon, observing that the pain-sensing neuron subtypes became increasingly reactive.
Technology Networks interviewed Wolfson to learn more about the study and how it could enhance the development of GI-targeting therapies.
MC: The connection between the gut and brain is a continuously growing area of science. Why have scientists found it challenging to study the neurons of the GI tract?
Rachel Wolfson (RW): There certainly has been some focus on the GI tract innervation previously, but one major challenge has been having mouse genetic tools that are specific for labeling the different subsets of neurons that innervate the GI tract.
The GI tract has extrinsic sensory neurons (from both the dorsal root ganglia – DRG – and nodose ganglia), extrinsic autonomic neurons (the sympathetic and parasympathetic nervous systems) and its own intrinsic neurons (the enteric nervous system). Many mouse lines label multiple of these subsets, so it has been challenging to find tools that can label each subset individually to begin to dissect their functions. We used multiple mouse models, many of which have been recently created, to find ways to specifically label the neurons we are interested in (the DRG sensory neurons) while preventing the labeling of these other populations.
The sensory innervation of the colon is important for mediating many internal sensations from the GI tract, in particular abdominal pain, and is implicated in multiple GI diseases, including IBD and GI infections.
MC: Can you talk about how you repurposed the mouse models developed in the Ginty lab to study colon neurons?
RW: I used mouse lines that the Ginty lab had created to label each different subset but intersected them with other lines that allowed me to exclude labeling of the other sources of innervation to the GI tract. This was essential for making sure that I was specifically labeling and manipulating the DRG subtypes without labeling all the other populations of neurons that innervate the GI tract.
MC: You found that five different subtypes of sensory neurons present in the skin are also found in the colon. Were you surprised?
RW: We also found three skin-innervating subtypes that do not innervate the GI tract. This was in fact more surprising to me and implies that there is some tissue specificity of the DRG neuron subtypes.
MC: Neuronal subtypes that responded to inflammation specifically could help us understand and treat pain in conditions such as IBD. Can you tell us more about this finding and whether previous research ever explored IBD from this angle?
RW: Yes, previous research has found that there is a behavioral and physiologic hypersensitivity to colon distension in the setting of inflammation, but the DRG neuron subtype that was responsible for mediating this was unknown.
We found that one of our identified DRG subtypes is necessary for mediating inflammation-induced mechanical hypersensitivity, which implies a potential and exciting drug target to treat abdominal pain in patients with IBD.
MC: The research is conducted in mice – if results are replicated in humans, how do you hope this data will be utilized in the short and long term?
RW: If indeed the functions of these DRG subtypes are conserved in humans, future work can start to delve into possible drug targets within each subtype. This could be helpful for treating various GI conditions, in particular abdominal pain that is triggered by different etiologies. For example, one could imagine finding a drug target within the Bmpr1b+ subtype that is necessary for inflammation-induced mechanical hypersensitivity to help treat patients with abdominal pain induced by IBD.
MC: Can you talk about your next steps for advancing this work?
RW: We plan to delve into the structural and gene expression differences between the colon and skin innervating DRG subtypes, which we believe will help us to better understand their functions and how they respond to various stimuli.
MC: Are there any limitations to the study that you think are important to highlight?
RW: While we identified five subtypes that innervate the colon, one of the subtypes was labeled using a mouse line that is sparse, so we were not able to do any functional experiments with this line and don’t know how it responds to colon distension. This will need to be addressed in future studies. Another limitation is that we focus on how these subtypes respond to colon distension, but future work is needed to understand how they respond to other stimuli.
Dr. Rachel Wolfson, research fellow at Harvard Medical School and a gastroenterology fellow at Massachusetts General Hospital, was speaking to Molly Campbell, Senior Science Writer for Technology Networks.