Advances in enteric neurobiology – the ‘brain’ in the gut in health and disease

Written by Sharon Salt, Editor

Subhash Kulkarni is an Assistant Professor in Medicine at the Center of Neurogastroenterology at Johns Hopkins University (MD, USA). His lab studies the pathways by which the enteric nervous system (ENS) develops and is maintained in adults. Subhash is also interested in what kind of disorders and cellular mechanisms are associated with the development of different diseases that affect not only gut function but also CNS function.
In this interview, Subhash speaks to us about the mini-symposium he co-chaired at SfN Neuroscience (3–7 November 2018, San Diego, CA, USA) on the advances in enteric neurobiology. He also discusses the challenges involved in the field of enteric neurobiology and how these challenges could be overcome.


1. You’ve co-chaired a mini-symposium here at SfN Neuroscience on the advances in enteric neurobiology – the ‘brain’ in the gut in health and disease. Could you provide us with an overview about this?

The ENS comprises of the largest nervous system outside of the brain. In normal human adults, it has approximately 500 million neurons, which by some calculations is a larger population of neurons than the spinal cord. Unfortunately, it’s also one of the most understudied regions of neurosciences. There is a lot that the ENS regulates and governs. Apart from gut motility, it also regulates immunity, intestinal secretions and hormones. Beyond the gut, it has an effect on various metabolic functions, as well as CNS behaviors.

“…we wanted to understand how the ENS shapes and reshapes the immune function in the gut, but also how the immune function shapes the ENS back.”

The idea for this mini-symposium was to present the ENS as a growing and extremely significant part of neuroscience to the larger SfN community. Within this mini-symposium, we featured talks on recent advances that break set dogma and further our understanding of the ENS biology. ENS biology has traditionally been an extremely small field but in recent years, there have been many new investigators, such as myself, who have been trying to understand questions that have not been looked at in significant depth. We showcased the work of six of these investigators within the symposium and they spanned from developmental ENS biology in zebrafish; my own talk that looked at mechanisms of maintenance of ENS in adults; and a talk on enteric glial biology (which is an extremely understudied area – most glial scientists don’t know there are enteric glia). Then we had people who spoke about enterochromaffin (EC) cells, which are extremely important in the mechanosensing and chemosensing in the gut, and have a direct link to the brain – so the brain communicates with EC cells in the gut through the gut–brain axis.

Finally, we wanted to understand how the ENS shapes and reshapes the immune function in the gut, but also how the immune function shapes the ENS back. Additionally, we were interested in how this crosstalk between the ENS and immune cells, which is regulated by the microbiota, is deformed during aging and causes GI disorders as well as CNS disorders.

We showcased only six of the most recent advances, however, there are many more new investigators doing amazing work out there. This mini-symposium was a way for us to bring ENS research to the attention of the SfN community in an effort to educate them with what we have been doing in our little cocoon of ENS biology.

2. You have also presented a talk within this mini-symposium on adult enteric neurogenesis. Please could you elaborate on that and provide us with an understanding for the biology of the ENS and the larger neuroscience community?

There has been a huge amount of work that has been done in the developmental biology of the ENS, but very little work that could explain how the ENS is maintained once it’s formed.”

As I mentioned previously, the ENS contains a vast number of neurons and there are a diverse subtype of neurons present, depending upon the neurochemistry and behavior. The ENS is situated between two layers of muscles, which are constantly contracting and relaxing. In a normal gut, the amount of sheer force that the ENS experiences, would cause concussion of the brain. On top of that, there is a proximity to microbiota toxins, drugs, food sources, nutrients and a whole host of abiotic factors that are present in the gut. Regardless of this, the ENS is stable and remains absolutely robust – it contributes to normal healthy function. The question we had was to figure out how the ENS is maintained in adults. There has been a huge amount of work that has been done in the developmental biology of the ENS, but very little work that could explain how the ENS is maintained once it’s formed.

There have been controversies in the past with regards to whether there is ongoing neuronal loss or if there is neurogenesis. The field has settled that no neuronal loss and no neurogenesis are present, without having an explanation for how it is maintained. Are there special neuroprotective mechanisms for the ENS? This is something that is relatively unknown.

What this means for us is that it gives us a way by which we are able to explain for the first time how the ENS is maintained, and more importantly, explain the cellular mechanisms by which chronic GI diseases of motility exist and persist in the gut.”

When I started working on the ENS, I looked at it with a fresh slate. By using a whole host of updated tools and techniques, we figured out there is a significant amount of neuronal loss that is constantly happening in the gut – as much as 10%, if not more in certain regions. However, the accepted dogma suggested that there are no mechanisms to generate new neurons in healthy gut to replenish populations of these significant numbers of dying neurons. We figured out that there were newer ways of labeling precursor cells in the gut, of detecting precursor cell activity in the gut and newer markers that could delineate a neural stem cell compared to the accepted dogma.

When we started looking at the system through these updated tools and techniques, we could actually see that there was an active process of neurogenesis, which was very robust and something that was very rapid and comprehensive. So fast that it actually replenished almost all of the neurons in the gut in approximately 2.5 weeks, which is unprecedented in most of the parts of the CNS biology except for those regions that show evidence of ongoing neurogenesis, such as the olfactory bulb. So this actually places the ENS on par with the olfactory system.

What this means for us is that it gives us a way by which we are able to explain for the first time how the ENS is maintained, and more importantly, explain the cellular mechanisms by which chronic GI diseases of motility exist and persist in the gut. In the construct of the constantly renewing ENS, we are now placing the blame on the neural stem cells and aberrations in the behavior of neural stem cells than on neurons themselves. So this leads us to a novel way of understanding and curing persistent gut disorders. There is a lot of interesting research it has led to, such as the interaction with immune mechanisms and the microbiome. This is something that I’m looking forward to in some of our own work.

3. What are the key challenges to be overcome in this field of enteric neurobiology?

Since we don’t have a good idea of the basic biology of the ENS, this has hampered our understanding of clinical presentations of a lot of diseases because we have only been looking at either loss of neurons or loss of fibers, and nothing beyond this.”

The key challenges that need to be overcome are that we don’t have a very good idea about defining the structure of the ENS. People have looked at the structure of the developing gut in mathematical models in order to study how the developing gut is innervated. However, once the gut is formed and is stable, what exactly is the architecture of the ENS is something that we know little about. This is very crucial because unless we know what the basal structure of the gut is, we won’t know what the aberration looks like. To understand disease, we have to understand what is healthy. Hence, there is a significant need to understand the anatomical structure of the ENS at a microscale level.

The second challenge is to understand the heterogeneity of the different cells that are participating normally in these mechanisms.

The third and most important thing is understanding circuits in the gut. It has been accepted dogma that ENS circuits are simplistic, where there is an input neuron (sensory neuron), a CPU neuron that processes this input (interneuron) and output or motor neurons that are effector neurons. This is the understanding that we have viewed ENS circuits in general. However, given the significant heterogeneity of difference types of ENS cells, the complex circuit of the ENS is yet to be elucidated. We really don’t know what the different circuits are and how they contribute to normal functioning of the gut.

Since we don’t have a good idea of the basic biology of the ENS, this has hampered our understanding of clinical presentations of a lot of diseases because we have only been looking at either loss of neurons or loss of fibers, and nothing beyond this. A better understanding of this will lead to a better understanding of clinical pathology of many of these disorders.

4. How could these challenges be overcome?

Single-cell sequencing has great potential to help us understand the underlying heterogeneity in cellular structures.”

These challenges can be overcome by a set of things that people have already been looking at, at least in the CNS biology field. Single-cell sequencing has great potential to help us understand the underlying heterogeneity in cellular structures. There is an urgent need to look at the adult gut and the developing gut, including a normal model and disease models, as well as pathogenic samples in humans, in order to advance anatomical tracing methods and to advance microscopy methods.

There have been novel viral tools that people have been looking into, such as cardiac innervations and CNS innervations. I know of a few labs that have been applying this to ENS biology with the hope to understand the circuits in much better detail and to understand how these circuits relate to different types of function that the ENS participates in.

I think that the future is bright, as there are new techniques and people coming into the field. The bandwidths of limitations in the past are no longer there so much, so I’m hopeful for the future.

5. Finally, what aspect of the field are you looking forward to most in the next few years?

Primarily single-cell sequencing; previously, we used to think there were only three types of neurons in the gut, and now preliminary data has suggested that there could be at least eight and at most 20 different types of neurons. This opens a whole Pandora’s box of understanding how this amazing system is regulating itself, and what kind of circuits are built. I really look forward to quite a lot of data that’s going to come out fairly soon.

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Disclaimer
The opinions expressed in this interview are those of the interviewee and do not necessarily reflect the views of Neuro Central or Future Science Group.