An insight into the topology of the Enteric Nervous System

The Enteric Nervous system (ENS) is one of the largest subdivisions of the Peripheral Nervous System. This remarkable system is embedded between sheets of smooth muscle cells of the intestinal tissue. It is also called as the “second brain” as it consists of millions of neurons that coalesce to form its vast network. The ENS originates from neural crest-derived progenitors that traverse through different spatio-temporal environments, expanding in number, colonizing the gut tissue and eventually generating a diverse array of neuronal and glial subtypes. The ENS is indispensable for key functions of gut physiology such as peristalsis, secretion of enzymes and absorption of food. It is a tremendously interactive system maintaining a long distance connection with the brain and communicating with the inside local immune, epithelial and vascular systems of the intestine. Additionally, it interacts with the outside world of the microbiome that resides within the gut lumen.

ENS development and structure

The ENS is both intriguing and important to study for various reasons. It is initiated from a small pool of progenitors and develops within an extensively expanding tissue (~8 metres in humans). Despite a random arrangement of diverse neuronal subtypes into interconnecting ganglia, reproducible patterns of secretory and motor function are observed. Absence and lack of ENS function can cause several gastrointestinal disorders, such as Hirschsprung disease and others of unknown aetiology. Recent work highlights the plausible interaction of the ENS with the microbiome that may influence the course of neurodegenerative diseases such as Parkinson’s and Alzheimer’s disease.

For a system that is tremendously interactive and extensively expanding, it is imperative to gain insight into how the ENS develops, organizes and functions. This would help us to understand diseases of the gut that may arise during development or later in adulthood. However, it is a daunting task to study this system. Anatomically, it develops between the muscle layers of the gut tissue and complicates several experimental approaches. We ingest a variety of food and drink every day, which affect our microbiome and their responses challenge the ENS making it a formidable system to study. The environment we face daily (stress) can also cause an imbalance to the system, thereby affecting its physiology and posing an additional challenge to understanding ENS function in our daily lives.

ENS cellular blueprint and structure

Our study aimed to unravel any basic principles that formed the basis of its cellular blueprint and how it contributes to function1. We decided to uncover the concealed mystery of this system by using single-cell approaches. We tracked the static behaviour of the progeny of individual ENS progenitors over time employing a multi-colour mosaic technique. We combined in vivo and ex-vivo approaches to capture the cellular and molecular properties of these cells.We introduced mosaic mutagenesis of the Ret gene (receptor tyrosine kinase), an established player in ENS development, to disrupt the ENS during development and understand its effect on ENS composition. The Ret molecule is important for the survival, migration and differentiation of enteric neurons.However, our study revealed yet another important role of role in neuronal commitment of ENS progenitors and how it can maintain the balance of ENS lineage generation.

Despite this study being lengthy and arduous, resembling the long gut tissue, we were able to gain insight into the organizational structure of the ENS. Close spatial restriction of clonal progeny suggested strong family ties. Overlapping clonal progeny were arranged into columns defining a columnar structure in the 3D space of the gut tissue. Our single-cell transcriptomic analysis helped us to gain insight into the generation of neuronal and glial lineages and their commitment to subtype fate. Neurons related to each other showed co-ordinate activity upon electrical stimulation, highlighting the requirement of lineage to function in the small intestine. Together, our study revealed the importance of lineage relationships as a pre-requisite for the spatial organization and the function of the ENS.

Conclusion

Perhaps misprints in the framework are the reason for gastrointestinal diseases with unknown aetiology. Now that we have a better understanding of how the ENS of the small intestine assembles and underpins its function, we can begin to probe and monitor the changes to this system at different stages of development. We understand that this architecture of the ENS helps its components to work and face the challenges of the system together as a family. Knowledge of its structure can reveal the systematic steps undertaken towards making specific choices and decisions; dissects the complexity underlying the formation of networks and builds the foundation on which the fundamental units function within the confines of the system.

Figure legend: Two non-overlapping neuronal subpopulations of the ENS highlight the extensive network of this system in the mouse gut tissue. Courtesy: Dr. Reena Lasrado.

Reference

Lasrado et al., Lineage-dependent spatial and functional organization of the enteric nervous system. Science 356 (2017) 722-726.