From the AGA Journals

Do myenteric neurons replicate in small intestine?

View on the News

Contrary to controversy

The enteric nervous system (ENS) is composed of neurons and glia along the GI tract that are responsible for coordinating its motility, absorption, secretion, and other essential functions. While new neurons are formed during gut development, enteric neurogenesis in adult animals has been a subject of controversy but is of fundamental importance to understanding ENS biology and pathophysiology.

Allan M. Goldstein, MD is chief of pediatric surgery at Massachusetts General Hospital, professor of surgery at Harvard Medical School

Dr. Allan M. Goldstein

The debate was sparked by a study from Kulkarni and colleagues in 2017 that showed a surprising rate of neuronal turnover, with 88% of all myenteric neurons in the ileum replaced every 2 weeks. Given the complexity of enteric neuronal network formation, the concept of continual neuronal death and rebirth came as a surprise to the field. That finding is in sharp contrast to multiple studies that show essentially no enteric neurogenesis in healthy adult intestine, and to recent transcriptomic studies of the ENS that, while supporting a high turnover of intestinal epithelial cells, have found no significant cycling population of enteric neurons.

To settle the debate, Virtanen et al. replicated the Kulkarni study using the same methods, with the addition of EdU-based click chemistry, and found no replicating neurons. The bulk of evidence thus supports the concept that enteric neurons in the adult gut are a stable population that undergo minimal turnover. Enteric neuronal progenitors, however, are present in the adult gut and can undergo neurogenesis in response to injury. Further research is needed to identify the signals that activate that neurogenic response and to understand how it can be leveraged to treat neurointestinal diseases.

Allan M. Goldstein, MD, is chief of pediatric surgery at Massachusetts General Hospital, professor of surgery at Harvard Medical School, principal investigator in the Pediatric Surgery Research Laboratories, and codirector of the Massachusetts General Center for Neurointestinal Health, all in Boston. He has no relevant conflicts.


 

FROM CELLULAR AND MOLECULAR GASTROENTEROLOGY AND HEPATOLOGY

A new study contradicts controversial findings from a 2017 study that had suggested around two-thirds of myenteric neurons replicate within 1 week under normal conditions, which – if true – would have an impact on research into several GI diseases and pathologies.

Previous research had suggested that enteric nerve cells, which help control peristalsis throughout the digestive tract, do not replicate in the small intestine under normal conditions, with some limited potential for it observed only after injury, wrote Heikki Virtanen, MD, of the University of Helsinki (Finland), and colleagues. Their report is in Cellular and Molecular Gastroenterology and Hepatology. However, a study by Subhash Kulkarni, PhD, published in 2017, “challenged this dogma, suggesting that almost 70% of myenteric neurons are replaced within 1 week under normal physiological conditions.” These findings were reportedly considered controversial and presented “possibly far-reaching impact on future research,” Dr. Virtanen and colleagues explained.

According to the researchers, the difference between the controversial study findings and other research results may be partially explained by differences in methodology such as DNA labeling times, antigen retrieval methods, and analyzed portions of the small intestine. Dr. Virtanen and colleagues initiated the current study because no systematic evaluation of those potential confounding variables or attempt at independently replicating the findings had been undertaken.

For example, Dr. Virtanen and colleagues administered the nucleoside analogue 5-iodo-2’-deoxyuridine (IdU) in drinking water with the same concentration and labeling period, DNA denaturation steps, and antibodies as Dr. Kulkarni’s 2017 study had used. However, they also examined additional areas of the small intestine, employed paraffin embedding, performed parallel analysis using “click chemistry”-based detection of 5-ethynyl-2’-deoxyuridine (EdU), and more.

The gut’s epithelial cells turn over within 1 week “and serve as an internal positive control for DNA replication,” the researchers noted. In this study, IdU-positive enteric nerve cells were not revealed in microscopic analysis of immunohistochemically labeled small intestines of both cryosections and paraffin-embedded sections or in measurement of 300 ganglia in the small intestine. In contrast, the researchers wrote that the epithelium demonstrated label retention.

In their discussion section of their paper, Dr. Virtanen and colleagues wrote that while “proliferating epithelial cells were readily detectable” in the study, they were unable to detect enteric neuronal proliferation. Although noting that they could not identify reasons for the observations by Kulkarni and colleagues, Dr. Virtanen and colleagues continued to suspect unnoticed variables in the 2017 study affected its findings.

“The fact that the repeat of exactly the same experiment with the same reagents and methods did not reproduce the finding, not even partially, supports this interpretation and is further supported by the same conclusion using EdU-based click chemistry data and previous studies.”

The authors disclose no conflicts.

Next Article: