The enteric nervous system (ENS), often called the “second brain”, controls most activities in our gastrointestinal tract.
A new study reveals important new information about how this brain-in-the-gut develops and changes as we age.
Key Facts:
- The ENS contains two main types of nerve cells, from different origins.
- The balance between these cell types changes over our lifespan.
- This balance affects how well the gut works, especially in older age.
- Drugs that alter the balance may treat common gut disorders.
Source: eLife August 2023
The ENS Controls Most Activities of the Digestive System
The enteric nervous system (ENS) is a vast network of over 100 million nerve cells that resides in the gut wall.
It contains as many nerve cells as the entire spinal cord.
For this reason, the ENS is often referred to as our “second brain”.
The ENS regulates most aspects of the digestive process.
Its nerve cells connect with the muscle layers of the gut to control patterns of contraction that move food through the intestines.
ENS nerves also communicate with the lining cells of the gut and blood vessels in the wall to influence secretion, absorption, and blood flow. In addition, they interact with the gut immune system.
Given its critical roles, damage to the ENS can lead to many problems with gut function.
These include serious conditions like chronic intestinal pseudo-obstruction, gastroparesis, irritable bowel syndrome and slow transit constipation.
Understanding how the ENS normally develops and is maintained provides crucial insight into why these disorders occur.
For decades, scientists believed that the ENS forms exclusively during fetal development from neural crest cells.
These cells arise early in embryogenesis and normally generate the entire peripheral nervous system.
According to dogma, neural crest cells migrate into the developing gut where they form interconnected ganglia and mature into the neurons and glia of the future ENS.
After this embryonic period, the basic structure of the ENS was thought to remain stable throughout life.
However, observations in mice that many gut nerve cells in adults lack expected genetic markers of neural crest origin challenged this dogma.
This implied that the standard view of ENS development was incomplete.
Kulkarni et al. have now proven this is true by showing that the cellular composition of the ENS changes dramatically after birth.
The Mature ENS Contains Neural Crest-Derived and Mesoderm-Derived Nerve Cells
To re-examine the origins of enteric nerve cells, Kulkarni et al. used genetic fate mapping in mice.
This involves tagging cells that express a chosen gene during embryogenesis with a fluorescent marker that permanently labels them and their descendants.
First, they confirmed that roughly half of enteric neurons in adult mice failed to glow when neural crest lineage markers were used.
This suggested either silencing of the fluorescent tag in some neural crest-derived cells or the existence of enteric neurons that actually came from a different source.
To distinguish between these possibilities, they performed complementary fate mapping using genes active in the neural crest during early development.
Both approaches yielded closely matching fractions of unlabeled nerve cells, strongly arguing against silencing.
In embryonic development, cells other than neural crest that populate the gut originate from the mesoderm, one of the primary tissue layers.
Kulkarni et al. therefore repeated their fate mapping using genes active in the embryonic mesoderm.
Indeed, these experiments labeled the ~50% of enteric neurons that lacked neural crest markers.
Together, these definitive genetic lineage tracing studies proved that the mature ENS contains two distinct populations of nerve cells, which the authors designated neural crest-derived (NENs) and mesoderm-derived (MENs).
Their relative abundance indicates that NENs and MENs together make up the full complement of enteric neurons in mice.
The Two Enteric Nerve Cell Types Have Different Properties and Functions
To determine whether NENs and MENs represent transient states or stable cell types, Kulkarni et al. performed single cell RNA analysis on adult mouse gut tissues.
This technique examines gene expression patterns in thousands of individual cells simultaneously.
The results revealed that NENs and MENs have very different transcriptional profiles and therefore constitute bona fide distinct cell populations rather than transitional variants of a single lineage.
For example, NENs uniquely express Ret, a receptor for the signaling factor GDNF that is essential for neural crest-derived nerve cells to develop in the gut.
MENs instead express Met, the receptor for HGF, as well as various other mesoderm-specific genes.
The two lineages also differ functionally based on their production of classic neurotransmitters and signaling molecules.
Most NOS+ inhibitory motor neurons and ChAT+ stimulatory motor neurons belong to the NEN population whereas most sensory neurons originate from MENs.
Despite these differences, MENs and NENs share common neuron-specific genes.
However, MENs exhibit a distinctive complement of channels, receptors, neuropeptides and structural components likely related to unique functional capabilities.
More research is needed to elucidate the precise roles of the two cell types.
The Balance Between NENs and MENs Shifts From Neural Crest to Mostly MENs After Birth
The existence of two lineages implied a major change duringdevelopment since the ENS arises initially from the neural crest alone.
Kulkarni et al. confirmed this by showing that NENs make up over 95% of enteric neurons at birth and two weeks postnatally, while MENs begin appearing around 10 days after birth.
The MEN population then expands dramatically, accounting for ~30% of the total by 3 weeks of age.
This proportion remains steady through young adulthood but then climbs further so that MENs represent >95% of all enteric neurons in aged 17-month old mice.
These dynamics prove that the NEN population established prenatally from the neural crest is gradually replaced by MENs in postnatal life through young adulthood and into old age.
Overall neuron numbers remain constant indicating that NENs dying off are counterbalanced by MENs newly forming.
Growth Factor Signaling Controls the Balance of NENs and MENs
The researchers next explored the signals that might control this remarkable shift in enteric neuron composition after birth.
GDNF is well known to be essential for the development of neural crest-derived nerve cells through its receptor RET.
Correspondingly, Kulkarni et al. found that GDNF levels in the gut diminish substantially between 2 weeks and 1 month after birth, coinciding with the initiation of NEN loss.
When juvenile mice were given extra GDNF, NENs were preserved longer whereas untreated pups exhibited the normal decrease.
In contrast, levels of HGF, which acts through the MET receptor present on MENs, steadily rise over the same postnatal period.
Experimentally elevating HGF accelerated MEN accumulation in pups to adult-like proportions.
Thus, the relative availability of GDNF versus HGF establishes the balance between NENs and MENs as the ENS transitions from purely neural crest-derived to predominantly MENs during maturation.
Shifting the Balance Alters Gut Motility
To determine whether lineage composition affects ENS function, Kulkarni et al. analyzed intestinal movement at different ages.
Juveniles possessed faster gut transit than adults, corresponding to their greater NEN dominance.
Old mice displayed severely slowed transit, consistent with their MENs-skewed ENS.
Remarkably, just 10 days of GDNF treatment in aged animals restored their transit time to normal adult levels, coinciding with partial replacement of MENs by NENs.
Hence, the progressive shift toward MENs is directly linked to impaired gut motility, which can be reversed by rescuing the NEN population.
Parallel changes occurred in patients with chronic intestinal pseudo-obstruction, a severe motility disorder.
Their tissues showed reduced GDNF pathway markers and elevated MEN-associated genes, indicating imbalance favoring MENs.
Updated understanding of ENS biology
The long-accepted model of a stable neural crest-derived ENS is overturned.
Instead, a second nerve cell lineage derived from the mesoderm emerges after birth to gradually become predominant into adulthood and aging.
ENS structure and function are dynamic outcomes of the lineage balance, which is controlled by competing growth factor signals.
Imbalance likely contributes to motility disorders.
These revelations fundamentally update our understanding of ENS biology.
Key goals going forward are to elucidate the distinct functional roles of NENs versus MENs and to determine whether manipulating their balance can effectively treat common gastrointestinal diseases.
References
- Study: Age-associated changes in the lineage composition of the enteric nervous system regulate gut health and disease
- Authors: Subhash Kulkarni et al. (2023)
