Alteration of cholinergic pathway in the enteric nervous system of a mouse model of Rett syndrome

Rett syndrome (RTT) is a severe neurological disorder caused by pathogenic variants in the MECP2 gene associated with gastrointestinal (GI) motility disorders strongly impacting patients' quality of life. Intestinal motility relies on coordinated neuronal activity in the enteric nervous system (ENS) where MECP2 is highly expressed. Here, we sought to determine whether cholinergic-mediated excitatory neuromuscular transmission could be affected in the ENS of a RTT mouse model and the functional consequences on intestinal motility.

In this study, 55 days-old wild type (WT) and Mecp2-knock-out (KO) mice were used. After confirming the  increase in GI transit time in Mecp2-KO mice in vivo, intestinal mechanical activity was assessed in vitro by evaluating contractile responses of small intestinal muscle to electrical field stimulation (EFS) of the ENS. The function of the cholinergic pathway was evaluated using gene expression, protein, and enzymatic activity quantifications of different factors of the acetylcholine (ACh) metabolism.

Our in vivo results confirm that the slowed GI transit previously identified in RTT patients and in the mouse model. In vitro, successive EFS lead to a progressive and rapid decrease in the amplitude of cholinergic contractions in Mecp2-KO mice contrary to WT mice. Levels of ACh are decreased in the ENS of Mecp2-KO mice (-47%, P-value<0.01) due to decreased activity of the synthesizing enzyme choline-acetyltransferase (-39%, P-value = 0.04). In addition to the deficit of ACh, we found that both nicotinic and muscarinic cholinergic receptors are also significantly impacted.

This is the first report of a major dysfunction of the enteric cholinergic pathway in a mouse model of RTT supporting its direct involvement in GI disorders in this disease. We propose that the rhythmic GI contractions occurring during fed conditions may cause a rapid depletion of the stock of enteric ACh that could largely depress the propulsive efficiency of, or even suppress, peristalsis during digestion.