Specification of enteric neuron subtypes in the developing gut

Abstract

The enteric nervous system (ENS) is the biggest subdivision of the peripheral nervous system and harbors more neurons than the spinal cord. Its intricate network of ganglia is spanning the entire length of the gastrointestinal wall from where it controls peristalsis, secretion and blood flow. The ENS is able to comply with these functions due to its organization into a full reflex arc. In the adult ENS different subtypes of neurons are defined according to their morphology, electrophysiology, neurotransmitter expression and function. Yet, discrimination of subtypes according to marker gene expression is not reliable as different neurotransmitters are employed by several classes of neurons, and their expressions are not always conserved between species. Developmentally, the ENS arises from the neural crest, a transient cell population that gives rise to numerous cell types throughout the whole body. Neural crest cells find their way into the developing foregut from where they migrate, proliferate and colonize the complete length of the gut. In certain ENS disorders, though, neural crest cells fail to colonize the whole gut leading to a complete absence of neurons in parts of the colon. In other enteric disorders specific enteric neuron subtypes are lost or dysregulated.

The initial progression of neural crest migration and colonization as well as general aspects of neurogenesis and neuronal differentiation are well understood. How different enteric neuron subtypes are specified during development is, however, not resolved. This PhD thesis aspires to reduce this gap of knowledge. In paper I we demonstrate that the transcription factor Ascl1 is needed for general neurogenesis and gliogenesis in addition to regulating CALB1, TH and VIP neuron subtype specification in the developing ENS. In paper II we identified a vast array of transcription and signaling regulators expressed in the developing mouse and human ENS. We further show that the transcription factor Sox6 is indispensable for the development of gastric dopaminergic neurons and normal gastric motility. In paper III we used high-throughput sequencing approaches to redefine the classification of small intestine myenteric plexus neurons. We established lineage trajectories for the identified enteric neuron classes during embryonic development and demonstrate the transcriptional regulation of subtype conversion from NOS1+ ENC2 neurons to CALB1+ ENC6 neurons via expression of Pbx3.

Taken together, the data collected in this thesis describe new regulatory mechanisms governing subtype specification in the developing ENS. This knowledge might help in future endeavors for finding molecular mechanisms in enteric neuropathies in which specific subtypes of enteric neurons are affected. Moreover, it will hopefully guide new cell-based regenerative approaches towards the generation of specific enteric neurons in vitro and in vivo.