An Enteroendocrine Cell – Enteric Glia Connection Revealed by 3D Electron Microscopy

Abstract

The enteroendocrine cell is the cornerstone of gastrointestinal chemosensation. In the intestine and colon, this cell is stimulated by nutrients, tastants that elicit the perception of flavor, and bacterial by-products; and in response, the cell secretes hormones like cholecystokinin and peptide YY – both potent regulators of appetite. The development of transgenic mice with enteroendocrine cells expressing green fluorescent protein has allowed for the elucidation of the apical nutrient sensing mechanisms of the cell. However, the basal secretory aspects of the enteroendocrine cell remain largely unexplored, particularly because a complete account of the enteroendocrine cell ultrastructure does not exist. Today, the fine ultrastructure of a specific cell can be revealed in the third dimension thanks to the invention of serial block face scanning electron microscopy (SBEM). Here, we bridged confocal microscopy with SBEM to identify the enteroendocrine cell of the mouse and study its ultrastructure in the third dimension. The results demonstrated that 73.5% of the peptide-secreting vesicles in the enteroendocrine cell are contained within an axon-like basal process. We called this process a neuropod. This neuropod contains neurofilaments, which are typical structural proteins of axons. Surprisingly, the SBEM data also demonstrated that the enteroendocrine cell neuropod is escorted by enteric glia – the cells that nurture enteric neurons. We extended these structural findings into an in vitro intestinal organoid system, in which the addition of glial derived neurotrophic factors enhanced the development of neuropods in enteroendocrine cells. These findings open a new avenue of exploration in gastrointestinal chemosensation by unveiling an unforeseen physical relationship between enteric glia and enteroendocrine cells.

Correlative Confocal and 3D Electron Microscopy of a Specific Sensory Cell

“…The method allowed us to document the entire ultrastructure of an enteroendocrine cell and its neuropod. Within neuropods, we found structural features of neuronal axons, and outside neuropods, we found a physical relationship to enteric glia 8 . Indeed, neuropods contain about 70% of all secretory vesicles suggesting an essential role in the secretory function of these cells.…”

“…Indeed, neuropods contain about 70% of all secretory vesicles suggesting an essential role in the secretory function of these cells. Building on the structural data, more recently we found that through these neuropods, enteroendocrine cells and neurons innervating the gut form a neuroepithelial circuit, similar to that of taste cells in the tongue 8,9 .…”

“…Segmentation for the videos and figures presented in Bohórquez et al 2014 8 was done manually and took about 500 hr of labor.…”

Targeting the gut to treat obesity and its metabolic consequences: view from the Chair

“…In some cases, however, these cells have a basal extension that extends in close apposition to neighboring epithelial cells, nerves and the processes of subepithelial enteric glial cells. 21,22 These basal processes have been termed 'neuropods' because they possess the proteins that are involved in vesicular release at synapses. 21,22 This anatomical arrangement suggests a tighter release-effector cell relationship than was previously appreciated, which allows for far more precise temporal and spatial signaling for gut peptides and their target cells.…”

“…21,22 These basal processes have been termed 'neuropods' because they possess the proteins that are involved in vesicular release at synapses. 21,22 This anatomical arrangement suggests a tighter release-effector cell relationship than was previously appreciated, which allows for far more precise temporal and spatial signaling for gut peptides and their target cells. In the case of enteric glia, this seems to be for the maintenance of epithelial barrier function.…”