What Is an L-Cell and How Do We Study the Secretory Mechanisms of the L-Cell?

Abstract

Synthetic glucagon-like peptide-1 (GLP-1) analogues are effective anti-obesity and anti-diabetes drugs. The beneficial actions of GLP-1 go far beyond insulin secretion and appetite, and include cardiovascular benefits and possibly also beneficial effects in neurodegenerative diseases. Considerable reserves of GLP-1 are stored in intestinal endocrine cells that potentially might be mobilized by pharmacological means to improve the body's metabolic state. In recognition of this, the interest in understanding basic L-cell physiology and the mechanisms controlling GLP-1 secretion, has increased considerably. With a view to home in on what an L-cell is, we here present an overview of available data on L-cell development, L-cell peptide expression profiles, peptide production and secretory patterns of L-cells from different parts of the gut. We conclude that L-cells differ markedly depending on their anatomical location, and that the traditional definition of L-cells as a homogeneous population of cells that only produce GLP-1, GLP-2, glicentin and oxyntomodulin is no longer tenable. We suggest to sub-classify L-cells based on their differential peptide contents as well as their differential expression of nutrient sensors, which ultimately determine the secretory responses to different stimuli. A second purpose of this review is to describe and discuss the most frequently used experimental models for functional L-cell studies, highlighting their benefits and limitations. We conclude that no experimental model is perfect and that a comprehensive understanding must be built on results from a combination of models.

Keywords: GLP-1 - glucagon-like peptide-1; L-cell; experimental - animal models; hormone secretion; in vitro model; peptide expression.

Figure 1

L-cells in non-human primate colon (Cynomolgus macaque). L-cells were identified based on proglucagon immunoreactivity (green). In upper panel two L-cells are shown. Lower panels shows a close-up of the L-cell in the upper panel (indicated by arrow). At the highest magnification, the individual GLP-1 granules are visible. Cell outlines are labelled by e-cadherin (red) and nuclei are stained with DAPI (grey). Cynomolgus necropsy and tissue collection was conducted at Charles River Laboratories, Montreal, Canada, according to regulations specified under the Protection of Animals Act by the Authority in the European Union (directive 2010/63/EU). Tissue samples were stained with antibodies against proglucagon (rabbit-anti-glucagon,Glu001, NovoNordisk A/S) and ecadherin (610182, BD Transduction Laboratories) detected with Cy3 and Cy5 conjugated secondary antibodies raised in donkey (Jackson ImmunoReserach) with DAPI nuclear contrast agent. The tissue section was imaged on a Leica TCS SP8 laser scanning confocal microscope with 10x/0.40 and 63x/1.30 objectives.

Figure 2

Products of proglucagon processing in L-cells and α-cells. GRPP, glicentin-related pancreatic polypeptide; and IP, intervening peptide.

Figure 3

Intestinal organoid. Left panel: Small intestine organoid generated from Glu-Venus mouse. L-cells are labelled by expression of Venus (shown in green) and have cone-like appearance similar to native L-cells in intact mucosa. Right panel: L-cells (green) in an organoid crypt. Luminal side and apical surface of cells is outlined by F-actin staining. Cell nuclei are labeled by DAPI (blue).

Figure 4

GLP-1 secretion from isolated perfused rat small intestine (lower half) in response to luminal infusion of taurodeoxycholic acid (TDCA) and vascular (inter-arterial) infusion of bombesin (BBS). Samples were collected at minute intervals, allowing the short lasting, but pronounced, GLP-1 response to BBS to be identified (A). Had a sampling frequency of 5-min intervals instead been used (B), the GLP-1 response would not have been noticed. Methods used are described in details elsewhere (204). Data are presented as means+SEM, n = 2. are presented as means+SEM, n = 2.