Evidence for a Neogenic Niche at the Periphery of Pancreatic Islets

Abstract

We recently discovered a novel subset of beta cells that resemble immature beta cells during pancreas development. We named these "virgin" beta cells as they do not stem from existing mature beta cells. Virgin beta cells are found exclusively at the islet periphery in areas that we therefore designated as the "neogenic niche." As beta cells are our only source of insulin, their loss leads to diabetes. Islets also contain glucagon-producing alpha cells and somatostatin-producing delta cells, that are important for glucose homeostasis and form a mantle surrounding the beta cell core. This 3D architecture is important and determines access to blood flow and innervation. We propose that the distinctive islet architecture may also play an important, but hitherto unappreciated role in generation of new endocrine cells, including beta cells. We discuss several predictions to further test the contribution of the neogenic niche to beta cell regeneration.

Keywords: Urocortin 3; alpha cells; lineage tracing; pancreas islets; stem cells; transdifferentiation; virgin beta cells.

Figure 1:

Two opposing scenarios for the origin of new beta cells and the experimental outcomes they generate. First, beta cells are labeled using -recombinase (Box 1). In this particular example, Cre labels only 30% of beta (β) cells (red beta cells), while the remaining 70% are not labeled (grey beta cells). Delta cells (δ, blue) and alpha cells (α, yellow) are also not labeled. Next, beta cells are allowed to turnover. In scenario A, beta cells derive from a non-beta cell precursor and the existing pool of beta cells slowly disappears, including the lineage-labeled beta cells. They are replaced by unlabeled beta cells. In scenario B, all beta cells, including lineage-labeled ones, contribute equally to new beta cells. Thus the frequency of lineage labeling does not change. Because of the inefficient lineage-labeling at the start of the experiment, a minor contribution of a non-beta cell source (green beta cells) may be overlooked.

Figure 2:

Embryonic development of the three main endocrine lineages in the pancreatic islet. Starting from a common endocrine progenitor, endocrine cells first split into an alpha and a beta/delta lineage, which then further differentiates into beta and delta cells. Well-known transcription factors and other markers are indicated under the cells in which they are expressed. Those indicated in bold letters are selective to certain endocrine lineages. In the adult islet, endocrine cells interactions are indicated by green arrows (activation) and red blocking arrows (inhibition).

Figure 3:

Neogenic niche beta cells lack the cell surface glucose transporter GLUT2. Pancreatic islet cells were stained for DNA (blue), UCN3 (green), insulin (red) and GLUT2 (white). Examples of UCN3-negative virgin beta cells (*) and beta cells expressing low levels of UCN3 (arrows) are indicated. All UCN3-negative cells lack cell- surface GLUT2, while some UCN3-low cells started expressing GLUT2. For two areas, two-color panels are shown to side and below the main panel.

Figure 4:

Alpha cells contribute to the niche. An islet with lineage-labeled alpha cells (green), stained for insulin (red) and UCN3 (white) features two transdifferentiated cells indicated by white boxes (A). One of these has not yet acquired UCN3 expression (top, arrow), while the other has further matured and expresses UCN3 (bottom, arrow). Reproduced with permission^[55].

Figure 5:

The neogenic niche is a physical space within the islet where endocrine cells change fate. (A) An islet with all cells labeled with a membrane-bound red fluorescent protein, except in those cells that saw Gcg-Cre activity marking alpha cells. These cells exclusively and permanently express a membrane-bound green fluorescent protein. The islet was stained for UCN3 (mature beta cells, blue) and glucagon (alpha cells, white). (B) Three different scenarios for the generation of lineage-alpha beta cells and the predicted distribution of the virgin beta cells across the islet surface. (C) Actual distribution of lineage-alpha beta cells across the islet surface supporting the idea of a local neogenic niche. (D) An islet with all cells labeled with a membrane-bound red fluorescent protein, unless Ucn3-Cre activity (mature beta cells) results in the exclusive and permanent expression of a membrane-bound green fluorescent protein. The islet was stained for UCN3 (mature beta cells, blue) and glucagon (alpha cells, white). Reproduced with permission^[55].

Figure 6:

Endocrine cell transdifferentiation in the neogenic niche. Depiction of two scenarios that can result in the presence of insulin positive, UCN3-negative beta cells in an islet. In the first, alpha cells lose glucagon expression and then acquire insulin expression, prior to UCN3 expression. In the other, mature beta cells sequentially lose expression of UCN3 and then insulin prior to acquisition of glucagon expression. Both scenarios occur within the neogenic niche.