550 Diabetes Volume 69, April 2020 The Local Paracrine Actions of the Pancreatic a-Cell Rayner Rodriguez-Diaz,1,2 Alejandro Tamayo,1,2 Manami Hara,3 and Alejandro Caicedo1,2,4,5 Diabetes 2020;69:550–558 | https://doi.org/10.2337/dbi19-0002 Secretion of glucagon from the pancreatic a-cells is of the islet is being revisited, with studies showing that conventionally seen as the first and most important there are vast species differences in islet cytoarchitecture defense against hypoglycemia. Recent findings, how- (2–6). A remarkable feature in all vertebrate species is that, ever, show that a-cell signals stimulate insulin secre- despite their variety, islets always contain a- and b-cells. In tion from the neighboring b-cell. This article focuses on our research group, we always wonder why it makes evo- these seemingly counterintuitive local actions of a-cells lutionary sense to have these two cells packed together. The and describes how they impact islet biology and glucose most common explanation is that it is important for b-cells metabolism. It is mostly based on studies published in to inhibit a-cell activity so that release of hormones with a the last decade on the physiology of -cells in human antagonistic effects does not overlap (7). Using findings on islets and incorporates results from rodents where ap- human a-cells published over the last decade, this article propriate. As this and the accompanying articles show, will try to show that this is probably not the whole story. the emerging picture of a-cell function is one of in- creased complexity that needs to be considered when Most, if Not All, Islet Cells Release Paracrine Signals developing new therapies aimed at promoting islet func- A common feature of islet endocrine cells is that they tion in the context of diabetes. SYMPOSIUM release molecules that serve as paracrine signals. In many cases, even the hormones they secrete have local effects. This paracrine function may not be exclusive to endocrine PARACRINOLOGICAL CONSIDERATIONS cells. Indeed, secretions from islet pericytes and local DIABETES a- and b-Cells: From This Day Forward, for Better, for macrophages have been shown to exert trophic effects Worse, for Richer, for Poorer, in Sickness and in Health, on b-cells (8–10). The effects of paracrine signals are being to Love and to Cherish investigated intensely because they will help us understand Hormone secretion from the pancreatic islet is central to how hormone secretion is orchestrated in the islet. The the regulation of glucose metabolism. Responsible for paracrine signaling pathways could in principle also be most of the islet’s hormonal output are the insulin- targeted for therapeutic intervention. Before we delve secreting b-cells and the glucagon-secreting a cells. Be- deeper into the subject, however, we need some termi- cause these hormones have opposite effects on plasma nological and conceptual definitions. glucose levels, a- and b-cells have long been considered In the pancreatic islet, paracrine signals can influence functional antagonists. The islet also contains other en- other cells within the same islet by diffusing through the docrine cells that secrete hormones, but their actions may interstitial space or circulating in local blood vessels. In be more local. Recent genome-wide association studies autocrine signaling, by contrast, a cell secretes a signaling point out that variants of genes expressed in the islet are molecule that binds to receptors on the same cell or other associated with diabetes. This, combined with the in- cells of the same cell type (e.g., b-cell to b-cell communi- creased availability of human islets for research (1), has cation). The same signaling molecule can be used in revitalized the field of diabetes research and led to a re- different contexts such as endocrine, paracrine, autocrine, naissance in islet biology studies. Even the basic anatomy or synaptic signaling. What then are the criteria to consider 1Department of Medicine, University of Miami Miller School of Medicine, Miami, FL Corresponding author: Rayner Rodriguez-Diaz, [email protected], or 2Diabetes Research Institute, University of Miami Miller School of Medicine, Miami, Alejandro Caicedo, [email protected] FL Received 15 May 2019 and accepted 16 December 2019 3Department of Medicine, University of Chicago, Chicago, IL © 2019 by the American Diabetes Association. Readers may use this article as 4Department of Physiology and Biophysics, University of Miami Miller School of long as the work is properly cited, the use is educational and not for profit, and the Medicine, Miami, FL work is not altered. More information is available at https://www.diabetesjournals 5Program in Neuroscience, University of Miami Miller School of Medicine, Miami, .org/content/license. FL diabetes.diabetesjournals.org Rodriguez-Diaz and Associates 551 that a molecule is a paracrine signal? By adopting standards a series of metabolic reactions to convert glucose stimu- used for decades in the neurosciences (11), we propose that lation into changes in membrane potential: glucose trans- to be a bona fide paracrine signal 1) the molecule must be port into the cells via low-affinity glucose transporters, present or produced in the islet cell, 2) the molecule must be glucose phosphorylation by the enzyme glucokinase, and released in response to cell stimulation, and 3)specific the subsequent metabolism of glucose that increases the receptors for the molecule must be present on target intracellular ATP-to-ADP ratio. This elevated ratio closes cells in the islet. These criteria seem obvious, but for KATP channels, depolarizes the membrane, and opens 1 many signaling candidates they have not been met. voltage-gated Ca2 channels, triggering hormone secretion A first step is to detect the signaling molecules in islet (15). The complex stimulus-secretion coupling in islet cells cells, but often it is easier to show that the cells express not only ensures that glucose metabolism is coupled to components of the synthetic machinery producing the membrane electrical activity but also provides multiple molecule. Because many of the signaling molecules also points where external signals can modulate hormone participate in general cellular metabolism (e.g., GABA or secretion. Because islet endocrine cells display complex glutamate), demonstrating the presence of a candidate electrical activity with bursts of action potentials, many molecule or its biosynthetic pathway may not be sufficient. signals impinging on islet endocrine cells can change the Perhaps the most rigorous approach is to show that secre- secretory behavior of the cells by altering membrane tory granules or vesicles contain the molecule, which electrical activity. Function of endocrine cells can also be requires challenging methodologies such as postembedding regulated at the level of the secretory pathway. Many immunoelectron microscopy. A cell’s secretory phenotype processes related to granule exocytosis are dependent on 1 can also be defined by its expression of vesicular trans- second messenger cascades (e.g., cAMP, Ca2 )thatcan porters for transmitters. A cell expressing the vesicular be activated by ligands binding to membrane receptors. acetylcholine transporter (vAChT) can thus be considered Paracrine or autocrine signals can therefore target recep- cholinergic (12,13). This approach, however, may overlook tors to manipulate different intracellular signaling cas- cells that use different mechanisms to secrete paracrine cades and thus regulate islet hormone secretion. molecules (e.g., ATP release through pannexins) (14). A second, more challenging criterion is to demonstrate The Particular Case of the Human Islet that the paracrine signal is released from the islet cell. This a- and d-cells are not segregated to the periphery in human is difficult because the molecule is very likely secreted at islets as they are in rodent islets (4–6). Most b-cells in the low concentrations, degraded by enzymes, or taken up by human islet are close to a-cells, d-cells, or both. The membrane transporters. Another challenge is to selectively association of b-cells with a-cells in human islets is so stimulate the islet cell in question and to locate the source close that, even after dispersionofisletsintosinglecells, for the paracrine signal in the intact islet. This may require most b-cells remain attached to an a-cell (16). This has measuring molecules secreted from single, dispersed islet profound implications for islet function. The close ap- cells, at a great loss of detectability. Here, islet cells of position allows endocrine cells to interact using mem- a given type could be enriched using cell sorting, with such brane-bound molecules, an interaction that promotes protocols being recently established for human islets. For function and survival (17). Because of these close con- several candidate molecules, these technical limitations tacts, electrical coupling between a-andb-cells is also have prevented demonstrating that they are bona fide conceivable (18). Although this remains to be estab- paracrine signals in the pancreatic islet. lished in the human islet, this direct communication A third criterion is that the target cell must express could contribute to control the biosynthesis and release receptors for the signaling molecule. The problem here is of secretory products, as well as to cell survival (19). that many receptor genes are expressed at very low levels Because many signaling molecules are rapidly degraded and may not be detected, and there are few reliable anti- in the interstitial space and in the bloodstream, the bodies for immunohistochemical visualization. A common proximity of b-cells with a-cells also promotes paracrine approach is to stimulate islets pharmacologically and interactions. 1 examine the effects on hormone release, Ca2 responses, or There are additional players that could play a role in changes in membrane potential. Most of the candidate a-cell to b-cell communication. Molecules released by paracrine signals have been proposed based on pharmaco- a-cells can be degraded by enzymes or bound by molecules logical stimulation of receptors on islet cells.