Summary of CMS Seminar Club presentation on Friday, July 29, 2022.
Title: Diabetes and Pancreatic Islet Crosstalk
Speaker: Prof. Mark O. Huising, PhD, Neurobiology, Physiology & Behavior, College of Biological Sciences and Physiology & Membrane Biology, School of Medicine, University of California, Davis, USA
On Friday, July 29, Prof. Huising gave a presentation at Fujita Health University. He showed us how cells in pancreatic islets interplay to regulate blood glucose levels and how their malfunctioning contributes to metabolic disorders like diabetes.
Recording: For members of Fujita University, a recording of the meeting (without the discussion part) will be available at our Manabi system. Unfortunately, we cannot open the recording for a wider audience.
There were 24 participants. Partly because I know Prof. Huising from our previous scientific lives, the atmosphere was very relaxed. I am very grateful that he was willing to give the presentation during what for him was the middle of the night. The presentation was excellent and told us about how different cells in the pancreatic islets interact to regulate blood glucose levels. At the meeting, I could immediately understand and appreciate the first part of his presentation, which was a general introduction to diabetes and the (simplified) textbook models on pancreatic islet functions. The second part of the presentation went too fast for me, in part because the positive and negative feedback loops somewhat dazzled me. However, I did learn that besides α and β cells the islets also contain δ cells that provide negative feedback to the insulin-producing β cells. Also, I immediately was fascinated by the elegance of these pancreatic islets, which consist of only a few major cell types, and by the impressive live ex vivo cell imaging using intact pancreatic islets by Prof. Huising and his group. They established fluorescent activation markers for the different pancreatic cell types, showing beautifully how they are regulated by different conditions, and how the β cells react together as a unit by being connected through gap junctions. Apart from my general appreciation, I only realized after rewatching the video that also in the second part of his presentation Prof. Huising’s explanations were mostly exceptionally clear and easy to understand. Therefore, I recommend everyone at Fujita Health University with an interest in this topic to watch the video, possibly at interrupted speed.
During my introduction of Prof. Huising, I was allowed to show a picture in which he, his wife Talitha, and their three children were wild water rafting on the Kern river in California (I always ask the speakers for their hobbies). During the discussion part of the meeting, Prof. David Alexander asked him about that. After the meeting, I got a nice e-mail from Prof. Alexander, of which I am allowed to show the contents here:
“I enjoyed the seminar very much. I made some notes and I need to go over his abstract and paper. I am just an amateur, but it made a lot of sense to me. AND, I was very happy to learn that the Kern river is OK.
Yes, I did quite a bit of wild water rafting when I lived in Calif. You buy a raft and from then on the adventures are free. AND, I almost died on the Kern only one single time – you can’t ask for more than that.
Of course, as Mark (river rafters are always on a first name basis) mentioned, when you are carrying precious cargo, you have to be more careful.”
The contents of the presentation
In the below paragraphs, I am only summarizing parts of Prof. Huising’s presentation. After a general introduction, his talk focused on how pancreatic islet α, β, and δ cells communicate to regulate blood glucose levels. By necessity, the presentation was a simplification in that some minor pancreatic islet cell populations and relevant stimuli other than glucose and amino acid levels were mostly left undiscussed (Huising 2020).
I. INTRODUCTION
Diabetes and pancreatic islets
The blood glucose levels vary in all of us during the day, depending on the type of and time after nutrition. Healthy people have mechanisms, involving cells in the pancreatic islets, to bring these levels back to set-point values. People with diabetes have deficiencies in lowering blood glucose levels, and high blood glucose levels over a prolonged period of time are toxic.
Insulin, secreted by the β-cells of the pancreatic islets, is a hormone that increases glucose uptake by cells and thereby reduces blood glucose levels; hepatocytes in the liver can convert the taken-up glucose to glycogen for energy storage. In the opposite direction, glucagon, secreted by the α-cells of the pancreatic islets, is a hormone that induces the liver to convert stored glycogen into glucose which is released into the blood and so increases blood glucose levels.
There are Type 1 and Type 2 Diabetes (Fig. 1). Type 1 is a hereditary autoimmune disease in which the own immune system kills the β-cells, so that insulin can’t be made. T1D was a lethal disease until in 1921 insulin was discovered by Banting and Best (and Macleod). From 1923, insulin, in that period isolated from pigs, became the standard treatment of T1D. There is still no permanent cure for T1D.
T2D, on the other hand, is predominantly determined by a modern sedentary, overeating lifestyle. However, genetic factors also play a role in T2D, while lifestyle also affects the course of T1D. T2D primarily is caused by insulin resistance, meaning that cells don’t follow the order of insulin anymore to take up glucose. In the early stages of T2D, the pancreatic islet β-cells respond by making more insulin. However, over time this stresses the β-cells too much which leads to β-cell failure (Brentki and Nolan 2006) so that also some T2D patients need to be supported by insulin therapy.
Pancreatic islets
The total number of islets distributed throughout the human pancreas (for a schematic figure see the figure at the top of this blogpost) has been estimated to be between 3.2 and 14.8 million, with around 3000 cells per islet and a total islet volume of 0.5 to 2.0 cm3 (Da Silva Xavier 2018). They have a thin fibrous capsule and are vascularized and innervated for direct communication with the blood circulation and nervous system, respectively (Fig. 2). The majority of cells in the pancreatic islets are β cells (Fig. 2), which form a network connected by gap junctions (see below). Especially in young mice the other cell types are located more in the islet periphery, whereas in humans the cell types are more dispersed (Fig. 2; Noguchi and Huising 2019).
The standard textbook model of how α and β cells regulate blood glucose levels
The typical textbook model for glucose regulation is very simple, with low and high glucose levels having opposite activating and inactivating effects on glucagon and insulin production by α and β cells, respectively (Fig. 3). The problem is that this is an oversimplification and also not entirely true. For example, α cells are not very sensitive to low glucose levels and are much more sensitive to the nutritional status through the monitoring of amino acids. This simplified model also fails to acknowledge that α and β cells directly interact, which is the reason for their residing together in pancreatic islets.
II. RESEARCH DATA
α Cells are stimulated by low concentrations of blood glucose as well as by conditions associated with high levels of blood glucose (after a meal); glucagon released by α cells stimulates β cells to make more insulin
Prof. Huising’s group made many ex vivo live cell imaging videos in which cell activation was monitored by fluorescence analysis of Ca2+ influx in isolated intact mouse pancreatic islets. For all the three cell types α, β, and δ, the Ca2+ influx is believed to correspond with the production of their specific hormones. These videos showed us that at low glucose concentrations (0.5 mM) some α cells are activated as expected indeed, but that these are only a small percentage of all available α cells (Fig. 4A). In contrast, upon incubation with amino acids—for which α cells are much more sensitive—most α cells were found to be activated (Fig. 4B). Thus, after a meal, when blood glucose and amino acid concentrations are high, not only most β cells (by high glucose) but also most α cells are activated. In Fig. 5, some of the interactions between the α, β, and δ cells within a pancreatic islet are shown, of which Prof. Huising only had time to discuss a few. He explained that glucagon secreted by α cells activates β cells, and therefore after a meal provides a booster for β cells to produce more insulin (Svendsen et al. 2018; Capozzi et al. 2019; Zhu et al. 2019).
α Cells do not necessarily affect the blood glucose set-point concentrations
Using a recombinant diphteria toxin sensitivity system, Prof. Huising and co-workers could in vivo ablate α cells (glucagon producing cells). They found that this did not change the resting glucose levels, as others had already shown before them (Shiota et al. 2013; Thorel et al. 2011). Naturally, α cells are necessary during periods of starvation and under those conditions they do affect blood glucose levels.
The negative feedback loop for β cell activation: Activated β cells express insulin but also Urocortin-3 (Ucn3), the latter stimulating δ cells to release somatostatin that inhibits β cells
β cells only make insulin from a certain level of glucose concentration (around 6 mM) which increases with further rises in glucose concentration (Fig. 6). When β cells are activated, they also make the peptide hormone Urocortin-3 (Ucn3) (Fig. 7a), which activates δ cells to release the peptide hormone somatostatin (Van der Meulen et al. 2015). Ucn3 was discovered by Prof. Huising’s former mentor Wylie Vale (Zhao et al. 1998), and somatostatin was discovered by Vale’s mentor the Nobel prize laureate Roger Guillemin. In humans but not in mice, also activated α cells make Ucn3 (Van der Meulen et al. 2012). Somatostatin inhibits α and β cell activities, and negative feedback loops are formed (Fig. 7a) (Van der Meulen et al. 2015). Prof. Huising told us that the discovery of this feedback system took him eight years after his initial assignment by his former mentor, Prof. Vale, to find out the function of Ucn3 in the pancreas. Their Van der Meulen et al. 2015 paper describes several lines of evidence for this model, some of which Prof. Huising explianed during his presentation.
It is important to realize that Ucn3 alone is not enough to activate δ cells, as they also need to be stimulated by high glucose (that is explained in Fig. 7b)
Fig. 8B shows how pancreatic islets of Ucn3 knockout mice produce more insulin upon glucose stimulation, and that this can be brought back to normal levels by adding recombinant Ucn3 (Van der Meulen et al. 2015). This agrees very well with data from other researchers for somatostatin knockout mice (Fig. 8A). These data confirm the role of Ucn3/δ cells/somatostatin in β cell inhibition.
Ucn3 does not change the maximum level of insulin produced by islets but right-shifts the glucose threshold in the glucose-insulin expression curve by about 1.5 mM towards a lesser sensitivity at lower glucose concentrations (schematically shown in Fig. 9).
δ Cell function reduces the blood glucose set-point concentrations; neonatal mice lack δ cells and therefore have lower blood glucose
So, δ cells suppress β cells to make insulin, but what is the end effect of this in terms of glucose levels? As shown in this paragraph, both in a recombinant system (in which δ cells were ablated) and in a natural system (neonatal mice) without functional δ cells the blood glucose set point values are lower than in controls (because the insulin levels are higher).
Prof. Huising and co-workers ablated the δ cells from pancreatic islets of adult mice in vivo (by using a recombinant diphtheria toxin sensitivity system), leading to an immediate drop in basal blood glucose levels (Fig. 10) (Huang et al. 2022). For isolated islets, they found, as Prof. Huising showed us in live imaging videos, that δ cell ablation caused the β cells to become more sensitive to glucose (become activated already at lower glucose concentrations); this is quite similar to what was found in neonatal mice.
In neonatal mice, it takes several weeks before most β cells express Ucn3 (Van der Meulen et al. 2012). In agreement, compared to older mice, neonatal mice have lower blood glucose (set-point) levels and higher insulin levels and their isolated islets show enhanced sensitivity to glucose (even at low concentrations) for inducing insulin secretion (Blum et al. 2012). When Prof. Huising and co-workers recombinantly expressed Ucn3 in neonatal mice, they found that the blood glucose levels became similar to those in older mice (Fig. 11) (Van der Meulen et al. 2015); meanwhile, the same study found that extra expression of Ucn3 in older mice did not have an impact on the blood glucose levels (Fig. 11). Prof. Huising described as the reason for these observations that in neonatal mice the “glucose set-point has not yet matured,” a process for which the interaction between β and δ cells is needed.
In summary: Ucn3-triggered somatostatin expressed by δ cells forms the natural break on insulin secretion and determines the set-point for blood glucose (schematically shown for neonatal versus adult mice in Fig. 12).
δ Cells react as single sentinel cells that are more sensitive to glucose increases, whereas the β cells of a single islet react together as a gap-junction-connected unit with a higher threshold value for activation
Prof. Huising explained well, showing several lines of evidence, how β and δ cells form different types of units when activated. I will only summarize that very shortly here in text, and, for example, not discuss frequencies of membrane depolarization (oscillation patterns).
The big difference between β and δ cells is that δ cells are not connected by gap junctions and β cells are, to the extent that upon activation (membrane depolarization) all the β cells of a single islet act in an “all or none” mode. This makes the threshold for β cell activation higher, so, in comparison, δ cells are sensitive to lower glucose concentrations (although they do need Ucn3 for functional activation). In an artificial situation where β cells are individually separated from the islets, they become equally sensitive to glucose as δ cells.
Professor Huising and his group keep studying how the cells of pancreatic islets are activated and communicate in the hope that a better understanding may help to cure metabolic diseases. Despite that many factors remain to be understood, with its few major cell types and major messenger molecules the pancreatic islet system does not seem to be all that complicated compared to some other bodily systems, and—as Prof. Huising expressed—it is amazing how in most people it manages to keep blood sugar levels within healthy (survivable) limits throughout our lives, despite our sometimes “crazy” eating habits.
