key: cord-0794181-8bqe5qpk
authors: Santos, Theodore dos; Galipeau, Maria; Gomes, Amanda Schukarucha; Greenberg, Marley; Larsen, Matthew; Lee, Daniel; Maghera, Jasmine; Mulchandani, Christina Marie; Patton, Megan; Perera, Ineli; Polischevska, Kateryna; Ramdass, Seeta; Shayeganpour, Kasra; Vafaeian, Kiano; Van Allen, Kyle; Wang, Yufeng; Weisz, Tom; Estall, Jennifer L.; Mulvihill, Erin E.; Screaton, Robert A.
title: Islet Biology during COVID-19: Progress and Perspectives
date: 2021-11-23
journal: Can J Diabetes
DOI: 10.1016/j.jcjd.2021.11.002
sha: 10414a324f43bf51193e3d5afb92b0e3edb6a681
doc_id: 794181
cord_uid: 8bqe5qpk

The coronavirus disease 2019 (COVID-19) pandemic had significant impact on research directions and productivity in the past year. Despite these challenges, since 2020, >2,500 peer-reviewed articles in pancreatic islet biology have been published. These include updates on the roles of isocitrate dehydrogenase, pyruvate kinase, and incretin hormones in insulin secretion, as well as the discovery of Inceptor and signaling by circulating RNAs. 2020 also brought advancements in in vivo and in vitro models, including a new transgenic mouse for assessing β-cell proliferation, a pancreas-on-a-chip to study glucose-stimulated insulin secretion, and successful genetic editing of primary human islet cells. Islet biologists evaluated the functionality of stem cell-derived islet-like cells coated with semi-permeable biomaterials to prevent autoimmune attack, revealing the importance of cell maturation following transplantation. Prompted by observations that COVID-19 symptoms can worsen for people with obesity or diabetes, researchers examined how islets are directly affected by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Herein, we highlight novel functional insights, technologies and therapeutic approaches emerging between March 2020 and July 2021, written for both scientific and lay audiences. We also include a response to these advancements from patient stakeholders, to help lend broader perspective to developments and challenges in islet research.

Introduction 2021 marks the 100th anniversary of the discovery of insulin, and to this day, we continue to gain new insight into the synthesis, secretion, and mechanism of action of this essential hormone. Although the regulation of insulin secretion from β-cells in response to nutrient intake has been studied for decades, new molecular clues that contribute to pathological pancreatic islet dysfunction underlying type 1 diabetes (T1D) and type 2 diabetes (T2D) are continually being identified. In this review, we discuss recent advances addressing significant gaps in knowledge surrounding islet biology. Papers highlighted in this review represent, in our opinion, research that shifts our fundamental understanding of islet biology or represents significant technological advances in the islet field, which would have relevance to Patient Partners with lived experience of diabetes.

We first focus on the central player in insulin homeostasis, the β-cell, identifying recent developments for mitochondrial control of insulin exocytosis, and addressing a long-standing paradox involving how βcells mitigate the effects of their own insulin secretion. We also explore new research showing how α-cells may be both a target of incretin hormone signaling, and controversially, a source of incretin release. Next, we elaborate on how extracellular vesicles from adipocytes and non-coding RNAs can influence β-cells and insulin secretion. We also highlight recent in vitro and in vivo technological advancements, including a new animal model to quantify β-cell proliferation, applications of CRISPR-Cas9 gene editing, and innovations to create a pancreas-on-a-chip. We discuss the current state of β-cell transplantation as a treatment for diabetes, integrating new research on stem cell generation and protective encapsulation of βcells. Lastly, we discuss recent findings on the effects of COVID-19 on patients living with diabetes and potential links between the infection and cases of new-onset diabetes. To ensure that these new research findings reached both the science community and important stakeholders, we include a lay description of our review and a summary of patient perspectives on this research from people living with diabetes.

Paul Langerhans's discovery of pancreatic islets in 1869 was pivotal in diabetes research history, initiating over 150 years of work in islet biology, insulin production, and glucose homeostasis (1) . However, the details of insulin exocytosis are still being unraveled. Zhang et al. provided new insight into the activation of insulin granule exocytosis, via ancillary signals produced by the "counterclockwise" flux of signaling molecules to and from the tricarboxylic acid (TCA) cycle (2) . These key signals are generated from citrate/isocitrate metabolism by isoforms of isocitrate dehydrogenase in mitochondria (IDH2) and the cytosol (IDH1). Metabolite flux via IDH1 and IDH2 generates cytosolic signals that trigger insulin granule exocytosis, which small molecule antagonists of IDH2 can block. The authors also show that IDH1mediated insulin secretion is enhanced by the presence of glutamate, an amino acid that leads to citrate generation via reversal of the TCA cycle. While such counterclockwise metabolite flux in the TCA cycle has been implicated in cancer cell growth (3, 4) and in cell function (5-7), here the authors provide additional evidence for this important mechanism in regulating insulin secretion and islet function.

Glucose sensing in β-cells describes how changing ambient glucose levels during the transition between fasting and fed states leads to glucose oxidation and coupling to regulate insulin exocytosis (1, 2, 8 (8) . In this revised model, pyruvate kinase (PK) directs metabolites between the phosphoenolpyruvate cycle and oxidative phosphorylation to modulate ADP production. Therefore, PK mediates insulin secretion by reducing ADP availability in mitochondria, altering cytosolic ATP/ADP ratios and causing the closure of ATP-sensitive K + channels (8) . They also show that pharmacologic activation of PK depolarizes the cell membrane, leading to enhanced insulin secretion independent of glucokinase activity (i.e., glucose sensing), in contrast to current canonical models. In a J o u r n a l P r e -p r o o f companion paper, the authors use mouse models to show that PK activators can amplify insulin secretion, regulate gluconeogenesis, and improve insulin sensitivity in vivo, further illustrating the importance of this pathway (9) .

A defining feature of insulin action is glucose uptake in peripheral tissues such as muscle and fat. One of the hallmarks of prediabetes is insulin resistance (10), a phenomenon whereby insulin-sensitive tissues downregulate or inactivate components of the insulin signaling pathway in the presence of consistently elevated insulin levels (10) . Insulin also induces the growth and proliferation of target cells, including βcells (11) . Thus, a long-standing mystery in islet biology is what prevents β-cells from autocrine/paracrine induced insulin resistance and hyperplasia in a healthy individual (11) (12) (13) . Key insight into this mystery came from the exciting discovery of a new protein expressed in β-cells, termed Inceptor, for Insulin Inhibitory Receptor/endosome-lysosome Associated Apoptosis and Autophagy Regulator 1 (14) . Using βcell lines and mice, Ansarullah et al. showed that Inceptor interacts with the receptors for insulin (INSR) and insulin growth factor 1 (IGF1R) to promote their internalization via clathrin-mediated endocytosis, protecting β-cells from autocrine/paracrine stimulation by their own insulin (14) . Knock-out of the inceptor gene in mice enhances β-cell proliferation and mass -a critical finding that could help to increase β-cell numbers for diabetes therapy (15) . In addition, if Inceptor is found in other insulin-sensitive tissues, blocking it could facilitate a reversal of insulin resistance.

A hallmark of T2D is dysfunction of both -and -cells (16, 17) . While the β-cell is often the primary focus in studies of islet dysfunction, people living with type 2 diabetes also exhibit inappropriate glucagon release from α-cells. Although mechanisms of -cell dysfunction in diabetes remain unclear, numerous studies suggest that complex crosstalk between α-and β-cells influences this phenomenon.

It is well established that incretin hormones released by intestinal cells during feeding amplify β-cell insulin secretion in a stimulatory endocrine process known as the "incretin effect" (18, 19) . Activation of incretin receptors on β-cells triggers insulin release only in high glucose, which has made incretin mimetics invaluable therapeutic agents (20) (21) (22) (23) . However, signalling from intestinal cells is not the only pathway that can stimulate insulin release from β-cells. Recent work from several groups revealed a pivotal paracrine role for α-cells in stimulating β-cell insulin secretion during postprandial hyperglycemia (24) (25) (26) . Until now, a link between the intestinal incretin effect and α-cell paracrine effects had not been discerned, suggesting that both pathways stimulate β-cell insulin release independently of each other. However, findings by El et al. show that part of the incretin effect stems from the action of gut hormone glucosedependent insulinotropic polypeptide (GIP) directly on α-cells, causing glucagon release to enhance insulin secretion from the β-cell (27) . Without this α-to β-cell communication, less insulin was secreted by β-cells in response to a mixed meal tolerance test in mice (27) . These recent findings demonstrate previously unappreciated pathways that work together to enhance β-cell insulin release.

Another intriguing, yet controversial, mediator of intra-islet communication involves the production of the incretin hormone glucagon-like peptide 1 (GLP-1) from α-cells. Generally believed to be synthesized and secreted by intestinal L-cells during feeding, recent studies provide new insight into the idea that GLP-1 may also be synthesized locally within the islet (28) (29) (30) . While most previous studies use either α-cell Interestingly, activation of GLP-1R also promotes the expression of additional β-cell-like genes in α-cells, such as INS, MAFA, and IAPP, pointing to a possible interconversion of these two islet cell types (26) .

Together, these new studies with incretin hormones, suggest that α-cells do not simply respond to hypoglycemia but also contribute to an integrated islet cell response to hyperglycemia, highlighting the therapeutic potential of targeting the -cell and this beneficial crosstalk.

There is also increasing evidence supporting roles for non-coding RNAs in modulating paracrine communication between different islet cell types. Stoll et al. report that insulin secretion is regulated by a conserved intronic circular RNA derived from insulin (INS) genes transcripts, dubbed ci-Ins2 in rodents and ci-INS in humans (33) . Deficiency in this circular RNA alters the expression of multiple genes involved in calcium signaling and insulin exocytosis, including reducing calcium channel subunit Cacna1d, two calcium sensors, Syt4, Sys7, and Pclo, and the insulin granule recruitment factor, Unc13a. In α-cells, ci-Ins2 is only marginally detected; however, new data suggest that non-coding RNAs can travel between islets cells via EVs (34) (35) (36) . Taken together, these discoveries reshape our fundamental understanding of the complexity of regulatory mechanisms for signaling events in islet function, dysfunction, and diabetes.

Islets also receive numerous signals from peripheral tissues to coordinate insulin release, and work from Gesmundo et al. has raised the possibility of targeting extra-islet tissues to improve insulin secretion.

They showed that extracellular vesicles (EVs) derived from healthy human adipocytes can have beneficial effects on β-cell function and survival (37) . In contrast, EVs derived from inflamed adipocytes collected from obese individuals exacerbate insulin resistance and β-cell dysfunction. By comparing the contents of EVs from healthy versus inflamed adipocytes, the authors pinpointed microRNAs as key signaling molecules that impact several key inflammatory genes in rodent (INS-1E) and human (EndoC-βH3) β-cell lines such as TNFα, IFNγ, IL-1B, adiponectin, and immune system complement factors. This work highlights an unappreciated role for regulatory crosstalk between adipocytes and β-cells.

March 2020-July 2021 also brought advancements in the development of in vivo and in vitro models to study the islet. The recently developed RIP-Cre; R26Fucci2aR mouse model promises to improve our ability to quantify β-cell proliferation (38) . Traditionally, measurement of proliferation relies on immunohistochemical methods, which lack specificity for -cells and are subject to high variability between studies (39). The R26Fucci2aR mouse addresses these caveats by expressing a fluorescent Fucci2a reporter in β-cells driven by the rat insulin promoter (RIP), specific to -cells. When activated, the fluorophore emits red light in the G1 phase and green light during the S/G2/M phases of the cell cycle, permitting clear definition and isolation of -cells undergoing active proliferation. This new tool allows for more reliable and specific quantification and characterization of replicating β-cells, which is highly useful when testing novel therapeutics claiming to expand β-cells mass (38) .

The "pancreas-on-a-chip" (PoC) was developed due to the need for an in vitro model of islet function recapitulating the in vivo microenvironment of human islets (40) . One recent model by Zbinden et al.

involves entrapping islet-like structures, or "pseudo-islets", composed of aggregated immortalized human EndoC-βH3 β-cells, into individual compartments of a microfluidic chip (40) . Unique to this PoC model is the incorporation of Raman microspectroscopy, a chemical analysis technique that assists in real-time monitoring of insulin release (40) . Experimental modeling demonstrated that PoC pseudo-islets are highly glucose-responsive and exhibit the biphasic glucose-stimulated insulin secretory responses characteristic of primary human islets (40) . Thus, the PoC model provides a new tool to study insulin secretion kinetics in a J o u r n a l P r e -p r o o f multitude of cells models, including primary human islets and non-immortalized human β-cell cultures, including stem cell (SC)-derived β-cells (40) .

Representing a major advancement, the genome editing technology CRISPR-Cas9 was used to generate the first genetically modified primary human islets (41) . As a proof-of-concept, Bevacqua et al.

deleted pancreatic and duodenal homeobox 1 (PDX1), a transcription factor important for maintaining βcell identity and function, impairing calcium channel activity, reducing total insulin content and glucosestimulated insulin secretion (41) . CRISPR-Cas9 constructs delivered by lentivirus were also used to delete the KCNJ11 gene encoding KATP channel subunit KIR6.2, or to introduce non-coding genetic variation at the ABCC8-KCNJ11 locus, both of which caused impaired KATP channel activity in human islets (41, 42) .

Successful editing of the human islet genome via CRISPR-Cas9 provides proof-of-principle for generating a wide variety of human primary islet knock-in and knock-out models, which will help bridge the gap between in vitro models and human islet biology.

As we cannot yet correct islet defects in vivo, pancreatic islet transplantation is an effective therapy to restore physiological glycemic control. However, several barriers still exist, such as a shortage of organ donors and the requirement for lifelong immunosuppression. Alternative islet sources, such as embryonic or induced pluripotent stem cell-derived islet-like cells (ESCs and iPSCs), are at the forefront; while generating -like cells from these stem cells that respond appropriately to glucose has been difficult (43, 44) . The Millman group recently tackled this challenge by investigating key maturation changes that occur throughout differentiation of iPSC or ESC stage 6 SC-islet cells before and after transplantation (45) .

Single-cell RNA sequencing determined that stem cell-derived β-cells six months after transplantation more closely resemble primary human β-cells than before transplant, and genes associated with β-cell maturation, information about the biology and function of these stem cell-derived islet cell populations, bringing us even closer to islet cells that more closely mimic primary islets (46, 47) .

Even as safe and effective sources of β-cells become available, cell replacement therapy still requires immunosuppression (48) . Biocompatible encapsulation devices, which allow efficient blood glucose regulation while protecting the islet mass from immune attack, are a major focus in islet transplantation (49) . Pancreatic islet encapsulation involves loading donor islets into a protective device or matrix that allows the exchange of glucose, insulin, oxygen, and other metabolic products, yet provides a physical barrier against immune cell infiltration (50) . A recent study by Stock et al. using cryo-recovered stage-6 stem cell-derived islet cells encapsulated with a 5% poly(ethylene glycol)-maleimide hydrogel shows that the function of coated stem cell derived-islet cells is similar to non-coated controls in vitro (51) . When transplanted into the fat pads of immunocompromised mice, coated and non-coated stem cells were equally effective in re-establishing glycemic control. Investigating this novel coating on islets in immunocompetent humanized mice and evaluating the safety of coated islet cells in larger species are important next steps before clinical trials.

In the early days of the pandemic, media reports claimed that COVID-19, the disease resulting from infection by Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2), may be more severe in people who are living with diabetes, and may even cause new-onset diabetes (52, 53) . Indeed, people living with diabetes, they were eventually prioritized for early vaccination in Canada (54) . For excellent reviews on the impact of SARS-CoV-2 on clinical outcomes in diabetes, please see (55, 56) . However, pathophysiological mechanisms underlying this observation remain unknown (57) . Once it became clear that people living with diabetes showed substandard outcomes during COVID-19 disease, there was great (63, 64) . They also report SARS-CoV-2 N-protein in the β cells of deceased patients with COVID-19 concurrent with increased islet cell apoptosis that may be due to the viral spike protein (64) .

Together, these studies provide evidence that β-cell infection may be involved in COVID-19 pathogenesis, or alternatively, that pancreatic infection may impact β-cells by changing their local microenvironment. This evidence raises the possibility that SARS-CoV-2 infection may contribute to pancreatic endocrine and/or exocrine dysfunction. While these studies all provide evidence toward a possible mechanistic link between COVID-19 and new-onset diabetes, it is important to note that recent data suggests that the incidence of type 1 diabetes has, so far, not increased following SARS-CoV-2 infection (65) . It is important to note that viral entry is not limited to mature pancreatic β-cells, but also occurs in iPSC-derived β-like cells. Examining how SARS-CoV-2 affects acute and chronic islet cell function will be of future interest, particularly the pathophysiology underlying new-onset cases of diabetes following SARS-CoV-2 infection. 

Paul Langerhans' discovery of the pancreatic islets in 1869 helped initiate over 150 years of research on insulin and blood sugar (glucose) control (1) . Islets are clusters of hormone-secreting cells within the pancreas responsible for releasing insulin (from β-cells, which lowers blood glucose) and glucagon (from α-cells, which increases blood glucose) to maintain blood glucose levels at a healthy level. Our understanding of insulin secretion and how it is controlled is constantly evolving. The period from March 2020-July 2021 gave us many advances in islet biology, including new molecular mechanisms, tools, and cell proteins that could target new drugs. Though the pandemic dramatically slowed research, studies on COVID-19 unexpectedly expanded our understanding of how the islet may be impacted by the virus that causes COVID-19, while also revealing new challenges that must be faced by those living with diabetes.

It is generally appreciated that glucose is the signal to β-cells to secrete insulin (1, 2, 66) . In back-toback articles, researchers revealed that pyruvate kinase, the enzyme that catalyzes the final step in glucose breakdown, is critical for controlling insulin release at the cell surface (9, 66) . The researchers went on to show that drugs targeting pyruvate kinase can enhance insulin secretion (even in the absence of glucose) and increase the effectiveness of insulin in mice. Along the same lines, new research from Zhang et al. showed that other signals from mitochondria, parts of a cell that use pyruvate to make energy, also help βcells secrete insulin (2) . The authors demonstrated that these signals, sent back and forth between mitochondria, help amplify insulin release.

This year, an emerging theme is that insulin secretion from β-cells can be controlled by other islet cell types. New research by El et al. revealed that while eating a meal, an intestinal hormone called glucosedependent insulinotropic polypeptide, or GIP, is secreted from the intestine and causes α-cells to release glucagon (27) . Glucagon is usually secreted when blood sugar is low, but they reveal that in this context, J o u r n a l P r e -p r o o f glucagon promotes insulin secretion from neighboring β-cells. While the role of α-cells in regulating insulin secretion received a lot of attention this year, El et al.'s work is one of the first to demonstrate the involvement of GIP in this α-and β-cell relationship (24) (25) (26) . This study also highlights the importance of mixed meals -those consisting of protein, carbohydrate, and fat, as opposed to carbohydrates alone -in insulin control, and emphasizes that focus should not be on sugar intake alone.

Another hallmark of diabetes is "insulin resistance", where cells stop responding to insulin partially because of over-stimulation. This is common in tissues such as muscle and fat and leads to poor glucose absorption. A long-standing puzzle is why β-cells do not develop insulin resistance, despite being bathed in high amounts of insulin they secrete (11, 12) . Exciting work from Ansarullah et al. identifies a new protein the authors named Insulin Inhibitory Receptor, or "Inceptor", (14) that hides the insulin receptor inside β-cells, preventing insulin overstimulation and the generation of new β-cells. Not only does this solve a mystery, it also suggests that interfering with inceptor function could increase insulin levels and allow more β-cells to form, which could be an added advantage for regenerative and stem cell-based therapies.

A growing body of evidence suggests that impaired islet function can result from signals originating within the pancreas as well as from other organs (16, 17, 36, 67) . Gesmundo et al. discovered that signals involving small pieces of ribonucleic acid, known as microRNA, are sent between the fat cells and β-cells (37) . Interestingly, when fat cells come from lean patients, the signals can promote insulin secretion and increase β-cell health, whereas fat cells from obese individuals send signals that can impair β-cell function and even cause cell death. Similarly, Stoll et al. also found a new signal from a microRNA called ci-INS that is essential for insulin to be properly secreted by β-cells (33) .

It was also shown this year that human α-cells not only secrete glucagon, but also secrete glucagonlike peptide 1 (GLP-1), a hormone that promotes insulin secretion and is thought to mainly come from the Thus, a continuing theme of findings over that last year is that α-cells seem to work together with β-cells to control high blood sugar levels.

This past year has brought new and exciting research tools to improve our understanding of how pancreatic islets work in the laboratory. It has always been a challenge to detect when new β-cells are being made. This year, a particular type of genetically modified mice was developed, in which β-cells appear green when they are dividing to create a new β-cell and red when they are not dividing. This allows researchers to better detect and count the β-cells that are making new copies -a critical step toward testing novel therapies designed to increase the number of β-cells (38) . Another exciting tool, popularly known as "pancreas-on-a-chip", involves trapping and growing human pancreatic cells on a small device, or 'chip', while also trying to mimic the natural environment of islets in the body (40) . This small chip has a set of micro-channels etched or molded into it, which are connected to allow blood flow. Islet cells can then detect changes in nutrients and release the necessary hormones to maintain normal blood sugar levels. While pancreas-on-a-chip is not a new development, the chip was modified this year to incorporate a technique called Raman imaging, which can measure insulin release on the chip in real time (40) . This exciting development allows the study of how these islets function in an environment that mimics the body.

For the first time, researchers have used an exciting Nobel Prize-winning technique called CRISPR-Cas9 mediated gene editing [see CRISPR-Cas9 explained for an easy-to-follow primer on how this technology works] to modify genes in pancreatic islets from human donors (41) . This allows islet biologists J o u r n a l P r e -p r o o f to quickly modify specific genes in human islet cells to understand their importance. This has the potential to allow scientists to improve human donor-derived pancreatic islets, such as by repairing harmful mutations, enhancing function or improving survival (41) . While this is a major technological advancement, this discovery again highlights the continual need to revisit and discuss ethical and socioeconomic concerns that underlie gene therapy.

Pancreatic islet transplantation can restore normal blood sugar regulation in patients with T1D (68).

However, barriers to using this therapy include a global shortage of organ donors and the need for immunosuppressants to prevent transplant rejection. To address organ shortage, scientists look to other islet sources, such as pig islets and human stem cells.

Generating islet cells from embryonic stem cells (ESCs) has been possible since the early 2000s (69, 70) . Induced pluripotent stem cell (iPSC) technology, which involves generating stem cells from mature cells taken directly from the patient involved, has further opened the door to using one's own cells to make islets, avoiding problems of rejection. However, generating new cells that function like normal β-cells has been challenging (43, 71) . This last year, the Millman group identified many new genes that are associated with maturation of stem cell-derived β-cells, and Lien et al. and Balboa et al. discovered several new aspects controlling how these cells behave in their environment, advancing our understanding of how these cells work and how best to generate them (45) (46) (47) .

As effective and safe sources of replacement β-cells become available, we will still need to address transplant rejection. Immunosuppressive drugs help prevent the body from attacking foreign islet transplants but come with significant negative side effects. Recently, scientists have investigated how to "coat" transplanted islet cells to prevent immune attack, while still allowing the islet to sense sugar and release insulin (50, 72) . A recent study found that pre-coating transplanted β-cells with a new protective gel did not prevent reversal of diabetes in mice, a critical first step in addressing transplant rejection (51) .

Researchers must now determine whether this coating also guards β-cells against immune attack, the next step in advancing this technology from bench to bedside.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has had a global impact on all aspects of society in the past year. It quickly became clear that living with diabetes made an individual more likely to have severe COVID-19 symptoms and experience worse blood glucose control (73) . In addition, SARS-CoV-2 infection has been associated with new-onset diabetes, yet whether this results directly from infection remains unclear (54) . The following section features responses to the lay review from a diverse group of seven Patient Partners living with Type 1 or Type 2 Diabetes. The group response synthesizes commonly held perspectives by all, while individual comments reflect unique personal perspectives. This effort was coordinated through Diabetes Action Canada.

Group Response: Several areas in this review piqued our interest -any strides towards applicable changes for diabetes are really exciting. We feel enthusiastic about advancements in understanding how SC-islet cells function and how to best generate them. The possibility for immune attack after islet cell transplants is an area of concern for us, so utilizing encapsulation Group Response: Collectively, we are concerned about the costs related to diabetes and innovative treatments. The financial accessibility of diabetes care advancements, funding for innovative ideas, and costs of SC-islet cell treatments were the areas that stuck out. COVID-19 related complications were also a concern for the group. We are worried about future complications for diabetics caused by COVID-19 and the potential for the virus to trigger the onset of other long-term chronic conditions (such as diabetes).

There was also concern around the responsible use and safety regarding some of these novel treatments, such as the idea of genetic modification. The need to ensure that safeguards are in place to avoid triggering uncontrolled cell proliferation with mitochondrial pathways for insulin secretion was also a fear.

"I am worried that these advancements may not be available to patients for a long time. I also worry that if these treatments do become available to people living with diabetes, they will be really commodified because it's a novel treatment." Megan Patton "After living through a pandemic, I am now concerned that COVID-19 may trigger the onset of long-term chronic conditions such as diabetes." Christina Marie Mulchandani

What new discoveries/advancements would you like to see in the next 3-5 years?

J o u r n a l P r e -p r o o f Group Response: Co-authoring this review made evident how critical collaboration between researchers and people living with diabetes is. To ensure new research is meeting the diverse needs of those living with the condition, as a group, we are hopeful to see more patient engagement occurring within the field. We are hopeful for the continued development of novel treatments mentioned throughout this review. The affordability of diabetes management supplies is a large contributing factor to proper care. We are hopeful that current supplies and future care strategies will be made accessible to all socioeconomic groups and allow everyone to adequately care for their condition. Establishing a reliable process for SC-islet cells to mimic human islet cells, deeper understanding around how GLP-1 is produced, further development in genetic modification to treat diabetes, and treatments that target insulin resistance were areas that the group was particularly interested in seeing further advancements. We would like to see more data around the correlation between viral infiltration in pancreatic islets and COVID-19 severe complications and deaths. Understanding how COVID-19 can affect the cells in the pancreas is also an area we would like to see be discovered more. Finally, we are increasingly hopeful that the novel treatments outlined in this review will put us on the path towards a cure for diabetes. 

Mechanisms controlling pancreatic islet cell function in insulin secretion

Reductive TCA cycle metabolism fuels glutamine-and glucose-stimulated insulin secretion

Reductive glutamine metabolism by IDH1 mediates lipogenesis under hypoxia

Reductive carboxylation supports redox homeostasis during anchorage-independent growth

LKB1 couples glucose metabolism to insulin secretion in mice

Beta Cells Enhances Glucose-stimulated Insulin Secretion Despite Profound Mitochondrial Defects

Toward Connecting Metabolism to the Exocytotic Site

Pyruvate kinase controls signal strength in the insulin secretory pathway

Multi-Tissue Acceleration of the Mitochondrial Phosphoenolpyruvate Cycle Improves Whole-Body Metabolic Health

Mechanisms of Insulin Action and Insulin Resistance

Ser/Thr phosphorylation of IRS proteins: a molecular basis for insulin resistance

Modulation of β-cell function: a translational journey from the bench to the bedside

Direct autocrine action of insulin on beta-cells: does it make physiological sense?

Inceptor counteracts insulin signalling in β-cells to control glycaemia

Harnessing Proliferation for the Expansion of Stem Cell-Derived Pancreatic Cells: Advantages and Limitations

Reduced Insulin Production Relieves Endoplasmic Reticulum Stress and Induces β Cell Proliferation

Pancreatic β cell dedifferentiation in diabetes and redifferentiation following insulin therapy

Pharmacology, physiology, and mechanisms of incretin hormone action

The incretin system in healthy humans: The role of GIP and GLP-1

Biology of incretins: GLP-1 and GIP

GLP-1 Analogs and DPP-4 Inhibitors in Type 2 Diabetes Therapy: Review of Head-to-Head

Efficacy and safety of glucagon-like peptide-1 receptor agonists in type 2 diabetes: A systematic review and mixed-treatment comparison analysis

A review of GLP-1 receptor agonists: Evolution and advancement, through the lens of randomised controlled trials

Repositioning the Alpha Cell in Postprandial Metabolism

The Local Paracrine Actions of the Pancreatic α-Cell

Alpha cell regulation of beta cell function

GIP mediates the incretin effect and glucose tolerance by dual actions on α cells and β cells

Pancreatic alpha Cell-Derived Glucagon-Related Peptides Are Required for beta Cell Adaptation and Glucose Homeostasis

GPR142 prompts glucagon-like Peptide-1 release from islets to improve beta cell function

Glucagon-like peptide 1 (GLP-1)

Human islets contain a subpopulation of glucagon-like peptide-1 secreting α cells that is increased in type 2 diabetes

GLP-1 receptor signaling increases PCSK1 and β cell features in human α cells

A circular RNA generated from an intron of the insulin gene controls insulin secretion

Noncoding RNAs Carried by Extracellular Vesicles in Endocrine Diseases

The Long and Short of It: The Emerging Roles of Non-Coding RNA in Small Extracellular Vesicles

The Role of Extracellular Vesicles in β-Cell Function and Viability: A Scoping Review

Adipocyte-derived extracellular vesicles regulate survival and function of pancreatic β cells

Generation and Characterization of a Novel Mouse Model That Allows Spatiotemporal Quantification of Pancreatic β-Cell Proliferation

Resolving Discrepant Findings on ANGPTL8 in β-Cell Proliferation: A Collaborative Approach to Resolving the Betatrophin Controversy

Non-invasive markerindependent high content analysis of a microphysiological human pancreas-on-a-chip model

CRISPR-based genome editing in primary human pancreatic islet cells

Generation of a KCNJ11 homozygous knockout human embryonic stem cell line WAe001-A-12 using CRISPR/Cas9

Emerging routes to the generation of functional β-cells for diabetes mellitus cell therapy

Advances Toward Engineering Functionally Mature Human Pluripotent Stem Cell-Derived beta Cells. Frontiers in bioengineering and biotechnology

Targeting the cytoskeleton to direct pancreatic differentiation of human pluripotent stem cells

Transcriptomic and Quantitative Proteomic Profiling Reveals Signaling Pathways Critical for Pancreatic Islet Maturation

Functional, metabolic and transcriptional maturation of stem cell derived beta cells

Advances in beta-cell replacement therapy for the treatment of type 1 diabetes

Advances in islet encapsulation technologies

Islet encapsulation therapy -racing towards the finish line?

Conformal Coating of Stem Cell-Derived Islets for β Cell Replacement in Type 1 Diabetes

New-Onset Diabetes in Covid-19

Association of Blood Glucose Control and Outcomes in Patients with COVID-19 and Pre-existing Type 2 Diabetes

New onset diabetes, type 1 diabetes and COVID-19

COVID-19 and diabetes mellitus: from pathophysiology to clinical management

COVID-19 in people with diabetes: understanding the reasons for worse outcomes

Autoantibody-negative insulin-dependent diabetes mellitus after SARS-CoV-2 infection: a case report

Tissue distribution of ACE2 protein, the functional receptor for SARS coronavirus. A first step in understanding SARS pathogenesis

SARS-CoV-2 Receptor

Converting Enzyme Type 2 (ACE2) Is Expressed in Human Pancreatic β-Cells and in the Human Pancreas Microvasculature

Expression Profile of SARS-CoV-2 Host Receptors in Human Pancreatic Islets Revealed Upregulation of ACE2 in Diabetic Donors

SARS-CoV-2 Cell Entry Factors ACE2 and TMPRSS2 Are Expressed in the Microvasculature and Ducts of Human Pancreas but Are Not Enriched in β Cells

SARS-CoV-2 infects and replicates in cells of the human endocrine and exocrine pancreas

A Human Pluripotent Stem Cell-based Platform to Study SARS-CoV-2 Tropism and Model Virus Infection in Human Cells and Organoids

SARS-CoV-2 infects human pancreatic β-cells and elicits β-cell impairment

Prognostic factors in patients with diabetes hospitalized for COVID-19: Findings from the CORONADO study and other recent reports

Pyruvate Kinase Controls Signal Strength in the Insulin Secretory Pathway

Islet transplantation in seven patients with type 1 diabetes mellitus using a glucocorticoid-free immunosuppressive regimen

Insulin production by human embryonic stem cells

Advances Toward Engineering Functionally Mature Human Pluripotent Stem Cell-Derived β Cells. Frontiers in bioengineering and biotechnology

Assessment of mesenchymal stem cell effect on foreign body response induced by intraperitoneally implanted alginate spheres

Novel Molecular Evidence Related to COVID-19 in Patients with Diabetes Mellitus