Petsavers – Pancreatic islet microenvironment – an unexplored therapeutic target

Diabetes mellitus (DM) is a disease syndrome that is commonly seen in clinical practice and affects a large number of cats (1 in 230) and dogs (1 in 300). Although there is potential for good disease control, and in cats it may be possible to achieve clinical remission in some cases, approximately one in ten pets affected by diabetes are euthanased at the time of diagnosis and a further one in ten within the following year. Even with devoted care from both pet owners and the veterinary team, a diagnosis of DM has a profound impact on the quality of life of a canine or feline patient and its owner. Furthermore, veterinary patients may develop a number of DM-associated complications including renal failure, neuropathies, and blindness due to cataracts.

The two main routes to developing DM are insulin deficiency or insulin resistance, the latter comprising a relative lack of insulin. Dogs tend to have a form of diabetes characterized by complete insulin deficiency, due to the destruction of pancreatic islets and ensuing loss of the insulin-secreting beta cells. This is similar to the human type 1 form of the disease in which islets are the targets of a destructive autoimmune process. Cats, on the other hand, are more likely to present with DM resembling human type 2 diabetes, with insulin resistance a key component of disease pathogenesis. There are a number of underlying aetiologies for canine and feline DM, although all can be distilled into two groupings relating to these concepts of insulin-deficient DM (beta cell-related disorders) and insulin-resistant DM (target organ disorders). The reader is directed to the European Society of Veterinary Endocrinology ‘Project ALIVE’ website (https://www.esve.org/alive/intro.aspx) for the current aetiologic classification system.

What is the pancreatic islet microenvironment and why is it important?

Pancreatic islets are the focus of pathological changes in diabetes. Islets of Langerhans are composed of insulin-producing beta cells and other endocrine cells, such as glucagon-secreting alpha cells, somatostatin-secreting delta cells, and pancreatic polypeptide cells. One particularly fascinating aspect of the pancreatic islets is that these cells do not exist in isolation, but rather constitute key components of a rich microenvironment including blood vessels, autonomic innervation, extracellular matrix, and resident or infiltrating immune cells. Pancreatic islets are therefore in a state of complex and delicate biological equilibrium and it is easy to imagine how perturbations to this balance can have a detrimental effect on the health of the islets. Investigating the DM-associated changes in the islets can offer valuable insights into the pathogenesis of the disease, particularly in terms of identifying key cellular players that contribute to islet pathology, and previously unrecognized cellular interactions.

What is currently known about the pancreatic islet microenvironment in DM?

In both diabetic cats and dogs there is a significant decrease in islet area and overall islet mass as a proportion of overall pancreatic mass when compared to non-diabetic control animals. In humans, there is extensive evidence demonstrating that not only are beta cell numbers reduced, but that other endocrine cells are also affected. Dogs seem to follow a similar pattern to human type 1 diabetic patients, where multiple islet cell types are destroyed, whereas cats seem to have more similarities to human type 2 diabetic patients, with beta cell loss occurring at a higher rate than alpha cells.

In canine DM, the pancreatic islet pathology is of sufficient magnitude that the islets are frequently almost impossible to identify beyond the time of diagnosis. By contrast, the islets of diabetic cats are often still discernible, although they may be affected by pathological processes such as amyloid deposition and vacuolation. Islet amyloidosis occurs as a result of the deposition of islet amyloid polypeptide (IAPP) in and around islet cells. In this process, beta cells secrete a peptide known as amylin, which in some cases polymerizes into IAPP and can be observed as eosinophilic, amorphous material replacing the islet. Islet amyloidosis is often reported in diabetic cats and humans affected by type 2 diabetes. However, its role in the pathogenesis of feline DM remains unclear, particularly as it is not exclusively detected in the islets of cats with DM and can be observed in older nondiabetic cats.

Inflammation is an essential step of the pathogenesis in human type 1 and 2 DM. The role of islet inflammation, termed insulitis, in the pathogenesis of DM is not as clear-cut in cats and dogs as it is in humans. Despite lymphocytic infiltration having been observed in a small number of feline and canine diabetic cases, it is not a consistent feature and, occasionally, lymphocytes can also be observed in non-diabetic cats. DM and overt inflammation of the exocrine pancreas (pancreatitis) may co-exist in some patients, but at present the relationship between these two conditions, in terms of cause versus effect, remains uncertain.

Macrophages are another class of immune cell of particular interest in human diabetic research. They are heavily implicated in the pathogenesis of type 2 diabetes and recent evidence suggests they may also play a part in the development of type 1 diabetes; however, the role of macrophages in canine and feline DM is less clear. One focus of our current PetSavers-funded research is the role of macrophages in the pathogenesis of both feline and canine diabetes. These immune cells are increasingly recognized to exhibit a wide range of phenotypes and to have diverse roles in the pathogenesis of many diseases. We hope that our current study will further elucidate the role of macrophages in the development of canine and feline DM.

New technologies, better understanding of disease, identification of new therapeutic targets…

In parallel with increased recognition of the role of macrophages in the pathogenesis of human DM, there has been an explosion of interest in human medicine in new technologies that allow optical tissue clearing of tissues obtained from patients. When combined with immunofluorescence and confocal microscopy, these techniques facilitate microscopic visualization of tissues and cells in three dimensions in a startling amount of detail and to an unprecedented degree of depth. We are adapting one such technique for use in canine and feline tissue. In addition to expanding the toolbox of imaging techniques available to assess canine and feline pancreatic islets, we are harnessing the power of artificial intelligence and machine learning to quantify changes in a robust and unbiased manner.

By better understanding the complex cellular interactions underpinning changes in the diabetic islets, we envisage that our research will help to identify new cellular dynamics and signalling pathways. It is hoped that these previously unrecognized cellular interactions may constitute potential novel therapeutic targets to offer new opportunities to treat or prevent cases of canine and feline DM, thus facilitating precision medicine. This is a particularly attractive and compelling prospect in patients where some functional beta cells remain at the time of diagnosis such that the need for administration of exogenous insulin might be reduced if the progress of the pathological process could be slowed or halted.

Acknowledgement: We would like to thank the colleagues who have contributed samples to this project, and the members of the Canine Diabetes Genetics Partnership for their ongoing support. We are extremely grateful to PetSavers for funding Valeria Bergomi through a Masters Degree by Research awarded to Kate Hughes (MDR 04.19). The study plan for the work detailed here was approved by the Ethics and Welfare Committee of the Department of Veterinary Medicine, University of Cambridge (CR315).