In Vivo, Ex Vivo and In Silico Methods to Study Pancreas Biology During Health and Disease

Editorial

The pancreas is an organ containing two distinct functional segments that include the exocrine and endocrine tissues. The main cellular components of the exocrine tissue include acinar and ductal cells, which release enzymes that aid in the digestion of food. The endocrine compartment consists of pancreatic islets of Langerhans, where alpha and beta cells play an essential role in the regulation of glucose metabolism by controlling the release of glucagon and insulin hormones. Cellular dysfunction of the exocrine and endocrine compartments leads to pathological conditions that include type-1 and type-2 diabetes, pancreatitis, and cancer. The mechanisms that regulate these distinct systems are not completely understood. The goal of this collection is to bring together complementary methods that will contribute to a better understanding of pancreas biology during health and disease.

Dysfunction or death of the insulin-secreting beta cell leads to diabetes, a devastating disease that affects roughly 10% of the population worldwide¹. It is therefore critical to understand the physiology of the beta cell to find treatments to restore cellular function and prevent beta cell loss. Intracellular calcium signaling regulates numerous cellular functions, including insulin secretion from beta cells. Glucose, the major insulin secretagogue, triggers intracellular calcium oscillations in beta cells. The synchronization of these oscillations is critical to islet function. As the critical micro-organ regulating glucose metabolism, pancreatic islets are conserved in many animal species. In the protocol by Delgadillo et al., the authors use a zebrafish model to study beta cell function in vivo. To perform in vivo imaging, they leverage the optical transparency of zebrafish during their larvae stage. Using transgenic lines expressing fluorescent reporters, the authors performed live Ca²⁺ imaging to track calcium oscillations in response to glucose stimulation². In higher vertebrate animals, unlike zebrafish, the pancreas at all stages is within a non-transparent abdominal cavity. To circumvent this biological limitation for imaging, a “living window” model was described, which required a surgical procedure to insert a transparent window into the abdominal cavity of mice³. The window was designed to create optical transparency and seclude the pancreas from the bowel in the abdominal cavity, which allowed the pancreas to remain fixed in place next to the imaging window. The authors describe the utility of the in vivo imaging methods by imaging multiple models including vascular biology, islets, acinar cell tissues, and pancreatic cancer³. The previously mentioned in vivo techniques^2,3 are orthogonal approaches with distinct advantages and limitations, which include expertise in animal surgical manipulations, ease of genetic manipulation, and genetic similarity when compared to human biology.

In vivo manipulation is possible in animal models; however, experimentation in a nonclinical setting using human tissues requires in vitro approaches. Recently, methods using living tissue slices derived from biological specimens obtained during surgical biopsies or post-mortem donors have provided invaluable insight into pancreas biology^4,5,6. While human tissue is most relevant to human disease, mouse tissues are also useful for modeling human disease, given that human tissues are a limited biological resource. As such, in Stožer et al., the authors perform calcium imaging of acinar, islet, and ductal components of tissue slices from mouse tissues⁷. These results have implications for studying multiple aspects of endocrine and exocrine biology. To model diabetes pathology using human diabetic tissues, tissue slices were prepared by Huber et al., who describe a method to track islet and immune cell interactions, as well as beta cell function during diabetes⁸. Using confocal microscopy, the authors performed live and dead imaging, and also recorded functional calcium responses from pancreatic islets in diabetic donors (mouse NOD and human T1D). Moreover, in the context of type-1 diabetes autoimmunity, which occurs when immune cells attack the pancreatic islets, the authors performed in situ cytolabeling to track immune cells in living slices⁸. Importantly, the slice models describe the only current model that contains all the cytoarchitectural components of the pancreatic tissue. Although blood flow is cut from the circulation, the authors importantly describe an ­in vitro­ model where paracrine interactions between immune, islet, acinar, and vascular elements can be modeled^5,9,10,11.

Distinct from in vitro and in vivo approaches, Felix-Martinez et al. describe an in silico approach to quantitatively study morphological and connectivity properties of reconstructed islets¹². Functional simulations were performed based on the pulsatile nature of islet hormone secretions (insulin, glucagon, and somatostatin). Applications of this approach will be particularly useful when modeling islets during normal and altered cell ratio states (i.e., loss of beta cells during diabetes), or cellular amalgamation-based organoid approaches^13,14,15.

While the endocrine pancreas controls glucose metabolism, the exocrine pancreas controls the secretion of enzymes into the gastrointestinal track to aid in digestion. While rare, inflammation of the pancreas caused by ductal obstruction can lead to acute pancreatitis. Zhou et al. describe a method to investigate acute pancreatitis using a surgical model. Here, the authors use a chemical induction technique by injecting sodium taurocholate into the biliopancreatic duct. The authors found a significant change in overall survival, the presence of inflammatory cells, and an increased accumulation of pancreatic enzymes such as amylase and lipase¹⁶. Given that the model recapitulates critical elements of pancreatitis biology, this method will be important for the validation of novel preclinical drugs for patients with this life-threatening condition.

Dysfunction of the endocrine and exocrine system has immediate, life-threatening consequences, and also presents long term chronic problems. Indeed, consequences of dysfunction in either system often lead to the impaired function of multiple systems, such as new-onset diabetes, that occurs in nearly 47% of patients during pancreatic cancer^17,18. Continuing efforts to better understand and model pancreas pathophysiology will improve our ability to understand and develop novel strategies for the treatment of pancreatic diseases such as diabetes, chronic pancreatitis, and cancer. The methods described here present an organized arrangement of tools to be used by researchers to study biology critical to pancreatic health and disease.