Immunocytochemical identification of endocrine cells in the pancreas of the fruit bat, Rousettus aegyptiacus

Comparative study of endocrine pancreatic tissue in bats: Assessing variations among frugivorous, insectivorous, and nectarivorous diets

Whether the endocrine pancreas exhibits structural features to couple with dietary patterns is not fully explored. Considering the lack of data comparing endocrine pancreas and islet cell distribution among different bat species in the same study, we considered this an opportunity to explore the topic, including five species within three different predominant diets. For this, we applied morphometric techniques to compare the islets of frugivorous Artibeus lituratus and Carollia perspicillata, insectivorous Molossus molossus and Myotis nigricans, and nectarivorous Glossophaga soricina bats. Data for islet size, cellularity, and mass were equivalent between frugivorous A. lituratus and nectarivorous G. soricina, which differed from insectivorous bats. The frugivorous C. perspicillata bat exhibited morphometric islet values between A. lituratus and the insectivorous species. A. lituratus and G. soricina but not C. perspicillata bats had higher islet mass than insectivorous species due to larger size, instead of a higher number of islets per area. Insectivorous bats, on the other hand, had a higher proportion of α-cells per islet. These differences in the endocrine pancreas across species with different eating habits indicate the occurrence of species-specific adjustments along the years of evolution, with the demand for α-cells higher in bats with higher protein intake.

Heterogeneity in the preferential diet of neotropical bats impacts the pancreatic islet mass and α and β cell distribution

Whether there is a relationship between bats' dietary patterns and evolutionary endocrine pancreas adaptation is not clearly understood. Aiming to contribute to this topic, we evaluated some metabolic and structural parameters in the following adult bats: the frugivorous Artibeus lituratus, the nectarivorous Anoura caudifer, the hematophagous Desmodus rotundus, and the insectivorous Molossus molossus. A. lituratus and A. caudifer diets consist of high amounts of simple carbohydrates, while D. rotundus and M. molossus diets consist of high amounts of proteins or protein and fat, respectively. In our results, A. lituratus and A. caudifer bats exhibited the highest values of relative islet mass (%), islet density (number of islets per pancreas area), and the lowest values of intestinal length among the four species. When adjusted by the body mass (mg/g of body mass), both D. rotundus and A. caudifer bats exhibited the highest islet mass values among the groups. Blood glucose was similar between A. lituratus, D. rotundus, and M. molossus, with the lowest values for the A. caudifer bats. M. molossus bats had the highest plasma cholesterol values among the studied species but exhibited similar plasma triacylglycerol with D. rotundus and A. caudifer bats. β- and α-cell distribution within A. lituratus, A. caudifer, and M. molossus islets achieved an approximate average value of ∼ 66% and ∼ 28%, respectively, a pattern inverted in D. rotundus islets (53% of α cells and 40% of β cells). A. caudifer and D. rotundus exhibited the highest and the lowest β/α-cells ratio per islet, respectively. We conclude that the macronutrient predominance in each bat-eating niche correlates with the morphophysiological pancreas features being the nectarivorous A. caudifer the species with the highest islet mass per body mass and β/α-cells ratio, while the hematophagous D. rotundus showed the highest α-cells apparatus.

Integrative single-cell characterization of a frugivorous and an insectivorous bat kidney and pancreas

Frugivory evolved multiple times in mammals, including bats. However, the cellular and molecular components driving it remain largely unknown. Here, we use integrative single-cell sequencing (scRNA-seq and scATAC-seq) on insectivorous (Eptesicus fuscus; big brown bat) and frugivorous (Artibeus jamaicensis; Jamaican fruit bat) bat kidneys and pancreases and identify key cell population, gene expression and regulatory differences associated with the Jamaican fruit bat that also relate to human disease, particularly diabetes. We find a decrease in loop of Henle and an increase in collecting duct cells, and differentially active genes and regulatory elements involved in fluid and electrolyte balance in the Jamaican fruit bat kidney. The Jamaican fruit bat pancreas shows an increase in endocrine and a decrease in exocrine cells, and differences in genes and regulatory elements involved in insulin regulation. We also find that these frugivorous bats share several molecular characteristics with human diabetes. Combined, our work provides insights from a frugivorous mammal that could be leveraged for therapeutic purposes.

Integrative single-cell characterization of frugivory adaptations in the bat kidney and pancreas

Frugivory evolved multiple times in mammals, including bats. However, the cellular and molecular components driving it remain largely unknown. Here, we used integrative single-cell sequencing on insectivorous and frugivorous bat kidneys and pancreases and identified key cell population, gene expression and regulatory element differences associated with frugivorous adaptation that also relate to human disease, particularly diabetes. We found an increase in collecting duct cells and differentially active genes and regulatory elements involved in fluid and electrolyte balance in the frugivore kidney. In the frugivorous pancreas, we observed an increase in endocrine and a decrease in exocrine cells and differences in genes and regulatory elements involved in insulin regulation. Combined, our work provides novel insights into frugivorous adaptation that also could be leveraged for therapeutic purposes.

Difference in glucose tolerance between phytophagous and insectivorous bats

Bats are mostly insectivorous or phytophagous. It is hypothesized that bats are evolved from small insectivorous mammals. Therefore, the digestive and metabolic systems of phytophagous and insectivorous bats must have evolved differently to adapt to their dietary habits. To investigate the difference in sugar tolerance in bats, we determined changes in blood glucose levels after intraperitoneal (i.p.) injection of glucose in three species of phytophagous and four species of insectivorous bats under resting conditions. Results showed that phytophagous bats eliminated blood glucose faster than insectivorous bats. All three species of fruit bats reduced blood glucose to fasting levels within 30-45 min, whereas all insectivorous bats failed to lower blood glucose to fasting levels even 120 min after i.p. glucose injection. Taken together, results of this study suggest that bats have undergone adaptations and become diversified in dietary habits.

Influence of feeding habits in the endocrine pancreas of insectivore bat Pteronotus personatus and nectarivore bat Anoura geoffroyi: A comparative stereological and immunohistochemical study

Pteronotus personatus as an insectivore bat and has a diet that consists of a high protein diet, whereas the diet of Anoura geoffroyi, a predominantly nectarivore bat, is rich in simple sugars like sucrose, glucose and fructose. Considering that diet influences the activation of different pathways, which may influence morphological adaptations in the gastrointestinal system, the aim of this study was to compare the morphology of the endocrine pancreas in P. personatus and A. geoffroyi. For this, histological, stereological and immunohistochemical methods were used. In P. personatus, the average diameter of the pancreatic islet was 40.47μm±13.94, while in A. geoffroyi was 88.16μm±36.40. The total number of pancreatic islets in P. personatus was 26150±2346 and in A. geoffroyi was 15970±1666. In P. personatus, the volume density of the pancreatic islets was 3.4%± 2.6, whereas in A. geoffroyi the volume density was 6.1%±3.7. In addition, the immunodensity of the α, β and δ cells, in P. personatus was 25.8%±11.9, 35.5%±13.5, 3.9%±0.7, respectively, and in A. geoffroyi was 33.10%±12.7, 55.08%±7.4, 6.2%±4.6, respectively. In conclusion, the results of this study indicate differences in the pancreatic weight/body, weight ratio, diameter and volume density of pancreatic islets and in immunodensity of the β and α cells between both species, which have different dietary habits.

Bats: Body mass index, forearm mass index, blood glucose levels and SLC2A2 genes for diabetes

Bats have an unusually large volume of endocrine tissue, with a large population of beta cells, and an elevated sensitivity to glucose and insulin. This makes them excellent animal models for studying diabetes mellitus. We evaluated bats as models for diabetes in terms of lifestyle and genetic factors. For lifestyle factors, we generated data sets of 149 body mass index (BMI) and 860 forearm mass index (FMI) measurements for different species of bats. Both showed negative inter-species correlations with blood glucose levels in sixteen bats examined. The negative inter-species correlations may reflect adaptation of a small insectivorous ancestor to a larger frugivore. We identified an 11 bp deletion in the proximal promoter of SLC2A2 that we predicted would disrupt binding sites for the transcription repressor ZNF354C. In frugivorous bats this could explain the relatively high expression of this gene, resulting in a better capacity to absorb glucose and decrease blood glucose levels.

Structural similarities and differences between the human and the mouse pancreas

Mice remain the most studied animal model in pancreas research. Since the findings of this research are typically extrapolated to humans, it is important to understand both similarities and differences between the 2 species. Beside the apparent difference in size and macroscopic organization of the organ in the 2 species, there are a number of less evident and only recently described differences in organization of the acinar and ductal exocrine tissue, as well as in the distribution, composition, and architecture of the endocrine islets of Langerhans. Furthermore, the differences in arterial, venous, and lymphatic vessels, as well as innervation are potentially important. In this article, the structure of the human and the mouse pancreas, together with the similarities and differences between them are reviewed in detail in the light of conceivable repercussions for basic research and clinical application.

Relaxed evolution in the tyrosine aminotransferase gene tat in old world fruit bats (Chiroptera: Pteropodidae)

Frugivorous and nectarivorous bats fuel their metabolism mostly by using carbohydrates and allocate the restricted amounts of ingested proteins mainly for anabolic protein syntheses rather than for catabolic energy production. Thus, it is possible that genes involved in protein (amino acid) catabolism may have undergone relaxed evolution in these fruit- and nectar-eating bats. The tyrosine aminotransferase (TAT, encoded by the Tat gene) is the rate-limiting enzyme in the tyrosine catabolic pathway. To test whether the Tat gene has undergone relaxed evolution in the fruit- and nectar-eating bats, we obtained the Tat coding region from 20 bat species including four Old World fruit bats (Pteropodidae) and two New World fruit bats (Phyllostomidae). Phylogenetic reconstructions revealed a gene tree in which all echolocating bats (including the New World fruit bats) formed a monophyletic group. The phylogenetic conflict appears to stem from accelerated TAT protein sequence evolution in the Old World fruit bats. Our molecular evolutionary analyses confirmed a change in the selection pressure acting on Tat, which was likely caused by a relaxation of the evolutionary constraints on the Tat gene in the Old World fruit bats. Hepatic TAT activity assays showed that TAT activities in species of the Old World fruit bats are significantly lower than those of insectivorous bats and omnivorous mice, which was not caused by a change in TAT protein levels in the liver. Our study provides unambiguous evidence that the Tat gene has undergone relaxed evolution in the Old World fruit bats in response to changes in their metabolism due to the evolution of their special diet.

Comparative Pancreatic Pathology

Spontaneous pathologies of the pancreas are important causes of morbidity and mortality in some veterinary species and rare in others. As in human beings, the pancreas of most domestic and exotic animals is a composite organ with both endocrine and exocrine functions. The similarities between structure and function of porcine, canine, and human pancreata are such that the pig and dog serve as valuable models in basic and translational studies, most recently for efforts aimed at modeling pancreatitis and diabetes, developing functional and sustainable replacement of endocrine functions, and in imaging and manipulation studies.

This article will provide a brief review of spontaneous veterinary diseases and their underlying mechanisms and the morphological features that reflect these alterations. Several species- or breed-specific conditions and the effects of selected systemic diseases on the pancreas are also discussed. The contributions to our knowledge of pancreatic physiology and pathology by small mammal (rodent) and engineered animal models and the in-depth mechanisms homologous to those in the human pancreas are covered in other sections of this article.

What difference exists in the pancreas of mammals with sanguivorous diet? A morphological, stereological and immunohistochemical study of the pancreatic islets of the hematophagous bat Diphylla ecaudata

Diphylla ecaudata is a vampire bat that mainly feeds on the blood of birds. This highly specialized diet - hematophagy - is accompanied by a series of morphological changes in the gastro-entero-pancreatic system, since the distribution and relative proportions of different pancreatic endocrine cell types can vary between species due to different physiological conditions and eating habits. The aim of this study was to examine for the first time the pancreas of the vampire bat D. ecaudata using morphological, stereological and immunohistochemical techniques. The pancreas of the D. ecaudata has an exocrine acinar portion in which the highest concentration of pancreatic islets is scattered. These pancreatic islets have irregular size and a mean diameter of 56.94 μm. The total number of islets in the pancreas was 23,900, with a volumetric density of 4.1%. Insulin-immunoreactive (IR) cells were located in the central pancreatic islet region and had the largest density (54.8%). Glucagon-IR cells were located mainly in the peripheral mantle region (16.2%), along with somatostatin-IR (SS) cells (14.3%). Cells immunoreactive to insulin, glucagon and somatostatin were also observed to have spread in isolated places in the exocrine pancreas. In the connective tissue near the pancreatic ducts, a high concentration was identified of insulin-IR cells and a low concentration of glucagon-IR and somatostatin-IR cells. These results indicate that although the pancreas of D. ecaudata has morphological similarities with that of other mammals, it has a differentiated islet structure, because there were a large number of islets and different volumetric densities of α, β and δ cells.

Reduced insulin secretion and glucose intolerance are involved in the fasting susceptibility of common vampire bats

Susceptibility during fasting has been reported for the common vampire bat (Desmodus rotundus), to the point of untimely deaths after only 2-3 nights of fasting. To investigate the underlying physiology of this critical metabolic condition, we analyzed serum insulin levels, pancreatic islets morphometry and immunocytochemistry (ICC), static insulin secretion in pancreas fragments, and insulin signaling mechanism in male vampire bats. A glucose tolerance test (ipGTT) was also performed. Serum insulin was found to be lower in fed vampires compared to other mammals, and was significantly reduced after 24h fasting. Morphometrical analyses revealed small irregular pancreatic islets with reduced percentage of β-cell mass compared to other bats. Static insulin secretion analysis showed that glucose-stimulated insulin secretion was impaired, as insulin levels did not reach significance under high glucose concentrations, whereas the response to the amino acid leucin was preserved. Results from ipGTT showed a failure on glucose clearance, indicating glucose intolerance due to diminished pancreatic insulin secretion and/or decreased β-cell response to glucose. In conclusion, data presented here indicate lower insulinemia and impaired insulin secretion in D. rotundus, which is consistent with the limited ability to store body energy reserves, previously reported in these animals. Whether these metabolic and hormonal features are associated with their blood diet remains to be determined. The peculiar food sharing through blood regurgitation, reported to this species, might be an adaptive mechanism overcoming this metabolic susceptibility.

Adaptive evolution in the glucose transporter 4 gene Slc2a4 in Old World fruit bats (family: Pteropodidae)

Frugivorous and nectarivorous bats are able to ingest large quantities of sugar in a short time span while avoiding the potentially adverse side-effects of elevated blood glucose. The glucose transporter 4 protein (GLUT4) encoded by the Slc2a4 gene plays a critical role in transmembrane skeletal muscle glucose uptake and thus glucose homeostasis. To test whether the Slc2a4 gene has undergone adaptive evolution in bats with carbohydrate-rich diets in relation to their insect-eating sister taxa, we sequenced the coding region of the Slc2a4 gene in a number of bat species, including four Old World fruit bats (Pteropodidae) and three New World fruit bats (Phyllostomidae). Our molecular evolutionary analyses revealed evidence that Slc2a4 has undergone a change in selection pressure in Old World fruit bats with 11 amino acid substitutions detected on the ancestral branch, whereas, no positive selection was detected in the New World fruit bats. We noted that in the former group, amino acid replacements were biased towards either Serine or Isoleucine, and, of the 11 changes, six were specific to Old World fruit bats (A133S, A164S, V377F, V386I, V441I and G459S). Our study presents preliminary evidence that the Slc2a4 gene has undergone adaptive changes in Old World fruit bats in relation to their ability to meet the demands of a high sugar diet.

Insulin and glucose sensitivity, insulin secretion and beta-cell distribution in endocrine pancreas of the fruit bat Artibeus lituratus

The fruit bat Artibeus lituratus absorbs large amounts of glucose in short periods of time and maintains normoglycemia even after a prolonged starvation period. Based on these data, we aimed to investigate various aspects related with glucose homeostasis analyzing: blood glucose and insulin levels, intraperitoneal glucose and insulin tolerance tests (ipGTT and ipITT), glucose-stimulated insulin secretion (2.8, 5.6 or 8.3 mmol/L glucose) in pancreas fragments, cellular distribution of beta cells, and the amount of pAkt/Akt in the pectoral muscle and liver. Blood glucose levels were higher in fed bats (6.88+/-0.5 mmol/L) than fasted bats (4.0+/-0.8 mmol/L), whereas insulin levels were similar in both conditions. The values of the area-under-the curve obtained from ipGTT were significantly higher when bats received 2 (5.5-fold) or 3g/kg glucose (7.5-fold) b.w compared to control (saline). These bats also exhibited a significant decrease of blood glucose values after insulin administration during the ipITT. Insulin secretion from fragments of pancreas under physiological concentrations of glucose (5.6 or 8.3 mmol/L) was similar but higher than in 2.8 mmol/L glucose 1.8- and 2.0-fold, respectively. These bats showed a marked beta-cell distribution along the pancreas, and the pancreatic beta cells are not exclusively located at the central part of the islet. The insulin-induced Akt phosphorylation was more pronounced in the pectoral muscle, compared to liver. The high sensitivity to glucose and insulin, the proper insulin response to glucose, and the presence of an apparent large beta-cell population could represent benefits for the management of high influx of glucose from a carbohydrate-rich meal, which permits appropriate glucose utilization.

Pancreatic islet plasticity: interspecies comparison of islet architecture and composition

The pancreatic islet displays diverse patterns of endocrine cell arrangement. The prototypic islet, with insulin-secreting beta-cells forming the core surrounded by other endocrine cells in the periphery, is largely based on studies of normal rodent islets. Recent reports on large animals, including humans, show a difference in islet architecture, in which the endocrine cells are randomly distributed throughout the islet. This particular species difference has raised concerns regarding the interpretation of data based on rodent studies to humans. On the other hand, further variations have been reported in marsupials and some nonhuman primates, which possess an inverted ratio of beta-cells to other endocrine cells. This review discusses the striking plasticity of islet architecture and cellular composition among various species including changes in response to metabolic states within a single species. We propose that this plasticity reflects evolutionary acquired adaptation induced by altered physiological conditions, rather than inherent disparities between species.

The comparative anatomy of islets

In the past 20 years, numerous publications on a variety of mammalian and non-mammalian species have appeared in the literature to supplement the excellent comparative work performed in the 70s and 80s by the Falkmer, Epple, and Youson groups. What emerges is that islets are much more complex than once thought and show a lot of similarities in rodents and higher primates. The diversity of lifestyles, metabolic demands, and diets has most likely influenced the great diversity in both structure and cell-type content of islets in lower vertebrate species. In this chapter, I try to provide an overview of the evolution from endocrine cell types in invertebrates to the higher mammals and focus on what has been reported in the literature and some of our own experiences and also include a description of other hormones reported to be found in islets.

Endocrine function in naturally long-living small mammals

The complex, highly integrative endocrine system regulates all aspects of somatic maintenance and reproduction and has been widely implicated as an important determinant of longevity in short-lived traditional model organisms of aging research. Genetic or experimental manipulation of hormone profiles in mice has been proven to definitively alter longevity. These hormonally induced lifespan extension mechanisms may not necessarily be relevant to humans and other long-lived organisms that naturally show successful slow aging. Long-lived species may have evolved novel anti-aging defenses germane to naturally retarding the aging process. Here, we examine the available endocrine data associated with the vitamin D, insulin, glucocorticoid and thyroid endocrine systems of naturally long-living small mammals. Generally, long-living rodents and bats maintain tightly regulated lower basal levels of these key pleiotropic hormones than shorter lived rodents. Similarities with genetically manipulated long-lived rodent models of aging suggest that evolutionary well-conserved hormonal mechanisms are integrally involved in lifespan determination.

A comparative immunohistochemical study of endocrine cells in the digestive tract of two frugivorous bats: Artibeus cinerius and Sturnira lilium

The purpose of the present study was to examine the serotonin (5-hydroxytryptamine, 5-HT), gastrin (GAS), cholecystokinin (CCK) and glucagon (GLUC) endocrine cells in the gastrointestinal tract of frugivorous Phillostomidae bats, Sturnira lilium and Artibeus cinerius, to clarify the correlation between distribution of cell types and their relative frequency, with feeding habits. Five portions of the gastrointestinal tract--fundus, pilorus, and three parts of the intestine, I, II and III--were examined. Most of the immunoreactive cells in the stomach and intestine were of triangular, oval or piriform shape. Serotonin-immunoreactive cells were most commonly found in the S. lilium intestine I (66.6+/-9.9) and the A. cinerius intestine III (35+/-18). Gastrin-immunoreactive cells were the most abundant cell type in the pyloric glands of both species. They were more numerous in A. cinerius (126.9+/-27.4) than in S. lilium (75.8+/-1.8). CCK-immunoreactive cells were found in the alimentary tract epithelia at moderate frequencies in both species. GLUC-immunoreactive cells were detected at very low or low frequencies. This study suggests that there is a correlation between endocrine cell distribution and frequency, and the feeding habits of the bats.

The pancreas of the naked mole-rat (Heterocephalus glaber): an ultrastructural and immunocytochemical study of the endocrine component of thermoneutral and cold acclimated animals

Endocrine cell distribution within the islets of Langerhans may vary both between species and under different energetically demanding conditions such as cold acclimation. The naked mole-rat, Heterocephalus glaber, lacking an effective insulatory pelage, is effectively a poikilotherm, yet it shows a typical mammalian cold-acclimation response by substantially increasing food intake to meet higher energy requirements when housed at lower temperatures. The endocrine component of the pancreas of thermoneutral and cold-acclimated naked mole-rats was thus characterized using immunocytochemistry and ultrastructural analyses. Four distinct endocrine cells were identified: alpha (glucagon-producing), beta (insulin-producing), delta (somatostatin-producing), and PP (pancreatic polypeptide-producing) cells. Distribution of these cells differed from that of other rodents, in that beta cells formed the mantle while alpha cells formed the core of the islets. This distribution may contribute to the observed insulin insensitivity of this species, as indicated in abnormal responses to glucose tolerance tests. Insulin-producing cells, however, were more numerous than glucagon-producing cells. This ratio was unchanged with cold acclimation. Immunoreactivity of alpha and beta cells was more intense in cold-acclimated than in thermoneutral animals, possibly indicative of a change in hormonal production in animals housed at a lower temperature.