Authors: Xiaofan Ding, Jianhui Chen, Wenwen Zeng
Categories: Review, Metabolism, Neuroimmune, Neuroimmune and pancreatic diseases, Pancreas, Pancreatic neuronal innervation
Source: Fundamental Research
The pancreas exerts endocrine and exocrine functions in energy balance. The neural innervation and immune milieu are both crucial in supporting pancreatic homeostasis. The neuronal network connects the pancreas with the central nervous system (CNS) and the enteric nervous system (ENS) and sustains metabolic activities. The nerves in the pancreas are categorized as spinal sensory afferent fibers, vagal sensory afferent nerves, autonomic fibers of both sympathetic and parasympathetic divisions, and fibers from the ENS and intrapancreatic ganglia. They innervate different regions and various cell types, which collectively determine physiological functions. Studies have established that the diverse pathological conditions, including pancreatitis, diabetes, and pancreatic tumor, are attributed to aberrant immune reactions; however, it is largely not clear how the neuronal network may influence the disease conditions. Enlightened by the recent advances illuminating the organ-wide neuronal architecture and the dysfunctions in pancreatic disorders, this review will highlight emerging opportunities to explore the cellular interrelationship, particularly the neuroimmune components in pancreatic health and diseases.
Keywords: Neuroimmune, Pancreas, Pancreatic neuronal innervation, Neuroimmune and pancreatic diseases, Metabolism
The neural innervation and immune cells support the pancreas in its functions of nutritional assimilation and glucose homeostasis. The abundant nerve supply of the pancreas was described back more than a century ago at the times of Paul Langerhans, and the secretory role of neural impulses was subsequently demonstrated by Ivan Pavlov and others [1,2]. Anatomical and functional studies reveal that the pancreas connects to the CNS and ENS, and also contains intrapancreatic ganglia. The nerve bundles entering the pancreas include sensory afferent fibers and autonomic fibers of both sympathetic and parasympathetic branches (Fig. 1). Histochemical, immunohistochemistry, and tracing studies show that their innervation targets differ, which influences the distinct physiological functions. Recent advances in volume tissue imaging begin to illuminate the organ-wide neuronal architecture and spatial organization [3], [4], [5], [6], which has opened up new opportunities to explore the cellular interrelationship, particularly the neuronal activity and immune components in health and diseases.
Fig. 1 Neuronal innervation of the pancreas. The pancreas receives sympathetic and parasympathetic efferent, spinal and vagal afferent, and also innervation from the ENS. The preganglionic sympathetic neurons reside in the intermediolateral nucleus (IML) in the spinal cord, and project to the sympathetic neuronal cell bodies located within the celiac ganglia (CG) and superior mesenteric ganglia (SMG). The postganglionic sympathetic fibers enter the pancreas and innervate the blood vessels, islets, and to a lesser extent, the exocrine pancreas. The parasympathetic fibers originate in the DMV and innervate intrapancreatic ganglia and the islets. Afferent vagal neurons have their cell bodies located within the nodose ganglion (NG), and the nerves traveling to innervate the islet. The spinal sensory nerves with cell bodies in dorsal root ganglia (DRG) predominantly innervate the exocrine pancreas. The intrapancreatic ganglionic neurons receive sensory and autonomic inputs and are also innervated by the ENS.
Within the organ, the pancreas is structurally organized into distinct compartments for the endocrine and exocrine functions [1]. Though their functions are similar, the gross anatomy shows the difference between the the human pancreas is a solitary organ divided into three parts, the head, the body, and the tail; in mice, the pancreas is located at the mesentery of the small intestine in a dendritic manner with three lobes, the duodenal, the gastric and the splenic lobe. The exocrine pancreas resides in the parenchyma and amounts to the majority of the pancreas volume. It consists of acinar cells for synthesizing and releasing digestive enzymes and ductal cells for producing chloride and bicarbonate into the small intestine to assist food digestion and assimilation. The endocrine pancreas contains major endocrine cell types, e.g., beta cells secrete insulin, alpha cells release glucagon, and delta cells produce somatostatin. Insulin and glucagon released into the bloodstream are vital for maintaining glucose homeostasis. The islets are highly innervated compared to exocrine tissue with regional variations and the duodenal lobe is more densely innervated than the splenic lobe [3].
The pancreas is innervated by various types of nerve fibers. Sensory nerves relay the tissue information to the CNS via both spinal and vagal pathways and also release neuropeptides for regulating pancreatic functions [1,5,7]. The cell bodies of the spinal afferent nerves are located in thoracic and lumbar dorsal root ganglia, and their fibers travel within the splanchnic nerves and celiac plexus predominantly to exocrine tissue in the pancreas. The pancreatic vagal afferent nerves originate from the nodose ganglia. Anterograde tracing of the nodose neurons in rat pancreas detects vagal afferent fibers in islets, and occasionally in acini and excretory ducts, supporting a role for these afferents in the endocrine pancreatic function [8]. Interestingly, injections of agglutinin-horseradish peroxidase conjugate into the right nodose ganglion lead to axonal labeling preferentially in the splenic lobe, whereas injections into the left ganglion mark fibers predominantly in the duodenal lobe. The regional distribution pattern of vagal afferent fibers perplexes the dissection of the precise functions of the spinal and vagal innervations, which remain to be fully uncovered.
The autonomic pancreatic innervation includes both parasympathetic and sympathetic divisions. The parasympathetic fibers originate from the dorsal motor nucleus of the vagus (DMV) and travel mostly in the hepatic and bilateral gastric branches and relatively few in the bilateral celiac branches of the vagus [7]. The nerves terminate in the intrapancreatic ganglia or enter the islet from the parenchyma [1,5]. The diversity in anatomical projections, as well as electrophysiological characteristics, was found within pancreatic-projecting neurons [7], studies of which will help clarify the parasympathetic control of the pancreatic functions.
Sympathetic preganglionic nerves descend from the lower thoracic and upper lumbar segments of the spinal cord and travel along the splanchnic nerves to the celiac and superior mesenteric ganglia. The postganglionic fibers then project from the ganglia and enter the pancreas. During development, sympathetic innervation appears to be important for the formation of pancreatic islet architecture and its functional maturation [9]. The sympathetic nerves innervate blood vessels, islets, and to a lesser extent, the exocrine pancreas, mostly characterized in early studies of dogs and cats and later in rodents and humans [4,[10], [11], [12]]. Adrenergic sympathetic stimulation causes vasoconstriction, through arteriovenous shunts between the interlobular arterioles and veins, resulting in reduced blood flow through the capillary beds, and inhibition of pancreatic exocrine secretion [13]. Particularly, islet pericytes wrap around the microvasculature and are innervated by sympathetic nerves [14]. The sympathetic adrenergic outflow induces the contractile activity of the pericytes, leading to reduced capillary diameter and local blood flow. The sympathetic nerves are also detected adjacent to the alpha cells and affect the endocrine pancreas [1]. The sympathetic neurotransmitter norepinephrine could stimulate glucagon release via the beta2-adrenergic receptor on alpha cells and inhibit insulin release via the alpha2-adrenergic receptor on beta cells. The sympathetic activity is considered to regulate exocrine secretion to a lesser extent, though the direct contribution to exocrine functions is often confounded by the vasoconstrictive effects [7]. Collectively, the contrasting yet tightly coordinated roles of the sympathetic and parasympathetic divisions in the pancreas await to be disentangled.
Intrapancreatic ganglia are dispersed in the pancreatic parenchyma and interlobular spaces, and some around pancreatic islets [1,5,15]. Those ganglia consist of 1–35 nerve cells, and the number varies with anatomical location and species [7,15]. They are mostly cholinergic and abundant in the perivascular plexus and fewer in the perineural plexus. The intrapancreatic neurons receive input from multimodal extrinsic neuronal types, including the parasympathetic, sympathetic, sensory, and enteric neurons [1,5]. In turn, their fibers reach vasculatures, endocrine cells in islets, exocrine acini, and the ductal system which express cholinergic receptors muscarinic 1 and 3 (M1R and M3R). Acetylcholine released by the nerve fibers function to promote exocrine secretion from acinar cells and the release of insulin and glucagon from endocrine cells. However, it remains largely elusive how the intrapancreatic neurons may integrate variable inputs to achieve various physiological responses.
The crosstalk of the pancreas with the brain may occur via autonomic, sensory, intrapancreatic neural network and indirectly through ENS. Retrograde viral transneuronal tracing using pseudorabies virus (PRV) shows that pancreas-innervating vagal sensory neurons project to the NTS and the DMV where the parasympathetic efferent pathways descend to the pancreas [16,17]; the pancreatic islets are innervated by efferent circuits that emanate from the hypothalamus [1,18]. Those anatomical pathways could serve as the communication routes between the brain and pancreas. For instance, pancreatic beta cells release serotonin together with insulin, and the former conveys information to vagal sensory neurons and further to the commissural NTS (cNTS) in the brainstem [19]. Microglia in the brain facilitates cephalic phase insulin release via stimulating vagal nerve transmission [20]. Pancreatic innervations coordinate the activity coupling between beta cells, an observation made in zebrafish in vivo [21]. However, though the early studies have revealed various brain sites involved in communicating with the pancreas, the precise brain-pancreas neurocircuitry remains ill-defined. Identifying and characterizing the functional pathways between the brain, pancreas, and gut would enrich our understanding of how pancreatic endocrine and exocrine functions are controlled and coordinated.
Neuroimmune surveillance starts to be recognized as an essential part of pancreatic homeostasis, dysfunction of which contributes to pancreatic diseases such as pancreatitis, diabetes, and tumors (Fig. 2). Inflammation is a hallmark of pancreatitis, and studies show that neurogenic inflammation participates in pancreatitis pathogenesis. Sensory nerve denervation ameliorates inflammation in rat models of secretagogue-induced pancreatitis and obstructive pancreatitis [22]. On the same line of neurogenic inflammation, chemogenetic activation of engineered M3R expressed in pancreatic acinar cells in vivo initiates acute pancreatitis to manifest widespread inflammation and progress to chronic pancreatitis [23]. M3R antagonist ameliorates the severity of acute pancreatitis, implicating a pro-inflammatory role of the M3R ligand acetylcholine which could be derived from parasympathetic or intrapancreatic neurons. While on the other hand, a “nicotinic anti-inflammatory pathway” was also proposed, based on previous findings such as that vagotomy or pretreatment with the nicotinic receptor antagonist leads to an enhanced severity of pancreatitis [24]. It is thus not clear how the neuronal signals may influence inflammation in different contexts, and it is likely that various neural types and their associated neurotransmitters could execute pro- or anti-inflammatory functions in concert with the responsive pancreatic-residential cells or infiltrating immune cells. For instance, most of the spinal afferents, vagal sensory axons, and intrapancreatic neurons express neuropeptides including calcitonin gene-related peptide (CGRP), which affects inflammation in multiple organs such as the lung and gut [1,25,26]. In the lung, CGRP works in concert with intrleukin-33 (IL-33) and neuromedin-U (NMU) and supports IL-5 but constrains IL-13 expression and group 2 innate lymphoid cells (ILC2) proliferation [27]. While in the gut, CGRP antagonizes ILC2 expansion but promotes IL-5 expression in intestinal type 2 immunity [28]. Future studies are imperative to understand how those neuron-derived neuropeptides or neurotransmitters may mediate the inflammatory response in the pancreas.
Fig. 2 Neuroimmune regulation in the pancreas. Neurogenic inflammation mediated by muscarinic receptor signal in acinar cells promotes acute pancreatitis, however, vagotomy or pretreatment with the nicotinic receptor antagonist results in an increased severity of pancreatitis. Most of the sensory axons and intrapancreatic neurons express neuropeptides including calcitonin gene-related peptide (CGRP), and its effects on pancreatic inflammation is unknown. A loss of sympathetic nerves in islets is observed in autoimmune diabetes both in mice and humans, while an increase is detected for the total nerve fiber density in the surviving islets in mice. TRPV1^+^ sensory neurons promote islet inflammation and beta cell stress, while pancreatic nerve electrical stimulation leads to reduced proliferation of autoreactive T cells in pancreatic lymph nodes and halts disease progress. Increased density of sympathetic nerves is observed in T2D mice, and increased total nerve density in islets is also observed in human T2D. Sensory neurons promote the initiation and progression of the early stages of pancreatic ductal adenocarcinoma (PDAC). The pancreas acinar-derived cells invade along sensory neurons into the spinal cord and sensory neuron ablation prevents perineural invasion and prolongs survival.
Both type 1 and type 2 diabetes (T1D and T2D) display features of dysregulated immune homeostasis and neuronal innervation. T1D is characterized by the loss of immune tolerance to pancreatic beta cells leading to their destruction. A loss of sympathetic nerves in islets is observed in autoimmune diabetes both in mice and humans [29], [30], [31], [32], while when the total nerve fiber density is determined with whole-organ assessment, an increase is detected in the surviving islets in mice [3]. Interestingly, in the autoimmune diabetic mice, pancreatic sensory neurons labeled by transient receptor potential cation channel subfamily V member 1 (TRPV1) promote islet inflammation and beta cell stress [33], however, pancreatic nerve electrical stimulation leads to reduced proliferation of autoreactive T cells in pancreatic lymph nodes and halts disease progress [34], indicating the opposing roles of pancreatic innervation in autoimmunity. T2D animal models, the db/db mice, show the increased density of pancreatic sympathetic nerves at weaning age visualized by 3D images [35], suggesting a potentially elevated sympathetic outflow. Increased total nerve density in islets is also observed in the pancreas of human donors with T2D [3]. The complex roles of neuroimmune interaction in diabetes are not well understood and further research is needed.
Other disease conditions are also affected by the immune and neuronal components in the pancreas. For instance, sensory neurons promote the initiation and progression of the early stages of pancreatic ductal adenocarcinoma (PDAC), one of the deadly malignancies with limited response to current treatments [36]. The pancreas acinar-derived cells frequently invade along sensory neurons into the spinal cord and sensory neuron ablation prevents perineural invasion and prolongs survival. New insights underlying the neuronal promotion of tumor aggression are in urgent need to inform the development of new diagnostic tools and immunotherapeutic strategies.
The pancreatic nerves are subjected to the damaging effects of disease conditions, such as the above-mentioned conditions of diabetes, pancreatitis, and cancer. Diabetic peripheral neuropathy (DPN) is the most common complication associated with neuropathic pain [37]. The past effort focusing on the somatosensory nerves implicates that chronic inflammation and metabolic dysregulation contribute to their pathological changes. It thus leads to an open question of whether the intrapancreatic nerves are susceptible to the same sets of inflicting factors, or whether a distinct mechanism exists considering the complex composition of the intrapancreatic nerves. As an inflammatory disease of the exocrine pancreas, abdominal pain is common in patients with chronic pancreatitis [38]. The neuroinflammatory response tends to enhance the pain sensation, as demonstrated by early studies in the somatosensory nerves [38], [39], [40], [41], and likely acts as a pathophysiological pain mechanism in chronic pancreatitis. For patients with chronic pancreatitis or pancreatic adenocarcinoma, increased neural density and hypertrophy have been observed. Neural alterations occur in the majority of pancreatic adenocarcinoma patients with intra- and extra-pancreatic perineural invasion of cancer cells [42]. Severe and enduring pain is associated with poor prognosis in pancreatic adenocarcinoma patients’ pain sensation [43]. The unanswered question is whether the neuroimmune reaction may contribute to the pain behavior and influence the tumor progression within the pancreas.
Direct interplay takes place between the pancreatic nerves and the immune system, which might influence the structural and functional integrity of the pancreatic nerves. The immune reaction can target the nerve components, not just proteins but also carbohydrates. For instance, gangliosides, the sialic acid-containing glycosphingolipids particularly abundant in the nervous system, represent a crucial factor bridging the immune and nervous systems. They are composed of hydrophobic ceramide portion predominantly which is inserted into the outer leaflet of the cellular membrane, and hydrophilic carbohydrate moieties facing the outside of the cells [44]. They interact with a diverse range of lipids and proteins laterally in the plasma membrane and through their head groups to regulate cellular function.
When binding to the complement regulatory protein factor, the ganglioside could prevent complement attack and thereby protect the host cells. Besides, they could function as pattern recognition receptors to mediate innate immune responses [45]. Further, the ganglioside such as a-GalCer could function as a lipid antigen for NKT. And a-glycosylceramides act as endogenous ligands for NKT cells. B-GlcCer could serve as ligands for Mincle to modulate immune activity [45]. Further, certain microbes and viruses exploit the gangliosides as the entry point for cellular infection, e.g., influenza viruses bind to the sialic acid and gain entrance to the host cells [44,46]. The crucial role of microbes present in the pancreas is increasingly appreciated, both in the cancer progress and immunotherapy [47,48]. However, whether the microbes might impact the neuroimmune network and affect the disease course remains unknown.
Moreover, the autoantibodies targeting gangliosides are identified [49]. Gangliosides are abundant in neuronal membranes and also present in myelin, principally enriched in the node of Ranvier which is often affected by autoimmune neuropathy. The autoantibodies bind glycan epitopes on peripheral nerve gangliosides, thus triggering complement cascade, recruiting macrophages, and causing disruption of nerve conductivity [46]. The self-reactive antibodies may arise spontaneously from the natural antibody repertoire, or be induced by infections that share structural similarities to gangliosides. Campylobacter jejuni displays mimics of gangliosides on its surface lipo-oligosaccharide and infection of Campylobacter jejuni is strongly associated with serum anti-gangliosides antibodies [50]. The pathophysiological role of anti-ganglioside autoantibodies in autoimmune demyelination neuropathy has been demonstrated in Guillain–Barré syndrome and the Miller–Fisher syndrome [44]. Further investigation might help elucidate whether pancreatic nerves are disrupted by the autoantibodies against gangliosides. Particularly, the anti-gangliosides antibodies have long been described in the autoimmune type 1 diabetes [50,51], while it is not known whether they may affect the pancreatic nerves and change energy balance.
The research efforts have greatly enhanced our knowledge regarding inter-cellular communications in nutritional homeostasis and also in pathophysiology. Recent advances in optogenetics, pharmacological and genetic approaches in combination with imaging techniques for functional perturbation and interrogation have empowered the endeavors to delineate the network between pancreatic innervation and the immune system. Future studies will clarify the neuroanatomical and neurochemical circuits within the pancreas and between the pancreas and other organs such as the brain. A better understanding of the neuroimmune regulation within and beyond the pancreas might offer possibilities for innovative therapy for pancreatic diseases, such as diabetes, pancreatitis, and tumors.
The authors declare that they have no conflicts of interest in this work.
This work was supported by the National Natural Science Foundation of China (32225019, 32394003 31822018, 31770936), the National Key R&D Program of China (2023YFC2306300), Beijing Natural Science Foundation (5222010), and the Tsinghua University (School of Medicine)-Xiamen Changgeng Hospital Co Ltd Joint Research Center for Anaphylactic Disease. The work was also supported by the Center for Life Sciences, the Institute for Immunology, and School of Medicine at Tsinghua University.
Wenwen Zeng(BRID: 05081.00.50061), Ph.D., is a principal investigator at Institute for Immunology and School of Medicine, Tsinghua University, principal investigator in Center for Life Sciences, Beijing, China. Her research interest is metabolic homeostasis and diseases, particularly neural and immune regulation of metabolism and neuroinflammation.
Xiaofan Ding(BRID: 01813.00.86917) is currently a post-doctoral fellow at the Institute for Immunology, Tsinghua University, China. She obtained a B.Sc. degree at Jiangnan University in 2015 and then a Ph.D. degree at Tsinghua University in 2020. Her research interests focus on neuroimmune regulation in physiology and disease, inflammation and innate immunity.
Jianhui Chen(BRID: 02017.00.85710) is a graduate student at the Institute for Immunology and School of Medicine at Tsinghua University, China. He got his B.Sc. degree in Tsinghua University and studied life science. Currently, he is investigating the sympathetic nervous system and inflammation.