Key points

Introduction

The cellular abnormalities that lead to diabetic gastroparesis are increasingly being understood. Several key cell types are affected by diabetes, leading to gastroparesis. These changes include abnormalities in the extrinsic innervation to the stomach, loss of key neurotransmitters at the level of the enteric nervous system, smooth muscle abnormalities, loss of ICC, and changes in the macrophage population resident in the muscle wall. This article reviews the current understanding with a focus on data from human studies when available.

Introduction

The cellular abnormalities that lead to diabetic gastroparesis are increasingly being understood. Several key cell types are affected by diabetes, leading to gastroparesis. These changes include abnormalities in the extrinsic innervation to the stomach, loss of key neurotransmitters at the level of the enteric nervous system, smooth muscle abnormalities, loss of ICC, and changes in the macrophage population resident in the muscle wall. This article reviews the current understanding with a focus on data from human studies when available.

Extrinsic innervation in diabetic gastroparesis

Diabetic gastroparesis was first described by Dr Kassander in 1958. After the initial description, investigations centered on the role of abnormalities in the extrinsic innervation to the stomach in the causation of diabetic gastroparesis. Both sympathetic and parasympathetic abnormalities were described, with increasing evidence over the years for a defect in the vagal innervation to the stomach and indeed the upper gastrointestinal tract. Damage to the vagal innervation of the stomach was shown by a sham feeding test, which takes advantage of the innervation of the pancreas by the vagus. During the cephalic phase of food digestion, stimulation of the vagus results in release of pancreatic polypeptide. Patients with advanced diabetic gastroparesis have a blunted pancreatic polypeptide response as well as reduced gastric secretion in response to sham feeding suggesting vagus nerve dysfunction. Abnormalities in vagal innervation of the stomach may contribute to the motor abnormalities seen, including abnormal relaxation of the pylorus. However, the initial histologic report in 1988 in 16 diabetic patients of whom 5 had gastroparesis failed to show any histologic defects. In retrospect, this was likely due to the small n value and the limited techniques available at that time (hematoxylin and eosin, Gomori trichrome, Luxol fast blue, and Holmes silver stains). In subsequent animal and human studies abnormalities have been described, including abnormalities at a histologic level both in myelinated and unmyelinated nerve fibers of the vagus nerve, which were also reported to be smaller in the biobreeding rat model of spontaneous diabetes. Sympathetic nervous system abnormalities have also been described, with changes in the axons and dendrites within the prevertebral sympathetic ganglia.

Smooth muscle

In the past relatively, rarely, patients with severe symptoms of diabetic gastroparesis, often unremitting nausea and vomiting, had gastrectomies as a treatment of their symptoms with variable results. An examination of the resected tissue showed evidence of smooth muscle degeneration and fibrosis, with eosinophilic inclusion bodies. In a study of 2 patients with severe diabetic gastroparesis, one had no fibrosis and the other showed fibrosis with the use of a trichrome stain. A study of full-thickness biopsies at the time of gastric stimulation implantation did not show significant fibrosis, suggesting that the fibrosis seen in the earlier studies may represent a more end-stage aspect of the disease.

Nonobese diabetic (NOD) mice are an often used model of diabetic gastroparesis. NOD mice develop a leukocytic infiltrate of the pancreatic islets, resulting in a type 1 type of diabetes. Studies on organotypic cultures from the stomachs of these mice have shown a loss of smooth muscle–derived insulin-like growth factor 1, suggesting that smooth muscle function may be impaired before the onset of overt fibrosis.

Enteric nerves

After the initial discovery that extrinsic nervous system defects are present in diabetic gastroparesis, work on animal models found that the intrinsic nervous system was also affected. Initial work was carried out in rats. Rats made diabetic with streptozotocin showed an increase in vasoactive intestinal peptide-like immunoreactivity in nerve cell bodies and nerve fibers, with no change in level of substance (SP). These changes were reversible with insulin administration. The same rat model also showed evidence for altered enteric nerve ion transport. A study using spontaneously non-insulin-dependent diabetic rats demonstrated depolarization of the smooth muscle membrane potential, an attenuation of nonadrenergic noncholinergic inhibitory neurotransmission, and a reduction in reactivity of adrenoceptors to noradrenaline. Work carried out using spontaneously diabetic biobreeding/Worcester (BB/W) rats showed that the number of neuronal nitric oxide synthase (nNOS)-containing neurons in the gastric myenteric plexus and NOS activity were significantly reduced in diabetic BB/W rats, suggesting a nitrergic defect. Similar results were found in streptozotocin-induced diabetes in rats. Gastric relaxation correlates better with the dimerized form of nNOS rather than with absolute nNOS levels, suggesting that posttranslational modification is important and in fact may be more relevant than overall quantification of nNOS. Work in mice has similarly shown loss of nNOS expression in diabetes, both in the stomach and in other regions of the gastrointestinal tract.

Studies in humans have also highlighted the role of nNOS in diabetic gastroenteropathy. Work using human colon showed enteric nerves cells with enhanced apoptosis and loss of peripherin, nNOS, neuropeptide Y, and choline acetyl transferase neurons, with evidence for increased oxidative stress. A study on male patients with gastric cancer, with and without type 2 diabetes, showed reduced number of ICC, nNOS, and substance P (SP) in the antrum of patients with diabetes. In a study of 16 patients with diabetic gastroparesis, 6 had reduced myenteric nerve cell bodies. The study from the Gastroparesis Clinical Research Consortium (GpCRC) funded by the National Institutes of Health examined tissue from the gastric body of 20 patients with diabetic gastroparesis. Overall, there was no statistical difference in the level of PGP9.5 (a marker of neurons) or nNOS-containing neurons between patients with diabetic gastroparesis and controls, although 4 patients did have a greater than 25% decrease in the number of nNOS-containing neurons. At the electron microscopic level, several patients had empty secretory vesicles in nerve terminals suggesting altered neurotransmission. These data suggest that, given the relative sparing of enteric neurons, the enteric neuronal abnormalities seen, such as loss of nNOS expression, may be more reversible than initially thought. One needs to understand the regulation of the expression of nNOS and other key proteins to target their expression.

Interstitial cells of Cajal

In the early 1990s, several studies reported on the requirement for an intact ICC network for normal gastrointestinal motility. Loss of ICC has been associated with several diseases, including chronic intestinal pseudo-obstruction and slow transit constipation. ICC generate an electric event known as the slow wave that sets the smooth muscle membrane potential, thereby regulating contractility. ICCs are also involved in cholinergic and nitrergic neurotransmission, with enteric nerves innervating both ICC and smooth muscle, and in mechanotransduction. Loss of ICC is the most common abnormality seen in diabetic gastroparesis. Loss of ICC was first reported in mouse models of diabetic gastroparesis, but it soon became apparent that loss of ICC is also seen in humans. The GpCRC study that reported on enteric nerve changes also studied the numbers of ICC and found that 50% of patients with diabetic gastroparesis had a significant decrease in the number of ICC. At an ultrastructural level, even when the number of ICC was not reduced there were significant changes to ICC and the surrounding stroma, with 95% (19/20) of patient tissue examined showing ICC abnormalities and a thick stroma separating ICC from smooth muscle cells and nerves. A protein key to the electrical function of ICC is Ano-1, a calcium-activated chloride channel. Ano-1 expression is altered in diabetic gastroparesis, and patients with diabetic gastroparesis have variants of Ano-1 different from those in diabetic patients without gastroparesis. These variants were associated with altered electrical activity of the ion channel, suggesting that even when structurally normal, the function of ICC may be impaired in diabetic gastroparesis.

Loss of ICC impairs gastric function. Loss of ICCs in diabetic gastroparesis is associated with disruption of the generation and propagation of electrical slow waves, resulting in gastric dysrhythmias. A decrease in frequency of the slow wave is referred to as bradygastria, with tachygastria referring to an increase in frequency. These changes are often transient, and both have been reported in diabetic gastroparesis with symptoms related to meals. Refractory diabetic gastroparesis was found to correlate with both loss of ICC and abnormal results in electrogastrogram. Animal studies have shown that not only the absolute number of ICCs leads to electrical dysrhythmias but also a patchy disruption of ICC networks may result in reentrant tachyarrhythmias as well as loss of generation of the slow waves resulting in bradyarrhythmias. A recent study reported severe ICC loss in 12 of 34 patients with refractory diabetic gastroparesis and correlated loss of ICC with abnormal results in electrogastrogram showing tachygastria. Loss of ICC is correlated with development of delayed gastric emptying, with a more severe loss of ICC associated with a more severe delay in gastric emptying.

Fibroblast-like cells

A recent addition to our understanding of the cell types required for normal gastric motor function is a type of interstitial cell with fibroblast-like ultrastructure that is referred to as fibroblast-like cells (FLCs). These cells were shown to have gap junctions with smooth muscle cells and to be close to but distinct from ICC. A distinct feature of this cell type is the high expression of SK3 (small conductance calcium-activated potassium channels type 3) channels and platelet-derived growth factor receptor α. FLCs are involved in enteric neurotransmission, specifically purinergic neurotransmission. Given that FLCs, like ICC, are involved in enteric neurotransmission and the number of ICCs is decreased in diabetic gastroparesis, the question was soon raised on whether FLCs are also altered in diabetic gastroparesis. The one study that addressed this question did not find any difference in the number or distribution of FLCs in diabetic gastroparesis, suggesting that diabetic gastroparesis is not due to structural changes to this cell type.