The effects of mechanical forces on intestinal physiology and pathology

The epithelial and non-epithelial cells of the intestinal wall experience a myriad of physical forces including strain, shear, and villous motility during normal gut function. Pathologic conditions alter these forces, leading to changes in the biology of these cells. The responses of intestinal epithelial cells to forces vary with both the applied force and the extracellular matrix proteins with which the cells interact, with differing effects on proliferation, differentiation, and motility, and the regulation of these effects involves similar but distinctly different signal transduction mechanisms. Although normal epithelial cells respond to mechanical forces, malignant gastrointestinal epithelial cells also respond to forces, most notably by increased cell adhesion, a critical step in tumor metastasis. This review will focus on the phenomenon of mechanical forces influencing cell biology and the mechanisms by which the gut responds these forces in both the normal as well as pathophysiologic states when forces are altered. Although more is known about epithelial responses to force, information regarding mechanosensitivity of vascular, neural, and endocrine cells within the gut wall will also be discussed, as will, the mechanism by which forces can regulate epithelial tumor cell adhesion.     

Interleukin-1beta activates specific populations of enteric neurons and enteric glia in the guinea pig ileum and colon

Fos expression was used to assess whether the proinflammatory cytokine interleukin-1beta (IL-1beta) activated specific, chemically coded neuronal populations in isolated preparations of guinea pig ileum and colon. Whether the effects of IL-1beta were mediated through a prostaglandin pathway and whether IL-1beta induced the expression of cyclooxygenase (COX)-2 was also examined. Single- and double-labeling immunohistochemistry was used after treatment of isolated tissues with IL-1beta (0.1-10 ng/ml). IL-1beta induced Fos expression in enteric neurons and also in enteric glia in the ileum and colon. For enteric neurons, activation was concentration-dependent and sensitive to indomethacin, in both the myenteric and submucosal plexuses in both regions of the gut. The maximum proportion of activated neurons differed between the ileal (approximately 15%) and colonic (approximately 42%) myenteric and ileal (approximately 60%) and colonic (approximately 75%) submucosal plexuses. The majority of neurons activated in the myenteric plexus of the ileum expressed nitric oxide synthase (NOS) or enkephalin immunoreactivity. In the colon, activated myenteric neurons expressed NOS. In the submucosal plexus of both regions of the gut, the majority of activated neurons were vasoactive intestinal polypeptide (VIP) immunoreactive. After treatment with IL-1beta, COX-2 immunoreactivity was detected in the wall of the gut in both neurons and nonneuronal cells. In conclusion, we have found that the proinflammatory cytokine IL-1beta specifically activates certain neurochemically defined neural pathways and that these changes may lead to disturbances in motility observed in the inflamed bowel.     

The morphology,physiology and function of suboesophageal neck motor neurons in the honeybee

We report some of the neural and muscular circuitry that allows honeybees to control head movements. We studied neck motor neurons with cell bodies in the suboesophageal ganglion, axons in the first cervical nerve (IK1) and terminals in neck muscles 44 and 51 (muscle classification: Snodgrass in Smithsonian Misc Coll 103:1-120, 1942). We show that muscle 44 actually comprises five separate bundles of muscle fibres (subunits), while muscle 51 is split into two subunits. Eight motor neurons innervate muscles 44 and 51. Two motor neurons have cell bodies in the ventral-median cell body group (one innervates a subunit in muscle 44, the other a subunit in muscle 51). One motor neuron has a ventrally located contralateral cell body (innervating a subunit in muscle 44) and five have laterally located ipsilateral cell bodies. Of the five lateral cells, one innervates a subunit in muscle 51, three selectively innervate subunits in muscle 44 and one co-innervates a subunit in muscle 44 with the contralateral cell. Extracellular recordings revealed three types of visually driven, direction-selective cell-types in each IK1 tuned for leftward, rightward and downward motion over the eyes. The spatiotemporal tuning of the units is similar to that of other visual interneurons in the bee brain.     

Giardiasis in dairy calves: effects of fenbendazole treatment on intestinal structure and function

Twelve Giardia duodenalis-infected Holstein dairy calves were allocated into a treatment (n=6) and placebo group (n=6) according to pre-study faecal cyst counts. Calves in the treatment group received an oral dose of 5 mg/kg fenbendazole once daily for 3 days, while placebo calves received a sterile saline solution. Calves were euthanised 7 days following the initiation of treatment and intestinal were collected and prepared for trophozoite quantitation, histology, electron microscopy, and disaccharidase assays. In all calves treated with fenbendazole, intestinal trophozoites were below detection limits, while in saline-treated calves, trophozoites were observed in all intestinal segments. Histologically, no significant difference was observed between treatment groups with respect to intestinal villus height or crypt depth. However, a significant decline in the number of intraepithelial lymphocytes (IEL) was observed in fenbendazole-treated calves when compared with placebo-treated calves in the duodenum (13.9+/-1.2 vs. 17.0+/-1.1 IEL/100 enterocytes) and jejunum (21.6+/-0.8 vs. 30.7+/-1.0 IEL/100 enterocytes). In addition, measurements from TEM micrographs demonstrated a significant increase in microvillus surface area in the jejunum of fenbendazole-treated calves compared with saline-treated calves (31.2+/-10.2 vs. 22.8+/-7.6 microm(2)). This increase in microvillus surface area was also associated with an increase in jejunal maltase activity in fenbendazole-treated calves compared with calves treated with saline. These results demonstrate that fenbendazole is an effective treatment for giardiasis in calves. fenbendazole treatment eliminated Giardia trophozoites from the small intestine of calves resulting in increased microvillus surface area and greater intestinal enzyme activity. This study also demonstrates that the pathogenesis of giardiasis in calves is similar to that observed in humans and laboratory animals, and provides further evidence that Giardia is a pathogen of cattle with potential economic importance.     

The role of histamine in vagal and gastrin effects on the secretory function of the rat stomach

In lumen-perfused stomachs of anaesthetized rat, acid and bicarbonate types of secretion were estimated on the basis of pH/PCO2 measurements. The data obtained reveal that, in anaesthetised rats, pentagastrin and cholinergic input affect acid secretion mainly indirectly via histamine release. Vagal effects on pepsinogen output are mediated partially via indirect histamine pathways. The direct action of pentagastrin, however, predominates. Bicarbonate production is subjected to cholinergic control alone.     

Neuroplasticity and neuroprotection in enteric neurons: Role of epithelial cells

Neurons of enteric nervous system (ENS) regulate intestinal epithelial cells (IEC) functions but whether IEC can impact upon the neurochemical coding and survival of enteric neurons remain unknown. Neuro-epithelial interactions were studied using a coculture model composed of IEC lines and primary culture of rat ENS or human neuroblastoma cells (SH-SY5Y). Neurochemical coding of enteric neurons was analysed by immunohistochemistry and quantitative PCR. Neuroprotective effects of IEC were tested by measuring neuron specific enolase (NSE) release or cell permeability to 7-amino-actinomycin D (7-AAD). Following coculture with IEC, the percentage of VIP-immunoreactive (IR) neurons but not NOS-IR and VIP mRNA expression were significantly increased. IEC significantly reduced dopamine-induced NSE release and 7-AAD permeability in culture of ENS and SH-SY5Y, respectively. Finally, we showed that NGF had neuroprotective effects but reduced VIP expression in enteric neurons. In conclusion, our study identified a novel role for IEC in the regulation of enteric neuronal properties.     

Migration and function of glia in the developing Drosophila eye.

Although glial cells have been implicated widely in the formation of axon tracts in both insects and vertebrates, their specific function appears to be context-dependent, ranging from providing essential guidance cues to playing a merely facilitory role. Here we examine the role of the retinal basal glia (RBG) in photoreceptor axon guidance in Drosophila. The RBG originate in the optic stalk and have been thought to migrate into the eye disc along photoreceptor axons, thus precluding any role in axon guidance. Here we show the following. (1) The RBG can, in fact, migrate into the eye disc even in the absence of photoreceptor axons in the optic stalk; they also migrate to ectopic patches of differentiating photoreceptors without axons providing a continuous physical substratum. This suggests that glial cells are attracted into the eye disc not through haptotaxis along established axons, but through another mechanism, possibly chemotaxis. (2) If no glial cells are present in the eye disc, photoreceptor axons are able to grow and direct their growth posteriorly as in wild type, but are unable to enter the optic stalk. This indicates that the RBG have a crucial role in axon guidance, but not in axonal outgrowth per se. (3) A few glia close to the entry of the optic stalk suffice to guide the axons into the stalk, suggesting that glia instruct axons by local interaction.     

Gastrointestinal secretory, motor and circulatory effects of corticotropin releasing factor (CRF)

This study was designed to determine the effects of CRF on the gastrointestinal functions such as secretion, motility and circulation in dogs. CRF was found to inhibit dose-dependently gastric acid response to pentagastrin but not to histamine. CRF stimulated pancreatic bicarbonate and protein secretion under basal conditions and in response to secretin or cholecystokinin (CCK). This stimulation was accompanied by an increase in plasma levels of pancreatic polypeptide (PP), but not of secretin or gastrin. CRF caused a partial inhibition of the migrating motor complexes in fasted dogs and increased spike activity of the small bowel. These motor effects of CRF probably resulted from the action of the released PP on the intestinal smooth muscle. CRF is also a potent and selective stimulant of the mesenteric blood flow. This effect may be secondary to the stimulation of intestinal motility and metabolism.     

The effects of fluorocitrate on renal glutamine, lactate, alanine, and oxygen metabolism in the dog

Acid-base status is considered the major factor controlling renal NH4+ production from glutamine, with maximal values found in chronic acidosis. However, metabolic inhibitors have been shown to increase NH4+ production without acid-base change; the mechanism for this increase is unclear. Fluorocitrate was administered to dogs with chronic metabolic alkalosis. Following fluorocitrate total renal NH4+ production rose from 32 +/- 5 to 104 +/- 15 mumol/(min.100 mL glomerular filtration rate (GFR] (p less than 0.01) and glutamine extraction rose from 26 +/- 8 to 65 +/- 8 mumol/(min.100 mL GFR) (p less than 0.01). These values approximate maximal values found in chronic acidosis. Lactate utilization fell from 165 +/- 19 to 99 +/- 7 mumol/(min.100 mL GFR) following fluorocitrate (p less than 0.01). Citrate extraction fell to zero and alanine production rose from 27 +/- 4 to 46 +/- 7 mumol/(min.100 mL GFR) (p less than 0.01). Oxygen consumption remained unchanged following fluorocitrate, 584 +/- 29 vs. 549 +/- 29 mumol/(min.100 mL GFR). These results demonstrate that in the presence of metabolic inhibition in the kidney, ATP production remains constant. This is achieved by increased utilization of one substrate, glutamine, when the ATP production from other substrates is reduced. Thus the necessity to maintain constant ATP production appears to modulate renal NH4+ production.     

Nervous control of intestinal fluid transport: physiology and pathophysiology

1. The enteric nervous system (10(8) neurones in man) consists of the myentric plexus, the submucosal plexus and other minor plexuses. Eighteen different chemicals are candidates for the role of neurotransmitters in the ENS. 2. The ENS together with the autonomic nervous system and the hormonal system controls gut epithelial transport systems.     

Steroid regulation of brain aromatase expression in glia: female preoptic and vocal motor nuclei

Expression of the enzyme aromatase, which converts androgens to estrogens, is known to be regulated by gonadal steroids in brain areas linked to reproduction and related behaviors in several groups of vertebrates. Previously, we demonstrated in a vocal fish, the plainfin midshipman, that both males and females undergo seasonal changes in brain aromatase mRNA expression in the preoptic area (POA) and the dimorphic sonic/vocal motor nucleus (SMN) that parallel seasonal variation in circulating steroid levels and reproductive behavior. We tested the hypothesis that steroids are directly responsible for seasonal modulation of aromatase in females because they show the most dramatic fluctuations of testosterone (T) and 17beta-estradiol (E2) throughout the year. Adult female midshipmen were ovariectomized and administered T, E2, or blank (control) implants. We then quantified aromatase mRNA expression within the POA and SMN by in situ hybridization. Both T- and E2-treated females had elevated mRNA expression levels in both brain areas compared to controls. T affected aromatase expression in a level-dependent manner, whereas E2 showed a decreased effect at higher circulating levels. This study demonstrates that seasonal differences in brain aromatase expression in female midshipman fish may be explained, in part, by changes in levels of circulating steroids.     

Effect of enteric micro-organisms on intestinal sugar and fatty acid absorption

The effect of micro-organisms contaminating the upper intestinal contents of malnourished children on intestinal absorption of 3-0 methyl-alpha-D-glucopyranose (3-M.G.) and oleic acid was studied in rats in vivo. Oleci acid absorption was unaffected by non-pathogenic E. coli but decreased by E. coli 0111, Salmonella paratyphi B., Shigella sonnei and Candida sp. This effect was probably explained by intestinal secretion diluting the test solution leading to a decreased diffusion gradient for solubilised fatty acid. Inhibition of sugar absorption occurred with bacterial suspensions of Staphylococcus aureus, Streptococcus faecalis, E. coli and Candida sp. and cell-free preparations of Staphylococcus aureus, Streptococcus faecalis, a non-pathogenic E. coli, Proteus sp., Klebsiella sp., Pseudomonas sp. and Candida sp. These effects were not explained by dilution of the test solution. This indicates that numerous micro-organisms and, in some instances, their cell-free preparations can interfere with intestinal active sugar transport. These findings may be relevant to the production of malabsorption in malnourished children who have a wide variety of micro-organisms contaminating their upper intestinal contents.     

Seasonal differences in the physiology and morphology of crayfish motor terminals

The physiology and morphology of identified crayfish motor terminals were compared at different seasons. We examined initial excitatory postsynaptic potential (EPSP) amplitudes, synaptic fatigue, and the frequency of synaptic varicosities along the motor terminals of an identified phasic motoneuron in animals collected over a period of 5 years. The physiology and morphology of identified crayfish motor terminals are different for animals collected in summer and winter. In winter animals, phasic axon motor terminals in the claw closer muscle produce large EPSPs initially, but show dramatic synaptic fatigue during repetitive stimulation. In summer animals, these terminals produce smaller initial EPSPs, but are more fatigue resistant. Due to their greater fatigue resistance, synaptic terminals have a greater over-all capacity for transmitter release in summer animals than do those of winter animals. Morphologically, terminals in summer animals have more synaptic varicosities, this result supports earlier studies that have shown that fatigue-resistant motor terminals have more synaptic varicosities. Experiments in which the electrical activity of the motoneuron was experimentally altered suggest that these differences in motor terminals may be due to seasonal differences in activity.     

Form and function in the trypanosomal secretory pathway

Recent advances in secretory biology of African trypanosomes reveal both similarities and striking differences with other model eukaryotic organisms. Secretion is streamlined for rapid and selective transport of the major cargo, VSG. Selectivity in the early and post-Golgi compartments is dependent on glycosylphosphatidyl inositol anchors. Streamlining includes reduced organellar abundance, and close association of ER exit sites with Golgi and with unique flagellar cytoskeletal elements that govern organellar replication and segregation. These elements include a novel centrin containing bilobe structure. Innate signals for post-Golgi sorting of biosynthetic lysosomal cargo trafficking have been defined, as have pathways for both biosynthetic and endocytic trafficking to the lysosome. Less well-defined secretory organelles such as the multivesicular body and acidocalcisomes are receiving closer scrutiny.     

Dietary effects of omega 3-fatty acids on intestinal transport function

Animals were fed for 2 weeks on one of four isocaloric and isocholesterolic semisynthetic diets: high 18:3 omega 3, low 18:3 omega 3, high 20:5 omega 3, or low 20:5 omega 3. The weight of the intestine and the percentage of the wall consisting of mucosa was greater in high 20:5 omega 3 than in high 18:3 omega 3, and greater in low 20:5 omega 3 than in low 18:3 omega 3, although the mucosal surface area was 26% lower in high 20:5 omega 3 than high 18:3 omega 3. The jejunal uptake of 40 mM glucose and ileal uptake of 40 mM galactose was greater in high 18:3 omega 3 than in high 20:5 omega 3, jejunal uptake of fatty acid 12:0 was higher, but 18:0 was lower in high 18:3 omega 3 than in high 20:5 omega 3. The jejunal or ileal uptake of cholesterol was not affected by 20:5 omega 3. However, 20:5 omega 3 had a variable effect on the uptake of medium- and long-chain fatty acids. Alterations in the uptake of fatty acids and glucose were not explained by any difference in the animals' food consumption, body weight gain, or intestinal weight, but the reduced jejunal uptake of 40 mM glucose in rats fed the high 20:5 omega 3 diet was associated with reduced mucosal surface area. Thus, (i) varying the source of omega 3-fatty acids (vegetable, 18:3 omega 3 versus fish oil, 20:5 omega 3) altered the mucosal mass of the intestine, and (ii) the source of the dietary omega 3-fatty acid (18:3 omega 3 versus 20:5 omega 3) influenced intestinal hexose uptake, with fish oil having an anti-absorptive effect on the jejunal uptake of D-glucose.