Supraphysiologic extracellular pressure inhibits intestinal epithelial wound healing independently of luminal nutrient flow

BACKGROUND

Luminal pressure may injure the gut mucosa in obstruction, ileus, or inflammatory bowel disease.

METHODS

We formed Roux-en-Y anastomoses in 19 mice, creating proximal and defunctionalized partially obstructed limbs and a distal limb to vary luminal pressure and flow. We induced mucosal ulcers by serosal acetic acid, and assessed proliferation (proliferating cell nuclear antigen) and ERK (immunoblotting). Parallel studies compared Caco-2 enterocyte migration and proliferation after pressure and/or ERK blockade.

RESULTS

At 3 days, anastomoses were probe-patent, proximal and distal limbs contained chyme, and defunctionalized limbs were empty. The proximal and defunctionalized limbs showed increased pressure and slower healing despite increased proliferation, ERK protein, and ERK activation. In vitro, pressure decreased Caco-2 migration across collagen or fibronectin, stimulated proliferation, and activated ERK. However, ERK blockade did not prevent pressure effects.

CONCLUSIONS

Luminal pressure during obstruction or ileus may impair mucosal healing independently of luminal flow despite increased mitosis and ERK activation.

Colchicine inhibits pressure-induced tumor cell implantation within surgical wounds and enhances tumor-free survival in mice

Iatrogenic tumor cell implantation within surgical wounds can compromise curative cancer surgery. Adhesion of cancer cells, in particular colon cancer cells, is stimulated by exposure to increased extracellular pressure through a cytoskeleton-dependent signaling mechanism requiring FAK, Src, Akt, and paxillin. Mechanical stimuli during tumor resection may therefore negatively impact patient outcome. We hypothesized that perioperative administration of colchicine, which prevents microtubule polymerization, could disrupt pressure-stimulated tumor cell adhesion to surgical wounds and enhance tumor-free survival. Ex vivo treatment of Co26 and Co51 colon cancer cells with colchicine inhibited pressure-stimulated cell adhesion to murine surgical wounds and blocked pressure-induced FAK and Akt phosphorylation. Surgical wound contamination with pressure-activated Co26 and Co51 cells significantly reduced tumor-free survival compared with contamination with tumor cells under ambient pressure. Mice treated with pressure-activated Co26 and Co51 cells from tumors preoperatively treated with colchicine in vivo displayed reduced surgical site implantation and significantly increased tumor-free survival compared with mice exposed to pressure-activated cells from tumors not pretreated with colchicine. Our data suggest that pressure activation of malignant cells promotes tumor development and impairs tumor-free survival and that perioperative colchicine administration or similar interventions may inhibit this effect.

The motogenic effects of cyclic mechanical strain on intestinal epithelial monolayer wound closure are matrix dependent

BACKGROUND & AIMS

Complex deformation during normal digestion due to peristalsis or villous motility may be trophic for the intestinal mucosa. Because tissue fibronectin is increased in inflammatory states that may accompany mucosal injury, we evaluated the effects of cyclic mechanical strain and fibronectin on intestinal epithelial monolayer wound closure in Caco-2 and IEC-6 intestinal epithelial cells.

METHODS

Wounds created in intestinal epithelial monolayers were subjected to cyclic deformation. Wound closure was assessed by morphometry using microscopic imaging. Cell signals were assessed by Western blot and confocal microscopy.

RESULTS

Mechanical strain stimulated wound closure on fibronectin but inhibited closure on collagen in Caco-2 and IEC-6 cells. The effect was independent of proliferation or cell spreading. Myosin light chain (MLC) and extracellular signal-regulated kinase (ERK) were phosphorylated in response to strain in confluent monolayers on both collagen and fibronectin. Blocking MLC or ERK phosphorylation inhibited the motogenic effect of strain on fibronectin. Although phosphorylated MLC was redistributed to the leading edge of migrating cells following 6 hours of strain on collagen and fibronectin, phosphorylated ERK was redistributed to the lamellipodial edge only on fibronectin.

CONCLUSIONS

Strain promotes intestinal epithelial wound closure by a pathway requiring ERK and MLC kinase. Fibronectin-dependent ERK redistribution in response to strain in confluent migrating cells may explain the matrix dependence of the motogenic effect. Repetitive deformation stimulates intestinal epithelial proliferation on a collagen substrate, but not fibronectin. Deformation may exert matrix-dependent effects on intestinal epithelial cells, promoting epithelial restitution in fibronectin-rich tissue and proliferation in fibronectin-poor mucosa.

PERK is responsible for the increased phosphorylation of eIF2alpha and the severe inhibition of protein synthesis after transient global brain ischemia

Reperfusion after global brain ischemia results initially in a widespread suppression of protein synthesis in neurons that is due to inhibition of translation initiation as a result of the phosphorylation of the alpha-subunit of eukaryotic initiation factor 2 (eIF2). To address the role of the eIF2alpha kinase RNA-dependent protein kinase-like endoplasmic reticulum kinase (PERK) in the reperfused brain, transgenic mice with a targeted disruption of the Perk gene were subjected to 20 min of forebrain ischemia followed by 10 min of reperfusion. In wild-type mice, phosphorylated eIF2alpha was detected in the non-ischemic brain and its levels were elevated threefold after 10 min of reperfusion. Conversely, there was no phosphorylated eIF2alpha detected in the non-ischemic transgenic mice and there was no sizeable rise in phosphorylated eIF2alpha levels in the forebrain after ischemia and reperfusion. Moreover, there was a substantial rescue of protein translation in the reperfused transgenic mice. Neither group showed any change in total eIF2alpha, phosphorylated eukaryotic elongation factor 2 or total eukaryotic elongation factor 2 levels. These data demonstrate that PERK is responsible for the large increase in phosphorylated eIF2alpha and the suppression of translation early in reperfusion after transient global brain ischemia.

Persistent eIF2alpha(P) is colocalized with cytoplasmic cytochrome c in vulnerable hippocampal neurons after 4 hours of reperfusion following 10-minute complete brain ischemia

Upon brain reperfusion following ischemia, there is widespread inhibition of neuronal protein synthesis that is due to phosphorylation of eukaryotic initiation factor 2alpha (eIF2alpha), which persists in selectively vulnerable neurons (SVNs) destined to die. Other investigators have shown that expression of mutant eIF2alpha (S51D) mimicking phosphorylated eIF2alpha induces apoptosis, and expression of non-phosphorylatable eIF2alpha (S51A) blocks induction of apoptosis. An early event in initiating apoptosis is the release of cytochrome c from mitochondria, and cytochrome c release corresponds to the selective vulnerability of hippocampal CA1 neurons in rats after transient global cerebral ischemia. At present the signaling pathways leading to this are not well defined. We hypothesized that persistent eIF2alpha(P) reflects injury mechanisms that are causally upstream of release of cytochrome c and induction of apoptosis. At 4 h of reperfusion following 10-min cardiac arrest, vulnerable neurons in the striatum, hippocampal hilus and CA1 showed colocalized intense immunostaining for both persistent eIF2alpha(P) and cytoplasmic cytochrome c, while resistant neurons in the dentate gyrus and elsewhere did not immunostain for either. A lower intensity of persistent eIF2alpha(P) immunostaining was present in cortical layer V pyramidal neurons without cytoplasmic cytochrome c, possibly reflecting the lesser vulnerability of this area to ischemia. We did not observe cytoplasmic cytochrome c in any neurons that did not also display persistent eIF2alpha(P) immunostaining. Because phosphorylation of eIF2alpha during early brain reperfusion is carried out by PERK, these findings suggest that there is prolonged activation of the unfolded protein response in the reperfused brain.

Molecular pathways of protein synthesis inhibition during brain reperfusion: implications for neuronal survival or death

Protein synthesis inhibition occurs in neurons immediately on reperfusion after ischemia and involves at least alterations in eukaryotic initiation factors 2 (eIF2) and 4 (eIF4). Phosphorylation of the alpha subunit of eIF2 [eIF2(alphaP)] by the endoplasmic reticulum transmembrane eIF2alpha kinase PERK occurs immediately on reperfusion and inhibits translation initiation. PERK activation, along with depletion of endoplasmic reticulum Ca2+ and inhibition of the endoplasmic reticulum Ca2+ -ATPase, SERCA2b, indicate that an endoplasmic reticulum unfolded protein response occurs as a consequence of brain ischemia and reperfusion. In mammals, the upstream unfolded protein response components PERK, IRE1, and ATF6 activate prosurvivial mechanisms (e.g., transcription of GRP78, PDI, SERCA2b ) and proapoptotic mechanisms (i.e., activation of Jun N-terminal kinases, caspase-12, and CHOP transcription). Sustained eIF2(alphaP) is proapoptotic by inducing the synthesis of ATF4, the CHOP transcription factor, through "bypass scanning" of 5' upstream open-reading frames in ATF4 messenger RNA; these upstream open-reading frames normally inhibit access to the ATF4 coding sequence. Brain ischemia and reperfusion also induce mu-calpain-mediated or caspase-3-mediated proteolysis of eIF4G, which shifts message selection to m 7 G-cap-independent translation initiation of messenger RNAs containing internal ribosome entry sites. This internal ribosome entry site-mediated translation initiation (i.e., for apoptosis-activating factor-1 and death-associated protein-5) can also promote apoptosis. Thus, alterations in eIF2 and eIF4 have major implications for which messenger RNAs are translated by residual protein synthesis in neurons during brain reperfusion, in turn constraining protein expression of changes in gene transcription induced by ischemia and reperfusion. Therefore, our current understanding shifts the focus from protein synthesis inhibition to the molecular pathways that underlie this inhibition, and the role that these pathways play in prosurvival and proapoptotic processes that may be differentially expressed in vulnerable and resistant regions of the reperfused brain.

Expression of the inducible nitric oxide synthase in distinct cellular types after traumatic brain injury: an in situ hybridization and immunocytochemical study

The Marmarou's acceleration traumatic brain injury (TBI) model, in situ hybridization and immunocytochemistry were utilized to study the temporal expression of the inducible form of nitric oxide synthase (iNOS) mRNA and protein in different cellular compartments of the rat brain. Four hours following TBI, expression of iNOS was observed in the endothelial cells of cerebral blood vessels, macrophages and many cortical and hippocampal neurons. In the cortex labeled neuronal and nonneuronal cells were primarily found in the superficial layers. In the hippocampus the strongest neuronal labeling was observed in the CAI and CA3 (lateral part) regions. By 24 h post TBI endothelial cells no longer expressed iNOS mRNA, and the macrophage and neuronal iNOS expression was reduced by 30-50%. The reduction was assessed by automated quantitation of the silver grains that occupy individual cellular profiles using an image analysis system. Immunocytochemistry revealed de novo iNOS synthesis in non-neuronal cells at the different time points, thus paralleling the changes in iNOS mRNA expression. In contrast, iNOS immunoreactivity in neurons was not observed before 24 h post TBI, suggesting failure of iNOS protein translation at 4 h after trauma. The results demonstrate complex spatial and temporal patterns of iNOS expression in discrete cellular populations, indicating different times of nitric oxide synthesis (and release) following TBI. Uncoupling of iNOS mRNA and protein synthesis in neurons suggests differential synthesis of nitric oxide in these cells as compared to non-neuronal cellular populations after trauma.

Ultrastructural localization of phosphorylated eIF2alpha [eIF2alpha(P)] in rat dorsal hippocampus during reperfusion

During post-ischemic brain reperfusion there is a substantial reduction of protein synthesis in selectively vulnerable neurons. Normal protein synthesis requires a functional translation initiation complex, a key element of which is eukaryotic initiation factor 2 (eIF2), which in a complex with GTP introduces the met-tRNA(i). Phosphorylation of Ser(51) on the alpha subunit of eIF2 [eIF2alpha(P)] generates a competitive inhibitor of eIF2B, thereby preventing the replenishment of GTP onto eIF2, thus blocking translation initiation. It has been shown that the conditional expression of an eIF2alpha mutant (Asp substituted for Ser(51)) imitating the negative charge of Ser(51) (P) induces apoptosis. During the first 10 min of post-ischemic reperfusion, there is an approximately 20-fold increase in eIF2alpha(P) seen in the cytoplasm of CA1 hippocampal neurons, and, by 1 h, there is also accumulation of eIF2alpha(P) in the nucleus. We utilized post-embedding electron microscopical immunogold methods to examine the localization of eIF2alpha(P) during reperfusion. Immunogold particles (10 nm) were concentrated chiefly along the rough endoplasmic reticulum and in association with the membranes of the nuclear envelope in CA1 neurons. Aggregations of gold particles in the nucleus were concentrated: (1) within and around the nucleolus, (2) associated to strands of heterochromatin, and (3) along putative nuclear filaments. The presence of eIF2alpha(P) in the nucleolus probably reflects its association with nascent ribosomal subunits. The beta-subunit of eIF2 has a zinc finger and polylysine blocks analogous to those on other proteins that affect transcription. The association of eIF2alpha(P) with chromatin may have important implications for transcription.

Enigmatic resistance of sheep (Ovis aries) to infection by virulent Francisella tularensis

Tularemia in range sheep is an occasional cause of severe economic loss from mortality and unthriftiness as well as a hazard to persons in contact with the animals. Epizootics are unpredictable and explosive, therefore, prophylaxis is more practical than therapy. Live vaccine of proven value in man and in beavers was inoculated into mature ewes and elicited antibodies without harm to the sheep. However, challenge of immunity was not possible because virulent Francisella tularensis in large doses did not cause significant mortality in healthy, well managed, unimmunized sheep. Evidence suggests that a complex of stresses such as inclement weather, lambing and concomitant ectoparasitism render sheep more susceptible to tularemia.

Yersinia philomiragia sp. n., a new member of the Pasteurella group of bacteria, naturally pathogenic for the muskrat (Ondatra zibethica)

A bacterium experimentally pathogenic for muskrats (Ondatra zibethica), white mice, mountain voles (Microtus montanus), and deer mice (Peromyscus maniculatus) was isolated from the tissues of a sick muskrat captured on the Bear River Migratory Bird Refuge (Brigham City, Utah) and from four surface water samples collected within 15 miles of that point. In culture, the cells are chiefly coccoid, but in the tissues of muskrats and voles they resemble the bizarre forms of Yersinia pestis, except for their smaller size. The characteristics of the organism are described and the name Yersinia philomiragia sp. n. is proposed.