Going your own way: Self-guidance mechanisms in cell migration

How cells and tissues migrate from one location to another is a question of significant biological and medical relevance. Migration is generally thought to be controlled by external hardwired guidance cues, which cells follow by polarizing their internal locomotory machinery in the imposed direction. However, a number of recently discovered ‘self-guidance’ mechanisms have revealed that migrating cells have more control over the path they follow than previously thought. Here, directional information is generated by the migrating cells themselves via a dynamic interplay of cell-intrinsic and -extrinsic regulators. In this review, we discuss how self-guidance can emerge from mechanisms acting at different levels of scale and how these enable cells to rapidly adapt to environmental challenges.     

Keeping your options open: Maintenance of thermal plasticity during adaptation to a stable environment

Phenotypic plasticity may allow species to cope with environmental variation. The study of thermal plasticity and its evolution helps understanding how populations respond to variation in temperature. In the context of climate change, it is essential to realize the impact of historical differences in the ability of populations to exhibit a plastic response to thermal variation and how it evolves during colonization of new environments. We have analyzed the real‐time evolution of thermal reaction norms of adult and juvenile traits in Drosophila subobscura populations from three locations of Europe in the laboratory. These populations were kept at a constant temperature of 18ºC, and were periodically assayed at three experimental temperatures (13ºC, 18ºC, and 23ºC). We found initial differentiation between populations in thermal plasticity as well as evolutionary convergence in the shape of reaction norms for some adult traits, but not for any of the juvenile traits. Contrary to theoretical expectations, an overall better performance of high latitude populations across temperatures in early generations was observed. Our study shows that the evolution of thermal plasticity is trait specific, and that a new stable environment did not limit the ability of populations to cope with environmental challenges.     

Epithelial sheet movement requires the cooperation of c-Jun and MAP3K1

Epithelial sheet movement is an essential morphogenetic process during mouse embryonic eyelid closure in which Mitogen-Activated Protein 3 Kinase 1 (MAP3K1) and c-Jun play a critical role. Here we show that MAP3K1 associates with the cytoskeleton, activates Jun N-terminal kinase (JNK) and actin polymerization, and promotes the eyelid inferior epithelial cell elongation and epithelium protrusion. Following epithelium protrusion, c-Jun begins to express and acts to promote ERK phosphorylation and migration of the protruding epithelial cells. Homozygous deletion of either gene causes defective eyelid closure, but non-allelic non-complementation does not occur between Map3k1 and c-Jun and the double heterozygotes have normal eyelid closure. Results from this study suggest that MAP3K1 and c-Jun signal through distinct temporal-spatial pathways and that productive epithelium movement for eyelid closure requires the consecutive action of MAP3K1-dependent cytoskeleton reorganization followed by c-Jun-mediated migration.     

Epithelial sheet movement: effects of tunicamycin on migration and glycoprotein synthesis

Corneas with central epithelial wounds, 3 mm in diameter, were organ cultured in the presence of tunicamycin (TM) (1 microgram/ml), an antibiotic that inhibits glycosylation of asparagine-linked glycoproteins. Compared with control corneas, which healed in 22 hr, corneas cultured in the presence of TM for the entire culture time or for only the first 6 hr displayed a progressively slower epithelial healing rate that essentially dropped to zero by 24 hr of culture time. At 24 hr, approximately 75% of the wound was covered. After repeated washings with TM-free culture media (6X, 10 min each), this effect could consistently be reversed in corneas exposed to TM for 6 hr. Incorporation of [3H]glucosamine into trichloroacetic acid-precipitable proteins of migrating epithelial sheets was reduced to 14% that of controls after 12 hr of culture with TM, whereas [14C]leucine incorporation was not significantly affected. The decreased glycosylation was reflected on the cell surface after 12 and 20 hr culture in the presence of TM: apical cell membranes of the first six cells of the leading edge of the migrating sheet bound significantly fewer ferritin-concanavalin A particles per micrometer of membrane than did controls. These results indicate that synthesis of asparagine-linked glycoproteins is required for continued migration of corneal epithelial sheets. The asparagine-linked glycoproteins that are required for migration probably include cell-surface glycoproteins.     

Epithelial permeability and the transepithelial migration of human neutrophils

Although polymorphonuclear leukocytes (PMN's) can migrate through every epithelium in the body regardless of its permeability, very little is known about the effect of epithelial permeability on PMN migration and the effect of emigrating PMN's on the permeability of the epithelium. In an in vitro model system of transepithelial migration, human PMN's were stimulated by 0.1 micrometer fMet-Leu-Phe to traverse confluent, polarized canine kidney epithelial monolayers of varying permeabilities. Epithelial permeability was determined by both conductance measurement and horseradish peroxidase (HRP) tracer studies. As epithelial permeability increased, the number of PMN invasion sites as well as the number of PMN's that traversed the monolayer increased. The effect of PMN migration on epithelial permeability was examined using the ultrastructural tracers HRP and lanthanum nitrate. PMN's traversing the monolayer made close cell-to- cell contacts with other invading PMNs and with adjacent epithelial cells. These close contacts appeared to prevent leakage of tracer across invasion sites. Following PMN emigration, epithelial junctional membranes reapproximated and were impermeable to the tracers. These results indicated that, in the absence of serum and connective tissue factors, (a) the number of PMN invasion sites and the number of PMN's that traversed an epithelium were a function of the conductance of the epithelium and (b) PMN's in the process of transepithelial migration maintained close cell-cell contacts and prevented the leakage of particles (greater than 5 nm in diameter) across the invasion site.     

Before your very eyes: illness, agency, and the management of Tourette Syndrome

In this article, I examine the ways that people with Tourette Syndrome (TS) manage the motor and vocal tics characteristic of this neurological disorder. To mitigate the powerful stigmas associated with TS, individuals must either remove tics from public view or strive to recast the way that they are perceived. Drawing on ethnographic research with TS sufferers in Indiana, I elaborate three strategies by which this is done, strategies referred to here as displacement, misattribution, and contextualization. These processes strongly affect both the symptoms themselves and the subjective experience of the illness. They also affect the perception of TS in the larger culture, associating the disease with florid symptoms like cursing--symptoms that, although not at all typical of TS, are the ones most resistant to these kinds of management. These patterns highlight how individual agency may actively shape the cultural construction of illness.     

Epithelial migration in organ culture : Effect of incubation in unsupplemented balanced salt solution

In a study of the metabolic requirements for epithelial migration earlier results had indicated that the minimal requirement for migration in organ culture would be a suitable glycolysable substrate. To test this, explants were incubated in a glucose-containing balanced salt solution (Earle's BSS). The epithelial cells consistently failed to migrate in the BSS but resumed migration when the explants were transferred to serum-supplemented medium after a 24 h incubation in BSS. The metabolism and morphology of the explants in BSS were analysed and it was found that protein and RNA synthetic capability were retained but that DNA synthetic capability was depressed. To determine the role of nucleic acid synthesis in epithelial migration explants in serum-supplemented medium were exposed to the inhibitors mitomycin C and actinomycin D. When nucleic acid synthesis was depressed below control values by low concentrations of inhibitor it was found that epithelial migration was not inhibited. It was concluded that, while direct depression of DNA synthesis by inhibitors did not inhibit migration, the results reported here indicate that failure to migrate in BSS may affect nuclear function.     

Retinoic acid inhibits cardiac neural crest migration by blocking c-Jun N-terminal kinase activation

Retinoic acid (RA), a potent teratogen, produces a characteristic set of embryonic cardiovascular malformations similar to those observed in neural crest ablated avians. While the effects of RA on neural crest are well described, the molecular mechanism(s) of RA action on these cells is less clear. The present study examines the relationship between RA and mitogen-activated protein kinase signaling in neural crest cells and demonstrates that c-Jun N-terminal kinase (JNK) activation is severely repressed by RA. RA suppressed migration and proliferation of primary cultures of mouse neural crest cells treated in vitro as well as from animals treated in vivo. On Western blots, JNK activation/phosphorylation in neural crest cultures was reduced, while neither extracellular signal-regulated kinase (ERK) nor p38 pathways were affected. Both the dose-dependent stimulation of neural crest outgrowth and JNK phosphorylation by platelet-derived growth factor AA, which promotes outgrowth but not proliferation of neural crest cultures, were completely abrogated by RA. To establish the relevance of the JNK signaling pathway to cardiac neural crest migration, dominant negative adenoviral constructs were used to inhibit upstream activation of JNK or c-Jun downstream responses. Both adenoviral constructs markedly reduced neural crest cell outgrowth, while a dominant negative inhibitor of the p38 pathway had no effect. These data demonstrate that the JNK signaling pathway and c-Jun activation are critical for cardiac neural crest outgrowth and are potential targets for the action of RA.     

JSAP1/JIP3 cooperates with focal adhesion kinase to regulate c-Jun N-terminal kinase and cell migration

c-Jun N-terminal kinase (JNK)/stress-activated protein kinase-associated protein 1 (JSAP1) (also termed JNK-interacting protein 3; JIP3) is a member of a family of scaffold factors for the mitogen-activated protein kinase (MAPK) cascades, and it also forms a complex with focal adhesion kinase (FAK). Here we demonstrate that JSAP1 serves as a cooperative scaffold for activation of JNK and regulation of cell migration in response to fibronectin (FN) stimulation. JSAP1 mediated an association between FAK and JNK, which was induced by either co-expression of Src or attachment of cells to FN. Complex formation of FAK with JSAP1 and p130 Crk-associated substrate (p130(Cas)) resulted in augmentation of FAK activity and phosphorylation of both JSAP1 and p130(Cas), which required p130(Cas) hyperphosphorylation and was abolished by inhibition of Src. JNK activation by FN was enhanced by JSAP1, which was suppressed by disrupting the FAK/p130(Cas) pathway by expression of a dominant-negative form of p130(Cas) or by inhibiting Src. We also documented the co-localization of JSAP1 with JNK and phosphorylated FAK at the leading edge and stimulation of cell migration by JSAP1 expression, which depended on its JNK binding domain and was suppressed by inhibition of JNK. The level of JSAP1 mRNA correlated with advanced malignancy in brain tumors, unlike other JIPs. We propose that the JSAP1.FAK complex functions cooperatively as a scaffold for the JNK signaling pathway and regulator of cell migration on FN, and we suggest that JSAP1 is also associated with malignancy in brain tumors.     

Epithelial migration in organ culture : Role of protein synthesis as determined by metabolic inhibitors

The role of protein synthesis in epithelial migration in the first 24 h after injury was assessed by exposing explants of rat palatal mucosa to the inhibitors puromycin, cycloheximide and 5-bromodeoxyuridine (BUdR). Epithelial migration was determined by morphological examination of fixed and sectioned explants and the extent of migration was estimated by counting the number of nuclei that had moved beyond the line of incision. The effects of these inhibitors on epithelial migration and on the relevant biochemical pathways were correlated by the use of dual label radioactive tracer technique. With puromycin and cycloheximide it was found that a significant depression of protein synthesis (greater than 50% of the control) was required before epithelial migration was completely inhibited. BUdR had no significant effect on the extent of epithelial migration or on protein synthesis at any concentration tested but significantly depressed thymidine incorporation at the higher concentrations of inhibitor (7.5 and 75 μg/ml). The results of these experiments are interpreted as indicating that ‘new’ protein synthesis is not required for the initiation of epithelial migration following injury and alternative mechanisms are discussed.