Many physiological processes require cells to polarize and move directionally. Examples include the directed migration of cells toward a chemoattractant, the polarized growth of neurites during development, or the polarization and migration of tissue cells at wound edges in a squamous epithelium. On a molecular level, polarization requires spatiotemporal subcellular regulation of signaling and cytoskeletal molecules. Polarized actin polymerization dynamics and myosin-mediated contractility provide the force for shape changes and migration. In addition, many polarized, migrating cell types show a characteristic polarization of the microtubule cytoskeleton in which the microtubule-organizing center and the bulk of microtubules orient forward in the direction of migration. Microtubules are also required for most cell types to polarize and directionally migrate. However, the function of the microtubule cytoskeleton during cell polarization and migration is incompletely understood.
+TIPs are a heterogenous group of proteins, which include EB1, CLIP-170, CLASPs and others. These proteins share the dynamic behavior that they specifically associate with growing microtubule plus ends. +TIPs interact with each other and it is thought that a +TIP complex may regulate microtubule polymerization dynamics, mediate cortical microtubule interactions, or act as signaling scaffold at microtubule ends. Because regulation of these processes is important in polarized cells, +TIPs are hypothesized to be required for cell polarization and migration.
Unlike most other +TIPs, CLASPs associate with microtubules in two distinct ways: In the cell body of epithelial cells, CLASPs track microtubule plus ends and rapidly dissociate from the lattice. In contrast, in the protruding lamella, CLASPs directly associate along the microtubule lattice independent of their plus end-tracking activity. In cells, the association of CLASPs with the microtubule lattice is controlled by the activity of glycogen synthase kinase 3β: GSK3β-inhibition promotes CLASP association with microtubule lattices, while expression of a constitutively active GSK3β mutant abolishes CLASP microtubule lattice-binding. Because GSK3β is inactivated in the protruding lamella, we propose that CLASP-phosphorylation in the cell body decreases CLASP affinity for microtubules, while association of non-phosphorylated CLASPs with microtubules in the leading edge may specifically stabilize these microtubules. We use biochemistry and live cell microscopy to test the implications of this model in in vitro tissue culture systems:
What is the molecular mechanism by which GSK3β regulates CLASP-function?
Are CLASPs (and other +TIPs) required for cytoskeletal and cell polarization?
Do physiological stimuli regulate CLASP-microtubule association?
Watch the dynamics of EGFP-tagged +TIPs in wound-edge epithelial cells.
Read why CLASPs are special.
Wittmann and Waterman-Storer,
J. Cell Biol. 169:929
