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Fuel-injected cell motility
News & Comment
TRENDS in Cell Biology Vol.11 No.7 July 2001
281
Journal Club
Location, location, location It has long been known that the differential localization of mRNA transcripts within a cell is an important way of establishing polarity, such as in asymmetric cell division and defining the polarity of the Drosophila egg. However, little is known about the significance of RNA localization in other cell types and how this is achieved. Recent experiments in Drosophila have shown that RNA can be actively targeted apically by the microtubule motor dynein1. The importance of this localization has also been highlighted by another group, which shows that the secreted developmental ligand Wingless requires its mRNA to be localized apically to function correctly2. Using a combination of high-resolution in situ hybridization and live imaging of fluorescently labeled RNA injected into early Drosophila embryos, Wilkie and Davis1 show that the transcripts of several key
developmental genes such as wingless, fushi tarazu and runt localize to the apical region of early Drosophila embryos. They also show that the transcripts diffuse non-directionally within the nucleus and disperse in all directions from the nucleus. The transcripts assemble into small particles in the cytoplasm that are transported apically. This apical transport can be inhibited by disrupting the microtubule network or by blocking the motor protein dynein by using either antibodies or mutants. They also show that transcripts of different genes that are targeted to the same region can occupy the same particle, suggesting some sort of common packing or adaptor protein. The importance of this apical localization is shown by Simmonds et al.2, who demonstrate that wingless mRNA requires two signals in its 3′ untranslated region in order to be targeted apically. When these
are replaced by sequences from basolateral or non-targeted transcripts and expressed in Drosophila embryos, Wingless protein is still produced at approximately normal levels and is secreted, but it is functionally inactive. Why this occurs is not known, but it suggests that apical RNA targeting is crucially required for some subsequent activating modification of the Wingless protein. RNA targeting could thus prove important for other signalling processes. 1 Wilkie, G. and Davis, I. (2001) Drosophila wingless and Pair-Rule transcripts localize apically by dynein-mediated transport of RNA particles. Cell 105, 209–219 2 Simmonds, A.J. et.al. (2001) Apical localization of wingless transcripts is required for Wingless signalling. Cell 105, 197–207
Adam Cliffe [email protected]
Fuel-injected cell motility Crawling cells use actin polymerization to generate a protrusive force at the front of the cell. Thus, motility requires a steady supply of actin monomers – the ‘fuel’ for cell locomotion. Peckham et al.1 reasoned that overexpressing β-actin (the isoform most heavily implicated in cell motility) would be the equivalent of upgrading the family car to one with a bigger engine, increasing speed by increasing the actin ‘fuel supply’ at the leading edge. Sure enough, increasing β-actin levels in myoblasts by around 60% led to a doubling of cell speed. At the same time, protrusive and retractive activity increased, suggesting that actin dynamics at the periphery were enhanced by the greater availability of β-actin. So far, so good. Everything fits the ‘fuel supply’ model. The trouble comes when one delves a little deeper. The first surprise is that
BDM (butanedione monoxime), an inhibitor of myosin motor proteins, reduces back to control levels the movement of cells overexpressing β-actin. Myosin inhibition does not abolish normal cell crawling as this event is driven by actin polymerization rather than actomyosin contraction. Thus, the additional motility induced by overexpressing β-actin appears to be driven by a myosindependent mechanism, unlike the normal motion of crawling cells. Even more surprising is the observation that overexpressing a mutated β-actin that is defective in its ability to polymerize also increases cell speed. Again, this increase can be eliminated with BDM treatment. In other words, the increase in speed induced by overexpressing β-actin does not result from a simple acceleration of the
normal cell-motility engine. Instead, the authors propose a model in which increased actin expression enhances fluid-driven protrusion. Contraction at the rear of the cell triggers a cytoplasmic ‘wave’ that sweeps through the cell and pushes the membrane forwards at the leading edge. This suggestion of a fundamentally different locomotive mechanism is a fascinating one. Perhaps the family car hasn’t had an upgrade after all – it’s been fitted with a booster engine instead. 1 Peckham, M. et al. (2001) Specific changes to the mechanism of cell locomotion induced by overexpression of β-actin. J. Cell Sci. 114, 1367–1377
Robin C. May [email protected]
Viruses at the centrosome: the birthplace of antigenic peptides? After treatment with proteasome inhibitors, proteasomes and their ubiquitinated substrates aggregate in a nuclear indentation around the centrosome – in a manner dependent on microtubular
transport and protein synthesis. Previously, it was proposed that the pericentrosomal area is either a place of enhanced ubiquitindependent proteolysis (the proteolysis center hypothesis) or a place where all
misfolded proteins aggregate by default (the aggresome hypothesis). Lacaille and Androlewicz1 have found that fusion proteins between HIV-1 Nef and ubiquitin (Ub) are targeted to the centrosome
http://tcb.trends.com 0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
TRENDS in Cell Biology Vol.11 No.7 July 2001
281
Journal Club
Location, location, location It has long been known that the differential localization of mRNA transcripts within a cell is an important way of establishing polarity, such as in asymmetric cell division and defining the polarity of the Drosophila egg. However, little is known about the significance of RNA localization in other cell types and how this is achieved. Recent experiments in Drosophila have shown that RNA can be actively targeted apically by the microtubule motor dynein1. The importance of this localization has also been highlighted by another group, which shows that the secreted developmental ligand Wingless requires its mRNA to be localized apically to function correctly2. Using a combination of high-resolution in situ hybridization and live imaging of fluorescently labeled RNA injected into early Drosophila embryos, Wilkie and Davis1 show that the transcripts of several key
developmental genes such as wingless, fushi tarazu and runt localize to the apical region of early Drosophila embryos. They also show that the transcripts diffuse non-directionally within the nucleus and disperse in all directions from the nucleus. The transcripts assemble into small particles in the cytoplasm that are transported apically. This apical transport can be inhibited by disrupting the microtubule network or by blocking the motor protein dynein by using either antibodies or mutants. They also show that transcripts of different genes that are targeted to the same region can occupy the same particle, suggesting some sort of common packing or adaptor protein. The importance of this apical localization is shown by Simmonds et al.2, who demonstrate that wingless mRNA requires two signals in its 3′ untranslated region in order to be targeted apically. When these
are replaced by sequences from basolateral or non-targeted transcripts and expressed in Drosophila embryos, Wingless protein is still produced at approximately normal levels and is secreted, but it is functionally inactive. Why this occurs is not known, but it suggests that apical RNA targeting is crucially required for some subsequent activating modification of the Wingless protein. RNA targeting could thus prove important for other signalling processes. 1 Wilkie, G. and Davis, I. (2001) Drosophila wingless and Pair-Rule transcripts localize apically by dynein-mediated transport of RNA particles. Cell 105, 209–219 2 Simmonds, A.J. et.al. (2001) Apical localization of wingless transcripts is required for Wingless signalling. Cell 105, 197–207
Adam Cliffe [email protected]
Fuel-injected cell motility Crawling cells use actin polymerization to generate a protrusive force at the front of the cell. Thus, motility requires a steady supply of actin monomers – the ‘fuel’ for cell locomotion. Peckham et al.1 reasoned that overexpressing β-actin (the isoform most heavily implicated in cell motility) would be the equivalent of upgrading the family car to one with a bigger engine, increasing speed by increasing the actin ‘fuel supply’ at the leading edge. Sure enough, increasing β-actin levels in myoblasts by around 60% led to a doubling of cell speed. At the same time, protrusive and retractive activity increased, suggesting that actin dynamics at the periphery were enhanced by the greater availability of β-actin. So far, so good. Everything fits the ‘fuel supply’ model. The trouble comes when one delves a little deeper. The first surprise is that
BDM (butanedione monoxime), an inhibitor of myosin motor proteins, reduces back to control levels the movement of cells overexpressing β-actin. Myosin inhibition does not abolish normal cell crawling as this event is driven by actin polymerization rather than actomyosin contraction. Thus, the additional motility induced by overexpressing β-actin appears to be driven by a myosindependent mechanism, unlike the normal motion of crawling cells. Even more surprising is the observation that overexpressing a mutated β-actin that is defective in its ability to polymerize also increases cell speed. Again, this increase can be eliminated with BDM treatment. In other words, the increase in speed induced by overexpressing β-actin does not result from a simple acceleration of the
normal cell-motility engine. Instead, the authors propose a model in which increased actin expression enhances fluid-driven protrusion. Contraction at the rear of the cell triggers a cytoplasmic ‘wave’ that sweeps through the cell and pushes the membrane forwards at the leading edge. This suggestion of a fundamentally different locomotive mechanism is a fascinating one. Perhaps the family car hasn’t had an upgrade after all – it’s been fitted with a booster engine instead. 1 Peckham, M. et al. (2001) Specific changes to the mechanism of cell locomotion induced by overexpression of β-actin. J. Cell Sci. 114, 1367–1377
Robin C. May [email protected]
Viruses at the centrosome: the birthplace of antigenic peptides? After treatment with proteasome inhibitors, proteasomes and their ubiquitinated substrates aggregate in a nuclear indentation around the centrosome – in a manner dependent on microtubular
transport and protein synthesis. Previously, it was proposed that the pericentrosomal area is either a place of enhanced ubiquitindependent proteolysis (the proteolysis center hypothesis) or a place where all
misfolded proteins aggregate by default (the aggresome hypothesis). Lacaille and Androlewicz1 have found that fusion proteins between HIV-1 Nef and ubiquitin (Ub) are targeted to the centrosome
http://tcb.trends.com 0962-8924/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.