At the interface of the actin and microtubule cytoskeleton lie a handful of signaling events – not the least of which is that of Rho-mDia1 signaling, which is the topic of a recent study in Molecular and Cellular Biology worth noting.
It’s been known that the small GTPase Rho influences the actin and microtubule cytoskeletons through Rho-associated Kinase (ROCK) and mammalian homologue of the Drosophila gene Diaphanous 1 (mDia1) signaling: ROCK by inducing actomyosin contraction and inhibiting actin filament disassembly and mDia1 by catalyzing actin polymerization and regulating MT dynamics. The specifics of mDia1’s role and the molecular mechanisms by which it regulates MT dynamics is unclear however.
Yamana et al. make the argument here that RNAi knockdown of mDia1 disrupted cell polarization and motility, including a loss of MT Organizing Center (MTOC) orientation towards the apical side of the nucleus. Makes sense, right? Well at first, maybe. But MTOC polarity is judged by its position relative to the nucleus, and it’s difficult to judge the direction the centroid of the microtubule cytoskeleton is point all by itself. If, as in this case, you go altering expression levels of key cytoskeletal-regulating proteins, you have to remember the caveat that what you see may be an artifact of the experiment.
Afterall, it could be just that some other determinant of cytoskeletal structure has been perturbed, throwing everything off. For instance, what else influences the arrangement of the MTOC and the nucleus, besides the MTOC and MTs in migrating cells? Yep, you guessed it, the actin cytoskeleton. You see, the nucleus is pretty much a big organelle floating in the soup that we call the cytoplasm, surrounded by filaments that create stability and motile forces in migrating cells, and actin filaments are driven rearward in the migrating cell, pushing nucleus backwards by retrograde forces as well. Gomes et al. make the case for this conclusion last year in Cell:
The velocity of rearward nuclear movement is similar to that of actin and MT retrograde flow (Mikhailov and Gundersen, 1995, Waterman-Storer and Salmon, 1997 and Salmon et al., 2002). We analyzed kymographs to see whether retrograde flow was activated by LPA and coupled with nuclear movement. Kymographs showed that LPA triggered rearward movement of some MTs in the lamella (Figures 2A and 2B). The slopes of the lines in the kymographs representing rearward-moving MTs and nuclei were nearly identical, indicating that both were moving at the same velocity (MTs, 0.23 μm/min; nucleus, 0.26 μm/min). As MT retrograde flow is driven by actin retrograde flow (Waterman-Storer and Salmon, 1997 and Salmon et al., 2002), this result suggests that actin retrograde flow might be responsible for the rearward movement of the nucleus. It also suggests that LPA triggers either actin retrograde flow itself or, alternatively, coupling of the nucleus (and MTs) to constitutively active actin retrograde flow.
In this case, the loss anterograde relocation of the MTOC doesn’t quite sound as good an explanation as the loss of regulated retrograde relocation of the nucleus through actin filaments, as the mechanism for loss of polarity by mDia1 knockdown.
Of course, however, the discovery of the molecular mechanisms downstream of mDia1 will resolve this question relating how Rho GTPases modulate MTs. And it makes sense that there be some cue other than retrograde actin flow pushing on the nucleus as the orienting factor. I’m just inclined to think that it’s at the cell membrane, relating to stabilized MT plus-ends and somehow sensed by the MTOC to effect orientation – but I’m just guessing while I play the wait-and-see game, waiting for more studies to clarify the issue.
- The Rho-mDia1 pathway regulates cell polarity and focal adhesion turnover in migrating cells through mobilizing Apc and c-Src. Yamana N, Arakawa Y, Nishino T, Kurokawa K, Tanji M, Itoh RE, Monypenny J, Ishizaki T, Bito H, Nozaki K, Hashimoto N, Matsuda M, Narumiya S. Mol Cell Biol. 2006 Sep ; 26(18): 6844-58. Pubmed.
- Nuclear movement regulated by Cdc42, MRCK, myosin, and actin flow establishes MTOC polarization in migrating cells. Gomes ER, Jani S, Gundersen GG. Cell. 2005 May 6; 121(3):451-63. Pubmed.