NIH3T3 fibroblast at the wound edge stained for microtubules (green), Par3 (red) and nuclei (blue). The oriented centrosome (not stained) localises at the focus of the microtubules "ahead" of the nucleus.

Image courtesy of Dr Gregg G. Gundersen, Columbia University, New York, USA.

The centrosome localises between the nucleus and the leading edge in many migrating cell types, thus contributing to cell polarity by positioning the endocytic compartment and the Golgi so that membrane traffic is directed towards the front of the cell. In wound-edge fibroblasts, reorientation results from the centrosome remaining at the cell centre while the nucleus moves rearwards. Both microtubules and the microtubule minus-end motor dynein are involved in maintaining the central position of the centrosome, but how their dynamics and activity are regulated remains unclear. In Current Biology, Jan Schmoranzer, Gregg Gundersen and colleagues now show that the partitioning defective 3 (PAR3) polarity protein functions together with cortical dynein to regulate microtubule dynamics for centrosome centration.

Cytoplasmic dynein is a large complex that contains heavy chains (DHCs), intermediate chains (DICs), light intermediate chains (LIC1 or LIC2) and light chains. The authors identified PAR3 as a factor that co-immunoprecipitated with dynein and show that the PAR3-dynein interaction requires the N-terminal regions of PAR3 and the LIC2 subunit.

LPA (lysophosphatidic acid) stimulation usually induces the repositioning of the nucleus and centrosome. However, when NIH 3T3 cells were depleted of PAR3 using siRNA, the centrosome failed to remain at the cell centre and instead moved rearward with the nucleus. This phenotype was similar to that previously observed for dynein inhibition, suggesting that PAR3 and dynein function in the same pathway for centrosome orientation. Furthermore, the authors show that centrosome positioning specifically requires LIC2, reinforcing the idea that dynein-PAR3 interactions regulate this process. Importantly, PAR3-dependent centrosome orientation was shown to be required for directed cell migration in a wound-closure experiment.

But how does PAR3 affect centrosome positioning? PAR3 is known to localise at cell–cell contacts. Interestingly, LPA stimulation increased the co-localisation of microtubule ends, dynein and PAR3 at cell-cell contacts, suggesting that PAR3 may interact with dynein to tether microtubules there. Microtubules go through phases of growth and shortening that are interspersed with short pauses. By measuring microtubule dynamics in cells expressing green fluorescent protein-labelled -tubulin, Schmoranzer, Gundersen and colleagues found that LPA specifically activates pausing at cell–cell contacts, and that this reduction of microtubule dynamics was dependent both on PAR3 and LIC2.

This work shows that the polarity protein PAR3 is involved in centrosome orientation in migrating cells. Furthermore, it shows that both PAR3 and LIC2-containing dynein regulate microtubule dynamics at cell–cell contacts. The authors propose that PAR3 and dynein tether microtubules at the cell-cell contact points, which might in turn prevent the displacement of the centrosome rearward.

Author: Kim Baumann - Copyright © 2009 Nature Publishing Group, a division of MacMillan Publishers Limited; used with permission