Mechanisms of interkinetic nuclear migration Y Kosodo et al 2005). In this cascade, the absence of Tpx2 might result in the aberrant localization of TACC (Barr and Gergely, 2007). Although centrosomal proteins are known to be involved in INM (Xie et al, 2007), we observed that the loss of Tpx2 function did not affect the localization of TACC3 protein at the centrosome during mitosis (Supplementary Figure S5E), suggesting that Tpx2 can act independently of centrosomal proteins. We instead found that microtubule distribution in the apical process becomes narrower in width in the G2-phase compared with the G1-phase and that this change depends on Tpx2 function. Considering the in vitro activity of Tpx2 in promoting microtubule bundling (Schatz et al, 2003) and Tpx2 functions in the organization of the microtubule cytoskeleton in the apical process of neural progenitor cells in vivo (Figure 4), how such organized microtubules promote nuclear migration in the apical process remains to be eluci- dated. Interestingly, an in vitro study shows that dynein- dynactin-coated microbeads are less likely to pass micro- tubule–microtubule intersections than to switch from one microtubule to another (Ross et al, 2008). This result indi- cates that dynein-dependent motor activity carries organelles more efﬁciently in cases where microtubules are uniformly orientated. Microtubules may be more randomly orientated in the absence of the bundling activity of Tpx2, as occurs in G1-phase or in Tpx2-knockdown cells. Thus, the alignment of microtubules by Tpx2 could possibly enhance the function of the microtubule-dynein system, accounting for efﬁcient nu- clear migration in the apical process. The most remarkable ﬁnding of this study is that nuclear migration in the apical-to-basal direction during G1-phase involves a passive, non-autonomous process dependent on the migration of nuclei during G2-phase, which takes place toward the apical surface and results in a higher nuclear density in the apical region and a lower nuclear density in the basal region of the VZ. In a packed tissue such as the VZ of the embryonic brain, free, mobile nuclei redistribute so that the nuclear density becomes homogeneous. Based on our experimental results and computational modelling, we pro- pose a displacement model of INM, in which, after mitosis, nuclei on the ventricular surface are driven away from this surface by a collective displacement effect generated by other nuclei migrating toward the ventricular surface. This scenario is supported by the recently reported observation in zebraﬁsh retina, suggesting that apical-to-basal nuclear movement dur- ing INM is a stochastic event (Norden et al, 2009). This does not mean that active, autonomous mechanisms are not in- volved in the apical-to-basal movement, but indicates a close link between migrations in two opposite directions, which is compatible with motor-dependent autonomous mechanisms in the movement in this direction. Recent work reported that kinesin-type microtubule plus-end-directed motors have a certain role in carrying the nucleus from the apical region to the basal region of the VZ (Tsai et al, 2010). Alternatively, it is demonstrated that myosin II is required for nuclear migration in the apical-to-basal direction (Schenk et al, 2009). However, another report documents that actomyosin drives the nucleus from the basal to the apical surface in zebraﬁsh retina (Norden et al, 2009). A simple explanation for the obvious contra- diction between those two studies is that involvement of the actomyosin system in the INM is not direction speciﬁc but rather is used to maintain general nucleokinesis. It is, however, also reported that inhibition of myosin II does not show any 1702 The EMBO Journal VOL 30 | NO 9 | 2011 effect on INM (Tsai et al, 2010). Although the idea that microtubule- and actomyosin motors show coordinated opera- tion in INM is attractive, further investigations are required to identify molecules that may control the direction and the cell- cycle dependency of actomyosin systems. Our model proposed here for INM of neural progenitor cells not only explains how nuclear migration is associated with the cell cycle but also manifests a robust system capable of maintaining neuroepithelial architecture during develop- ment. Prior to and during the neurogenic period, neural progenitor cells expand their numbers by self-renewing divi- sions. Given that most mitoses of self-renewing neural pro- genitor cells take place at the apical surface throughout brain development (Kowalczyk et al, 2009), it would be advanta- geous for daughter cells to leave the mitotic area until their next mitosis, as this would increase the efﬁciency of mitosis in the region of the ventricular surface. According to our model, only two phases of INM are motor dependent, active processes: nuclear migration toward the ventricular surface and mitosis. During the remaining phases of INM, G1- and S-phase, the distribution of nuclei is non-autonomously determined by the positions of other nuclei. The G1- and S-phases of the cell cycle therefore operate as adjustable phases in INM that allow nuclei in the VZ to be dynamically arranged as a whole to minimize the imbalance in nuclear density and avoid disorder in the neuroepithelium. The idea that INM might be related to the process of cell fate determination of neural progenitor cells in the develop- ing brain is attractive (Latasa et al, 2009). Studies of mouse brain and zebraﬁsh retina indicate that perturbation of INM by downregulation of centrosomal and microtubule motor- associated proteins, respectively, caused enhanced neuro- genesis at the expense of neural progenitor cells (Xie et al, 2007; Del Bene et al, 2008). In a recent study, it was reported that neural progenitor cells of mice genetically deﬁcient for SUN-domain and KASH-domain proteins show a proliferation defect (Zhang et al, 2009). Given that INM has a role in the cell fate decisions of neural progenitor cells, it is important to determine which phase of migration is crucial and what molecules are involved in cell fate choice. Our study suggests that autonomous nuclear movement toward the apical region affects migration in both directions. This implies that the rate of neurogenesis can be affected during INM in both direc- tions. Del Bene et al (2008) proposed that perturbation of INM alters the rate of exposure of cells to Notch signalling, leading to premature neurogenesis. This study suggested that Notch becomes activated as nuclei move toward the apical surface, whereas it is downregulated as cells move in the opposite direction (Del Bene et al, 2008). In the mouse cortex, however, Notch-active nuclei are found in the basal region of the VZ (Tokunaga et al, 2004; Kawaguchi et al, 2008). Further studies are necessary to clarify how Notch signals are trans- duced during INM progression. From a mechanistic perspec- tive, the role of centrosomes in INM is noteworthy. A close relationship between neurogenesis and centrosomal func- tions is suggested from the identiﬁcation of causative genes of human autosomal-recessive primary microcephaly (MCPH genes; several MCPH genes, such as ASPM, CDK5rap2 and CENPJ, encode centrosomal proteins (Thornton and Woods, 2009)). It would be interesting to examine whether mutations in these genes result in defective INM in addition to their effect on mitotic cells, thus leading to impaired neurogenesis. & 2011 European Molecular Biology Organization