The Hot Topic of Asymmetric Cell Division
Two papers were recently released as early epub on the Nature Cell Biology website. Both of these papers the first by Izumi and collegues and the other from Siller and colleagues. Although the two papers provide a nearly identical story, they are each worth reading as you will notice some small details that differ between the two papers.
As asymmetric cell division is a hallmark of self renewing cells, this topic has garnered much interest in recent years. Although many of the molecualar components regulating asymmetric cell division have been identified (there are dozens of reviews - for example Knoblich 2001 in Nat. Rev. Mol Cell Biol.), the story is still not clear. These papers add more complexity to the story by identifying a new player. The take home message from each of these papers is that a protein called Mud, a NuMA related protein, plays an essential role in regulating spindle orientation during asymmetric cell division of drosophila neuroblasts.
Both labs show that Mud binds directly to the TPR region of Pins and further show that the endogeneous proteins interact by immunoprecipitation. Additionally both papers show that during interphase Mud localizes to the apical surface, and that this localization is dependent on Pins. During mitosis Mud maintains its cortical localization but is now also found at the centrosomes. The Izumi paper finds that this relocalization is microtubule dependent. Both papers find that Mud has no effect on Pins localization but the Izumi paper finds that in the absence of Mud neuroblasts will often contain more than two centrosomes.
Finally both papers show that Mud is essential for the spindle to properly align during asymmetric cell division. Although both papers provide convincing data to this end, the Siller paper takes things a step further using live cell imaging to show that not only does the spindle not align, but it also does not move.
Both papers are highly recommended!