Cyclin Dependent Kinase (Cdk) family members play a role in CNS development. Cyclin Dependent Kinase 5 (Cdk5) is well known for its fundamental role in neuronal development and axogenesis, as well as, cell death. Other Cdks include Pctaire and Pftaire. Inhibition of Pctaire results in increased axon outgrowth, however, the role and function of Pftaire is unknown. Pftaire1 is a novel member of the Cdk family that was initially detected in a screen for cdc2-like kinases. Unpublished data from our lab reveals that Pftaire1 (Eip63E) deficiency in Drosophila melanogaster results in defects in the axon and neuronal structure of the ventral nerve cord (VNC). In mammals, Pftaire1 is highly, expressed in the CNS. Here, we proposed that Pftaire1 might have a role in axon outgrowth. To investigate the role of Pftaire1 in mammals, the first germline Pftaire1 knockout mice were generated. Considering the severe effects of Eip63E deficiency in Drosophila and the homology between mammalian and fly Pftaire1, CNS defects in the mouse were anticipated. However, to date, no gross abnormalities have been detected in the overall morphology, fertility, life span, or anatomical brain structures of the Pftaire1 deficient mice. This may be due to the presence of other post-mitotic Cdk proteins that are highly similar to Pftaire1. For instance, mammals possess Pftaire (1, and 2), as well as, Pctaire (1, 2, and 3), while Drosophila only possess the Pftaire1 orthologue where the Pftaire2 and Pctaire (1, 2, and 3) are absent. Furthermore, the mice were of mixed background. In spite of this, we demonstrated that Pftaire1 deficient neurons showed increased axon length, in the initial phases of culture. This was confirmed by expression of dominant negative (DN) D228N-Pftaire1 in wild type neurons. Also classification of axons into different ranges, reveals a higher percentage of hyperextended neurites in D228N and Pftaire1 knockout mice. The mechanism by which Pftaire1 controls axon outgrowth is unknown. In this study we show that, Pftaire1 interacts physically with the small GTPase proteins Rac1, Cdc42, and RhoA. Importantly, we showed that Pftaire1 phosphorylates GDP-RhoA on a serine residue. We propose that this regulates RhoA activity, which in turn controls axon outgrowth.

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