Biochemical and functional characterisation of αPIX, a specific regulator of axonal and dendritic branching in hippocampal neurons.

BACKGROUND INFORMATION: PIX proteins are exchange factors for Rac and Cdc42 GTPases that are differentially expressed in the brain, where they are implicated in neuronal morphogenesis. The PIX family includes the two members αPIX and βPIX, and the gene of αPIX is mutated in patients with intellectual disability.
RESULTS: We have analysed the expression of PIX proteins in the developing brain and addressed their role during early hippocampal neuron development. Mass spectrometry identified several βPIX isoforms and a major p75 αPIX isoform in brain and hippocampal cultures. PIX proteins expression increased with time during neuronal differentiation in vitro. The PIX partners GIT1 and GIT2 are also found in brain and their expression was increased during neuronal differentiation. We found that αPIX, but not βPIX, was required for proper hippocampal neuron differentiation, since silencing of αPIX specifically hampered dendritogenesis and axonal branching. Interestingly, the depletion of GIT2 but not GIT1 mimicked the phenotype observed after αPIX knock-down. Over-expression of αPIX specifically enhanced dendritic branching, while both αPIX and βPIX over-expression affected axonal morphology. Again, only over-expression of GIT2, but not GIT1, affected neuritic morphology.
CONCLUSIONS: The results indicate that αPIX and GIT2 are required for neuronal differentiation, and suggest that they are part of the same pathway, while GIT1 and βPIX are dispensable for early hippocampal neurons development.