SUMMARY Neuronal arborization is regulated by cell autonomous and non-autonomous mechanisms including endosomal signaling via BDNF/TrkB. The endosomal Na+/H+ exchanger 6 (NHE6) is mutated in a new autism-related disorder. NHE6 functions to permit proton leak from endosomes yet the mechanisms causing disease are unknown. We demonstrate that loss of NHE6 results in over-acidification of the endosomal compartment and attenuated TrkB signaling. Mouse brains with disrupted NHE6 display reduced axonal and dendritic branching, reduced synapse number and circuit strength. Site-directed mutagenesis shows that the proton leak function of NHE6 is required for neuronal arborization. We find that TrkB receptor co-localizes to NHE6-associated endosomes. TrkB protein and phosphorylation are reduced in NHE6 mutant neurons in response to BDNF signaling. Finally, exogenous BDNF rescues defects in neuronal arborization. We propose that NHE6 mutation leads to circuit defects that are in part due to impoverished neuronal arborization that may be treatable by enhanced TrkB signaling.

SUMMARYMicrocephaly affects ~1% of the population and is associated with mental retardation, motor defects and, in some cases, seizures. We analyzed the mechanisms underlying brain size determination in a mouse model of human microcephaly. The Hertwig's anemia (an) mutant shows peripheral blood cytopenias, spontaneous aneuploidy and a predisposition to hematopoietic tumors. We found that the an mutation is a genomic inversion of exon 4 of Cdk5rap2, resulting in an in-frame deletion of exon 4 from the mRNA. The finding that CDK5RAP2 human mutations cause microcephaly prompted further analysis of Cdk5rap2 an/an mice and we demonstrated that these mice exhibit microcephaly comparable to that of the human disease, resulting from striking neurogenic defects that include proliferative and survival defects in neuronal progenitors. Cdk5rap2 an/an neuronal precursors exit the cell cycle prematurely and many undergo apoptosis. These defects are associated with impaired mitotic progression coupled with abnormal mitotic spindle pole number and mitotic orientation. Our findings suggest that the reduction in brain size observed in humans with mutations in CDK5RAP2 is associated with impaired centrosomal function and with changes in mitotic spindle orientation during progenitor proliferation.

The guanosine tri-phosphatase Ran stimulates assembly of microtubule spindles. However, it is not known what aspects of the microtubule cytoskeleton are subject to regulation by Ran in mitosis. Here we show that Ran-GTP stimulates microtubule assembly by increasing the rescue frequency of microtubules three- to eightfold. In addition to changing microtubule dynamics, Ran-GTP also alters the balance of motor activities, partly as a result of an increase in the amount of motile Eg5, a plus-end-directed microtubule motor that is essential for spindle formation. Thus, Ran regulates multiple processes that are involved in spindle assembly.