RanBP9 overexpression reduces dendritic arbor and spine density.

RanBP9 is a multi-domain scaffolding protein known to integrate extracellular signaling with intracellular targets. We previously demonstrated that RanBP9 enhances Aβ generation and amyloid plaque burden which results in loss of specific pre- and postsynaptic proteins in vivo in a transgenic mouse model. Additionally, we showed that the levels of spinophilin, a marker of dendritic spines were inversely proportional to the RanBP9 protein levels within the synaptosomes isolated from AD brains. In the present study, we found reduced dendritic intersections within the layer 6 pyramidal neurons of the cortex as well as hippocampus of RanBP9 transgenic mice compared to age-matched wild-type (WT) controls at 12-months of age but not at 6 months. Similarly, the dendritic spine numbers were reduced in the cortex at only 12-months of age by 30% (p<0.01), but not at 6 months. In the hippocampus also the spine densities were reduced at 12-months of age (38%, p<0.01) in the RanBP9 transgenic mice. Interestingly, the levels of phosphorylated form of cofilin, an actin binding protein that plays crucial role in the regulation of spine numbers were significantly decreased in the cortical synaptosomes at only 12 months of age by 26% (p<0.01). In the hippocampal synaptosomes, the decrease in cofilin levels were 36% (p<0.01) at 12-months of age. Thus dendritic arbor and spine density were directly correlated to the levels of phosphorylated form of cofilin in the RanBP9 transgenic mice. Similarly, cortical synaptosomes showed a 20% (p<0.01) reduction in the levels of spinophilin in the RanBP9 transgenic mice. These results provided the physical basis for the loss of synaptic proteins by RanBP9 and most importantly it also explains the impaired spatial learning and memory skills previously observed in the RanBP9 transgenic mice.