Neuronal distribution of spatial in the developing cerebellum and hippocampus and its somatodendritic association with the kinesin motor KIF17.

We identified the Spatial (Stromal Protein Associated with Thymii and Lymph-node) gene from an adult thymus mouse library of cDNA clones. By RT-PCR, we reported that Spatial was highly expressed in restricted areas of the central nervous system. Here, we characterize the precise cellular localization of Spatial during mouse brain development in the cerebellum, hippocampus and cortex. Five different transcript isoforms have been described for Spatial and among those, only Spatial-epsilon and -beta present a tightly controlled expression. In the cerebellum, Spatial expression is detected in the external precursor granular layer and persists as these cells migrate and differentiate to form the internal granular layer. It is also expressed in differentiating Purkinje cells with a specific somatodendritic distribution. Spatial expression in the hippocampus is spatially and temporally regulated: it is first expressed in the CA3 field, then in CA1 and later in the dentate gyrus. Interestingly, Spatial-beta expression tightly overlaps with the beginning of neuronal differentiation in both structures. Using cultured hippocampal neurons, we show that Spatial also exhibits a somatodendritic distribution and it is concentrated in some synaptic regions. Moreover, the vesicle-like cellular distribution of Spatial protein in dendrites is similar to that described for the kinesin motor protein KIF17. Immunofluorescence analyses show that Spatial colocalizes with KIF17 in dendrites of hippocampal neurons in primary culture. Additionally, coimmunoprecipitation experiments of endogenous proteins from hippocampus confirmed that Spatial and KIF17 physically interact. These findings suggest that Spatial may play a role in neuronal morphogenesis and synaptic plasticity through its interaction with the kinesin motor KIF17 in dendrites.