Expression of Trio, a member of the Dbl family of Rho GEFs in the developing rat brain.

Trio, a member of the Dbl family of guanine nucleotide exchange factors (GEFs), has a series of spectrin repeats, two GEF domains, protein interaction domains, and a putative kinase domain, potentially important in neuronal axon guidance and cell migration. Most knowledge about Trio is based on studies of Caenorhabditis elegans and Drosophila, while the function of Trio in vertebrates is unclear. The aim of these experiments was to establish the patterns of Trio expression in the postnatal rat brain. During postnatal (P) development, high levels of Trio mRNA are found in the cerebral cortex, hippocampus, thalamus, caudate/putamen, and olfactory bulb, with lower levels in the septal nucleus, nucleus accumbens, amygdala, and hypothalamus. Except for the cerebellum, Trio mRNA in major brain areas is highest at P1, decreasing gradually during development, with low but detectable levels at P30. In P14 cerebral cortex, pyramidal neurons with strongly staining soma and dendrites are observed primarily in layer 5. In hippocampus, strong staining is observed in pyramidal neurons, granule cells, and isolated interneurons. Cerebellar Purkinje neurons exhibit intense staining in the soma and in extensive dendritic arbors at P7 and P14. Levels of Trio mRNA and the intensity of Trio immunostaining in cerebellar Purkinje cells increase from P1 to P30. Consistent with the in situ hybridization pattern, Western blot analyses show that Trio levels in the hippocampus and cortex are high at P1, decreasing until P30. The data suggest that Trio plays an important role during neuronal development.