Regional distribution of importin subtype mRNA expression in the nervous system: Study of early postnatal and adult mouse

Importin-α and β1 mediate the translocation of macromolecules bearing nuclear localization signals across the nuclear pore complex. Five importin-α isoforms have been identified in mice and six in human. Some of these importins play an important role in neural activity such as long term potentiation, but the functional differences of each isoform in the CNS are still unclear. We performed in situ hybridization (ISH) using non-isotopic probes to clarify the expression patterns of importin-α subtypes (α5, α7, α1, α4, α3) and importin-β1 in the mouse CNS of adult and early postnatal stages. The mRNAs of the importin-α subtypes and importin β1 were expressed throughout the CNS with specific patterns; importin-α5, α7, α3, and β1 showed moderate to high expression levels throughout the brain and spinal cord; importin-α4 showed a lack of expression in limited regions; and importin-α1 showed a low expression level throughout the brain and spinal cord but with a moderate expression level in the olfactory bulb and reticular system. We also demonstrated that importin-αs and β1 mRNAs were predominantly expressed in neurons in the adult mouse brain by using double-labeling fluorescence ISH and immunohistochemistry. Moreover, importin-αs and β1 mRNAs were detected throughout the CNS of postnatal mice and were highly expressed in the external granule layer of the cerebellar cortex on postnatal days 0, 4, and 10. This is the first report of importin-αs and β1 expression throughout the CNS of adult mice, as well as in the developing brain, including cell type specific localization.