Expression of heat-shock protein Hsp25 in mouse Purkinje cells during development reveals novel features of cerebellar compartmentation.

The small heat shock protein Hsp25 is constitutively expressed in the adult mouse cerebellum by parasagittal stripes of Purkinje cells confined to the caudal central zone ( approximately lobules VI and VII), the nodular zone ( approximately ventral lobule IX and lobule X), and the paraflocculi/flocculi. During development several distinct phases in Hsp25 expression can be distinguished. Hsp25-immunopositive Purkinje cells are first seen at birth, when four clusters are visible in the vermis of lobules IV/V, and scattered Hsp25-immunoreactive Purkinje cells are seen in lobule VIII. By postnatal day 2/3, six narrow parasagittal stripes of Hsp25-immunopositive Purkinje cells are seen in the vermis of the anterior lobe. In the posterior lobules, most Purkinje cells in the vermis of lobules VIII and IX express Hsp25. This initial limited expression is followed by a phase of widespread expression (postnatal days 6-9) in which Hsp25 immunoreactivity is detected in virtually all Purkinje cells. This global cerebellar expression of Hsp25 then gradually disappears, first in the anterior zone and the hemispheres and subsequently in the posterior zone, to leave the restricted adult expression pattern. Western blotting analysis and immunoprecipitation with anti-Hsp25 suggest that all immunocytochemistry can be attributed the expression of Hsp25. Furthermore, visual deprivation had no effect on the development of Hsp25 expression in Purkinje cells, suggesting that visuomotor input is not responsible for the establishment of constitutive Hsp25 expression in the cerebellar cortex.