Differential expression and induction of small heat shock proteins in rat brain and cultured hippocampal neurons.

The so-called stress response involving up-regulation of heat shock proteins (Hsps) is a powerful mechanism of cells to deal with harmful conditions to which they are exposed throughout life, such as hyperthermia, hypoxia, or oxidative stress. Some members of the group of small Hsps (sHsps) seem to play a neuroprotective role in the brain. Here we analyzed the expression of all 11 sHsps in the rat brain by using RNA in situ hybridization and quantitative real-time RT-PCR. Additionally, we investigated sHsps in cultured neurons exposed to heat shock. We found seven sHsps to be expressed in the rat brain, with HspB5 (αB-crystallin), HspB6 (Hsp20), and HspB11 (Hsp16.2) showing the highest expression levels (4-24% of reference genes) followed by HspB1 (Hsp25) and HspB8 (Hsp22; 0.1-2% of reference genes), all being widely expressed in the brain areas investigated. HspB2 (MKBP) and HspB3, however, showed selective expression in only some regions (B2: cortex and hippocampus, B3: cortex and cerebellum). Whereas HspB5 was expressed mainly in the white matter, HspB6 showed the greatest expression in the cerebellar cortex, and HspB11 was widely distributed over the whole brain. In cultured hippocampal neurons, heat shock led to an increase of HspB1 and HspB8 mRNA and additionally HspB5 protein. Our data indicate that the sHsps induced by heat shock, HspB1, B5, and B8, might be especially involved in neuroprotection under stress conditions. The other sHsps showing constant neuronal expression may play a constitutive role or may be up-regulated and important in types of stresses other than heat shock.