EVIDENCE FOR DIFFERENT MECHANISMS OF INDUCTION OF HSP70I - A COMPARISON OF CULTURED RAT CORTICAL-NEURONS WITH ASTROCYTES

This study is a follow-up of previous work which demonstrated that cul tured cortical neurons did not synthesize HSP70i immediately after hea t stress when compared with cultured cortical astrocytes. We have exte nded the period of observation for HSP70i induction of cultured cortic al neurons and astrocytes up to 24 h after heat stress. Cultured rat c ortical neurons derived from 16-day-old fetal rats respond differently to heat stress than cultured rat astrocytes derived from newborn rats . They showed a delayed HSP70i induction in the majority of cultured n eurons and the response was heterogeneous and was absent in most small er neurons. The delayed neuronal induction was accompanied by a prolon ged activation of heat-shock transcription factor 1 (HSF-1) and prolon ged transcription of HSP70i mRNA. In comparison astrocytes showed a ma rked early induction of HSP70i mRNA and protein. In addition the induc tion of HSP70i in astrocytes was followed by translocation of the prot ein into the nucleus, a finding which we failed to demonstrate in neur ons. Immunostaining for HSP70i was more uniform in astrocytes than neu rons. Many neurons did not stain for up to 24 h after heat shock in th is study. Immunocytocytochemical staining of HSF-1 and 2 showed major differences between neurons and astrocytes. Astrocytes showed localiza tion of HSF-1 to the nucleus before and after heat stress, while neuro ns showed HSF-1 localization to the cytoplasm and nucleus before and a fter heat stress. Finally HSF-2 was undetectable in neurons when compa red with astrocytes by Western immunoblot analysis. However, astrocyte s and neurons revealed weak immunostaining of HSF-2 in the cytoplasm a nd nucleus. The staining in the neurons was likely secondary to cross- reactivity to an unidentified protein. We conclude that HSP70i express ion after heat shock is delayed in rat cortical neurons when compared with rat cortical astrocytes. In addition most small neurons did not s ynthesize HSP70i after heat shock. This difference in induction of HSP 70i may be secondary to localization and activation of HSF-1 but not H SF-2. Neuronal susceptibility to injury may be related to the delayed induction of HSP70i and also the possible failure of newly synthesized HSP70i to translocate into the nucleus.