SELECTIVE POSTMORTEM DEGRADATION OF INDUCIBLE HEAT-SHOCK PROTEIN-70 (HSP70) MESSENGER-RNAS IN RAT-BRAIN

1. Altered mRNA levels in postmortem brain tissue from persons with Al zheimer's disease (AD) or other neurological diseases are usually pres umed to be characteristic of the disease state, even though both agona l state (the physiological state immediately premortem) and postmortem interval (PMI) (the time between death and harvesting the tissue) hav e the potential to affect levels of mRNAs measured in postmortem tissu e. Although the possible effect of postmortem interval on mRNA levels has been more carefully evaluated than that of agonal state, many stud ies assume that all mRNAs have similar rates of degradation postmortem . 2. To determine the postmortem stability of inducible heat shock pro tein 70 (hsp70) mRNAs, themselves unstable in vivo at normal body temp erature, rats were heat shocked in order to induce synthesis of the hs p70 mRNAs. hsp70 mRNA levels in cerebellum and cortex were then compar ed to those of their heat shock cognate 70 (hsc70) mRNAs, as well as t o levels of 18S rRNAs, at 0 and at 24 hr postmortem. 3. Quantiation of northern blots after hybridization with an hsp70 mRNA-specific oligo probe indicated a massive loss of hsp70 mRNA signal in RNAs isolated f rom 24-hr postmortem brains; quantitation by slot-blot hybridization w as 5- to 15-fold more efficient. Even using the latter technique, hsp7 0 mRNA levels were reduced by 59% in 24-hr-postmortem cerebellum and b y 78% in cortex compared to mRNA levels in the same region of 0-hr-pos tmortem brain. There was little reduction postmortem in levels of the hsp70 mRNAs or of 18S rRNAs in either brain region. 4. In situ hybridi zation analysis indicated that hsp70 mRNAs were less abundant in all m ajor classes of cerebellar cells after 24 hr postmortem and mRNAs had degraded severalfold more rapidly in neurons than in glia. There was n o corresponding loss of intracellular 18S rRNA in any cell type. 5. We conclude from these results that the effect of postmortem interval on mRNA degradation must be carefully evaluated when analyzing levels of inducible hsp70 mRNAs, and perhaps other short-lived mRNAs, in human brain.