The 70‐kDa Heat Shock Cognate Protein (HSC70) Is a Major Constituent of the Central Nervous System and Is Up‐Regulated Only at the mRNA Level in Acute Experimental Autoimmune Encephalomyelitis

Abstract

The expression of the 70-kDa heat shock cognate (HSC70) and stress-inducible (HSP70) proteins, and their mRNAs, was examined in experimental autoimmune encephalomyelitis, a model of inflammatory demyelination in the CNS. This study was undertaken as an extension of previous work demonstrating an abrupt decline in mRNA levels of both glial fibrillary acidic protein and the low-molecular-weight neurofilament subunit in experimental autoimmune encephalomyelitis spinal cord at 12 days after inoculation, the height of inflammation and clinical signs. Using the same total RNA preparations as our previous study, we report here that mRNA levels for HSC70 increased approximately sixfold over control values at the same time that glial fibrillary acidic protein and low-molecular-weight neurofilament subunit messages decreased and were similar to controls by 21 days after inoculation. In situ hybridization experiments showed that HSC70 mRNA was predominantly expressed in neurons and that the influx of inflammatory cells into the CNS was not responsible for the large increase in HSC70 message. Despite this elevation in mRNA, only small (if any) increases in protein levels for HSC70 were detected by both western blotting and in vitro cell-free translation systems. However, by quantitative immunoblotting, we determined that constitutive levels of HSC70 comprised a substantial portion of CNS proteins, representing 2-3% of the total protein content of spinal cord. Immunohistochemical staining illustrated that the distribution of HSC70 was consistent with that of its message. In contrast, no HSP70 mRNA or protein was detected in either control or experimental animals.