In situ hybridization and immunohistochemical localization of heme oxygenase-2 mRNA and protein in normal rat brain: Differential distribution of isozyme 1 and 2.

Heme oxygenase isozymes, HO-1 (HSP32) and HO-2, stereospecifically bind and degrade the potent prooxidant, the heme molecule, and convert it to the effective antioxidant, biliverdin, and the potential cellular messenger, carbon monoxide. In the present study we have examined the pattern of expression of the two HO-2 transcripts and protein in normal rat brain by in situ hybridization and immunochemical analysis, respectively. We have found by Northern blot analysis that HO-2 isozyme is by far the most prevalent form in the brain. Analysis of HO-2 1.3- and 1.9-kb mRNAs by in situ hybridization histochemistry showed that these transcripts are abundantly expressed in many neuronal and nonneuronal cell populations in forebrain, diencephalon, cerebellum, and brain stem regions. Furthermore, the pattern of expression of HO-2 transcripts, as detected by oligonucleotide probes, is in good agreement with that of immunoreactive protein detected by immunohistochemical analysis. Impressive levels of HO-2 transcripts and immunoreactive protein were observed in Purkinje cells of cerebellum, red nucleus, superior and inferior colliculus, nucleus of the trapezoid body, cochlear neurons, and facial nucleus of brain stem. Furthermore, in certain select brain cell populations the pattern of expression of HO-1- and HO-2-immunoreactive proteins overlapped. We suggest that the high levels of heme degradation activity and the localization of HO-2 transcripts and protein in the brain may reflect the functions of this enzyme in processes such as production of cellular messenger, regulation of the activity of heme-dependent enzymes catalyzing intracellular signaling molecule synthesis, and production of antioxidants.