CONSTITUTIVE EXPRESSION OF METALLOTHIONEIN GENES IN MOUSE-BRAIN

Metallothioneins (MTs) are ubiquitous low-molecular-weight proteins th at are induced by a variety of inducers, including metals, lipopolysac charides (LPS), cytokines, oxidative stress, etc., and are thought to play a protective role against various toxic insults. The constitutive level of metallothionein is an important determinant of a tissue's su sceptibility to toxic insults. In the present study, we report the con stitutive expression of MT mRNAs in adult mouse brain. Analysis of tot al RNA from whole brain by Northern blot and solution hybridization sh owed that mRNAs for all three MT isoforms (I, II, III) were constituti vely expressed in mouse brain, and there was no remarkable difference in their expression. However, in quantitative terms the order of expre ssion was MT-I > MT-III > MT-II. The expression of MT-III and MT-II wa s about 70 and 50% of that of MT-I, respectively. Examination of their constitutive expression in different brain regions revealed that the three isoforms were expressed in all seven brain regions studied (olfa ctory bulb, cortex, caudate, hippocampus, thalamus, cerebellum, and br ain stem), and there was only about a twofold difference in MT mRNA ex pression from one region of the brain to another. However, olfactory b ulb had the highest mRNA expression for all three isoforms, as reveale d by slot blot analysis. Constitutive expression of MT-I and -II mRNA, but not MT-III mRNA, was high in cerebellum. In order to study the ce llular localization of MT mRNA, in situ hybridization of MT-I and MT-I II mRNA was performed. For comparison, LPS was used to enhance MT-I mR NA signal because LPS is a good inducer of MT-I mRNA expression in mou se brain. In situ hybridization revealed that certain brain regions ha d distinctly localized high levels of expression of MT mRNAs. In brain s of untreated mice, the constitutive expression of MT-I mRNA was high in the ependymal cell layer lining the lateral ventricles and in the Purkinje cell layer of cerebellum. The signal in the Purkinje cell lay er was not on the Purkinje cells themselves, but was on locations cons istent with that of glial cells. In LPS-treated mice, the signal in ce rebellum was higher, and distinct signal appeared in the choroid plexu s. However, signal in ependyma was similar to that in untreated mice. Pia mater in LPS-treated, but not in untreated, mouse brain showed enh anced signal for MT-I mRNA. In contrast to MT-I, distinct localized si gnal for MT-III mRNA was obtained in the granular cell layer of dentat e gyrus of the hippocampus and not in the cerebellum, even though cere bellum had low amounts of MT-III mRNA as revealed by slot blot analysi s. Thus, the nonuniform and unique distribution of MT-I, -II, and -III mRNAs in brain regions and their cellular elements suggest specific, but yet unknown, functions for MT isoforms in each region of the brain . (C) 1995 Academic Press, Inc.