MATRIX METALLOPROTEINASES IN DOG BRAINS EXHIBITING ALZHEIMER-LIKE CHARACTERISTICS

We have previously reported that the amount of the neuronal matrix met alloproteinase (MMP) MMP-9, capable of cleaving beta-amyloid(1-40) pre dominantly at Leu(34)-Met(35), is increased in a latent form in hippoc ampal specimens from AD patients and have suggested that the lack of a ctivation of this enzyme may contribute to the deposition of beta-amyl oid in plaques. The current study addresses whether similar matrix pro teinases are detectable in amyloid-positive and -negative brain specim ens of aged beagles. Using quantitative zymography, three major neutra l proteinases with molecular masses of 60, 95, and 280 kDa were readil y detected. These enzymes have the characteristics of MMPs because the y were inhibited by EDTA and 1,10-phenanthroline, and their activities were restored by addition of both Ca2+ and Zn2+. The 95- and 280-kDa proteinases cross-reacted with specific monoclonal antibodies to human MMP-9 (gelatinase B; EC 3.4.24.35). Canine MMP-9 was latent because a ctivation by organomercurial treatment resulted in a characteristic de crease in molecular mass. Statistical analysis revealed no difference in the 60-kDa proteinase activity in amyloid-positive and -negative br ain specimens. However, significantly increased amounts of latent MMP- 9 were observed in amyloid-positive brain specimens (p less than or eq ual to 0.05) compared with amyloid-negative brain specimens. The obser vations document that changes in MMP-9 expression in amyloid-positive beagle brains are similar to those reported in the human Alzheimer's d isease hippocampus and suggest the possibility that insufficient activ ation of MMP-9 may contribute to beta-amyloid accumulation, a hypothes is that needs to be further investigated.