Involvement of nitric oxide released from microglia-macrophages in pathological changes of cathepsin D-deficient mice

Cathepsin D (CD) deficiency has been shown to induce ceroid-lipofuscin stor age in lysosomes of mouse CNS neuron (Koike et a[., 2000). To understand th e behavior of microglial cells corresponding to these neuronal changes, CD- deficient (CD-/-) mice, which die at approximately postnatal day (P) 25 by intestinal necrosis, were examined using morphological as well as biochemic al approaches. Light and electron microscopic observations revealed that mi croglia showing large round cell bodies with few processes appeared in the cerebral cortex and thalamus after P16. At P24, microglia often encircled n eurons that were occupied with autolysosomes, indicating increased phagocyt ic activity. These morphologically transformed microglia markedly expressed inducible nitric oxide synthase (iNOS), which was also detected in the int estine of the mice. To assess the role of microglial nitric oxide (NO) in n europathological changes in CD-/- mice, L-N-G-nitro-arginine methylester (L -NAME), a competitive NOS inhibitor, or S-methylisothiourea hemisulfate (SM T), an iNOS inhibitor, was administered intraperitoneally for 13 consecutiv e days. The total number of terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling-positive cells counted in the thalamus w as found to be significantly decreased by chronic treatment of L-NAME or SM T, whereas neither the neuronal accumulation of ceroid-lipofuscin nor the m icroglial phagocytic activity was affected by these treatments. Moreover, t he chronic treatment Of L-NAME or SMT completely suppressed hemorrhage-necr otic changes in the small intestine of CD-/- mice, resulting in normal grow th of the body weight of the mice. These results suggest that NO production via iNOS activity in microglia. and peripheral macrophages contributes to secondary tissue damages such as neuronal apoptosis and intestinal necrosis , respectively.