Classical late-infantile neuronal ceroid lipofuscinosis (LINCL) is a fatal neurodegenerative disease whose defective gene has remained elusive. A molecular basis for LINCL was determined with an approach applicable to other lysosomal storage diseases. When the mannose 6-phosphate modification of newly synthesized lysosomal enzymes was used as an affinity marker, a single protein was identified that is absent in LINCL. Sequence comparisons suggest that this protein is a pepstatin-insensitive lysosomal peptidase, and a corresponding enzymatic activity was deficient in LINCL autopsy specimens. Mutations in the gene encoding this protein were identified in LINCL patients but not in normal controls.

Microglia are the main immune cells of the brain, and under some circumstances they can play an important role in removal of fibrillar Alzheimer amyloid ␤ peptide (fA␤). Primary mouse microglia can internalize fA␤, but they do not degrade it efficiently. We compared the level of lysosomal proteases in microglia and J774 macrophages, which can degrade fA␤ efficiently, and we found that microglia actually contain higher levels of many lysosomal proteases than macrophages. However, the microglial lysosomes are less acidic (average pH of ϳ6), reducing the activity of lysosomal enzymes in the cells. Proinflammatory treatments with macrophage colony-stimulating factor (MCSF) or interleukin-6 acidify the lysosomes of microglia and enable them to degrade fA␤. After treatment with MCSF, the pH of microglial lysosomes is similar to J774 macrophages (pH of ϳ5), and the MCSF-induced acidification can be partially reversed upon treatment with an inhibitor of protein kinase A or with an anion transport inhibitor. Microglia also degrade fA␤ if lysosomes are acidified by an ammonia pulse-wash or by treatment with forskolin, which activates protein kinase A. Our results indicate that regulated lysosomal acidification can potentiate fA␤ degradation by microglia.

The neuronal ceroid lipofuscinoses (NCLs) constitute a group of neurodegenerative storage diseases characterized by progressive psychomotor retardation, blindness and premature death. Pathologically, there is accumulation of autofluorescent material in lysosome-derived organelles in a variety of cell types, but neurons in the central nervous system appear to be selectively affected and undergo progressive death. In this report we show that a novel form of NCL, congenital ovine NCL, is caused by a deficiency in the lysosomal aspartyl proteinase cathepsin D. A single nucleotide mutation in the cathepsin D gene results in conversion of an active site aspartate to asparagine, leading to production of an enzymatically inactive but stable protein. This results in severe cerebrocortical atrophy and early death, providing strong evidence for an important role of cathepsin D in neuronal development and/or homeostasis.

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