Nuclear Localization of Cystatin B, the Cathepsin Inhibitor Implicated in Myoclonus Epilepsy (EPM1)

• Published in: Experimental cell research Vol. 262; no. 2; pp. 84 - 94
• Main Authors: Riccio, Massimo, Di Giaimo, Rossella, Pianetti, Simona, Palmieri, Pier Paolo, Melli, Marialuisa, Santi, Spartaco
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15-01-2001
• Subjects: Animals; Cathepsin B - antagonists & inhibitors; Cell Compartmentation; Cell Differentiation - drug effects; Cell Division - drug effects; Cell Nucleus - metabolism; Cells, Cultured; Cerebellum - cytology; Cerebellum - metabolism; CLSM; Cystatin B; Cystatins - metabolism; cysteine proteases; Cytoplasm - metabolism; Cytoskeleton - metabolism; Cytoskeleton - ultrastructure; differentiation; Epilepsies, Myoclonic - etiology; Epilepsies, Myoclonic - metabolism; EPM1; Fluorescent Antibody Technique; hereditary neurodegenerative disease; Humans; Immunohistochemistry; Muscle, Skeletal - cytology; Muscle, Skeletal - metabolism; Nerve Growth Factor - pharmacology; Neuroblastoma - metabolism; Neurons - cytology; Neurons - metabolism; nuclear matrix; Nuclear Matrix - metabolism; Osteosarcoma - metabolism; protein interaction; Rats; Rats, Sprague-Dawley; cystatin B; protein interaction; differentiation; nuclear matrix; hereditary neurodegenerative disease; cysteine proteases; CLSM; EPM1
• ISSN: 0014-4827; 1090-2422
• DOI: 10.1006/excr.2000.5085

Cystatin B is an anti-protease implicated in myoclonus epilepsy, a degenerative disease of the central nervous system. In vitro, cystatin B interacts with and inhibits proteases of the cathepsin family. Confocal microscopy analysis of the subcellular localization of cystatin B and cathepsin B shows that, in vivo, the two proteins are concentrated in different cell compartments. In fact, cystatin B is found mainly in the nucleus of proliferating cells and both in the nucleus and in the cytoplasm of differentiated cells, while cathepsin B, in either case, is essentially cytoplasmic. However, colocalization of cystatin and cathepsin B is observed in the isolated cell matrix and in the nuclear scaffold of differentiated neuroblastoma cells but not of proliferating cells. This suggests that at least a fraction of cystatin B is bound to the protease in differentiated cells. The electron microscopy analysis of the cell matrix confirms the observation made with confocal microscopy. The cellular activity of cathepsin B was analyzed with a fluorogenic cytochemical assay. A fluorescent signal is observed in the cytoplasm of proliferating cells but is undetectable in the cytoplasm of differentiated cells, suggesting that cathepsin B is active mainly during the cell cycle. This result is consistent with the separate compartimentalization of cystatin B and cathepsin B that we have observed in growing cells.