###### Abstract  
The association of cathepsin proteases in neurobiology is increasingly recognized. Our previous studies indicated that cathepsin-K-deficient (_Ctsk\({}^{-/-}\)_) mice have learning and memory impairments. Alterations in cathepsin expression are known to result in compensatory changes in levels of related cathepsins. To gain insight into the therapeutic usefulness of cathepsin inhibitors in aging individuals with osteoporosis or neurodegenerative diseases, we studied for variations in cathepsin expression and activity in aged (18-20 months) versus young (5-7 months) wild-type (WT) and cathepsin-deficient mice brains. There were age-dependent increases in cathepsin B, D, and L and cystatin C protein levels in various brain regions, mainly of WT and _Ctsk\({}^{-/-}\)_ mice. This corresponded with changes in activity levels of cathepsins B and L, but not cathepsin D. In contrast, very little age-dependent variation was observed in cathepsin-B- and cathepsin-L-deficient mouse brain, especially at the protein level. The observed alterations in cathepsin protein amounts and activity are likely contributing to changes in important aging-related processes such as autophagy. In addition, the results provide insight into the potential impact of cathepsin inhibitor therapy in aged individuals, as well as in long-term use of cathepsin inhibitor therapy. Keywords:aging disorders; aspartic proteases; brain; cystatin C; cysteine peptides; protease inhibitor therapy; lysosomes +
Footnote †: journal:  
It is accepted that endolysosomes, and the hydrolases they contain, change during aging, also in the central nervous system (CNS), although the outcomes of such changes are not yet fully understood. After the first biochemical and preclinical studies, a major expansion of research occurred in the last 20 years, which was facilitated by the development of cathepsin-deficient and/or other murine disease models and by using new approaches [2; 9; 10; 11; 12; 13; 14; 15]. This research field led to a major advancement in our understanding of the molecular mechanisms underlying physiological and degenerative processes entailing cathepsin functions in the brain, beyond other organs. Cathepsins B, D, K, and L are essential for the central nervous system (CNS). While cathepsin-B- (\(Ctsb^{-/-}\)) and cathepsin-L-deficient (\(Ctsl^{-/-}\)) mice are viable, double cathepsin B/L-deficient mouse pups die between 2 and 4 weeks due to brain atrophy and massive apoptosis of brain cells [16], suggesting that cathepsin B and L have overlapping neurological roles. Cathepsin D knockout mice are also lethal due to a severe neurological phenotype [17]. In contrast, cathepsin-K-deficient (\(Ctsk^{-/-}\)) mice show milder afflictions, in that impairments of memory formation and neurobehavior were observed in comparison with wild-type (WT) controls [18]. At the molecular level, there are alterations in expression of important molecules such as synaptotagmin, glial fibrillary acidic protein (GFAP), cyclic nucleotide phosphodiesterase, and tyrosine hydroxylase in the \(Ctsk^{-/-}\) murine model [18]. Hence, the exact role of cathepsin in the CNS is yet to be fully elucidated. For instance, while cathepsins have been reported to play a role in increasing neurodegenerative protein aggregate formation [9; 19; 20], there is also mounting evidence that cathepsins can have neuroprotective functions [21; 22; 23], probably through aggregate-clearance mechanisms as part of their involvement in the autophagy-lysosomal system [23; 24]. Cathepsin B, for example, is associated with neuroinflammation-mary response and oxidative stress in inflammatory brain diseases and brain aging, where it is found to be upregulated particularly in microglia [11]. Furthermore, cathepsin D plays an important role in neuronal cell homeostasis because of its proteolytic action towards target proteins such as \(\alpha\)-synuclein, apolipoprotein E, huntingin, lipofuscin, or tau, namely proteins involved in neurological disorders, including [PERSON]'s and [PERSON]'s disease [3; 25].