Neutral pH-Selective Inhibition of Cytosolic Cathepsin B: A Novel Drug Targeting Strategy for Traumatic Brain Injury and Alzheimer’s Disease

Cathepsin B contributes to the behavioral deficits and neuropathology that occur in traumatic brain injury (TBI) and Alzheimer's disease (AD). TBI and AD patients display elevated levels of cathepsin B that correlate with the severity of injury or cognitive deficits, respectively. In animal models of TBI and AD, cathepsin B gene knockout ameliorates behavioral deficits and improves neuropathology. While cathepsin B is normally located in acidic lysosomes, during TBI and AD, lysosomal leakage results in the translocation of cathepsin B to the neutral pH environment of the cytosol, thereby initiating neurodegeneration. Neutral pH-selective inhibitors are hypothesized to specifically target the pathogenic cytosolic cathepsin B without affecting its normal lysosomal form. Therefore, this review focuses on a novel strategy to utilize pH-dependent substrate cleavage properties of cathepsin B for the design of a neutral pH-selective inhibitor. Investigation of the enzymatic properties of cathepsin B at different pH conditions led to the development of Z-Arg-Lys-AOMK, a neutral pH-selective inhibitor that does not affect the enzyme's activity at normal lysosomal acidic pH. Z-Arg-Lys-AOMK potently inhibits cathepsin B at nM concentrations and effectively inhibits cellular cathepsin B in neuronal cell cultures at similar levels. In mice subjected to controlled cortical impact (CCI) brain injury, a model of TBI, cytosolic cathepsin B activity was significantly elevated in the brain. Treatment of the CCI-TBI mice with Z-Arg-Lys-AOMK reduced cytosolic cathepsin B activity and resulted in less motor dysfunction. These findings show that pH-dependent cleavage properties of cathepsin B can be utilized for the development of selective inhibitors to target the neutral cytosolic form of cathepsin B. The new concept of pH-selective inhibitors of cathepsin B reveals novel opportunities for targeting pathogenic, cytosolic cathepsin B involved in brain disorders.