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.

Autoimmune diseases, immune-mediated inflammatory diseases, and cancer are distinct pathological conditions that can intersect through immune dysregulation, chronic inflammation, impaired host defense, and altered host–microbe interactions. In selected settings, autoimmune and immune-mediated inflammatory diseases are associated with increased gastrointestinal cancer risk. Although bacterial and viral contributions to inflammation-associated cancer have been extensively studied, the role of fungi remains less investigated. This Opinion article discusses host–fungus interactions through two experimental mouse models that connect altered immune tolerance, innate antifungal responses, and gastrointestinal tumorigenesis. Kinase-dead IKKα K44A knock-in mice develop defective central immune tolerance, APECED-like autoimmune manifestations, mucosal fungal infection, epithelial EGFR signaling, and esophageal squamous cell carcinoma. By contrast, Card9^−/− mice provide a distinct model in which impaired innate antifungal immunity alters intestinal fungal ecology and myeloid-cell responses in colitis-associated colorectal cancer. In the CARD9 setting, fungi may exert pathogenic or protective effects depending on host genotype, immune context, fungal species, and mycobiome structure. Together, these models provide useful tools for investigating how fungi may contribute to gastrointestinal tumorigenesis through cancer-promoting inflammation and immune suppression in a context-dependent manner. However, these findings do not yet validate clinical strategies and should not be interpreted as broadly representative of all autoimmune or immune-mediated inflammatory diseases. Further mechanistic and human studies are needed.