In the face of the newly emergent COVID-19 pandemic, researchers around the world are racing to identify efficacious drugs capable of preventing or treating its infection. They are doing that by testing already available and approved antimicrobials for their rapid repurposing against COVID-19. Using the data emerging on the comparable efficacy of various compounds having different mechanisms of action and indications, I suggest in this report, their potential mechanistic convergence. Specifically, I highlight the lysosome as a key possible therapeutic target for COVID-19, proposing one of the lysosomal storage disorders, Niemann-Pick type C disease (NPC), as a prototypical condition with inherent resistance or an “unfavorable” host cell environment for viral propagation. The included reasoning evolves from previously generated data in NPC, along with the emerging data on COVID-19. The aim of this report is to suggest that pharmacological induction of a “transient” NPC-like lysosomal dysfunction, could hold answers for targeting the ongoing COVID-19 pandemic.

Cysteine cathepsins, a class of proteinaceous enzymes, regulate a wide variety of metabolic processes in human including protein breakdown and turnover and immune functions. Eleven cysteine cathepsins have been identified so far and a wide array of studies related to identifying their specific functions, regulation and distribution patterns in tissues have been conducted. However, in recent past, the association of cysteine cathepsins in occurrence and progression of cancers have been identified and this has caused unrest in scientists triggering them to investigate the physiology, biochemical pathways and interactions of these cathepsins in cancer metastasis and therefore has become a noteworthy topic of intensive research. This review focusses and collects together the published work on molecular functional and structural characterization studies that have been done so far on in vitro expression of genes encoding for cysteine cathepsins in the Escherichia coli bacterial expression system. Accordingly, it was found out that all cathepsins except for cathepsins K, C, H, X and W have been expressed this way and the majority of them were found to be expressed in E. coli BL21(DE3) pLysS expression host via pET3 expression vector. In addition, it was also noted that in most of the expression studies, the substrate that was used to validate the enzymatic activity of the recombinant enzyme that was produced was a cysteine residue along with a benzyloxy-carbonyl salt. Through this review, the authors suggest that there is a very high need that all cysteine cathepsins need to be characterized both structurally and functionally on a molecular platform to better understand their interactions including the biochemical pathways. It is also momentous that the mass production of the recombinant forms of these enzymes are facilitated via expression in such bacterial expression systems and in turn, would also provide a strong platform for the development and progression of studies related to human physiology including oncological studies such as cancer metastasis. Moreover, as per biochemical features of the enzymes that could be identified, the production of efficient inhibitors or inducers as per the necessity to improve health and promote wellbeing among the mankind could be facilitated.

Papain-like lysosomal cysteine proteases include 11 human cysteine cathepsins which act as endopeptidases and/or exopeptidases. They are involved in numerous physiological and pathological processes. Among them, only cathepsins B, H, C, and X/Z exhibit exopeptidase activity. Their activities are tightly regulated in various ways to prevent potentially hazardous effects on the cell. In this review, we focus on the structural and functional aspects of these four cysteine cathepsins and their role in neurodegeneration and cancer. Neurodegenerative disorders of aging share an endolysosomal dysfunction and accumulation and spread of oligomeric forms of neurotoxic proteins. The accumulation of various protein aggregates activates the microglia, thus inducing the activation and release of cysteine cathepsins and proinflammatory cytokines, leading to neurodegeneration. In cancer, cysteine cathepsins participate in tumor progression and metastasis. Tumor–stromal crosstalk leads to activation of the stroma and overexpression and secretion of proteolytic enzymes, triggering extracellular matrix degradation and the release of soluble factors. Activated stromal cells (i.e., macrophages, fibroblasts, mast cells) secrete additional growth factors, cytokines, and chemokines that are responsible for the regulation of processes leading to tumor progression and metastasis. Therefore, a better understanding of the role of cysteine cathepsins in neurodegeneration and cancer could lead to novel targeted therapeutic approaches.