Demonstration of elevated levels of active cathepsin S in dextran sulfate sodium colitis using a new activatable probe

Cathepsins in the extracellular space: Focusing on non-lysosomal proteolytic functions with clinical implications

Cathepsins can be found in the extracellular space, cytoplasm, and nucleus. It was initially suspected that the primary physiological function of the cathepsins was to break down intracellular protein, and that they also had a role in pathological processes including inflammation and apoptosis. However, the many actions of cathepsins outside the cell and their complicated biological impacts have garnered much interest. Cathepsins play significant roles in a number of illnesses by regulating parenchymal cell proliferation, cell migration, viral invasion, inflammation, and immunological responses through extracellular matrix remodeling, signaling disruption, leukocyte recruitment, and cell adhesion. In this review, we outline the physiological roles of cathepsins in the extracellular space, the crucial pathological functions performed by cathepsins in illnesses, and the recent breakthroughs in the detection and therapy of specific inhibitors and fluorescent probes in associated dysfunction.

Mice expressing fluorescent PAR2 reveal that endocytosis mediates colonic inflammation and pain

G protein-coupled receptors (GPCRs) regulate many pathophysiological processes and are major therapeutic targets. The impact of disease on the subcellular distribution and function of GPCRs is poorly understood. We investigated trafficking and signaling of protease-activated receptor 2 (PAR₂) in colitis. To localize PAR₂ and assess redistribution during disease, we generated knockin mice expressing PAR₂ fused to monomeric ultrastable green fluorescent protein (muGFP). PAR₂-muGFP signaled and trafficked normally. PAR₂ messenger RNA was detected at similar levels in Par₂-mugfp and wild-type mice. Immunostaining with a GFP antibody and RNAScope in situ hybridization using F2rl1 (PAR₂) and Gfp probes revealed that PAR₂-muGFP was expressed in epithelial cells of the small and large intestine and in subsets of enteric and dorsal root ganglia neurons. In healthy mice, PAR₂-muGFP was prominently localized to the basolateral membrane of colonocytes. In mice with colitis, PAR₂-muGFP was depleted from the plasma membrane of colonocytes and redistributed to early endosomes, consistent with generation of proinflammatory proteases that activate PAR₂ PAR₂ agonists stimulated endocytosis of PAR₂ and recruitment of Gα_q, Gα_i, and β-arrestin to early endosomes of T84 colon carcinoma cells. PAR₂ agonists increased paracellular permeability of colonic epithelial cells, induced colonic inflammation and hyperalgesia in mice, and stimulated proinflammatory cytokine release from segments of human colon. Knockdown of dynamin-2 (Dnm2), the major colonocyte isoform, and Dnm inhibition attenuated PAR₂ endocytosis, signaling complex assembly and colonic inflammation and hyperalgesia. Thus, PAR₂ endocytosis sustains protease-evoked inflammation and nociception and PAR₂ in endosomes is a potential therapeutic target for colitis.

Toward Molecular Imaging of Intestinal Pathology

Inflammatory bowel disease (IBD) is defined by a chronic relapsing and remitting inflammation of the gastrointestinal tract, with intestinal fibrosis being a major complication. The etiology of IBD remains unknown, but it is thought to arise from a dysregulated and excessive immune response to gut luminal microbes triggered by genetic and environmental factors. To date, IBD has no cure, and treatments are currently directed at relieving symptoms and treating inflammation. The current diagnostic of IBD relies on endoscopy, which is invasive and does not provide information on the presence of extraluminal complications and molecular aspect of the disease. Cross-sectional imaging modalities such as computed tomography enterography (CTE), magnetic resonance enterography (MRE), positron emission tomography (PET), single photon emission computed tomography (SPECT), and hybrid modalities have demonstrated high accuracy for the diagnosis of IBD and can provide both functional and morphological information when combined with the use of molecular imaging probes. This review presents the state-of-the-art imaging techniques and molecular imaging approaches in the field of IBD and points out future directions that could help improve our understanding of IBD pathological processes, along with the development of efficient treatments.

Activatable fluorescence: From small molecule to nanoparticle

Molecular imaging has emerged as an indispensable technology in the development and application of drug delivery systems. Targeted imaging agents report the presence of biomolecules, including therapeutic targets and disease biomarkers, while the biological behaviour of labelled delivery systems can be non-invasively assessed in real time. As an imaging modality, fluorescence offers additional signal specificity and dynamic information due to the inherent responsivity of fluorescence agents to interactions with other optical species and with their environment. Harnessing this responsivity is the basis of activatable fluorescence imaging, where interactions between an engineered fluorescence agent and its biological target induce a fluorogenic response. Small molecule activatable agents are frequently derivatives of common fluorophores designed to chemically react with their target. Macromolecular scale agents are useful for imaging proteins and nucleic acids, although their biological delivery can be difficult. Nanoscale activatable agents combine the responsivity of fluorophores with the unique optical and physical properties of nanomaterials. The molecular imaging application and overall complexity of biological target dictate the most advantageous fluorescence agent size scale and activation strategy.

Pathophysiological roles of proteases in gastrointestinal disease

Gastrointestinal diseases, such as irritable bowel syndrome, inflammatory bowel disease, and colorectal cancer, affect a large proportion of the population and are associated with many unpleasant symptoms. Although the causes of these diseases remain largely unknown, there is increasing evidence to suggest that dysregulated protease activity may be a contributing factor. Proteases are enzymes that cleave other proteins, and their activity is normally very tightly regulated. During disease, however, the balance between proteases and their inhibitors is often shifted, leading to altered spatial and temporal control of substrate cleavage. Evaluating protease levels in normal physiology and disease has relied heavily on the use of chemical tools. Although these tools have greatly advanced the field, they are not without caveats. This review provides an introduction to these tools, their application in the gut, and a summary of the current knowledge on the contribution of protease activity to gastrointestinal disease.