A novel photoaffinity-based probe for selective detection of cathepsin L active form

Molecular probes for selective detection of cysteine cathepsins

Cysteine cathepsins are proteases critical in physiopathological processes and show potential as targets or biomarkers for diseases and medical conditions. The 11 members of the cathepsin family are redundant in some cases but remarkably independent of others, demanding the development of both pan-cathepsin targeting tools as well as probes that are selective for specific cathepsins with little off-target activity. This review addresses the diverse design strategies that have been employed to accomplish this tailored selectivity among cysteine cathepsin targets and the imaging modalities incorporated. The power of these diverse tools is contextualized by briefly highlighting the nature of a few prominent cysteine cathepsins, their involvement in select diseases, and the application of cathepsin imaging probes in research spanning basic biochemical studies to clinical applications.

Dual-Mechanism Quenched Fluorogenic Probe Provides Selective and Rapid Detection of Cathepsin L Activity*

Cathepsin L (CTL) is a cysteine protease demonstrating upregulated activity in many disease states. Overlapping substrate specificity makes selective detection of CTL activity difficult to parse from that of its close homologue CTV and the ubiquitous CTB. Current probes of CTL activity have limited applications due to either poor contrast or extra assay steps required to achieve selectivity. We have developed a fluorogenic probe, CTLAP, that displays good selectivity for CTL over CTB and CTV while exhibiting low background fluorescence attributed to dual quenching mechanisms. CTLAP achieves optimum CTL selectivity in the first 10 min of incubation, thus suggesting that it is amenable for rapid detection of CTL, even in the presence of competing cathepsins.

A Review of Small Molecule Inhibitors and Functional Probes of Human Cathepsin L

Human cathepsin L belongs to the cathepsin family of proteolytic enzymes with primarily an endopeptidase activity. Although its primary functions were originally thought to be only of a housekeeping enzyme that degraded intracellular and endocytosed proteins in lysosome, numerous recent studies suggest that it plays many critical and specific roles in diverse cellular settings. Not surprisingly, the dysregulated function of cathepsin L has manifested itself in several human diseases, making it an attractive target for drug development. Unfortunately, several redundant and isoform-specific functions have recently emerged, adding complexities to the drug discovery process. To address this, a series of chemical biology tools have been developed that helped define cathepsin L biology with exquisite precision in specific cellular contexts. This review elaborates on the recently developed small molecule inhibitors and probes of human cathepsin L, outlining their mechanisms of action, and describing their potential utilities in dissecting unknown function.

Imaging of extracellular cathepsin S activity by a selective near infrared fluorescence substrate-based probe

We designed a near-infrared fluorescent substrate-based probe (SBP), termed MG101, for monitoring extracellular cathepsin S (CatS) activity. We conceived a fused peptide hairpin loop-structure, combining a CatS recognition domain, an electrostatic zipper (with complementary charges of a polyanionic (D-Glu)₅ segment and a polycationic (D-Arg)₅ motif, as well as a N and C terminal Förster resonance energy transfer pair (donor: AlexaFluor680; quencher: BHQ3) to facilitate activity-dependent imaging. MG101 showed excellent stability since no fluorescence release corresponding to a self-dequenching was observed in the presence of either 2 M NaCl or after incubation at a broad range of pH (2.2-8.2). Cathepsins B, D, G, H, and K, neutrophil elastase and proteinase 3 did not cleave MG101, while CatS, and to a lesser extent CatL, hydrolysed MG101 at pH 5.5. However MG101 was fully selective for CatS at pH 7.4 (k_cat/K_m = 140,000 M^-1 s^-1) and sensitive to low concentration of CatS (<1 nM). The selectivity of MG101 was successfully endorsed ex vivo, as it was hydrolysed in cell lysates derived from wild-type but not knockout CatS murine spleen. Furthermore, application of the SBP probe with confocal microscopy confirmed the secretion of active CatS from THP-1 macrophages, which could be abrogated by pharmacological CatS inhibitors. Taken together, present data highlight MG101 as a novel near-infrared fluorescent SBP for the visualization of extracellular active CatS from macrophages and other cell types.

Cell penetrable, clickable and tagless activity-based probe of human cathepsin L

Human cathepsin L is a ubiquitously expressed endopeptidase and is known to play critical roles in a wide variety of cellular signaling events. Its overexpression has been implicated in numerous human diseases, including highly invasive forms of cancer. Inhibition of cathepsin L is therefore considered a viable therapeutic strategy. Unfortunately, several redundant and even opposing roles of cathepsin L have recently emerged. Selective cathepsin L probes are therefore needed to dissect its function in context-specific manner before significant resources are directed into drug discovery efforts. Herein, the development of a clickable and tagless activity-based probe of cathepsin L is reported. The probe is highly efficient, active-site directed and activity-dependent, selective, cell penetrable, and non-toxic to human cells. Using zebrafish model, we demonstrate that the probe can inhibit cathepsin L function in vivo during the hatching process. It is anticipated that the probe will be a highly effective tool in dissecting cathepsin L biology at the proteome levels in both normal physiology and human diseases, thereby facilitating drug-discovery efforts targeting cathepsin L.

The Life of Pi Star: Exploring the Exciting and Forbidden Worlds of the Benzophenone Photophore

The widespread applications of benzophenone (BP) photochemistry in biological chemistry, bioorganic chemistry, and material science have been prominent in both academic and industrial research. BP photophores have unique photochemical properties: upon n-π* excitation at 365 nm, a biradicaloid triplet state is formed reversibly, which can abstract a hydrogen atom from accessible C-H bonds; the radicals subsequently recombine, creating a stable covalent C-C bond. This light-directed covalent attachment process is exploited in many different ways: (i) binding/contact site mapping of ligand (or protein)-protein interactions; (ii) identification of molecular targets and interactome mapping; (iii) proteome profiling; (iv) bioconjugation and site-directed modification of biopolymers; (v) surface grafting and immobilization. BP photochemistry also has many practical advantages, including low reactivity toward water, stability in ambient light, and the convenient excitation at 365 nm. In addition, several BP-containing building blocks and reagents are commercially available. In this review, we explore the "forbidden" (transitions) and excitation-activated world of photoinduced covalent attachment of BP photophores by touring a colorful palette of recent examples. In this exploration, we will see the pros and cons of using BP photophores, and we hope that both novice and expert photolabelers will enjoy and be inspired by the breadth and depth of possibilities.

Ultrasensitive internally quenched substrates of human cathepsin L

Internally quenched cathepsin L (Cat L) substrate ABZ-Bip-Arg-Ala-Gln-Tyr(3-NO2)-NH2 with high specificity constant (kcat/KM=2.6×10(7)M(-1)s(-1)) was synthesized. The resultant compound displayed high selectivity over other members of the cathepsin family (B, S, X, V, C, K, H, F, D, and A). Activity of Cat L at picomolar (pM) concentrations was found using this substrate. Moreover, it was established that the presence of the selective Cat L inhibitor suppressed the proteolysis of the substrate to a non-detectable level. Incubation of the synthesized compound with a cell lysate of healthy and cancer cell lines indicated significant differences in Cat L activity. Based on the obtained results, it is proposed that this substrate could be used for selective monitoring of Cat L activity in biological systems.

FRET-based and other fluorescent proteinase probes

The continuous detection of enzyme activities and their application in medical diagnostics is one of the challenges in the translational sciences. Proteinases represent one of the largest groups of enzymes in the human genome and many diseases are based on malfunctions of proteolytic activity. Fluorescent sensors may shed light on regular and irregular proteinase activity in vitro and in vivo and provide a deeper insight into the function of these enzymes and their role in pathophysiological processes. The focus of this review is on Förster resonance energy transfer (FRET)-based proteinase sensors and reporters because these probes are most likely to provide quantitative data. The medical relevance of proteinases are discussed using lung diseases as a prominent example. Probe design and probe targeting are described and fluorescent probe development for disease-relevant proteinases, including matrix-metalloproteinases, cathepsins, caspases, and other selected proteinases, is reviewed.