Cathepsin B Turning Bioluminescence "On" For Tumor Imaging

Cathepsin B(CTSB)is a lysosomal cysteine protease associated with protein turnover and degradation in lysosomes.It plays important roles in a lot of biological events such as initiation,growth,angiogenesis,invasion,and metastasis.Several human cancers such as esophageal,gastric,prostate,pancreas,colon,glioblastoma,and breast cancers are reported to highly express CTSB.Therefore,as an attractive biomarker,CTSB can be used for cancer diagnosis.To date,many optical methods have been developed for CTSB detection(or imaging)including fluorescence,colorimetry,and chemiluminescence.Among them,fluorescence is commonly used due to its operational easiness and real-time feedback.However,autofluorescence from the detecting samples,as well as the photobleaching of the fluorophores,interferes with the detection accuracy of fluorescence.Bio luminescence(BL)originates from a natural phenomenon in some living invertebrates.In which,a visible light is emitted after an enzymatic reaction.In contrast with fluorescence,light excitation is not needed for BL.Therefore,BL has a much lower background but a higher signal-to-noise ratio.Additionally,BL probes are usually derivatives of their biological precursors and thus less toxic than fluorescence probes.Recently,the luciferase-luciferin BL system has been employed to detect various analytes such as glucose,hypochlorite,cysteine,and proteases with high sensitivity.As an effective optical detection method,bio luminescence has a wide application prospect.Inspired by the above studies,in this work,we designed a BL probe Val-Cit-AL for CTSB sensing in vitro,in living cells,and in tumors.When the amino group of the AL structure is caged by the carboxyl group of citrulline(Cit),Val-Cit-AL is BL inactive.CTSB cleavage of the Val-Cit substrate from Val-Cit-AL yields AL which generates BL upon the catalysis of firefly luciferase(fLuc).We anticipate that our probe Val-Cit-AL might be applied to diagnose CTSB-related diseases in rodent models and evaluate the roles of CTSB in more biological processes in the near future.