A Universal Assay for Aminopeptidase Activity and Its Application for Dipeptidyl Peptidase-4 Drug Discovery

Portable bioluminescent platform for in vivo monitoring of biological processes in non-transgenic animals

Bioluminescent imaging (BLI) is one of the most powerful and widely used preclinical imaging modalities. However, the current technology relies on the use of transgenic luciferase-expressing cells and animals and therefore can only be applied to a limited number of existing animal models of human disease. Here, we report the development of a “portable bioluminescent” (PBL) technology that overcomes most of the major limitations of traditional BLI. We demonstrate that the PBL method is capable of noninvasive measuring the activity of both extracellular (e.g., dipeptidyl peptidase 4) and intracellular (e.g., cytochrome P450) enzymes in vivo in non-luciferase-expressing mice. Moreover, we successfully utilize PBL technology in dogs and human cadaver, paving the way for the translation of functional BLI to the noninvasive quantification of biological processes in large animals. The PBL methodology can be easily adapted for the noninvasive monitoring of a plethora of diseases across multiple species.

Bioluminescence imaging tends to rely on transgenic luciferase-expressing cells and animals. Here the authors report a portable bioluminescent system to non-invasively measure intra- and extracellular enzymes in vivo in non-transgenic animals which do not express luciferase.

General Strategy for Bioluminescence Sensing of Peptidase Activity In Vivo Based on Tumor-Targeting Probiotic

The abnormally expressed peptidases in human tissues are associated with many kinds of cancers. Monitoring of endogenous peptidase activity could allow us for pathophysiology elucidation and early clinical diagnosis. Herein, we developed a general strategy for bioluminescence (BL) sensing of peptidase activity in vivo based on tumor-targeting probiotics. The probiotic that harbored a luciferase-encoding plasmid was used to target and colonize tumor and provide luciferase for BL imaging. The peptide-based probes Lc and GPc were applied to track leucine aminopeptidase (LAP) and dipeptidyl peptidase IV (DPPIV) activity, respectively, by simply adding l-leucine and Gly-Pro dipeptides at the N-terminus of d-cysteine, which were specifically controlled by peptidase cleavage and released free d-cysteine to conduct a subsequent click condensation reaction with 2-cyano-6-hydroxybenzothiazole (HCBT) to produce firefly luciferin in situ, giving rise to a strong BL signal. Neither gene modification of cells of interest nor complicated synthesis was required in this BL system. Encouraged by these advantages, we successfully used our probes to monitor LAP and DPPIV activities in vitro and in vivo, respectively. A good linearity between BL and peptidase was obtained in the concentration range of 2.5-40.0 mU/mL with a limit of detection (LOD) of 1.1 mU/mL (55 ng/mL) for LAP and 2.0-40.0 mU/mL with a LOD of 0.78 mU/mL (1.15 ng/mL) for DPPIV, respectively. Additionally, approximately 5-fold (LAP) and 10-fold (DPPIV) differences in the BL signal before and after treatment with a specific inhibitor were also obtained for in vivo BL imaging. All these results reflected the potential application value of our probes in BL sensing of peptidase activity. We envision that our strategy may be a useful approach for monitoring a wide range of peptidases in tumors, especially in primary tumors.

Real-Time Imaging and Quantification of Peptide Uptake in Vitro and in Vivo

Peptides constitute an important class of drugs for the treatment of multiple metabolic, oncological, and neurodegenerative diseases, and several hundred novel therapeutic peptides are currently in the preclinical and clinical stages of development. However, many leads fail to advance clinically because of poor cellular membrane and tissue permeability. Therefore, assessment of the ability of a peptide to cross cellular membranes is critical when developing novel peptide-based therapeutics. Current methods to assess peptide cellular permeability are limited by multiple factors, such as the need to introduce rather large modifications (e.g., fluorescent dyes) that require complex chemical reactions as well as an inability to provide kinetic information on the internalization of a compound or distinguish between internalized and membrane-bound compounds. In addition, many of these methods are based on end point assays and require multiple sample manipulation steps. Herein, we report a novel "Split Luciferin Peptide" (SLP) assay that enables the real-time noninvasive imaging and quantification of peptide uptake both in vitro and in vivo using a very sensitive bioluminescence readout. This method is based on a straightforward, stable chemical modification of the peptide of interest with a d-cysteine tag that preserves the overall peptidic character of the original molecule. This method can be easily adapted for screening peptide libraries and can thus become an important tool for preclinical peptide drug development.