Development of a synthetic biosensor for chemical exchange MRI utilizing in silico optimized peptides

Abstract

Chemical exchange saturation transfer (CEST) MRI has been identified as a novel alternative to classical diagnostic imaging. Over the last several decades, many studies have been conducted to determine possible CEST agents, such as endogenously expressed compounds or proteins, that can be utilized to produce contrast with minimally invasive procedures and reduced or non‐existent levels of toxicity. In recent years there has been an increased interest in the generation of genetically engineered CEST contrast agents, typically based on existing proteins with CEST contrast or modified to produce CEST contrast. We have developed an in silico method for the evolution of peptide sequences to optimize CEST contrast and showed that these peptides could be combined to create de novo biosensors for CEST MRI. A single protein, superCESTide, was designed to be 198 amino acids. SuperCESTide was expressed in E. coli and purified with size exclusion chromatography. The magnetic transfer ratio asymmetry generated by superCESTide was comparable to levels seen in previous CEST reporters, such as protamine sulfate (salmon protamine) and human protamine. These data show that novel peptides with sequences optimized in silico for CEST contrast that utilize a more comprehensive range of amino acids can still produce contrast when assembled into protein units expressed in complex living environments.

The Proton Resonance Enhancement for CEST imaging and Shift Exchange (PRECISE) family of RF pulse shapes for Chemical Exchange Saturation Transfer MRI

“…Chemical exchange saturation transfer MRI has matured as a contrast mechanism that enables the detection of low-concentration contrast agents by applying saturation pulses on resonance with labile protons and measuring the resulting water signal loss based on the chemical exchange transfer of this saturation [1][2][3][4] . Several studies have established the utility of CEST MRI, including measuring mobile protein content of brain tumors for diagnosis 5,6 and discrimination of radiation necrosis from tumor regrowth 7,8 , visualizing the perfusion of tumors by glucose and similar sugars [9][10][11][12][13] , mapping the pH of tumor microenvironment [14][15][16] , mapping the pH of the kidneys to detect changes in function 17,18 , monitor pH changes after cerebral ischemia, monitor gene expression through CEST reporter genes [19][20][21] among others. The number of adopters of CEST MRI technology has grown further due to new technologies that can improve the speed, selectivity, and sensitivity.…”