The Almighty Xenon!

Researchers in the Wemmer Lab, in collaboration with Alex Pines’ and Matt Francis’ groups at UC Berkeley, are utilizing the natural affinity of xenon for hydrophobic cavities and the sensitivity of laser-polarized xenon nuclear magnetic resonance (NMR) to develop sensors capable of detecting biological and chemical analytes of interest.[1]  When combined with optimized detection schemes, these so-called xenon biosensors can be detected in sub-picomolar concentrations—orders of magnitude below the threshold for detecting traditional magnetic resonance (MR) contrast agents.





1) Development of Biosensors

 

Xenon biosensors typically consist of a hydrophobic xenon host molecule attached to a targeting moiety via a solubilizing linker.  Due to the sensitivity of xenon to its environment, it is possible to detect the biosensors due to their unique resonance frequency in solution as well as changes in this resonance frequency when a sensor binds to its molecular target.  The biosensors used in the Wemmer lab are synthesized in the Francis group and the xenon MR studies are conducted with a dedicated setup in the Pines lab.  Long-term research aims include optimizing the sensors for solubility, detection sensitivity, targeting specificity, and biocompatibility.[2]  Currently, sensors are being developed that utilize naturally occurring scaffolds such as viral capsids[3] and bacteriophages for solubility, biocompatibility, and significant improvements in the threshold for detection sensitivity





2) Molecular Imaging with Hyper-CEST

The current clinical standard for visualizing soft tissue in the body is proton-based MR imaging (MRI).  In spite of its superiority in resolving anatomical structures, MRI has not enjoyed the same success as a method for localizing functionality; e.g., localizing tumor-specific molecules within the body for early cancer detection.  For these events, the spatial resolution of proton-based MRI is typically sacrificed for increased detection sensitivity by using radioactive agents with positron emission tomography (PET) and single photon emission computed tomography (SPECT).  Development of an MR technique that offers the sensitivity of PET and SPECT without radioisotopes would be invaluable in expanding the use of MRI for molecular imaging.

Xenon-based sensors targeting an array of biological and chemical markers are being developed to push the detection threshold of MR contrast agents towards that of PET and SPECT. Hyperpolarized xenon gas is either bubbled into solutions containing the sensors or dissolved into solution using hydrophobic membranes.  A method developed in the Wemmer and Pines labs is used to take advantage of the exchange of xenon in and out of the cage that dramatically increases the sensitivity for detecting sensors in significantly less time compared to directly looking at the xenon-cage resonance frequency.  This technique, termed Hyper-CEST (chemical exchange saturation transfer of hyperpolarized nuclei, [4]), combined with standard imaging pulse sequences can be used to selectively image the presence of the sensors.

Because the exchange kinetics of xenon with sensor molecules and the chemical shift of xenon bound in sensor molecules are both highly temperature dependent, additional information can be gained by controlling the temperature of the system being studied.  In doing so, it is possible to tune the detection of sensors via their chemical shift dependence on this parameter.[5]  Furthermore, detecting sensors with the Hyper-CEST technique is improved at increased temperatures.[6]  Applications here include using the sensors for MR thermometry.[7]





3) Remote detection of Xenon-based Sensors

Development of lab-on-a-chip techniques has exploded in the past decade due to low reagent costs, small sample size requirements, and the potential to multiplex on one device.  In combination with Hyper-CEST and optimization of the detected signal via remote detection NMR [8], xenon-based molecular sensors could provide a powerful tool for analysis of chemical, environmental, and biological processes on a microfluidic chip.  Current work involves development of the hardware required to detect xenon-based molecular sensors remotely, with the long-term goal of creating a platform compatible with microfluidic devices




[1] Spence, MM, et al. Functionalized xenon as a biosensor. Proc. Natl. Acad. Sci. USA 98, 10654-10657 (2001).
[2] Lowery, TJ, et al. Optimization of xenon biosensors for detection of protein interactions. ChemBioChem 7, 65-73 (2006).
[3] Meldrum, T, et al. A Xenon-Based Molecular Sensor Assembled on an MS2 Viral Capsid Scaffold, J. Am. Chem. Soc. 132 (17), 5936-5937 (2010).
[4] Schröder, L, et al. Molecular Imaging using a Targeted Magnetic Resonance Hyperpolized Biosensor, Science 314, 446-449 (2006).
[5] Schröder, L, et al. Temperature-Controlled Molecular Depolarization Gates in Nuclear Magnetic Resonance, Phys. Rev. Lett. 100(25), 257603 (2008).
[6] Schröder, L, et al. Temperature response of 129Xe depolarization transfer and application for unltra-sensitive NMR detection, Angew. Chem. Int. Ed. 47, 4316-4320 (2008).
[7] Schilling, F, et al. MRI Thermometry Based on Encapsulated Hyperpolarized Xenon, ChemPhysChem 11(16), 3529-3533 (2010).
[8] Granwehr, J, et al. Time-of-Flight Flow Imaging Using NMR Remote Detection. PRL. 95:075503 (2005).