A Xenon-Based Molecular Sensor Assembled on an MS2 Viral Capsid Scaffold

Abstract

In MRI, anatomical structures are most often differentiated by variations in their bulk magnetic properties. Alternatively, exogenous contrast agents can be attached to chemical moieties that confer affinity to molecular targets; the distribution of such contrast agents can be imaged by magnetic resonance. Xenon-based molecular sensors are molecular imaging agents that rely on the reversible exchange of hyperpolarized xenon between the bulk and a specifically targeted host-guest complex. We have incorporated approximately 125 xenon sensor molecules in the interior of an MS2 viral capsid, conferring multivalency and other properties of the viral capsid to the sensor molecule. The resulting signal amplification facilitates the detection of sensor at 0.7 pM, the lowest to date for any molecular imaging agent used in magnetic resonance. This amplification promises the detection of chemical targets at much lower concentrations than would be possible without the capsid scaffold.

Development of an antibody-based, modular biosensor for129Xe NMR molecular imaging of cells at nanomolar concentrations

“…Given the potential for high numbers of CrA per biosensor to mediate unspecific binding (21) and cell toxicity (9,24), this quantification also speaks to the potential benefit of tuning the CrA load to achieve suitable detection, whether via this modular system or through other scaffolds (20,23), while minimizing undesirable effects such as unspecific binding of the hydrophobic cage, which is prevented in our case. The higher CrA content of cells incubated with the complete construct is likely the result of a more efficient connection of the two modules, which is done before cell incubation in a phosphate buffer solution, for these samples.…”

“…In response to these developments, there have been a number of recent studies describing the development of new xenon contrast agents (17)(18)(19) and biosensors (10,(20)(21)(22)(23) for xenon NMR applications. Critically however, only recently have images of cells been achieved with xenon MRI, with a nontargeted approach using the contrast agent CrA (24).…”

Cell-compatible, integrin-targeted cryptophane-129XeNMR biosensors

“…49,50 Another reason for the “bound” signal being weak is the broad line width compared to some xenon biosensors bound to other protein targets (avidin, 0.15 ppm; carbonic anhydrase I and II, 0.19–0.27 ppm 16 ). In other cases, it has been observed that “bound” peaks for many xenon biosensors are considerably broadened (10–30 Hz to 100–150 Hz, 14 and even 1–5 kHz 20 ). Although peak broadening tends to be an obstacle in identifying peaks in high-resolution NMR spectra, this problem will be mitigated in a saturation transfer scheme (Hyper-CEST).…”

“…In a recent example, MS2 viral capsid modified with 125 cryptophane molecules was detected at concentrations as low as 0.7 picomolar. 20 …”

Advances in bio-imaging: A survey from WWMR 2010

“…Another development of the Hyper-CEST methodology dealt with the use of functionalized viral capsids as carriers for Xe sensors [12]. The coat protein of a virus has a porous structure that can incorporate several Xe-containing cages.…”