Increasing the optical transparency of a living mouse brain (and other nanotechnologies)

Abstract

Many methods for increasing the optical transparency of non-living brain tissue have come into widespread use because of their utility in enabling better anatomical brain imaging. In the first part of this thesis, we explore whether this is also possible for living brain tissue. We report a general principle for doing so, namely the reduction of refractive index mismatch between cellular membranes and the extracellular space, using high refractive index biocompatible reagents that have high molecular weights, so that they can be used at low concentrations. We implement this via multiple reagents that satisfied these criteria, including the iodinated radiocontrast agent iodixanol, high molecular weight polyethylene glycol (PEG), high molecular weight Dextran, and PEG-ylated Silicon nanoparticles. We achieve ~2x increases in the brightness of cells expressing red fluorescent proteins in vivo in mice, as measured by conventional one-photon epifluorescence imaging, using concentrations of reagents that increased the refractive index of the extracellular space by just 0.01. Lastly, We show that Dextran does not have a statistically significant effect on neural physiology or neural network properties. We expect such strategies to not only facilitate live imaging of the brains of mice and other mammals, but open up a new class of strategies for changing the electromagnetic properties of living systems. We conclude this thesis with two nanotechnologies that may be leveraged for making higher performance reagents for increasing the optical transparency of living brain tissue. (1) A method for the synthesis of high-yield and high-monodispersity nanoparticles of a variety of materials with tailored surface ligands, using common benchtop equipment. This method may be useful for developing nanoparticles with better biosafety, efficacy and performance. (2) A method for the delivery of hydrophobic NVNDs to neural cell membranes using PEG-ylated liposomes. These PEG-ylated liposomes may be used for delivery of hydrophyllic nanoparticles to neural soma and achieve maximal transparency.