Multi-Modal Nano Particle Labeling of Neurons

Abstract

The development of imaging methodologies for single cell measurements over extended timescales of up to weeks, in the intact animal, will depend on signal strength, stability, validity and specificity of labeling. Whereas light-microscopy can achieve these with genetically-encoded probes or dyes, this modality does not allow mesoscale imaging of entire intact tissues. Non-invasive imaging techniques, such as magnetic resonance imaging (MRI), outperform light microscopy in field of view and depth of imaging, but do not offer cellular resolution and specificity, suffer from low signal-to-noise ratio and, in some instances, low temporal resolution. In addition, the origins of the signals measured by MRI are either indirect to the process of interest or hard to validate. It is therefore highly warranted to find means to enhance MRI signals to allow increases in resolution and cellular-specificity. To this end, cell-selective bi-functional magneto-fluorescent contrast agents can provide an elegant solution. Fluorescence provides means for identification of labeled cells and particles location after MRI acquisition, and it can be used to facilitate the design of cell-selective labeling of defined targets. Here we briefly review recent available designs of magneto-fluorescent markers and elaborate on key differences between them with respect to durability and relevant cellular highlighting approaches. We further focus on the potential of intracellular labeling and basic functional sensing MRI, with assays that enable imaging cells at microscopic and mesoscopic scales. Finally, we illustrate the qualities and limitations of the available imaging markers and discuss prospects for in vivo neural imaging and large-scale brain mapping.

Tissue expansion of ‘cleared’ whole brains reduces contrast in ex vivo MRI

“…We also suggest that multiple clearing techniques can be combined and that multiple rounds of LM imaging-hydration-MR imaging and clearing can be performed 38,43 . This protocol should allow users to create atlases composed of registered MRI and LM images of entire tissues, as well as accelerate the development of novel multimodal agents by providing means to quickly validate MRI signals by uorescence imaging 1 . Lastly, we envision that this method could be implemented with LM to examine clinical histological specimens by ex vivo MRI to detect pathologies at various scales 26,37 , analogous to pathology-optimized expansion microscopy 20,78 .…”

“…However, the opaque nature of the brain and its large dimensions limit the use of high-resolution uorescence light microscopy (LM) to image the brain in its entirety [1][2][3][4] . This drawback has been addressed by emerging tissue-clearing techniques (e.g., CLARITY [5][6][7] ) combined with advanced LM imaging methods, such as light-sheet microscopy [8][9][10] .…”

“…One attractive method is ex vivo Magnetic Resonance Imaging (ex vivo MRI) 1,23 . Ex vivo MRI provide anatomical images of tissues (without the need to label) relatively fast (< minutes) and is non-damaging.…”

“…These make the method highly suitable for integration of data for precise image registration by, for instance, providing set-coordinates for correction of deformities and outlining major brain regions 12,[25][26][27][28][29] . Thus, multimodal imaging of cleared-tissues by LM and ex-vivo MRI should ease on the convergence between the micro-and mesoscale and aiding in framing the context of the uorescence information 1,30 . Despite these advantages, dual LM-ex vivo MRI is not common (more common are in vivo MRI followed by ex vivo LM 24,26,30 ).…”

Contrast Agents Delivery: An Up-to-Date Review of Nanodiagnostics in Neuroimaging

“…As future perspectives, specialists in the field of neuroimaging should consider the application of nanotechnology for single-neuron detection [152,153] and phase-contrast X-ray imaging techniques [154] which could offer the possibility of visualizing brain areas and activities that are currently obscure or undetectable for conventional methods. In the case of single-neuron detection, current studies focus on the use of diamond chips with quantum defects as sensors that can detect time-varying magnetic fields generated by action potentials of neurons [155]. Similarly, magnetic induction tomography performed with optical atomic magnetometry allows for the detection across a large frequency range in low-conductivity targets, such as the biological tissues [156].…”