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Adaptive Optics Two-Photon Endomicroscopy Achieves Synaptic Resolution Imaging of Brain Tissue

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Scientists from the Hong Kong University of Science and Technology (HKUST) developed a technology for in vivo imaging of deep brain structures at synaptic resolution. The technology, “adaptive optics two-photon endomicroscopy,” enables scientists to acquire information necessary to elucidating the ways in which the brain functions, overcoming limitation with existing imaging methods to shed light on brain functionality in previously unexplored regions of the brain.

The technology pairs a miniature endoscope (GRIN lens) with existing adaptive optics methods. GRIN lens resolution is low, and because the imaging field of view is limited, the lens is unable to clearly image certain very small structures in the brain, including dendritic spines — neuronal protrusions that obtain information from neighboring neurons.

Adaptive optics was first developed and deployed for use in/with ground-based astronomical telescopes to compensate for the atmosphere’s natural distortion of light. Because of its origins, it uses what is known as a “guide star” — a predetermined bright star — to quantify the light distortion of the atmosphere, and then makes up for the distortion by using a deformable mirror to interact with light.

Adaptive optics two-photon endomicroscopy enabled in vivo imaging of the hippocampus at synaptic resolution over a large FOV Credit. The image shows investigation of a mouse hippocampus. Courtesy of HKUST.
Adaptive optics two-photon endomicroscopy enabled in vivo imaging of the hippocampus at synaptic resolution over a large field of view. The image shows investigation of a mouse hippocampus. Courtesy of HKUST.
In developing their imaging technology, HKUST researchers paralleled the way light functions in adaptive optics, using a localized fluorescent signal as the “guide star” inside biological tissues to measure the aberration of the endoscope and tissue.

The technique delivered the high-resolution images necessary to accelerate progress in understanding the mechanisms of neurodegenerative diseases and developing related treatments. The study of neurons in particular, critical to understanding the full scope of how information is transferred in the brain itself, relies on morphological and functional imaging at high resolution — both spatial as well as temporal.

Adaptive optics two-photon endomicroscopy delivered on both, the researchers said. Where electron microscopy provides the requisite high spatial resolution, it is unsuitable for live imaging tissue. Noninvasive technologies, such as MRI, positron emission tomography (PET), and ultrasound have limited synaptic resolution.

Optical microscopy, though advantageous in its ability to provide subcellular resolution and nontoxicity (to human tissue), is influenced by optical scattering aberrations induced by tissues and the imaging system that detract from its imaging depth capabilities. Two-photon microscopy alone is effective in imaging the cortex regions of the human brain, but not the subcortical and deep-brain structures.

The researchers tested adaptive optics two-photon endomicroscopy on mice to investigate neuronal plasticity in the deep brain (specifically, the hippocampus). The application clearly revealed the relationship between somatic and dendritic activity of pyramidal neurons.

Other regions of the brain that members of the research team identified as potential sites of investigation with the new technology include the striatum, substantia nigra, and hypothalamus, said Qu Jianan, a professor in HKUST’s Department of Electronic and Computer Engineering. Jianan collaborated with Nancy Ip, vice president for research and development and the Morningside Professor of Life Science at HKUST, on the research.

Additional and planned research using the technology will aim to understand the impairment of neuronal communications at the hippocampus in Alzheimer’s disease, and to develop ways to improve neuronal communication in that region. The ability to visualize changes in the number and size of dendritic spines, where the synapses are located, will improve understanding of how neurotransmission is regulated during memory impairment and/or after treatment with a drug candidate.

The research was published in Science Advances (www.doi:10.1126/sciadv.abc6521).

Photonics Handbook
GLOSSARY
adaptive optics
Optical components or assemblies whose performance is monitored and controlled so as to compensate for aberrations, static or dynamic perturbations such as thermal, mechanical and acoustical disturbances, or to adapt to changing conditions, needs or missions. The most familiar example is the "rubber mirror,'' whose surface shape, and thus reflective qualities, can be controlled by electromechanical means. See also active optics; phase conjugation.
Research & TechnologyeducationAsia PacificimagingMicroscopyBiophotonicsmedicalmedicineHong Kong University of Science and Technologyadaptive opticsopticsendomicroscopytwo photon microscopyGRIN lensAlzheimer's diseaseneuronal activitytwo-photon microscopy

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