Holographic two-photon activation for synthetic optogenetics

Non-convex optimization for inverse problem solving in computer-generated holography

Computer-generated holography is a promising technique that modulates user-defined wavefronts with digital holograms. Computing appropriate holograms with faithful reconstructions is not only a problem closely related to the fundamental basis of holography but also a long-standing challenge for researchers in general fields of optics. Finding the exact solution of a desired hologram to reconstruct an accurate target object constitutes an ill-posed inverse problem. The general practice of single-diffraction computation for synthesizing holograms can only provide an approximate answer, which is subject to limitations in numerical implementation. Various non-convex optimization algorithms are thus designed to seek an optimal solution by introducing different constraints, frameworks, and initializations. Herein, we overview the optimization algorithms applied to computer-generated holography, incorporating principles of hologram synthesis based on alternative projections and gradient descent methods. This is aimed to provide an underlying basis for optimized hologram generation, as well as insights into the cutting-edge developments of this rapidly evolving field for potential applications in virtual reality, augmented reality, head-up display, data encryption, laser fabrication, and metasurface design.

Modern Microscopic Approaches to Astrocytes

Microscopy started as the histological analysis based on intrinsic optical properties of tissues such as the refractive index and light absorption, and is expanding to include the visualization of organelles by chemical staining, localization of molecules by immunostaining, physiological measurements such as Ca²⁺ imaging, functional manipulation by optogenetics, and comprehensive analysis of chemical composition by Raman spectra. The microscope is one of the most important tools in neuroscience, which aims to reveal the complex intercellular communications underlying brain function and pathology. Many aspects of astrocytes, including the structures of their fine processes and physiological activities in concert with neurons and blood vessels, were revealed in the course of innovations in modern microscopy. The evolution of modern microscopy is a consequence of breakthroughs in spatiotemporal resolutions and expansions in molecular and physiological targets due to the progress in optics and information technology, as well as the inventions of probes using organic chemistry and molecular biology. This review overviews the modern microscopic approach to astrocytes.

Polarized branched Actin modulates cortical mechanics to produce unequal-size daughters during asymmetric division

The control of cell shape during cytokinesis requires a precise regulation of mechanical properties of the cell cortex. Only few studies have addressed the mechanisms underlying the robust production of unequal-sized daughters during asymmetric cell division. Here we report that unequal daughter-cell sizes resulting from asymmetric sensory organ precursor divisions in Drosophila are controlled by the relative amount of cortical branched Actin between the two cell poles. We demonstrate this by mistargeting the machinery for branched Actin dynamics using nanobodies and optogenetics. We can thereby engineer the cell shape with temporal precision and thus the daughter-cell size at different stages of cytokinesis. Most strikingly, inverting cortical Actin asymmetry causes an inversion of daughter-cell sizes. Our findings uncover the physical mechanism by which the sensory organ precursor mother cell controls relative daughter-cell size: polarized cortical Actin modulates the cortical bending rigidity to set the cell surface curvature, stabilize the division and ultimately lead to unequal daughter-cell size.

Spectral-envelope modulated double-phase method for computer-generated holography

Computer-generated holography provides an approach to modulate the optical wavefront with computationally synthesized holograms. Since the hardware implementation for complex wavefronts is not yet available, double-phase decomposition is utilized as a complex encoding method of converting a complex wavefront to a double-phase hologram. The double-phase hologram adapts a complex wavefront for the phase-type devices, but the reconstruction is plagued by the noise caused by spatial-shifting errors. Here, a spectral-envelope modulated double-phase method is proposed to suppress the spatial-shifting noise with an off-axis envelope modulation on the Fourier spectrum of a double-phase hologram. This proposed method out-performs conventional on-axis double-phase method in optical reconstructing accuracy with indicated 9.54% improvement in PSNR and 196.86% improvement in SSIM.

New tricks and emerging applications from contemporary azobenzene research

Azobenzenes have many faces. They are well-known as dyes, but most of all, azobenzenes are versatile photoswitchable molecules with powerful photochemical properties. Azobenzene photochemistry has been extensively studied for decades, but only relatively recently research has taken a steer towards applications, ranging from photonics and robotics to photobiology. In this perspective, after an overview of the recent trends in the molecular design of azobenzenes, we highlight three research areas where the azobenzene photoswitches may bring about promising technological innovations: chemical sensing, organic transistors, and cell signaling. Ingenious molecular designs have enabled versatile control of azobenzene photochemical properties, which has in turn facilitated the development of chemical sensors and photoswitchable organic transistors. Finally, the power of azobenzenes in biology is exemplified by vision restoration and photactivation of neural signaling. Although the selected examples reveal only some of the faces of azobenzenes, we expect the fields presented to develop rapidly in the near future, and that azobenzenes will play a central role in this development.

Improved phase hologram generation of multiple 3D objects

We demonstrate the generation of phase holograms of multiple 3D objects at different axial positions without cross talk and significant improvements in performance over conventional methods. We first obtain the phase hologram of two 3D objects, each one comprising 50 layers, using the global Gerchberg-Saxton algorithm. Then, we discuss and demonstrate a propagation approach based on the singular value decomposition of the Fresnel impulse response function that enables fast computation of small distance propagations. Finally, we propose a new iterative hologram generation algorithm, to the best of our knowledge, that takes advantage of this propagation approach and use it to make the hologram of the same scene previously obtained with the global Gerchberg-Saxton algorithm. We perform numerical and experimental reconstructions to compare both methods, demonstrating that our proposal achieves 4 times faster computation, as well as improved reconstruction quality.

Alternative constraints for improved multiplane hologram generation

In this work, we introduce a modified hologram plane constraint to improve the accuracy of the global Gerchberg-Saxton (GGS) algorithm used for multiplane phase-only hologram generation. This constraint consists of a modified phase factor that depends on the amplitude of the field in the hologram plane. We demonstrate that this constraint produces an increase in the mean correlation coefficient between the reconstructed planes from a multiplane hologram and the corresponding amplitude targets for each plane. Furthermore, this constraint can be applied together with a mixed constraint in the reconstruction planes, leading to a more uniform and controllable reproduction of a target intensity distribution. To confirm the validity of our proposal, we show numerical and experimental results for multiplane holograms with six discrete planes, using both high and low contrast targets. For the experimental results, we implement a holographic projection scheme based on a phase-only spatial light modulator.

Holographic microscope and its biological application

Holographic structured illumination combined with optogenetics enables patterned stimulation of neurons and glial cells in an intact living brain. Moreover, in vivo functional imaging of cellular activity with recent advanced microscope technologies allows for visualization of the cellular responses during learning, emotion and cognition. Integrating these techniques can be used to verify the link between cell function and behavior output. However, there are technical limitations to stimulate multiple cells with high spatial and temporal resolution with available techniques of optogenetic stimulation. Here, we summarized a two-photon microscope combined with holographic system to stimulate multiple cells with high spatial and temporal resolution for living mice and their biological application.

All-optical approaches to studying psychiatric disease

Improvements in all-optical means of monitoring and manipulating neural activity have generated new ways of studying psychiatric disease. The combination of calcium imaging techniques with optogenetics to concurrently record and manipulate neural activity has been used to create new disease models that link distinct circuit abnormalities to specific disease dimensions. These approaches represent a new path towards the development of more effective treatments, as they allow researchers to identify circuit manipulations that normalize pathological network activity. In this review we highlight the utility of all-optical approaches to generate new psychiatric disease models where the specific circuit abnormalities associated with disease symptomology can be assessed in vivo and in response to manipulations designed to normalize disease states. We then outline the principles underlying all-optical interrogations of neural circuits and discuss practical considerations for experimental design.

Red Photoactivatable Genetic Optical-Indicators

Emerging genetically-encoded Ca²⁺-indicators (GECIs) are intensiometric reporters that increase in fluorescence when bound to Ca²⁺; highly suited for studying calcium-signaling in many cell types, notably neurons. Today, major efforts are devoted toward optimizing red-emitting [red fluorescent protein (RFP)-based] GECIs (R-GECI), as these provide several advantages over GFP-based reporters, for instance, increased imaging depth, reduced photodamage by longer imaging wavelengths and, in principle, are better suited for use with prevalent blue-absorbing optogenetic tools (e.g., channelrhodopsin). However, excessive fluorescence from intersecting neighboring cells in very dense tissues, notably the brain, hinders the ability to collect signals from single cells and their processes. This challenge can be addressed by photoactivatable (PA) fluorescent proteins that can be rendered fluorescent on demand by user-defined targeted light. This allows activation and, thereby, collection of fluorescent signals exclusively from desired cells and their processes, while leaving all neighboring cells in the dark (i.e., non-fluorescent). Nevertheless, there are no PA R-GECIs. Here, we sought to develop PA-R-GECIs. To do so, we initially explored a recently discovered phenomenon of Ca²⁺-independent increases in fluorescence (i.e., artifacts) in an emerging R-GECI, which has led us to rationally engineer several functional PA-R-GECIs. We also take advantage of our findings to quickly engineer a novel PA-RFP, namely, PA-mRuby3.

Two-Photon Absorption Enhancement by the Inclusion of a Tropone Ring in Distorted Nanographene Ribbons

A new family of distorted ribbon-shaped nanographenes was designed, synthesized, and their optical and electrochemical properties were evaluated, pointing out an unprecedented correlation between their structural characteristics and the two-photon absorption (TPA) responses and electrochemical band gaps. Three nanographene ribbons have been prepared: a seven-membered-ring-containing nanographene presenting a tropone moiety at the edge, its full-carbon analogue, and a purely hexagonal one. We have found that the TPA cross-sections and the electrochemical band gaps of the seven-membered-ring-containing compounds are higher and lower, respectively, than those of the fully hexagonal polycyclic aromatic hydrocarbon (PAH). Interestingly, the inclusion of additional curvature has a positive effect in terms of non-linear optical properties of those ribbons.

Two-Photon Excitation of Azobenzene Photoswitches for Synthetic Optogenetics

Synthetic optogenetics is an emerging optical technique that enables users to photocontrol molecules, proteins, and cells in vitro and in vivo. This is achieved by use of synthetic chromophores—denoted photoswitches—that undergo light-dependent changes (e.g., isomerization), which are meticulously designed to interact with unique cellular targets, notably proteins. Following light illumination, the changes adopted by photoswitches are harnessed to affect the function of nearby proteins. In most instances, photoswitches absorb visible light, wavelengths of poor tissue penetration, and excessive scatter. These shortcomings impede their use in vivo. To overcome these challenges, photoswitches of red-shifted absorbance have been developed. Notably, this shift in absorbance also increases their compatibility with two-photon excitation (2PE) methods. Here, we provide an overview of recent efforts devoted towards optimizing azobenzene-based photoswitches for 2PE and their current applications.

Can NMDA Spikes Dictate Computations of Local Networks and Behavior?

Intelligence is the ability to learn appropriate responses to stimuli and the capacity to master new skills. Synaptic integration at the dendritic level is thought to be essential for this ability through linear and non-linear processing, by allowing neurons to be tuned to relevant information and to maximize adaptive behavior. Showing that dendrites are able to generate local computations that influence how animals perceive the world, form a new memory or learn a new skill was a break-through in neuroscience, since in the past they were seen as passive elements of the neurons, just funneling information to the soma. Here, we provide an overview of the role of dendritic integration in improving the neuronal network and behavioral performance. We focus on how NMDA spikes are generated and their role in neuronal computation for optimal behavioral output based on recent in vivo studies on rodents.

Optogenetics in Brain Research: From a Strategy to Investigate Physiological Function to a Therapeutic Tool

Dissecting the functional roles of neuronal circuits and their interaction is a crucial step in basic neuroscience and in all the biomedical field. Optogenetics is well-suited to this purpose since it allows us to study the functionality of neuronal networks on multiple scales in living organisms. This tool was recently used in a plethora of studies to investigate physiological neuronal circuit function in addition to dysfunctional or pathological conditions. Moreover, optogenetics is emerging as a crucial technique to develop new rehabilitative and therapeutic strategies for many neurodegenerative diseases in pre-clinical models. In this review, we discuss recent applications of optogenetics, starting from fundamental research to pre-clinical applications. Firstly, we described the fundamental components of optogenetics, from light-activated proteins to light delivery systems. Secondly, we showed its applications to study neuronal circuits in physiological or pathological conditions at the cortical and subcortical level, in vivo. Furthermore, the interesting findings achieved using optogenetics as a therapeutic and rehabilitative tool highlighted the potential of this technique for understanding and treating neurological diseases in pre-clinical models. Finally, we showed encouraging results recently obtained by applying optogenetics in human neuronal cells in-vitro.