Fluorescence microscopy shadow imaging for neuroscience

Fluorescence microscopy remains one of the single most widely applied experimental approaches in neuroscience and beyond and is continuously evolving to make it easier and more versatile. The success of the approach is based on synergistic developments in imaging technologies and fluorophore labeling strategies that have allowed it to greatly diversify and be used across preparations for addressing structure as well as function. Yet, while targeted labeling strategies are a key strength of fluorescence microscopy, they reciprocally impose general limitations on the possible types of experiments and analyses. One recent development that overcomes some of these limitations is fluorescence microscopy shadow imaging, where membrane-bound cellular structures remain unlabeled while the surrounding extracellular space is made to fluoresce to provide a negative contrast shadow image. When based on super-resolution STED microscopy, the technique in effect provides a positive image of the extracellular space geometry and entire neuropil in the field of view. Other noteworthy advantages include the near elimination of the adverse effects of photobleaching and toxicity in live imaging, exhaustive and homogeneous labeling across the preparation, and the ability to apply and adjust the label intensity on the fly. Shadow imaging is gaining popularity and has been applied on its own or combined with conventional positive labeling to visualize cells and synaptic proteins in their parenchymal context. Here, we highlight the inherent limitations of fluorescence microscopy and conventional labeling and contrast these against the pros and cons of recent shadow imaging approaches. Our aim is to describe the brief history and current trajectory of the shadow imaging technique in the neuroscience field, and to draw attention to its ease of application and versatility.

Super-Resolution Microscopy Opens New Doors to Life at the Nanoscale

Super-resolution fluorescence microscopy holds tremendous potential for discovery in neuroscience. Much of the molecular machinery and anatomic specializations that give rise to the unique and bewildering electrochemical activity of neurons are nanoscale by design, ranging somewhere between 1 nm and 1 μm. It is at this scale where most of the unknown and exciting action is and where cell biologists flock to in their dreams, but it was off limits for light microscopy until recently. While the optical principles of super-resolution microscopy are firmly established by now, the technology continues to advance rapidly in many crucial areas, enhancing its performance and reliability, and making it more accessible and user-friendly, which is sorely needed. Indeed, super-resolution microscopy techniques are nowadays widely used for visualizing immunolabeled protein distributions in fixed or living cells. However, a great potential of super-resolution microscopy for neuroscience lies in shining light on the nanoscale structures and biochemical activities in live-tissue settings, which should be developed and harnessed much more fully. In this review, we will present several vivid examples based on STED and RESOLFT super-resolution microscopy, illustrating the possibilities and challenges of nano-imaging in vivo to pique the interest of tech-developers and neurobiologists alike. We will cover recent technical progress that is facilitating in vivo applications, and share new biological insights into the nanoscale mechanisms of cellular communication between neurons and glia.

Unveiling the Extracellular Space of the Brain: From Super-resolved Microstructure to In Vivo Function

The extracellular space occupies approximately one-fifth of brain volume, molding a spider web of gaps filled with interstitial fluid and extracellular matrix where neurons and glial cells perform in concert. Yet, very little is known about the spatial organization and dynamics of the extracellular space, let alone its influence on brain function, owing to a lack of appropriate techniques (and a traditional bias toward the inside of cells, not the spaces in between). At the same time, it is clear that understanding fundamental brain functions, such as synaptic transmission, memory, sleep, and recovery from disease, calls for more focused research on the extracellular space of the brain. This review article highlights several key research areas, covering recent methodological and conceptual progress that illuminates this understudied, yet critically important, brain compartment, providing insights into the opportunities and challenges of this nascent field.