Engineering Photoactivatability in Genetically Encoded Voltage and pH Indicators

Conserved Amino Acids Residing Outside the Voltage Field Can Shift the Voltage Sensitivity and Increase the Signal Speed and Size of Ciona Based GEVIs

To identify potential regions of the voltage-sensing domain that could shift the voltage sensitivity of Ciona intestinalis based Genetically Encoded Voltage Indicators (GEVIs), we aligned the amino acid sequences of voltage-gated sodium channels from different organisms. Conserved polar residues were identified at multiple transmembrane/loop junctions in the voltage sensing domain. Similar conservation of polar amino acids was found in the voltage-sensing domain of the voltage-sensing phosphatase gene family. These conserved residues were mutated to nonpolar or oppositely charged amino acids in a GEVI that utilizes the voltage sensing domain of the voltage sensing phosphatase from Ciona fused to the fluorescent protein, super ecliptic pHluorin (A227D). Different mutations shifted the voltage sensitivity to more positive or more negative membrane potentials. Double mutants were then created by selecting constructs that shifted the optical signal to a more physiologically relevant voltage range. Introduction of these mutations into previously developed GEVIs resulted in Plos6-v2 which improved the dynamic range to 40% ΔF/F/100 mV, a 25% increase over the parent, ArcLight. The onset time constant of Plos6-v2 is also 50% faster than ArcLight. Thus, Plos6-v2 appears to be the GEVI of choice.

Photo-transformable genetically-encoded optical probes for functional highlighting in vivo

Studying the brain requires knowledge about both structure (i.e., connectome) and function of its constituents (neurons and glia alike). This need has prompted the development of novel tools and techniques, in particular optical techniques to examine cells remotely. Early works (1900's) led to the development of novel cell-staining techniques that, when combined with the use of a very simple light microscope, visualized individual neurons and their subcellular compartments in fixed tissues. Today, highlighting of structure and function can be performed on live cells, notably in vivo, owing to discovery of GFP and subsequent development of genetically encoded fluorescent optical tools. In this review, we focus our attention on a subset of optical biosensors, namely probes whose emission can be modified by light. We designate them photo-transformable genetically encoded probes. The family of photo-transformable probes embraces current probes that undergo photoactivation (PA), photoconversion (PC) or photoswitching (PS). We argue that these are particularly suited for studying multiple features of neurons, such as structure, connectivity and function concomitantly, for functional highlighting of neurons in vivo.

Imaging voltage and brain chemistry with genetically encoded sensors and modulators

Neurons and glia are functionally organized into circuits and higher-order structures that allow the precise information processing required for complex behaviors. To better understand the structure and function of the brain, we must understand synaptic connectivity, action potential generation and propagation, as well as well-orchestrated molecular signaling. Recently, dramatically improved sensors for voltage, intracellular calcium, and neurotransmitters/modulators, combined with advanced microscopy provide new opportunities for in vivo dissection of cellular and circuit activity in awake, behaving animals. This review focuses on the current trends in genetically encoded sensors for molecules and cellular events and their potential applicability to the study of nervous system in health and disease.

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.