FNCaMP, ratiometric green calcium indicator based on mNeonGreen protein

Split genetically encoded calcium indicators for interorganellar junctions

Genetically encoded calcium indicators (GECIs) have revolutionized the study of cellular calcium signaling, offering powerful tools for real-time optical monitoring of calcium dynamics. Although contemporary GECIs can be targeted to various organelles, there are no means to obtain active and functional GECIs exclusively at interorganellar junctions. To address this gap, we have developed a toolbox of split versions of green and red GECIs designed to reassemble only when the two "halves" come into proximity. We developed split probes to investigate interorganellar connectivity and activity between mitochondria and the ER (via split-MEGIC) or between the plasma membrane and the ER (via split-sf-MEMBER). We employ the various split-sensors to image neural Ca2+ activity in vitro and in vivo and, in the process, identify Mito-ER junctions and calcium activity within individual dendritic spines by use of split-MEGIC.

Development of novel indicators and molecular systems for calcium sensing through protein engineering

Intracellular calcium (Ca2+) is involved in a plethora of cell signalling processes and physiological functions. Increases in Ca2+ concentration are bona fide biomarkers of neuronal activity, reflecting the spike count, timing, frequency, and the intensity of synaptic input. The development of genetically encoded calcium indicators (GECIs) was a significant advancement in modern neuroscience that enabled real-time visualisation of neuronal activity at single-cell resolution. These indicators leverage the conformational changes induced by calcium-binding proteins, such as calmodulin (CaM) or troponin C (TnC). Harnessing protein engineering approaches such as directed evolution yielded new GECIs with enhanced sensitivity, kinetics, and brightness. Notably, the development of calcium-based integrators, such as scFLARE (single-chain fast light- and activity-regulated expression), convert transient raises in cytosolic Ca2+ into a transcriptional readout rather than an optical signal. This review summarises the latest efforts in protein engineering to develop new indicators and molecular systems to sense changes in Ca2+ concentrations.

Calcium Indicators with Fluorescence Lifetime-Based Signal Readout: A Structure-Function Study

The calcium cation is a crucial signaling molecule involved in numerous cellular pathways. Beyond its role as a messenger or modulator in intracellular cascades, calcium's function in excitable cells, including nerve impulse transmission, is remarkable. The central role of calcium in nervous activity has driven the rapid development of fluorescent techniques for monitoring this cation in living cells. Specifically, genetically encoded calcium indicators (GECIs) are the most in-demand molecular tools in their class. In this work, we address two issues of calcium imaging by designing indicators based on the successful GCaMP6 backbone and the fluorescent protein BrUSLEE. The first indicator variant (GCaMP6s-BrUS), with a reduced, calcium-insensitive fluorescence lifetime, has potential in monitoring calcium dynamics with a high temporal resolution in combination with advanced microscopy techniques, such as light beads microscopy, where the fluorescence lifetime limits acquisition speed. Conversely, the second variant (GCaMP6s-BrUS-145), with a flexible, calcium-sensitive fluorescence lifetime, is relevant for static measurements, particularly for determining absolute calcium concentration values using fluorescence lifetime imaging microscopy (FLIM). To identify the structural determinants of calcium sensitivity in these indicator variants, we determine their spatial structures. A comparative structural analysis allowed the optimization of the GCaMP6s-BrUS construct, resulting in an indicator variant combining calcium-sensitive behavior in the time domain and enhanced molecular brightness. Our data may serve as a starting point for further engineering efforts towards improved GECI variants with fine-tuned fluorescence lifetimes.

Fluorescent Protein-Based Sensors for Detecting Essential Metal Ions across the Tree of Life

Genetically encoded fluorescent metal ion sensors are powerful tools for elucidating metal dynamics in living systems. Over the last 25 years since the first examples of genetically encoded fluorescent protein-based calcium indicators, this toolbox of probes has expanded to include other essential and non-essential metal ions. Collectively, these tools have illuminated fundamental aspects of metal homeostasis and trafficking that are crucial to fields ranging from neurobiology to human nutrition. Despite these advances, much of the application of metal ion sensors remains limited to mammalian cells and tissues and a limited number of essential metals. Applications beyond mammalian systems and in vivo applications in living organisms have primarily used genetically encoded calcium ion sensors. The aim of this Perspective is to provide, with the support of historical and recent literature, an updated and critical view of the design and use of fluorescent protein-based sensors for detecting essential metal ions in various organisms. We highlight the historical progress and achievements with calcium sensors and discuss more recent advances and opportunities for the detection of other essential metal ions. We also discuss outstanding challenges in the field and directions for future studies, including detecting a wider variety of metal ions, developing and implementing a broader spectral range of sensors for multiplexing experiments, and applying sensors to a wider range of single- and multi-species biological systems.

YTnC2, an improved genetically encoded green calcium indicator based on toadfish troponin C

Genetically encoded calcium indicators based on truncated troponin C are attractive probes for calcium imaging due to their relatively small molecular size and twofold reduced calcium ion buffering. However, the best-suited members of this family, YTnC and cNTnC, suffer from low molecular brightness, limited dynamic range, and/or poor sensitivity to calcium transients in neurons. To overcome these limitations, we developed an enhanced version of YTnC, named YTnC2. Compared with YTnC, YTnC2 had 5.7-fold higher molecular brightness and 6.4-fold increased dynamic range in vitro. YTnC2 was successfully used to reveal calcium transients in the cytosol and in the lumen of mitochondria of both mammalian cells and cultured neurons. Finally, we obtained and analyzed the crystal structure of the fluorescent domain of the YTnC2 mutant.