ChrimsonR

ChrimsonR is a genetically encoded, red‑shifted channelrhodopsin that functions as a light‑gated cation channel in optogenetics. It was engineered by mutagenesis of the wild‑type channelrhodopsins originally isolated from marine algae; the key modifications shifted its peak spectral sensitivity from the blue-green range to around 590–610 nm, allowing activation with far‑red light. This spectral shift reduces phototoxicity and scattering in deep tissue, enabling deeper and more precise stimulation in living organisms.

The ion conductance of ChrimsonR is largely non‑selective, permitting a mixture of Na⁺, K⁺, and Ca²⁺ to enter the cell, which typically depolarizes neurons and elicits action potentials. Its kinetic properties include a slow deactivation time (hundreds of milliseconds), which makes it suitable for sustained stimulation protocols. Compared with its predecessor Chrimson, ChrimsonR shows improved membrane trafficking in mammalian cells, higher expression levels, and a faster rise time, thereby providing stronger photocurrents with lower light intensities.

In research, ChrimsonR is widely used to manipulate neuronal populations in vivo, especially in combination with other optogenetic tools. By pairing it with blue‑light responsive opsins, researchers can achieve two‑color, bidirectional control of neural circuits. Its red‑shifted spectrum also permits concurrent optical imaging, as the excitation wavelengths for fluorescence imaging typically lie in the blue/green range and thus minimally overlap with ChrimsonR’s activation window.

Commercially, ChrimsonR plasmids and viral vectors are available from several biotech suppliers, and it is frequently incorporated into adeno‑associated virus (AAV) vectors for brain‑wide delivery. Current studies continue to refine its kinetics and expression, and novel variants are being reported that further enhance photocurrent amplitude and reduce light‑induced heating.