Optogenetic behavioral studies in depression research: A systematic review.

Optogenetics has made substantial contributions to our understanding of the mechanistic underpinnings of depression. This systematic review employs quantitative analysis to investigate the impact of optogenetic stimulation in mice and rats on behavioral alterations in social interaction, sucrose consumption, and mobility. The review analyses optogenetic behavioral studies using standardized behavioral tests to detect behavioral changes induced via optogenetic stimulation in stressed or stress-naive mice and rats. Behavioral changes were evaluated as either positive, negative, or not effective. The analysis comprises the outcomes of 248 behavioral tests of 168 studies described in 37 articles, including negative and null results. Test outcomes were compared for each behavior, depending on the animal cohort, applied type of stimulation and the stimulated neuronal circuit and cell type. The presented synthesis contributes toward a comprehensive picture of optogenetic behavioral research in the context of depression.

The Functionality of the DC Pair in a Rhodopsin Guanylyl Cyclase from Catenaria anguillulae.

Rhodopsin guanylyl cyclases (RGCs) belong to the class of enzymerhodopsins catalyzing the transition from GTP into the second messenger cGMP, whereas light-regulation of enzyme activity is mediated by a membrane-bound microbial rhodopsin domain, that holds the catalytic center inactive in the dark. Structural determinants for activation of the rhodopsin moiety eventually leading to catalytic activity are largely unknown. Here, we investigate the mechanistic role of the D283-C259 (DC) pair that is hydrogen bonded via a water molecule as a crucial functional motif in the homodimeric C. anguillulae RGC. Based on a structural model of the DC pair in the retinal binding pocket obtained by MD simulation, we analyzed formation and kinetics of early and late photocycle intermediates of the rhodopsin domain wild type and specific DC pair mutants by combined UV-Vis and FTIR spectroscopy at ambient and cryo-temperatures. By assigning specific infrared bands to S-H vibrations of C259 we are able to show that the DC pair residues are tightly coupled. We show that deprotonation of D283 occurs already in the inactive L state as a prerequisite for M state formation, whereas structural changes of C259 occur in the active M state and early cryo-trapped intermediates. We propose a comprehensive molecular model for formation of the M state that activates the catalytic moiety. It involves light induced changes in bond strength and hydrogen bonding of the DC pair residues from the early J state to the active M state and explains the retarding effect of C259 mutants.

Diversity of rhodopsin cyclases in zoospore-forming fungi.

Light perception for orientation in zoospore-forming fungi is linked to homo- or heterodimeric rhodopsin-guanylyl cyclases (RGCs). Heterodimeric RGCs, first identified in the chytrid Rhizoclosmatium globosum, consist of an unusual near-infrared absorbing highly fluorescent sensitizer neorhodopsin (NeoR) that is paired with a visual light-absorbing rhodopsin responsible for enzyme activation. Here, we present a comprehensive analysis of the distribution of RGC genes in early-branching fungi using currently available genetic data. Among the characterized RGCs, we identified red-sensitive homodimeric RGC variants with maximal light activation close to 600 nm, which allow for red-light control of GTP to cGMP conversion in mammalian cells. Heterodimeric RGC complexes have evolved due to a single gene duplication within the branching of Chytridiales and show a spectral range for maximal light activation between 480 to 600 nm. In contrast, the spectral sensitivity of NeoRs is reaching into the near-infrared range with maximal absorption between 641 and 721 nm, setting the low energy spectral edge of rhodopsins so far. Based on natural NeoR variants and mutational studies, we reevaluated the role of the counterion-triad proposed to cause the extreme redshift. With the help of chimera constructs, we disclose that the cyclase domain is crucial for functioning as homo- or heterodimers, which enables the adaptation of the spectral sensitivity by modular exchange of the photosensor. The extreme spectral plasticity of retinal chromophores in native photoreceptors provides broad perspectives on the achievable spectral adaptation for rhodopsin-based molecular tools ranging from UVB into the near-infrared.

NeoR, a near-infrared absorbing rhodopsin.

The Rhizoclosmatium globosum genome encodes three rhodopsin-guanylyl cyclases (RGCs), which are predicted to facilitate visual orientation of the fungal zoospores. Here, we show that RGC1 and RGC2 function as light-activated cyclases only upon heterodimerization with RGC3 (NeoR). RGC1/2 utilize conventional green or blue-light-sensitive rhodopsins (λmax = 550 and 480 nm, respectively), with short-lived signaling states, responsible for light-activation of the enzyme. The bistable NeoR is photoswitchable between a near-infrared-sensitive (NIR, λmax = 690 nm) highly fluorescent state (QF = 0.2) and a UV-sensitive non-fluorescent state, thereby modulating the activity by NIR pre-illumination. No other rhodopsin has been reported so far to be functional as a heterooligomer, or as having such a long wavelength absorption or high fluorescence yield. Site-specific mutagenesis and hybrid quantum mechanics/molecular mechanics simulations support the idea that the unusual photochemical properties result from the rigidity of the retinal chromophore and a unique counterion triad composed of two glutamic and one aspartic acids. These findings substantially expand our understanding of the natural potential and limitations of spectral tuning in rhodopsin photoreceptors.