Our ultimate goal in the Multiscale Sensory Structures Laboratory is to understand how brain function is created based on mechanisms at lower levels of hierarchy, including genes, molecules, cell types, circuits, and neural computation. Such hierarchical longitudinal studies will allow us to identify the cell types and circuits responsible for neurological diseases that may be subject to genetic repair, and furthermore, to identify genomic differences that are responsible for species-specific brain functions. The knowledge gained from these species comparisons will be particularly useful in understanding human systems and diseases using mouse models. Directionally selective neural circuits in the retina are ideal models for studying neural circuit computation at various levels of hierarchy, including genes, synapses, circuits, and behavioral control.
1. Emergence of spatial asymmetries in neural circuits
The animal body is formed in an orderly fashion along the body axis by a strict genetic program. Neural connections also have specificity along the body axis, and in particular spatial asymmetry of connections is a fundamental component of neuronal processing. Direction-selective (DS) cells in the retina have the property of responding selectively to visual motion direction and are involved in the detection of optic flow and optokinetic eye movements (Yonehara et al., PLoS One 2009). Starburst cell processes in the retina make an angle-dependent inhibitory coupling of processes to four types of DS cells (each with selectivity for dorsal, ventral, nasal, and ear directions), and this asymmetric inhibitory coupling is one of the key mechanisms underlying directional selectivity in the retina. This asymmetric coupling develops before eye-opening at 1 to 2 weeks of age (Yonehara et al., Nature 2011).
Previously, we found that Frmd7, a known causative gene of congenital nystagmus, is essential for the development of horizontally asymmetric inhibitory connections (Yonehara et al., Neuron 2016). In addition, we have identified several other important plasma membrane proteins and transcription factors (unpublished). Our future goal is to use these molecules as a starting point to identify completely new inter-synaptic and intracellular signaling pathways involved in the development of asymmetric connections in mammalian central nervous system neuronal circuits. To achieve these goals, we will combine a variety of techniques, including large-scale mouse genetics, biochemistry, cell biology, transcriptomics, proteomics, electrophysiology, and two-photon functional imaging, using the NIG's excellent animal breeding facilities.
2. Sensory processing by retinal neural circuits
We have previously identified important neural circuit mechanisms for visual motor information processing in the mammalian retina. First, we performed two-photon glutamate imaging from the retina to identify space-time-wiring from excitatory interneurons, bipolar cells, to DS cells, and showed that this excitatory circuit motif is important for the formation of velocity tuning in DS cells ( Matsumoto et al., Curr Biol 2019). Next, we performed the first two-photon acetylcholine imaging from retinal DS cell dendrites and showed that the balance of excitatory/inhibitory inputs at the microsegmental level of DS cell dendrites is important for the calculation of directional selectivity (Sethuramanujam et al, Nat Commun 2021). More recently, we made the unexpected discovery that direction selectivity in the retina has already been calculated in bipolar cell axon terminals (Matsumoto et al., Neuron 2021). Since axons are often thought of as simple conduits for transmitting signals, it was a surprising finding that tuning to sensory stimuli occurs de novo in axon terminals. We will continue to perform two-photon imaging and patch-clamp recordings of calcium and neurotransmitter release from genetically labeled cell types to gain a comprehensive understanding of neural circuit computation in the retina.
Two-photon imaging system for recording optical responses from explanted retinas
3. Visual-motor transformation in the superior colliculus
The mouse superior colliculus is an ideal model system for understanding how visual feature signals extracted by retinal neural circuits and encoded in spike trains of 40 different ganglion cell types are translated into visually dependent innate behaviors in the visual center. To date, we have identified at least five novel mouse lines in which a single morphological cell type is labeled by Cre-recombinase (unpublished). Future work will include in vivo two-photon imaging under head position immobilization, imaging with a microendoscope mounted above the head under free movement, transsynaptic labeling, and behavioral analysis to elucidate the function of cell types and circuits in the superior colliculus.
At the National Institute of Genetics we will also start a new analysis of the genetic mechanism of environmental adaptation of visual neural circuits. In collaboration with Dr. Tsuyoshi Koide, we will compare visually dependent behaviors (such as prey hunting and escape defense behaviors) of various mouse strains from the wild-collected from all over the world by tracking with DeepLabCut (is there a difference in the visual stimuli that cause escape defense behaviors in areas where owls are natural enemies and areas where foxes are natural enemies? Isn't it?) ). Next, quantitative trait loci analysis will be used to narrow down the loci and genes responsible for the differences in behavior, and their gene expression will be mapped histologically to reveal the cell types and circuits underlying environmental adaptation.
4. Sensory processing in the visual cortex
We have shown that signals from retinal DS cells selectively reach the RL of the higher visual cortex (Rasmussen and Matsumoto et al., Nat Commun 2020), and that the combination of signals from retinal DS cells from both eyes in this region forms the responsiveness to rotational optical flow (Rasmussen and Matsumoto et al., Curr Biol 2021). (Rasmussen and Matsumoto et al., Curr Biol 2021), and that the combination of signals from retinal DS cells from both eyes in this region results in the formation of responsiveness to rotational optic flow (Rasmussen and Matsumoto et al., Curr Biol 2021). We will continue to study the circuit mechanism of motor information processing in RL and its contribution to behavior.
5. Cell types and circuits of the marmoset visual system
What functional cell types are present in the primate retina and epiretinal membrane are not comprehensively understood. We will comprehensively identify the physiological types of retinal ganglion cells in the marmoset retina by ex vivo two-photon calcium imaging and unsupervised clustering. Second, by utilizing the well-equipped core facilities of NIG, we will conduct single-cell transcriptome analysis of mouse and marmoset epiretinal cortex for interspecies comparison. The information obtained will provide an important stepping stone for future studies of the role of species-specific cell types in visual computation and behavior.
6. RNA-based strategies for treating genetic diseases
About 20% of genetic diseases are caused by nonsense mutation. By synthesizing artificial suppressor-tRNAs or snoRNAs, we aim to develop an efficient readthrough strategy that can be used for treating genetic diseases with nonsense mutation. We also develop animal models to use as an in vivo readthrough assay system, which will facilitate high-throughput screening of RNA variants. Our target diseases include blindness, muscular dystrophy, heart failure, and special types of cancer.