Periodic Reporting for period 4 - CIRCUITASSEMBLY (Development of functional organization of the visual circuits in mice)
Reporting period: 2019-10-01 to 2020-03-31
how do they contribute to behavior? Second, what are the molecular and synaptic mechanisms that underlie developmental circuit plasticity for forming parallel topographic neuronal maps in the brain? Third, what are the molecular mechanisms that set up spatially asymmetric circuit connectivity without the need for sensory experience? I predict that my insights into the developmental mechanism of asymmetric, parallel, and topographic connectivity and circuit plasticity will be instructive when studying other brain circuits which contain similar organizing principles.
Impairment of neuronal computation, neuronal connectivity or synaptic physiology would lead to psychiatric and neurological disorders. Efficient treatment of such disorders does not exist, and the disorders burden human society with significant medical and social expenses. Gained mechanistic insight into function and development of nervous system would be very helpful for understanding and developing treatment for such diseases in future.
We have done retrograde mono-trans-synaptic tracing from the 4 Cre mouse lines. This is a first demonstration of cell type-to-cell type connectivity pattern between retina and superior colliculus in mammals, providing insight into how information about individual visual features could be processed by dedicated neural circuits in the visual center. Currently we are performing confocal scanning of labelled ganglion cells to reconstruct dendritic structure and classifying into distinct ganglion cell types.
For the investigation of function of cell types in visually-guided behaviour, we have built an automated optomotor and optokinetic reflex assessment box, a looming-dependent escaping behavior assessment box, and prey capture measurement system. We have ablated Cre-labeled collicular cell types with Cre-dependent AAV expressing diphtheria toxin A (DTA) and are testing the effect of cell type ablation on these behaviors.
Aim 2. We have established galvo scanning 2-photon imaging system in my laboratory for neonatal in vivo imaging. We have characterized dendritic and axonal morphology of Cre-labeled cell types in superior colliculus in neonatal transgenic mouse lines by targeting the cell types with transgenically expressed fluorescent markers followed by immunohistochemistry and confocal scanning. We confirmed that in most of the transgenic mouse lines the cell types are already labelled in neonatal periord.
Aim 3. We have established galvo scanning two photon imaging system combined with LED visual stimulation system and microelectrode array spike recording system in my laboratory for exo vivo retinal recordings. We have identified Frmd7 knockout mice in which horizontal optokinetic reflex is lost and retinal direction selectivity is lost along horizontal axis, and this work was published in Neuron last year as my first authored paper (Yonehara et al., 2016). Expression of Frmd7 is localized to starburst amacrine cells in mice and primates, and mutation of Frmd7 leads to congenital nystagmus, accompanied by lack of horizontal optokinetic reflex, in human. We will analyse Frmd7 mice further to understand the molecular mechanisms of circuit assembly of retinal direction-selective circuits.
By understanding cell type basis of visual processing and visually guided behaviors mediated by superior colliculus, we expect that we will understand principles underlying how neuronal inputs from retina is processed by central target to evoke ethologically relevant visually guided behaviors. Such insight would add better understanding of how our visual system works and how impairment of specific cell type or circuits could lead to dysfunction of selective symptom in visual disorders such as cerebral visual impairment, which is a leading cause for blindness in children.
By understanding how topographic projection from retina to superior colliculus is established and what is the cell type and synaptic basis for its developmental mechanism, we aim to understand how our brain circuit is shaped by genetic mechanisms and activity-dependent mechanisms. Particularly, we aim to understand molecular basis of activity-dependent synaptic formation. Gained knowledge would provide a hint into how we could restore plasticity of neuronal circuits and ability to learn and memorize, which is impaired in many neurological diseases such as Alzheimer´s disease.