Utilizing a culture system for horizontal cells to study neural circuit assembly in the developing mouse retina

The precise wiring of the nervous system relies on neurons extending their processes at the right time and place to find their appropriate synaptic partner. The mechanisms that determine when and where neurons extend their neurites during synaptogenesis remains a central question in the field. In the present study, we used a cell culture system coupled with live imaging to investigate the wiring mechanisms in the developing mouse retina. We focused on horizontal cells which are a class of interneurons in the outer mouse retina known to synapse selectively to the distinct types of photoreceptors. Previous research has shown horizontal cells extend their neurites and make connections to their respective photoreceptor partner in a temporal- and spatial-dependent manner. However, the mechanisms responsible for their selective wiring to photoreceptors during development remains poorly understood. To answer this question, we developed a horizontal cell culture system to investigate the cellular mechanisms responsible for neurite outgrowth during circuit assembly. Our data shows cultured horizontal cells extend neurites with a similar morphology as in vivo. Moreover, neurite extension of horizontal cells is limited to early developmental stages as young mice extend more complex processes compared to those from adolescent retinas. We also found that horizontal cells, unlike retinal ganglion cells, do not extend neurites when cultured alone and require other retinal neurons to promote neurite outgrowth. In summary, we established a horizontal cell culture system that can be used to decipher the mechanisms involved in neural circuit assembly of the mouse retina.