Molecular Identity and Development of Layer IV Thalamorecipient Neurons in the Normal and Disorganized Neocortex
Final Report Abstract
During development of the mammalian central nervous system, neurons are born from two distinct types of germinal zones: The ventricular zone (VZ) is evolutionary old and lines the walls of the ventricular system. For several brain regions like the neocortex, thalamus and ganglionic eminences a second, evolutionary more advanced germinal zone has been described. This sub-ventricular zone (SVZ) is located away from the ventricular cavities and amplifies neuronal expansion, especially during later stages of brain development. Understanding the dynamic relationships between VZ and SVZ-born neurons across brain regions during development is important to understand brain structure and circuit function, yet classical methods of neuronal birth dating using thymidine analogues to label DNA do not distinguish between VZ- and SVZ-born neurons. For our current project, we take advantage of the FlashTag (FT) birth dating approach, developed in Denis Jabaudon’s laboratory in Geneva. VZ-born neurons are specifically labelled by a membrane permeable dye (CFSE), as they are in contact with the ventricular wall at the moment of their birth. By combining FlashTag labeling with chronic BrdU administration, we are generating a dynamic spatio-temporal map of VZ-born neurons between embryonic days (E) 10.5 and E17.5 in the mouse brain. The atlas will be provided as an online resource to systematically investigate the relationship between final spatial position of neurons and their date of birth. It hopefully will allow to connect these features to other neuronal characteristics such as wiring and ultimately function. The FT-method labels time-locked homogenous cohorts of VZ-born neurons and makes them accessible to transcriptional investigations. We were intrigued by our birth dating results in the diencephalic thalamus: at one given birth date, neurons in very different thalamic nuclei are born from one homogenous cohort of thalamic progenitors. We therefore followed the transcriptional sequence of such a cohort in early thalamic development. We performed microdissections of developing thalamic tissue and fluorescence-activated cell sorting and finally single cell RNA-sequencing, in order to access the developing transcriptome of these cells. We are about to unravel the underlying early transcriptional programs in becoming a postmitotic thalamic neuron. In theory, every neuron highlighted in our atlas with its birth date and spatial position is accessible to the same transcriptional investigation, making use of the described techniques. We thus hope that our dynamic online atlas will be equally inspirational for other researchers and a starting point for other exciting functional investigations.
