A dendritic-like microtubule network is organized from swellings of the basal fiber in neural progenitors

Abstract

19Neurons of the neocortex are generated by neural progenitors called radial glial cells. 20These polarized cells extend a short apical process towards the ventricular surface and a long 21 basal fiber that acts as a scaffold for neuronal migration. How the microtubule cytoskeleton is 22 organized in these cells to support long-range transport in unknown. Using subcellular live 23 imaging within brain tissue, we show that microtubules in the apical process uniformly emanate 24 for the pericentrosomal region, while microtubules in the basal fiber display a mixed polarity, 25 reminiscent of the mammalian dendrite. We identify acentrosomal microtubule organizing 26 centers localized in swellings of the basal fiber. We characterize their distribution and 27 demonstrate that they accumulate the minus end stabilizing factor CAMSAP3 and TGN-related 28 membranes, from which the majority of microtubules grow. Finally, using live imaging of 29 human fetal cortex, we show that this organization is conserved in basal radial glial (bRG) cells, 30 a highly abundant progenitor cell population associated with human brain size expansion. 31 32 33

Ariadne’s Thread in the Developing Cerebral Cortex: Mechanisms Enabling the Guiding Role of the Radial Glia Basal Process during Neuron Migration

“…Live imaging of microtubule plus-end markers in rat and mouse organotypic brain slices confirms the existence of an active microtubule network within the basal process [ 54 , 55 , 56 ] ( Figure 4 ). These live imaging studies show that microtubule growth is largely basally directed.…”

“…These live imaging studies show that microtubule growth is largely basally directed. While the nucleation of microtubules is very active in the radial glia soma, and also particularly around the centrosome located in apical endfeet [ 54 , 56 ], other nucleation sites exist within the basal process [ 55 , 56 ]. What are those nucleation sites?…”

“…What are those nucleation sites? Live imaging of organotypic brain slices demonstrates that microtubules can be nucleated within swellings decorating the radial glia basal process [ 56 ] ( Figure 4 ). These swellings, also called varicosities, have been a mysterious structure until a recent study.…”

“…These swellings, also called varicosities, have been a mysterious structure until a recent study. Baffet’s group showed that these structures contain Trans Golgi Network proteins [ 56 ]. The presence of Trans Golgi Network proteins is interesting given that Golgi outposts are known microtubule nucleation sites in neurons [ 57 ].…”

“…The presence of Trans Golgi Network proteins is interesting given that Golgi outposts are known microtubule nucleation sites in neurons [ 57 ]. However, certain Golgi outpost markers are absent from radial glia basal process swellings [ 56 , 58 ], suggesting that nucleation within the basal process differs from that found in neurons.…”