Determining the degree of synapse formation and elimination is essential for understanding the structural basis of brain plasticity and pathology. We show that in vivo imaging of dendritic spine dynamics through an open-skull glass window, but not a thinned-skull window, is associated with high spine turnover and substantial glial activation during the first month after surgery. These findings help to explain existing discrepancies in the degree of dendritic spine plasticity observed in the mature cortex.

Highlights d Knockdown of Ptbp1 converts Mu ¨ller glia into retinal ganglion cells in mature retinas d Central projections of converted retinal ganglion cells restore visual responses d Induction of neurons with dopaminergic features in PD model mice

A fundamental feature of the mammalian neocortex is its columnar organization1. In the visual cortex, functional columns consisting of neurons with similar orientation preference have been characterized extensively2-4, but how these columns are constructed during development remains unclear5. The ‘radial unit hypothesis’6 posits that the ontogenetic columns formed by clonally related neurons migrating along the same radial glial fiber during corticogenesis7 provide the basis for functional columns in adult neocortex1. However, direct correspondence between the ontogenetic and functional columns has not been demonstrated8. Here we show that, despite the lack of discernible orientation map in mouse visual cortex4,9,10, sister neurons in the same radial clone exhibit similar orientation preference. Using a retroviral vector encoding green fluorescent protein (GFP) to label radial clones of excitatory neurons and in vivo two-photon calcium imaging to measure the neuronal response properties, we found that sister neurons preferred similar orientations, while nearby non-sisters showed no such relationship. Interestingly, disruption of gap junction coupling by viral expression of a dominant-negative mutant of Cx26 or by daily administration of a gap junction blocker carbenoxolone (CBX) during the first postnatal week greatly diminished the functional similarity between sister neurons, suggesting that the maturation of ontogenetic into functional columns requires intercellular communication through gap junctions. Together with the recent finding of preferential excitatory connections among sister neurons11, our results support the radial unit hypothesis and unify the ontogenetic and functional columns in the visual cortex.

Despite rapid progresses in the genome-editing field, in vivo simultaneous overexpression of multiple genes remains challenging. We generated a transgenic mouse using an improved dCas9 system that enables simultaneous and precise in vivo transcriptional activation of multiple genes and long noncoding RNAs in the nervous system. As proof of concept, we were able to use targeted activation of endogenous neurogenic genes in these transgenic mice to directly and efficiently convert astrocytes into functional neurons in vivo. This system provides a flexible and rapid screening platform for studying complex gene networks and gain-of-function phenotypes in the mammalian brain.