Establishment of high reciprocal connectivity between clonal cortical neurons is regulated by the Dnmt3b DNA methyltransferase and clustered protocadherins

Abstract

BackgroundThe specificity of synaptic connections is fundamental for proper neural circuit function. Specific neuronal connections that underlie information processing in the sensory cortex are initially established without sensory experiences to a considerable extent, and then the connections are individually refined through sensory experiences. Excitatory neurons arising from the same single progenitor cell are preferentially connected in the postnatal cortex, suggesting that cell lineage contributes to the initial wiring of neurons. However, the postnatal developmental process of lineage-dependent connection specificity is not known, nor how clonal neurons, which are derived from the same neural stem cell, are stamped with the identity of their common neural stem cell and guided to form synaptic connections.ResultsWe show that cortical excitatory neurons that arise from the same neural stem cell and reside within the same layer preferentially establish reciprocal synaptic connections in the mouse barrel cortex. We observed a transient increase in synaptic connections between clonal but not nonclonal neuron pairs during postnatal development, followed by selective stabilization of the reciprocal connections between clonal neuron pairs. Furthermore, we demonstrate that selective stabilization of the reciprocal connections between clonal neuron pairs is impaired by the deficiency of DNA methyltransferase 3b (Dnmt3b), which determines DNA-methylation patterns of genes in stem cells during early corticogenesis. Dnmt3b regulates the postnatal expression of clustered protocadherin (cPcdh) isoforms, a family of adhesion molecules. We found that cPcdh deficiency in clonal neuron pairs impairs the whole process of the formation and stabilization of connections to establish lineage-specific connection reciprocity.ConclusionsOur results demonstrate that local, reciprocal neural connections are selectively formed and retained between clonal neurons in layer 4 of the barrel cortex during postnatal development, and that Dnmt3b and cPcdhs are required for the establishment of lineage-specific reciprocal connections. These findings indicate that lineage-specific connection reciprocity is predetermined by Dnmt3b during embryonic development, and that the cPcdhs contribute to postnatal cortical neuron identification to guide lineage-dependent synaptic connections in the neocortex.Electronic supplementary materialThe online version of this article (doi:10.1186/s12915-016-0326-6) contains supplementary material, which is available to authorized users.

Regulation of neural circuit formation by protocadherins

“…Thus, it seems clear that, in addition to mediating trans homophilic adhesion themselves, Pcdhs can act in cis to regulate the stability or turnover of other proteins with synaptic functions. At the same time, the clustered Pcdhs also appear to be important for reciprocal connectivity between cortical “sister neurons” derived from the same progenitor [109], so it is possible that Pcdhs help stabilize only the “proper” synaptic connections during development.…”

“…Finally, an interesting report by Tarusawa et al [109] presents evidence that the clustered Pcdhs are important for the preferential synaptic connections made by ‘sister neurons’ deriving from the same cortical progenitor and found in a clonal column. These authors generated chimeric mice using GFP+ induced pluripotent stem (iPS) cells derived from mice lacking either the DNA methyltransferase Dnmt3b, or all 3 Pcdh clusters (Pcdha, Pcdhb, and Pcdhg), whose genes are known to be regulated by Dnmt3b [105].…”

Distinct and Cooperative Functions for the Protocadherin-α, -β and -γ Clusters in Neuronal Survival and Axon Targeting

“…We retrieved the 16-kb BAC DNA fragments that contained the Myc-tagged Venus and floxed neo r genes, and inserted them into pBRSDT. The retrieved plasmid was used as a targeting vector (Tarusawa et al, 2016). …”

The development of local circuits in the neocortex: recent lessons from the mouse visual cortex

“…For example, a recent study implicated Dnmt3b DNA methyltransferase in stabilizing reciprocal chemical connections among clonally related layer 4 sister neurons in somatosensory cortex [47]. The authors proposed that methylation patterns influence the expression patterns of clustered protocadherins, cell adhesion molecules thought to play roles in self-recognition and non-self-recognition [47].…”