The polarization of neurons, which mainly includes the differentiation of axons and dendrites, is regulated by cell-autonomous and non-cell-autonomous factors. In the developing central nervous system, neuronal development occurs in a heterogeneous environment that also comprises extracellular matrices, radial glial cells, and neurons. Although many cell-autonomous factors that affect neuronal polarization have been identified, the microenvironmental cues involved in neuronal polarization remain largely unknown. Here, we show that neuronal polarization occurs in a microenvironment in the lower intermediate zone, where the cell adhesion molecule transient axonal glycoprotein-1 (TAG-1) is expressed in cortical efferent axons. The immature neurites of multipolar cells closely contact TAG-1-positive axons and generate axons. Inhibition of TAG-1-mediated cell-to-cell interaction or its downstream kinase Lyn impairs neuronal polarization. These results show that the TAG-1-mediated cell-to-cell interaction between the unpolarized multipolar cells and the pioneering axons regulates the polarization of multipolar cells partly through Lyn kinase and Rac1.

Highlights d More than 400 CBP-interacting proteins, including Npas4, are identified d MAPK phosphorylates Npas4 and increases the interaction between Npas4 and CBP d Phosphorylation of Npas4 enhances the transcriptional activity of Npas4 d Phosphorylation of Npas4 in D1R-MSNs regulates rewardrelated learning and memory

Dopamine type 1 receptor (D1R) signaling activates protein kinase A (PKA), which then activates mitogen‐activated protein kinase (MAPK) through Rap1, in striatal medium spiny neurons (MSNs). MAPK plays a pivotal role in reward‐related behavior through the activation of certain transcription factors. How D1R signaling regulates behavior through transcription factors remains largely unknown. CREB‐binding protein (CBP) promotes transcription through hundreds of different transcription factors and is also important for reward‐related behavior. To identify transcription factors regulated by dopamine signaling in MSNs, we performed a phosphoproteomic analysis using affinity beads coated with CBP. We obtained approximately 40 novel candidate proteins in the striatum of the C57BL/6 mouse brain after cocaine administration. Among them, the megakaryoblastic leukemia‐2 (MKL2) protein, a transcriptional coactivator of serum response factor (SRF), was our focus. We found that the interaction between CBP and MKL2 was increased by cocaine administration. Additionally, MKL2, CBP and SRF formed a ternary complex in vivo. The C‐terminal domain of MKL2 interacted with CBP‐KIX and was phosphorylated by MAPK in COS7 cells. The activation of PKA‐MAPK signaling induced the nuclear localization of MKL2 and increased SRF‐dependent transcriptional activity in neurons. These results demonstrate that dopamine signaling regulates the interaction of MKL2 with CBP in a phosphorylation‐dependent manner and thereby controls SRF‐dependent gene expression. Cover Image for this issue: https://doi.org/10.1111/jnc.15067.

Dopamine (DA) activates MAPK via PKA/Rap1 in medium spiny neurons (MSNs) expressing the dopamine D1 receptor (D1R)in the nucleus accumbens (NAc), thereby regulating reward-related behavior.However, howMAPKregulates reward-relatedlearning and memory through gene expression is poorly understood. Here, to identify the relevant transcriptional factors, we performed proteomic analysis using affinity beads coated with CREBbinding protein (CBP), a transcriptional coactivator involved in reward-related behavior. We identified more than 400 CBP-interacting proteins, including Neuronal Per Arnt Sim domain protein 4 (Npas4). We found that MAPK phosphorylated Npas4 downstream of PKA, increasing the Npas4-CBP interaction and the transcriptional activity of Npas4 at the brain-derived neurotrophic factor (BDNF) promoter. The deletion of Npas4 in D1R-expressing MSNs impaired cocaine-induced place preference, which was rescued by Npas4-WT but not by a phospho-deficient Npas4 mutant. These observations suggest that MAPK phosphorylates Npas4 in D1R-MSNs and increases transcriptional activity to enhance reward-related learning and memory. (Funahashi et al., Cell Reports, 2019) YIA

Dopamine (DA) is necessary for motor function, motivation, working memory, and reward. However, how DA regulates reward-related learning and memorythrough the gene expression is not fully understood. Neuronal Per Arnt Sim domain protein 4 (Npas4), a brain-specific basic helix-loop-helix transcription factor, plays a role in synaptic plasticity by regulating the expression of activity-dependent genes,such as BDNF.Although Npas4 required forcontextual fear memory formation in mice, the role of Npas4 in reward-relatedlearning and memoryremains unknown. To this purpose, we used the conditioned place preference (CPP) paradigm in which animals learn to prefer a context associated with cocaine. Here, we found that the deletion of Npas4 in the accumbal D1R-expressing MSNs (D1R-MSNs) suppressedCPP. This phonotype was rescued by the D1R-MSNs specific expression of Npas4-WT but not phospho-deficient Npas4 mutant. These results suggestthat Npas4 and its phosphorylation regulate reward-related learning and memory in the accumbal D1R-MSNs.