YB-1 Binds to GluR2 mRNA and CaM1 mRNA in the Brain and Regulates their Translational Levels in an Activity-Dependent Manner

The translational regulator YB-1 binds to mRNAs. In the brain, YB-1 is prominently expressed from the prenatal stage until the first week after birth, being associated with polysomes and distributed in neuronal dendrites, but its expression declines to a much lower level thereafter. It is therefore of interest to identify the mRNAs whose translation is controlled by YB-1 in the postnatal growing brain. In this study we found that YB-1 interacted with the mRNAs for glutamate receptor subunit 2 (GluR2) and calmodulin1 (CaM1) in both brain and NG108-15 cells. Overexpression or knockdown of YB-1 altered the levels of these proteins significantly in cultured cells without any change in their mRNA levels. When the cells were treated with neurotransmitters, translation of these proteins was induced within a short time, and a change in the amount of YB-1 on its target mRNAs was observed in the heavy-sedimenting polysome fractions on a sucrose gradient. Depletion of YB-1 expression by siRNA abrogated the translational activation. Furthermore, in the brain of kainic acid-treated mice, the distribution of YB-1 was shifted to much heavier fractions associated with polysomes within 30 min to 1 h after the treatment, and the distribution returned to lighter fractions within the following 2 h. The protein levels of GluR2 and CaM1 were also increased transiently when the distribution of YB-1 on the gradient changed. These results suggest that in the brain of growing mice, YB-1 binds to GluR2 and CaM1 mRNAs and regulates their translation in an activity-dependent manner.