MAP2d mRNA is expressed in identified neuronal populations in the developing and adult rat brain and its subcellular distribution differs from that of MAP2b in hippocampal neurones

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Abstract
The brain microtubule‐associated protein MAP2 family is composed of high‐molecular‐weight (MAP2a and MAP2b) and low‐molecular‐weight (MAP2c and MAP2d) isoforms. The common C‐terminal region of HMW MAP2 and MAP2c contains three repeated microtubule‐binding domains while MAP2d comprises four repeats. MAP2c mRNA is known to be expressed at high levels in the immature brain. We show that in the brains of rat pups, MAP2c mRNAs are indeed expressed at high levels compared with MAP2d. However, in adult rat brains, MAP2d mRNA levels are higher than MAP2c. In order to identify the neural cells expressing MAP2d, we used in situ hybridization. In vivo, we show that MAP2d mRNA is expressed in well‐identified neuronal populations of the brain. In primary cultures of hippocampal neurones, double‐labelling experiments confirm that MAP2d is clearly expressed in neurones. We also evaluated in this study the subcellular distribution of the MAP2d mRNAs in cultured hippocampal neurones and we report that in contrast with MAP2b mRNAs, mostly localized in dendrites, MAP2d mRNAs are essentially located in neuronal cell bodies.