Developmental regulation of MAP2 variants during neuronal differentiation in vitro

Microtubule-associated protein 2 (MAP2) is a major component of the neuronal cytoskeleton and is known to promote the assembly and stabilization of microtubules, functions which have important implications in neuronal differentiation. MAP2 consists of high molecular weight (HMW) proteins MAP2a, MAP2b and a low molecular weight (LMW) isoform MAP2c which are produced from a single gene by alternative splicing. In this study, we describe the expression of the various MAP2 mRNA isotypes and protein isoforms during the development of rat cerebellar granule cell neurons over a 21-day period in vitro. In situ hybridization was used to detect MAP2 mRNA isotypes which corresponded to HMW- and LMW-MAP2 proteins. The distribution of MAP2 mRNAs in the developing P7 cerebellar cortex was related to the different stages of granule neuron development in situ. During early stages of neuronal differentiation in vitro, high levels of MAP2c mRNA were observed which gradually decreased as development progressed. Throughout the period studied, MAP2ab mRNA concentrations remained low although a small transient rise was noted during the first 14 days in vitro (div). The profile of MAP2 protein variants showed further developmental regulation. The expression of the LMW-MAP2c isoform closely mirrored that of its mRNA whilst HMW-MAP2b protein concentrations rose during the first 10 div and were maintained in older cultures. HMW-MAP2a appeared after 4 div and gradually increased throughout the remainder of the study. Clearly, the outline of HMW-MAP2 protein did not relate to its encoding mRNA and such disparity may be due to the operation of different transcriptional and/or posttranslational mechanisms. Immunocytochemical analyses of MAP2 variants provided further information concerning their localization during neurite outgrowth. These results describe the developmental regulation of MAP2 mRNA and protein variants and that the profile of their expression relates to the formation of processes during the differentiation of granule neurons in vitro.