DEVELOPMENTAL REGULATION OF MAP2 VARIANTS DURING NEURONAL DIFFERENTIATION IN-VITRO

Microtubule-associated protein 2 (MAP2) is a major component of the ne uronal cytoskeleton and is known to promote the assembly and stabiliza tion of microtubules, functions which have important implications in n euronal differentiation. MAP;! 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 a nd protein isoforms during the development of rat cerebellar granule c ell neurons over a 21-day period in vitro. In situ hybridization was u sed to detect MAP2 mRNA isotypes which corresponded to HMW- and LMW-MA P2 proteins. The distribution of MAP2 mRNAs in the developing P7 cereb ellar cortex was related to the different stages of granule neuron dev elopment in situ. During early stages of neuronal differentiation in v itro, high levels of MAP2c mRNA were observed which gradually decrease d as development progressed. Throughout the period studied, MAP2ab mRN A concentrations remained low although a small transient rise was note d during the first 14 days in vitro (div). The profile of MAP2 protein variants showed further developmental regulation. The expression of t he LMW-MAP2c isoform closely mirrored that of its mRNA whilst HMW-MAP2 b protein concentrations rose during the first 10 div and were maintai ned in older cultures. HMW-MAP2a appeared after 4 div and gradually in creased throughout the remainder of the study. Clearly, the outline of HMW-MAP2 protein did not relate to its encoding mRNA and such dispari ty may be due to the operation of different transcriptional and/or pos ttranslational mechanisms. Immunocytochemical analyses of MAP;! varian ts provided further information concerning their localization during n eurite outgrowth. These results describe the developmental regulation of MAP2 mRNA and protein variants and that the profile of their expres sion relates to the formation of processes during the differentiation of granule neurons in vitro.