In this study we investigate the effects of the branched-chain keto acids (BCKA) alpha-ketoisocaproic (KIC), alpha-ketoisovaleric (KIV), and alpha-keto-beta-methylvaleric (KMV) acids, metabolites accumulating in maple syrup urine disease (MSUD), on the in vitro phosphorylation of glial fibrillary acidic protein (GFAP) and cytoskeletal reorganization in C6-glioma cells. We observed that after 3 h treatment with KIC, KIV, or KMV cells showed retracted cytoplasm with bipolar processes containing packed GFAP filaments as revealed by immunocytochemistry. Western Blot analysis by anti-GFAP monoclonal antibody demonstrated that BCKA were not able to alter GFAP immunocontent in total cell homogenate, but the immunocontent as well as the in vitro (32)P incorporation into GFAP recovered into the high salt Triton-insoluble cytoskeletal fraction were significantly increased. Western Blot using monoclonal antiphosphoserine antibody showed that BCKA induced increased immunocontent of phosphoserine-containing amino acids in several proteins in total cell homogenate. In addition, the immunocontent of phosphoserine-containing amino acids was also greatly increased in GFAP recovered in the high-salt Triton insoluble cytoskeletal fraction, corresponding to the polymerized intermedite filament (IF) proteins present in the cell. In conclusion, our results indicate that KIC, KIV, or KMV increased the serine/threonine in vitro phosphorylation of GFAP leading to increased Triton-insoluble GFAP immunocontent and cytoskeletal reorganization. Considering IF networks can be regulated by phosphorylation of polypeptide subunits leading to reorganization of the IF filamentous structure, we could suppose that GFAP hyperphosphorylation and disorganization of cellular structure could be involved in the brain damage characteristic of MSUD patients.