1. A method of reducing activity of GSK3? in a cell, the method comprising:contacting a cell with a composition comprising an isolated GSK3? polypeptide and a carrier, wherein the polypeptide is a full-length GSK3? protein or a fragment of a full-length GSK3? protein and comprises a phosphorylated residue that corresponds to residue Thr390 in a full-length, wild-type, human GSK3? amino acid sequence, or corresponds to residue Ser389 in a full-length, wild-type, mouse GSK3? amino acid sequence, wherein the contacting results in the isolated GSK3? polypeptide in the cell and reduces GSK3? activity in the cell.