An alternatively spliced form of glycogen synthase kinase-3β is targeted to growing neurites and growth cones

The serine/threonine kinase glycogen synthase kinase-3β (GSK-3β) is expressed in two, alternatively spliced, isoforms: a short form (GSK-3β1) and a long form containing a 13 amino acid insert in the catalytic domain (GSK-3β2). We examined the expression of these isoforms in the rat using specific antibodies and found that GSK-3β2, in contrast to GSK-3β1, is only expressed in the nervous system. The highest levels of GSK-3β2 are found in the developing nervous system but expression persists into adulthood. In the adult central nervous system the highest expression of GSK-3β2 occurs in regions with a high proportion of white matter, suggesting that GSK-3β2 is expressed in axons. Consistent with this finding, sub-cellular fractionation of neonatal rat brain showed that GSK-3β2 is present in fractions enriched in neurites and growth cones. Furthermore, we found that when we separated neuronal cell bodies from neurites by culturing embryonic cortical neurons in neurite outgrowth inserts, GSK-3β2 was present in both compartments. Finally, a rabbit polyclonal antibody raised to the 13 amino acid insert of GSK-3β2 (anti-8A) that specifically recognises GSK-3β2, labels the cell body, including the nucleus, neurites and growth cones of embryonic neurons in culture. To compare functionally the two isoforms, we performed in vitro kinase assays. These showed that GSK-3β1 is more efficient at phosphorylating the microtubule-associated protein MAP1B than GSK-3β2, consistent with previous findings with the microtubule-associated protein tau. However, when co-expressed with MAP1B in COS-7 cells, both GSK-3β isoforms equally efficiently phosphorylated MAP1B and had a similar influence on the regulation of microtubule dynamics by MAP1B in these cells. We conclude that the alternatively spliced isoform of GSK-3β, GSK-3β2, is neuron-specific and has overlapping activities with GSK-3β1.