GSK3β also exists as longer splice variants (Mukai et al., 2002; Schaffer et al., 2003). GSK3α and GSK3β are constitutively active, proline-directed serine/threonine kinases involved in a variety of cellular processes including glycogen metabolism (Welsh and Proud, 1993), gene transcription (Troussard et al., 1999), apoptosis (Turenne and Price, 2001) and microtubule stability (Anderton et al., 2001; Brion et al., 2001).
GSK3 activity is negatively regulated by the insulin, Wnt and reelin signaling pathways and GSK3 also plays a pivotal role in the hedgehog signaling cascade. Many, but not all GSK3 substrates require pre-phosphorylation (priming) before phosphorylation by GSK3 can occur.
Insulin signaling activates phosphatidylinositol 3-kinase (PI3-kinase). PI3-kinase is a lipid kinase comprising a regulatory subunit (p85) and a catalytic subunit (p110) (Saltiel and Kahn, 2001; Lizcano and Alessi, 2002). The catalytic subunit of PI3-kinase phosphorylates PtdIns(4,5)P2 leading to the formation of PtdIns(3,4,5)P3.
A key downstream effector of PtdIns(3,4,5)P3 is AKT (otherwise known as Protein kinase B: PKB), which is recruited to the plasma membrane. Activation of AKT requires the protein kinase 3-phosphoinositide-dependent protein kinase-1 (PDPK1), which in combination with an as yet unidentified kinase leads to the phosphorylation of AKT.
Once active, AKT enters the cytoplasm where it triggers the phosphorylation of free cytoplasmic GSK3β and GSK3α at serine residues 9 and 21 respectively rendering the kinases inactive (Saltiel and Kahn, 2001; Lizcano and Alessi, 2002). Regulatory serine phosphorylation results in the generation of an intra-molecular pseudo-substrate, which blocks part of the active site preventing the enzymatic activity of GSK3 towards primed substrates.
This in turn leads to the de-phosphorylation of downstream substrates such as glycogen synthase and eukaryotic protein synthesis initiation factor-2B (eIF-2B) eliciting an increase in glycogen and protein synthesis. (Doble and Woodgett, 2003).
Wnt signaling regulates GSK3 activity by physically displacing complexed GSK3 from its regulatory binding partners, in the so called destruction complex, consequently preventing the phosphorylation and degradation of β-catenin.
In the absence of Wnt, the signaling pool of β-catenin is maintained at low levels through degradation (Dale, 1998;Huelsken and Behrens, 2002;Nusse, 2005). β-catenin is targeted for ubiquitination by the β-transducin repeat containing protein (βTrCP) and is then degraded by the proteosome. β-catenin is phosphorylated by the serine/threonine kinases casein kinase 1 (CK1) and GSK3β. Phosphorylation of β-catenin occurs in a multi-protein complex (the destruction complex), comprising axin, adenomatous polyposis coli (APC) and diversin.
Wnt ligands bind to Frizzled/LRP5-6 receptor complexes at the cell surface, which couple to dishevelled inducing the recruitment of GBP/FRAT1 to the destruction complex, which in turn displaces GSK3β; precluding the phosphorylation and degradation of β-catenin. Stabilized β-catenin is then free to enter the nucleus and associates with T cell factor (TCF)/lymphoid enhancer factor (LEF) transcription factors leading to the transcription of Wnt target genes.
GSK3α as well as GSK3β can function in the Wnt signaling pathway and destruction complex, suggesting that GSK3α is equally as important in Wnt biology as GSK3β (Asuni et al., 2006; Doble et al., 2007). GSK3β and GSK3α can also be regulated by tyrosine (Tyr) phosphorylation at residues 216 or 279 respectively. Normally, GSK3 is phosphorylated at these sites; however increases in Tyr phosphorylation augment GSK3 activity (Bhat et al., 2000; Bijur and Jope, 2001).
Reelin binds to the very-low density lipoprotein receptor (VLDLR) and to the Apolipoprotein E receptor 2 (APOER2) and induces the activation of disabled-1 (DAB1), a cytoplasmic adaptor protein that interacts with NPxY motifs in both receptor tails (Herz and Chen, 2006; Tissir and Goffinet, 2003).
The clustering of DAB1 activates SRC family tyrosine kinases (SFKs), which potentiates tyrosine phosphorylation of DAB1. Phosphorylated DAB1 further activates PI3-kinase and subsequently AKT, which in turn inhibits the activity of GSK3β. As a result, phosphorylation of tau is reduced, thus promoting microtubule stability.
The Hedgehog gene was first identified in Drosophila melanogaster (Nusslein-Volhard et al., 1980). In Drosophila, Hedgehog signaling is initiated by the binding of Hedgehog ligand to Patched (Ptc), which is a 12-transmembrane protein receptor (Hooper and Scott 2005; Østerlund and Kogerman 2006). Ptc acts as an inhibitor of Smoothened (Smo), a 7-transmembrane protein.
Downstream of Smo is a multi-protein complex known as the Hedgehog signaling complex (HSC), which comprises the transcription factor Cubitus interruptus (Ci), the serine/threonine kinase Fused (Fu), the kinesin-like molecule Costal 2 (Cos2) and the Supressor of fused (Sufu). Cos2 also binds to protein kinase A (PKA), protein kinase CK1 (formerly casein kinase 1) and GSK3.
In the absence of ligand, Ptc represses Smo preventing the activation of Hedgehog signaling (Hooper and Scott 2005; Østerlund and Kogerman 2006). The HSC is bound to microtubules/membranes and associates with Smo through Cos2. The full length form of Ci is prevented from nuclear translocation through interactions with Sufu and Cos2.
A portion of full length Ci is proteolytically cleaved to produce a repressor form of Ci, which enters the nucleus leading to the inhibition of Hedgehog target gene expression. Proteolytic processing of Ci is mediated by PKA, CK1 and GSK3. In the presence of Hedgehog, the inhibitory effects of Ptc on Smo are relieved and the HSC is freed from microtubules and membranes.
Smo becomes phosphorylated by PKA and CK1 and PKA, CK1 and GSK3 are released from Cos2, precluding the generation of the repressor from of Ci. Full length Ci is no longer inhibited by Sufu and is therefore free to enter the nucleus to induce the transcription of Hedgehog target genes.
The Hedgehog signaling pathways in vertebrates share many common features with Drosophila Hedgehog signaling, although distinct differences are also apparent (Hooper and Scott 2005). In mammals there are three Hedgehog genes, Sonic, Indian and Desert Hedgehog. There are also 2 Ptc genes (Ptc 1 and Ptc 2) as well as three Ci homologues known as Gli1, Gli2 and Gli3. Gli1 is a transcriptional activator as is Gli2, whereas Gli3 functions as a transcriptional repressor.
GSK3 plays a pivotal role in the pathology of Alzheimer’s disease (AD), being involved in memory impairment at the synaptic level, tau hyper-phosphorylation and neurofibrillary tangle formation (NFT) as well as the increased production of β-amyloid (Aβ) and hence senile plaque deposition (Hooper et al., 2008).
GSK3 displays enhanced activity in the frontal cortex in AD (Leroy et al., 2007) and is up-regulated in peripheral lymphocytes in both AD and in mild cognitive impairment (MCI) (Hye et al., 2005). GSK3 is also implicated in the pathology of Schizophrenia and psychosis (Lovestone et al., 2007).
GSK3 is regulated by the neurotransmitter dopamine. Augmented dopamine levels lead to the reduction of inhibitory serine phosphorylation of GSK3 increasing its enzymatic activity; a phenomenon that is mediated by D2 receptors (Beaulieu et al., 2004, 2007). D2 receptors stimulate the assembly of a complex containing β-arrestin 2, protein phosphatase 2A (PP2A), AKT and probably GSK3. PP2A negatively regulates AKT, which in turn augments GSK3 activity.