Neuronal polarity is regulated by glycogen synthase kinase-3 (GSK-3β) independently of Akt/PKB serine phosphorylation

Abstract

An essential step during the development of hippocampal neurons is the polarised outgrowth of a single axon. Recently, it has been suggested that inhibition of glycogen synthase kinase-3β (GSK-3β) via Akt/PKB-dependent phosphorylation of Ser9, specifically at the tip of the presumptive axon, is required for selective axonal outgrowth. We now report that, by using neurons from double knock-in mice in which Ser9 and Ser21 of the two GSK-3β isoforms have been replaced by Ala, polarity develops independently of phosphorylation at these sites. Nevertheless, global inhibition of GSK-3β disturbs polarity development by leading to the formation of multiple axon-like processes in both control and knock-in neurons. This unpolarised outgrowth is accompanied by the symmetric delivery of membrane components to all neurites. Finally, the adenomatous polyposis coli (APC) protein accumulates at the tip of one neurite before and during axon elongation, but global inhibition of GSK-3β leads to APC protein accumulation in all neurites. We conclude that GSK-3β inhibition promotes the development of neuronal polarity, but that this is not mediated by Akt/PKB-dependent phosphorylation.

Protein Kinase Cζ and Glycogen Synthase Kinase-3β Control Neuronal Polarity in Developing Rodent Enteric Neurons, whereas SMAD Specific E3 Ubiquitin Protein Ligase 1 Promotes Neurite Growth But Does Not Influence Polarity

“…In parallel with PI3 kinase/PKC-mediated signaling, localized inhibition of GSK3␤ is also essential for polarized outgrowth of single axons from hippocampal (Shi et al, 2004;Jiang et al, 2005;Yoshimura et al, 2005;Gartner et al, 2006) and dorsal root ganglion (Zhou et al, 2004) neurons and for migration of astrocytes (EtienneManneville and Hall, 2003b). This localized GSK3␤ inhibition appears to be required for adenomatous polyposis coli-and kinesinmediated transport of PAR3 to the plus ends of rapidly growing microtubules (Shi et al, 2004;Zhou et al, 2004).…”

Initiating and Growing an Axon

“…Using double knockin mice bearing single point mutations in GSK-3b S9A and GSK-3a S21A , Gartner et al reported no obvious deficits in neuronal morphogenesis in vivo, or in vitro (Gartner et al 2006). However, using inhibitors of GSK-3a/b such as lithium chloride or, more specifically, SB-415286, SB-216763, and AR-A014418, Gartner et al (2006) were able to replicate the multiple axon phenotype obtained by others (Jiang et al 2005;Garrido et al 2007).…”

“…Using double knockin mice bearing single point mutations in GSK-3b S9A and GSK-3a S21A , Gartner et al reported no obvious deficits in neuronal morphogenesis in vivo, or in vitro (Gartner et al 2006). However, using inhibitors of GSK-3a/b such as lithium chloride or, more specifically, SB-415286, SB-216763, and AR-A014418, Gartner et al (2006) were able to replicate the multiple axon phenotype obtained by others (Jiang et al 2005;Garrido et al 2007). These results indicate that although the exact role of Ser9/Ser21 phosphorylation in GSK-3 inactivation remains to be understood, or may involve an alternate site (Thornton et al 2008), it is clear that the catalytic activity of GSK-3 is a critical regulator of neuronal polarity in these in vitro paradigms.…”

Singar1, a Novel RUN Domain-containing Protein, Suppresses Formation of Surplus Axons for Neuronal Polarity

“…Localization, overexpression, and loss of function studies showed that spatially localized intracellular signals of a number of molecules, such as phosphoinositide-3-kinase (PI 3-kinase) 2 (9), phosphatidylinositol (3,4,5) triphosphate (10), Akt (11), glycogen synthase kinase-3␤ (11)(12)(13), collapsin response mediator protein-2 (CRMP-2) (12,14), mPar3/mPar6/atypical protein kinase C complex (9,15), Cdc42 (15,16), Rap1B (16), STEF/Tiam1 (15,17), Rac (15), adenomatous polyposis coli (18), JNK (19), and DOCK7 (20) are involved in axon specification for neuronal polarity formation. As downstream events, these pathways are thought to regulate cytoskeletal networks of actin filaments (21) and microtubules (20,22).…”