Distinct Molecular Regulation of Glycogen Synthase Kinase-3α Isozyme Controlled by Its N-terminal Region

Abstract

Glycogen synthase kinase-3 (GSK-3) is expressed as two isozymes ␣ and ␤. They share high similarity in their catalytic domains but differ in their N-and C-terminal regions, with GSK-3␣ having an extended glycine-rich N terminus. Here, we undertook live cell imaging combined with molecular and bioinformatic studies to understand the distinct functions of the GSK-3 isozymes focusing on GSK-3␣ N-terminal region. We found that unlike GSK-3␤, which shuttles between the nucleus and cytoplasm, GSK-3␣ was excluded from the nucleus. Deletion of the N-terminal region of GSK-3␣ resulted in nuclear localization, and treatment with leptomycin B resulted in GSK-3␣ accumulation in the nucleus. GSK-3␣ rapidly accumulated in the nucleus in response to calcium or serum deprivation, and accumulation was strongly inhibited by the calpain inhibitor calpeptin. This nuclear accumulation was not mediated by cleavage of the N-terminal region or phosphorylation of GSK-3␣. Rather, we show that calcium-induced GSK-3␣ nuclear accumulation was governed by GSK-3␣ binding with as yet unknown calpain-sensitive protein or proteins; this binding was mediated by the N-terminal region. Bioinformatic and experimental analyses indicated that nuclear exclusion of GSK-3␣ was likely an exclusive characteristic of mammalian GSK-3␣. Finally, we show that nuclear localization of GSK-3␣ reduced the nuclear pool of ␤-catenin and its target cyclin D1. Taken together, these data suggest that the N-terminal region of GSK-3␣ is responsible for its nuclear exclusion and that binding with a calcium/calpain-sensitive product enables GSK-3␣ nuclear retention. We further uncovered a novel link between calcium and nuclear GSK-3␣-mediated inhibition of the canonical Wnt/␤-catenin pathway.

Dynamic range of GSK3α not GSK3β is essential for bidirectional synaptic plasticity at hippocampal CA3‐CA1 synapses

“…However, nonphysiological levels of GSK3 may have various unforeseen knock‐on effects, especially between the isoforms. Distinct cellular localisation of GSK3α has been demonstrated, which would be consistent with functions independent of the β isoform (Azoulay‐Alfaguter et al, ). Yet, most previous studies have largely ignored the potential role of GSK3α, perhaps because its expression levels are lower than GSK3β and decrease with age (Giese, ).…”

Targeted Disruption of Glycogen Synthase Kinase 3a (Gsk3a) in Mice Affects Sperm Motility Resulting in Male Infertility1

“…While GSK3A is present in several species, its unique glycine rich N-terminus is highly conserved only in mammals. Therefore, it appears likely that the N-terminus could be responsible for an isoform-specific function for GSK3A in mammals [56,57]. Whether our observations suggest that GSK3A has an isoform specific function during spermatogenesis and in mature sperm or whether the lack of substitution of GSK3B for GSK3A in sperm is a gene dosage effect remains to be verified.…”

β-N-methylamino-L-alanine induces changes in both GSK3 and TDP-43 in human neuroblastoma

“…In the same figure, it can be appreciated that an increase in nuclear GSK3α occurs. Although in recent years GSK3β has been the main isoform related with neurodegeneration, GSK3α has also been described to play a possible role in the pathological process such as ALS and AD (Chung et al, 2008;Azoulay-Alfaguter et al, 2011). In order to determine whether L-BMAA treatment may result in altered forms of TDP-43, appearing frequently in neurodegenerative diseases forming pathological aggregates, high molecular weight, phosphorylated and truncated forms were analyzed.…”