Unique and overlapping functions of GSK-3 isoforms in cell differentiation and proliferation and cardiovascular development

Abstract

Intensive study over the past 30 years has helped define the role of the GSK-3 (glycogen synthase kinase-3) family in a variety of physiological and pathophysiological processes. However, the majority of these studies have relied upon overexpression approaches or nonselective small molecule inhibitors. Herein, we examine recent data derived from studies in gene-targeted embryonic stem cells and knock-out mice in an attempt to define the role these protein kinases play in critical decisions made by stem/progenitor cells and by early lineage-committed cardiomyocytes during development.

FIGURE 1.

Regulators of self-renewal/pluripotency versus differentiation in mouse ES cells. FGF4-mediated activation of ERK drives lineage commitment. LIF and BMP signal maintenance of the self-renewing/pluripotent state, blocking differentiation by interfering with ERK signaling. Critical to the maintenance of self-renewal is inhibition of GSK-3, whether achieved via activation of PI3K/PKB/Akt signaling or by small molecule inhibitors, and this appears to be particularly important when ERK signaling is inhibited (13, 14). Although stabilization of β-catenin and ill defined metabolic effects of GSK-3 inhibition are important, other (Wnt-independent) effects of GSK-3 inhibition, including stabilization of c-Myc and possibly other components driving proliferation over differentiation (e.g. D- and E-type cyclins), may also contribute to maintenance of self-renewal/pluripotency. FGF-R, fibroblast growth factor receptor.

FIGURE 2.

Regulation of cardiomyoblast proliferation/differentiation in the developing heart. Several growth factors, including Nrg1, insulin-like growth factor 1 (IGF-1), retinoic acid, fibroblast growth factors (FGFs), and BMP10, acting through their cognate receptors, regulate cardiomyoblast proliferation in the developing heart (reviewed in Ref. 42). Inhibition of GSK-3β, likely mediated via activation of PI3K and PKB/Akt, appears to be critical to the proliferative response. GSK-3β negatively regulates a number of factors, including transcription factors and cell cycle regulators, via ubiquitination and degradation (Ub/Deg) and/or or nuclear (Nuc) export. Thus, inhibition of GSK-3β appears to connect signaling at the cardiomyoblast membrane to nuclear events driving proliferation. Loss of the negative input contributed by GSK-3β leads to the near obliteration of the left and right ventricle cavities, heart failure, and embryonic death. GSK-3α appears to be able to compensate in the developing heart for loss of GSK-3β as regards regulation of NF-AT (nuclear factor of activated T cells) family members and β-catenin, as evidenced by the normal valve development in the GSK-3β KO (7). It is not clear why GSK-3α and GSK-3β can compensate for loss of the other isoform in some cells and tissues but not in others. This tissue-specific dominant effect is not simply due to relative levels of expression because, as noted above, GSK-3α primarily regulates glycogen storage in the liver, whereas GSK-3β primarily regulates this in skeletal muscle, yet expression levels of the isoforms are comparable in the two tissues. Other possibilities include tissue-specific scaffolds that facilitate binding of one versus the other isoform to specific targets.