Abstract
Neurodegeneration is often associated with DNA synthesis in neurons, the latter usually remaining for a long time as tetraploid cells before dying by apoptosis. The molecular mechanism preventing G2/M transition in these neurons remains unknown, but it may be reminiscent of the mechanism that maintains tetraploid retinal ganglion cells (RGCs) in a G2-like state during normal development, thus preventing their death. Here we show that this latter process, known to depend on brain-derived neurotrophic factor (BDNF), requires the inhibition of cdk1 by TrkB. We demonstrate that a subpopulation of chick RGCs previously shown to become tetraploid co-expresses TrkB and cdk1 in vivo. By using an in vitro system that recapitulates differentiation and cell cycle re-entry of chick retinal neurons we show that BDNF, employed at concentrations specific for the TrkB receptor, reduces the expression of cdk1 in TrkB-positive, differentiating neurons. In this system, BDNF also inhibits the activity of both endogenous cdk1 and exogenously-expressed cdk1/cyclin B1 complex. This inhibition correlates with the phosphorylation of cdk1 at Tyr15, an effect that can be prevented with K252a, a tyrosine kinase inhibitor commonly used to prevent the activity of neurotrophins through their Trk receptors. The effect of BDNF on cdk1 activity is Tyr15-specific since BDNF cannot prevent the activity of a constitutively active form of cdk1 (Tyr15Phe) when expressed in differentiating retinal neurons. We also show that BDNF-dependent phosphorylation of cdk1 at Tyr15 could not be blocked with MK-1775, a Wee1-selective inhibitor, indicating that Tyr15 phosphorylation in cdk1 does not seem to occur through the canonical mechanism observed in proliferating cells. We conclude that the inhibition of both expression and activity of cdk1 through a BDNF-dependent mechanism contributes to the maintenance of tetraploid RGCs in a G2-like state.
Highlights
Reactivation of cell cycle and DNA synthesis in neurons represents a common feature of certain neuropathological states [1], including Alzheimer’s disease (AD) and ischemia/hypoxia [2,3,4,5]
In this study we have described two mechanisms used by brain-derived neurotrophic factor (BDNF) to prevent G2/M transition in tetraploid neurons, which rely on the reduction of cdk1 and cyclin B1 [17] expression, as well as the induction of Tyr15 phosphorylation in cdk1, inhibiting its kinase activity (Fig. 10)
We have demonstrated the presence of cdk1 in cells that undergo ectopic mitosis at the basal portion of the retina, previously identified as differentiating retinal ganglion cells (RGCs) that have reactivated the cell cycle and are about to undergo apoptosis [15], indicating that the common mechanism for G2/M transition can be active in these cells
Summary
Reactivation of cell cycle and DNA synthesis in neurons represents a common feature of certain neuropathological states [1], including Alzheimer’s disease (AD) and ischemia/hypoxia [2,3,4,5]. Cell cycle re-entry in these neurons occurs as they migrate from the apical portion of the neuroepithelium, where they are born, to the basal neuroepithelium, where the ganglion cell layer (GCL) arises [15] These neurons are known to express E2F1 and E2F4 in the absence of retinoblastoma protein (Rb) and, after DNA duplication, they remain in a G2-like state in the GCL [12,15]. Cell cycle reentry in differentiating RGCs and maintenance of these cells in a G2-like state can be considered as part of a physiological process taking place in the developing nervous system aimed at inducing somatic tetraploidy in specific neuronal types [15,20,21] Overall, these observations are compatible with BDNF being responsible for the maintenance in a G2-like state of pathologically-generated tetraploid neurons, preventing their death [22]
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