Id1 and Sonic Hedgehog Mediate Cell Cycle Reentry and Apoptosis Induced by Amyloid Beta-Peptide in Post-mitotic Cortical Neurons

Abstract

Amyloid beta-peptide (Aβ), the neurotoxic component of senile plaques in Alzheimer's disease (AD) brains, is known to trigger cell cycle reentry in post-mitotic neurons followed by apoptosis. However, the underlying mechanisms remain unclear. Recently, we have reported that Aβs stimulate the expression of inhibitor of differentiation-1 (Id1) to induce sonic hedgehog (SHH) (Hung et al., Mol Neurobiol 53(2):793-809, 2016), and both are mitogens capable of triggering cell cycle progression. In this work, we tested the hypothesis that Aβ-induced Id1 and SHH contribute to cell cycle reentry leading to apoptosis in neurons. We found that Aβ triggered cell cycle progression in the post-mitotic neurons, as indicated by the increased expression of two G1-phase markers including cyclin D1 and phosphorylated retinoblastoma protein (pRb), two G2-phase markers such as proliferating cell nuclear antigen (PCNA) and incorporation of 5-bromo-2'-deoxyuridine (BrdU) into newly synthesized DNA, as well as the mitotic marker histone H3 phosphorylated at Ser-10. As expected, Aβ also enhanced caspase-3 cleavage in the cortical neurons. Id1 siRNA, the neutralization antibody against SHH (SHH-Ab), and the cyclin-dependent kinase (CDK)-4/6 inhibitor PD0332991 all attenuated, in part or in full, the Aβ-induced expression of these cell cycle markers. Indeed, exogenous recombinant Id1 protein and the biologically active N-terminal fragment of SHH (SHH-N) were both sufficient to enhance the expression of cell cycle markers independent of Aβ. Taken together, our results revealed the critical roles of Id1 and SHH mediating Aβ-dependent cell cycle reentry and subsequently caspase-dependent apoptosis in the fully differentiated post-mitotic neurons, at least in vitro.

Emerging Roles of Inhibitor of Differentiation-1 in Alzheimer’s Disease: Cell Cycle Reentry and Beyond

“…Cobalt chloride, known to stabilize HIF-1α, is sufficient to initiate cell cycle reentry in the postmitotic neurons with the enhanced expression of both cyclin D1 and PCNA independent of Aβ; furthermore, Id1-Tag-mediated cyclin D1 induction can be blocked by HIF-1α siRNA [ 34 ]. Overall, these results, along with our earlier studies [ 32 , 33 ], firmly established the signaling cascade of “Aβ → Id1 → HIF-1 → Shh → cell cycle reentry → apoptosis” in the fully differentiated postmitotic neurons.…”

“…In our recent studies, we have shown that Aβ may induce Id1 expression in differentiated rat cortical neurons [ 32 ], which contributes to the induction of hypoxia-inducible factor-1 (HIF-1) and the expression of sonic hedgehog (Shh). We further validated that both Id1 and Shh mediate cell cycle reentry and apoptosis induced by Aβ in the fully differentiated postmitotic cortical neurons [ 33 ]. Moreover, both Id1 and cyclin-dependent kinase-5 (CDK5) act upstream of HIF-1 to regulate the cell cycle reentry induced by Aβ [ 34 ].…”

“…We showed that Id1 siRNA, the neutralization antibody against Shh (Shh-Ab), and the CDK-4/6 inhibitor PD0332991 all exerted partial or full competence to attenuate the Aβ-induced cell cycle markers and caspase-3 cleavage in the fully differentiated neurons. Notably, both the recombinant human Id1 protein with an 11-arginine tag on its C-terminal (Id1-Tag), thus rendering it capable of membrane permeation, and the biologically active N-terminal fragment of Shh (Shh-N) were able to increase the expression of cell cycle markers independent of Aβ [ 33 ]. These observations further imply that Id1/Shh may represent a common pathway for cell cycle reentry and the accompanied apoptosis in neurodegenerative diseases other than AD.…”

“…These observations further imply that Id1/Shh may represent a common pathway for cell cycle reentry and the accompanied apoptosis in neurodegenerative diseases other than AD. Thus, we have uncovered the vital roles of Id1 and Shh mediating Aβ-dependent cell cycle reentry and, consequently, leading to apoptosis, at least in an in vitro AD model of fully differentiated postmitotic neurons [ 33 ].…”

“…Given our earlier report demonstrating Aβ-induced Id1 activation with HIF-1α stabilization and subsequent Shh expression in primary rat cortical cultures [ 32 ], coupled with the observations that Id1 and Shh both mediate Aβ-dependent cell cycle reentry and apoptosis [ 33 ], we therefore were interested to know whether Id1/HIF-1α may also be involved in cell cycle regulation in this experimental paradigm. We found that, in the primary cortical cultures exposed to Aβ, two cell cycle markers, namely cyclin D1 and PCNA, are colocalized with microtubule-associated protein-2 (MAP-2)-positive cells, suggesting cell cycle reentry in the mature neurons [ 34 ].…”