Regulation of Neuronal Survival by Nucleophosmin 1 (NPM1) Is Dependent on Its Expression Level, Subcellular Localization, and Oligomerization Status

Abstract

NPM1 (nucleophosmin 1) is a nucleolar phosphoprotein that regulates many cellular processes, including ribosome biogenesis, proliferation, and genomic integrity. Although its role in proliferating cell types and tissues has been extensively investigated, little is known about its function in neurons and in the brain where it is highly expressed. We report that NPM1 protein expression is increased selectively in the striatum in both the R6/2 transgenic and 3-nitropropionic acid-injected mouse models of Huntington's disease. Examination of the effect of ectopic expression on cultured neurons revealed that increasing NPM1 is toxic to otherwise healthy cerebellar granule and cortical neurons. Toxicity is dependent on its cytoplasmic localization and oligomerization status. Forced retention of NPM1 in the nucleus, as well as inhibiting its ability to oligomerize, not only neutralizes NPM1 toxicity but also renders it protective against apoptosis. Although not blocked by pharmacological inhibition of the pro-apoptotic molecules, JNK, glycogen synthase kinase 3 beta, or caspases, toxicity is blocked by compounds targeting cyclin-dependent kinases (CDKs), as well as by dominant-negative forms of CDK1 and CDK2 and the pan-CDK inhibitor, p21Cip1/Waf1 . Although induced in in vivo Huntington's disease models, NPM1 protein levels are unchanged in cultured cerebellar granule and cortical neurons induced to die by low potassium or homocysteic acid treatment, respectively. Moreover, and counterintuitively, knockdown of its expression or inhibition of endogenous NPM1 oligomerization in these cultured neurons is toxic. Taken together, our study suggests that although neurons need NPM1 for survival, an increase in its expression beyond physiological levels and its translocation to the cytoplasm leads to death through abortive cell cycle induction.NPM1, also known as B23, is an abundant, ubiquitously expressed, and evolutionarily conserved non-ribosomal nucleolar phosphoprotein (1, 2). It is one of three members of the nucleophosmin/nucleoplasmin family of proteins, the other two being NPM2 and NPM3. NPM1 is an important regulator of a variety of cellular processes including centrosome duplication, genomic stability, cell proliferation, and the response to genotoxic stress. Inactivation of the NPM1 gene in mice causes death at midgestation, indicating its requirement for normal development. Fibroblasts cultured from NPM1 knock-out mice display genomic instability and reduced growth and cell proliferation (3). Other studies have shown that NPM1 can shuttle between the nucleolus and the nucleoplasm and from the nucleus to the cytoplasm. Nuclear export is believed to be required for cellular proliferation (4, 5). Mutations of NPM1 are associated with cancers and are responsible for approximately a third of cases of acute myeloid leukemia in humans (3, 6 -8). These mutations, which result in an altered C terminus containing an addition of a nuclear export signal, have been reported to aberrantly localize NPM1 to the cytoplasm (8). Ove...

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“…Human NPM1 is thought to have key roles in cell cycle regulation (Zhao et al. 2015; Pfister and D’Mello 2016). Tetrahymena thermophila cells lacking Cyc2 and Cyc17 are arrested at early crescent (∼2–3.5 h postmixing) and diakinesis-like metaphase I (∼5 h postmeiotic induction) stages of meiosis, respectively (Xu et al.…”

Catalytic-independent neuroprotection by SIRT1 is mediated through interaction with HDAC1

“…In conjunction with the mild toxicity to HK, this suggests that rather than losing neuroprotective activity, Δ8 may gain a “toxic” cell cycle stimulating activity. It is well established that abortive re-entry of neurons into the cell cycle contributes to neurodegeneration both in vitro and in vivo [44, 62, 63]. We have also recently shown that a SIRT1-interacting protein, JAZ, protects neurons through the induction of p21 expression and cell cycle suppression [44].…”

Genetic effects on brain traits impact cell-type specific gene regulation during neurogenesis

“…Among RBP binding sites specifically enriched for progenitor sQTLs, we found LIN28B, known to play a role in neural progenitor proliferation and differentiation 45 . On the other hand, for neurons, we detected enrichment of the NPM1 regulating neuronal survival 46 . These observations suggest that sQTLs alter the function of RBPs with cell-type specific splicing roles during neural development.…”

“…CC-BY 4.0 International license perpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for this this version posted October 28, 2020. ; https://doi.org/10.1101/2020.10.21.349019 doi: bioRxiv preprint NPM1 regulating neuronal survival 54 . These observations suggest that sQTLs alter the function of RBPs with cell type specific splicing roles during neural development.…”