Ku80 is required but not sufficient for Galpha13-mediated endodermal differentiation in P19 embryonic carcinoma cells

Puromycin-resistant lentiviral control shRNA vector, pLKO.1 induces unexpected cellular differentiation of P19 embryonic stem cells

RNA silencing is used as a common method for investigating loss-of-function effects of genes of interest. In mammalian cells, RNA interference (RNAi) or RNA silencing can be achieved by transient siRNA (small or short interfering RNA) transfection or by stable shRNA (short hairpin RNA) systems. Various vectors are used for efficient delivery of shRNA. Lentiviral vectors offer an efficient delivery system for stable and long-term expression of the shRNA in mammalian cells. The widely used lentiviral pLKO.1 plasmid vector is very popular in RNAi studies. A large RNAi database, a TRC (the RNAi Consortium) library, was established based on the pLKO.1-TRC plasmid vector. This plasmid (also called pLKO.1-puro) has a puromycin-resistant gene for selection in mammalian cells along with designs for generating lentiviral particles as well for RNA silencing. While using the pLKO.1-puro TRC control shRNA plasmid for transfection in murine P19 embryonic stem (ES) cells, it was unexpectedly discovered that this plasmid vector induced robust endodermal differentiation. Since P19 ES cells are pluripotent and respond to external stimuli that have the potential to alter the phenotype and thus its stemness, other cell types used in RNA silencing studies do not display the obvious effect and therefore, may affect experiments in subtle ways that would go undetected. This study for the first time provides evidence that raises concern and warrants extreme caution while using the pLKO.1-puro control shRNA vector because of its unexpected non-specific effects on cellular integrity.

DNA-PK Deficiency in Alzheimer's Disease

Alzheimer's disease (AD) is characterized by neuronal death with an accumulaton of intra-cellular neurofibrillary tangles (NFT) and extracellular amyloid plaques. Reduced DNA repair ability has been reported in AD brains. In neurons, the predominant mechanism to repair double-strand DNA breaks (DSB) is non-homologous end joining (NHEJ) that requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK is a holoenzyme comprising the p460 kD DNA-PK catalytic subunit (DNA-PKcs) and its activator Ku, a heterodimer of p86 (Ku80) and p70 (Ku70) subunits. Upon binding to double-stranded DNA ends, Ku recruits DNA-PKcs to process NHEJ. In AD brains, reduced NHEJ activity as well as DNA-PKcs and Ku protein levels have been shown. Normal aging brains also show a reduction in both DNA-PKcs and Ku levels questioning a direct link between NHEJ ability and AD, and suggesting additional players/events in AD pathogenesis. Deficiency of Ku80, a somatostatin receptor, can disrupt somatostatin signaling thus inducing amyloid beta (Aβ) generation, which in turn can potentiate DNA-PKcs degradation and consequently loss of NHEJ activity, an additional step negatively affecting DSB repair. Trigger of these two different pathways culminating in genome instability may differentiate the outcomes between AD and normal aging.

DNA-dependent protein kinase and DNA repair: relevance to Alzheimer's disease

The pathological hallmark of Alzheimer's disease (AD), the leading cause of senile dementia, involves region-specific neuronal death and an accumulation of neuronal and extracellular lesions termed neurofibrillary tangles and senile plaques, respectively. One of the biochemical abnormalities observed in AD is reduced DNA end-joining activity. The reduced capacity of post-mitotic neurons for some types of DNA repair is further compromised by aging. The predominant mechanism to repair double-strand DNA (dsDNA) breaks (DSB) is non-homologous end joining (NHEJ), which requires DNA-dependent protein kinase (DNA-PK) activity. DNA-PK is a holoenzyme comprising the p460 kDa DNA-PK catalytic subunit (DNA-PKcs) and the Ku heterodimer consisting of p86 (Ku 80) and p70 (Ku 70) subunits. Ku binds to DNA ends first and then recruits DNA-PKcs during NHEJ. However, in AD brains, reduced NHEJ activity has been reported along with reduced levels of DNA-PKcs and the Ku proteins, indicating a potential link between AD and dsDNA damage. Since age-matched control brains also show a reduction in these protein levels, whether there is a direct link between NHEJ ability and AD remains unknown. Possible mechanisms involving the role of DNA-PK in neurodegeneration, a benchmark of AD, are the focus of this review.

Menin induces endodermal differentiation in aggregated P19 stem cells by modulating the retinoic acid receptors

Menin, a ubiquitously expressed protein, is the product of the multiple endocrine neoplasia type I (Men1) gene, mutations of which cause tumors primarily of the parathyroid, endocrine pancreas, and anterior pituitary. Menin-null mice display early embryonic lethality, and thus imply a critical role for menin in early development. In this study, using the P19 embryonic carcinoma stem cells, we studied menin's role in cell differentiation. Menin expression is induced in P19 cell aggregates by retinoic acid (RA). Menin over-expressing stable clones proliferated in a significantly reduced rate compared to the empty vector harboring cells. RA induced cell death in aggregated menin over-expressing cells. However, in the absence of RA, specific populations of the aggregated menin over-expressing cells displayed the characteristic of an endodermal phenotype by the acquisition of cytokeratin Endo A expression (TROMA-1), a marker for the primitive endoderm, with a concomitant loss of the stem cell marker SSEA-1. Menin's ability to induce endodermal differentiation in specific populations of the aggregated cells in the absence of RA implied that menin could substitute RA by inducing a set of target genes that are RA responsive. Menin over-expressing cells upon aggregation showed a robust expression of RA receptors (RAR), RARα, β, and γ relative to the empty vector-harboring cells. Moreover, endodermal differentiation was inhibited by the pan-RAR antagonist Ro41-5253, suggesting that menin could induce endodermal differentiation of uncommitted cells by functionally modulating the RARs.