Anabolism-Associated Mitochondrial Stasis Driving Lymphocyte Differentiation over Self-Renewal

Regeneration requires related cells to diverge in fate. We show that activated lymphocytes yield sibling cells with unequal elimination of aged mitochondria. Disparate mitochondrial clearance impacts cell fate and reflects larger constellations of opposing metabolic states. Differentiation driven by an anabolic constellation of PI3K/mTOR activation, aerobic glycolysis, inhibited autophagy, mitochondrial stasis, and ROS production is balanced with self-renewal maintained by a catabolic constellation of AMPK activation, mitochondrial elimination, oxidative metabolism, and maintenance of FoxO1 activity. Perturbations up and down the metabolic pathways shift the balance of nutritive constellations and cell fate owing to self-reinforcement and reciprocal inhibition between anabolism and catabolism. Cell fate and metabolic state are linked by transcriptional regulators, such as IRF4 and FoxO1, with dual roles in lineage and metabolic choice. Instructing some cells to utilize nutrients for anabolism and differentiation while other cells catabolically self-digest and self-renew may enable growth and repair in metazoa

Regulation of ubiquitin ligase dynamics by the nucleolus

Cellular pathways relay information through dynamic protein interactions. We have assessed the kinetic properties of the murine double minute protein (MDM2) and von Hippel-Lindau (VHL) ubiquitin ligases in living cells under physiological conditions that alter the stability of their respective p53 and hypoxia-inducible factor substrates. Photobleaching experiments reveal that MDM2 and VHL are highly mobile proteins in settings where their substrates are efficiently degraded. The nucleolar architecture converts MDM2 and VHL to a static state in response to regulatory cues that are associated with substrate stability. After signal termination, the nucleolus is able to rapidly release these proteins from static detention, thereby restoring their high mobility profiles. A protein surface region of VHL's β-sheet domain was identified as a discrete [H+]-responsive nucleolar detention signal that targets the VHL/Cullin-2 ubiquitin ligase complex to nucleoli in response to physiological fluctuations in environmental pH. Data shown here provide the first evidence that cells have evolved a mechanism to regulate molecular networks by reversibly switching proteins between a mobile and static state

Recapitulating the tumor ecosystem along the metastatic cascade using 3D culture models

Advances in cancer research have shown that a tumor can be likened to a foreign species that disrupts delicately balanced ecological interactions, compromising the survival of normal tissue ecosystems. In efforts to mitigate tumor expansion and metastasis, experimental approaches from ecology are becoming more frequently and successfully applied by researchers from diverse disciplines to reverse engineer and re-engineer biological systems in order to normalize the tumor ecosystem. We present a review on the use of 3D biomimetic platforms to recapitulate biotic and abiotic components of the tumor ecosystem, in efforts to delineate the underlying mechanisms that drive evolution of tumor heterogeneity, tumor dissemination, and acquisition of drug resistance.ope

Ethanolamine phosphoglycerol attachment to eEF1A is not essential for normal growth of Trypanosoma brucei

Eukaryotic elongation factor 1A (eEF1A) is the only protein modified by ethanolamine phosphoglycerol (EPG). In mammals and plants, EPG is attached to conserved glutamate residues located in eEF1A domains II and III, whereas in the unicellular eukaryote, Trypanosoma brucei, a single EPG moiety is attached to domain III. A biosynthetic precursor of EPG and structural requirements for EPG attachment to T. brucei eEF1A have been reported, but the role of this unique protein modification in cellular growth and eEF1A function has remained elusive. Here we report, for the first time in a eukaryotic cell, a model system to study potential roles of EPG. By down-regulation of EF1A expression and subsequent complementation of eEF1A function using conditionally expressed exogenous eEF1A (mutant) proteins, we show that eEF1A lacking EPG complements trypanosomes deficient in endogenous eEF1A, demonstrating that EPG attachment is not essential for normal growth of T. brucei in culture

Down syndrome is an oxidative phosphorylation disorder

Down syndrome is the most common genomic disorder of intellectual disability and is caused by trisomy of chromosome 21. Several genes in this chromosome repress mitochondrial biogenesis. The goal of this study was to evaluate whether early overexpression of these genes may cause a prenatal impairment of oxidative phosphorylation negatively affecting neurogenesis. Reduction in the mitochondrial energy production and a lower mitochondrial function have been reported in diverse tissues or cell types, and also at any age, including early fetuses, suggesting that a defect in oxidative phosphorylation is an early and general event in Down syndrome individuals. Moreover, many of the medical conditions associated with Down syndrome are also frequently found in patients with oxidative phosphorylation disease. Several drugs that enhance mitochondrial biogenesis are nowadays available and some of them have been already tested in mouse models of Down syndrome restoring neurogenesis and cognitive defects. Because neurogenesis relies on a correct mitochondrial function and critical periods of brain development occur mainly in the prenatal and early neonatal stages, therapeutic approaches intended to improve oxidative phosphorylation should be provided in these periods.Funding sources: This work was supported by grants from Instituto de Salud Carlos III [FIS-PI17/00021, FIS-PI17/00166]; Fundación Mutua Madrileña [MMA17/01]; Precipita-FECYT crowdfunding program [PR194]; Gobierno de Aragón [LMP135_18, Grupos Consolidados B33_17R] and FEDER 2014–2020 “Construyendo Europa desde Aragón”. CIBERER is an initiative of the ISCIII

eEF1A2 and neuronal degeneration

Translation elongation factor eEF1A (eukaryotic elongation factor 1A) exists as two individually encoded variants in mammals, which are 98% similar and 92% identical at the amino acid level. One variant, eEF1A1, is almost ubiquitously expressed, the other variant, eEF1A2, shows a very restricted pattern of expression. A spontaneous mutation was described in 1972, which gives rise to the wasted phenotype: homozygous wst/wst mice develop normally until shortly after weaning, but then lose muscle bulk, acquire tremors and gait abnormalities and die by 4 weeks. This mutation has been shown to be a deletion of 15 kb that removes the promoter and first exon of the gene encoding eEF1A2. The reciprocal pattern of expression of eEF1A1 and eEF1A2 in muscle fits well with the timing of onset of the phenotype of wasted mice: eEF1A1 declines after birth until it is undetectable by 3 weeks, whereas eEF1A2 expression increases over this time. No other gene is present in the wasted deletion, and transgenic studies have shown that the phenotype is due to loss of eEF1A2. We have shown that eEF1A2, but not eEF1A1, is also expressed at high levels in motor neurons in the spinal cord. Wasted mice develop many pathological features of motor neuron degeneration and may represent a good model for early onset of motor neuron disease. Molecular modelling of the eEF1A1 and eEF1A2 protein structures highlights differences between the two variants that may be critical for functional differences. Interactions between eEF1A2 and ZPR1 (zinc-finger protein 1), which interacts with the SMN (survival motor neuron) protein, may be important in motor neuron biology

The translation elongation factor eEF1A1 couples transcription to translation during heat shock response.

Translation elongation factor eEF1A has a well-defined role in protein synthesis. In this study, we demonstrate a new role for eEF1A: it participates in the entire process of the heat shock response (HSR) in mammalian cells from transcription through translation. Upon stress, isoform 1 of eEF1A rapidly activates transcription of HSP70 by recruiting the master regulator HSF1 to its promoter. eEF1A1 then associates with elongating RNA polymerase II and the 3′UTR of HSP70 mRNA, stabilizing it and facilitating its transport from the nucleus to active ribosomes. eEF1A1-depleted cells exhibit severely impaired HSR and compromised thermotolerance. In contrast, tissue-specific isoform 2 of eEF1A does not support HSR. By adjusting transcriptional yield to translational needs, eEF1A1 renders HSR rapid, robust, and highly selective; thus, representing an attractive therapeutic target for numerous conditions associated with disrupted protein homeostasis, ranging from neurodegeneration to cancer. DOI: http://dx.doi.org/10.7554/eLife.03164.00

Subcellular trafficking of VHL and oxygen homeostasis: Discovery of a new nuclear export pathway

Degradation of nuclear proteins by the ubiquitylation system often requires nuclear-cytoplasmic trafficking of E3 ubiquitin-ligases. The von Hippel-Lindau (VHL) tumor suppressor protein is the substrate recognition component of a Cullin-2-containing E3 ubiquitin-ligase that recruits the hypoxia-inducible factor (HIF) for oxygen-dependent degradation. The dynamic properties of VHL are essential for its ability to mediate efficient degradation of HIF. Interestingly, nuclear export of VHL requires ongoing transcription and is independent of the classical NES/CRM1 pathway. Examining this uncharacterized nuclear export pathway led to the identification of a discreet motif, "DxGx2Dx 2L", that directs transcription-dependent nuclear export of VHL. The "DxGx2Dx2L" motif is also found in other proteins, including Poly(A) Binding Protein (PABP1) to direct transcription-dependent nuclear export. The DxGx2Dx2L motif is denoted as TD-NEM (Transcription-Dependent Nuclear Export Motif) since inhibition of transcription by ActD or DRB abrogates its nuclear export activity. In VHL, TD-NEM is targeted by naturally-occurring mutations associated with renal carcinoma and polycythemia in humans. Disease-causing mutations of key residues of TD-NEM restrain the ability of VHL to efficiently mediate oxygen-dependent degradation of HIF by altering its nuclear export dynamics without affecting interaction with its substrate or core components of the E3 ubiquitin-ligase complex. Further studies aimed at understanding the mechanism of TD-NEM-mediated nuclear export led to the identification of a novel VHL and PABP1 interacting protein, the cytoplasmic translation elongation factor eEF1A. eEF1A, which has been implicated in the nuclear export of RNA species in lower eukaryotes, is involved in nuclear export of proteins encoding a TD-NEM in mammalian cells. eEF1A interacts specifically with TD-NEM and disrupting this interaction, by point mutations of the key residues within TD-NEM or siRNA-mediated knockdown of eEF1A, suppresses nuclear export. ActD suppresses eEF1A/TD-NEM interaction and abrogates eEF1A-mediated nuclear export of TD-NEM, providing a possible explanation for the inhibitory effect of ActD on nuclear export of TD-NEM-containing proteins. These results identify a novel and potentially ubiquitous, nuclear export motif, further highlight the role of nuclear-cytoplasmic shuttling of E3 ubiquitin-ligases in degradation of nuclear substrates and provide evidence that disease-causing mutations can target subcellular trafficking. Furthermore, these findings demonstrate that eEF1A, a mediator of RNA export in yeast, has an additional role in the nuclear export of proteins in mammalian cells

Subcellular dynamics of the VHL tumor suppressor: on the move for HIF degradation

The von Hippel-Lindau (VHL) tumor suppressor protein, the recognition component of an E3 ubiquitin ligase complex, recruits the alpha-subunit of the hypoxia-inducible factor (HIFalpha) for oxygen-dependent degradation. The ability of VHL to mediate efficient degradation of HIFalpha is also dependent on its oxygen/pH-regulated subcellular trafficking. Under aerobic conditions, VHL engages in nuclear-cytoplasmic trafficking that requires ongoing transcription and is mediated by a novel nuclear export motif, the transcription-dependent nuclear export motif (TD-NEM). Disease-causing mutations targeting TD-NEM restrain VHL from mediating efficient oxygen-dependent degradation of HIFalpha by altering its subcellular dynamics. In addition, decreasing the extracellular pH, during anaerobic metabolism, stabilizes HIFalpha by triggering the relocalization and static detention of VHL to nucleoli. Together, these recent findings support the critical role of subcellular trafficking and dynamic properties for the function of VHL in promoting HIF regulation and tumor suppression