Thomas E. Dever

The integrated stress response (ISR) is essential for cellular homeostasis and cognitive function. We investigated how persistent ISR activation affects cognitive performance by studying the PPP1R15B ^R658C genetic variant associated with intellectual disability. To model this condition, we generated a mouse line with the pathogenic allele inserted. This variant destabilized the PPP1R15B•PP1 phosphatase complex, causing persistent ISR activation, impaired protein synthesis, and long-term memory deficits. We demonstrated that the cognitive and synaptic impairments in Ppp1r15b ^R658C mice arise directly from ISR activation. Furthermore, we characterized DP71L, a viral ortholog of PPP1R15B, which acted as a potent pan-ISR inhibitor. DP71L reversed the cognitive and synaptic deficits across mouse models of Down syndrome, Alzheimer’s disease, and aging, and enhanced synaptic plasticity and memory in healthy mice.

Polyamines, small organic polycations, are essential for life, and cells maintain polyamines via synthesis and uptake. Recently, we identified Hol1 as the high-affinity polyamine transporter in Saccharomyces cerevisiae . Hol1 is a conserved fungal-specific transporter with Candida albicans having two HOL1 homologs (orf19.4889 and orf19.2991) and no identifiable HOL1 homolog in mammals. Deleting both HOL1 homologs blocked efficient polyamine uptake in C. albicans , establishing Hol1 as the high-affinity polyamine transporter in C. albicans . Combined deletion of HOL1 and SPE1, encoding ornithine decarboxylase (ODC), the first enzyme in the polyamine synthesis pathway, resulted in a severe growth defect, confirming the importance of polyamines for C. albicans growth. In addition, cells lacking HOL1 and SPE1 failed to form hyphae when exposed to serum, suggesting a role for polyamines in C. albicans virulence. In accord with this hypothesis, in the mouse model of disseminated candidiasis, the homozygous spe1Δ mutant, like the WT strain, readily colonized the kidney and all mice died within 2 weeks following intravenous inoculation. In contrast, the homozygous spe1Δ hol1Δ mutant was avirulent with all mice surviving the infection. Consistent with these genetic results, simultaneously treating WT cells with l -α-difluoromethylornithine, an irreversible inhibitor of ODC, to inhibit polyamine biosynthesis and with the polyamine transport inhibitor Trimer44NMe to inhibit uptake substantially impaired C. albicans growth and hyphal differentiation. We conclude that polyamines are critical for C. albicans virulence and could be of potential therapeutic interest via combined targeting of polyamine synthesis and the fungal-specific polyamine transporter Hol1. IMPORTANCE: Fungal infections are a growing concern, and a predominant human opportunistic pathogen is the fungus Candida albicans . Current antifungals commonly target cell wall and cell membrane biosynthesis or integrity; however, resistant strains are emerging. Polyamines are essential small organic cations that cells synthesize and import. We identify polyamines as a possible new target for antifungal therapies. The Hol1 polyamine transporter is unique to fungi and is distinct from mammalian transporters, so it is an intriguing antifungal target. As proof of concept, we show that combined knock-out of Hol1 and a polyamine biosynthesis gene impairs C. albicans growth and hyphal differentiation in culture and virulence in mouse infection assays. Moreover, we identify a polyamine analog that robustly inhibits Hol1 function, providing insights into potential new therapeutics.

Human translation initiation requires accurate recognition of translation start sites. While AUG codons are canonical start sites, non-AUG codons are also used, typically with lower efficiency. The initiator tRNA and initiation factors eIF1 and eIF5 control recognition. How they distinguish different start sites yet allow flexible recognition remains unclear. Here we used real-time single-molecule assays and an in vitro reconstituted human system to reveal how eIF1 and eIF5 direct start site selection. eIF1 binds initiation complexes in two modes: stable binding during scanning, followed by transient, concentration-dependent rebinding after start site recognition. Termination of eIF1 rebinding requires transient and concentration-dependent binding by eIF5, which allows the formation of translation competent ribosomes. Non-AUG start sites differentially stabilize eIF1 and destabilize eIF5 binding, blocking initiation at multiple points. We confirmed these opposing effects in human cells. Collectively, our findings uncover that eIF1 and eIF5 directly compete to bind initiation complexes and illuminate how their dynamic interplay tunes the fidelity of start site recognition, which has broad connections to health and disease.

Translation initiation defines the identity of a synthesized protein through selection of a translation start site on a messenger RNA. This process is essential to well-controlled protein synthesis, modulated by stress responses, and dysregulated in many human diseases. The eukaryotic initiation factors eIF1 and eIF5 interact with the initiator methionyl-tRNA _i ^Met on the 40S ribosomal subunit to coordinate start site selection. Here, using single-molecule analysis of in vitro reconstituted human initiation combined with translation assays in cells, we examine eIF1 and eIF5 function. During translation initiation on a panel of RNAs, we monitored both proteins directly and in real time using single-molecule fluorescence. As expected, eIF1 loaded onto mRNAs as a component of the 43S initiation complex. Rapid (∼ 2 s) eIF1 departure required a translation start site and was delayed by alternative start sites and a longer 5’ untranslated region (5’UTR). After its initial departure, eIF1 rapidly and transiently sampled initiation complexes, with more prolonged sampling events on alternative start sites. By contrast, eIF5 only transiently bound initiation complexes late in initiation immediately prior to association of eIF5B, which allowed joining of the 60S ribosomal subunit. eIF5 association required the presence of a translation start site and was inhibited and destabilized by alternative start sites. Using both knockdown and overexpression experiments in human cells, we validated that eIF1 and eIF5 have opposing roles during initiation. Collectively, our findings demonstrate how multiple eIF1 and eIF5 binding events control start-site selection fidelity throughout initiation, which is tuned in response to changes in the levels of both proteins.

In addition to the main, protein-coding, open reading frame (mORF), many eukaryotic mRNAs contain upstream ORFs (uORFs) initiated at AUG or near-cognate codons residing 5′ of the mORF start site. Whereas translation of uORFs generally represses translation of the mORFs, a subset of uORFs serves as a nexus for regulating translation of the mORF. In this review, we summarize the mechanisms by which uORFs can repress or stimulate mRNA translation, highlight uORF-mediated translational repression involving ribosome queuing, and critically evaluate recently described alternatives to the delayed reinitiation model for uORF-mediated regulation of theGCN4/ATF4mRNAs.

Diphthamide (DPH), a conserved amino acid modification on eukaryotic translation elongation factor eEF2, is synthesized via a complex, multi-enzyme pathway. While DPH is non-essential for cell viability and its function has not been resolved, diphtheria and other bacterial toxins ADP-ribosylate DPH to inhibit translation. Characterizing Saccharomyces cerevisiae mutants that lack DPH or show synthetic growth defects in the absence of DPH, we show that loss of DPH increases resistance to the fungal translation inhibitor sordarin and increases –1 ribosomal frameshifting at non-programmed sites during normal translation elongation and at viral programmed frameshifting sites. Ribosome profiling of yeast and mammalian cells lacking DPH reveals increased ribosomal drop-off during elongation, and removal of out-of-frame stop codons restores ribosomal processivity on the ultralong yeast MDN1 mRNA. Finally, we show that ADP-ribosylation of DPH impairs the productive binding of eEF2 to elongating ribosomes. Our results reveal that loss of DPH impairs the fidelity of translocation during translation elongation resulting in increased rates of ribosomal frameshifting throughout elongation and leading to premature termination at out-of-frame stop codons. We propose that the costly, yet non-essential, DPH modification has been conserved through evolution to maintain translational fidelity despite being a target for inactivation by bacterial toxins.

Translation initiation defines the identity and quantity of a synthesized protein. The process is dysregulated in many human diseases^1,2. A key commitment step is when the ribosomal subunits join at a translation start site on a messenger RNA to form a functional ribosome. Here, we combined single-molecule spectroscopy and structural methods using an in vitro reconstituted system to examine how the human ribosomal subunits join. Single-molecule fluorescence revealed when the universally conserved eukaryotic initiation factors eIF1A and eIF5B associate with and depart from initiation complexes. Guided by single-molecule dynamics, we visualized initiation complexes that contained both eIF1A and eIF5B using single-particle cryo-electron microscopy. The resulting structure revealed how eukaryote-specific contacts between the two proteins remodel the initiation complex to orient the initiator aminoacyl-tRNA in a conformation compatible with ribosomal subunit joining. Collectively, our findings provide a quantitative and architectural framework for the molecular choreography orchestrated by eIF1A and eIF5B during translation initiation in humans.

Metabolic programs contribute to hematopoietic stem and progenitor cell (HSPC) fate but it is not known whether the metabolic regulation of protein synthesis controls HSPC differentiation. We discovered that SLC7A1/CAT1-dependent arginine uptake and its catabolism to spermidine control the erythroid specification of HSPCs via activation of eukaryotic translation initiation factor 5A (eIF5A). eIF5A activity is dependent on the metabolism of spermidine to hypusine and inhibiting hypusine synthesis abrogates erythropoiesis and diverts EPO-stimulated HSPCs to a myeloid fate. Proteomic profiling reveals mitochondrial translation to be a critical target of hypusinated eIF5A and induction of mitochondrial function partially rescues erythropoiesis in the absence of hypusine. Within the hypusine network, ribosomal proteins are highly enriched and we identify defective eIF5A hypusination in erythroid pathologies caused by abnormal ribosome biogenesis. Thus, eIF5A-dependent protein synthesis is critical in the branching of erythro-myeloid differentiation and attenuated eIF5A activity characterizes ribosomal protein-linked disorders of ineffective erythropoiesis.