Autophagy-linked plasma and lysosomal membrane protein PLAC8 is a key host factor for SARS-CoV-2 entry into human cells.

Better understanding on interactions between SARS-CoV-2 and host cells should help to identify host factors that may be targetable to combat infection and COVID-19 pathology. To this end, we have conducted a genome-wide CRISPR/Cas9-based loss-of-function screen in human lung cancer cells infected with SARS-CoV-2-pseudotyped lentiviruses. Our results recapitulate many findings from previous screens that used full SARS-CoV-2 viruses, but also unveil two novel critical host factors: the lysosomal efflux transporter SPNS1 and the plasma and lysosomal membrane protein PLAC8. Functional experiments with full SARS-CoV-2 viruses confirm that loss-of-function of these genes impairs viral entry. We find that PLAC8 is a key limiting host factor, whose overexpression boosts viral infection in eight different human lung cancer cell lines. Using single-cell RNA-Seq data analyses, we demonstrate that PLAC8 is highly expressed in ciliated and secretory cells of the respiratory tract, as well as in gut enterocytes, cell types that are highly susceptible to SARS-CoV-2 infection. Proteomics and cell biology studies suggest that PLAC8 and SPNS1 regulate the autophagolysosomal compartment and affect the intracellular fate of endocytosed virions.

Comparative genomics of mortal and immortal cnidarians unveils novel keys behind rejuvenation.

Turritopsis dohrnii is the only metazoan able to rejuvenate repeatedly after its medusae reproduce, hinting at biological immortality and challenging our understanding of aging. We present and compare whole-genome assemblies of T. dohrnii and the nonimmortal Turritopsis rubra using automatic and manual annotations, together with the transcriptome of life cycle reversal (LCR) process of T. dohrnii. We have identified variants and expansions of genes associated with replication, DNA repair, telomere maintenance, redox environment, stem cell population, and intercellular communication. Moreover, we have found silencing of polycomb repressive complex 2 targets and activation of pluripotency targets during LCR, which points to these transcription factors as pluripotency inducers in T. dohrnii. Accordingly, we propose these factors as key elements in the ability of T. dohrnii to undergo rejuvenation.

Prelamin A causes aberrant myonuclear arrangement and results in muscle fiber weakness.

Physiological and premature aging are frequently associated with an accumulation of prelamin A, a precursor of lamin A, in the nuclear envelope of various cell types. Here, we aimed to underpin the hitherto unknown mechanisms by which prelamin A alters myonuclear organization and muscle fiber function. By experimentally studying membrane-permeabilized myofibers from various transgenic mouse lines, our results indicate that, in the presence of prelamin A, the abundance of nuclei and myosin content is markedly reduced within muscle fibers. This leads to a concept by which the remaining myonuclei are very distant from each other and are pushed to function beyond their maximum cytoplasmic capacity, ultimately inducing muscle fiber weakness.

Cancer Susceptibility Models in Protease-Deficient Mice.

For decades, proteases have been associated with cancer progression due to the ability of some members of this large group of enzymes to degrade tumor cell surroundings, thereby facilitating cancer invasion and dissemination. However, the generation of mouse models deficient in proteases has revealed the existence of a great variety of functions among proteolytic enzymes in cancer biology, including important tumor-suppressive roles. Therefore, in this chapter, we describe methods to chemically induce different types of cancer (lung adenocarcinoma, hepatocellular carcinoma, oral and esophageal carcinoma, colorectal carcinoma, skin cancer, and fibrosarcoma) in genetically modified mouse models to efficiently evaluate the specific pro- or antitumoral function of proteases in cancer.

The deubiquitinase USP54 is overexpressed in colorectal cancer stem cells and promotes intestinal tumorigenesis.

Ubiquitin-Specific Proteases (USPs) are deubiquitinating enzymes frequently deregulated in human malignancies. Here, we show that USP54 is overexpressed in intestinal stem cells and demonstrate that its downregulation in colorectal carcinoma cells impedes tumorigenesis. We have generated mutant mice deficient for this deubiquitinase, which are viable and fertile, and protected against chemically-induced colorectal carcinoma. Furthermore, we show that USP54 is upregulated in human colon cancer and associates with poor prognosis. In agreement with these results, Usp54 downregulation in mouse melanoma cells inhibits lung metastasis formation. Collectively, this work has uncovered the pro-tumorigenic properties of USP54, highlighting the importance of deubiquitinating enzymes as promising targets for the development of specific anti-cancer therapies.

The Degradome database: expanding roles of mammalian proteases in life and disease.

Since the definition of the degradome as the complete repertoire of proteases in a given organism, the combined effort of numerous laboratories has greatly expanded our knowledge of its roles in biology and pathology. Once the genomic sequences of several important model organisms were made available, we presented the Degradome database containing the curated sets of known protease genes in human, chimpanzee, mouse and rat. Here, we describe the updated Degradome database, featuring 81 new protease genes and 7 new protease families. Notably, in this short time span, the number of known hereditary diseases caused by mutations in protease genes has increased from 77 to 119. This increase reflects the growing interest on the roles of the degradome in multiple diseases, including cancer and ageing. Finally, we have leveraged the widespread adoption of new webtools to provide interactive graphic views that show information about proteases in the global context of the degradome. The Degradome database can be accessed through its web interface at http://degradome.uniovi.es.

ATP-dependent Lon protease controls tumor bioenergetics by reprogramming mitochondrial activity.

We generated mice deficient in Lon protease (LONP1), a major enzyme of the mitochondrial quality control machinery. Homozygous deletion of Lonp1 causes early embryonic lethality, whereas its haploinsufficiency protects against colorectal and skin tumors. Furthermore, LONP1 knockdown inhibits cellular proliferation and tumor and metastasis formation, whereas its overexpression increases tumorigenesis. Clinical studies indicate that high levels of LONP1 are a poor prognosis marker in human colorectal cancer and melanoma. Additionally, functional analyses show that LONP1 plays a key role in metabolic reprogramming by remodeling OXPHOS complexes and protecting against senescence. Our findings demonstrate the relevance of LONP1 for cellular and organismal viability and identify this protease as a central regulator of mitochondrial activity in oncogenesis.

ATG4B/autophagin-1 regulates intestinal homeostasis and protects mice from experimental colitis.

The identification of inflammatory bowel disease (IBD) susceptibility genes by genome-wide association has linked this pathology to autophagy, a lysosomal degradation pathway that is crucial for cell and tissue homeostasis. Here, we describe autophagy-related 4B, cysteine peptidase/autophagin-1 (ATG4B) as an essential protein in the control of inflammatory response during experimental colitis. In this pathological condition, ATG4B protein levels increase in parallel with the induction of autophagy. Moreover, ATG4B expression is significantly reduced in affected areas of the colon from IBD patients. Consistently, atg4b (-/-) mice present Paneth cell abnormalities, as well as an increased susceptibility to DSS-induced colitis. atg4b-deficient mice exhibit significant alterations in proinflammatory cytokines and mediators of the immune response to bacterial infections, which are reminiscent of those found in patients with Crohn disease or ulcerative colitis. Additionally, antibiotic treatments and bone marrow transplantation from wild-type mice reduced colitis in atg4b (-/-) mice. Taken together, these results provided additional evidence for the importance of autophagy in intestinal pathologies and describe ATG4B as a novel protective protein in inflammatory colitis. Finally, we propose that atg4b-null mice are a suitable model for in vivo studies aimed at testing new therapeutic strategies for intestinal diseases associated with autophagy deficiency.

Molecular reconstruction of a fungal genetic code alteration.

Fungi of the CTG clade translate the Leu CUG codon as Ser. This genetic code alteration is the only eukaryotic sense-to-sense codon reassignment known to date, is mediated by an ambiguous serine tRNA (tRNACAG(Ser)), exposes unanticipated flexibility of the genetic code and raises major questions about its selection and fixation in this fungal lineage. In particular, the origin of the tRNACAG(Ser) and the evolutionary mechanism of CUG reassignment from Leu to Ser remain poorly understood. In this study, we have traced the origin of the tDNACAG(Ser) gene and studied critical mutations in the tRNACAG(Ser) anticodon-loop that modulated CUG reassignment. Our data show that the tRNACAG(Ser) emerged from insertion of an adenosine in the middle position of the 5'-CGA-3'anticodon of a tRNACGA(Ser) ancestor, producing the 5'-CAG-3' anticodon of the tRNACAG(Ser), without altering its aminoacylation properties. This mutation initiated CUG reassignment while two additional mutations in the anticodon-loop resolved a structural conflict produced by incorporation of the Leu 5'-CAG-3'anticodon in the anticodon-arm of a tRNA(Ser). Expression of the mutant tRNACAG(Ser) in yeast showed that it cannot be expressed at physiological levels and we postulate that such downregulation was essential to maintain Ser misincorporation at sub-lethal levels during the initial stages of CUG reassignment. We demonstrate here that such low level CUG ambiguity is advantageous in specific ecological niches and we propose that misreading tRNAs are targeted for degradation by an unidentified tRNA quality control pathway.

Micromanaging aging with miRNAs: new messages from the nuclear envelope.

Over the last years, the discovery of microRNAs (miRNAs) has revolutionized the classic concepts of gene expression regulation and has introduced a new group of molecules that may contribute to the complex changes observed during aging. Although several Caenorhabditis elegans miRNAs have been proved to influence the nematode life span, the current knowledge about miRNA-mediated regulation of mammalian aging is still limited. Recently, we have analyzed the functional relevance of miRNAs in accelerate aging by using Zmpste24-/- mice, a murine model that phenocopies Hutchinson-Gilford progeria syndrome. These studies have revealed that the nuclear abnormalities present in these mice affect the expression levels of several miRNAs, including a marked upregulation of miR-1 and miR-29. Furthermore, we have found that the altered expression of these miRNAs may contribute to the progeroid phenotype of mutant mice by modulating the levels of key components of the somatroph axis and DNA damage response pathways. Here, we discuss these recent discoveries and summarize the present evidences regarding the involvement of aging-associated miRNAs or geromiRs in senescence and longevity regulation.

3-(3-amino-3-carboxypropyl)-5,6-dihydrouridine is one of two novel post-transcriptional modifications in tRNALys(UUU) from Trypanosoma brucei.

tRNA is the most heavily modified of all RNA types, with typically 10-20% of the residues being post-transcriptionally altered. Unravelling the modification pattern of a tRNA is a challenging task; there are 92 currently known tRNA modifications, many of which are chemically similar. Furthermore, the tRNA has to be investigated with single-nucleotide resolution in order to ensure complete mapping of all modifications. In the present work, we characterized tRNA(Lys)(UUU) from Trypanosoma brucei, and provide a complete overview of its post-transcriptional modifications. The first step was MALDI-TOF MS of two independent digests of the tRNA, with RNase A and RNase T1, respectively. This revealed digestion products harbouring mass-changing modifications. Next, the modifications were mapped at the nucleotide level in the RNase products by tandem MS. Comparison with the sequence of the unmodified tRNA revealed the modified residues. The modifications were further characterized at the nucleoside level by chromatographic retention time and fragmentation pattern upon higher-order tandem MS. Phylogenetic comparison with modifications in tRNA(Lys) from other organisms was used through the entire analysis. We identified modifications on 12 nucleosides in tRNA(Lys)(UUU), where U47 exhibited a novel modification, 3-(3-amino-3-carboxypropyl)-5,6-dihydrouridine, based on identical chromatographic retention and MS fragmentation as the synthetic nucleoside. A37 was observed in two versions: a minor fraction with the previously described 2-methylthio-N(6)-threonylcarbamoyl-modification, and a major fraction with A37 being modified by a 294.0-Da moiety. The latter product is the largest adenosine modification reported so far, and we discuss its nature and origin.

A mechanism for functional segregation of mitochondrial and cytosolic genetic codes.

The coexistence of multiple gene translation machineries is a feature of eukaryotic cells and a result of the endosymbiotic events that gave rise to mitochondria, plastids, and other organelles. The conditions required for the integration of these apparatuses within a single cell are not understood, but current evidence indicates that complete ablation of the mitochondrial protein synthesis apparatus and its substitution by its cytosolic equivalent is not possible. Why certain mitochondrial components and not others can be substituted by cytosolic equivalents is not known. In trypanosomatids this situation reaches a limit, because certain aminoacyl-tRNA synthetases are mitochondrial specific despite the fact that all tRNAs in these organisms are shared between cytosol and mitochondria. Here we report that a mitochondria-specific lysyl-tRNA synthetase in Trypanosoma has evolved a mechanism to block the activity of the enzyme during its synthesis and translocation. Only when the enzyme reaches the mitochondria is it activated through the cleavage of a C-terminal structural extension, preventing the possibility of the enzyme being active in the cytosol.

Solid-phase combinatorial synthesis of a lysyl-tRNA synthetase (LysRS) inhibitory library.

The solid-phase combinatorial synthesis of a new library with potential inhibitory activity against the cytoplasmic lysyl-tRNA synthetase (LysRS) isoform of Trypanosoma brucei is described. The library has been specifically designed to mimic the lysyl adenylate complex. The design was carried out by dividing the complex into four modular parts. Proline derivatives (cis-gamma-amino-L-proline or trans-gamma-hydroxy-L-proline) were chosen as central scaffolds. After primary screening, three compounds of the library caused in vitro inhibition of the tRNA aminoacylation reaction in the low micromolar range.