Localization of a TORC1-eIF4F translation complex during CD8+ T cell activation drives divergent cell fate.

Activated CD8+ T lymphocytes differentiate into heterogeneous subsets. Using super-resolution imaging, we found that prior to the first division, dynein-dependent vesicular transport polarized active TORC1 toward the microtubule-organizing center (MTOC) at the proximal pole. This active TORC1 was physically associated with active eIF4F, required for the translation of c-myc mRNA. As a consequence, c-myc-translating polysomes polarized toward the cellular pole proximal to the immune synapse, resulting in localized c-myc translation. Upon division, the TORC1-eIF4A complex preferentially sorted to the proximal daughter cell, facilitating asymmetric c-Myc synthesis. Transient disruption of eIF4A activity at first division skewed long-term cell fate trajectories to memory-like function. Using a genetic barcoding approach, we found that first-division sister cells often displayed differences in transcriptional profiles that largely correlated with c-Myc and TORC1 target genes. Our findings provide mechanistic insights as to how distinct T cell fate trajectories can be established during the first division.

LAG3 associates with TCR-CD3 complexes and suppresses signaling by driving co-receptor-Lck dissociation.

LAG3 is an inhibitory receptor that is highly expressed on exhausted T cells. Although LAG3-targeting immunotherapeutics are currently in clinical trials, how LAG3 inhibits T cell function remains unclear. Here, we show that LAG3 moved to the immunological synapse and associated with the T cell receptor (TCR)-CD3 complex in CD4+ and CD8+ T cells, in the absence of binding to major histocompatibility complex class II-its canonical ligand. Mechanistically, a phylogenetically conserved, acidic, tandem glutamic acid-proline repeat in the LAG3 cytoplasmic tail lowered the pH at the immune synapse and caused dissociation of the tyrosine kinase Lck from the CD4 or CD8 co-receptor, which resulted in a loss of co-receptor-TCR signaling and limited T cell activation. These observations indicated that LAG3 functioned as a signal disruptor in a major histocompatibility complex class II-independent manner, and provide insight into the mechanism of action of LAG3-targeting immunotherapies.

Cytoneme delivery of Sonic Hedgehog from ligand-producing cells requires Myosin 10 and a Dispatched-BOC/CDON co-receptor complex.

Morphogens function in concentration-dependent manners to instruct cell fate during tissue patterning. The cytoneme morphogen transport model posits that specialized filopodia extend between morphogen-sending and responding cells to ensure that appropriate signaling thresholds are achieved. How morphogens are transported along and deployed from cytonemes, how quickly a cytoneme-delivered, receptor-dependent signal is initiated, and whether these processes are conserved across phyla are not known. Herein, we reveal that the actin motor Myosin 10 promotes vesicular transport of Sonic Hedgehog (SHH) morphogen in mouse cell cytonemes, and that SHH morphogen gradient organization is altered in neural tubes of Myo10-/- mice. We demonstrate that cytoneme-mediated deposition of SHH onto receiving cells induces a rapid, receptor-dependent signal response that occurs within seconds of ligand delivery. This activity is dependent upon a novel Dispatched (DISP)-BOC/CDON co-receptor complex that functions in ligand-producing cells to promote cytoneme occurrence and facilitate ligand delivery for signal activation.

During development, cells must work together and talk to each other to build the organs and tissues of the growing embryo. To communicate precisely with long-distance targets, cells can project a series of thin finger-like structures known as cytonemes. Cells use these miniature highways to exchange cargo and signals, such as the protein sonic hedgehog (SHH for short). Alterations to the way SHH is exchanged during development predispose to cancer and lead to disorders of the nervous system. Yet, the mechanisms by which cytonemes work in mammals remain to be fully elucidated. In particular, it is still unclear how the structures start to form, and how the proteins are loaded and transported from one end to another. A 'molecular motor' called myosin 10, which can carry cargo along the internal skeleton of cells, may be involved in these processes. To find out, Hall et al. used fluorescent probes to track both myosin 10 and SHH in mouse cells, showing that myosin 10 carries SHH from the core of the signal-producing cell to the tips of cytonemes. There, the protein is passed to the target cell upon contact, triggering a quick response. SHH also appeared to be more than just passive cargo, interacting with another group of proteins in the signal-emitting cell before reaching its target. This mechanism then encourages the signalling cells to produce more cytonemes towards their neighbours. SHH is crucial during development, but also after birth: in fact, changes to SHH transport in adulthood can also disrupt tissue balance and hinder healing. Understanding how healthy tissues send this signal may reveal why and how disease emerges.

MEKK3-MEK5-ERK5 signaling promotes mitochondrial degradation.

Mitochondria are vital organelles that coordinate cellular energy homeostasis and have important roles in cell death. Therefore, the removal of damaged or excessive mitochondria is critical for maintaining proper cellular function. The PINK1-Parkin pathway removes acutely damaged mitochondria through a well-characterized mitophagy pathway, but basal mitochondrial turnover occurs via distinct and less well-understood mechanisms. Here we report that the MEKK3-MEK5-ERK5 kinase cascade is required for mitochondrial degradation in the absence of exogenous damage. We demonstrate that genetic or pharmacological inhibition of the MEKK3-MEK5-ERK5 pathway increases mitochondrial content by reducing lysosome-mediated degradation of mitochondria under basal conditions. We show that the MEKK3-MEK5-ERK5 pathway plays a selective role in basal mitochondrial degradation but is not required for non-selective bulk autophagy, damage-induced mitophagy, or restraint of mitochondrial biogenesis. This illuminates the MEKK3-MEK5-ERK5 pathway as a positive regulator of mitochondrial degradation that acts independently of exogenous mitochondrial stressors.

Loss of MAGEL2 in Prader-Willi syndrome leads to decreased secretory granule and neuropeptide production.

Prader-Willi syndrome (PWS) is a developmental disorder caused by loss of maternally imprinted genes on 15q11-q13, including melanoma antigen gene family member L2 (MAGEL2). The clinical phenotypes of PWS suggest impaired hypothalamic neuroendocrine function; however, the exact cellular defects are unknown. Here, we report deficits in secretory granule (SG) abundance and bioactive neuropeptide production upon loss of MAGEL2 in humans and mice. Unbiased proteomic analysis of Magel2pΔ/m+ mice revealed a reduction in components of SG in the hypothalamus that was confirmed in 2 PWS patient-derived neuronal cell models. Mechanistically, we show that proper endosomal trafficking by the MAGEL2-regulated WASH complex is required to prevent aberrant lysosomal degradation of SG proteins and reduction of mature SG abundance. Importantly, loss of MAGEL2 in mice, NGN2-induced neurons, and human patients led to reduced neuropeptide production. Thus, MAGEL2 plays an important role in hypothalamic neuroendocrine function, and cellular defects in this pathway may contribute to PWS disease etiology. Moreover, these findings suggest unanticipated approaches for therapeutic intervention.

Chronic optogenetic induction of stress granules is cytotoxic and reveals the evolution of ALS-FTD pathology.

Stress granules (SGs) are non-membrane-bound RNA-protein granules that assemble through phase separation in response to cellular stress. Disturbances in SG dynamics have been implicated as a primary driver of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD), suggesting the hypothesis that these diseases reflect an underlying disturbance in the dynamics and material properties of SGs. However, this concept has remained largely untestable in available models of SG assembly, which require the confounding variable of exogenous stressors. Here we introduce a light-inducible SG system, termed OptoGranules, based on optogenetic multimerization of G3BP1, which is an essential scaffold protein for SG assembly. In this system, which permits experimental control of SGs in living cells in the absence of exogenous stressors, we demonstrate that persistent or repetitive assembly of SGs is cytotoxic and is accompanied by the evolution of SGs to cytoplasmic inclusions that recapitulate the pathology of ALS-FTD. Editorial note: This article has been through an editorial process in which the authors decide how to respond to the issues raised during peer review. The Reviewing Editor's assessment is that all the issues have been addressed (see decision letter).

An unexpected protein interaction promotes drug resistance in leukemia.

The overall survival of patients with acute myeloid leukemia (AML) is poor and identification of new disease-related therapeutic targets remains a major goal for this disease. Here we show that expression of MPP1, a PDZ-domain-containing protein, highly correlated with ABCC4 in AML, is associated with worse overall survival in AML. Murine hematopoietic progenitor cells overexpressing MPP1 acquired the ability to serially replate in methylcellulose culture, a property crucially dependent upon ABCC4. The highly conserved PDZ-binding motif of ABCC4 is required for ABCC4 and MPP1 to form a protein complex, which increased ABCC4 membrane localization and retention, to enhance drug resistance. Specific disruption of this protein complex, either genetically or chemically, removed ABCC4 from the plasma membrane, increased drug sensitivity, and abrogated MPP1-dependent hematopoietic progenitor cell replating in methylcellulose. High-throughput screening identified Antimycin A as a small molecule that disrupted the ABCC4-MPP1 protein complex and reversed drug resistance in AML cell lines and in primary patient AML cells. In all, targeting the ABCC4-MPP1 protein complex can lead to new therapies to improve treatment outcome of AML, a disease where the long-term prognosis is poor.

TIA1 Mutations in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia Promote Phase Separation and Alter Stress Granule Dynamics.

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are age-related neurodegenerative disorders with shared genetic etiologies and overlapping clinical and pathological features. Here we studied a novel ALS/FTD family and identified the P362L mutation in the low-complexity domain (LCD) of T cell-restricted intracellular antigen-1 (TIA1). Subsequent genetic association analyses showed an increased burden of TIA1 LCD mutations in ALS patients compared to controls (p = 8.7 × 10-6). Postmortem neuropathology of five TIA1 mutations carriers showed a consistent pathological signature with numerous round, hyaline, TAR DNA-binding protein 43 (TDP-43)-positive inclusions. TIA1 mutations significantly increased the propensity of TIA1 protein to undergo phase transition. In live cells, TIA1 mutations delayed stress granule (SG) disassembly and promoted the accumulation of non-dynamic SGs that harbored TDP-43. Moreover, TDP-43 in SGs became less mobile and insoluble. The identification of TIA1 mutations in ALS/FTD reinforces the importance of RNA metabolism and SG dynamics in ALS/FTD pathogenesis.

Drebrin-mediated microtubule-actomyosin coupling steers cerebellar granule neuron nucleokinesis and migration pathway selection.

Neuronal migration from a germinal zone to a final laminar position is essential for the morphogenesis of neuronal circuits. While it is hypothesized that microtubule-actomyosin crosstalk is required for a neuron's 'two-stroke' nucleokinesis cycle, the molecular mechanisms controlling such crosstalk are not defined. By using the drebrin microtubule-actin crosslinking protein as an entry point into the cerebellar granule neuron system in combination with super-resolution microscopy, we investigate how these cytoskeletal systems interface during migration. Lattice light-sheet and structured illumination microscopy reveal a proximal leading process nanoscale architecture wherein f-actin and drebrin intervene between microtubules and the plasma membrane. Functional perturbations of drebrin demonstrate that proximal leading process microtubule-actomyosin coupling steers the direction of centrosome and somal migration, as well as the switch from tangential to radial migration. Finally, the Siah2 E3 ubiquitin ligase antagonizes drebrin function, suggesting a model for control of the microtubule-actomyosin interfaces during neuronal differentiation.

C9orf72 Dipeptide Repeats Impair the Assembly, Dynamics, and Function of Membrane-Less Organelles.

Expansion of a hexanucleotide repeat GGGGCC (G4C2) in C9ORF72 is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Transcripts carrying (G4C2) expansions undergo unconventional, non-ATG-dependent translation, generating toxic dipeptide repeat (DPR) proteins thought to contribute to disease. Here, we identify the interactome of all DPRs and find that arginine-containing DPRs, polyGly-Arg (GR) and polyPro-Arg (PR), interact with RNA-binding proteins and proteins with low complexity sequence domains (LCDs) that often mediate the assembly of membrane-less organelles. Indeed, most GR/PR interactors are components of membrane-less organelles such as nucleoli, the nuclear pore complex and stress granules. Genetic analysis in Drosophila demonstrated the functional relevance of these interactions to DPR toxicity. Furthermore, we show that GR and PR altered phase separation of LCD-containing proteins, insinuating into their liquid assemblies and changing their material properties, resulting in perturbed dynamics and/or functions of multiple membrane-less organelles.

IRGB10 Liberates Bacterial Ligands for Sensing by the AIM2 and Caspase-11-NLRP3 Inflammasomes.

The inflammasome is an intracellular signaling complex, which on recognition of pathogens and physiological aberration, drives activation of caspase-1, pyroptosis, and the release of the pro-inflammatory cytokines IL-1ß and IL-18. Bacterial ligands must secure entry into the cytoplasm to activate inflammasomes; however, the mechanisms by which concealed ligands are liberated in the cytoplasm have remained unclear. Here, we showed that the interferon-inducible protein IRGB10 is essential for activation of the DNA-sensing AIM2 inflammasome by Francisella novicida and contributed to the activation of the LPS-sensing caspase-11 and NLRP3 inflammasome by Gram-negative bacteria. IRGB10 directly targeted cytoplasmic bacteria through a mechanism requiring guanylate-binding proteins. Localization of IRGB10 to the bacterial cell membrane compromised bacterial structural integrity and mediated cytosolic release of ligands for recognition by inflammasome sensors. Overall, our results reveal IRGB10 as part of a conserved signaling hub at the interface between cell-autonomous immunity and innate immune sensing pathways.

Phase separation by low complexity domains promotes stress granule assembly and drives pathological fibrillization.

Stress granules are membrane-less organelles composed of RNA-binding proteins (RBPs) and RNA. Functional impairment of stress granules has been implicated in amyotrophic lateral sclerosis, frontotemporal dementia, and multisystem proteinopathy-diseases that are characterized by fibrillar inclusions of RBPs. Genetic evidence suggests a link between persistent stress granules and the accumulation of pathological inclusions. Here, we demonstrate that the disease-related RBP hnRNPA1 undergoes liquid-liquid phase separation (LLPS) into protein-rich droplets mediated by a low complexity sequence domain (LCD). While the LCD of hnRNPA1 is sufficient to mediate LLPS, the RNA recognition motifs contribute to LLPS in the presence of RNA, giving rise to several mechanisms for regulating assembly. Importantly, while not required for LLPS, fibrillization is enhanced in protein-rich droplets. We suggest that LCD-mediated LLPS contributes to the assembly of stress granules and their liquid properties and provides a mechanistic link between persistent stress granules and fibrillar protein pathology in disease.

Outer Hair Cell Lateral Wall Structure Constrains the Mobility of Plasma Membrane Proteins.

Nature's fastest motors are the cochlear outer hair cells (OHCs). These sensory cells use a membrane protein, Slc26a5 (prestin), to generate mechanical force at high frequencies, which is essential for explaining the exquisite hearing sensitivity of mammalian ears. Previous studies suggest that Slc26a5 continuously diffuses within the membrane, but how can a freely moving motor protein effectively convey forces critical for hearing? To provide direct evidence in OHCs for freely moving Slc26a5 molecules, we created a knockin mouse where Slc26a5 is fused with YFP. These mice and four other strains expressing fluorescently labeled membrane proteins were used to examine their lateral diffusion in the OHC lateral wall. All five proteins showed minimal diffusion, but did move after pharmacological disruption of membrane-associated structures with a cholesterol-depleting agent and salicylate. Thus, our results demonstrate that OHC lateral wall structure constrains the mobility of plasma membrane proteins and that the integrity of such membrane-associated structures are critical for Slc26a5's active and structural roles. The structural constraint of membrane proteins may exemplify convergent evolution of cellular motors across species. Our findings also suggest a possible mechanism for disorders of cholesterol metabolism with hearing loss such as Niemann-Pick Type C diseases.

Cross-species genomic and epigenomic landscape of retinoblastoma.

Genetically engineered mouse models (GEMMs) of human cancer are important for advancing our understanding of tumor initiation and progression as well as for testing novel therapeutics. Retinoblastoma is a childhood cancer of the developing retina that initiates with biallelic inactivation of the RB1 gene. GEMMs faithfully recapitulate the histopathology, molecular, cellular, morphometric, neuroanatomical and neurochemical features of human retinoblastoma. In this study, we analyzed the genomic and epigenomic landscape of murine retinoblastoma and compared them to human retinoblastomas to gain insight into shared mechanisms of tumor progression across species. Similar to human retinoblastoma, mouse tumors have low rates of single nucleotide variations. However, mouse retinoblastomas have higher rates of aneuploidy and regional and focal copy number changes that vary depending on the genetic lesions that initiate tumorigenesis in the developing murine retina. Furthermore, the epigenetic landscape in mouse retinoblastoma was significantly different from human tumors and some pathways that are candidates for molecular targeted therapy for human retinoblastoma such as SYK or MCL1 are not deregulated in GEMMs. Taken together, these data suggest there are important differences between mouse and human retinoblastomas with respect to the mechanism of tumor progression and those differences can have significant implications for translational research to test the efficacy of novel therapies for this devastating childhood cancer.

Distinct TCR signaling pathways drive proliferation and cytokine production in T cells.

The physiological basis and mechanistic requirements for a large number of functional immunoreceptor tyrosine-based activation motifs (ITAMs; high ITAM multiplicity) in the complex of the T cell antigen receptor (TCR) and the invariant signaling protein CD3 remain obscure. Here we found that whereas a low multiplicity of TCR-CD3 ITAMs was sufficient to engage canonical TCR-induced signaling events that led to cytokine secretion, a high multiplicity of TCR-CD3 ITAMs was required for TCR-driven proliferation. This was dependent on the formation of compact immunological synapses, interaction of the adaptor Vav1 with phosphorylated CD3 ITAMs to mediate the recruitment and activation of the oncogenic transcription factor Notch1 and, ultimately, proliferation induced by the cell-cycle regulator c-Myc. Analogous mechanistic events were also needed to drive proliferation in response to weak peptide agonists. Thus, the TCR-driven pathways that initiate cytokine secretion and proliferation are separable and are coordinated by the multiplicity of phosphorylated ITAMs in TCR-CD3.

Anti-apoptotic MCL-1 localizes to the mitochondrial matrix and couples mitochondrial fusion to respiration.

MCL-1, an anti-apoptotic BCL-2 family member that is essential for the survival of multiple cell lineages, is also among the most highly amplified genes in cancer. Although MCL-1 is known to oppose cell death, precisely how it functions to promote survival of normal and malignant cells is poorly understood. Here, we report that different forms of MCL-1 reside in distinct mitochondrial locations and exhibit separable functions. On the outer mitochondrial membrane, an MCL-1 isoform acts like other anti-apoptotic BCL-2 molecules to antagonize apoptosis, whereas an amino-terminally truncated isoform of MCL-1 that is imported into the mitochondrial matrix is necessary to facilitate normal mitochondrial fusion, ATP production, membrane potential, respiration, cristae ultrastructure and maintenance of oligomeric ATP synthase. Our results provide insight into how the surprisingly diverse salutary functions of MCL-1 may control the survival of both normal and cancer cells.

A unified model of mammalian BCL-2 protein family interactions at the mitochondria.

During apoptosis, the BCL-2 protein family controls mitochondrial outer membrane permeabilization (MOMP), but the dynamics of this regulation remain controversial. We employed chimeric proteins composed of exogenous BH3 domains inserted into a tBID backbone that can activate the proapoptotic effectors BAX and BAK to permeabilize membranes without being universally sequestered by all antiapoptotic BCL-2 proteins. We thus identified two "modes" whereby prosurvival BCL-2 proteins can block MOMP, by sequestering direct-activator BH3-only proteins ("MODE 1") or by binding active BAX and BAK ("MODE 2"). Notably, we found that MODE 1 sequestration is less efficient and more easily derepressed to promote MOMP than MODE 2. Further, MODE 2 sequestration prevents mitochondrial fusion. We provide a unified model of BCL-2 family function that helps to explain otherwise paradoxical observations relating to MOMP, apoptosis, and mitochondrial dynamics.

Directed evolution of an extremely stable fluorescent protein.

In this paper we describe the evolution of eCGP123, an extremely stable green fluorescent protein based on a previously described fluorescent protein created by consensus engineering (CGP: consensus green protein). eCGP123 could not be denatured by a standard thermal melt, preserved almost full fluorescence after overnight incubation at 80 degrees C and possessed a free energy of denaturation of 12.4 kcal/mol. It was created from CGP by a recursive process involving the sequential introduction of three destabilizing heterologous inserts, evolution to overcome the destabilization and finally 'removal' of the destabilizing insert by gene synthesis. We believe that this approach may be generally applicable to the stabilization of other proteins.

Measuring an antibody affinity distribution molecule by molecule.

Single molecule fluorescence microscopy was used to observe the binding and unbinding of hapten decorated quantum dots to individual surface immobilized antibodies. The fluorescence time history from an individual antibody site can be used to calculate its binding affinity. While quantum dot blinking occurs during these measurements, we describe a simple empirical method to correct the apparent/observed affinity to account for the blinking contribution. The combination of many single molecule affinity measurements from different antibodies yields not only the average affinity, it directly measures the full shape and character of the surface affinity distribution function.

The creation of a novel fluorescent protein by guided consensus engineering.

Consensus engineering has been used to increase the stability of a number of different proteins, either by creating consensus proteins from scratch or by modifying existing proteins so that their sequences more closely match a consensus sequence. In this paper we describe the first application of consensus engineering to the ab initio creation of a novel fluorescent protein. This was based on the alignment of 31 fluorescent proteins with >62% homology to monomeric Azami green (mAG) protein, and used the sequence of mAG to guide amino acid selection at positions of ambiguity. This consensus green protein is extremely well expressed, monomeric and fluorescent with red shifted absorption and emission characteristics compared to mAG. Although slightly less stable than mAG, it is better expressed and brighter under the excitation conditions typically used in single molecule fluorescence spectroscopy or confocal microscopy. This study illustrates the power of consensus engineering to create stable proteins using the subtle information embedded in the alignment of similar proteins and shows that the benefits of this approach may extend beyond stability.