Topics overview: Hypothalamic stem cells and ageing speed, The role of AMPA receptors in excitatory neurotransmission, The application of integrative genomics to medulloblastoma, In vivo genetic screens in tumour models, The treatment of blinding retinal diseases.
1. Hypothalamic stem cells control ageing speed partly through exosomal miRNAs
It has been proposed that the hypothalamus helps to control ageing, but the mechanisms responsible remain unclear. Here Yalin Zhang at Albert Einstein College of Medicine in New York, USA and her colleagues develop several mouse models in which hypothalamic stem/progenitor cells that co-express Sox2 and Bmi1 are ablated, as they observed that ageing in mice started with a substantial loss of these hypothalamic cells. Each mouse model consistently displayed acceleration of ageing-like physiological changes or a shortened lifespan. Conversely, ageing retardation and lifespan extension were achieved in mid-aged mice that were locally implanted with healthy hypothalamic stem/progenitor cells that had been genetically engineered to survive in the ageing-related hypothalamic inflammatory microenvironment. Mechanistically, hypothalamic stem/progenitor cells contributed greatly to exosomal microRNAs (miRNAs) in the cerebrospinal fluid, and these exosomal miRNAs declined during ageing, whereas central treatment with healthy hypothalamic stem/progenitor cell-secreted exosomes led to the slowing of ageing. In conclusion, ageing speed is substantially controlled by hypothalamic stem cells, partially through the release of exosomal miRNAs.
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2. Channel opening and gating mechanism in AMPA-subtype glutamate receptors
AMPA-subtype ionotropic glutamate receptors mediate fast excitatory neurotransmission throughout the central nervous system. Gated by the neurotransmitter glutamate, AMPA receptors are critical for synaptic strength and dysregulation of AMPA receptor-mediated signalling is linked to numerous neurological diseases. Here, Edward C. Twomey at Columbia University in New York, USA and her colleagues use cryo-electron microscopy to solve the structures of AMPA receptor-auxiliary subunit complexes in the apo, antagonist and agonist-bound states and elucidate the iris-like mechanism of ion channel opening. The ion channel selectivity filter is formed by the extended portions of the re-entrant M2 loops, while the helical portions of M2 contribute to extensive hydrophobic interfaces between AMPA receptor subunits in the ion channel. They show how the permeation pathway changes upon channel opening and identify conformational changes throughout the entire AMPA receptor that accompany activation and desensitization. Their findings provide a framework for understanding gating across the family of ionotropic glutamate receptors and the role of AMPA receptors in excitatory neurotransmission.
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3. The whole-genome landscape of medulloblastoma subtypes
Current therapies for medulloblastoma, a highly malignant childhood brain tumour, impose debilitating effects on the developing child, and highlight the need for molecularly targeted treatments with reduced toxicity. Previous studies have been unable to identify the full spectrum of driver genes and molecular processes that operate in medulloblastoma subgroups. Here Paul A. Northcott at German Cancer Research Center in Heidelberg, Germany and his colleagues analyse the somatic landscape across 491 sequenced medulloblastoma samples and the molecular heterogeneity among 1,256 epigenetically analysed cases, and identify subgroup-specific driver alterations that include previously undiscovered actionable targets. Driver mutations were confidently assigned to most patients belonging to Group 3 and Group 4 medulloblastoma subgroups, greatly enhancing previous knowledge. New molecular subtypes were differentially enriched for specific driver events, including hotspot in-frame insertions that target KBTBD4 and ‘enhancer hijacking’ events that activate PRDM6. Thus, the application of integrative genomics to an extensive cohort of clinical samples derived from a single childhood cancer entity revealed a series of cancer genes and biologically relevant subtype diversity that represent attractive therapeutic targets for the treatment of patients with medulloblastoma.
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4. In vivo CRISPR screening identifies Ptpn2 as a cancer immunotherapy target
Immunotherapy with PD-1 checkpoint blockade is effective in only a minority of patients with cancer, suggesting that additional treatment strategies are needed. Here Robert T. Manguso at Dana-Farber Cancer Institute in Massachusetts, USA and his colleagues use a pooled in vivo genetic screening approach using CRISPR–Cas9 genome editing in transplantable tumours in mice treated with immunotherapy to discover previously undescribed immunotherapy targets. They tested 2,368 genes expressed by melanoma cells to identify those that synergize with or cause resistance to checkpoint blockade. They recovered the known immune evasion molecules PD-L1 and CD47, and confirmed that defects in interferon-γ signalling caused resistance to immunotherapy. Tumours were sensitized to immunotherapy by deletion of genes involved in several diverse pathways, including NF-κB signalling, antigen presentation and the unfolded protein response. In addition, deletion of the protein tyrosine phosphatase PTPN2 in tumour cells increased the efficacy of immunotherapy by enhancing interferon-γ-mediated effects on antigen presentation and growth suppression. In vivo genetic screens in tumour models can identify new immunotherapy targets in unanticipated pathways.
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5. Stimulation of functional neuronal regeneration from Müller glia in adult mice.
Many retinal diseases lead to the loss of retinal neurons and cause visual impairment. The adult mammalian retina has little capacity for regeneration. By contrast, teleost fish functionally regenerate their retina following injury, and Müller glia (MG) are the source of regenerated neurons. The proneural transcription factor Ascl1 is upregulated in MG after retinal damage in zebrafish and is necessary for regeneration. Although Ascl1 is not expressed in mammalian MG after injury, forced expression of Ascl1 in mouse MG induces a neurogenic state in vitro and in vivo after NMDA (N-methyl-D-aspartate) damage in young mice. However, by postnatal day 16, mouse MG lose neurogenic capacity, despite Ascl1 overexpression. Loss of neurogenic capacity in mature MG is accompanied by reduced chromatin accessibility, suggesting that epigenetic factors limit regeneration. Here Nikolas L. Jorstad at University of Washington in Washington, USA and his colleagues show that MG-specific overexpression of Ascl1, together with a histone deacetylase inhibitor, enables adult mice to generate neurons from MG after retinal injury. The MG-derived neurons express markers of inner retinal neurons, synapse with host retinal neurons, and respond to light. Using an assay for transposase-accessible chromatin with high-throughput sequencing (ATAC–seq), they show that the histone deacetylase inhibitor promotes accessibility at key gene loci in the MG, and allows more effective reprogramming. Their results thus provide a new approach for the treatment of blinding retinal diseases.
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