Trazodone rescues dysregulated synaptic and mitochondrial nascent proteomes in prion neurodegeneration

The unfolded protein response (UPR) is rapidly gaining momentum as a therapeutic target for protein misfolding neurodegenerative diseases, in which its overactivation results in sustained translational repression leading to synapse loss and neurodegeneration. In mouse models of these disorders, from Alzheimer's to prion disease, modulation of the pathway-including by the licensed drug, trazodone-restores global protein synthesis rates with profound neuroprotective effects. However, the precise nature of the translational impairment, in particular the specific proteins affected in disease, and their response to therapeutic UPR modulation are poorly understood. We used non-canonical amino acid tagging (NCAT) to measure de novo protein synthesis in the brains of prion-diseased mice with and without trazodone treatment, in both whole hippocampus and cell-specifically. During disease the predominant nascent proteome changes occur in synaptic, cytoskeletal and mitochondrial proteins in both hippocampal neurons and astrocytes. Remarkably, trazodone treatment for just 2 weeks largely restored the whole disease nascent proteome in the hippocampus to that of healthy, uninfected mice, predominantly with recovery of proteins involved in synaptic and mitochondrial function. In parallel, trazodone treatment restored the disease-associated decline in synapses and mitochondria and their function to wild-type levels. In conclusion, this study increases our understanding of how translational repression contributes to neurodegeneration through synaptic and mitochondrial toxicity via depletion of key proteins essential for their function. Further, it provides new insights into the neuroprotective mechanisms of trazodone through reversal of this toxicity, relevant for the treatment of neurodegenerative diseases via translational modulation.

HNRNPH1 regulates the neuroprotective cold-shock protein RBM3 expression through poison exon exclusion

Enhanced expression of the cold-shock protein RNA binding motif 3 (RBM3) is highly neuroprotective both in vitro and in vivo. Whilst upstream signalling pathways leading to RBM3 expression have been described, the precise molecular mechanism of RBM3 cold induction remains elusive. To identify temperature-dependent modulators of RBM3, we performed a genome-wide CRISPR-Cas9 knockout screen using RBM3-reporter human iPSC-derived neurons. We found that RBM3 mRNA and protein levels are robustly regulated by several splicing factors, with heterogeneous nuclear ribonucleoprotein H1 (HNRNPH1) being the strongest positive regulator. Splicing analysis revealed that moderate hypothermia significantly represses the inclusion of a poison exon, which, when retained, targets the mRNA for nonsense-mediated decay. Importantly, we show that HNRNPH1 mediates this cold-dependent exon skipping via its thermosensitive interaction with a G-rich motif within the poison exon. Our study provides novel mechanistic insights into the regulation of RBM3 and provides further targets for neuroprotective therapeutic strategies.

ASO targeting RBM3 temperature-controlled poison exon splicing prevents neurodegeneration in vivo

Neurodegenerative diseases are increasingly prevalent in the aging population, yet no disease-modifying treatments are currently available. Increasing the expression of the cold-shock protein RBM3 through therapeutic hypothermia is remarkably neuroprotective. However, systemic cooling poses a health risk, strongly limiting its clinical application. Selective upregulation of RBM3 at normothermia thus holds immense therapeutic potential. Here we identify a poison exon within the RBM3 gene that is solely responsible for its cold-induced expression. Genetic removal or antisense oligonucleotide (ASO)-mediated manipulation of this exon yields high RBM3 levels independent of cooling. Notably, a single administration of ASO to exclude the poison exon, using FDA-approved chemistry, results in long-lasting increased RBM3 expression in mouse brains. In prion-diseased mice, this treatment leads to remarkable neuroprotection, with prevention of neuronal loss and spongiosis despite high levels of disease-associated prion protein. Our promising results in mice support the possibility that RBM3-inducing ASOs might also deliver neuroprotection in humans in conditions ranging from acute brain injury to Alzheimer's disease.

Evaluating data integrity in ribosome footprinting datasets through modelled polysome profiles

The assessment of transcriptome-wide ribosome binding to mRNAs is useful for studying the dynamic regulation of protein synthesis. Two methods frequently applied in eukaryotic cells that operate at different levels of resolution are polysome profiling, which reveals the distribution of ribosome loads across the transcriptome, and ribosome footprinting (also termed ribosome profiling or Ribo-Seq), which when combined with appropriate data on mRNA expression can reveal ribosome densities on individual transcripts. In this study we develop methods for relating the information content of these two methods to one another, by reconstructing theoretical polysome profiles from ribosome footprinting data. Our results validate both approaches as experimental tools. Although we show that both methods can yield highly consistent data, some published ribosome footprinting datasets give rise to reconstructed polysome profiles with non-physiological features. We trace these aberrant features to inconsistencies in RNA and Ribo-Seq data when compared to datasets yielding physiological polysome profiles, thereby demonstrating that modelled polysomes are useful for assessing global dataset properties such as its quality in a simple, visual approach. Aside from using polysome profile reconstructions on published datasets, we propose that this also provides a useful tool for validating new ribosome footprinting datasets in early stages of analyses.

Trazodone and patient outcomes in dementia-Limitations of naturalistic cohort data

The unfolded protein response has been increasingly implicated as an important pathological pathway and target for therapeutic intervention in neurodegeneration. The licensed antidepressant trazodone is one drug which has been proposed to act on this pathway and may therefore be a potential therapy. Previous examination of existing data for patients with dementia prescribed trazodone did not find a signal suggesting a disease modifying effect. Here we add to that literature by examining the electronic patient record of patients with dementia in Cambridgeshire UK. We found that trazodone is rarely prescribed and where it is used it is at a dose less than half that predicted to be disease modifying. We also found that patients prescribed trazodone had higher levels of neuropsychiatric symptoms and were relatively late in the disease course, likely beyond the optimal point for therapeutic intervention. We suggest it is therefore premature to discard potential therapies based on observational data alone, particularly when experimental medicine approaches to examine the effects of trazodone are feasible.

Targeting the Unfolded Protein Response as a Disease-Modifying Pathway in Dementia

Dementia is a global medical and societal challenge; it has devastating personal, social and economic costs, which will increase rapidly as the world's population ages. Despite this, there are no disease-modifying treatments for dementia; current therapy modestly improves symptoms but does not change the outcome. Therefore, new treatments are urgently needed-particularly any that can slow down the disease's progression. Many of the neurodegenerative diseases that lead to dementia are characterised by common pathological responses to abnormal protein production and misfolding in brain cells, raising the possibility of the broad application of therapeutics that target these common processes. The unfolded protein response (UPR) is one such mechanism. The UPR is a highly conserved cellular stress response to abnormal protein folding and is widely dysregulated in neurodegenerative diseases. In this review, we describe the basic machinery of the UPR, as well as the evidence for its overactivation and pathogenicity in dementia, and for the marked neuroprotective effects of its therapeutic manipulation in murine models of these disorders. We discuss drugs identified as potential UPR-modifying therapeutic agents-in particular the licensed antidepressant trazodone-and we review epidemiological and trial data from their use in human populations. Finally, we explore future directions for investigating the potential benefit of using trazodone or similar UPR-modulating compounds for disease modification in patients with dementia.

Rpl24Bst mutation suppresses colorectal cancer by promoting eEF2 phosphorylation via eEF2K

Increased protein synthesis supports the rapid cell proliferation associated with cancer. The Rpl24Bst mutant mouse reduces the expression of the ribosomal protein RPL24 and has been used to suppress translation and limit tumorigenesis in multiple mouse models of cancer. Here, we show that Rpl24Bst also suppresses tumorigenesis and proliferation in a model of colorectal cancer (CRC) with two common patient mutations, Apc and Kras. In contrast to previous reports, Rpl24Bst mutation has no effect on ribosomal subunit abundance but suppresses translation elongation through phosphorylation of eEF2, reducing protein synthesis by 40% in tumour cells. Ablating eEF2 phosphorylation in Rpl24Bst mutant mice by inactivating its kinase, eEF2K, completely restores the rates of elongation and protein synthesis. Furthermore, eEF2K activity is required for the Rpl24Bst mutant to suppress tumorigenesis. This work demonstrates that elevation of eEF2 phosphorylation is an effective means to suppress colorectal tumorigenesis with two driver mutations. This positions translation elongation as a therapeutic target in CRC, as well as in other cancers where the Rpl24Bst mutation has a tumour suppressive effect in mouse models.

Developing Therapies for Neurodegenerative Disorders: Insights from Protein Aggregation and Cellular Stress Responses

As the world's population ages, neurodegenerative disorders are poised to become the commonest cause of death. Despite this, they remain essentially untreatable. Characterized pathologically both by the aggregation of disease-specific misfolded proteins and by changes in cellular stress responses, to date, therapeutic approaches have focused almost exclusively on reducing misfolded protein load-notably amyloid beta (Aβ) in Alzheimer's disease. The repeated failure of clinical trials has led to despondency over the possibility that these disorders will ever be treated. We argue that this is in fact a time for optimism: Targeting various generic stress responses is emerging as an increasingly promising means of modifying disease progression across these disorders. New treatments are approaching clinical trials, while novel means of targeting aggregates could eventually act preventively in early disease.

TrkB signaling regulates the cold-shock protein RBM3-mediated neuroprotection

Increasing levels of the cold-shock protein, RNA-binding motif 3 (RBM3), either through cooling or by ectopic over-expression, prevents synapse and neuronal loss in mouse models of neurodegeneration. To exploit this process therapeutically requires an understanding of mechanisms controlling cold-induced RBM3 expression. Here, we show that cooling increases RBM3 through activation of TrkB via PLCγ1 and pCREB signaling. RBM3, in turn, has a hitherto unrecognized negative feedback on TrkB-induced ERK activation through induction of its specific phosphatase, DUSP6. Thus, RBM3 mediates structural plasticity through a distinct, non-canonical activation of TrkB signaling, which is abolished in RBM3-null neurons. Both genetic reduction and pharmacological antagonism of TrkB and its downstream mediators abrogate cooling-induced RBM3 induction and prevent structural plasticity, whereas TrkB inhibition similarly prevents RBM3 induction and the neuroprotective effects of cooling in prion-diseased mice. Conversely, TrkB agonism induces RBM3 without cooling, preventing synapse loss and neurodegeneration. TrkB signaling is, therefore, necessary for the induction of RBM3 and related neuroprotective effects and provides a target by which RBM3-mediated synapse-regenerative therapies in neurodegenerative disorders can be used therapeutically without the need for inducing hypothermia.

Targeting the kinase insert loop of PERK selectively modulates PERK signaling without systemic toxicity in mice

Chronic activation of the unfolded protein response (UPR), notably the branch comprising the kinase PERK and the translation initiation factor eIF2α, is a pathological feature of many neurodegenerative diseases caused by protein misfolding. Partial reduction of UPR signaling at the level of phosphorylated eIF2α is neuroprotective and avoids the pancreatic toxicity caused by full inhibition of PERK kinase activity. However, other stress pathways besides the UPR converge on phosphorylated eIF2α in the integrated stress response (ISR), which is critical to normal cellular function. We explored whether partial inhibition of PERK signaling may be a better therapeutic option. PERK-mediated phosphorylation of eIF2α requires its binding to the insert loop within PERK's kinase domain, which is, itself, phosphorylated at multiple sites. We found that, as expected, Akt mediates the phosphorylation of Thr⁷⁹⁹ in PERK. This phosphorylation event reduced eIF2α binding to PERK and selectively attenuated downstream signaling independently of PERK activity and the broader ISR. Induction of Thr⁷⁹⁹ phosphorylation with a small-molecule activator of Akt similarly reduced PERK signaling and increased both neuronal and animal survival without measurable pancreatic toxicity in a mouse model of prion disease. Thus, promoting PERK phosphorylation at Thr⁷⁹⁹ to partially down-regulate PERK-eIF2α signaling while avoiding widespread ISR inhibition may be a safe therapeutic approach in neurodegenerative disease.

Control of translation elongation in health and disease

Regulation of protein synthesis makes a major contribution to post-transcriptional control pathways. During disease, or under stress, cells initiate processes to reprogramme protein synthesis and thus orchestrate the appropriate cellular response. Recent data show that the elongation stage of protein synthesis is a key regulatory node for translational control in health and disease. There is a complex set of factors that individually affect the overall rate of elongation and, for the most part, these influence either transfer RNA (tRNA)- and eukaryotic elongation factor 1A (eEF1A)-dependent codon decoding, and/or elongation factor 2 (eEF2)-dependent ribosome translocation along the mRNA. Decoding speeds depend on the relative abundance of each tRNA, the cognate:near-cognate tRNA ratios and the degree of tRNA modification, whereas eEF2-dependent ribosome translocation is negatively regulated by phosphorylation on threonine-56 by eEF2 kinase. Additional factors that contribute to the control of the elongation rate include epigenetic modification of the mRNA, coding sequence variation and the expression of eIF5A, which stimulates peptide bond formation between proline residues. Importantly, dysregulation of elongation control is central to disease mechanisms in both tumorigenesis and neurodegeneration, making the individual key steps in this process attractive therapeutic targets. Here, we discuss the relative contribution of individual components of the translational apparatus (e.g. tRNAs, elongation factors and their modifiers) to the overall control of translation elongation and how their dysregulation contributes towards disease processes.

Small Molecules to Improve ER Proteostasis in Disease

Abnormally high levels of misfolded proteins in the endoplasmic reticulum (ER) lumen result in a stress state that contributes to the progression of several pathological conditions including diabetes, cancer, neurodegeneration, and immune dysregulation. ER stress triggers a dynamic signaling pathway known as the unfolded protein response (UPR). The UPR enforces adaptive or cell death programs by integrating information about the intensity and duration of the stress stimuli. Thus, depending on the disease context, ER stress signaling can be beneficial or detrimental. We discuss current efforts to develop small molecules to target distinct components of the UPR, and their possible applications in treating human disease, focusing on neurodegenerative diseases, metabolic disorders, and cancer.

Noncanonical Modulation of the eIF2 Pathway Controls an Increase in Local Translation during Neural Wiring

Local translation is rapidly regulated by extrinsic signals during neural wiring, but its control mechanisms remain elusive. Here we show that the extracellular cue Sema3A induces an initial burst in local translation that precisely controls phosphorylation of the translation initiation factor eIF2α via the unfolded protein response (UPR) kinase PERK. Strikingly, in contrast to canonical UPR signaling, Sema3A-induced eIF2α phosphorylation bypasses global translational repression and underlies an increase in local translation through differential activity of eIF2B mediated by protein phosphatase 1. Ultrasensitive proteomics analysis of axons reveals 75 proteins translationally controlled via the Sema3A-p-eIF2α pathway. These include proteostasis- and actin cytoskeleton-related proteins but not canonical stress markers. Finally, we show that PERK signaling is needed for directional axon migration and visual pathway development in vivo. Thus, our findings reveal a noncanonical eIF2 signaling pathway that controls selective changes in axon translation and is required for neural wiring.

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eIF2α phosphorylation underlies Sema3A-induced upregulation of global translation

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pSILAC-SP3 reveals 75 nascent proteins regulated by the Sema3A-p-eIF2α pathway

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eIF2B modulation is the key node between Sema3A and canonical stress responses

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PERK signaling is required for axon guidance and neural connectivity

Promoting the clearance of neurotoxic proteins in neurodegenerative disorders of ageing

Neurodegenerative disorders of ageing (NDAs) such as Alzheimer disease, Parkinson disease, frontotemporal dementia, Huntington disease and amyotrophic lateral sclerosis represent a major socio-economic challenge in view of their high prevalence yet poor treatment. They are often called 'proteinopathies' owing to the presence of misfolded and aggregated proteins that lose their physiological roles and acquire neurotoxic properties. One reason underlying the accumulation and spread of oligomeric forms of neurotoxic proteins is insufficient clearance by the autophagic-lysosomal network. Several other clearance pathways are also compromised in NDAs: chaperone-mediated autophagy, the ubiquitin-proteasome system, extracellular clearance by proteases and extrusion into the circulation via the blood-brain barrier and glymphatic system. This article focuses on emerging mechanisms for promoting the clearance of neurotoxic proteins, a strategy that may curtail the onset and slow the progression of NDAs.

Nitric oxide-mediated posttranslational modifications control neurotransmitter release by modulating complexin farnesylation and enhancing its clamping ability

Nitric oxide (NO) regulates neuronal function and thus is critical for tuning neuronal communication. Mechanisms by which NO modulates protein function and interaction include posttranslational modifications (PTMs) such as S-nitrosylation. Importantly, cross signaling between S-nitrosylation and prenylation can have major regulatory potential. However, the exact protein targets and resulting changes in function remain elusive. Here, we interrogated the role of NO-dependent PTMs and farnesylation in synaptic transmission. We found that NO compromises synaptic function at the Drosophila neuromuscular junction (NMJ) in a cGMP-independent manner. NO suppressed release and reduced the size of available vesicle pools, which was reversed by glutathione (GSH) and occluded by genetic up-regulation of GSH-generating and de-nitrosylating glutamate-cysteine-ligase and S-nitroso-glutathione reductase activities. Enhanced nitrergic activity led to S-nitrosylation of the fusion-clamp protein complexin (cpx) and altered its membrane association and interactions with active zone (AZ) and soluble N-ethyl-maleimide-sensitive fusion protein Attachment Protein Receptor (SNARE) proteins. Furthermore, genetic and pharmacological suppression of farnesylation and a nitrosylation mimetic mutant of cpx induced identical physiological and localization phenotypes as caused by NO. Together, our data provide evidence for a novel physiological nitrergic molecular switch involving S-nitrosylation, which reversibly suppresses farnesylation and thereby enhances the net-clamping function of cpx. These data illustrate a new mechanistic signaling pathway by which regulation of farnesylation can fine-tune synaptic release.

The unfolded protein response in neurodegenerative disorders - therapeutic modulation of the PERK pathway

The unfolded protein response (UPR) is a highly conserved protein quality control mechanism, activated in response to Endoplasmic Reticulum (ER) stress. Signalling is mediated through three branches, PERK, IRE1, and ATF6, respectively, that together provide a coordinated response that contributes to overcoming disrupted proteostasis. PERK branch activation predominantly causes a rapid reduction in global rates of translation, while the IRE1 and ATF6 branch signalling induce a transcriptional response resulting in expression of chaperones and components of the protein degradation machinery. Protein misfolding neurodegenerative diseases show disruption of proteostasis as a biochemical feature. In the brains of animal models of disease and in human post mortem tissue from many of these disorders, markers of UPR induction, particularly, the PERK pathway can be observed in close association with disease progression. Recent research has revealed dysregulated UPR signalling to be a major pathogenic mechanism in neurodegeneration, and that genetic and pharmacological modulation of the PERK pathway results in potent neuroprotection. Targeting aberrant UPR signalling is the focus of new therapeutic strategies, which importantly could be beneficial across the broad spectrum of neurodegenerative diseases.

Fine-tuning PERK signaling for neuroprotection

Protein translation and folding are tightly controlled processes in all cells, by proteostasis, an important component of which is the unfolded protein response (UPR). During periods of endoplasmic reticulum stress because of protein misfolding, the UPR activates a coordinated response in which the PERK branch activation restricts translation, while a variety of genes involved with protein folding, degradation, chaperone expression and stress responses are induced through signaling of the other branches. Chronic overactivation of the UPR, particularly the PERK branch, is observed in the brains of patients in a number of protein misfolding neurodegenerative diseases, including Alzheimer's, and Parkinson's diseases and the tauopathies. Recently, numerous genetic and pharmacological studies in mice have demonstrated the effectiveness of inhibiting the UPR for eliciting therapeutic benefit and boosting memory. In particular, fine-tuning the level of PERK inhibition to provide neuroprotection without adverse side effects has emerged as a safe, effective approach. This includes the recent discovery of licensed drugs that can now be repurposed in clinical trials for new human treatments for dementia. This review provides an overview of the links between UPR overactivation and neurodegeneration in protein misfolding disorders. It discusses recent therapeutic approaches targeting this pathway, with a focus on treatments that fine-tune PERK signaling.

Repurposed drugs targeting eIF2α-P-mediated translational repression prevent neurodegeneration in mice

See Mercado and Hetz (doi:10.1093/brain/awx107) for a scientific commentary on this article.Signalling through the PERK/eIF2α-P branch of the unfolded protein response plays a critical role in controlling protein synthesis rates in cells. This pathway is overactivated in brains of patients with Alzheimer’s disease and related disorders and has recently emerged as a promising therapeutic target for these currently untreatable conditions. Thus, in mouse models of neurodegenerative disease, prolonged overactivation of PERK/eIF2α-P signalling causes sustained attenuation of protein synthesis, leading to memory impairment and neuronal loss. Re-establishing translation rates by inhibition of eIF2α-P activity, genetically or pharmacologically, restores memory and prevents neurodegeneration and extends survival. However, the experimental compounds used preclinically are unsuitable for use in humans, due to associated toxicity or poor pharmacokinetic properties. To discover compounds that have anti-eIF2α-P activity suitable for clinical use, we performed phenotypic screens on a NINDS small molecule library of 1040 drugs. We identified two compounds, trazodone hydrochloride and dibenzoylmethane, which reversed eIF2α-P-mediated translational attenuation in vitro and in vivo. Both drugs were markedly neuroprotective in two mouse models of neurodegeneration, using clinically relevant doses over a prolonged period of time, without systemic toxicity. Thus, in prion-diseased mice, both trazodone and dibenzoylmethane treatment restored memory deficits, abrogated development of neurological signs, prevented neurodegeneration and significantly prolonged survival. In tauopathy-frontotemporal dementia mice, both drugs were neuroprotective, rescued memory deficits and reduced hippocampal atrophy. Further, trazodone reduced p-tau burden. These compounds therefore represent potential new disease-modifying treatments for dementia. Trazodone in particular, a licensed drug, should now be tested in clinical trials in patients.

M1 muscarinic allosteric modulators slow prion neurodegeneration and restore memory loss

The current frontline symptomatic treatment for Alzheimer's disease (AD) is whole-body upregulation of cholinergic transmission via inhibition of acetylcholinesterase. This approach leads to profound dose-related adverse effects. An alternative strategy is to selectively target muscarinic acetylcholine receptors, particularly the M1 muscarinic acetylcholine receptor (M1 mAChR), which was previously shown to have procognitive activity. However, developing M1 mAChR-selective orthosteric ligands has proven challenging. Here, we have shown that mouse prion disease shows many of the hallmarks of human AD, including progressive terminal neurodegeneration and memory deficits due to a disruption of hippocampal cholinergic innervation. The fact that we also show that muscarinic signaling is maintained in both AD and mouse prion disease points to the latter as an excellent model for testing the efficacy of muscarinic pharmacological entities. The memory deficits we observed in mouse prion disease were completely restored by treatment with benzyl quinolone carboxylic acid (BQCA) and benzoquinazoline-12 (BQZ-12), two highly selective positive allosteric modulators (PAMs) of M1 mAChRs. Furthermore, prolonged exposure to BQCA markedly extended the lifespan of diseased mice. Thus, enhancing hippocampal muscarinic signaling using M1 mAChR PAMs restored memory loss and slowed the progression of mouse prion disease, indicating that this ligand type may have clinical benefit in diseases showing defective cholinergic transmission, such as AD.

Mitofusin-mediated ER stress triggers neurodegeneration in pink1/parkin models of Parkinson's disease

Mutations in PINK1 and PARKIN cause early-onset Parkinson's disease (PD), thought to be due to mitochondrial toxicity. Here, we show that in Drosophila pink1 and parkin mutants, defective mitochondria also give rise to endoplasmic reticulum (ER) stress signalling, specifically to the activation of the protein kinase R-like endoplasmic reticulum kinase (PERK) branch of the unfolded protein response (UPR). We show that enhanced ER stress signalling in pink1 and parkin mutants is mediated by mitofusin bridges, which occur between defective mitochondria and the ER. Reducing mitofusin contacts with the ER is neuroprotective, through suppression of PERK signalling, while mitochondrial dysfunction remains unchanged. Further, both genetic inhibition of dPerk-dependent ER stress signalling and pharmacological inhibition using the PERK inhibitor GSK2606414 were neuroprotective in both pink1 and parkin mutants. We conclude that activation of ER stress by defective mitochondria is neurotoxic in pink1 and parkin flies and that the reduction of this signalling is neuroprotective, independently of defective mitochondria. A video abstract for this article is available online in the supplementary information

The unfolded protein response: mechanisms and therapy of neurodegeneration

Activation of the unfolded protein response is emerging as a common theme in protein-misfolding neurodegenerative diseases, with relevant markers observed in patient tissue and mouse models. Genetic and pharmacological manipulation of the pathway in several mouse models has shown that this is not a passive consequence of the neurodegeneration process. Rather, overactivation of the protein kinase RNA-like ER kinase (PERK, encoded by EIF2AK3) branch of the unfolded protein response directly contributes to disease pathogenesis through the critical reduction in neuronal protein synthesis rates, essential for learning and memory and for neuronal survival. The pharmacological inhibition of this process in these models is strikingly neuroprotective, resulting in the discovery of the first small molecule preventing neurodegeneration and clinical disease in vivo. This now represents a potential generic approach for boosting memory and preventing neurodegeneration across the spectrum of these disorders, albeit with some exceptions, independent of disease-specific proteins. Targeting the unfolded protein response, and particularly PERK-branch mediated translational failure is thus an increasingly compelling strategy for new treatments for dementia and neurodegenerative disease.

Cooling-induced SUMOylation of EXOSC10 down-regulates ribosome biogenesis

The RNA exosome is essential for 3' processing of functional RNA species and degradation of aberrant RNAs in eukaryotic cells. Recent reports have defined the substrates of the exosome catalytic domains and solved the multimeric structure of the exosome complex. However, regulation of exosome activity remains poorly characterized, especially in response to physiological stress. Following the observation that cooling of mammalian cells results in a reduction in 40S:60S ribosomal subunit ratio, we uncover regulation of the nuclear exosome as a result of reduced temperature. Using human cells and an in vivo model system allowing whole-body cooling, we observe reduced EXOSC10 (hRrp6, Pm/Scl-100) expression in the cold. In parallel, both models of cooling increase global SUMOylation, leading to the identification of specific conjugation of SUMO1 to EXOSC10, a process that is increased by cooling. Furthermore, we define the major SUMOylation sites in EXOSC10 by mutagenesis and show that overexpression of SUMO1 alone is sufficient to suppress EXOSC10 abundance. Reducing EXOSC10 expression by RNAi in human cells correlates with the 3' preribosomal RNA processing defects seen in the cold as well as reducing the 40S:60S ratio, a previously uncharacterized consequence of EXOSC10 suppression. Together, this work illustrates that EXOSC10 can be modified by SUMOylation and identifies a physiological stress where this regulation is prevalent both in vitro and in vivo.

Review: Modulating the unfolded protein response to prevent neurodegeneration and enhance memory

Recent evidence has placed the unfolded protein response (UPR) at the centre of pathological processes leading to neurodegenerative disease. The translational repression caused by UPR activation starves neurons of the essential proteins they need to function and survive. Restoration of protein synthesis, via genetic or pharmacological means, is neuroprotective in animal models, prolonging survival. This is of great interest due to the observation of UPR activation in the post mortem brains of patients with Alzheimer's, Parkinson's, tauopathies and prion diseases. Protein synthesis is also an essential step in the formation of new memories. Restoring translation in disease or increasing protein synthesis from basal levels has been shown to improve memory in numerous models. As neurodegenerative diseases often present with memory impairments, targeting the UPR to both provide neuroprotection and enhance memory provides an extremely exciting novel therapeutic target.

PERK inhibition prevents tau-mediated neurodegeneration in a mouse model of frontotemporal dementia

The PERK-eIF2α branch of the Unfolded Protein Response (UPR) mediates the transient shutdown of translation in response to rising levels of misfolded proteins in the endoplasmic reticulum. PERK and eIF2α activation are increasingly recognised in postmortem analyses of patients with neurodegenerative disorders, including Alzheimer’s disease, the tauopathies and prion disorders. These are all characterised by the accumulation of misfolded disease-specific proteins in the brain in association with specific patterns of neuronal loss, but the role of UPR activation in their pathogenesis is unclear. In prion-diseased mice, overactivation of PERK-P/eIF2α-P signalling results in the sustained reduction in global protein synthesis, leading to synaptic failure, neuronal loss and clinical disease. Critically, restoring vital neuronal protein synthesis rates by inhibiting the PERK-eIF2α pathway, both genetically and pharmacologically, prevents prion neurodegeneration downstream of misfolded prion protein accumulation. Here we show that PERK-eIF2α-mediated translational failure is a key process leading to neuronal loss in a mouse model of frontotemporal dementia, where the misfolded protein is a form of mutant tau. rTg4510 mice, which overexpress the P301L tau mutation, show dysregulated PERK signalling and sustained repression of protein synthesis by 6 months of age, associated with onset of neurodegeneration. Treatment with the PERK inhibitor, GSK2606414, from this time point in mutant tau-expressing mice restores protein synthesis rates, protecting against further neuronal loss, reducing brain atrophy and abrogating the appearance of clinical signs. Further, we show that PERK-eIF2α activation also contributes to the pathological phosphorylation of tau in rTg4510 mice, and that levels of phospho-tau are lowered by PERK inhibitor treatment, providing a second mechanism of protection. The data support UPR-mediated translational failure as a generic pathogenic mechanism in protein-misfolding disorders, including tauopathies, that can be successfully targeted for prevention of neurodegeneration.

Partial restoration of protein synthesis rates by the small molecule ISRIB prevents neurodegeneration without pancreatic toxicity

Activation of the PERK branch of the unfolded protein response (UPR) in response to protein misfolding within the endoplasmic reticulum (ER) results in the transient repression of protein synthesis, mediated by the phosphorylation of the alpha subunit of eukaryotic initiation factor 2 (eIF2α). This is part of a wider integrated physiological response to maintain proteostasis in the face of ER stress, the dysregulation of which is increasingly associated with a wide range of diseases, particularly neurodegenerative disorders. In prion-diseased mice, persistently high levels of eIF2α cause sustained translational repression leading to catastrophic reduction of critical proteins, resulting in synaptic failure and neuronal loss. We previously showed that restoration of global protein synthesis using the PERK inhibitor GSK2606414 was profoundly neuroprotective, preventing clinical disease in prion-infected mice. However, this occured at the cost of toxicity to secretory tissue, where UPR activation is essential to healthy functioning. Here we show that pharmacological modulation of eIF2α-P-mediated translational inhibition can be achieved to produce neuroprotection without pancreatic toxicity. We found that treatment with the small molecule ISRIB, which restores translation downstream of eIF2α, conferred neuroprotection in prion-diseased mice without adverse effects on the pancreas. Critically, ISRIB treatment resulted in only partial restoration of global translation rates, as compared with the complete restoration of protein synthesis seen with GSK2606414. ISRIB likely provides sufficient rates of protein synthesis for neuronal survival, while allowing some residual protective UPR function in secretory tissue. Thus, fine-tuning the extent of UPR inhibition and subsequent translational de-repression uncouples neuroprotective effects from pancreatic toxicity. The data support the pursuit of this approach to develop new treatments for a range of neurodegenerative disorders that are currently incurable.

Oral treatment targeting the unfolded protein response prevents neurodegeneration and clinical disease in prion-infected mice

During prion disease, an increase in misfolded prion protein (PrP) generated by prion replication leads to sustained overactivation of the branch of the unfolded protein response (UPR) that controls the initiation of protein synthesis. This results in persistent repression of translation, resulting in the loss of critical proteins that leads to synaptic failure and neuronal death. We have previously reported that localized genetic manipulation of this pathway rescues shutdown of translation and prevents neurodegeneration in a mouse model of prion disease, suggesting that pharmacological inhibition of this pathway might be of therapeutic benefit. We show that oral treatment with a specific inhibitor of the kinase PERK (protein kinase RNA-like endoplasmic reticulum kinase), a key mediator of this UPR pathway, prevented UPR-mediated translational repression and abrogated development of clinical prion disease in mice, with neuroprotection observed throughout the mouse brain. This was the case for animals treated both at the preclinical stage and also later in disease when behavioral signs had emerged. Critically, the compound acts downstream and independently of the primary pathogenic process of prion replication and is effective despite continuing accumulation of misfolded PrP. These data suggest that PERK, and other members of this pathway, may be new therapeutic targets for developing drugs against prion disease or other neurodegenerative diseases where the UPR has been implicated.

Targeting synaptic pathology with a novel affinity mass spectrometry approach

We report a novel strategy for studying synaptic pathology by concurrently measuring levels of four SNARE complex proteins from individual brain tissue samples. This method combines affinity purification and mass spectrometry and can be applied directly for studies of SNARE complex proteins in multiple species or modified to target other key elements in neuronal function. We use the technique to demonstrate altered levels of presynaptic proteins in Alzheimer disease patients and prion-infected mice.

Prions: generation and spread versus neurotoxicity

Neurodegenerative diseases are characterized by the aggregation of misfolded proteins in the brain. Among these disorders are the prion diseases, which are transmissible, and in which the misfolded proteins (“prions”) are also the infectious agent. Increasingly, it appears that misfolded proteins in Alzheimer and Parkinson diseases and the tauopathies also propagate in a “prion-like” manner. However, the association between prion formation, spread, and neurotoxicity is not clear. Recently, we showed that in prion disease, protein misfolding leads to neurodegeneration through dysregulation of generic proteostatic mechanisms, specifically, the unfolded protein response. Genetic and pharmacological manipulation of the unfolded protein response was neuroprotective despite continuing prion replication, hence dissociating this from neurotoxicity. The data have clear implications for treatment across the spectrum of these disorders, targeting pathogenic processes downstream of protein misfolding.

Enhancing nucleotide metabolism protects against mitochondrial dysfunction and neurodegeneration in a PINK1 model of Parkinson's disease

Mutations in PINK1 cause early-onset Parkinson's disease (PD). Studies in Drosophila melanogaster have highlighted mitochondrial dysfunction on loss of Pink1 as a central mechanism of PD pathogenesis. Here we show that global analysis of transcriptional changes in Drosophila pink1 mutants reveals an upregulation of genes involved in nucleotide metabolism, critical for neuronal mitochondrial DNA synthesis. These key transcriptional changes were also detected in brains of PD patients harbouring PINK1 mutations. We demonstrate that genetic enhancement of the nucleotide salvage pathway in neurons of pink1 mutant flies rescues mitochondrial impairment. In addition, pharmacological approaches enhancing nucleotide pools reduce mitochondrial dysfunction caused by Pink1 deficiency. We conclude that loss of Pink1 evokes the activation of a previously unidentified metabolic reprogramming pathway to increase nucleotide pools and promote mitochondrial biogenesis. We propose that targeting strategies enhancing nucleotide synthesis pathways may reverse mitochondrial dysfunction and rescue neurodegeneration in PD and, potentially, other diseases linked to mitochondrial impairment.

miRNAs-19b, -29b-2* and -339-5p show an early and sustained up-regulation in ischemic models of stroke

Stroke, the loss of neurons after ischemic insult to the brain, is one of the leading causes of death and disability worldwide. Despite its prevalence and severity, current therapy is extremely limited, highlighting the importance of further understanding the molecular events underlying ischemia-induced neuronal cell death. An ischemic area can be subdivided into two separate pathophysiological regions: the rapidly dying necrotic core, and the potentially salvageable apoptotic penumbra. Understanding molecular events occurring in the apoptotic ischemic penumbra may give greater insight into mechanisms controlling this salvageable tissue. miRNAs are known to have key roles in the regulation of gene expression in numerous pathological conditions, including the modulation of distinct pathways in stroke. However, previous studies have profiled miRNAs in the whole ischemic infarct, and do not differentiate between miRNA regulation in the necrotic core versus the apoptotic penumbra. We asked if there were unique miRNAs that are differentially regulated following ischemic insults in the salvageable apoptotic penumbra. miRNA expression profiles were compared in the whole infarct from in vivo stroke models, using the three vessel occlusion approach, to an in vitro model of the ischemic penumbra, prior to apoptotic induction. Multiple miRNAs were found to be differentially regulated following ischemic insults in each system. However, miR-19b, miR-29b-2* and miR-339-5p were significantly up-regulated in both model systems. Further, we confirmed these results in a neuroblastoma cell line subjected to a penumbra-like ischemic insult that induced the apoptotic cell death pathway. The data show that miR-19b, miR-29b-2* and miR-339-5p are up-regulated following ischemic insults and may be regulating gene expression to control important cellular pathways in the salvageable ischemic penumbra. Further investigation of their role and mRNA target identification may lead to new insights into the molecular mechanisms taking place in the salvageable apoptotic penumbra.

RNAi for the treatment of prion disease: a window for intervention in neurodegeneration?

Effective treatment of neurodegenerative disease is one of the major challenges facing biomedical research. These disorders, which include Alzheimer's, Huntington's and Parkinson's diseases - as well as the rarer prion diseases - constitute an ever-increasing burden in the developed world, socially, medically and economically. The key barrier to effective therapy is that they present clinically when neuronal loss is advanced, and irreversible. Current treatments are almost all directed at modifying symptoms; few address underlying pathogenic mechanisms and are inevitably delivered too late to rescue dying neurons. In the field of prion diseases, however, insights into the molecular basis and the temporal evolution of prion neurotoxicity are increasing. Recent work in mice leads to new hope for the treatment of these disorders, and potentially for rescuing neurodegeneration more broadly. Using lentivirally mediated RNA interference (RNAi) against native prion protein (PrP), White et al report the first intervention resulting in neuronal rescue, prevention of symptoms and increased survival in mice with established prion disease. Central to the effectiveness of this strategy are both the target and the timing of the intervention: the treatment prevents the formation of the neurotoxic prion agent at a point when diseased neurons can still be saved from death. This review introduces the basic concepts of prion pathogenesis, emerging insights into mechanisms of prion neurotoxicity and the rationale for targeting endogenous prion protein (PrP) in prion therapeutics. It describes the discovery of a window of reversibility of early neuronal damage in prion disease and how together these advances led to the subsequent development of the strategy using RNAi based therapy for these disorders. It discusses the use and relevance of this approach more broadly in neurodegeneration.

Prion neurodegeneration: starts and stops at the synapse

Synaptic dysfunction is a key process in the evolution of many neurodegenerative diseases, with synaptic loss preceding that of neuronal cell bodies. In Alzheimer, Huntington, and prion diseases early synaptic changes correlate with cognitive and motor decline, and altered synaptic function may also underlie deficits in a number of psychiatric and neurodevelopmental conditions. The formation, remodelling and elimination of spines and synapses are continual physiological processes, moulding cortical architecture, underpinning the abilities to learn and remember. In disease, however, particularly in protein misfolding neurodegenerative disorders, lost synapses are not replaced and this loss is followed by neuronal death. These two processes are separately regulated, with mechanistic, spatial and temporal segregation of the death 'routines' of synapses and cell bodies. Recent insights into the reversibility of synaptic dysfunction in a mouse model of prion disease at neurophysiological, behavioral and morphological levels call for a deeper analysis of the mechanisms underlying neurotoxicity at the synapse, and have important implications for therapy of prion and other neurodegenerative disorders.

The role of GPI-anchored PrP C in mediating the neurotoxic effect of scrapie prions in neurons

There are two central phenomena in prion disease: prion replication and prion neurotoxicity. Underlying them both is the conversion of a host-encoded ubiquitously expressed protein, prion protein (PrP(C)), into a partially-protease resistant isoform, PrP(Sc), which accumulates in the brain. PrP(Sc) is associated with both pathology and infectivity. In the absence of PrP(C), PrP(Sc) cannot be generated and PrP-null mice do not propagate infectivity or develop pathology on infection with scrapie. However, while PrP(C) expression is fundamental to both prion infectivity and neurodegeneration, the uncoupling of these processes is a growing concept in the field. This dissociation is evident in subclinical states of prion infection, where PrP(Sc) levels are high in the absence of disease, and in transgenic mice experiments, where, despite extra-neuronal PrP(Sc) accumulation, even in very high amounts, there is no neurotoxicity. Both these models have further implications. Thus depleting PrP(C) from neurons (but not glia) of prion-infected mice prevents clinical disease, and detaching it from the surface of cells by removing its anchor does the same. The uncoupling toxicity and infectivity has implications for the nature of the neurotoxic species; these mouse models suggest that the site for the generation of this species is intra-neuronal. This review considers the role of neuronal surface-expressed PrP(C) in mediating toxicity in prion infection, and the dissociation of this from the deposition of PrP(Sc).

Therapy for prion diseases: Insights from the use of RNA interference

Insights into the molecular basis and the temporal evolution of neurotoxicity in prion disease are increasing, and recent work in mice leads to new avenues for targeting treatment of these disorders. Using lentivirally mediated RNA interference (RNAi) against native prion protein (PrP), White et al. report the first therapeutic intervention that results in neuronal rescue, prevents symptoms and increases survival in mice with established prion disease.(1) Both the target and the timing of treatment here are crucial to the effectiveness of this strategy: the formation of the neurotoxic prion agent is prevented at a point when diseased neurons can still be saved from death. But the data also give new insights into the timing of treatment in the context of the pattern of spread of prion infection throughout the brain, with implications for developing the most effective treatments.

Targeting cellular prion protein reverses early cognitive deficits and neurophysiological dysfunction in prion-infected mice

Currently, no treatment can prevent the cognitive and motor decline associated with widespread neurodegeneration in prion disease. However, we previously showed that targeting endogenous neuronal prion protein (PrP(C)) (the precursor of its disease-associated isoform, PrP(Sc)) in mice with early prion infection reversed spongiform change and prevented clinical symptoms and neuronal loss. We now show that cognitive and behavioral deficits and impaired neurophysiological function accompany early hippocampal spongiform pathology. Remarkably, these behavioral and synaptic impairments recover when neuronal PrP(C) is depleted, in parallel with reversal of spongiosis. Thus, early functional impairments precede neuronal loss in prion disease and can be rescued. Further, they occur before extensive PrP(Sc) deposits accumulate and recover rapidly after PrP(C) depletion, supporting the concept that they are caused by a transient neurotoxic species, distinct from aggregated PrP(Sc). These data suggest that early intervention in human prion disease may lead to recovery of cognitive and behavioral symptoms.

Inherited prion disease with an alanine to valine mutation at codon 117 in the prion protein gene

A large English family with autosomal dominant segregation of presenile dementia, ataxia and other neuropsychiatric features is described. Diagnoses of demyelinating disease, Alzheimer's disease, Creutzfeldt-Jakob disease (CJD) and Gerstmann-Sträussler-Scheinker syndrome have been attributed to particular individuals at different times. An Irish family, likely to be part of the same kindred, is also described, in which diagnoses of multiple sclerosis, dementia, corticobasal degeneration and new variant CJD have been considered in affected individuals. Molecular genetic studies have enabled the classification of this disease at the molecular level as one of the group of inherited prion diseases, with the substitution of valine for alanine at codon 117 of the prion protein gene (PRNP). Only three other kindreds have been described world-wide with this mutation and only limited phenotypic information has been reported. Here we describe the phenotypic spectrum of inherited prion disease (PrPA117V). The diversity of phenotypic expression seen in this kindred emphasizes the logic of molecular classification of the inherited prion diseases rather than classification by specific clinicopathological syndrome. Indeed, inherited prion disease should be excluded by PRNP analysis in any individual presenting with atypical presenile dementia or neuropsychiatric features and ataxia, including suspected cases of new variant CJD.