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

A small molecule targeting protein translation does not rescue spatial learning and memory deficits in the hAPP-J20 mouse model of Alzheimer's disease

A small molecule named ISRIB has recently been described to enhance memory in rodents. In this study we aimed to test whether ISRIB would reverse learning and memory deficits in the J20 mouse model of human amyloid precursor protein (hAPP) overexpression, a model that simulates many aspects of Alzheimer’s disease in which memory deficits are a hallmark feature. We did not observe a significant rescue effect with ISRIB treatment on spatial learning and memory as assessed in the Morris water maze in J20 mice. We also did not observe a significant enhancement of spatial learning or memory in nontransgenic mice with ISRIB treatment, although a trend emerged for memory enhancement in one cohort of mice. Future preclinical studies with ISRIB would benefit from additional robust markers of target engagement in the brain.

Synthesis, in Vitro Evaluation, and Radiolabeling of Fluorinated Puromycin Analogues: Potential Candidates for PET Imaging of Protein Synthesis

There is currently no ideal radiotracer for imaging of protein synthesis rate (PSR) by positron emission tomography (PET). Existing fluorine-18-labeled amino acid-based radiotracers predominantly visualize amino acid transporter processes, and in many cases they are not incorporated into nascent proteins at all. Others are radiolabeled with the short-half-life positron emitter carbon-11, which is rather impractical for many PET centers. Based on the puromycin (6) structural manifold, a series of 10 novel derivatives of 6 was prepared via Williamson ether synthesis from a common intermediate. A bioluminescence assay was employed to study their inhibitory action on protein synthesis, which identified the fluoroethyl analogue 7b as a lead compound. The fluorine-18 analogue was prepared via nucleophilic substitution of the corresponding tosylate precursor in a modest radiochemical yield of 2 ± 0.6% with excellent radiochemical purity (>99%) and showed complete stability over 3 h at ambient temperature.

The effects of proteasomal inhibition on synaptic proteostasis

Synaptic function crucially depends on uninterrupted synthesis and degradation of synaptic proteins. While much has been learned on synaptic protein synthesis, little is known on the routes by which synaptic proteins are degraded. Here we systematically studied how inhibition of the ubiquitin-proteasome system (UPS) affects the degradation rates of thousands of neuronal and synaptic proteins. We identified a group of proteins, including several proteins related to glutamate receptor trafficking, whose degradation rates were significantly slowed by UPS inhibition. Unexpectedly, however, degradation rates of most synaptic proteins were not significantly affected. Interestingly, many of the differential effects of UPS inhibition were readily explained by a quantitative framework that considered known metabolic turnover rates for the same proteins. In contrast to the limited effects on protein degradation, UPS inhibition profoundly and preferentially suppressed the synthesis of a large number of synaptic proteins. Our findings point to the importance of the UPS in the degradation of certain synaptic proteins, yet indicate that under basal conditions most synaptic proteins might be degraded through alternative pathways.

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.

Gene therapy to target ER stress in brain diseases

Gene therapy based on the use of Adeno-associated viruses (AAVs) is emerging as a safe and stable strategy to target molecular pathways involved in a variety of brain diseases. Endoplasmic reticulum (ER) stress is proposed as a transversal feature of most animal models and clinical samples from patients affected with neurodegenerative diseases. Manipulation of the unfolded protein response (UPR), a major homeostatic reaction under ER stress conditions, had proved beneficial in diverse models of neurodegeneration. Although increasing number of drugs are available to target ER stress, the use of small molecules to treat chronic brain diseases is challenging because of poor blood brain barrier permeability and undesirable side effects due to the role of the UPR in the physiology of peripheral organs. Gene therapy is currently considered a possible future alternative to circumvent these problems by the delivery of therapeutic agents to selective regions and cell types of the nervous system. Here we discuss current efforts to design gene therapy strategies to alleviate ER stress on a disease context. This article is part of a Special Issue entitled SI:ER stress.

Injury to the nervous system: A look into the ER

Injury to the central or peripheral nervous systems leads to the loss of cognitive and/or sensorimotor capabilities that still lack an effective treatment. Although injury to the nervous system involves multiple and complex molecular factors, alteration to protein homeostasis is emerging as a relevant pathological mechanism. In particular, chronic endoplasmic reticulum (ER) stress is proposed as a possible driver of neuronal dysfunction in conditions such as spinal cord injury, stroke and damage to peripheral nerves. Importantly, manipulation of the unfolded protein response (UPR), a homeostatic pathway engaged by ER stress, has proved effective in improving cognitive and motor recovery after nervous system injury. Here we provide an overview on recent findings depicting a functional role of the UPR to the functional recovery after injury in the peripheral and central nervous systems. This article is part of a Special Issue entitled SI:ER stress.

Activating transcription factor 6 derepression mediates neuroprotection in Huntington disease

Deregulated protein and Ca2+ homeostasis underlie synaptic dysfunction and neurodegeneration in Huntington disease (HD); however, the factors that disrupt homeostasis are not fully understood. Here, we determined that expression of downstream regulatory element antagonist modulator (DREAM), a multifunctional Ca2+-binding protein, is reduced in murine in vivo and in vitro HD models and in HD patients. DREAM downregulation was observed early after birth and was associated with endogenous neuroprotection. In the R6/2 mouse HD model, induced DREAM haplodeficiency or blockade of DREAM activity by chronic administration of the drug repaglinide delayed onset of motor dysfunction, reduced striatal atrophy, and prolonged life span. DREAM-related neuroprotection was linked to an interaction between DREAM and the unfolded protein response (UPR) sensor activating transcription factor 6 (ATF6). Repaglinide blocked this interaction and enhanced ATF6 processing and nuclear accumulation of transcriptionally active ATF6, improving prosurvival UPR function in striatal neurons. Together, our results identify a role for DREAM silencing in the activation of ATF6 signaling, which promotes early neuroprotection in HD.