One of the pathways of the unfolded protein response, initiated by PKR-like endoplasmic reticulum kinase (PERK), is key to neuronal homeostasis in neurodegenerative diseases. PERK pathway activation is usually accomplished by inhibiting eIF2α-P dephosphorylation, after its phosphorylation by PERK. Less tried is an approach involving direct PERK activation without compromising long-term recovery of eIF2α function by dephosphorylation. Here we show major improvement in cellular (STHdh Q111/111 ) and mouse (R6/2) Huntington's disease (HD) models using a potent small molecule PERK activator that we developed, MK-28. MK-28 showed PERK selectivity in vitro on a 391-kinase panel and rescued cells (but not PERK−/− cells) from ER stress-induced apoptosis. Cells were also rescued by the commercial PERK activator CCT020312 but MK-28 was significantly more potent. Computational docking suggested MK-28 interaction with the PERK activation loop. MK-28 exhibited remarkable pharmacokinetic properties and high BBB penetration in mice. Transient subcutaneous delivery of MK-28 significantly improved motor and executive functions and delayed death onset in R6/2 mice, showing no toxicity. Therefore, PERK activation can treat a most aggressive HD model, suggesting a possible approach for HD therapy and worth exploring for other neurodegenerative disorders.HD is a neurodegenerative disease arising from an expanded CAG repeat in the exon 1 of the huntingtin gene, which translates into a polyglutamine (polyQ) tract in the huntingtin (Htt) protein 1,2 . HD is a genetic, autosomal dominant disease with late onset and progressive motor dysfunction, cognitive decline and behavioral abnormalities. In addition to these, other systemic impairments such as weight loss, muscle wasting and glucose regulation impairment were also reported 2 . The expansion of the polyQ repeats causes mutant Htt (mHtt) to aggregate in HD tissues when it includes above 35 glutamine residues, with a consequent induction of cellular stress, toxicity and cell death especially in the brain striatum and extending later to the cortex. This reflects progressively in the deterioration of the individual's biological functions 3,4 . Although several therapeutic approaches are currently being pursued, including ongoing clinical trials for lowering mHtt levels using antisense oligonucleotides (ASOs), there is currently no effective treatment for HD 5 .One of the important consequences of the gradual accumulation of misfolded mHtt is its inhibition of ER-associated degradation (ERAD), causing endoplasmic reticulum (ER) stress and induction of a conserved stress response known as the unfolded protein response (UPR) 6-12 . The function of the UPR is to either re-establish cellular homeostasis or, if this fails, to trigger cell death in order to prevent further accumulation of 1 PERK modulator, with only Tun. The graphs show the average relative apoptosis rate of at least 3 independent experiments for each compound.Cell Cycle FACS analysis. Cells were washed with PBS and fixed with ...

Increasing evidence in recent years indicates that protein misfolding and aggregation, leading to ER stress, are central factors of pathogenicity in neurodegenerative diseases. This is particularly true in Huntington's disease (HD), where in contrast with other disorders, the cause is monogenic. Mutant huntingtin interferes with many cellular processes, but the fact that modulation of ER stress and of the unfolded response pathways reduces the toxicity, places these mechanisms at the core and gives hope for potential therapeutic approaches. There is currently no effective treatment for HD and it has a fatal outcome a few years after the start of symptoms of cognitive and motor impairment. Here we will discuss recent findings that shed light on the mechanisms of protein misfolding and aggregation that give origin to ER stress in neurodegenerative diseases, focusing on Huntington's disease, on the cellular response and on how to use this knowledge for possible therapeutic strategies.

With the extension of life span in recent decades, there is an increasing burden of late-onset neurodegenerative diseases, for which effective treatments are lacking. Neurodegenerative diseases include the widespread Alzheimer’s disease (AD) and Parkinson’s disease (PD), the less frequent Huntington’s disease (HD) and Amyotrophic Lateral Sclerosis (ALS) and also rare early-onset diseases linked to mutations that cause protein aggregation or loss of function in genes that maintain protein homeostasis. The difficulties in applying gene therapy approaches to tackle these diseases is drawing increasing attention to strategies that aim to inhibit cellular toxicity and restore homeostasis by intervening in cellular pathways. These include the unfolded protein response (UPR), activated in response to endoplasmic reticulum (ER) stress, a cellular affliction that is shared by these diseases. Special focus is turned to the PKR-like ER kinase (PERK) pathway of the UPR as a target for intervention. However, the complexity of the pathway and its ability to promote cell survival or death, depending on ER stress resolution, has led to some confusion in conflicting studies. Both inhibition and activation of the PERK pathway have been reported to be beneficial in disease models, although there are also some reports where they are counterproductive. Although with the current knowledge a definitive answer cannot be given on whether it is better to activate or to inhibit the pathway, the most encouraging strategies appear to rely on boosting some steps without compromising downstream recovery.

There is currently no disease-modifying therapy for Huntington’s disease (HD) and two recent clinical trials testing antisense oligonucleotides failed. We recently described a small molecule, MK-28, which restored homeostasis in HD models by specifically activating PKR‐like ER kinase (PERK) and thus boosting neuroprotection by the unfolded protein response (UPR), and reducing endoplasmic reticulum (ER) stress, a central cytotoxic mechanism in HD and other neurodegenerative diseases. Here we have tested the long-term effects of MK-28 in HD model mice. R6/2 CAG (160) mice were treated by lifetime IP injection, 3 times a week. CatWalk measurements of motor function showed significant improvement after only two weeks of MK-28 treatment and continued with time, most significantly at 1 mg/kg MK-28, approaching WT values. Seven weeks treatment significantly improved paw grip strength. Body weight recovered and glucose levels, which are elevated in HD mice, were significantly lowered. Treatment with another PERK activator, CCT020312, also caused amelioration, although less significant than with MK-28 in some of the parameters. Lifespan, measured in more resilient R6/2 CAG (120) mice with daily IP injection, was significantly extended by 16 days (20%) with 0.3 mg/kg MK-28, and by 38 days (46%) with 1 mg/kg MK-28. No toxicity, measured by weight, blood glucose levels and blood liver function markers, was detectable in WT mice treated for 6 weeks with 6 mg/kg MK-28. Boosting of PERK activity by long-term treatment with MK-28 appears to be a safe and promising therapeutic approach for HD.

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