Increased levels of the proapoptotic kinase PKR have recently been detected in the brains of patients with Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, HIV, dementia and prion diseases. PKR inhibition could represent a new target to reduce human neurodegeneration.
PKR, or double-stranded RNA-dependent protein kinase, is a serine–threonine kinase formed by a kinase domain and two double-stranded RNA-binding domains that are able to control its activity. This activation results in dimerization of the protein and autophosphorylation at the Thr 446 and Thr 451 sites. PKR is a constitutive enzyme in mammalian cells and was cloned at the Pasteur Institute in 1990 Citation[1]. PKR has been implicated in translational control through the phosphorylation of its major substrate eukaryotic initiation factor 2a (eIF2α). The phosphorylation of eIF2α on Ser 51 leads to the blockade of protein synthesis. In addition, PKR is involved in transcriptional control as well as in the regulation of apoptosis, cell growth and proliferation Citation[2]. Viruses and dsRNA are the major activators of PKR, but this kinase can also be induced by interferon, TNF-a, IL-1, calcium and reactive oxygen species. PKR is a stress kinase and, in the absence of virus, the PKR activator PACT (or RAX in mice) can produce PKR dimerization and autophosphorylation Citation[3]. PACT could interact with the PACT binding domain of the kinase domain of PKR, annealing an inactive conformation of the PKR molecule linked to an inhibitory interaction between the kinase domain and the dsRNA binding motif 2.
Previous data have shown that PKR can induce apoptosis during viral infection but also in the absence of virus, when cells are exposed to different types of insults. Many stresses can activate PKR, such as TNF-α, lipopolysaccharide, tunicamycin or serum withdrawal. PKR activation can induce apoptosis through the phosphorylation of its substrate eIF2α, leading to translation attenuation, or via the FADD/caspase 8 pathway and the APAF/caspase 9 pathway, suggesting that both intrinsic and extrinsic apoptotic pathways can be induced by PKR activation Citation[2].
The first report revealing that PKR could be linked to neurodegenerative diseases was published in 2001 and showed that levels of activated PKR were enhanced in the brains of patients suffering from Huntington’s disease Citation[4]. In 2002 our research team was the first to report increased stainings of activated PKR and phosphorylated eIF2α in the brains of patients affected by Alzheimer’s disease Citation[5]. Abnormal labelings were noted mainly in degenerating hippocampal neurons. Phosphorylated PKR or eIF2α accumulations were sometimes associated with hyperphosphorylated tau stainings. At the same time, we also demonstrated that the amyloid-β (Aβ) peptide could activate neuronal PKR and eIF2α in vitro and that blocking PKR could reduce Aβ neuronal toxicity Citation[6]. Neuronal cultures from knockout PKR mice were more resistant to Aβ toxicity than neuronal cultures from wild-type mice. In addition, we revealed that caspase 3 and 8 were involved in the apoptotic process caused by Aβ-induced PKR activation Citation[7]. The involvement of PKR in Alzheimer’s disease was confirmed by other groups Citation[8,9]. The levels of activated PKR were also increased in the brains of APP/PS1 knock-in transgenic mice and two PKR antagonists, the oxindol–imidazol compound C16 and the inhibitory peptide PRI, were able to reduce Aβ-induced PKR activation and apoptosis in neural cell cultures Citation[10]. Surprisingly, the concentrations of activated PKR and eIF2α were elevated in blood lymphocytes of Alzheimer’s disease patients compared with control individuals, and were inversely correlated with the decline of cognitive and memory tests Citation[11]. More recently, a report demonstrated that minocycline reduces PKR activation and neuronal death and also improves cognitive impairment in Tg2576 mice, a transgenic model of Alzheimer’s disease Citation[12]. Enhanced neuronal immunostaining of phosphorylated PKR was also observed in extrastriatal brain regions in Parkinson’s disease, suggesting that the activation of PKR could occur in several human neurodegenerative diseases and could contribute to a common neuronal machinery leading to cell apoptosis and death Citation[13].
Recently, a similar detrimental mechanism has been proposed for neurodegeneration observed in HIV-infected patients and patients with prion diseases. The HIV surface glycoprotein gp120 can produce neuronal apoptosis. In 2007, a publication revealed that gp120 can induce PKR expression and activation in mixed cortical cells and that the brains of patients with HIV-associated dementia, but not HIV infection alone, were strongly immunoreactive for the activated form of PKR Citation[14]. Cortical neurons from PKR knockout mice were significantly protected from gp120 neurotoxicity and two PKR inhibitors strongly attenuated gp120 toxicity. This finding in HIV patients and the studies carried out in patients suffering from Alzheimer’s disease, demonstrate that both gp120 and Aβ can be added to the list of cell stressors able to trigger neuronal apoptosis through PKR activation. This situation has also been reproduced very recently in prion diseases. We have analyzed the brains of patients with Creutzfeldt–Jakob disease (CJD) and control individuals. Neuronal immunostainings for activated PKR were detected in all CJD cases and not in controls. The number and repartition of PKR-positive neurons correlated with the extent of neuronal apoptosis, spongiosis and astrocytosis Citation[15]. Although the trigger of neuronal loss in CJD is not completely understood, these findings are in agreement with a recent report showing that prion protein aggresomes can induce a PKR-mediated deficient cell stress response Citation[16]. In view of the recent discovery that Aβ and prion protein can interact to induce synaptic dysfunctions Citation[17], one can suggest that PKR could concur in this neuronal demise.
Viable PKR knockout mice have been produced and it is possible to propose that PKR inhibition could reduce the neuropathological consequences following the repeated brain exposure to cell stressors such as the Aβ peptide, gp120, mutated huntingtin or prion protein. In addition to 2-aminopurate, two PKR inhibitors have recently been used to attenuate PKR activation, the compound C16, which could also interact with other kinases, and the inhibitory peptide PRI Citation[10]. Further studies will be necessary to evaluate appropriate compounds in preclinical models.
The fact that PKR activation could be implicated in various human diseases in which neurodegeneration is a prominent aspect of the pathology could reinforce the potential of this kinase as a suitable target for neuroprotection.
Financial & competing interests disclosure
The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.
No writing assistance was utilized in the production of this manuscript.
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