Some people find this blog too complicated and would prefer it to be simplified; it would be great if all the science could be accurately described in very simple terms.
This blog has ended up going into far more detail than I had ever intended, because if you want to get to the bottom of a problem you have to keep digging until you get to what is relevant. The relevant part is not near the surface, as you will see in today’s post, but many potential therapeutic options are sitting there in plain view, obscured only by the scientific jargon.
eIF4E, ADNP, Alzheimer’s, Tauopathy and Autism
In today’s post I am drawing together material from autism, Alzheimer’s and other so-called tauopathies. The post ends up with the suggestion that an existing antiviral drug called Ribavirin, which affects a very specific part of mTOR signaling, could be a useful autism therapy and should be the subject of a serious clinical trial.
Tauopathies sound interesting. Tau protein is present in the brains of all humans, but it can dysfunction (hyperphosphorylation) and form tangles. When tau behaves like this it leads to so-called tauopathies, like Alzheimer’s. Tangles form inside dying cells; they are twisted fibers of a protein tau. In areas where tangles are forming, the twisted strands of tau block nutrients from moving through the cells, causing cell death.
Most people develop some amyloid plaques and tau tangles as they get older, but people with Alzheimer’s tend to develop far more. Plaques and tangles tend to form in a pattern, starting in areas related to learning and memory and then spreading to other regions of the brain.
The question is to what extent are infantile tauopathies present in autism? Particularly autism with MR/ID?
Tuberous Sclerosis (TSC) is a widely used research model of autism. TSC is a genetic disorder that is usually caused by the TSC2 gene, but can be caused by TSC1. TSC1 and TSC2 are growth supressors and dysfunction leads to the growth of benign tumors. TSC is associated with seizures, autism, MR/ID and other issues. TSC is a tauopathy.
It is unkown to what extent tauopathy may be present in autism, or those with mental retardation/intellectual disability. This question was also posed in the blog written by Dr Emily Casanova, wife of the neurologist/blogger Dr Manuel Casanova; the latter normally seems to get the most media attention.
A very expensive drug called Everolimus, is being used to treat TSC. Everolimus is a potent mTOR inhibitor. mTOR is part of a key constellation of signaling pathways implicated in cancer and autism. mTOR is extremely complex and even highly intelligent people will need quite some time to figure it out.
Eukaryotic translation initiation factor 4E (eIF4E)
Eukaryotic translation initiation factor 4E (eIF4E), is a protein that in humans is encoded by the EIF4E gene.
eIF4E seems to play a critical role in the mTOR pathway to trigger the excitatory/inhibitory imbalance in autism.
There are multiple pathways involved in this process and we previously looked at RORa.
The Purkinje-RORa-Estradiol-Neuroligin-KCC2 axis in Autism
We have already seen that in most autism the mTOR pathway is over active. The problem is that this pathway is highly complex and affects very many aspects of your body. You would ideally intervene in a highly selective manner.
eIF4E is just one small part of the mTOR pathway and it appears that by selectively inhibiting it, good things should happen.
In the chart below we would inhibit eIF4E (green box) and then expect a reduction in neuroligins (NLGNS), leading to more inhibition on neurons, resulting in better cognition and milder autism.
Over expression of eIF4E in mice leads to autistic behaviors.
Inhibition of eIF4E works in a mouse model of autism.
Inhibitors of eIF4E exist today.
ADNP
Activity-dependent neuroprotective protein (ADNP) is the most frequent autism associated gene and the only protein significantly decrease in the serum of Alzheimer's disease patients.
Israeli researchers investigating Alzheimer’s and otherTauopathies identified binding sites on ADNP for eIF4E.
ADNP expression is suggested as a master regulator of key ASD and AD risk genes.
It is also suggested, based on mouse research, that ADNP expression may contribute to the male/female variations in autism and Alzheimer’s (women are more affected by Alzheimer’s, but less by autism). Increased male ADNP expression was replicated in human postmortem hippocampal samples.
Choice of of eIF4E Inhibitor
Thanks to all the cancer research there is detailed knowledge of eIF4E inhibitors.
As usual a key issue is bioavailability.
I thought ribavirin looks very interesting and I am not the only one (see later studies). It is an old generic anti-viral medication, often used to treat hepatitis C. An expensive version is being developed as a cancer therapy.
Conclusion
All the evidence points towards eIF4E Inhibitors, but as the professionals would tell us, more research and validation is required. A clinical trial of Ribavirin would seem in order.
There may be different types of E/I imbalance in autism and different therapies are likely to suit different people.
The supporting science :-
There may be different types of E/I imbalance in autism and different therapies are likely to suit different people.
The supporting science :-
Researchers at McGill University have mouse data showing a causal link between eIF4E-mediated translational dysregulation and autism-related deficits. The group also corrected the dysregulation—and the associated autistic phenotype—with a small molecule.1
The McGill group, led by Nahum Sonenberg, has been studying the role of eukaryotic translation initiation factor 4E (eIF4E) in protein synthesis for over three decades and has primarily focused on the factor's relevance in cancer. eIF4E binds to the cap structure on mRNA and helps to initiate the translation of the mRNA. Sonenberg is a professor in the Department of Biochemistry and at the Rosalind and Morris Goodman Cancer Research Centre at McGill.
The team previously reported that eIF4E-mediated protein translation is modulated by the phosphoinositide 3-kinase (PI3K), protein kinase B (PKB; PKBA; AKT; AKT1) and mammalian target of rapamycin (mTOR; FRAP; RAFT1) pathway, which is commonly disrupted in cancer.2
He said the initial connection to autism came after other research groups showed that autistic children carry mutations in genes upstream of mTOR. These genes included PTEN (MMAC1; TEP1) and tuberous sclerosis complex tumor suppressor 1 (TSC1).3, 4, 5
Separately, a 2009 study from a research group in the U.K. showed an association between mutations that increased eIF4E promoter activity and autism.6
With multiple studies pointing to eIF4E-dependent processes in autism, the McGill group sought to determine whether dysregulation of eIF4E activity itself could cause an autistic phenotype. Indeed, past studies suggested that dysregulated translation of mRNA could be an underlying cause of autism7 but never showed a causal relationship.
In a new study published in Nature, the McGill researchers showed that increasing eif4e activity in mice—by knocking out the gene encoding an eif4e repressor called eif4e binding protein 2 (eif4ebp2)—led to autism-associated electrophysiological abnormalities and behaviors.
In these mice, as well as mice that overexpressed eif4e, translation of neuroligin proteins was greater than that seen in wild-type controls. Alterations in neuroligin signaling occur in autism.8, 9
In the mouse models, a small molecule inhibitor of eIF4E signaling called 4EGI-1 reversed the electrophysiological abnormalities and decreased autistic behaviors compared with vehicle. Knockdown of neuroligin 1 (Nlgn1) had similar effects.
Importantly, inhibition of eif4e and Nlgn1 activity did not affect electrophysiological and behavioral parameters in wild-type mice.
Autism spectrum disorders (ASDs) are a group of clinically and genetically heterogeneous neurodevelopmental disorders characterized by impaired social interactions, repetitive behaviors and restricted interests. The genetic defects in ASDs may interfere with synaptic protein synthesis. Synaptic dysfunction caused by aberrant protein synthesis is a key pathogenic mechanism for ASDs Understanding the details about aberrant synaptic protein synthesis is important to formulate potential treatment for ASDs. The mammalian target of the Rapamycin (mTOR) pathway plays central roles in synaptic protein. Recently, Gkogkas and colleagues published exciting data on the role of downstream mTOR pathway in autism
Previous studies have indicated that upstream mTOR signaling is linked to ASDs. Mutations in tuberous sclerosis complex (TSC) 1/TSC2, neurofibromatosis 1 (NF1), and Phosphatase and tensin homolog (PTEN) lead to syndromic ASD with tuberous sclerosis, neurofibromatosis, or macrocephaly, respectively. TSC1/TSC2, NF1, and PTEN act as negative regulators of mTOR complex 1 (mTORC1), which is activated by phosphoinositide-3 kinase (PI3K) pathway. Activation of cap-dependent translation is a principal downstream mechanism of mTORC1. The eIF4E recognizes the 5′ mRNA cap, recruits eIF4G and the small ribosomal subunit. The eIF4E-binding proteins (4E-BPs) bind to eIF4E and inhibit translation initiation. Phosphorylation of 4E-BPs by mTORC1 promotes eIF4E release and initiates cap-dependent translation. A hyperactivated mTORC1–eIF4E pathway is linked to impaired synaptic plasticity in fragile X syndrome, an autistic disorder caused by lack of fragile X mental retardation protein (FMRP) due to mutation of the FMR1 gene, suggesting that downstream mTOR signaling might be causally linked to ASDs. Notably, one pioneering study has identified a mutation in the EIF4E promoter in autism families, implying that deregulation of downstream mTOR signaling (eIF4E) could be a novel mechanism for ASDs.As an eIF4E repressor downstream of mTOR, 4E-BP2 has important roles in synaptic plasticity, learning and memory. Writing in their Nature article, Gkogkas and colleagues reported that deletion of the gene encoding 4E-BP2 (Eif4ebp2) leads to autistic-like behaviors in mice. Pharmacological inhibition of eIF4E rectifies social behavior deficits in Eif4ebp2 knockout mice. Their study in mouse models has provided direct evidence for the causal link between dysregulated eIF4E and the development of ASDs.Are these ASD-like phenotypes of the Eif4ebp2 knockout mice caused by altered translation of a subset mRNAs due to the release of eIF4E? To test this, Gkogkas et al. measured translation initiation rates and protein levels of candidate genes known to be associated with ASDs in hippocampi from Eif4ebp2 knockout and eIF4E-overexpressing mice. They found that the translation of neuroligin (NLGN) mRNAs is enhanced in both lines of transgenic mice. Removal of 4E-BP2 or overexpression of eIF4E increases protein amounts of NLGNs in the hippocampus, whereas mRNA levels are not affected, thus excluding transcriptional effect. In contrast, the authors did not observe any changes in the translation of mRNAs coding for other synaptic scaffolding proteins. Interestingly, treatment of Eif4ebp2 knockout mice with selective eIF4E inhibitor reduces NLGN protein levels to wild-type levels. These data thus indicate that relief of translational suppression by loss of 4E-BP2 or by the overexpression of eIF4E selectively enhances the NLGN synthesis. However, it cannot be ruled out that other proteins (synaptic or non-synaptic) may be affected and contribute to animal autistic phenotypes.Aberrant information processing due to altered ratio of synaptic excitation to inhibition (E/I) may contribute to ASDs. The increased or decreased E/I ratio has been observed in ASD animal models In relation to these E/I shifts, Gkogkas et al then examined the synaptic transmission in hippocampal slices of Eif4ebp2 knockout mice. They found that 4E-BP2 de-repression results in an increased E/I ratio, which can be explained by the increase of vesicular glutamate transporter and spine density in hippocampal pyramidal neurons. As expected, application of eIF4E inhibitor restores the E/I balanceFinally, in view of the facts that genetic manipulation of NLGNs results in ASD-like phenotypes with altered E/I balance in mouse models and NLGN mRNA translation is enhanced concomitant with increased E/I ratio in Eif4ebp2 knockout mice, Gkogkas et al. tested the effect of NLGN knockdown on synaptic plasticity and behaviour in these mice . NLGN1 is predominantly postsynaptic at excitatory synapses and promotes excitatory synaptic transmission. The authors found that NLGN1 knockdown reverses changes at excitatory synapses and partially rescues the social interaction deficits in Eif4ebp2 knockout mice. These findings thus established a strong link between eIF4E-dependent translational control of NLGNs, E/I balance and the development of ASD-like animal behaviors (Figure 1).
In summary, Gkogkas et al. have provided a model for mTORC1/eIF4E-dependent autism-like phenotypes due to dysregulated translational control (Gkogkas et al., 2013). This novel regulatory mechanism will prompt investigation of downstream mTOR signaling in ASDs, as well as expand our knowledge of how mTOR functions in human learning and cognition. It may narrow down therapeutic targets for autism since targeting downstream mTOR signaling reverses autism. Pharmacological manipulation of downstream effectors of mTOR (eIF4E, 4E-BP2, and NLGNs) may eventually provide therapeutic benefits for patients with ASDs.
In summary, Gkogkas et al. have provided a model for mTORC1/eIF4E-dependent autism-like phenotypes due to dysregulated translational control (Gkogkas et al., 2013). This novel regulatory mechanism will prompt investigation of downstream mTOR signaling in ASDs, as well as expand our knowledge of how mTOR functions in human learning and cognition. It may narrow down therapeutic targets for autism since targeting downstream mTOR signaling reverses autism. Pharmacological manipulation of downstream effectors of mTOR (eIF4E, 4E-BP2, and NLGNs) may eventually provide therapeutic benefits for patients with ASDs.
Ribavirin Inhibitsthe Activity of mTOR/eIF4E, ERK/Mnk1/eIF4E Signaling Pathway and Synergizeswith Tyrosine Kinase Inhibitor Imatinib to Impair Bcr-Abl MediatedProliferation and Apoptosis in Ph+ Leukemia
Abstract
Background: Autism is a common childhood onset neurodevelopmental disorder, characterised by severe and sustained impairment of social interaction and social communication, as well as a notably restricted repertoire of activities and interests. Its aetiology is multifactorial with a strong genetic basis. EIF4E is the rate limiting component of eukaryotic translation initiation, and plays a key role in learning and memory through its control of translation within the synapse. EIF4E mediated translation is the final common process modulated by the mammalian target of rapamycin (mTOR), PTEN and fragile X mental retardation protein (FMRP) pathways, which are implicated in autism. Linkage of autism to the EIF4E region on chromosome 4q has been found in genome wide linkage studies.
Methods and results: The authors present evidence that directly implicates EIF4E in autism. In a boy with classic autism, the authors observed a de novo chromosome translocation between 4q and 5q and mapped the breakpoint site to within a proposed alternative transcript of EIF4E. They then screened 120 autism families for mutations and found two unrelated families where in each case both autistic siblings and one of the parents harboured the same single nucleotide insertion at position −25 in the basal element of the EIF4E promoter. Electrophoretic mobility shift assays and reporter gene studies show that this mutation enhances binding of a nuclear factor and EIF4E promoter activity.
Conclusions: These observations implicate EIF4E, and more specifically control of EIF4E activity, directly in autism. The findings raise the exciting possibility that pharmacological manipulation of EIF4E may provide therapeutic benefit for those with autism caused by disturbance of the converging pathways controlling EIF4E activity.
Abstract
Activity-dependent neuroprotective protein (ADNP) is a most frequent autism spectrum disorder (ASD)-associated gene and the only protein significantly decreasing in the serum of Alzheimer's disease (AD) patients. Is ADNP associated with ASD being more prevalent in boys and AD more prevalent in women? Our results revealed sex-related learning/memory differences in mice, reflecting hippocampal expression changes in ADNP and ADNP-controlled AD/ASD risk genes. Hippocampal ADNP transcript content was doubled in male vs female mice, with females showing equal expression to ADNP haploinsufficient (ADNP+/−) males and no significant genotype-associated reduction. Increased male ADNP expression was replicated in human postmortem hippocampal samples. The hippocampal transcript for apolipoprotein E (the major risk gene for AD) was doubled in female mice compared with males, and further doubled in the ADNP+/− females, contrasting a decrease in ADNP+/− males. Previously, overexpression of the eukaryotic translation initiation factor 4E (eIF4E) led to ASD-like phenotype in mice. Here, we identified binding sites on ADNP for eIF4E and co-immunoprecipitation. Furthermore, hippocampal eIF4E expression was specifically increased in young ADNP+/− male mice. Behaviorally, ADNP+/− male mice exhibited deficiencies in object recognition and social memory compared with ADNP+/+ mice, while ADNP+/− females were partially spared. Contrasting males, which preferred novel over familiar mice, ADNP+/+ females showed no preference to novel mice and ADNP+/− females did not prefer mice over object. ADNP expression, positioned as a master regulator of key ASD and AD risk genes, introduces a novel concept of hippocampal gene-regulated sexual dimorphism and an ADNP+/− animal model for translational psychiatry.
