Fragile X mental retardation syndrome (FXS), the most common form of inheritable retardation, results from a trinucleotide repeat expansion in the FMR1 gene, which ultimately results in transcriptional silencing of the Fragile X mental retardation protein (FMRP). Recent research has made great strides in understanding the function of FMRP and this has shed light on the molecular mechanisms of the cognitive deficits observed in FXS (O'Donnell and Warren, 2002).

FMRP knockout mice exhibit many symptoms similar to human patients, including dendritic spine abnormalities and cognitive dysfunction. Deficits in synaptic plasticity have also been observed in knockout mice, including enhancement of long-term depression (LTD) in the hippocampus. FMRP is an RNA binding protein that has been shown to bind selective mRNAs in dendrites. Indeed, many neuronal mRNAs localize to dendrites where they undergo local translation. FMRP has been shown to both transport and regulate translation of these specific mRNAs, many of which include important synaptic proteins such as MAP1, CaMKII and Arc...  Read more

Fragile X mental retardation syndrome (FXS), the most common form of inheritable retardation, results from a trinucleotide repeat expansion in the FMR1 gene, which ultimately results in transcriptional silencing of the Fragile X mental retardation protein (FMRP). Recent research has made great strides in understanding the function of FMRP and this has shed light on the molecular mechanisms of the cognitive deficits observed in FXS (O'Donnell and Warren, 2002).

FMRP knockout mice exhibit many symptoms similar to human patients, including dendritic spine abnormalities and cognitive dysfunction. Deficits in synaptic plasticity have also been observed in knockout mice, including enhancement of long-term depression (LTD) in the hippocampus. FMRP is an RNA binding protein that has been shown to bind selective mRNAs in dendrites. Indeed, many neuronal mRNAs localize to dendrites where they undergo local translation. FMRP has been shown to both transport and regulate translation of these specific mRNAs, many of which include important synaptic proteins such as MAP1, CaMKII and Arc (Zalfa et al., 2003). In tandem with these findings, overactivation of group 1 metabotropic glutamate receptor (mGluR) signaling has been shown to mimic certain Fragile X symptoms, leading to the mGluR theory (recently reviewed in Bear et al., 2004) that posits a leading role for mGluR signaling in FXS. A form of LTD requires mGluR activation and is dependent on protein synthesis, linking mGluR signaling to FMRP-dependent protein synthesis (Huber et al., 2000).

A Drosophila model of Fragile X has been developed by knocking out the Drosophila FMRP homologue dfmr1. Studies of these flies have uncovered neuronal and behavioral phenotypes with parallels to symptoms observed in FXS patients, including alterations in circadian rhythms and synaptic branching. This paper uses this model to show that mGluR antagonists can treat behavioral deficits.

The authors utilized courtship mating to address whether dfmr1 knockout exhibited deficits in learning and memory. Courtship behavior in Drosophila is innate and involves a complex set of behaviors in order for copulation to occur. The authors find that dfmr1 KO flies learn normally but are deficient in retaining memory in two paradigms of courtship behavior. Surprisingly, when the authors fed the KO flies MPEP (a selective mGluR5 antagonist in mammals), these deficits were reversed. This is surprising because the only known Drosophila mGluR (dmGluRA) is most similar to group II mammalian mGluRs and shows little homology to mGLuR5, a group 1 mGluR. Three other mGluR antagonists were used and exhibited similar beneficial effects in the KO flies.

Feeding flies MPEP during and after development had similar beneficial effects, although restoration was greatest in developmentally fed flies. Intriguingly, mGluR antagonists fed during development also restored an anatomical deficit in the mushroom bodies of KO flies. Mushroom bodies are brain structures that are thought to be equivalent to the hippocampus in vertebrates. However, MPEP fed post-development did not restore the anatomical deficits, suggesting that the anatomical abnormality is not responsible for the memory deficits.

Although the findings from this study provide suggestive evidence that the mGluR theory of Fragile X applies across species, further work will be needed to verify this. The signaling pathways downstream of dmGlurA are unclear, and it will be interesting to determine if FMRP-dependent protein synthesis is also controlled by dmGluRA. A limitation of this paper is that the authors do not definitively show that the mGluR antagonists are indeed acting directly on the dmGluRA receptor. Genetically reducing dmGluRA in flies will be needed to verify that the observed benefits of the drugs are acting through dmGluRA. However, the study does suggest that pharmacotherapy may be a viable treatment for FXS and is a promising step forward in alleviating the burden of this disease.

References:
Bear MF, Huber KM, Warren ST (2004) The mGluR theory of fragile X mental retardation. Trends Neurosci 27:370-377. Abstract

Huber KM, Kayser MS, Bear MF (2000) Role for rapid dendritic protein synthesis in hippocampal mGluR-dependent long-term depression. Science 288:1254-1257. Abstract

O'Donnell WT, Warren ST (2002) A decade of molecular studies of fragile X syndrome. Annu Rev Neurosci 25:315-338. Abstract

Zalfa F, Giorgi M, Primerano B, Moro A, Di Penta A, Reis S, Oostra B, Bagni C (2003) The fragile X syndrome protein FMRP associates with BC1 RNA and regulates the translation of specific mRNAs at synapses. Cell 112:317-327. Abstract

View all comments by Jason Shepherd

This work is an impressive confirmation of the metabotropic glutamate receptor (mGluR) theory of Fragile X syndrome, and it extends our understanding of the therapeutic mechanisms of this important new drug class. This is the first report of chronic treatment of this duration, made possible by this new agent (CTEP), which is significantly more potent and much longer acting than any other available research agent. Other studies have shown excellent preclinical efficacy later in (mouse) life, and studies of conditional knockout mice also strongly suggest that most symptoms of Fragile X are potentially reversible, but this article demonstrates that the kind of therapeutic intervention that we can realistically implement in patients is extraordinarily effective in reversing the major Fragile X phenotypes at all levels (cellular, synaptic, neural circuit, and whole animal/behavioral).

One of the past critiques of the mGluR5 antagonist treatment strategy for Fragile X was that some animal studies showed development of tolerance over the course of a few days of chronic treatment....  Read more

This work is an impressive confirmation of the metabotropic glutamate receptor (mGluR) theory of Fragile X syndrome, and it extends our understanding of the therapeutic mechanisms of this important new drug class. This is the first report of chronic treatment of this duration, made possible by this new agent (CTEP), which is significantly more potent and much longer acting than any other available research agent. Other studies have shown excellent preclinical efficacy later in (mouse) life, and studies of conditional knockout mice also strongly suggest that most symptoms of Fragile X are potentially reversible, but this article demonstrates that the kind of therapeutic intervention that we can realistically implement in patients is extraordinarily effective in reversing the major Fragile X phenotypes at all levels (cellular, synaptic, neural circuit, and whole animal/behavioral).

One of the past critiques of the mGluR5 antagonist treatment strategy for Fragile X was that some animal studies showed development of tolerance over the course of a few days of chronic treatment. One paper on which I was a coauthor (Yan et al., 2005) showed some tolerance, though we did not believe this was necessarily a predictor of human tolerance. No tolerance was seen with this long-term, high-level mGluR5 antagonism by CTEP, which bodes quite well for human clinical trials currently underway. CTEP also appears to be quite similar pharmacokinetically to the Roche drug currently under development for Fragile X, and this study appears to justify the use of long-half-life drugs that achieve steady antagonism of mGluR5. It is also remarkable that high-level antagonism of mGluR5 (well over 50 percent) results in no apparent adverse effects, in line with clinical observation of excellent tolerability of these drugs in patients. This is a powerful research tool, since far fewer administrations of drug are required in animal models—frequent administration of drugs results in powerful stress-related confounds in this type of chronic dosing study, and that was clearly absent here. CTEP will likely be in great demand for this use. Frequent drug administration and fluctuating drug levels are also problematic in clinical therapeutics as well, so this type of drug may be superior for many reasons, not least of which are adherence and efficacy.

There may be a great many potential uses for mGluR5 antagonists for a wide range of neuropsychiatric conditions. Clinical trials of this class in L-dopa-induced dyskinesia of Parkinson's disease (PD-LID) have been uniquely successful; this alone should justify marketing. This class also has great promise for the treatment of a broad range of addictions, and long-acting agents such as this would be ideal for this application. Of course, we are most intrigued by the potential of mGluR5 antagonists for treating other forms of autism spectrum disorders; it appears likely that some (though not all) cases of non-fragile-X-autism involve similar abnormalities in these same signaling pathways. Biomarkers such as APP may reveal individuals without the Fragile X mutation who also have hyperactive signaling similar to Fragile X (APP translation is known to be regulated by FMRP via mGluR, as Cara Westmark has shown (see ARF Webinar); Sokol et al. have shown elevated APP levels in a subset of autism patients—usually more severely affected individuals (Sokol et al., 2006). These individuals may be potential responders to this type of treatment. It is also possible that mGluR5 antagonists could be an effective component of a cocktail treatment for Alzheimer's disease, for example, by decreasing translation of APP while co-administration of a secretase inhibitor could reduce abnormal metabolism.

Intriguingly, Fatemi et al. recently published several postmortem studies suggesting that low levels of FMRP are associated with many different neuropsychiatric disorders—not just autism, but also Alzheimer's, schizophrenia, bipolar disorder, and other mood disorders (Fatemi et al., 2010). Decreased FMRP levels, from whatever primary cause, could lead to excessive signaling in these same pathways, and mGluR5 antagonists could be useful treatments in these cases. FMRP itself could end up as an important biomarker, even in cases other than developmental disorders

View all comments by Michael Tranfaglia