Historically, drug discovery in disorders of brain development has been unproductive largely due to the lack of mechanistic understanding of these disorders as well as the absence of predictive animal models. Seaside Therapeutics is changing this paradigm through scientific exploration that focuses on identifying the fundamental pathophysiology of brain development disorders and application of this knowledge to develop targeted therapeutics. Recent discoveries by the Company's scientific founder, Dr. Mark Bear, have revealed a molecular pathway, the mGluR5 signaling cascade, that is disrupted in a specific disorder of brain development – Fragile X syndrome. With this knowledge, further research has provided insights for developing novel medications to normalize the function of this pathway, which Seaside believes may extend beyond Fragile X into a number of other developmental disorders including autism.
Initial Seaside drug candidates include STX209, which reduces glutamate signaling in the brain and STX107, a highly potent and selective mGluR5 antagonist. STX209 entered a Phase 2 clinical study in adults, adolescents and children with Fragile X in late 2008 and initiated a second trial in adults, adolescents and children with autism spectrum disorders in March of 2009. STX107 entered Phase 1 clinical studies in healthy volunteers in October 2009.
From disease model to therapeutic target
Despite the high heritability estimates of autism, identifying the genetic etiology of the disease has been challenging because autism is an umbrella diagnosis, encompassing a large number and heterogeneity of causative mutations. Of these, single gene causes that are relatively common, like Fragile X Syndrome, have been the most tractable. Fragile X is caused by a mutation in the FMR1 gene which leads to transcriptional silencing and loss of the protein product, the Fragile X mental retardation protein (FMRP). The Fragile X mouse model recapitulates the human mutation by insertional deletion of the Fmr1 gene; this Fmr1 knockout has been validated for Fragile X and is currently one of the leading models of autism1.
Autism is a neurodevelopmental disorder. Onset of symptoms is temporally coincident with a dynamic phase of brain development, which includes myelination of axons, neurite outgrowth, maturation of the inhibitory connections between neurons, synaptic plasticity and maturation of the receptor proteins. These processes are tightly regulated during development by the complex interplay of molecular programs and experience. Perhaps because of this tight regulation, this period is also particularly vulnerable to molecular and environmental insults; thus it has been tempting to speculate that the pathogenesis of autism involves a derailment of at least one of these processes.
FIGURE 1: Brain development (adapted from2)
Given this basic neuroscience framework, early studies of the Fmr1 knockout model examined a possible deficit in hippocampal synaptic plasticity. A potential breakthrough came with the discovery that a novel form of long-term synaptic depression in the hippocampus3 was exaggerated in the Fmr1 knockout mouse4. Unlike the N-methyl D-aspartate receptor dependent forms of plasticity examined previously5,6, this synaptic depression is induced by activation of group I metabotropic glutamate receptors (Gp I mGluRs), and is normally protein synthesis dependent. Meanwhile, a number of studies showed that FMRP is an mRNA binding protein, is enriched postsynaptically at glutamatergic synapses and functions as a repressor of protein synthesis.
Taken together, these results suggested the possibility that FMRP normally functions to balance the activity of the GpI mGluRs; when it is missing, the balance between the two proteins is disrupted and unchecked mGluR activity leads to the characteristic symptoms of Fragile X and autism7. The therapeutic potential of this so called "mGluR theory" is that symptoms of Fragile X could potentially be reversed by turning down GpI mGluR activity, thereby restoring the balance of protein synthesis at the synapse8.
FIGURE 2: Model for pathogenesis and correction of Fragile X Syndrome (adapted from9)
This theory has now been tested in the Fmr1 knockout model, using both molecular genetic manipulation of mGluR510 (one of the two GpI mGluRs) and with selective mGluR5 antagonists11. Although a range of phenotypes have been studied, a simple way to conceptualize the constellation of findings is that Fragile X is a disorder of excess—excessive sensitivity to environmental change, synaptic connectivity, memory extinction, protein synthesis, body growth and excitability. Remarkably, these metabolic, morphologic, synaptic, circuit and behavioral excesses in the Fmr1 knockout mouse can all be corrected by reducing mGluR5 signaling.
These results provide the first real hope of global therapy for Fragile X — increased mGluR5 signaling provides a thread that connects diverse manifestations of the disease. Of course this model awaits validation in forthcoming clinical trials in humans. For now, it is important to note that metabotropic glutamate receptors are particularly amenable to pharmacologic manipulation12 and these studies provide compelling evidence that these receptors, if targeted appropriately (i.e. with antagonists), will have significant therapeutic value for the treatment of Fragile X and related disorders.
Applications Beyond Fragile X Syndrome
The current mGluR5 program at Seaside Therapeutics is based on the scientific findings, described above, using the Fmr1 knockout model of autism. This bottom-up approach focused our attention on mGluR5 signaling as a regulator of protein synthesis at the synapse. Similarly, studies using mouse models of other single gene causes of autism including Angelman syndrome, tuberous sclerosis and Rett's syndrome, have identified other relevant transduction cascades which, interestingly, intersect with mGluR5 cascades and converge at the synapse at the level of protein synthesis regulation. Regardless of the direction of the imbalance (too much or too little protein synthesis), these alterations disrupt neuronal network performance and interfere with cognitive function. The prediction of the model is that while some single gene causes of autism, like Fragile X, would respond to therapies aimed at reducing synaptic protein synthesis, others like Rett's syndrome, may respond to therapies that increase synaptic protein synthesis13.
FIGURE 3: Bidirectional alterations in synaptic protein synthesis (adapted from13)
Genetic linkage and association studies represent a second, top-down, approach to understanding the pathogenesis of autism. These studies have identified a number of autism candidate genes, including HOMER, SHANK, Neuroligin and Neurexin14,15. These genes encode structural proteins that either directly or indirectly tether mGluRs to the synapse2.
FIGURE 4: Autism as a synapsopathy (adapted from2)
While purely speculative at this point, these converging signaling cascades identified by both bottom-up and top-down approaches, raise three interesting possibilities: 1) that the symptomatic overlap in clinical presentation between the various forms of autism reflect derailments in common pathways that regulate protein synthesis at the synapse 2) that other causes of autism, including single gene disorders and mutations of autism candidate genes, when modeled in mice, will show disruptions in mGluR5 signaling and protein synthesis and 3) that treatments developed for one cause will show efficacy for some, if not all, other causes of autism. Furthermore as the signaling cascades are further delineated and the theory broadened to include other relevant neurotransmitter systems, additional targets may be identified. Exploring these intersecting pathways and putative targets continues Seaside Therapeutics' tradition of seeking treatments based on a mechanistic understanding of the pathogenesis of neurodevelopmental disease.