Peter C. Kind

Reversal of Disease-Related Pathologies in the Fragile X Mouse Model by Selective Activation of GABAB Receptors with Arbaclofen

Pharmacological activation of the GABAB receptor with arbaclofen in a mouse model of fragile X syndrome corrects neuronal defects associated with the disease. A Fragile Balancing Act A wide array of symptoms—including intellectual disability, anxiety, seizures, and autistic behavior—are associated with fragile X syndrome (FXS). Although some symptoms can be managed (or masked) with drugs or other therapies, treatments that target the fundamental impairments are not available. Henderson et al. and Berry-Kravis et al. now provide evidence that activation of a particular neuronal receptor can improve symptoms in both mice and humans. FXS is caused by silencing of the FMR1 gene, which encodes FMRP, an RNA binding protein that inhibits protein synthesis. In a mouse model of FXS, dendritic protein synthesis is abnormally high; FMRP is believed to regulate mRNAs important for neuronal development. Furthermore, these mice—and some humans with FXS—have an increased density of dendritic spines, which are dynamic structures that make neuronal connections. Dendritic spine plasticity is linked to learning and memory. Normally, FMRP may balance mRNA translation that is stimulated by activation of synaptic receptors that respond to glutamate, an excitatory neurotransmitter. Indeed, inhibitors of these receptors rescue many irregular phenotypes in the animal models but are not yet approved for human use. These mice also exhibit deficient signaling through a different set of receptors, which respond to the inhibitory neurotransmitter GABA—and for which clinically approved agonists already exist. Henderson et al. tested one such GABAB receptor agonist, STX209, in the mouse model and found that it decreased mRNA translation in the cortex and corrected the increased dendritic spine density. Berry-Kravis et al. studied the effects of STX209 in a double-blind, placebo-controlled crossover trial, in which 63 FXS patients received placebo or drug for 4 weeks and then switched to the other treatment. Although a measure of irritability and aggression was unchanged, social avoidance improved; the drug was well tolerated. Thus, this targeted approach, which may help restore the balance between excitatory and inhibitory neurotransmission, has promise for improving social function in FXS. Fragile X syndrome (FXS), the most common inherited cause of intellectual disability and autism, results from the transcriptional silencing of FMR1 and loss of the mRNA translational repressor protein fragile X mental retardation protein (FMRP). Patients with FXS exhibit changes in neuronal dendritic spine morphology, a pathology associated with altered synaptic function. Studies in the mouse model of fragile X have shown that loss of FMRP causes excessive synaptic protein synthesis, which results in synaptic dysfunction and altered spine morphology. We tested whether the pharmacologic activation of the γ-aminobutyric acid type B (GABAB) receptor could correct or reverse these phenotypes in Fmr1-knockout mice. Basal protein synthesis, which is elevated in the hippocampus of Fmr1-knockout mice, was corrected by the in vitro application of the selective GABAB receptor agonist STX209 (arbaclofen, R-baclofen). STX209 also reduced to wild-type values the elevated AMPA receptor internalization in Fmr1-knockout cultured neurons, a known functional consequence of increased protein synthesis. Acute administration of STX209 in vivo, at doses that modify behavior, decreased mRNA translation in the cortex of Fmr1-knockout mice. Finally, the chronic administration of STX209 in juvenile mice corrected the increased spine density in Fmr1-knockout mice without affecting spine density in wild-type mice. Thus, activation of the GABAB receptor with STX209 corrected synaptic abnormalities considered central to fragile X pathophysiology, a finding that suggests that STX209 may be a potentially effective therapy to treat the core symptoms of FXS.

Critical Period Plasticity Is Disrupted in the Barrel Cortex of Fmr1 Knockout Mice

Alterations in sensory processing constitute prominent symptoms of fragile X syndrome; however, little is known about how disrupted synaptic and circuit development in sensory cortex contributes to these deficits. To investigate how the loss of fragile X mental retardation protein (FMRP) impacts the development of cortical synapses, we examined excitatory thalamocortical synapses in somatosensory cortex during the perinatal critical period in Fmr1 knockout mice. FMRP ablation resulted in dysregulation of glutamatergic signaling maturation. The fraction of silent synapses persisting to later developmental times was increased; there was a temporal delay in the window for synaptic plasticity, while other forms of developmental plasticity were not altered in Fmr1 knockout mice. Our results indicate that FMRP is required for the normal developmental progression of synaptic maturation, and loss of this important RNA binding protein impacts the timing of the critical period for layer IV synaptic plasticity.

The Role of Activity in Development of the Visual System

Neuronal activity is important for both the initial formation and the subsequent refinement of anatomical and physiological features of the mammalian visual system. Here we examine recent evidence concerning the role that spontaneous activity plays in axonal segregation, both of retinogeniculate afferents into eye-specific layers and of geniculocortical afferents into ocular dominance bands. We also assess the role of activity in the generation and plasticity of orientation selectivity in the primary visual cortex. Finally, we review recent challenges to textbook views on how inputs representing the two eyes interact during the critical period of visual cortical plasticity.

The subtype of GluN2 C-terminal domain determines the response to excitotoxic insults.

Summary It is currently unclear whether the GluN2 subtype influences NMDA receptor (NMDAR) excitotoxicity. We report that the toxicity of NMDAR-mediated Ca2+ influx is differentially controlled by the cytoplasmic C-terminal domains of GluN2B (CTD2B) and GluN2A (CTD2A). Studying the effects of acute expression of GluN2A/2B-based chimeric subunits with reciprocal exchanges of their CTDs revealed that CTD2B enhances NMDAR toxicity, compared to CTD2A. Furthermore, the vulnerability of forebrain neurons in vitro and in vivo to NMDAR-dependent Ca2+ influx is lowered by replacing the CTD of GluN2B with that of GluN2A by targeted exon exchange in a mouse knockin model. Mechanistically, CTD2B exhibits stronger physical/functional coupling to the PSD-95-nNOS pathway, which suppresses protective CREB activation. Dependence of NMDAR excitotoxicity on the GluN2 CTD subtype can be overcome by inducing high levels of NMDAR activity. Thus, the identity (2A versus 2B) of the GluN2 CTD controls the toxicity dose-response to episodes of NMDAR activity.

Correlated binocular activity guides recovery from monocular deprivation

Monocular deprivation (MD) has much more rapid and severe effects on the ocular dominance of neurons in the primary visual cortex (V1) than does binocular deprivation. This finding underlies the widely held hypothesis that the developmental plasticity of ocular dominance reflects competitive interactions for synaptic space between inputs from the two eyes. According to this view, the relative levels of evoked activity in afferents representing the two eyes determine functional changes in response to altered visual experience. However, if the deprived eye of a monocularly deprived kitten is simply reopened, there is substantial physiological and behavioural recovery, leading to the suggestion that absolute activity levels, or some other non-competitive mechanisms, determine the degree of recovery from MD. Here we provide evidence that correlated binocular input is essential for such recovery. Recovery is far less complete if the two eyes are misaligned after a period of MD. This is a powerful demonstration of the importance of cooperative, associative mechanisms in the developing visual cortex.

Altered maturation of the primary somatosensory cortex in a mouse model of fragile X syndrome

Fragile X syndrome (FXS) is the most common inherited form of intellectual disability and results from the loss of the fragile X mental retardation protein (FMRP). Many fragile X-related cognitive and behavioral features emerge during childhood and are associated with abnormal synaptic and cellular organization of the cerebral cortex. Identifying the roles of FMRP in cortical development will provide a basis for understanding the pathogenesis of the syndrome. However, how the loss of FMRP influences the developmental trajectory of cortical maturation remains unclear. We took advantage of the stereotyped and well-characterized development of the murine primary somatosensory cortex to examine cortical maturation during a time-window that corresponds to late embryonic and early postnatal development in the human. In the Fmr1 knockout mouse, we find a delay in somatosensory map formation, alterations in the morphology profile of dendrites and spines of layer 4 neurons and a decrease in the synaptic levels of proteins involved in glutamate receptor signaling at times corresponding to the highest levels of FMRP expression. In contrast, cortical arealization, synaptic density in layer 4 and early postnatal regulation of mRNAs encoding synaptic proteins are not altered in Fmr1 knockout mice. The specificity of the developmental delay in Fmr1 knockout mice indicates that the loss of FMRP does not result in a general stalling of cerebral cortex maturation. Instead, our results suggest that inaccurate timing of developmental processes caused by the loss of FMRP may lead to alterations in neural circuitry that underlie behavioral and cognitive dysfunctions associated with FXS.

mGluR5 Regulates Glutamate-Dependent Development of the Mouse Somatosensory Cortex

We have previously reported that mGluR5 signaling via PLC-β1 regulates the development of whisker patterns within S1 (barrel) cortex of mice (Hannan et al., 2001). However, whether these defects arise from the loss of postsynaptic mGluR5 signaling, and whether the level of mGluR5 is important for barrel formation, was not examined. Furthermore, whether mGluR5 regulates other developmental processes that occur before or after barrel development is not known. We now show that mGluR5 is present postsynaptically at thalamocortical synapses during barrel formation. In addition, Mglur5+/− mice exhibit normal TCA patch formation but reduced cellular segregation in layer 4, indicating a dose-dependent role for mGluR5 in the regulation of pattern formation. Furthermore Mglur5−/− and Mglur5+/− mice display normal cortical arealization, layer formation, and size of PMBSF indicating the defects within S1 do not result from general abnormalities of cortical mapping during earlier stages of development. At P21 layer 4 neurons from Mglur5−/− and Mglur5+/− mice show a significant reduction in spine density but normal dendritic complexity compared with Mglur5+/+ mice indicating a role in synaptogenesis during cortical development. Finally, mGluR5 regulates pattern formation throughout the trigeminal system of mice as the representation of the AS whiskers in the PrV, VpM, and S1 cortex was disrupted in Mglur5−/− mice. Together these data indicate a key role for mGluR5 at both early and late stages of neuronal development in the trigeminal system of mice.

Brief daily periods of binocular vision prevent deprivation-induced acuity loss

The role of experience in the development of the central visual pathways has been explored in the past through examination of the consequences of imposed periods of continuously abnormal or biased visual input. The massive changes in the visual cortex (area 17) induced by selected early visual experience, especially monocular deprivation (MD) or experience (ME) where patterned visual input is provided to just one eye, are accompanied by profound and long-standing visual deficits. Although the use of exclusively abnormal experience permits identification of those aspects of the visual cortex and of visual function that can be influenced by visual experience during development, this approach may provide a distorted view of the nature of the role of visual experience because of the absence of any normal visual input. In this study a different approach was used whereby animals were provided daily with separate periods of normal (i.e., binocular exposure) and abnormal (monocular exposure) visual experience. We show that 2 hr of daily normal concordant binocular experience (BE) can outweigh or protect against much longer periods of monocular deprivation (MD) and permit the development of normal visual acuities in the two eyes. This result is not what would be expected if all visual input had equal influence on visual development.

Initial recovery of vision after early monocular deprivation in kittens is faster when both eyes are open

A comparison was made of the speed of visual recovery in the deprived eye of kittens after a 6-day period of monocular deprivation imposed at 5–9 weeks of age in two postdeprivation conditions. In one condition, binocular recovery (BR), both eyes were open, whereas in the other condition, reverse lid-suture (RLS), the formerly nondeprived eye was closed to force the animal to use the originally deprived eye. In littermate pairs, BR kittens began to recover form vision 12 to 30 h before those subjected to RLS. The vision of the deprived eye of the BR animals remained superior to that of their RLS littermates for 4–8 days. Although this finding is difficult to reconcile with competitive mechanisms of synaptic plasticity, it supports a prediction of an alternative model of synaptic plasticity [Bienenstock, E. L., Cooper, L. N. & Munro, P. W. (1982) J. Neurosci. 2, 32–48] for slower initial recovery with RLS because of the time required to reset the modification threshold.

Phospholipase C-β1 expression correlates with neuronal differentiation and synaptic plasticity in rat somatosensory cortex

Abstract Receptor-mediated signal transduction is thought to play an important role in neuronal differentiation and the modification of synaptic connections during brain development. The intracellular signalling molecule phospholipase C-β1 (PLC-β1), which is activated via specific neurotransmitter receptors, has recently been implicated in activity-dependent plasticity in the cat visual cortex. PLC-β1 has been shown to be concentrated in an intermediate compartment-like organelle, the botrysome, which is present in 5-week-old, but not adult, cat cortical neurons. We have characterized the spatial and temporal regulation of PLC-β1 expression in the developing rat cerebral cortex. PLC-β1-positive botrysome-like organelles are observed during early postnatal cortical development, but not at postnatal day 14 or later stages. In the postnatal somatosensory cortex, there is also striking spatial variation in diffuse neuropilar immunoreactivity of layer IV and above, in a pattern corresponding to the thalamocortical recipient zones known as barrels. This expression pattern is specific to the developing barrel field and is most distinct at postnatal days 4–7, when cellular components of barrels are capable of activity-dependent modification. During later stages of cortical maturation, stained botrysomes disappear, expression of PLC-β1 is down-regulated and only diffuse immunoreactivity remains in dendritic processes. Our results are consistent with a role for PLC-β1 in activity-dependent, receptor-mediated neuronal plasticity during development of the somatosensory cortex.