The Developmental Switch in GABA Polarity Is Delayed in Fragile X Mice

199

179

Rett syndrome is a severe form of autism spectrum disorder, mainly caused by mutations of a single gene methyl CpG binding protein 2 (MeCP2) on the X chromosome. Patients with Rett syndrome exhibit a period of normal development followed by regression of brain function and the emergence of autistic behaviors. However, the mechanism behind the delayed onset of symptoms is largely unknown. Here we demonstrate that neuron-specific K+-Cl− cotransporter2 (KCC2) is a critical downstream gene target of MeCP2. We found that human neurons differentiated from induced pluripotent stem cells from patients with Rett syndrome showed a significant deficit in KCC2 expression and consequently a delayed GABA functional switch from excitation to inhibition. Interestingly, overexpression of KCC2 in MeCP2-deficient neurons rescued GABA functional deficits, suggesting an important role of KCC2 in Rett syndrome. We further identified that RE1-silencing transcriptional factor, REST, a neuronal gene repressor, mediates the MeCP2 regulation of KCC2. Because KCC2 is a slow onset molecule with expression level reaching maximum later in development, the functional deficit of KCC2 may offer an explanation for the delayed onset of Rett symptoms. Our studies suggest that restoring KCC2 function in Rett neurons may lead to a potential treatment for Rett syndrome.

Section: Discussionsupporting

confidence: 92%

KCC2 rescues functional deficits in human neurons derived from patients with Rett syndrome

Tang

Kim

Zhou

et al. 2016

199

179

“…A developmental shift in the expression of these proteins toward SLC12A5 regulates the transition of GABAergic transmission from depolarizing to hyperpolarizing. The diuretic NKCC1 inhibitor bumetanide has been proposed as a potential therapeutic in FXS and autism (56), and our data corroborate recent work suggesting that FXS may indeed involve an improper balance of SLC12A2 and SLC12A5 (10). Interestingly, at P45, both proteins were comparably expressed in WT and KO synapses, suggesting that the ratio imbalance evident in young cortex may continue into adulthood even as absolute levels of each decrease.…”

Section: Discussionsupporting

confidence: 89%

“…Substantial evidence indicates that FMRP is particularly critical as a suppressor of activity-dependent mRNA translation at glutamatergic synapses (6,7) and that loss of this function results in abnormalities in dendritic spine shape and several forms of long-term synaptic plasticity (8,9). In addition, significant changes have been described regarding the structure and/or function of other synaptic systems, including GABAergic and endocannabinoid synapses (10,11). Loss of FMRP expression has been recently linked to abnormalities in adult neurogenesis, migration, neural differentiation, and cortical maturation (12).…”

mentioning

confidence: 99%

Fmr1 deficiency promotes age-dependent alterations in the cortical synaptic proteome

Tang

Wang

Wan

et al. 2015

Fragile X syndrome (FXS) is an X-linked neurodevelopmental disorder characterized by severe intellectual disability and other symptoms including autism. Although caused by the silencing of a single gene, Fmr1 (fragile X mental retardation 1), the complexity of FXS pathogenesis is amplified because the encoded protein, FMRP, regulates the activity-dependent translation of numerous mRNAs. Although the mRNAs that associate with FMRP have been extensively studied, little is known regarding the proteins whose expression levels are altered, directly or indirectly, by loss of FMRP during brain development. Here we systematically measured protein expression in neocortical synaptic fractions from Fmr1 knockout (KO) and wild-type (WT) mice at both adolescent and adult stages. Although hundreds of proteins are up-regulated in the absence of FMRP in young mice, this up-regulation is largely diminished in adulthood. Up-regulated proteins included previously unidentified as well as known targets involved in synapse formation and function and brain development and others linked to intellectual disability and autism. Comparison with putative FMRP target mRNAs and autism susceptibility genes revealed substantial overlap, consistent with the idea that the autism endophenotype of FXS is due to a "multiple hit" effect of FMRP loss, particularly within the PSD95 interactome. Through studies of de novo protein synthesis in primary cortical neurons from KO and WT mice, we found that neurons lacking FMRP produce nascent proteins at higher rates, many of which are synaptic proteins and encoded by FMRP target mRNAs. Our results provide a greatly expanded view of protein changes in FXS and identify age-dependent effects of FMRP in shaping the neuronal proteome.fragile X | quantitative mass spectrometry | synaptic protein synthesis | autism | stable isotope labeling F ragile X syndrome (FXS) is an X-linked monogenic disorder that leads to highly debilitating changes in neurodevelopment. Affected individuals exhibit mental retardation, attention deficit and hyperactivity, anxiety, autism spectrum behaviors, and other symptoms that compound overall impairment (1). In the vast majority of cases, FXS is caused by an mRNA-dependent epigenetic silencing of the Fmr1 (fragile X mental retardation 1) gene (2), which occurs secondarily to a CGG repeat expansion in the 5′ UTR region of Fmr1 (3) and results in absence of the encoded protein FMRP. FMRP is an RNA-binding protein that regulates several aspects of mRNA translation (1), transport (4), and stability (5) in neurons. Substantial evidence indicates that FMRP is particularly critical as a suppressor of activity-dependent mRNA translation at glutamatergic synapses (6, 7) and that loss of this function results in abnormalities in dendritic spine shape and several forms of long-term synaptic plasticity (8, 9). In addition, significant changes have been described regarding the structure and/or function of other synaptic systems, including GABAergic and endocannabinoid synapses (10, 11). Loss of FMR...

“…(21,22). Increased intracellular chloride concentration and a change in the postsynaptic impact of GABAergic synapses have been demonstrated during development in mouse models of ASD, including fragile-X syndrome (23,24). Indirect evidence also points to this mechanism affecting RTT: Lower levels of KCC2 relative to NKCC1 have been found by immunoblot analysis of cerebrospinal fluid samples in patients with RTT (25); depolarizing GABAergic synapses in development follow from down-regulation of brain-derived neurotrophic factor (BDNF) (26), which is also an early consequence of MeCP2 deletion in mouse models (27); and insulin-like growth factor-1 (IGF1) treatment, which partially rescues behavioral and synaptic deficits in mutant mouse models of RTT (28,29), also activates KCC2 expression and restores the inhibitory action of GABAergic synapses in the hippocampus (30).…”

Section: Significancementioning

confidence: 99%

Jointly reduced inhibition and excitation underlies circuit-wide changes in cortical processing in Rett syndrome

Banerjee

Rikhye

Breton-Provencher

et al. 2016

159

202

Rett syndrome (RTT) arises from loss-of-function mutations in methyl-CpG binding protein 2 gene (Mecp2), but fundamental aspects of its physiological mechanisms are unresolved. Here, by whole-cell recording of synaptic responses in MeCP2 mutant mice in vivo, we show that visually driven excitatory and inhibitory conductances are both reduced in cortical pyramidal neurons. The excitation-to-inhibition (E/I) ratio is increased in amplitude and prolonged in time course. These changes predict circuit-wide reductions in response reliability and selectivity of pyramidal neurons to visual stimuli, as confirmed by two-photon imaging. and increases KCC2 expression to normalize the KCC2/NKCC1 ratio. Thus, loss of MeCP2 in the brain alters both excitation and inhibition in brain circuits via multiple mechanisms. Loss of MeCP2 from a specific interneuron subtype contributes crucially to the cell-specific and circuit-wide deficits of RTT. The joint restoration of inhibition and excitation in cortical circuits is pivotal for functionally correcting the disorder.MeCP2 | E/I balance | parvalbumin neurons | IGF1 | chloride transporters S ynaptic excitation (E) and inhibition (I), along with the neuronal balance of excitation and inhibition (E/I), is key to the function of brain circuits, and is often disrupted in neurodevelopmental disorders, including autism spectrum disorders (ASDs) (1-3). Rett syndrome (RTT) is a severe neurodevelopmental and adult disorder that arises from sporadic loss-of-function mutations in the X-linked (Xq28) methyl-CpG binding protein 2 gene (Mecp2) encoding the protein MeCP2 (4-7). MeCP2 is a critical regulator of brain development and adult neural function (8), and arrested brain maturation due to synaptic dysfunction is one of the hallmarks of RTT (3). However, the effects of MeCP2 on excitatory and inhibitory synaptic mechanisms in vivo, and on neuronal and circuit function underlying RTT pathophysiology, are unknown.MeCP2 is ubiquitously expressed in multiple cell types and subregions of the brain (4, 6, 9), including inhibitory interneurons, and has cell-autonomous as well as non-cell-autonomous effects (10); thus, it has been particularly challenging to identify its role in cell-specific brain circuits. Anatomically diverse inhibitory interneuron subtypes with distinct physiological signatures influence different aspects of neocortical function and behavior (11,12). Soma-targeting parvalbumin-expressing (PV + ) and dendritetargeting somatostatin-expressing (SOM + ) interneurons are the two major nonoverlapping populations of interneurons in mice that target cortical pyramidal neurons (13). Inhibition by PV + and SOM + neurons powerfully influences neuronal responses and circuit computations in visual cortex (14-17). Deletion of MeCP2 from all forebrain GABAergic interneurons recapitulates key aspects of RTT (18), demonstrating that altered inhibitory function is an important facet of RTT pathophysiology. Indeed, a major phenotype of MeCP2 reduction in individuals with RTT and in mouse models is ...