Sensory Hypersensitivity in Fragile X Syndrome Due to Deficits in Tonic Inhibition Reversed by Neuroactive Steroids

脆性X综合征（FXS）是遗传性智力残疾的最常见形式，也是一种主要的遗传性疾病。 自闭症谱系障碍的原因。FXS的症状包括感觉过敏， 刺激和癫痫FXS的原因是脆性X智力低下蛋白（FMRP）的丢失，但FXS的基因突变是一个非常重要的因素。 这些症状的潜在途径和机制描述不多。GABAA受体（GABAAR） 是大脑中主要的抑制性离子通道。来自FXS患者和动物模型的研究表明， GABAAR α4亚基的表达水平改变，伴随着滋补剂疗效的降低。 抑制是一种非突触类型的抑制，对于确定神经元输出的增益很重要，因此 调节神经回路的兴奋性和活性。我们的初步数据表明，在Fmr 1 KO小鼠中， α4亚基磷酸化水平降低，导致紧张性电流降低， 激发紧张性抑制的缺陷是由于含有α4-GABAAR的运输中断， 突触外到突触部位。FXS患者和Fmr 1 KO小鼠均显示听觉处理缺陷， 他们对声音非常敏感。听觉信息是由丘脑内侧核的听觉神经元处理的 膝状体（MGB），因为它上升到听觉皮层。听觉丘脑神经元的兴奋是 由依赖于含有α4亚基的GABAAR的紧张性抑制控制。减少强直性抑制， FXS患者的MGB可以解释听觉灵敏度的增加，并可以作为治疗的目标，以减少 感觉过敏我们实验室的研究表明，神经活性类固醇（NASs）通过 代谢型，激酶依赖性机制，以增加α4亚基中丝氨酸443的磷酸化， 促进GABAAR插入质膜，导致其在细胞膜上的积累持续升高。 细胞表面，并增加了紧张性抑制的功效。这些初步研究使我们能够 提出一个中心假设，将在这里进行测试; FXS中对感官刺激的超敏反应是由 由于α4亚单位错误转运至 GABA能突触。暴露于神经活性类固醇会增加α4亚单位的运输 含有GABAAR的细胞膜，以增强紧张性抑制，并减少 disorder. 目标1.表征含α4亚基的磷酸化和亚细胞定位 Fmr 1 KO小鼠中的GABAAR。目标2.检查Fmr 1 KO的脑电图（EEG）活动 小鼠目标3：确定NAS治疗逆转GABA能抑制缺陷的功效， 神经元兴奋性这项研究的结果将提供新的见解调节机制， 紧张性抑制功效以及这些改变是否有助于FXS的病理生理学。等 这些见解可能会促进新疗法的开发，以减轻FXS的负担。

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and a major cause for a diagnosis of autism spectrum disorders. The symptoms of FXS include hypersensitivity to sensory stimuli, and seizures. The cause of FXS is a loss of the fragile X mental retardation protein (FMRP) yet the underlying pathways and mechanisms of these symptoms are poorly described. GABAA receptors (GABAARs) are major inhibitory ion channels in the brain. Studies from both FXS patients and animal models have revealed altered expression levels of GABAAR α4 subunits with a concomitant reduced efficacy of tonic inhibition, a non-synaptic type of inhibition important for determining the gain of the neuronal output, thus regulating the excitability and activity of neuronal circuits. Our preliminary data suggest that in Fmr1 KO mice there is a decrease in the phosphorylation of α4 subunits leading to decreased tonic current, and an increased excitation. The deficits of tonic inhibition are due to a disruption in trafficking of α4-containing GABAARs from extrasynaptic to synaptic sites. Both FXS patients and Fmr1 KO mice show auditory processing deficits making them hypersensitive to sound. Acoustic information is processed by thalamic auditory neurons in the medial geniculate body (MGB) as it ascends to the auditory cortex. Excitation of auditory thalamic neurons is controlled by tonic inhibition dependent upon GABAARs containing α4 subunits. Reduced tonic inhibition in MGB of FXS patients could explain the increase in auditory sensitivity and be a target for therapy to reduce sensory hypersensitivity. Studies from our laboratory have revealed that neuro-active steroids (NASs) act via a metabotropic, kinase dependent mechanism to increase the phosphorylation of serine 443 in the α4 subunit to promote GABAAR insertion into the plasma membrane leading to sustained elevations in their accumulation on the cell surface and increases in the efficacy of tonic inhibition. These preliminary studies have allowed us to formulate a central hypothesis that will be tested here; Hypersensitivity to sensory stimuli in FXS is caused by reduced tonic inhibition in the central auditory system due to mis-trafficking of α4 subunits to GABAergic synapses. Exposure to neuroactive steroids will increase the trafficking of α4 subunit containing GABAARs to the membrane to boost tonic inhibition and diminish the severity of the disorder. Aim 1. Characterize the phosphorylation and sub-cellular localization of α4 subunit containing GABAARs in Fmr1 KO mice. Aim 2. Examine the electroencephalographic (EEG) activity of Fmr1 KO mice. Aim 3. Determine the efficacy of NAS treatment to reverse deficits in GABAergic inhibition and neuronal excitability. The result of this study will provide new insights into the mechanisms that regulate the efficacy of tonic inhibition and if that alterations in these contribute to the pathophysiology of FXS. Such insights may promote the development of new therapeutics to alleviate the burdens of FXS.