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

脆性X综合征(FXS)是遗传性智能障碍的最常见形式，也是一种主要的 自闭症谱系障碍的诊断原因。FXS的症状包括感官过敏 刺激和癫痫发作。FXS的原因是脆性X智力低下蛋白(FMRP)的丢失 这些症状的潜在途径和机制没有得到很好的描述。GABA受体(GABA受体) 是大脑中主要的抑制离子通道。来自FXS患者和动物模型的研究已经 GABAARα4亚基表达水平的改变伴随着补药疗效的降低 抑制，一种非突触类型的抑制，对决定神经元输出的增益很重要，因此 调节神经元回路的兴奋性和活性。我们的初步数据表明，在Fmr1 KO小鼠中 α4亚基的磷酸化程度降低，导致紧张性电流降低，而 激动人心。紧张性抑制的缺陷是由于含有α4的GABAARs从 突触外至突触部位。FXS患者和Fmr1 KO小鼠都显示出听觉加工缺陷 它们对声音高度敏感。听觉信息由丘脑内侧听神经元处理 膝状小体(MGB)上升至听觉皮质。听觉丘脑神经元的兴奋 受紧张性抑制控制，依赖于含有α4亚基的GABA受体。减少紧张性抑制 FXS患者的MGB可以解释听力敏感度的增加，并可作为治疗的靶点 感觉过敏症。我们实验室的研究表明，神经活性类固醇(NASS)通过 促进α4亚基丝氨酸443磷酸化的代谢性、依赖于激酶的机制 促进GABA插入质膜，使其在细胞膜上的积累量持续增加 改善细胞表面，增加紧张性抑制的功效。这些初步研究使我们能够 阐述一个将在这里进行检验的中心假设；FXS对感觉刺激的过敏是由 通过减少中枢听觉系统中由于α4亚基被错误运输到 GABA能突触。暴露于神经活性类固醇会增加α4亚单位的转运 含有GABA受体的膜，以加强紧张性抑制和减轻严重的 无序。 目的1.研究α-4亚基的磷酸化和亚细胞定位 Fmr1KO小鼠体内的GABAARs。目的2.检测Fmr1KO的脑电活动 老鼠。目的3.确定NAS治疗逆转GABA能抑制缺陷和 神经元兴奋性。这项研究的结果将为我们提供新的见解，以调节 紧张性抑制的有效性以及这些变化是否有助于FXS的病理生理学。是这样的 洞察力可能会促进新疗法的发展，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.