漢丁頓式舞蹈症(HD)是一種自體顯性遺傳的神經退化性疾病，目前在治療上只有有限的症狀緩解藥物，病人仍持續退化。γ-氨基丁酸(GABA)是中樞神經系統重要的抑制性傳導物質，其缺損會導致降低抑制性訊號，進而喪失與興奮性訊號的平衡。而在大腦-基底核路徑中，有許多需要透過抑制性GABA訊號來調控動作及行為;此路徑在HD已知出現問題，這樣的異常可能導致臨床上HD相關的動作行為缺損表徵。在HD，對GABA訊號在動作上的調控了解仍有限。其中有兩個功能相對的離子傳輸通道-鉀/氯離子(KCC2)和鈉鉀/氯離子傳輸通道(NKCC1)能調控GABA作用在突觸後神經傳導為抑制性或興奮性訊號。透過幾種不同的HD老鼠模型和HD病人死後的腦組織，我們得以研究異常的GABA訊號對HD動作上的調控及影響。我們發現HD鼠相較正常鼠對兩種作用機轉不同的GABA-A型受體藥物(Diazepam 和 Gaboxadol)皆呈現較弱的反應。進一步探究其可能機轉，我們發現在HD鼠的大腦和紋狀體腦區相較正常鼠的同腦區有較少的α1/α2- 和 δ-GABA-A型受體。這現象同時可在HD病人死後的腦組織偵測到。此外調控GABA抑制訊號的其中一個重要分子- KCC2，其功能和表現量在HD鼠腦皆呈現較正常鼠下降。KCC2的下降常伴隨NKCC1的上升進而導致去極化的GABA訊號。我們更進一步研究NKCC1在HD的病理機轉調控。首先，我們在不同的HD老鼠模型和HD病人的紋狀體腦區偵測到明顯上升的NKCC1。經由電生理測量到在HD鼠的紋狀體棘狀突起投射神經元(Medium spiny neurons)相較正常鼠有更加去極化的GABA訊號(EGABA)，同時加入抑制NKCC1藥物(Bumetanide)證實上升的NKCC1與此現象相關。透過(1) 腹腔內給藥-Bumetanide 和(2)直接紋狀體注射腺相關病毒(Adeno-associated virus)載體攜帶抑制NKCC1物質來達到抑制HD鼠腦區上升的NKCC1，發現抑制NKCC1可改善HD鼠的動作障礙。在調控NKCC1表現的機轉方面，我們利用細胞和動物實驗發現帶有變異性漢丁頓式蛋白(Htt)的星形膠質細胞和神經發炎因子會誘發NKCC1的上升。此研究提出透過基因或藥物方式來平衡KCC2和NKCC1功能能調節GABA傳導訊號進而改善HD中一些與抑制性訊號異常有關的症狀。
Huntington’s disease (HD) is an autosomal dominant neurodegenerative disease without effective therapy. Altered GABAergic signaling in the striatum is believed to disrupt the balance of excitation/inhibition within the corticostriatal circuit, which might be associated with motor symptoms in patients with HD. To date, the pathophysiological role of GABAergic signaling in motor control has attracted limited attention. KCC2 and NKCC1 are two key molecules that regulate the intracellular chloride homeostasis to affect the GABAAR-mediated neurotransmission. Using different HD mouse models and post-morten brain tissues of HD patients, we can study deeply regarding the aberrant GABAergic signaling in HD brains. In our study, we found altered behavioral responses to GABAA receptor agents (diazepam and gaboxadol specifically targeting synaptic α1/α2- and extrasynaptic δ-containing GABAAR, respectively) in a mouse model of HD (R6/2 mice). Further biochemical studies in different HD models (R6/2, N171-82Q and Hdh150Q mice) consistently detected altered expression levels of α1/α2- and δ-containing GABAARs and KCC2, which partly explain the inferior efficacy of GABAAR agents in HD mice. A reduced KCC2 is usually associated with an enhanced NKCC1, which contributes to aberrant GABAAergic signaling. We further demonstrated that striatal NKCC1 is increased both in different mouse models of HD (R6/2, Hdh150Q and GFAP-HD mice) and patients with HD. Accordingly, increased striatal NKCC1 led to an elevated EGABA in the medium spiny neurons (MSNs) of R6/2 mice at the manifest stage. The aberrant polarity of GABAAR signaling in MSNs might affect the overall output of striatum in HD through altering the local excitability of MSNs. Notably, the increased NKCC1 was pathogenic in HD because both pharmacological and genetic inhibition of NKCC1 rescued the motor phenotype of R6/2 mice. Further in vitro and in vivo mechanistic studies suggest that pro-inflammatory cytokines and mHTT-expressing astrocytes are likely to evoke neuronal NKCC1 production. Our findings advance the current knowledge of GABAergic signaling in HD pathogenesis and provide a new therapeutic avenue toward HD via modulating the function of NKCC1.
