Akt1基因剔除小鼠背側紋狀體medium spiny neurons型態生理功能研究

Abstract

AKT，又稱為protein kinase B，在中樞神經系統調控神經生長扮演重要角色。近年研究指出，精神分裂症(schizophrenia)病人的AKT1蛋白有較低的表現量，因此被視為精神分裂症的候選基因。然而少有研究探討AKT1蛋白不正常功能在神經功能的直接影響。於此我們比較Akt1基因剔除小鼠、Akt1 雜合型小鼠與正常小鼠在紋狀體medium spiny neurons (MSN) 之型態與生理特性。MSN神經元以大量的樹突spine以及相對較負的靜止膜電位(約~ -80 mV)為特色。首先比較MSN基本的膜生理特性與其興奮性。Akt1基因剔除鼠之MSN其輸入電阻相較正常小鼠之MSN輸入電阻高；而Akt1基因剔除鼠之MSN其rheobase相較正常小鼠之MSN低。比較打入不同強度正電流引發放電頻率之輸入/輸出關係(I/O relationship)，以曲線回歸值為指標gain值，兩者並無顯著差異，Akt1基因剔除鼠之MSN其輸入/輸出關係曲線顯著向左偏移，此偏移可能是來自較低的rheobase造成。除此之外Akt1基因剔除鼠之MSN其主要樹突數目以及複雜度較正常小鼠少。這些型態差異顯示不正常的AKT功能在早期神經發育造成較少的樹突數目，進一步改變細胞基本的膜生理的特性(如輸入電阻與rheobase)。興奮性與抑制性神經傳導研究指出，興奮性神經傳導之傳導物質的釋放效能以及突觸間特性在正常小鼠以及Akt1基因剔除鼠無明顯差異，而抑制性神經傳導則受到Akt1基因剔除而改變。在皮質紋狀體路徑引發長效抑制作用(long-term depression, LTD)，需要多巴胺第二型受體(D2 receptor)參與。然而結果發現抑制Akt signaling會阻礙皮質紋狀體路徑的長效抑制作用，顯示Akt signaling亦參與引發長效抑制作用。

AKT, also known as Protein Kinas B, plays many important roles in regulation of neuronal growth in the central nervous system. Recent evidence has shown that there is a decreased AKT1 protein level in schizophrenia patients, suggesting that it might be one of the susceptibility genes for schizophrenia. However, the direct impact of AKT1 dysfunction on the neuronal function is poorly understood. In the present study, the physiological and morphological properties of medium spiny neurons (MSN) in the striatum were compared among wild-type (WT), Akt1-heterozyous (Akt1 -/+) and Akt1 knockout (Akt1-/-) mice. The MSNs were characterized by the presence of large number of dendritic spines and having relatively hyperpolarized resting membrane potential (~ -80 mV). The intrinsic membrane properties and excitability of MSN were firstly examined. The input resistance (Rn) of MSN in Akt1-/- was significantly higher than that in WT mice, while the rheobase (Rh) was remarkably lower in Akt1-/- than in WT mice. As for the input-output (I/O) relationship, defined as the intensity of injected depolarizing current versus the resulted instantaneous firing frequency, no significant difference in the gain was found between WT and Akt1-/- mice, though significant left-shifting of the I/O relationship was observe in Akt1-/- mice, which may be attributed to the lower Rh of MSN in Akt1-/- mice. In addition, lower number of primary dendrites and lower complexity of dendrites were observed in Akt1-/- mice. These morphological changes indicate that AKT dysfunction impairs the growth of dendritic trees in early neuronal development and leads to alter membrane properties such as higher Rn and lower Rh in MSNs in Akt1 -/- mice. With regard to the excitatory and inhibitory neurotransmission, there were no significant differences of excitatory neurotransmission in efficacy of transmitter release and overall property at synaptic sites between WT and Akt1 -/- mice, however Akt deficit affected inhibitory neurotransmission at both pre- and post-synaptic sites. Data from the LTD of corticotriatal pathway in MSNs showed a requirement for D2 receptor activation. Blockade of AKT signaling did not elicit the formation of LTD, which indicates AKT signaling may be involved in this form of LTD.