NMDA離子通道去敏感化閘門之位置及其運作機制

Abstract

NMDA離子通道分別是由兩個GluN1和GluN2所組成,兩個不同的次單元間有著不同的角色與功能。在正常的生理功能下,NMDA離子通道屬於非選擇性陽離子通道,能夠讓突觸後神經細胞興奮且傳遞重要訊息,且與神經的可塑性和記憶學習息息相關。NMDA離子通道的去敏感化現象在正常生理情況下有重要的地位,因為當通透鈣離子進入細胞後若沒有產生去敏感化現象,將會有更大量的鈣離子進入細胞,而鈣離子在細胞質中扮演著訊息傳遞物質,大量的鈣離子濃度將會造成細胞錯誤的訊息傳遞。然而去敏感化現象的成因,尤其是GluN1和GluN2在其中切確的調控機轉至今仍然不是很清楚。因此我們試著透過tetraalkylammonium (TAA)不同側鏈長度的碳來探究activation gate和dsensitization gate之間的相對位置。我們發現tetrapentylammonium chloride(TpentA)能夠與關閉和開啟的NMDA離子通道作用,而且具有門閥修飾作用,能夠將NMDA離子通道維持在開啟態。換句話說就是TpentA的大小與結合位置剛好可以讓dsensitization gate無法關閉,讓NMDA離子通道無法待在去敏感化狀態。而且經過不同側鏈長度的TAA離子,我們推測dsensitization gate在activation gate的相當位置或更偏外面且可能位在洞口附近,另外我們透過TpentA的結合以及改變不同濃度的ligands發現,NMDA離子通道在開啟狀態和去敏感化狀態之間有著高度的耦合關係。在GluN1未活化或僅稍活化時,GluN2在接合NMDA之後通道就會短暫打開,但卻一下子就會關閉。只是此種關閉並非去敏感化現象,通道仍可隨時被NMDA+glycine所再次打開,因此GluN1對GluN2的門閥管制較有空間,亦即去敏感化現象需要GluN1和GluN2皆已活化之協同合作下才會出現。整體而言,GluN2可能直接負責通道的閘門開關,而GluN1卻對GluN2的活動扮演著重要角色。

Desensitization of NMDA receptor channel an obligatory heterotetramer formed by two GluN1 and two GluN2 subunits may play an important in neurophysiology and pathophysiology.However, the molecular mechanism of desensitization, especially the exact role of GluN1 and GluN2 subunits,are still unclear.We explored the relative position of the activation gate and the desensitization gate with tetraalkylammonium (TAA) ions of different side-chain lengths.We found that external tetrapentylammonium (TpentA) is a pore blocker.It binds to both closed and open NMDA channel with the same affinity,but precludes channel desensitization.In other words, TpentA makes the dsensitization gate unable to close,but leaves the activation gate relatively untouched.The dsensitization gate therefore is located outside the activation gate in the bundle crossing region at the external pore mouth.In addition,we found that NMDA channel have a high desensitization is hightly coupled to the activation of both GluN1 and GluN2 subunits.When the GluNl subunit is not activated or only slightly activated,activation of GluN2 subunit by NMDA binding would open the channel for a short period of time followed by channel closure.There closed channels are not desensitized,and could still be opened again by the presence of both activating ligands NMDA and glycine.We conclude that desensitization requires the cooperative GluN1 and GluN2 subunits.GluN2 subunit may be directly responsible for the gate itself,while GluN1 subunit may assume a control on GluN2 movement.