Department of Pharmacology, University of Oxford, Oxford, UK.
Graduate School of Pharmaceutical Sciences, Kyoto University, Kyoto, Japan.
J Physiol. 2019 May;597(10):2691-2705. doi: 10.1113/JP277241. Epub 2019 Apr 14.
There are two subtypes of trimeric intracellular cation (TRIC) channels but their distinct single-channel properties and physiological regulation have not been characterized. We examined the differences in function between native skeletal muscle sarcoplasmic reticulum (SR) K -channels from wild-type (WT) mice (where TRIC-A is the principal subtype) and from Tric-a knockout (KO) mice that only express TRIC-B. We find that lone SR K -channels from Tric-a KO mice have a lower open probability and gate more frequently in subconducting states than channels from WT mice but, unlike channels from WT mice, multiple channels gate with high open probability with a more than six-fold increase in activity when four channels are present in the bilayer. No evidence was found for a direct gating interaction between ryanodine receptor and SR K -channels in Tric-a KO SR, suggesting that TRIC-B-TRIC-B interactions are highly specific and may be important for meeting counterion requirements during excitation-contraction coupling in tissues where TRIC-A is sparse or absent.
The trimeric intracellular cation channels, TRIC-A and TRIC-B, represent two subtypes of sarcoplasmic reticulum (SR) K -channel but their individual functional roles are unknown. We therefore compared the biophysical properties of SR K -channels derived from the skeletal muscle of wild-type (WT) or Tric-a knockout (KO) mice. Because TRIC-A is the major TRIC-subtype in skeletal muscle, WT SR will predominantly contain TRIC-A channels, whereas Tric-a KO SR will only contain TRIC-B channels. When lone SR K -channels were incorporated into bilayers, the open probability (Po) of channels from Tric-a KO mice was markedly lower than that of channels from WT mice; gating was characterized by shorter opening bursts and more frequent brief subconductance openings. However, unlike channels from WT mice, the Po of SR K -channels from Tric-a KO mice increased as increasing channel numbers were present in the bilayer, driving the channels into long sojourns in the fully open state. When co-incorporated into bilayers, ryanodine receptor channels did not directly affect the gating of SR K -channels, nor did the presence or absence of SR K -channels influence ryanodine receptor activity. We suggest that because of high expression levels in striated muscle, TRIC-A produces most of the counterion flux required during excitation-contraction coupling. TRIC-B, in contrast, is sparsely expressed in most cells and, although lone TRIC-B channels exhibit low Po, the high Po levels reached by multiple TRIC-B channels may provide a compensatory mechanism to rapidly restore K gradients and charge differences across the SR of tissues containing few TRIC-A channels.
有两种三聚体细胞内阳离子（TRIC）通道亚型，但它们的单一通道特性和生理调节尚未得到表征。我们研究了来自野生型（WT）小鼠的原生骨骼肌肌浆网（SR）K-通道（其中 TRIC-A 是主要亚型）和仅表达 TRIC-B 的 Tric-a 敲除（KO）小鼠之间功能的差异。我们发现，Tric-a KO 小鼠的孤立 SR K-通道的开放概率较低，并且在亚导通状态下更频繁地打开，而 WT 小鼠的通道则不然，但与 WT 小鼠的通道不同，当双层中存在四个通道时，多个通道以高开放概率打开，活性增加超过六倍。在 Tric-a KO SR 中，没有发现肌浆网钙释放通道与 SR K-通道之间的直接门控相互作用的证据，这表明 TRIC-B-TRIC-B 相互作用具有高度特异性，并且可能对满足兴奋-收缩偶联过程中的反离子要求很重要，在 TRIC-A 稀疏或不存在的组织中。
三聚体细胞内阳离子通道，TRIC-A 和 TRIC-B，代表两种肌浆网（SR）K-通道亚型，但它们各自的功能作用尚不清楚。因此，我们比较了来自野生型（WT）或 Tric-a 敲除（KO）小鼠骨骼肌的 SR K-通道的生物物理特性。由于 TRIC-A 是骨骼肌中的主要 TRIC 亚型，因此 WT SR 将主要包含 TRIC-A 通道，而 Tric-a KO SR 将仅包含 TRIC-B 通道。当单独的 SR K-通道被掺入双层时，来自 Tric-a KO 小鼠的通道的开放概率（Po）明显低于来自 WT 小鼠的通道；门控的特点是打开爆发时间更短，并且更频繁的短暂亚电导打开。然而，与 WT 小鼠的通道不同，当双层中存在越来越多的通道时，来自 Tric-a KO 小鼠的 SR K-通道的 Po 增加，从而使通道长时间保持完全打开状态。当共同掺入双层时，肌浆网钙释放通道不会直接影响 SR K-通道的门控，也不会影响 SR K-通道的存在或不存在对肌浆网钙释放通道活性的影响。我们认为，由于在横纹肌中高表达，TRIC-A 产生了兴奋-收缩偶联过程中所需的大部分反离子通量。相比之下，TRIC-B 在大多数细胞中表达稀疏，尽管单独的 TRIC-B 通道的 Po 较低，但多个 TRIC-B 通道达到的高 Po 水平可能提供了一种补偿机制，可快速恢复含有少量 TRIC-A 通道的组织中 SR 的 K 梯度和电荷差异。
