Abrams C K, Bennett M V L, Verselis V K, Bargiello T A
Department of Neuroscience, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461, USA.
Proc Natl Acad Sci U S A. 2002 Mar 19;99(6):3980-4. doi: 10.1073/pnas.261713499. Epub 2002 Mar 12.
The X-linked form of Charcot-Marie-Tooth disease (CMTX) is an inherited peripheral neuropathy that arises in patients with mutations in the gene encoding the gap junction protein connexin 32 (Cx32), which is expressed by Schwann cells. We recently showed that Cx32 containing the CMTX-associated mutation, Ser-85-Cys (S85C), forms functional cell-cell channels in paired Xenopus oocytes. Here, we describe that this mutant connexin also shows increased opening of hemichannels in nonjunctional surface membrane. Open hemichannels may damage the cells through loss of ionic gradients and small metabolites and increased influx of Ca(2+), and provide a mechanism by which this and other mutant forms of Cx32 may damage cells in which they are expressed. Evidence for open hemichannels includes: (i) oocytes expressing the Cx32(S85C) mutant show greatly increased conductance at inside positive potentials, significantly larger than in oocytes expressing wild-type Cx32 (Cx32WT); and (ii) the induced currents are similar to those previously described for several other connexin hemichannels, and exhibit slowly developing increases with increasing levels of positivity and reversible reduction when intracellular pH is decreased or extracellular Ca(2+) concentration is increased. Although increased currents are seen, oocytes expressing Cx32(S85C) have lower levels of the protein in the surface and in total homogenates than do oocytes expressing Cx32WT; thus, under the conditions examined here, hemichannels in the surface membrane formed of the Cx32(S85C) mutant have a higher open probability than hemichannels formed of Cx32WT. This increase in functional hemichannels may damage Schwann cells and ultimately lead to loss of function in peripheral nerves of patients harboring this mutation.
夏科-马里-图斯病（CMTX）的X连锁型是一种遗传性周围神经病变，发生于编码缝隙连接蛋白连接蛋白32（Cx32）的基因突变患者中，该蛋白由施万细胞表达。我们最近发现，含有与CMTX相关突变Ser-85-Cys（S85C）的Cx32在爪蟾卵母细胞对中形成功能性细胞间通道。在此，我们描述这种突变连接蛋白在非连接表面膜中的半通道开放也增加。开放的半通道可能通过离子梯度和小代谢物的丧失以及Ca²⁺内流增加而损害细胞，并提供了一种机制，通过该机制，这种和其他突变形式的Cx32可能损害其表达的细胞。开放半通道的证据包括：（i）表达Cx32（S85C）突变体的卵母细胞在膜内正电位时电导大大增加，显著大于表达野生型Cx32（Cx32WT）的卵母细胞；（ii）诱导电流与先前描述的其他几种连接蛋白半通道的电流相似，并且随着正电位水平的增加而缓慢增加，当细胞内pH降低或细胞外Ca²⁺浓度增加时可逆性降低。尽管观察到电流增加，但表达Cx32（S85C）的卵母细胞表面和总匀浆中的蛋白水平低于表达Cx32WT的卵母细胞；因此，在此处检查的条件下，由Cx32（S85C）突变体形成的表面膜中的半通道比由Cx32WT形成的半通道具有更高的开放概率。功能性半通道的这种增加可能损害施万细胞，并最终导致携带这种突变的患者周围神经功能丧失。
