The amphibian kidney's filtration barrier: where is the glomerular basement membrane?
The amphibian kidney's filtration barrier: where is the glomerular basement membrane?
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两栖动物肾脏的过滤屏障：肾小球基底膜在哪里？
DOI：
10.1152/ajprenal.00236.2009
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发表时间：
2009
期刊：
影响因子：
--
通讯作者：
Miner,JeffreyH
中科院分区：
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作者：
Miner,JeffreyH
致编辑：坦纳及其同事（8）将体内双光子显微镜的成像灵敏度与蝾螈肾小球毛细血管壁（GCW）的独特结构相结合，以研究有关肾小球滤过和选择性渗透机制的几个重要问题。由于两栖动物生理学是哺乳动物生理学的代表，这可能是一个强大的，高度信息化的方法。然而，纳瓦尔的编辑重点（4）伴随着文件表示严重怀疑的一些结论。虽然我不同意坦纳等人的发现。(8)关于筛选系数和电荷选择性，我确实看到了Navar和可能还有裁判忽略的数据解释中的一个重要错误。基于这一缺陷，这涉及到Necturus肾小球基底膜（GBM）是如何定义的，我不相信体内研究已经真正回答，甚至解决了GBM是否作为白蛋白的屏障的问题，无论是在Necturus，在小鼠，还是在人。Necturus肾小球成像的一个优点是GCW非常宽。坦纳等人(8)GBM具有3.5 m的非凡宽度;这与小鼠中的0.2 m GBM相比，相差18倍。然而，坦纳等人对图5D所示超微结构的仔细观察表明，海葵属GCW是非常不典型的。尽管具有缝隙隔膜和薄内皮的足细胞足突是熟悉的，但介入的细胞外基质并不令人想起哺乳动物GBM。不仅绝大多数细胞外物质是电子透明的，而且其中也有许多细胞突起（8）。这些特征通常与先前报道的蟾蜍（Bufo）、青蛙（Lemnodynastes）和牛蛙（Rana）中的GCW超微结构一致（5，6）。然而，在研究基底膜超过17年后，我不同意内皮细胞和足细胞之间的异质3.5-m基质应被视为Necturus GBM。尽管在Necturus GCW中存在非典型的细胞间隙，但在足细胞足突附近存在明显的、结构良好的、连续的致密层。在部分分离的内皮细胞的基底面，有另一个致密层，相比之下，它看起来不连续（8）。足细胞下方的致密薄层肯定是基底膜的特征，而GCW中相对较大的非细胞部分则不是。事实上，这以前被称为毛细血管周围空间（5）和内皮下空间（6）。为了比较，图1显示了赖歇特膜（3）的电子显微镜照片，赖歇特膜是胚胎大鼠和小鼠卵黄囊中非常厚的基底膜。整个细胞间隙的广泛致密层是明显的。从图4（8）的荧光可以很容易地接受，无论是GCW细胞间隙的巨大电子透明部分还是不连续的内皮下基底膜都不能有效地阻挡大分子。然而，与足突相邻的致密层太薄，无法通过体内双光子显微镜进行区分，就像哺乳动物中的致密层太薄一样。因此，存在这样的可能性，即致密层是大分子的关键屏障，与我们和其他人的建议一致（1，2，4，7）。不幸的是，这个几十年的老问题仍然没有答案。
TO THE EDITOR: Tanner and colleagues (8) combined the imaging sensitivity of in vivo two-photon microscopy with the distinctive architecture of the salamander Necturus’ glomerular capillary wall (GCW) to investigate several important issues regarding the mechanisms of glomerular filtration and permselectivity. Insofar as amphibian physiology is representative of mammalian physiology, this is potentially a powerful, highly informative approach. Yet Navar’s editorial focus (4) accompanying the paper expresses significant doubts about some of the conclusions. Although I do not disagree with the findings of Tanner et al.(8) with respect to sieving coefficients and charge selectivity, I do see an important error in data interpretation that Navar and perhaps also the referees overlooked. Based on this flaw, which relates to how the Necturus glomerular basement membrane (GBM) was defined, I do not believe that the in vivo studies have really answered, or even addressed, the question of whether the GBM serves as a barrier to albumin, either in Necturus, in Mus, or in Homo. One advantage of the Necturus glomerulus for imaging is the very wide GCW. Tanner et al.(8) state that the GBM has an extraordinary width of 3.5 m; this compares to the 0.2-m GBM in the mouse, an 18-fold difference. However, a careful examination of the ultrastructure shown in Fig. 5D in Tanner et al. reveals that the Necturus GCW is quite atypical. Although the podocyte foot processes with slit diaphragms and the thin endothelium are familiar, the intervening extracellular matrix is not reminiscent of the mammalian GBM. Not only is the great majority of the extracellular material electron lucent, but there are also numerous cell processes within it (8). These features are generally consistent with the previously reported GCW ultrastructure in the toad (Bufo), frog (Lemnodynastes) and bullfrog (Rana)(5, 6). However, having studied basement membranes for over 17 years, I disagree that the heterogeneous 3.5-m matrix between the endothelial cell and the podocyte should be viewed in toto as the Necturus GBM. Despite the atypical intercellular space in the Necturus GCW, there is an obvious, well-structured, continuous lamina densa adjacent to the podocyte foot processes. There is another lamina densa, which by comparison appears discontinuous, at the basal aspect of the partly detached endothelial cell (8). Whereas the thin lamina densa immediately beneath the podocytes is certainly characteristic of a basement membrane, the comparatively huge remaining acellular portion of the GCW is not. In fact, this has previously been referred to as the pericapillary space (5) and the subendothelial space (6). For comparison, Fig. 1 shows an electron micrograph of Reichert’s membrane (3), a very thick basement membrane in the yolk sac of embryonic rats and mice. The extensive lamina densa throughout the intercellular space is evident. I can easily accept from the fluorescence in Fig. 4 (8) that neither the vast electron lucent portion of the GCW’s intercellular space nor the discontinuous subendothelial basement membrane make efficient barriers to macromolecules. However, the lamina densa adjacent to the foot processes is much too thin to have been discriminated by in vivo two-photon microscopy, just as it is too thin in mammals. The possibility therefore exists that this lamina densa is serving as a crucial barrier to macromolecules, consistent with suggestions by us and others (1, 2, 4, 7). Unfortunately, the decades-old question remains unanswered.