In their recent review in TiNS, Thrane et al. [1] discussed the role of a proposed ‘glymphatic’ system of convective solute flow in the pathology of acute brain edema. Controversies relating to the relative importance of convective versus diffusive transport in clearing toxic metabolic byproducts from brain parenchyma have been comprehensively reviewed elsewhere [2, 3] and it is not our intention to repeat the arguments made in these papers but, instead, to clarify aspects of the physiological function of the glial water channel aquaporin-4 (AQP4) and its contribution to brain edema that we believe have been muddied by the glymphatic hypothesis. As originally stated [4], the glymphatic hypothesis pro-poses convective flow in brain parenchyma generated by hydrostatic pressure-driven trans-astrocytic water move-ment facilitated by AQP4 in perivascular endfeet. There are several problems with this hypothesis (Figure 1). First, the hydrostatic pressure is extremely small compared with the osmotic pressure generated by ion transport across cell membranes. Arterial wall pulsations that have been pro-posed to generate hydrostatic pressure in the paravascular space are on the order of 1 µm [5]; modeling studies with larger wall displacements of 5 µm suggest maximal hydro-static pressure transients of approximately 1 kPa in the paravascular space [6], equivalent to the osmotic pressure generated by a 0.4-mOsm difference across the endfoot membrane. Osmotic gradients of 20 mOsm or more are routinely generated across the astrocyte plasma mem-brane in response to metabolic activity and K+ redistribution, which provide the dominant driving force for water transport. A related issue is the ‘salt accumulation problem’ [3, 7], where pressure-driven water flow across a salt-impermeable membrane concentrates solutes on the high-pressure side and dilutes solutes on the low-pressure side, creating an opposing osmotic gradient that largely counter-acts the effectiveness of hydrostatic pressure in driving water transport within confined volumes. Furthermore, because AQP4 transports water only and is concentrated on the perivascular face of endfeet, where it facilitates uptake of water into the astrocyte and cellular swelling, it is hard to envision how convective flow across the endfoot itself could occur. Finally, the cell impermeant tracers used to track convective flow move through the small gaps between endfeet and not through the endfeet themselves. Any pulsation-driven flow into endfeet would reduce the driving force for convection through the gaps and, hence, tracer transport into the interstitial space. Therefore, pulsation-driven water flow through AQP4 in astrocyte endfeet is unlikely at physiologically relevant hydrostatic pressures and, if it did occur, would not be expected to generate convective flow in the interstitial space or in-crease tracer movement from paravascular cerebrospinal fluid (CSF) to interstitial fluid (ISF). Figure 1 The glymphatic hypothesis versus conventional understanding of the role of aquaporin-4 (AQP4) in brain water movement. (A) The glymphatic hypothesis proposes a major role for hydrostatic pressure-driven water flow through astrocyte endfeet; conventionally, ... Application of the glymphatic hypothesis in the context of brain edema led Thrane et al. to propose that convective pumping of CSF into ischemic areas, rather than osmotic mechanisms, is responsible for acute edema following ischemia. As the authors indicate, studies to measure astrocyte volume changes using two-photon optical micros-copy are limited by spatial resolution; however, electron microscopy consistently demonstrates endfoot swelling as an early consequence of ischemia [8, 9], supporting AQP4-mediated osmotic uptake of water into astrocytes during the early stages of edema. As the authors correctly point out, much remains to be understood about the functional significance of AQP4 enrichment in astrocyte endfeet, both in normal physiology and during edema. Experimental data suggesting that AQP4 facilitates fluid exchange between CSF and ISF are intriguing but difficult to interpret because of baseline differences in parenchymal extracellular volume fraction between wild type and AQP4-null mice [10, 11]. Quantitative studies have failed to find evidence for directional convective flow in cortical gray matter under normal conditions [12, 13], so perhaps the results of Iliff et al. [4] are a consequence of differences in extracellular space (ECS) structure between wild type and AQP4-null mice. Future studies of the role of AQP4 in diffusive and convective fluid transport in the brain will benefit from the application of quantitative imaging methods and biophysically realistic spatial modeling.

中文翻译：

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脑水肿水混浊？

在他们最近对 TiNS 的评论中，Thrane 等人。[1] 讨论了提出的对流溶质流的“glymphatic”系统在急性脑水肿病理中的作用。关于对流运输与扩散运输在清除脑实质有毒代谢副产物方面的相对重要性的争议已在其他地方进行了全面审查 [2, 3]，我们无意重复这些论文中的论点，而是澄清各个方面胶质水通道水通道蛋白 4 (AQP4) 的生理功能及其对脑水肿的贡献，我们认为已经被 glymphatic 假说弄糊涂了。正如最初所述[4]，glymphatic 假说提出了由静水压力驱动的跨星形细胞水运动在血管周围尾足中由 AQP4 促进的脑实质中的对流流动。这个假设有几个问题（图 1）。首先，与离子跨细胞膜转运产生的渗透压相比，静水压非常小。已提议在血管旁空间产生静水压力的动脉壁脉动约为 1 µm [5]；具有 5 µm 更大壁位移的建模研究表明，血管旁空间中的最大静水压力瞬变约为 1 kPa [6]，相当于跨足底膜的 0.4 mOsm 差异产生的渗透压。20 mOsm 或更高的渗透梯度通常会在星形胶质细胞质膜上产生，以响应代谢活动和 K+ 重新分布，这为水运输提供了主要驱动力。一个相关的问题是“盐积聚问题”[3, 7]，其中压力驱动的水流穿过盐不渗透膜，在高压侧浓缩溶质并在低压侧稀释溶质，形成相反的渗透梯度在很大程度上抵消了静水压力在限制体积内驱动水运输的有效性。此外，因为 AQP4 只运输水并集中在尾足的血管周围表面，在那里它促进水进入星形胶质细胞和细胞肿胀，很难想象穿过脚掌本身的对流是如何发生的。最后，用于跟踪对流的细胞不透性示踪剂通过尾脚之间的小间隙移动，而不是通过尾脚本身。任何进入尾足的脉动驱动流都会降低通过间隙的对流驱动力，因此，示踪剂传输到间隙空间。因此，在生理学相关的静水压力下，脉动驱动的水流通过星形胶质细胞尾足中的 AQP4 不太可能发生，如果确实发生了，则预计不会在间质空间中产生对流或增加来自血管旁脑脊液 (CSF) 的示踪剂运动) 到间质液 (ISF)。图 1 水通道蛋白 4 (AQP4) 在脑水运动中的作用的 glymphatic 假设与传统理解。(A) glymphatic 假说提出了静水压力驱动的水流通过星形胶质细胞尾足的主要作用；传统上，... 在脑水肿的背景下应用 glymphatic 假设导致 Thrane 等人。提出脑脊液对流泵入缺血区域，而不是渗透机制，是造成缺血后急性水肿的原因。正如作者所指出的，使用双光子光学显微镜测量星形胶质细胞体积变化的研究受到空间分辨率的限制。然而，电子显微镜始终表明，足底肿胀是缺血的早期后果 [8, 9]，支持在水肿的早期阶段 AQP4 介导的水进入星形胶质细胞的渗透吸收。正如作者正确指出的那样，关于星形胶质细胞尾足中 AQP4 富集的功能意义，无论是在正常生理学还是在水肿期间，还有很多需要了解。表明 AQP4 促进 CSF 和 ISF 之间的液体交换的实验数据很有趣，但由于野生型和 AQP4 缺失小鼠之间实质细胞外体积分数的基线差异而难以解释 [10, 11]。定量研究未能找到正常条件下皮质灰质中定向对流流动的证据 [12, 13]，因此也许 Iliff 等人的结果。[4] 是野生型和 AQP4-null 小鼠细胞外空间 (ECS) 结构差异的结果。