The CD34-Related Molecule Podocalyxin Is a Potent Inducer of Microvillus Formation

BackgroundPodocalyxin is a CD34-related transmembrane protein involved in hematopoietic cell homing, kidney morphogenesis, breast cancer progression, and epithelial cell polarization. Although this sialomucin has been shown to block cell adhesion, the mechanisms involved remain enigmatic. It has, however, been postulated that the adaptor proteins NHERF-1 and 2 could regulate apical targeting of Podocalyxin by linking it to the actin cytoskeleton.
Principal FindingsHere, in contrast, we find that full-length Podocalyxin acts to recruit NHERF-1 to the apical domain. Moreover, we show that ectopic expression of Podocalyxin in epithelial cells leads to microvillus formation along an expanded apical domain that extends laterally to the junctional complexes. Removal of the C-terminal PDZ-binding domain of Podocalyxin abolishes NHERF-1 recruitment but, surprisingly, has no effect on the formation of microvilli. Instead, we find that the extracellular domain and transmembrane region of Podocalyxin are sufficient to direct recruitment of filamentous actin and ezrin to the plasma membrane and induce microvillus formation.
Conclusions/SignificanceOur data suggest that this single molecule can modulate NHERF localization and, independently, act as a key orchestrator of apical cell morphology, thereby lending mechanistic insights into its multiple roles as a polarity regulator, tumor progression marker, and anti-adhesin.

背景
Podocalyxin是一种与CD34相关的跨膜蛋白，参与造血细胞归巢、肾脏形态发生、乳腺癌进展以及上皮细胞极化过程。尽管该唾液酸黏蛋白（sialomucin）已被证实可阻断细胞黏附，但其介导该抗黏附功能的具体分子机制仍未明确。此前有研究假说提出，衔接蛋白NHERF-1与NHERF-2可通过将Podocalyxin与肌动蛋白细胞骨架相连，从而调控Podocalyxin的顶端靶向转运过程。
主要研究结果
与之相反，本研究发现全长Podocalyxin可将NHERF-1招募至细胞顶端结构域。此外，我们证实，在上皮细胞中异位表达Podocalyxin，会使细胞在侧向延伸至细胞连接复合体的扩展型顶端结构域处形成微绒毛。去除Podocalyxin的C端PDZ结合结构域后，虽可完全阻断NHERF-1的招募过程，但令人意外的是，这并不会对微绒毛的形成造成显著影响。进一步研究发现，Podocalyxin的胞外结构域与跨膜区域足以招募丝状肌动蛋白（filamentous actin）和埃兹蛋白（ezrin）至质膜，并诱导微绒毛形成。
结论与意义
本研究数据表明，该单分子可调控NHERF家族蛋白的细胞定位，并可独立作为细胞顶端形态的关键调控因子，从而为其作为极性调控因子、肿瘤进展标志物以及抗黏附分子的多重生理病理功能提供了机制层面的新见解。