極性蛋白VANG-1促進Frizzled胞吞以拮抗Wnt訊息傳遞

Abstract

神經對於環境中的導引因子產生反應進而觸發下游訊息傳遞，使神經細胞或軸突生長錐可以移動或延伸到正確的位置。這些導引因子所觸發的訊息傳遞，在時序和空間上須受到嚴密精確的調控，以確保訊息傳遞能夠在正確的時間和位置上活化並適時停止。前人的研究發現，物種間共同性極高的分泌性醣蛋白Wnt透過和其受體Frizzled的結合，可以調控神經細胞的移動和軸突生長錐的延伸。我們在本篇研究中發現，調控平面細胞極性的關鍵穿膜蛋白VANG-1(Vangl2/Strabismus)會拮抗並且減弱Wnt訊息傳遞，而此作用可發生在線蟲神經母細胞移動或觸覺神經細胞極化並發展出不對稱的突起時。在缺乏Wnt-Frizzled訊息時，原本往身體後端移行的QL神經母細胞的子代細胞會異常地往身體前端移行，而往前移行的QR神經母細胞的子代細胞和HSN運動神經元則提早終止其移動，造成其分布位置的異常。 vang-1的突變可以改正因缺少Wnt-Frizzled訊息時所造成的神經移行或極化的失常，而過度表現VANG-1則會引起和Wnt或Frizzled突變時相近似的缺陷。根據我們所做的遺傳分析結果，我們假設VANG-1專一地拮抗Frizzled受體。以下所觀察到的兩個現象支持我們的假設 : 第一，VANG-1會和MIG-1或MOM-5/Frizzled形成蛋白質複合體。第二，增加VANG-1的表現可促使Frizzled進入細胞質;反之，在vang-1突變的狀態下，Frizzled滯留在神經細胞膜上的比例有顯著增加。統整所有實驗結果後，我們認為VANG-1會促進Frizzled受體的胞吞作用，來阻止Wnt訊息過度或過久的活化，藉此達到精準控制線蟲中神經母細胞的移動，使其能抵達正確位置。

Signaling of neuronal guidance cues through their receptors is tightly regulated both temporally and spatially, allowing the migrating neurons or growth cones to respond promptly to changing guidance cues in the environments. The conserved Wnt family of secreted glycoproteins control neuronal migration and axon guidance through Frizzled receptors. We report here that VANG-1/Vangl2/Strabismus, a membrane protein important for planar tissue polarity, antagonized Frizzled functions in C. elegans neuronal migration. In the absence of Wnt-Frizzled signaling, descendants of the QL neuroblast mismigrated towards the anterior, and those of the QR neuroblast, as well as the HSN motor neurons, stopped prematurely in their anterior migration. Mutations in vang-1 significantly rescued these phenotypes, and cell-specific overexpression of VANG-1 phenocopied Wnt and Frizzled mutants for neuronal migration defects. Our genetic analysis suggested that VANG-1 specifically targeted the Frizzled receptors, but not the Wnt ligands or components downstream of the Frizzleds. This conclusion was further substantiated by the following observations: First, VANG-1 formed protein complexes with the Frizzleds MIG-1 and MOM-5. Second, VANG-1 facilitated Frizzled localization to the cytosol, and Frizzleds tended to accumulate on the neuronal membrane in the vang-1 mutant. We propose that VANG-1 promotes Frizzled endocytosis and prevents prolonged or excessive Wnt signaling to achieve proper regulation of neuroblast migration in C. elegans.