A positive feedback loop between Flower and PI(4,5)P2 at periactive zones controls bulk endocytosis in Drosophila

Membrane structure and internalization dynamics of human Flower isoforms hFWE3 and hFWE4 indicate a conserved endocytic role for hFWE4

Human Flower (hFWE) isoforms hFWE1-4 are putative transmembrane (TM) proteins that reportedly mediate fitness comparisons during cell competition through extracellular display of their C-terminal tails. Isoform topology, subcellular localization, and duration of plasma membrane presentation are essential to this function. However, disagreement persists regarding the structure of orthologous fly and mouse FWEs, and experimental evidence for hFWE isoform subcellular localization or membrane structure is lacking. Here, we used AlphaFold2 and subsequent molecular dynamics-based structural predictions to construct epitope-tagged hFWE3 and hFWE4, the most abundant human isoforms, for experimental determination of their structure and internalization dynamics. We demonstrate that hFWE3 resides in the membrane of the endoplasmic reticulum (ER), while hFWE4 partially colocalizes with Rab4-, Rab5-, and Rab11-positive vesicles as well as with the plasma membrane. An array of imaging techniques revealed that hFWE4 positions both N- and C-terminal tails and a loop between second and third TM segments within the cytosol, while small (4-12aa) loops between the first and second and the third and fourth TM segments are either exposed to the extracellular space or within the lumen of cytoplasmic vesicles. Similarly, we found hFWE3 positions both N- and C-terminal tails in the cytosol, while a short loop between TM domains extends into the ER lumen. Finally, we demonstrate that hFWE4 exists only transiently at the cell surface and is rapidly internalized in an AP-2- and dynamin-1-dependent manner. Collectively, these data are consistent with a conserved role for hFWE4 in endocytic processes.

Minibrain kinase and calcineurin coordinate activity-dependent bulk endocytosis through synaptojanin

Neurons use multiple modes of endocytosis, including clathrin-mediated endocytosis (CME) and activity-dependent bulk endocytosis (ADBE), during mild and intense neuronal activity, respectively, to maintain stable neurotransmission. While molecular players modulating CME are well characterized, factors regulating ADBE and mechanisms coordinating CME and ADBE activations remain poorly understood. Here we report that Minibrain/DYRK1A (Mnb), a kinase mutated in autism and up-regulated in Down's syndrome, plays a novel role in suppressing ADBE. We demonstrate that Mnb, together with calcineurin, delicately coordinates CME and ADBE by controlling the phosphoinositol phosphatase activity of synaptojanin (Synj) during varying synaptic demands. Functional domain analyses reveal that Synj's 5'-phosphoinositol phosphatase activity suppresses ADBE, while SAC1 activity is required for efficient ADBE. Consequently, Parkinson's disease mutation in Synj's SAC1 domain impairs ADBE. These data identify Mnb and Synj as novel regulators of ADBE and further indicate that CME and ADBE are differentially governed by Synj's dual phosphatase domains.