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Activity-regulated cytoskeleton-associated protein (Arc/Arg3.1) is an immediate early gene that plays a vital role in learning and memory. Arc protein has structural and functional properties similar to viral Group-specific antigen (Gag) protein and mediates the intercellular RNA transfer through virus-like capsids. However, the regulators and secretion pathway through which Arc capsids maneuver cargos are unclear. Here, we identified that phosphatidylinositol-3-phosphate (PI3P) mediates Arc capsid assembly and secretion through the endosomal–multivesicular body (MVB) pathway. Indeed, reconstituted Arc protein preferably binds to PI3P. In HEK293T cells, Arc forms puncta that colocalize with FYVE, an endosomal PI3P marker, as well as Rab5 and CD63, early endosomal and MVB markers, respectively. Superresolution imaging resolves Arc accumulates within the intraluminal vesicles of MVB. CRISPR double knockout of RalA and RalB, crucial GTPases for MVB biogenesis and exocytosis, severely reduces the Arc-mediated RNA transfer efficiency. RalA/B double knockdown in cultured rat cortical neurons increases the percentage of mature dendritic spines. Intake of extracellular vesicles purified from Arc-expressing wild-type, but not RalA/B double knockdown, cells in mouse cortical neurons reduces their surface GlutA1 levels. These results suggest that unlike the HIV Gag, whose membrane targeting requires interaction with plasma-membrane-specific phosphatidyl inositol (4,5) bisphosphate (PI(4,5)P2), the assembly of Arc capsids is mediated by PI3P at endocytic membranes. Understanding Arc’s secretion pathway helps gain insights into its role in intercellular cargo transfer and highlights the commonality and distinction of trafficking mechanisms between structurally resembled capsid proteins.

Necroptosis is a form of inflammatory lytic cell death involving active cytokine production and plasma membrane rupture. Progression of necroptosis is tightly regulated in time and space, and its signaling outcomes can shape the local inflammatory environment of cells and tissues. Pharmacological induction of necroptosis is well established, but the diffusive nature of chemical death inducers makes it challenging to study cell‐cell communication precisely during necroptosis. Receptor‐interacting protein kinase 3, or RIPK3, is a crucial signaling component of necroptosis, acting as a crucial signaling node for both canonical and non‐canonical necroptosis. RIPK3 oligomerization is crucial to the formation of the necrosome, which regulates plasma membrane rupture and cytokine production. Commonly used necroptosis inducers can activate multiple downstream signaling pathways, confounding the signaling outcomes of RIPK3‐mediated necroptosis. Opsin‐free optogenetic techniques may provide an alternative strategy to address this issue. Optogenetics uses light‐sensitive protein‐protein interaction to modulate cell signaling. Compared to chemical‐based approaches, optogenetic strategies allow for spatiotemporal modulation of signal transduction in live cells and animals. We developed an optogenetic system that allows for ligand‐free optical control of RIPK3 oligomerization and necroptosis. This article describes the sample preparation, experimental setup, and optimization required to achieve robust optogenetic induction of RIPK3‐mediated necroptosis in colorectal HT‐29 cells. We expect that this optogenetic system could provide valuable insights into the dynamic nature of lytic cell death. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC.

Basic Protocol 1: Production of lentivirus encoding the optogenetic RIPK3 system

Support Protocol: Quantification of the titer of lentivirus

Basic Protocol 2: Culturing, chemical transfection, and lentivirus transduction of HT‐29 cells

Basic Protocol 3: Optimization of optogenetic stimulation conditions

Basic Protocol 4: Time‐stamped live‐cell imaging of HT‐29 lytic cell death

Basic Protocol 5: Quantification of HT‐29 lytic cell death