The Rhizarian amoeba Filoreta ramosa develops a neuron-like arborized network using conserved cytoskeletal mechanisms
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Abstract
Cells must spatially differentiate to organize their organelles, maintain function, and interact with their environment efficiently. One type of spatial differentiation used throughout eukaryotes is branched morphology, as seen in neuronal arbors and fungal hyphae. However, the mechanisms driving branched morphogenesis remain undefined in most lineages. The Rhizaria are a eukaryotic lineage including numerous amoebae with branched network-forming pseudopodia called ‘reticulopodia’. Our Rhizarian isolate, Filoreta ramosa , develops an intricately branched network covering multiple centimeters in surface area. We investigated the development of its arborized morphology, focusing on cytoskeletal structure and organelle transport. Through live imaging, immunofluorescence, and drug perturbations, we show that conserved cytoskeletal proteins drive branching and development in a manner that mirrors the cytoskeleton of neuronal arbors, despite an estimated 1.2 billion years of evolutionary distance. Additionally, we demonstrate that its dynamic morphology is interdependent on branched microtubule arrays that facilitate rapid organelle transport, while actin-filled pseudopods enable nutrient uptake, new branch formation, and intercellular interactions. These findings suggest an ancient, shared strategy for long-distance spatial organization in arborizing cell types.
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DISCUSSION
This is an exceptionally cool and ambitious piece of work. It’s impressive to see such a deep level of structural and dynamic characterization developed from an organism that was essentially uncharacterized and self-isolated by the authors. The combination of live imaging, high-resolution cytoskeletal staining, and careful morphometric analysis provides a remarkably detailed view of Filoreta’s network architecture across different states. This represents a substantial contribution both technically and conceptually, and it’s exciting to see such a comprehensive foundation being built for a new model system. Amazing work!
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Organelles are rapidly and bidirectionally transported throughout the syncytial network.
Incredibly cool analysis of this amazing phenotype!
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These observations indicate that actin-dense patches are formed as a response to external stimuli (i.e. food, neighboring syncytia) which then generate new branchlets for the network to respond dynamically to its environment.
Do you see evidence of these actin-dense patches staying at the base of newly formed branchlets? I was curious whether you have observations that help distinguish patches functioning as nucleation sites for new protrusions versus representing increased endocytic activity in the nutrient-enriched conditions. Also, do you think these puncta might correspond to Arp2/3-mediated branched actin networks, or is that still unclear?
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When grown in cultures with five times the normal nutrient concentration (0.05% YET), individual amoebae were present and readily fused to the network (Figure 1F).
I was curious whether you ever see a change in feeding behavior when the medium is enriched with YET. Since the network responds so quickly to added nutrients, do think cells would reduce their phagocytosis of bacteria under these conditions, or do you think feeding branchlets would stay just as active?
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Branchlets at nodes became the thicker stabilized branches of the network as the syncytium grew outwards (Figure 1G).
Just curious if you ever see the "loops" close overtime or if "feeding" branchlets might extend into the loop incase a food source is captured within, or if you believe these are fully functioning as support systems for the reticulated network?
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