ESCRT Disruption Reveals Synaptic Exosomes Primarily Serve Proteostatic Functions, Not Intercellular Signaling

Belangrijkste concepten

Disruption of ESCRT machinery leads to accumulation of exosome cargoes within presynaptic terminals, without depleting their signaling activities, suggesting synaptic exosomes primarily serve proteostatic functions rather than intercellular signaling.

Samenvatting

The study investigates the role of ESCRT (Endosomal Sorting Complex Required for Transport) machinery in the release of synaptic exosomes and the functional consequences of exosome disruption at the Drosophila larval neuromuscular junction (NMJ).

Key findings:

1. Knockdown of ESCRT components Tsg101, Hrs, Shrub, and Vps4 disrupts the release of exosome cargoes (Syt4, Evi, APP, Nrg) from presynaptic terminals, causing their accumulation within the presynaptic compartment.
2. The autophagic pathway is induced as a compensatory mechanism in response to ESCRT dysfunction, leading to the accumulation of arrested autophagic structures within presynaptic terminals. However, autophagy is separable from the exosome release defects.
3. Despite the dramatic depletion of exosomal cargoes from the postsynaptic compartment, many signaling functions of Evi and Syt4 are maintained, suggesting these cargoes can function cell-autonomously within the presynaptic neuron.
4. Neuronally-derived Syt4 is not detected in the muscle cytoplasm, further supporting the idea that exosome transfer is not required for its signaling functions.
5. Tsg101 and Shrub knockdown partially phenocopy the developmental defects seen in evi mutants, suggesting some aspects of postsynaptic development may require trans-synaptic transfer of Evi/Wnt in exosomes.

Overall, the results indicate that synaptic exosome release primarily serves proteostatic functions for at least some cargoes, rather than being essential for their intercellular signaling activities.

Statistieken

Tsg101 knockdown leads to a significant increase in the number of autophagic vacuoles within presynaptic boutons (58.9% of mutant boutons compared to 2.5% of control boutons).
Tsg101 knockdown causes a dramatic decrease in the postsynaptic levels of exosome cargoes Syt4, Evi, APP, and Nrg, while their presynaptic levels accumulate.
Hrs mutants exhibit a decrease in both pre- and post-synaptic levels of the exosome cargo Nrg.
Atg1 and Atg2 mutants show a modest but significant decrease in pre- and post-synaptic Nrg levels, without the dramatic depletion of postsynaptic exosomes seen in ESCRT mutants.

Citaten

"Surprisingly, exosome cargoes Synaptotagmin-4 (Syt4) and Evenness Interrupted (Evi) retain many of their signaling activities upon ESCRT depletion, despite being trapped in presynaptic terminals."
"Thus, these cargoes may not require intercellular transfer, and instead are likely to function cell autonomously in the motor neuron."

Belangrijkste Inzichten Gedestilleerd Uit

ESCRT disruption provides evidence against signaling functions for synaptic exosomes

by Dresselhaus,... om www.biorxiv.org 04-24-2023

https://www.biorxiv.org/content/10.1101/2023.04.22.537920v1

Diepere vragen

If synaptic exosomes primarily serve proteostatic functions, what are the mechanisms by which neurons achieve local control of signaling pathways in a rapid and spatially directed manner

In the context of synaptic exosomes primarily serving proteostatic functions, neurons achieve local control of signaling pathways through mechanisms that allow for rapid and spatially directed regulation. One key mechanism is the regulation of endosomal sorting via retromer, which controls the amount of cargo loaded into exosomes. By modulating the endosomal sorting process, neurons can selectively package specific signaling molecules into exosomes for release. Additionally, the rate of endocytic flux into the recycling pathway, such as the Rab11-dependent pathway, can be controlled to further fine-tune the cargo content of exosomes. This precise control over cargo selection and release allows neurons to locally regulate signaling pathways at synapses in a dynamic and efficient manner.

What are the potential signaling functions of neuronal exosomes in other cellular contexts or neuronal cell types, beyond the Drosophila larval NMJ

While the study focused on the Drosophila larval NMJ, the potential signaling functions of neuronal exosomes in other cellular contexts or neuronal cell types are diverse and multifaceted. In various cellular contexts, neuronal exosomes have been implicated in intercellular communication, including the transfer of signaling molecules, genetic material, and proteins. For example, in the context of neurodegenerative diseases, neuronal exosomes have been shown to propagate pathological factors and contribute to disease progression. In neuronal cell types beyond the Drosophila larval NMJ, exosomes may play roles in synaptic plasticity, neuronal development, and neuroprotection. Additionally, exosomes released from neurons can interact with other cell types in the brain, such as glial cells, to modulate immune responses, synaptic function, and neuronal survival. Overall, the potential signaling functions of neuronal exosomes are vast and may vary depending on the specific cellular context and physiological conditions.

How do the findings from this study on the dispensability of exosome transfer for Evi and Syt4 signaling compare to the established literature supporting a requirement for exosome-mediated transfer of these cargoes

The findings from this study on the dispensability of exosome transfer for Evi and Syt4 signaling provide new insights into the functional roles of these cargoes. While previous literature has supported the requirement for exosome-mediated transfer of Evi and Syt4 for their signaling functions, the current study challenges this notion by demonstrating that the signaling activities of these cargoes are maintained even in the absence of exosome transfer. This suggests that Evi and Syt4 may function cell autonomously within the motor neuron, rather than relying on trans-synaptic transfer via exosomes for their signaling activities. The discrepancy between the established literature and the current findings highlights the complexity of exosome-mediated signaling and the need for further research to elucidate the precise mechanisms by which these cargoes regulate synaptic function and plasticity.