Impaired $$\alpha$$-Synuclein aggregate clearance in neuronal cells drive their spread to microglia through tunneling nanotubes

Tunneling nanotubes (TNTs) play a crucial role in intercellular communication, enabling transfer of molecular cargoes over long distances between connected cells. Previous studies have demonstrated efficient, directional transfer of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Synuclein (<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Syn) aggregates from neurons to microglia, with endosomal trafficking and lysosomal processing identified as the primary events following <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Syn internalization. Using human neuronal and microglial cell lines, we show that microglia exhibit higher lysosomal turnover, particularly through lysophagy, whereas neuronal lysosomes display compromised degradative capacity and impaired autophagic flux upon <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Syn exposure, resulting in compromised aggregate clearance. Such a response to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Syn aggregates is also conserved in human iPSC-derived neurons and microglia. Moreover, perturbing aggregate clearance via autophagy inhibition enhances TNT-mediated transfer of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Syn from neuronal cells to microglia. Microglia co-cultured with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Syn-containing neurons upregulate autophagy flux, enabling efficient degradation of the transferred aggregates. These results highlight dysfunctional autophagy in neurons as a key driver outsourcing <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>α</mml:mi></mml:math>-Syn aggregates to microglia.