Organelle dysfunction and TNT-mediated aggregate spreading in neurodegeneration

Organelle dysfunction is a central hallmark of neurodegenerative diseases (NDs), which are characterized by the pathological accumulation of misfolded proteins capable of inducing aggregation in healthy cells. This process generates a self-perpetuating cycle of protein misfolding and spreading across interconnected neuronal networks. In this review, we provide an integrated overview of organelle alterations associated with major NDs, emphasizing the pivotal roles of lysosomes, mitochondria, and the endoplasmic reticulum (ER) at the crossroads of proteostasis, metabolism, and stress signaling. We examine how defects in these organelles create conditions that favor aggregate formation and cellular vulnerability, with a focus on α-synuclein and Tau, the main aggregating proteins in Parkinson's and Alzheimer's diseases, respectively. We then explore mechanisms of intercellular protein transfer, highlighting the emerging role of tunneling nanotubes (TNTs). We discuss how organelle status influences TNT formation and cargo selection, and how TNTs may act as conduits for the propagation of pathogenic aggregates. Finally, we summarize the downstream consequences of TNT-mediated transfer in recipient cells, including alterations in the autophagy-lysosomal pathway, TFEB-dependent transcription, mitochondrial stress responses, calcium homeostasis, and inflammatory or senescent signaling. Together, these insights underscore the intertwined roles of organelle dysfunction and TNT-mediated communication in driving the progression of NDs and suggest new therapeutic avenues aimed at restoring organelle function and limiting aggregate spread.