Presymptomatic lysosomal failure appears before overt degeneration in C9orf72 ALS/FTD
Hexanucleotide repeat expansions in C9orf72 remain the most common known genetic cause of amyotrophic lateral sclerosis and frontotemporal dementia, and this preprint adds a mechanistic layer to the disorder’s biology. In Drosophila models carrying toxic C9orf72 repeats, the earliest disturbance was not a late-stage collapse of neuronal architecture but a presymptomatic defect in lysosome function. Autophagosome formation stayed largely intact, which matters because it separates cargo generation from cargo disposal: the cell still seems able to package material for degradation, yet the terminal degradative compartment is already compromised.
A single sentence captures the central problem: the waste-disposal system fails before the neuron looks sick.
That temporal sequence offers a plausible explanation for why toxic species continue to accumulate despite apparently preserved autophagy initiation. Lysosomes are the point where sequestration becomes clearance, and once they stall, the cell is left with autophagic intermediates and pathogenic protein species that can be diverted into other trafficking routes. In neurodegenerative disease, this shift is not merely inefficient housekeeping. It can redirect toxic material toward secretion and spread.
Valproic acid partially blunted neurodegeneration when lysosomal biogenesis remained available
Pharmacologic treatment with valproic acid partially prevented neurodegenerative and locomotor phenotypes in animals expressing the toxic repeats, but only in the presence of Mitf, the transcriptional regulator required for lysosomal biogenesis. The dependence on Mitf is biologically important. It implies that any benefit from valproic acid likely runs through restoration or support of the lysosomal compartment rather than through a broad, nonspecific rescue of neuronal stress alone.
Valproic acid has long been discussed in neurological contexts for its epigenetic and signaling effects, but here the relevant signal is tied to lysosomal competence. If lysosome biogenesis can be sustained, the cell has a chance to keep degradative flux moving. If it cannot, the same treatment loses its protective edge. The study therefore frames lysosomal capacity as a gatekeeper for therapeutic response rather than a passive downstream marker of injury.
Impaired degradation amplified toxicity, while blocking secretory routes reduced it
Genetic inhibition of residual lysosomal degradation intensified neurotoxicity. The direction of effect is consistent with a model in which partially functioning lysosomes still do useful work, even when they are impaired, and further reducing that residual activity pushes the cell deeper into dysfunction. In parallel, downregulation of autophagy and secretory autophagy genes suppressed toxicity, revealing an unexpected contribution from secretion itself.
That result shifts the interpretation of the autophagy-lysosome pathway. It is not only a clearance route; when lysosomal degradation falters, the same machinery can be repurposed toward export. In a disease driven by toxic repeat products, this matters because secretion can convert a cell-autonomous problem into a network problem. A neuron that cannot digest its cargo may externalize it instead.
DPR release rose when lysosomes were disabled, not when autophagy was simply induced
In vitro experiments sharpened that distinction. Disabling lysosome function increased neuronal release of autophagy-associated proteins and dipeptide repeat proteins, or DPRs, whereas autophagy induction alone did not produce the same effect. The result points to lysosomal failure, not autophagy activation by itself, as the trigger for aberrant secretion. Once that export pathway is engaged, the toxic species do not disappear; they become available to nearby cells.
Nearby neurons and glia were shown in vivo to take up these released proteins and DPRs. The finding has direct mechanistic weight for C9orf72 disease because it provides a route for intercellular propagation. Repeat-expanded RNA and its translated products are already toxic inside the producing cell, but extracellular escape may allow them to seed dysfunction in surrounding neural tissue as well. In practical terms, this creates a mechanism for local spread that complements cell-intrinsic injury.
A second sentence captures the clinical implication in blunt terms: secretion can make one injured neuron a source for many more.
Secretory autophagy now sits beside loss of lysosomal clearance in the disease model
The preprint argues for a sequence in which repeat-driven pathology first compromises lysosomal function, then promotes secretory autophagy, and finally facilitates extracellular release and uptake of neurotoxic DPRs. This is a useful refinement of the usual autophagy story. Rather than treating lysosomes only as failing garbage disposals, the model shows how failure can reroute traffic outward. The biology is especially compelling in a disease where toxic protein species are already linked to both cell-autonomous stress and non-cell-autonomous propagation.
For ALS/FTD biology, the value of the work lies in its timing. Presymptomatic dysfunction means the system is already deranged before overt phenotypes dominate the animal, which strengthens the argument that lysosomal impairment is an early event rather than a terminal byproduct. If the same sequence holds in human neurons, then interventions aimed at preserving lysosomal biogenesis or preventing secretory diversion would need to begin early, before repeated toxic export becomes entrenched.
What this preprint can and cannot support
The data come from Drosophila models and in vitro experiments, so they define mechanism rather than clinical efficacy. They do not establish a treatment strategy for people with C9orf72-linked ALS/FTD, and they do not show that valproic acid will alter disease course in humans. What they do show is a coherent molecular chain: presymptomatic lysosomal dysfunction, preserved but insufficient autophagy formation, secretion of DPR-associated material, and uptake by neighboring neural cells.
That chain matters because it connects intracellular failure to intercellular spread with unusual clarity. The next questions are narrow and concrete: whether the same secretory switch occurs in mammalian neurons, whether lysosomal support can be separated from broader toxicity, and whether DPR release can be interrupted without impairing normal proteostatic traffic. Until those questions are answered, the work stands as a strong preclinical argument that lysosomal competence is not peripheral to C9orf72 disease biology; it is embedded in the mechanism of spread itself.