Exosome secretion is a key pathway for clearance of pathological TDP-43

Abstract

Cytoplasmic TDP-43 aggregation is a pathological hallmark of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Here we investigated the role of exosomes in the secretion and propagation of TDP-43 aggregates. TDP-43 was detected in secreted exosomes from Neuro2a cells and primary neurons but not from astrocytes or microglia. Evidence is presented that protein aggregation and autophagy inhibition are factors that promote exosomal secretion of TDP-43. We also report that levels of exosomal TDP-43 full length and C-terminal fragment species are upregulated in human amyotrophic lateral sclerosis brains. Exposure of Neuro2a cells to exosomes from amyotrophic lateral sclerosis brain, but not from control brain, caused cytoplasmic redistribution of TDP-43, suggesting that secreted exosomes might contribute to propagation of TDP-43 proteinopathy. Yet, inhibition of exosome secretion by inactivation of neutral sphingomyelinase 2 with GW4869 or by silencing RAB27A provoked formation of TDP-43 aggregates in Neuro2a cells. Moreover, administration of GW4869 exacerbated the disease phenotypes of transgenic mice expressing human TDP-43 mutant. Thus, even though results suggest that exosomes containing pathological TDP-43 may play a key role in the propagation of TDP-43 proteinopathy, a therapeutic strategy for amyotrophic lateral sclerosis based on inhibition of exosome production would seem inappropriate, as in vivo data suggest that exosome secretion plays an overall beneficial role in neuronal clearance of pathological TDP-43.

The role of ubiquitinated TDP-43 in amyotrophic lateral sclerosis

“…TDP-35 participates in the aggregation of mRNA precursors, which makes the transformation of proteins into polymers easier. The insoluble fraction of ALS acts as a seed of TDP-43 aggregation when it is introduced in SH-SY5Y cells, and subsequently transmitted to other co-cultured cells [19,29] . Such extracellular accumulation, in a potentially more harmful way, is similar to the prion infections [30,31] .…”

“…The pathological TDP-43 protein aggregation and autophagy inhibition promote exosomal secretion of TDP-43 [29] . The levels of exosomal full length TDP-43 and C-terminal fragment species are upregulated in the brain of ALS patients.…”

“…Although the tardbp gene mutation is uncommon, other gene mutations related to ALS can also lead to abnormal intracellular TDP-43 levels [29,34] . Therefore, it is necessary to analyze the clearing pathway of pathological TDP-43.…”

Prions on the run: How extracellular vesicles serve as delivery vehicles for self-templating protein aggregates

“…TDP-43 oligomers or aggregates also transmit from donor to recipient cells in culture, either through tunneling nanotubes or exosomes 86-89 . Cell culture experiments suggest that the ceramide-dependent exosomal pathway is involved in exosomal TDP-43 release 88 . As TDP-43 is also present in exosomal fractions from brains and CSF of healthy controls, its assembly into disease-associated aggregates might not cause the sorting into EVs 87,89 .…”

Proteostasis and Diseases of the Motor Unit

“…Recent studies have shown that they can transfer insoluble aggregates to neighboring cells, resulting in a decreased toxic load on the proteome: whether the spreading of protein aggregates via exosomes could provide the molecular basis for the prion-like mechanism of disease propagation in neurodegeneration is certainly very intriguing (Yuyama et al, 2012; Schneider and Simons, 2013; Iguchi et al, 2016) and has the potential to be exploited for therapeutic and biomarker discovery (Boukouris and Mathivanan, 2015; Yuyama et al, 2015). …”