The AAA-ATPase VPS4 Regulates Extracellular Secretion and Lysosomal Targeting of α-Synuclein

Abstract

Many neurodegenerative diseases share a common pathological feature: the deposition of amyloid-like fibrils composed of misfolded proteins. Emerging evidence suggests that these proteins may spread from cell-to-cell and encourage the propagation of neurodegeneration in a prion-like manner. Here, we demonstrated that α-synuclein (αSYN), a principal culprit for Lewy pathology in Parkinson's disease (PD), was present in endosomal compartments and detectably secreted into the extracellular milieu. Unlike prion protein, extracellular αSYN was mainly recovered in the supernatant fraction rather than in exosome-containing pellets from the neuronal culture medium and cerebrospinal fluid. Surprisingly, impaired biogenesis of multivesicular body (MVB), an organelle from which exosomes are derived, by dominant-negative mutant vacuolar protein sorting 4 (VPS4) not only interfered with lysosomal targeting of αSYN but facilitated αSYN secretion. The hypersecretion of αSYN in VPS4-defective cells was efficiently restored by the functional disruption of recycling endosome regulator Rab11a. Furthermore, both brainstem and cortical Lewy bodies in PD were found to be immunoreactive for VPS4. Thus, VPS4, a master regulator of MVB sorting, may serve as a determinant of lysosomal targeting or extracellular secretion of αSYN and thereby contribute to the intercellular propagation of Lewy pathology in PD.

The Extracellular α-Synuclein and its Biological Significance

“…As an ATPase, VPS4 releases the ESCRT-III machinery from the endosomal membrane, promotes the membrane invagination and the formation of MVB vesicles. The dominant-negative mutant of VPS4 interfered with the lysosomal targeting of α-synuclein, and facilitated α-synuclein secretion [15].…”

“…Part of early endosome is casted out of the cells through the Ras-related protein Rab11a-dependent recycling endosome pathway, especially in the physiological conditions [15]. For the remainders, the peripheral membrane around the endosome is invaginated into the endosome lumen, which forms luminal vesicles.…”

“…In the physiological condition, MVB fuses with the autophagosome for lysosomal degradation [15]. However, when α-synuclein is excessively produced or lysosomes fail to clear α-synuclein, the superfluous α-synuclein is secreted out of the cells.…”

“…It has been found that α-synuclein is also aggregated in the recycling endosome [15]. Although the recycling endosome pathway is mainly intended to clear up the extracellular materials via the endocytosis, accumulating evidences indicate that it may be also involved in the secretion of α-synuclein.…”

“…Firstly, the engulfed endosomes can be re-secreted out of the cells. Secondly, by the unknown mechanism, the α-synuclein in early endosomes is transported into the recycling endosome, and then is casted into the extracellular milieu [15]. This pathway is found to be regulated by rab11a and heat shock protein 90 (HSP90) [18].…”