Essentially all neurodegenerative diseases are associated with misaccumulation of cellular waste products. Of these, misfolded or hyperphosphorylated proteins are among the most difficult for the brain to dispose. For example, tau and β-amyloid can accumulate as stable aggregates that are neurotoxic in conditions such as Alzheimer's disease . Intracellular proteasomal degradation and autophagy are considered the principal means for removing proteins in the central nervous system, and the dysfunction of each has been causally associated with neurodegeneration .Yet many cytosolic proteins are released into the interstitial space in the brain, suggesting that extracellular disposal routes may also eliminate waste .