The proteasome has justifiably become a target of Parkinson's researchers. There is much evidence implicating this particular arm of the cellular protein degradation machinery in the disease (see ARF related news story). Not the least of this evidence is the fact that parkin and UCH-L1 are both involved in the ubiquitin-proteasome system. However, since some studies have failed to show that α-synuclein levels are affected by proteasomal inhibition, Ana Maria Cuervo of Albert Einstein College of Medicine in Bronx, New York, and colleagues at several other institutions, suggest we turn our attention to the lysosomal autophagy arm of the protein recycling system. They note that the proteasome is typically only responsible for short-lived proteins, whereas the lysosome generally mops up cytosolic proteins with a half-life longer than 10 hours.

Working with cultured rat neurons, Cuervo and colleagues found that endogenous α-synuclein has a half-life of about 17 hours. They also found evidence that wild-type α-synuclein, both native and human, is degraded by lysosomes. The authors then asked whether α-synuclein is presented for proteolysis via macroautophagy—an indiscriminate process wherein entire cytosol regions and everything in them is vacuumed up and delivered to lysosomes—or via chaperone-mediated autophagy (CMA). A series of experiments in isolated lysosomes supported the notion that α-synuclein is chaperoned to lysosomes. For example, the researchers severely reduced lysosomal uptake of the protein by mutating a putative chaperone recognition motif on α-synuclein.

When the authors turned their attention to PD-causing α-synuclein mutations, they found that both A30P and A53T mutants had trouble getting into the lysosomes, despite the fact that they were readily recognized by CMA receptors and bound tightly to the organelle surface. Moreover, these mutants blocked lysosomal uptake and degradation of other proteins, "which may further contribute to cellular stress, perhaps causing the cell to rely on alternate degradation pathways or to aggregate damaged neurons," write the authors.

But the proteasome-PD connection may not go quietly into the night, judging by an article published online August 18 in the Journal of Biological Chemistry by Hardy J. Rideout, of Columbia University in New York City, and colleagues. In earlier work, these researchers were among the groups that demonstrated that they could induce a PD model—featuring Lewy body-like intracellular inclusions and neuronal apoptosis—by inhibiting the proteasome. The fibrillar inclusions in this model contained α-synuclein, as in Parkinson disease.

In their current study, the researchers raise the possibility that α-synuclein is not actually required for the formation of intracellular aggregates, at least in this model. Inclusions still formed—and cells died—in α-synuclein -null cells exposed to a proteasome inhibitor. However, the inclusions were not fibrillar, and the results of further experiments by Rideout and colleagues suggest that fibrillization may be α-synuclein's contribution. "The lack of effect on survival in α-synuclein knockout cultures further suggests that the fibrillar nature of the inclusions does not contribute to neuronal degeneration in this model," the authors conclude.—Hakon Heimer.

References:
Cuervo AM, Stefanis L, Fredenburg R, Lansbury PT, Sulzer D. Impaired degradation of mutant alpha-synuclein by chaperone-mediated autophagy. Science. 2004 Aug 27;305:1292-5. Abstract

Rideout HJ, Dietrich P, Wang Q, Dauer WT, Stefanis L. alpha -synuclein is required for the fibrillar nature of ubiquitinated inclusions induced by proteasomal inhibition in primary neurons. J Biol Chem. 2004 Aug 18 [Epub ahead of print] Abstract