Typically, when a chain of ubiquitin molecules gets hitched to a protein—usually a misfolded or damaged one—that protein becomes doomed for destruction in the cell. But although some α-synuclein does get ubiquitinated, much of it somehow escapes degradation, perhaps as a result of a faulty mechanism (see ARF related news story on Liu et al., 2002 and ARF related news story on Liu et al., 2009 and Cullen et al., 2009). To investigate this possibility, researchers have long been trying to pin down the enzyme that tacks ubiquitin onto α-synuclein.

Nedd4 is one of many E3 ubiquitin ligases, a class of enzymes that catalyze the last of three steps required for the addition of ubiquitin to proteins. George Tofaris, a postdoc in Goldberg’s lab, now at Oxford University in the U.K., started to have a hunch that Nedd4 might be involved in α-synuclein ubiquitination when he noticed that the protein contains a C-terminal region sequence similar to those found in other Nedd4 substrates. Tofaris and another postdoc in Goldberg’s lab, Hyoung Tae Kim, decided to test a possible α-synuclein-Nedd4 connection.

When they expressed high levels of Nedd4 in neuronal cells, the added Nedd4 activity boosted α-synuclein degradation. Conversely, when Nedd4 levels were down, α-synuclein content shot up. Having established that Nedd4 ubiquitinates α-synuclein and targets it for degradation, the researchers next turned to the mechanism involved.

Ubiquitinated proteins are degraded either by lysosomes or proteasomes, depending on the type of ubiquitin chain attached to them. Nedd4 tags proteins with ubiquitin chains linked to one another through their lysine 63 residues. That configuration normally targets membrane proteins to lysosomes. To test if this is true for synuclein, Goldberg and colleagues exposed nerve cells to different inhibitors of protein degradation. Chloroquine, a chemical that blocks lysosomal function, reduced α-synuclein degradation resulting from the increased Nedd4 activity. On the other hand, bortezomib, which blocks proteasome function, did not. Thus, Nedd4 seems to target α-synuclein for destruction in the lysosome.

Ubiquitinated membrane proteins are typically engulfed by endosomes and carried to the lysosome via a pathway known as ESCRT (which stands for endosomal sorting complex required for transport). According to Goldberg, no one had implicated the ESCRT pathway in the degradation of α-synuclein, but using RNA interference to block various molecules involved in ESCRT, his group showed that Nedd4 acts via this route. Previously, researchers reported that α-synuclein is degraded in lysosomes by chaperone-mediated autophagy—which clears cytosolic proteins independently of ubiquitination (see ARF related news story on Cuervo et al., 2004). Since α-synuclein can exist as both a membrane-associated and cytosolic protein, it is probably degraded in cells via multiple pathways, said Goldberg.

“A role for the endocytic-lysosomal pathway in neurodegeneration is emerging. Goldberg’s study further highlights the importance of this pathway and offers specific mechanistic insights that are valuable for further research in this area,” wrote Dimitri Krainc at Massachusetts General Hospital in Boston in an e-mail to ARF (see complete comment below).

To approach whether Nedd4 activity could play a role in disease, Tofaris and Goldberg first turned to a widely used model system: yeast cells overexpressing human α-synuclein (see ARF related news story on Outeiro and Lindquist, 2003). They showed that expression of Nedd4’s yeast ortholog, Rsp5p, protected cells from α-synuclein-induced toxicity. Moving to study healthy humans, they were unable to detect Nedd4 in pigmented neurons of the locus coeruleus and substantia nigra, brain areas affected by PD. In contrast, they did find Nedd4 strongly expressed in these brain regions in neurons containing Lewy bodies. “We think this increased expression of Nedd4 might reflect a compensatory change that occurs to reduce the toxic protein in these cells,” said Goldberg. “It will be important to confirm this finding in a physiological model of disease.”

More experiments will be needed before Nedd4 is confirmed to be the α-synuclein ubiquitin ligase in vivo and to play a role in slowing the development of PD. As Sulzer pointed out, other enzymes, most notably parkin, had been implicated in α-synuclein degradation but did not stand the test of time. If Nedd4 does turn out to play a role in neurodegeneration, the finding might be relevant not only to PD and other Lewy body-associated pathologies, but also other diseases involving protein aggregation. “This [endosomal-lysosomal degradation] pathway may also target the amyloid precursor protein and tau,” said Goldberg.—Laura Bonetta.

Reference:
Tofaris GK, Kim HT, Houreza R, Jung J-W, Kim KP, Goldberg AL. Ubiquitin Ligase Nedd4 Ubiquitinates alpha-synuclein and Promotes its Degradation by the Endosomal-Lysosomal Pathway. PNAS Early Edition. Abstract