Ubiquitin, lysosomes and neurodegenerative diseases

Reverse engineering Lewy bodies: how far have we come and how far can we go?

Lewy bodies (LBs) are α-synuclein (α-syn)-rich intracellular inclusions that are an important pathological hallmark of Parkinson disease and several other neurodegenerative diseases. Increasing evidence suggests that the aggregation of α-syn has a central role in LB formation and is one of the key processes that drive neurodegeneration and pathology progression in Parkinson disease. However, little is known about the mechanisms underlying the formation of LBs, their biochemical composition and ultrastructural properties, how they evolve and spread with disease progression, and their role in neurodegeneration. In this Review, we discuss current knowledge of α-syn pathology, including the biochemical, structural and morphological features of LBs observed in different brain regions. We also review the most used cellular and animal models of α-syn aggregation and pathology spreading in relation to the extent to which they reproduce key features of authentic LBs. Finally, we provide important insights into molecular and cellular determinants of LB formation and spreading, and highlight the critical need for more detailed and systematic characterization of α-syn pathology, at both the biochemical and structural levels. This would advance our understanding of Parkinson disease and other neurodegenerative diseases and allow the development of more-reliable disease models and novel effective therapeutic strategies.

Biogenesis and function of multivesicular bodies

The two major cellular sites for membrane protein degradation are the proteasome and the lysosome. Ubiquitin attachment is a sorting signal for both degradation routes. For lysosomal degradation, ubiquitination triggers the sorting of cargo proteins into the lumen of late endosomal multivesicular bodies (MVBs)/endosomes. MVB formation occurs when a portion of the limiting membrane of an endosome invaginates and buds into its own lumen. Intralumenal vesicles are degraded when MVBs fuse to lysosomes. The proper delivery of proteins to the MVB interior relies on specific ubiquitination of cargo, recognition and sorting of ubiquitinated cargo to endosomal subdomains, and the formation and scission of cargo-filled intralumenal vesicles. Over the past five years, a number of proteins that may directly participate in these aspects of MVB function and biogenesis have been identified. However, major questions remain as to exactly what these proteins do at the molecular level and how they may accomplish these tasks.

UBIQUITIN-DEPENDENT REGULATION OF THE SYNAPSE

Posttranslational modification of cellular proteins by the covalent attachment of ubiquitin regulates protein stability, activity, and localization. Ubiquitination is rapid and reversible and is a potent mechanism for the spatial and temporal control of protein activity. By sculpting the molecular composition of the synapse, this versatile posttranslational modification shapes the pattern, activity, and plasticity of synaptic connections. Synaptic processes regulated by ubiquitination, as well as ubiquitination enzymes and their targets at the synapse, are being identified by genetic, biochemical, and electrophysiological analyses. This work provides tantalizing hints that neuronal activity collaborates with ubiquitination pathways to regulate the structure and function of synapses.

Ubiquitin-positive inclusions in different types of multiple system atrophy: distribution and specificity

Multiple system atrophy (MSA) is a neurodegenerative disorder that encompasses different clinicopathological syndromes, either occurring alone or with a variable degree of overlap. Oligodendroglial, intracytoplasmic argyrophilic and ubiquitin-reactive inclusions are regarded as a histologic hallmark. We examined the distribution and specificity of these ubiquitin-reactive inclusions (UBRI) in 20 cases of MSA (7 cases of sporadic adult olivoponto-cerebellar atrophy [OPCA], 1 case of hereditary adult OPCA, 4 cases of infantile OPCA, 2 cases of Shy-Drager-Syndrome [SDS], 4 cases of striatonigral degeneration [SND] and 2 cases of non-classified atrophy of multiple systems) and 93 control cases with various disorders. Antibodies were used against ubiquitin, PGP 9.5, TAU-protein, glutathion-S-transferase (GST) and metallothionein (MT). Oligodendroglial UBRI were detected in all but 2 cases of sporadic adult MSA and 3 controls, whereas they were absent in hereditary and infantile OPCA. They could also be recognized with Gallyas stain and anti-TAU. Immunopositivity was also seen with GST (11 cases), PGP 9.5 (4 cases) and MT (1 case). Distribution of oligodendroglial UBRI, although not showing topographical linkage to neuronal degeneration in all cases, does not seem to occur in a haphazard pattern.