Mutations in the parkin gene are responsible for a common familial form of Parkinson's disease. As parkin encodes an E3 ubiquitin ligase, defects in proteasome-mediated protein degradation are believed to have a central role in the pathogenesis of Parkinson's disease. Here, we report a novel role for parkin in a proteasome-independent ubiquitination pathway. We have identified a regulated interaction between parkin and Eps15, an adaptor protein that is involved in epidermal growth factor (EGF) receptor (EGFR) endocytosis and trafficking. Treatment of cells with EGF stimulates parkin binding to both Eps15 and the EGFR and promotes parkin-mediated ubiquitination of Eps15. Binding of the parkin ubiquitin-like (Ubl) domain to the Eps15 ubiquitin-interacting motifs (UIMs) is required for parkin-mediated Eps15 ubiquitination. Furthermore, EGFR endocytosis and degradation are accelerated in parkin-deficient cells, and EGFR signalling via the phosphoinositide 3-kinase (PI(3)K)-Akt pathway is reduced in parkin knockout mouse brain. We propose that by ubiquitinating Eps15, parkin interferes with the ability of the Eps15 UIMs to bind ubiquitinated EGFR, thereby delaying EGFR internalization and degradation, and promoting PI(3)K-Akt signalling. Considering the role of Akt in neuronal survival, our results have broad new implications for understanding the pathogenesis of Parkinson's disease.

Mutations in the parkin gene result in an autosomal recessive juvenile-onset form of Parkinson's disease. As an E3 ubiquitin-ligase, parkin promotes the attachment of ubiquitin onto specific substrate proteins. Defects in the ubiquitination of parkin substrates are therefore believed to lead to neurodegeneration in Parkinson's disease. Here, we identify the PSD-95/Discs-large/Zona Occludens-1 (PDZ) protein PICK1 as a novel parkin substrate. We find that parkin binds PICK1 via a PDZ-mediated interaction, which predominantly promotes PICK1 monoubiquitination rather than polyubiquitination. Consistent with monoubiquitination and recent work implicating parkin in proteasome-independent pathways, parkin does not promote PICK1 degradation. However, parkin regulates the effects of PICK1 on one of its other PDZ partners, the acid-sensing ion channel (ASIC). Overexpression of wild-type, but not PDZ binding-or E3 ubiquitinligase-defective parkin abolishes the previously described, protein kinase C-induced, PICK1-dependent potentiation of ASIC2a currents in non-neuronal cells. Conversely, the loss of parkin in hippocampal neurons from parkin knockout mice unmasks prominent potentiation of native ASIC currents, which is normally suppressed by endogenous parkin in wild-type neurons. Given that ASIC channels contribute to excitotoxicity, our work provides a mechanism explaining how defects in parkin-mediated PICK1 monoubiquitination could enhance ASIC activity and thereby promote neurodegeneration in Parkinson's disease. INTRODUCTIONParkinson's disease (PD) is characterized by the selective and progressive loss of midbrain dopamine neurons resulting in motor dysfunction and disability. Mutations in the parkin gene cause autosomal recessive juvenile parkinsonism, which accounts for a large proportion of genetically linked PD cases (Kitada et al., 1998). Parkin encodes a 465-amino acid protein (ϳ52 kDa) that is expressed in multiple tissues and functions in the ubiquitin (Ub) system as an E3 Ub-ligase (Shimura et al., 2000). Ubiquitination of substrate proteins is a tightly regulated process, requiring the combined activity of three enzymes: an E1 Ub-activating enzyme, E2 Ub-conjugating enzymes, and E3 Ub-ligases (Hershko and Ciechanover, 1998). E3 Ub-ligases bind substrate proteins and therefore regulate and confer specificity to the ubiquitination reaction. Typically, ubiquitination leads to the assembly of a K48-linked polyubiquitin chain on the substrate, which targets it for degradation by the 26S proteasome, a large multimeric proteolytic complex (Voges et al., 1999). Accordingly, defects in parkin-mediated ubiquitination have been proposed to result in the failure to target parkin substrates to the proteasome for degradation (Kahle et al., 2000;Feany and Pallanck, 2003;Giasson and Lee, 2003). The ensuing accumulation of parkin substrates is believed, in turn, to induce the cellular toxicity and dopamine neuron loss seen in PD. Although numerous parkin substrates have been identified (Zhang et al., 2000;Chung et al., 2001...

The ephrin receptors EphA4 and EphB2 have been implicated in synaptogenesis and long‐term potentiation in the cerebral cortex and hippocampus, where they are generally viewed as post‐synaptic receptors. To determine the precise distribution of EphA4 and EphB2 in mature brain synapses, we used subcellular fractionation and electron microscopy to examine the adult mouse forebrain/midbrain. EphA4 and EphB2 were both enriched in microsomes and synaptosomes. In synaptosomes, they were present in the membrane and the synaptic vesicle fractions. While EphA4 was tightly associated with PSD‐95‐enriched post‐synaptic density fractions, EphB2 was easily extracted with detergents. In contrast, both receptors were found in the pre‐synaptic active zone fraction. By electron microscopy, EphA4 was mainly detected in axon terminals, whereas EphB2 was more frequently detected in large dendritic shafts, in the hippocampus and cerebral cortex. However, in the ventrobasal thalamus, EphB2 was detected most frequently in axon terminals and thin dendritic shafts. The localization of EphA4 and EphB2 in multiple compartments of neurons and synaptic junctions suggests that they interact with several distinct scaffolding proteins and play diverse roles at synapses.