Ji Won Um

Parkin Ubiquitinates and Promotes the Degradation of RanBP2

Parkinson disease (PD) is a common neurodegenerative disorder, which involves the deterioration of dopaminergic neurons in the pars compacta of the substantia nigra. The etiology of PD is still unknown, but recent identification of mutations in familial cases of PD has advanced the understanding of the molecular mechanisms of this neurological disease. Mutations in the parkin gene, which encodes for ubiquitin-protein ligase (E3), have been implicated in autosomal recessive juvenile Parkinsonism, an early onset and common familial form of PD. Here we reported that Parkin selectively binds to RanBP2, which is localized in the cytoplasmic filament of the nuclear pore complex and belongs to the small ubiquitin-related modifier E3 ligase family. We also demonstrated that RanBP2 becomes a target for Parkin E3 ubiquitin-ligase and is processed via Parkin-mediated ubiquitination and subsequent proteasomal degradation. Furthermore, Parkin controls the intracellular levels of sumoylated HDAC4, as a result of the ubiquitination and degradation of RanBP2. Our findings suggested that the intracellular levels of RanBP2 and its functional activity may be modulated by Parkin-mediated ubiquitination and proteasomal pathways.

LAR-RPTPs: synaptic adhesion molecules that shape synapse development

The synapse is the most elementary operating unit in neurons, creating neural circuits that underlie all brain functions. Synaptic adhesion molecules initiate neuronal synapse connections, promote their stabilization and refinement, and control long-term synaptic plasticity. Leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) have previously been implicated as essential elements in central nervous system (CNS) development. Recent studies have demonstrated that LAR-RPTP family members are also involved in diverse synaptic functions, playing a role in synaptic adhesion pathways together with a host of distinct transmembrane proteins and serving as major synaptic adhesion molecules in governing pre- and postsynaptic development, dysfunctions of which may underlie various disorders. This review highlights the emerging role of LAR-RPTPs as synapse organizers in orchestrating synapse development.

Functional modulation of parkin through physical interaction with SUMO‐1

Parkinson disease (PD) is the second most common neurodegenerative disorder and is characterized by the extensive and progressive loss of dopaminergic neurons in the CNS substantia nigra pars compacta region. Mutations in the parkin gene, which encodes for E3 ubiquitin ligase, have been implicated in autosomal recessive juvenile parkinsonism, an early‐onset and common familial form of PD. Although several parkin substrates have already been identified, the molecular mechanism underlying the regulation of enzymatic activity of parkin has yet to be clarified. In a previous study, we demonstrated that RanBP2 becomes a new target for parkin E3 ubiquitin ligase and is processed via parkin‐mediated ubiquitination and subsequent proteasomal degradation. RanBP2, which is localized in the cytoplasmic filament of the nuclear pore complex, belongs to the small ubiquitin‐related modifier (SUMO) E3 ligase family. Here we show that parkin appears to bind selectively to the SUMO‐1 in vivo and in vitro. Moreover, the physical association of SUMO‐1 with parkin results in an increase in the nuclear transport of parkin as well as its self‐ubiquitination. Our findings suggest that the E3 ubiquitin ligase activity of parkin and its intracellular localization may be modulated through the SUMO‐1 association.

Molecular interaction between parkin and PINK1 in mammalian neuronal cells.

Parkinsons disease (PD) is characterized by the deterioration of dopaminergic neurons in the pars compacta of substantia nigra and the formation of intraneuronal protein inclusions. The etiology of PD is not known, but the recent identification of several mutation genes in familial PD has provided a rich understanding of the molecular mechanisms of PD pathology. Mutations in PTEN-induced putative kinase 1 (PINK1) and parkin are linked to early-onset autosomal recessive forms of familial PD. Here we show molecular and functional interactions between parkin and PINK1. Parkin selectively binds to PINK1 and upregulates PINK1 levels. In addition, PINK1 reduces the solubility of parkin, which induces the formation of microtubule-dependent cytoplasmic aggresomes. Our findings reveal that parkin and PINK1 affect each others stability, solubility and tendency to form aggresomes, and have important implications regarding the formation of Lewy bodies.

Parkin Directly Modulates 26S Proteasome Activity

Parkinsons disease (PD) is a common neurodegenerative disease that involves the deterioration of dopaminergic neurons in the substantia nigra pars compacta. Although the etiology of PD remains poorly understood, recent genetic, postmortem, and experimental evidence shows that abnormal protein accumulation and subsequent aggregate formation are prominent features of both sporadic and familial PD. While proteasome dysfunction is observed in PD, diverse mutations in the parkin gene are linked to early-onset autosomal-recessive forms of familial PD. We demonstrate that parkin, an E3 ubiquitin ligase, activates the 26S proteasome in an E3 ligase activity-independent manner. Furthermore, an N-terminal ubiquitin-like domain within parkin is critical for the activation of the 26S proteasome through enhancing the interaction between 19S proteasomal subunits, whereas the PD-linked R42P mutant abolishes this action. The current findings point to a novel role for parkin for 26S proteasome assembly and suggest that parkin mutations contribute to the proteasomal dysfunction in PD.

Structural basis for LAR-RPTP/Slitrk complex-mediated synaptic adhesion

Synaptic adhesion molecules orchestrate synaptogenesis. The presynaptic leukocyte common antigen-related receptor protein tyrosine phosphatases (LAR-RPTPs) regulate synapse development by interacting with postsynaptic Slit- and Trk-like family proteins (Slitrks), which harbour two extracellular leucine-rich repeats (LRR1 and LRR2). Here we identify the minimal regions of the LAR-RPTPs and Slitrks, LAR-RPTPs Ig1-3 and Slitrks LRR1, for their interaction and synaptogenic function. Subsequent crystallographic and structure-guided functional analyses reveal that the splicing inserts in LAR-RPTPs are key molecular determinants for Slitrk binding and synapse formation. Moreover, structural comparison of the two Slitrk1 LRRs reveal that unique properties on the concave surface of Slitrk1 LRR1 render its specific binding to LAR-RPTPs. Finally, we demonstrate that lateral interactions between adjacent trans-synaptic LAR-RPTPs/Slitrks complexes observed in crystal lattices are critical for Slitrk1-induced lateral assembly and synaptogenic activity. Thus, we propose a model in which Slitrks mediate synaptogenic functions through direct binding to LAR-RPTPs and the subsequent lateral assembly of LAR-RPTPs/Slitrks complexes.

PTPσ functions as a presynaptic receptor for the glypican-4/LRRTM4 complex and is essential for excitatory synaptic transmission

Significance This paper documents and systematically characterizes the molecular interactions of protein tyrosine phosphatase σ (PTPσ) with glypicans (GPCs). The identified interactions require heparan sulfate (HS), suggesting that GPCs are a major source of HS for PTPσ at excitatory synapses. Strikingly, we found that leucine-rich repeat transmembrane protein 4 (LRRTM4) induces presynaptic differentiation via the PTPσ/GPC interaction, suggesting that PTPσ may function as a coreceptor for GPCs in presynaptic neurons. More importantly, we found that HS-binding ability of PTPσ is critical for excitatory synaptic transmission. These results expand our previous understanding of how synaptic adhesion pathways regulate excitatory synapse development and shed light on GPCs/LRRTM4 trans-synaptic signaling. Moreover, to our knowledge, this is the first study to document the physiological significance of HS in the presynaptic function of mammalian neurons. Leukocyte common antigen-related receptor protein tyrosine phosphatases—comprising LAR, PTPδ, and PTPσ—are synaptic adhesion molecules that organize synapse development. Here, we identify glypican 4 (GPC-4) as a ligand for PTPσ. GPC-4 showed strong (nanomolar) affinity and heparan sulfate (HS)-dependent interaction with the Ig domains of PTPσ. PTPσ bound only to proteolytically cleaved GPC-4 and formed additional complex with leucine-rich repeat transmembrane protein 4 (LRRTM4) in rat brains. Moreover, single knockdown (KD) of PTPσ, but not LAR, in cultured neurons significantly reduced the synaptogenic activity of LRRTM4, a postsynaptic ligand of GPC-4, in heterologous synapse-formation assays. Finally, PTPσ KD dramatically decreased both the frequency and amplitude of excitatory synaptic transmission. This effect was reversed by wild-type PTPσ, but not by a HS-binding–defective PTPσ mutant. Our results collectively suggest that presynaptic PTPσ, together with GPC-4, acts in a HS-dependent manner to maintain excitatory synapse development and function.

NF-κB-inducing Kinase Phosphorylates and Blocks the Degradation of Down Syndrome Candidate Region 1

Down syndrome, the most frequent genetic disorder, is characterized by an extra copy of all or part of chromosome 21. Down syndrome candidate region 1 (DSCR1) gene, which is located on chromosome 21, is highly expressed in the brain of Down syndrome patients. Although its cellular function remains unknown, DSCR1 expression is linked to inflammation, angiogenesis, and cardiac development. To explore the functional role of DSCR1 and the regulation of its expression, we searched for novel DSCR1-interacting proteins using a yeast two-hybrid assay. Using a human fetal brain library, we found that DSCR1 interacts with NF-κB-inducing kinase (NIK). Furthermore, we demonstrate that NIK specifically interacts with and phosphorylates the C-terminal region of DSCR1 in immortalized hippocampal cells as well as in primary cortical neurons. This NIK-mediated phosphorylation of DSCR1 increases its protein stability and blocks its proteasomal degradation, the effects of which lead to an increase in soluble and insoluble DSCR1 levels. We show that an increase in insoluble DSCR1 levels results in the formation of cytosolic aggregates. Interestingly, we found that whereas the formation of these inclusions does not significantly alter the viability of neuronal cells, the overexpression of DSCR1 without the formation of aggregates is cytotoxic.

ASK1 Negatively Regulates the 26 S Proteasome

The 26 S proteasome, composed of the 20 S core and 19 S regulatory particle, plays a central role in ubiquitin-dependent proteolysis. Disruption of this process contributes to the pathogenesis of the various diseases; however, the mechanisms underlying the regulation of 26 S proteasome activity remain elusive. Here, cell culture experiments and in vitro assays demonstrated that apoptosis signal-regulating kinase 1 (ASK1), a member of the MAPK kinase kinase family, negatively regulated 26 S proteasome activity. Immunoprecipitation/Western blot analyses revealed that ASK1 did not interact with 20 S catalytic core but did interact with ATPases making up the 19 S particle, which is responsible for recognizing polyubiquitinated proteins, unfolding them, and translocating them into the 20 S catalytic core in an ATP-dependent process. Importantly, ASK1 phosphorylated Rpt5, an AAA ATPase of the 19 S proteasome, and inhibited its ATPase activity, an effect that may underlie the ability of ASK1 to inhibit 26 S proteasome activity. The current findings point to a novel role for ASK1 in the regulation of 26 S proteasome and offer new strategies for treating human diseases caused by proteasome malfunction.

Elfn1 recruits presynaptic mGluR7 in trans and its loss results in seizures

GABAergic interneurons are highly heterogeneous, and much is unknown about the specification and functional roles of their neural circuits. Here we show that a transinteraction of Elfn1 and mGluR7 controls targeted interneuron synapse development and that loss of Elfn1 results in hyperactivity and sensory-triggered epileptic seizures in mice. Elfn1 protein increases during postnatal development and localizes to postsynaptic sites of somatostatin-containing interneurons (SOM-INs) in the hippocampal CA1 stratum oriens and dentate gyrus (DG) hilus. Elfn1 knockout (KO) mice have deficits in mGluR7 recruitment to synaptic sites on SOM-INs, and presynaptic plasticity is impaired at these synapses. In patients with epilepsy and attention deficit hyperactivity disorder (ADHD), we find damaging missense mutations of ELFN1 that are clustered in the carboxy-terminal region required for mGluR7 recruitment. These results reveal a novel mechanism for interneuron subtype-specific neural circuit establishment and define a common basis bridging neurological disorders.