Mini-Review |
2 Mclean Hospital, Harvard Medical School, Belmont, MA 02478
Address correspondence to Dan Lindholm, Dept. of Neuroscience, Unit of Neurobiology, Biomedical Centre, Box 587, Uppsala University, S-75123 Uppsala, Sweden. Tel.: 46-18-471-4435. Fax: 46-18-559-017. email: dan.lindholm{at}neuro.uu.se
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Abstract |
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Key Words: neurodegeneration; synapse function; Wallerian degeneration; polyubiquitination; monoubiquitination
Neurodegenerative diseases are characterized by the selective loss of neurons due to aggregation of different intra- or extracellular proteins (Bence et al., 2001). Apart from the deleterious effects of the protein deposits on the nerve cell soma, the axons and dendrites also degenerate in these diseases. The functional changes observed early in neurodegenerative disorders are reflected by alterations in synaptic dysfunctions and loss of connectivity. Recent evidence indicates that the degeneration of axons and synapses plays an important role both in chronic diseases and after injuries. The molecular mechanisms governing axonal degeneration and synaptic maintenance are not fully understood, but a crucial role has been ascribed to the activity of the UPS.
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UPS and neurological disorders |
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UPS and the function of synapses |
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However, the time window and specific proteins regulated by the UPS vary between different studies. In the work on the PSD proteins, a time scale of 2448 h was used. In this time, the UPS acts on different targets, including ubiquitination of proteins controlling transcription or translation that may indirectly influence protein abundance in the synapse. In their study of the neuromuscular system in Drosophila, Speese et al. (2003) noted a relatively short time window for the degradation of synaptic proteins by the UPS. Using drugs to inhibit the activity, in addition to genetic manipulation of the proteasome, DUNC-13 was identified as a selective target for UPS in the presynaptic terminal. Electrophysiological recordings showed that proteasome inhibition also enhanced synaptic efficacy and presynaptic transmitter release. This data demonstrates that the activity of the UPS locally regulates the levels of DUNC-13 and influences presynaptic efficacy in Drosophila. The correlative changes observed in synaptic function and regulation of the DUNC-13 by the UPS also suggest that the levels of DUNC-13 may be the crucial mediator for increased neurotransmitter release. However, this functional link has so far not been directly shown neither are other protein excluded as targets for the UPS in this context.
Members of the UNC protein family are found in different species, and are involved in synaptic vesicle priming and regulation of neurotransmitter release. It would be important to study whether the mouse homologue MUNC-1, shown to be important for proper function of glutamatergic synaptic vesicles (Augustin et al., 1999), also undergoes UPS-mediated down-regulation. Mice carrying a gene deletion for Munc-18-1, exhibit loss of neurotransmitter secretion during development, without affecting the initial assembly of the synapse (Verhage et al., 2000). However, after birth there is a widespread neurodegeneration with the loss of synapses. The contribution of the UPS to this phenotype has yet not been studied.
Protein modification by ubiquitin at nerve endings may go beyond effects on protein degradation. Polyubiquitination targets proteins for degradation, but monoubiquitination regulates protein trafficking, involving endosomes, as well as other functions (Hicke, 2001). Receptor tyrosine kinases, such as the EGF receptor are monoubiquitinated upon ligand binding (Haglund et al., 2003; Mosesson et al., 2003). This leads to receptor tyrosine kinase internalization and degradation in lysosomes, preventing recycling to the plasma membrane. Monoubiquitination of receptor proteins can thus exert an important control step in the action of trophic factors. Although not yet studied in brain, this may also occur for neurotransmitter receptors, and other molecules at the synapse. It is crucial to study whether alterations in monoubiquitination per se can affect protein trafficking and synaptic connectivity. Wilson et al. (2002) reported recently that mutation in Usp14, encoding an ubiquitin-specific protease, causes synaptic dysfunction leading to ataxia in mice.
In recent years, novel proteins have been discovered, such as the ubiquitin-like proteins and those carrying ubiquitin interacting domains that influence the efficacy of the UPS (Weissman, 2001; Adams, 2003). So far little is known about these proteins in the nervous system or locally in the function of the synapse.
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Axonal degeneration and the UPS |
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Supporting evidence for the involvement of the UPS in axonal degeneration comes from studies of the Gracile axonal dystrophy (Gad) mutant mice. In these mice, there is an inactivation of the ubiquitin carboxy-terminal hydrolase, UCH-L1, causing, among others, degeneration of the gracile tract of the spinal cord (Saigoh et al., 1999). UCH-1 displays dual activities as a deubiquitination enzyme and as an ubiquitin ligase (Liu et al., 2002). The exact roles of UCH-1 and the corresponding protein substrates in the Gad mice are so far not known.
Recent studies in rats using transection of cultured neurons or crush lesions of the optic nerve in vivo showed that inhibition of the UPS by drugs targeting the proteasome can retard the onset of Wallerian degeneration (Zhai et al., 2003). Similar results were obtained by viral expression of an ubiquitin protease that can reverse ubiquitination of protein substrates. Using specific antibodies for axonal components, the fragmentation of microtubuli was identified as an early sign of the axonal degeneration that was also sensitive to proteasome inhibition (Zhai et al., 2003). These results lend credence to the view that the local activity of UPS, acting on selective protein targets, is important during axonal degeneration.
Further evidence comes from studies in Drosophila, showing that the degeneration of axons occurring during pruning requires the UPS (Watts et al., 2003). Axon pruning is an important process for the refinement of the neuronal connections in both vertebrates and invertebrates. Studying development of particular projecting neurons of the Drosophila mushroom bodies it was shown that overexpression of an ubiquitin protease or a mutant form of the Drosophila E1 ubiquitin-activating enzyme, inhibited pruning (Watts et al., 2003). Likewise, mutations in two of the subunits in the 19S regulatory particle (Fig. 1 A) also impaired pruning. This genetic evidence strongly suggests that local degeneration of protein via the UPS activity is a necessary requirement for axon pruning during development of the mushroom bodies. The relevance of these findings for Wallerian degeneration and axonal reactions in neurodegenerative diseases is not straightforward. However, there are similarities between these processes both with regard to molecular mechanisms and the cellular reactions involved. First, studying different fly mutants, it was shown that axon pruning is independent of the activity of grim, hid, and rpr that regulate cell death in Drosophila. This is in keeping with results obtained in rodents in the Wld s mice (Finn et al., 2000; Raff et al., 2002), and shows that the programs governing axonal degeneration and death of the nerve cell body via apoptosis are inherently different. Second, local UPS activity seems instrumental for both pruning and Wallerian degeneration to occur. Third, looking at the cellular changes, the disruption of the microtubuli was identified as an early step in both pruning and Wallerian degeneration. Thus, it is tempting to suggest that the two processes may be governed by similar mechanisms involving local regulation of axonal proteins via the UPS.
The results on inhibition of the UPS in axons, also suggest therapeutic targets to preserve axons after injury and in degeneration. The caveat is that a general inhibition of proteasome activity may be harmful to the cell. To arrive at a specific inhibition of the UPS, the exact nature of the different molecules involved in ubiquitination and degradation of protein targets in the axons needs to be studied in more detail. The first attempt toward this was made by Watts et al. (2003) in Drosophila. However, of the over a dozen of E2 and E3 enzymes studied, none was found important for axon pruning. This does not come as a surprise in view of the fact that in Drosophila, as in mammalian cells the diversity of these enzymes and their corresponding protein substrates is large. However, the use of modern large-scale techniques for proteome analysis may provide new insights into these important questions.
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Concluding remarks |
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Acknowledgments |
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Submitted: 18 November 2003
Accepted: 26 February 2004
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References |
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Adams, J. 2003. The proteasome: structure, function, and role in the cell. Cancer Treat. Rev. 29(Suppl. 1):39.[Medline]
Augustin, I., C. Rosenmund, T.C. Sudhof, and N. Brose. 1999. Munc13-1 is essential for fusion competence of glutamatergic synaptic vesicles. Nature. 400:457461.[CrossRef][Medline]
Bence, N.F., R.M. Sampat, and R.R. Kopito. 2001. Impairment of the ubiquitin-proteasome system by protein aggregation. Science. 292:15521555.
Coleman, M.P., and V.H. Perry. 2002. Axon pathology in neurological disease: a neglected therapeutic target. Trends Neurosci. 25:532537.[CrossRef][Medline]
Coleman, M.P., L. Conforti, E.A. Buckmaster, A. Tarlton, R.M. Ewing, M.C. Brown, M.F. Lyon, and V.H. Perry. 1998. An 85-kb tandem triplication in the slow Wallerian degeneration (Wlds) mouse. Proc. Natl. Acad. Sci. USA. 95:99859990.
Ciechanover, A., and P. Brundin. 2003. The ubiquitin proteasome system in neurodegenerative diseases. Sometimes the chicken, sometimes the egg. Neuron. 40:427446.[Medline]
Dawson, T.M., and V. Dawson. 2003. Molecular pathways of neurodegeneration in Parkinson's disease. Science. 302:819822.
Ehlers, M.D. 2003. Activity level controls postsynaptic composition and signaling via the ubiquitin-proteasome system. Nat. Neurosci. 6:231242.[CrossRef][Medline]
Eriksen, J.L., T.M. Dawson, D.W. Dickson, and L. Petrucelli. 2003. Caught in the act: -synuclein is the culprit in Parkinson's disease. Neuron. 40:453456.[Medline]
Finn, J.T., M. Weil, F. Archer, R. Siman, A. Srinivasan, and M.C. Raff. 2000. Evidence that Wallerian degeneration and localized axon degeneration induced by local neurotrophin deprivation do not involve caspases. J. Neurosci. 20:13331341.
Gunawardena, S., L.S. Her, R.G. Brusch, R.A. Laymon, I.R. Niesman, B. Gordesky-Gold, L. Sintasath, N.M. Bonini, and L.S. Goldstein. 2003. Disruption of axonal transport by loss of huntingtin or expression of pathogenic polyQ proteins in Drosophila. Neuron. 40:2540.
Haglund, K., S. Sigismund, S. Polo, I. Szymkiewicz, P.P. Di Fiore, and I. Dikic. 2003. Multiple monoubiquitination of RTKs is sufficient for their endocytosis and degradation. Nat. Cell Biol. 5:461466.[CrossRef][Medline]
Hegde, A.N., and A. DiAntonio. 2002. Ubiquitin and the synapse. Nat. Rev. Neurosci. 3:854861.[CrossRef][Medline]
Hicke, L. 2001. Protein regulation by monoubiquitin. Nat. Rev. Mol. Cell Biol. 2:195201.[CrossRef][Medline]
Kaplan, B., V. Ratner, and E. Haas. 2003. Alpha-synuclein: its biological function and role in neurodegenerative diseases. J. Mol. Neurosci. 20:8392.[Medline]
Lindsten, K., F.M. de Vrij, L.G. Verhoef, D.F. Fischer, F. W. van Leeuwen, E.M. Hol, M.G. Masucci, and N.P. Dantuma. 2002. Mutant ubiquitin found in neurodegenerative disorders is a ubiquitin fusion degradation substrate that blocks proteasomal degradation. J. Cell Biol. 157:417427.
Liu, Y., L. Fallon, H.A. Lashuel, Z. Liu, and P.T. Lansbury, Jr. 2002. The UCH-L1 gene encodes two opposing enzymatic activities that affect -synuclein degradation and Parkinson's disease susceptibility. Cell. 111:209218.[Medline]
Mack, T.G., M. Reiner, B. Beirowski, W. Mi, M. Emanuelli, D. Wagner, D. Thomson, T. Gillingwater, F. Court, L. Conforti, et al. 2001. Wallerian degeneration of injured axons and synapses is delayed by a Ube4b/Nmnat chimeric gene. Nat. Neurosci. 4:11991206.
Mosesson, Y., K. Shtiegman, M. Katz, Y. Zwang, G. Vereb, J. Szollosi, and Y. Yarden. 2003. Endocytosis of receptor tyrosine kinases is driven by monoubiquitylation, not polyubiquitylation. J. Biol. Chem. 278:2132321326.
Miura, K., T. Kishino, E. Li, H. Webber, P. Dikkes, G.L. Holmes, and J. Wagstaff. 2002. Neurobehavioral and electroencephalographic abnormalities in Ube3a maternal-deficient mice. Neurobiol. Dis. 9:149159.[CrossRef][Medline]
Raff, M.C., A.V. Whitmore, and J.T. Finn. 2002. Axonal self-destruction and neurodegeneration. Science. 296:868871.
Saigoh, K., Y.L. Wang, J.G. Suh, T. Yamanishi, Y. Sakai, H. Kiyosawa, T. Harada, N. Ichihara, S. Wakana, T. Kikuchi, and K. Wada. 1999. Intragenic deletion in the gene encoding ubiquitin carboxy-terminal hydrolase in gad mice. Nat. Genet. 23:4751.[CrossRef][Medline]
Singleton, A.B., M. Farrer, J. Johnson, A. Singleton, S. Hague, J. Kachergus, M. Hulihan, T. Peuralinna, A. Dutra, R. Nussbaum, et al. 2003. -Synuclein locus triplication causes Parkinson's disease. Science. 302:841.
Skinner, P.J., C.A. Vierra-Green, H.B. Clark, H.Y. Zoghbi, and H.T. Orr. 2001. Altered trafficking of membrane proteins in purkinje cells of SCA1 transgenic mice. Am. J. Pathol. 159:905913.
Song, C., Y. Zhang, C.G. Parsons, and Y.F. Liu. 2003. Expression of polyglutamine-expanded huntingtin induces tyrosine phosphorylation of N-methyl-D-aspartate receptors. J. Biol. Chem. 278:3336433369.
Speese, S.D., N. Trotta, C.K. Rodesch, B. Aravamudan, and K. Broadie. 2003. The ubiquitin proteasome system acutely regulates presynaptic protein turnover and synaptic efficacy. Curr. Biol. 13:899910.[Medline]
Szebenyi, G., G.A. Morfini, A. Babcock, M. Gould, K. Selkoe, D.L. Stenoien, M. Young, P.W. Faber, M.E. MacDonald, M.J. McPhaul, and S.T. Brady. 2003. Neuropathogenic forms of huntingtin and androgen receptor inhibit fast axonal transport. Neuron. 40:4152.
Verhage, M., A.S. Maia, J.J. Plomp, A.B. Brussaard, J.H. Heeroma, H. Vermeer, R.F. Toonen, R.E. Hammer, T. K. van den Berg, M. Missler, et al. 2000. Synaptic assembly of the brain in the absence of neurotransmitter secretion. Science. 287:864869.
Watts, R.J., E.D. Hoopfer, and L. Luo. 2003. Axon pruning during Drosophila metamorphosis. Evidence for local degeneration and requirement of the ubiquitin-proteasome system. Neuron. 38:871885.[Medline]
Weissman, A.M. 2001. Themes and variations on ubiquitylation. Nat. Rev. Mol. Cell Biol. 2:169178.[CrossRef][Medline]
Wilson, S.M., B. Bhattacharyya, R.A. Rachel, V. Coppola, L. Tessarollo, D.B. Householder, C.F. Fletcher, R.J. Miller, N.G. Copeland, and N.A. Jenkins. 2002. Synaptic defects in ataxia mice result from a mutation in Usp14, encoding a ubiquitin-specific protease. Nat. Genet. 32:420425.[CrossRef][Medline]
Zhai, Q., J. Wang, A. Kim, Q. Liu, R. Watts, E. Hoopfer, T. Mitchison, L. Luo, and Z. He. 2003. Involvement of the ubiquitin-proteasome system in the early stages of Wallerian degeneration. Neuron. 39:217225.[Medline]