麻省理工大学：《蛋白质标志—分解机制与人类疾病》（英文版）Huntington s disease(HD)

von-Hippel Lindau (Vhl) hereditary cancer syndrome Images removed due to copyright considerations. See Figures 1 through 5, Table 1 and Box 1 in Kaelin Jr. WG. 200. 2002. Molecular basis of the vhl hereditary cancer syndrome. Nature Rev. Cancer 2: 673-682

von-Hippel Lindau (VHL) hereditary cancer syndrome Images removed due to copyright considerations. See Figures 1 through 5, Table 1, and Box 1 in Kaelin Jr., WG. 200. 2002. Molecular basis of the VHL hereditary cancer syndrome. Nature Rev. Cancer 2; 673-682

Huntington s disease(HD) insolubility. Administration of cystamine, a transglutaminase inhibitor, reduces the To date, 10 neurological diseases, including aggregate formation, retarding the Huntington's and several ataxias, are caused development of neurological phenotype and by the lengthening of glutamine(@) tracts in prolonging the life span of brain cells in various proteins with no obvious functional or transgenic mouse models evolutionary relationships. This phenomenon results from a mutation involving a CA e corresponding ge though the Q expansions arise in proteins, the diseases share three striking Image removed due to copyright considerations features See Figure 1 in Tarlac, V and Storey, E. 2003. Role of (1) The existence of a stretch of proteolysis in polyglutar 35-45 glutamine residues in the Res.74:406-416. mutant protein. (2) The Q-expanded proteins are expressed in many tissues pathology is largely restricted to neurons (3) The Q-expanded protei Several hypotheses have been advanced to fragments thereof form explain why expanded glutamine regions cause inclusions that also neuronal degeneration The polyQ fragments may form cationic channels in membranes Although they differ in their clinical Intranuclear aggregation may presentation and neuropathological profile, the interfere with the function of patients display different combinations of transcription factors that also motor, psychiatric, cognitive, an contain glutamine tracts, thereby symptoms causing misregulation In Huntingtons, the disease is caused by a mutation in the gene encoding for Huntingtin (a protein of unknown function, although has neurodegenerative diseases may been recently implicated in the control of gene result from impaired proteolysis transcription). Huntingtin has been found to either because the protein/fragment ubiquitinated and also interacts with the becomes inherently difficultto E2-25 grade or bed In principle, the poly Q disease result from functional inactivation eof cac overwhelming and inhibiting the ubiquitin-proteasome pathw protein. However, expression of polyQ sequences attached to other proteins(such as As we have seen for other neurodegenerative GFP)can cause cell death. Thus, it seems tha diseases, evidences point out to the possibility neurodegeneration is, in fact, a toxic manifestation of the expanded Q tract. Poly Q represent a means for the cell to effectively forms polar zippers (amyloid-like fibrils sequester toxic misfolded proteins, thereby consisting of B strands of the mutant protein shielding organelles from damage. In such that result in protein aggregation, in the form case. the inclusions would have a of intranuclear or cytoplasmic inclusions neuroprotective function. However, continued Transglutaminase-mediated cross-linking of production/accumulation of the aberrant glutamine in poly Q tracts to lysines in the proteins would, as in othe er neuro same or other proteins can account for both disease activate stress-related pathways the formation of inclusions and their which would finally end up in neuronal

Huntington’s disease (HD) To date, 10 neurological diseases, including Huntington’s and several ataxias, are caused by the lengthening of glutamine (Q) tracts in various proteins with no obvious functional or evolutionary relationships. This phenomenon results from a mutation involving a CAG repeat expansion in the corresponding genes. Even though the Q expansions arise in unrelated proteins, the diseases share three striking features: (1) The existence of a stretch of 35-45 glutamine residues in the mutant protein. (2) The Q-expanded proteins are expressed in many tissues, yet pathology is largely restricted to neurons. (3) The Q-expanded proteins or fragments thereof form nuclear inclusions that also contain ubiquitin, proteasomes and chaperones. Although they differ in their clinical presentation and neuropathological profile, the patients display different combinations of motor, psychiatric, cognitive, and sensory symptoms. In Huntington’s, the disease is caused by a mutation in the gene encoding for Huntingtin (a protein of unknown function, although has been recently implicated in the control of gene transcription). Huntingtin has been found to be ubiquitinated and also interacts with the ubiquitin-conjugating enzyme E2-25. In principle, the polyQ diseases could result from functional inactivation of each protein. However, expression of polyQ sequences attached to other proteins (such as GFP) can cause cell death. Thus, it seems that neurodegeneration is, in fact, a toxic manifestation of the expanded Q tract. PolyQ forms polar zippers (amyloid-like fibrils consisting of β strands of the mutant protein) that result in protein aggregation, in the form of intranuclear or cytoplasmic inclusions. Transglutaminase-mediated cross-linking of glutamine in polyQ tracts to lysines in the same or other proteins can account for both the formation of inclusions and their insolubility. Administration of cystamine, a transglutaminase inhibitor, reduces the aggregate formation, retarding the development of neurological phenotype and prolonging the life span of brain cells in transgenic mouse models. Image removed due to copyright considerations. See Figure 1 in Tarlac, V. and Storey, E. 2003. Role of proteolysis in polyglutamine disorders. J. Neurosci. Res. 74: 406-416. Several hypotheses have been advanced to explain why expanded glutamine regions cause neuronal degeneration: • The polyQ fragments may form cationic channels in membranes. • Intranuclear aggregation may interfere with the function of transcription factors that also contain glutamine tracts, thereby causing misregulation of gene expression. • PolyQ and several other neurodegenerative diseases may result from impaired proteolysis, either because the protein/fragment becomes inherently difficult to degrade or because it can end up overwhelming and inhibiting the ubiquitin-proteasome pathway. As we have seen for other neurodegenerative diseases, evidences point out to the possibility that intracellular protein inclusions could represent a means for the cell to effectively sequester toxic misfolded proteins, thereby shielding organelles from damage. In such case, the inclusions would have a neuroprotective function. However, continued production/accumulation of the aberrant proteins would, as in other neurodegenerative disease activate stress-related pathways which would finally end up in neuronal

autophagy and cell death (it seems that in a last attempt of the cells to get rid of the accumulated huntingtin, cleavage of the protein as a result of caspases activation also akes place) The presence of ubiquitin in the aggregates of most poly Q diseases would seem to indicate Image removed due to copyright considerations. that ubiquitination is not impaired, although See Figure 5 in Rechsteiner M, Realini C rates of proteolysis could be affected by Ustrell V. The proteasome activator 11 S REG hanges in the poly Ub chain length, so the (PA28)and class I antigen presentation. Biochem mere presence of Ub doesn't necessarily mean J.2000Jan1;345Pt1:1-15 that an adequate degradation signal has been generated. However certain evidences point out to the hypothesis that poly@ tracts might reduce the rate of polypeptide chain transfer nto the central proteolytic chamber by either nhibiting the ATPases in the 19s subunits of the proteasome or by being difficult to unfold An interesting hypothesis involves alternative proteasome central subunit can associate with Could the functional status of the UPs differ produce the classical 26s to genetIc proteasomes, but instead it can also bind the epigenetic influences unrelated to the donut-shaped 11s REG or pA28 heptamers polyglutamine disease, and can this difference Hybrids binding 11s REGs to one end of the play a role in governing the age of disease 205 proteasome and 19s subunit to the other onset? end of the 20s can also be formed REGo The CAG repeat length in the mutant subunits are thought to play a role in antigen accounts for -70% of the variance of age of presentation by class I MHC molecules and onset for hD (the number of CAG they activate proteasomal hydrolysis following inversely correlated with the age hydrophobic, acidic or basic residues. REGy is onset, suggesting that the rate at which the by contrast, found in the nucleus and is mutant proteins misfold is related to the particularly enriched in nervious tissue. As a length of the poly Q tract). But individual activates hydrolysis after differences in UPS activity could also basic residues but suppress the influence the time it takes for mutant proteins responsible for the cleavage of Gin bond to accumulate in the patient's brain. Thus, the hypothesis is that hybrid 26s pr ate of age-related decline in UPS activity have little difficulty in pumping soluble poly Q could define the efficacy with which the brain tracts into the central proteolytic chamber but if there is impaired cleavage within Gln inf luencing the age of onset of symptoms in an tracts due to bound REGy poly Q peptides ndividual patient would accumulate within the proteasomes, nac tIva

autophagy and cell death (it seems that in a last attempt of the cells to get rid of the accumulated huntingtin, cleavage of the protein as a result of caspases activation also takes place). The presence of ubiquitin in the aggregates of most polyQ diseases would seem to indicate that ubiquitination is not impaired, although rates of proteolysis could be affected by changes in the polyUb chain length, so the mere presence of Ub doesn’t necessarily mean that an adequate degradation signal has been generated. However, certain evidences point out to the hypothesis that polyQ tracts might reduce the rate of polypeptide chain transfer into the central proteolytic chamber by either inhibiting the ATPases in the 19S subunits of the proteasome or by being difficult to unfold. An interesting hypothesis involves alternative forms of the proteasome: The 20S proteasome central subunit can associate with the 19S to produce the classical 26S proteasomes, but instead it can also bind the donut-shaped 11S REG or PA28 heptamers. Hybrids binding 11S REGs to one end of the 20S proteasome and 19S subunit to the other end of the 20S can also be formed. REGα/β subunits are thought to play a role in antigen presentation by class I MHC molecules and they activate proteasomal hydrolysis following hydrophobic, acidic or basic residues. REGγ is, by contrast, found in the nucleus and is particularly enriched in nervious tissue. As a homo-heptamer activates hydrolysis after basic residues but suppress the sites responsible for the cleavage of Gln bonds. The hypothesis is that hybrid 26S proteasomes have little difficulty in pumping soluble polyQ tracts into the central proteolytic chamber, but if there is impaired cleavage within Gln tracts due to bound REGγ, polyQ peptides would accumulate within the proteasomes, inactivating them. Image removed due to copyright considerations. See Figure 5 in Rechsteiner M, Realini C, Ustrell V. The proteasome activator 11 S REG (PA28) and class I antigen presentation. Biochem J. 2000 Jan 1; 345 Pt 1: 1-15. Could the functional status of the UPS differ between individuals due to genetic or epigenetic influences unrelated to the polyglutamine disease, and can this difference play a role in governing the age of disease onset? The CAG repeat length in the mutant accounts for ~70% of the variance of age of onset for HD (the number of CAG repeats is inversely correlated with the age of disease onset, suggesting that the rate at which the mutant proteins misfold is related to the length of the polyQ tract). But individual differences in UPS activity could also influence the time it takes for mutant proteins to accumulate in the patient’s brain. Thus, the rate of age-related decline in UPS activity could define the efficacy with which the brain handles the mutant proteins, thereby influencing the age of onset of symptoms in an individual patient