Neurochemical Mechanisms in Disease P36 ppt

Trinucleotide-Expansion Diseases 335 proteinaceous deposits are especially prominent in this disease (Rolfs et al., 2003). The disease is interesting because some patients present first with exclusively pure psychiatric symptoms while having no signs of ataxia or movement disorders (Rolfs et al., 2003). The authors point out that SCA17 represents one of the very few psychiatric diseases for which there is a known monogenic cause (Rolfs et al., 2003). SCA17 also appears to be a risk factor for Parkinson-like symptoms (Lee et al., 2009). 4 Possible Factors Contributing to Neurodegeneration in the (CAG) n /Q n -Expansion Diseases As noted above, the neurodegenerative disease phenotypes in the Q n -expansion dis- eases are probably caused mostly by a toxic gain of function. This is suggested by the autosomal dominant mode of inheritance (except BMA) and by most experi- ments with cell and animal models (Ross, 2002). HD is common enough in the Lake Maracaibo area where HD homozygotes are sometimes encountered (The US– Venezuela Collaborative Research Project and Wexler, 2004). In agreement with the gain of function hypothesis, these individuals have an almost identical disease phenotype to heterozygotes. However, as also noted above, some loss of normal function may also occur in some cases (Ross, 2002). It is also possible that the aberrant protein expressed by the mutant allele interacts with the normal protein expressed by the other allele. This dominant-negative interaction might lead to a partial loss of function i n some cases (Ross, 2002). Although the expanded Q n domains impart a toxic gain of function, this does not explain the selective vulnerability in the various Q n -expansion diseases. In all cases, the mutated Q n domain is expressed throughout the brain. Therefore, the selective vulnerability must reside in a toxic gain of function that somehow involves the non- mutated part of the protein (Cummings and Zoghbi, 2001; Zoghbi and Orr, 2009). Many excellent reviews and discussions on possible mechanisms contribut- ingtoQ n -expansion diseases have been published (e.g., Gatchel and Zoghbi, 2005; Di Prospero and Fischbeck, 2005; Pearson et al., 2005). Possible mecha- nisms are discussed below. Because much of the work on Q n -expansion diseases has been directed to HD, much of the following discussion is heavily weighted toward HD. 4.1 Toxic Protein Aggregates A characteristic feature of all the Q n -expansion diseases is the presence of insolu- ble protein aggregates (inclusion bodies) in the affected brain regions. Some authors have suggested that these insoluble aggregates are toxic and thereby contribute to the disease process (e.g., Bates, 2003). However, other studies indicate that the insol- uble aggregates per se may not be toxic (e.g., Saudou et al., 1998; Kuemmerle 336 A.J.L. Cooper and J.P. Blass et al., 1999; Kaytor et al., 2004; Mitra et al., 2009), and that their concentrations do not necessarily correlate with damaged regions of the brain. The insoluble pro- tein aggregates may represent the end-stage of a cascade of previous events that relate more directly to toxicity than the aggregates themselves (Ross and Poirier, 2004). Nevertheless, an understanding of the mechanism of aggregate formation may provide clues as to pathological mechanisms. Two theories have been proposed concerning the origin of aggregates in the Q n -expansion diseases. The first was put forward by Max Perutz and is known as the “polar zipper hypothesis” (e.g., Perutz and Windle, 2001). Proteins/polypeptides containing Q n domains in vitro form hydrogen bonds that link the glutaminyl side chains of the Q n domain to the peptide backbone in an adjacent protein/polypeptide resulting in highly ordered β-pleated sheets that are often extremely insoluble. β-Pleated sheets occur in bacteria overexpressing proteins containing Q n domains (Scherzinger et al., 1999). However, the aggregates in HD brain appear to be less ordered (Karpuj et al., 1999). The second theory involves the action of the enzyme family transglutaminases (TGs). Q n domains are excellent substrates of TGs (Kahlem et al., 1998; Cooper et al., 2002), which catalyze calcium-dependent protein cross-linking between glu- taminyl (Q) and lysyl (K) residues. Often, but not always, such cross-linked proteins are insoluble. In HD mice lacking TG 2 (the major TG in brain; also known as tissue TG) (R6/2 TGase 2 –/– ) exhibit as many, and possibly more, insoluble inclusions in the brain than their R/6 TGase 2 +/+ littermates (Mastroberardino et al., 2002). The R6/2 TGase 2 –/– mice live longer than the R6/2 TGase 2 +/+ mice, suggesting that insoluble aggregates are not the only toxic manifestation of Q n domains. The finding also suggests that TG 2 may not be responsible for most of the aggregate formation in HD mouse brain. It should be noted, however, that the brain also contains TGs 1 and 3 and possibly other TGs that may also contribute to protein cross-linking (Zainelli et al., 2005). A possible explanation for the findings of increased insolu- ble cerebral proteinaceous aggregates in the R6/2 TGase 2 –/– mice compared to the R6/2 TGase 2 +/+ mice relates to the work of Lai et al. (2004). These authors found that a thioredoxin fusion protein containing a Q 62 polypeptide has a tendency in vitro to aggregate into insoluble polymers. However, this aggregation was arrested by TG2, which cross-linked the Q 62 fusion protein into soluble high-M r polymers. TG2-catalyzed conversion of the Q 62 fusion protein into soluble aggregates was decreased in the presence of amine substrates (Lai et al., 2004). The authors sug- gested that TGs may contribute to the pathogenicity of mutant Htt by catalyzing the formation of toxic, soluble Q n -containing fragments. Most probably, the aggregates in the Q n -expansion disease are formed by a combination of (a) noncovalent ordered interactions (polar zippers), (b) noncova- lent interactions of disordered misfolded proteins, and (c) covalent modifications (TG-catalyzed cross-linking). We suggest the following mechanism implicating TGs in the neuropathology of HD and other Q-expansion diseases. The cross-linking activity of brain TGs is nor- mally very low or quiescent in vivo. However, with aging calcium dysregulation begins to occur (Foster and Kumar, 2002) and inherent TG activity is increased Trinucleotide-Expansion Diseases 337 (Park et al., 1999). Both factors will lead to increased protein cross-linking in the aging brain. Misfolded mutant Htt has a tendency with time to generate insolu- ble protein aggregates. However, activation of TG competes with this process, such that a threshold is reached later in life in which toxic soluble, cross-linked mutant Htt fragments begin to accumulate. With time, excessive removal of “normal” Q n -containing protein, including transcription factors through both noncovalent interactions and covalent cross-linking may occur. Possibly, protein synthesis can keep up with “lost” proteins early in life, but with aging the process of replen- ishment may be less efficient. This would lead to loss of function through loss of biologically active Q n -containing proteins. 4.2 Disrupted Proteasome Function The aggregates in each of the Q n -expansion diseases (except in the case of SCA6) are immunopositive for ubiquitin (reviewed by Cummings and Zoghbi, 2000). In addition, the aggregates often contain parts of the proteasome machinery and chap- erones (reviewed by Cooper et al., 2002). The possibility therefore exists that the ubiquitin proteasome pathway is disrupted in HD and other Q n -expansion diseases (Cooper et al., 2002; Mandrusiak et al., 2003; Wang et al., 2008). Clogging of the proteasome might occur if the aggregates contain polar zippers or TG-catalyzed cross-links that cannot be “unzipped.” Clogging of proteasomes could prevent the removal of damaged and misfolded proteins. Such proteins, including those contain- ing the expanded Q n domain, might therefore accumulate and exert toxicity, perhaps by interacting aberrantly with other proteins/polypeptides. In support of this hypoth- esis, interference with the ubiquitin-tagging pathway, or interference of clearance by the proteasome machinery, either singly or in combination would be expected to increase the toxicity of mutant Htt in cell culture (Saudou et al., 1998). Moreover, SCA1 transgenic mice, with a block in the ubiquitin pathway, had markedly fewer intranuclear aggregates, but markedly worse SCA1 pathology (Cummings et al., 1998; Cummings and Zoghbi, 2001). Inasmuch as chaperones help fold proteins into their “correct” configurations, one might infer that chaperones would help in lessening the toxicity associated with misfolded proteins containing Q n expansions (Cummings and Zoghbi, 2000). Indeed, there is some evidence that this is the case both f or cell culture mod- elsofQ n -expansion diseases and for a Drosophila retina model of HD (reviewed by Cummings and Zoghbi, 2002). However, chaperones do not appear to miti- gate disease phenotype in at least one animal model of a Q n -expansion disease (Helmlinger et al., 2004). As in human SCA7, transgenic SCA7 mice develop retinopathy. Helmlinger et al. (2004) developed transgenic mice, which specifically overexpress Hsp70 and HDJ2 along with the Q n -containing protein. Although coex- pression prevented aggregate formation in a cell model it did not prevent either neuronal toxicity or aggregate formation in intact mice. Moreover, protein aggre- gates in SCA7 mice contained cleaved mutant ataxin-7, whereas in the transfected 338 A.J.L. Cooper and J.P. Blass cells the aggregates contained full-length ataxin-7. Thus, the possibility that dis- rupted proteasome dysfunction contributes to the neuropathology of Q n -expansion diseases remains controversial. Moreover, in vitro-generated Q n aggregates failed to inhibit purified proteasomes, whereas filamentous Htt aggregates isolated from mouse brain resulted in inhibition (Ortega et al., 2007). Perhaps formation of inclu- sion bodies is a protective mechanism to remove potentially harmful aggregates from solution (Mitra et al., 2009). However, this mechanism may eventually fail in HD brain. Indeed the activity of the proteasome machinery is s ignificantly lower in postmortem HD brain tissue (Ortega et al., 2007). Macroautophagy (sometimes more simply referred to as autophagy) is an addi- tional mechanism for degrading damaged or misfolded cellular proteins (Renna et al., 2010). It has been suggested that autophagy may be especially useful in degrading mutant Htt-containing fragments (Renna et al., 2010). If this hypothesis is correct then small molecule stimulators of autophagy (e.g., rapamycin, rilmeni- dine) might be useful in treating HD and other Q n -expansion diseases (Renna et al., 2010; Rose et al., 2010). In this regard, rilmenidine may be especially efficacious as it has a long clinical safety use (Rose et al., 2010). 4.3 Interference with Gene Expression Many transcription factors, such as CBP (CREB binding protein) and TBP (TATA binding protein), contain Q n domains (Perutz et al., 1994; Schaffar et al., 2004), which may assist in the assembly of the transcriptosome. Therefore, it is conceivable that the pathological gain of function in Q n -expansion diseases is due at least in part to aberrant interaction among the mutated protein and various transcription factors (e.g., Sugars and Rubinsztein, 2003; Li and Li, 2004). Indeed, proteins containing aberrant Q n domains have been shown to interact with various transcription factors including CREB, CBP, TAF μ 130, SP1, and TP53, some of which have been shown to be present in the protein aggregates in affected brain regions (Gatchel and Zoghbi, 2005). Moreover, one of the Q n -expansion diseases is due to a mutation within a transcription factor itself (TBP in SCA17; Table 2). In the case of HD, N-terminal fragments of mutated Htt (containing the expanded Q n domain) accumulate in the nucleus (Zainelli et al., 2003), apparently as a result of interference with the nuclear export machinery (Cornett et al., 2005). Moreover, nuclear-targeting of mutant Htt fragments produces a Huntington-disease-like phenotype in HD transgenic mice (Schilling et al., 2004). Thus, it is possible that one of the toxic effects of expanded Q n domains is the alteration of transcription factor interactions (e.g., Schaffar et al., 2004). It has been shown that mutant Htt binds to CBP and p53. The latter protein regulates transcription of various mitochondrial proteins (Sawa, 2001). Wild-type, but not mutant Htt, stimulates transcription of brain-derived neu- rotrophic factor (BDNF), and neuronal restrictive silencer element (NRSE) is the target of wild-type Htt activity on the BDNF promoter II (Zuccato et al., 2003). Trinucleotide-Expansion Diseases 339 Moreover, it was shown that mutant Htt in a mouse model of HD facilitates CRE- dependent transcription (Obrietan and Hoyt, 2004). Thus, mutated Htt may cause either increases (Obrietan and Hoyt, 2004) or decreases (Zuccato et al., 2003) in transcriptional regulation. Such alterations may contribute to the pathologi- cal response in HD and other CAG-expansion diseases. A particularly intriguing Q n -containing protein is PQBP-1, which binds to both Q n expansions and to brain- specific transcription factor Brn-2 (Waragai et al., 1999). Thus, aberrant interactions between an expanded Q n domain and PQBP-1 may, in turn, result in aberrant transcription of Brn-2 and neuropathology. Glutamine-rich transcription factor Sp1 is readily cross-linked by TG 2 (Han and Park, 2000). Inasmuch as some TG 2 is present in the nucleus, and Q n domains are excellent substrates, it is possible that TGs may modulate the activity of at least some transcription factors in vivo. Because TG activity is increased in HD brain, and because the expanded Q n domain of Htt is an excellent TG substrate, the possibility exists that TGs play a critical role in altered transcription level and properties in Q n -expansion diseases. Recent work has suggested that REST [RE1 (repressor element 1)-silencing tran- scription factor] function is disrupted in HD brain (Bithell et al., 2009). As discussed by Bithell et al. (2009), REST is a master regulator of many neuronal genes, includ- ing BDNF. In addition, recent work suggests that REST regulates transcription of regulatory miRNAs (microRNAs), many of which are involved in neuronal protein expression. Thus, mutant Htt not only appears to directly dysregulate target genes of REST, but also to indirectly dysregulate neuronal transcription (Bithell et al., 2009). Polyalanine (A n ) expansions also give rise to disease phenotypes (Section 5). In eight of these diseases, the expansion is in a transcription factor and the dis- ease phenotype is evident at birth. If transcription factor dysfunction contributes to Q n -expansion diseases, then a hypothesis explaining the disease phenotype must account for the fact that the disease phenotype is present at birth in the A n -expansion diseases, but is typically adult onset in Q n -expansion diseases. 4.4 Interference with Mitochondrial Function Marked interference with mitochondrial function is a feature of HD brain (Browne and Beal, 2004; Browne et al., 1997; Browne, 2008; Nicholls, 2009; Reddy et al., 2009; Quintanilla and Johnson, 2009; Su et al., 2010). For example, Browne et al. (1997) showed that citrate synthase-corrected complex II–III activity is markedly reduced in both HD caudate (–29%) and putamen (–67%), and complex IV specific activity is reduced in HD putamen (–62%). Tabrizi et al. (1999) reported that aconi- tase specific activity is reduced to 8, 27, and 52% of control activities in HD caudate, putamen, and cerebral cortex, respectively. Tabrizi et al. (2000) also reported that aconitase and complex IV activities are decreased in the striatum of 12-wk HD transgenic (R6/2) mice, and complex IV activity is decreased in cerebral cortex. As noted previously for human HD, oxidative stress indicators (increased inducible NO 340 A.J.L. Cooper and J.P. Blass synthase and nitrotyrosine) were detected in brains of HD-transgenic mice (Tabrizi et al., 2000). Deficits in energy metabolism in human HD brain also occur in human HD muscle. Thus, Lodi et al. (2000) demonstrated by means of in vivo magnetic resonance spectroscopy (MRS) a decreased ATP/(PCr + P i ) ratio in skeletal muscle of HD patients. During recovery from exercise the maximal rate of ATP synthesis was decreased by 44% in symptomatic patients and by 35% in presymptomatic HD carriers compared to controls (Lodi et al., 2000). Gårseth et al. (2000) reported small but significant decreases in lactate and citrate in the CSF of HD patients as assessed by MRS. Dietary supplementation with 2% creatine significantly improved survival and improved symptoms in HD-transgenic mouse models, possibly through redress in part of the energy deficits (Andreassen et al., 2001). Panov et al. (2002) reported t hat lymphoblast mitochondria from patients with HD have a lower membrane potential and depolarize at lower calcium loads than do mitochondria from controls. These authors also showed a defect in brain mito- chondria similar to those isolated from HD-transgenic mice. This defect preceded behavioral and pathological abnormalities. Panov et al. (2003) also showed that GST (glutathione S-transferase) constructs with a pathological-length Q n insert induced a small but significant reduction in membrane potential (State 4) of mitochondria isolated from normal rat liver and normal human lymphoblasts. With succes- sive increments of Ca 2+ aliquots, mitochondria exposed to pathological-length Q n domains depolarized much earlier and to a greater extent than did mitochondria exposed to nonpathological constructs (Panov et al., 2003). The striatum is particu- larly (“selectively”) vulnerable in HD, and mitochondria isolated from striatum of rat brain may be more susceptible to the Ca 2+ -induced permeability transition (PT) than are cortical mitochondria. The susceptibility of striatal mitochondria has been demonstrated by measurements of depolarization, swelling, Ca 2+ uptake, reactive oxygen species, and respiration (Brustovetsky et al., 2003). For a recent review of mitochondrial calcium function and dysfunction in neurodegenerative diseases (including HD) see Nicholls (2009). 4.5 Aberrant Caspase Activity Several studies have shown that cystamine is beneficial in mouse models of HD (e.g., Karpuj et al., 2002; Dedeoglu et al., 2002; Bailey and Johnson, 2006;Van Raamsdonk et al., 2005). In one study, brain aggregates were reduced by cystamine treatment in the HD mice (Dedeoglu et al., 2002). Cystamine is an in vitro inhibitor of Ca 2+ -dependent TGs and is an inhibitor of caspases in cells in culture (Lesort et al., 2003). Some studies have suggested a role for caspases in HD and that inhi- bition of these enzymes may be beneficial in HD mouse models (e.g., Ona et al., 1999; Chen et al., 2000; Lesort et al., 2003). Caspase activity has also been impli- cated in SCA-3 (Shoesmith Berke et al., 2004). Because caspases contain a crucial cysteine residue at the active site they are expected to be sensitive to inhibition by cystamine through disulfide interchange reactions (Lesort et al., 2003). However, cystamine does not accumulate to any great extent in the brains of mice treated with Trinucleotide-Expansion Diseases 341 pharmacological levels of cystamine (Pinto et al., 2005). Moreover, the magnitude of the protective effect of cystamine is similar in R6/2 TGase 2 +/+ mice to that in R6/2 TGase 2 –/– mice (Bailey and Johnson, 2006). Thus, the mechanism by which cystamine exerts its beneficial effects in the intact HD transgenic mice, especially in regard to caspase and TG activity, must await further study. Nevertheless, recent evidence does suggest a possible pivotal role for caspases in HD neuropathology. For example, there is some evidence that certain prote- olytic fragments generated from Htt are neurotoxic (Ratovitski et al., 2009, and references cited therein). Htt undergoes proteolysis by calpains and caspases at the N-terminus between amino acid residues 460 and 470 (Ratovitski et al., 2009). Evidently the proteolytic cleavage is heterogeneous. Htt can be phosphorylated at serine-421 (S421) by the prosurvival signaling kinases Akt and SGY (Warby et al., 2009). Interestingly, within the brain, phosphorylation of Htt is lowest in the stria- tum. Caspase 6-cleavage of Htt at amino acid 586 appears to be a crucial factor in Htt neurotoxicity (Warby et al., 2009). Phosphorylation of Htt reduces the nuclear accumulation of caspase-6-generated Htt fragments by reducing caspase-6 cleav- age. Inasmuch as different cells contain different complements of calpains, caspases, and phosphorylation/dephosphorylation machinery, it is possible that production of toxic fragments will be cell-specific (Ratovitski et al., 2009; Warby et al., 2009) and may explain in part the remarkable selectivity of different neuronal populations in the various Q n -expansion diseases. 4.6 Increased Excitotoxicity/Oxidative Stress Quinolinate has been known for more than 25 years to produce HD-like pathol- ogy in rodents (e.g., Beal et al., 1986). Thus, it has been suggested that the genetic defect in HD may result in heightened neuronal susceptibility to excitotoxic injury. Guidetti et al. (2004) have shown that the levels of quinolinate (an endogenous neuroactive metabolite of the kynurenine pathway of tryptophan metabolism) and 3-hydroxykynurenate (a free radical generator and additional metabolite of the kynurenine pathway) are elevated three- to fourfold in low-grade HD brain (grade 0/1) in the neocortex and neostriatum, but not in the cerebellum. In contrast, lev- els of these compounds tended to decrease in HD brain in advanced grades (grades 2–4). NAD(P)H oxidase has been suggested to contribute to neurotoxicity in an excitotoxic/pro-oxidant model of HD in rats (intrastriatal injection of quinolinate) (Maldonado et al., 2010) Calcineurin is a calcium-dependent serine/threonine phosphatase involved in the regulation of glutamate receptor signaling (Xifró et al., 2009). It has been sug- gested that reduction of calcineurin A (the catalytic subunit of the calcineurin heterodimer) activity may contribute to the pathophysiology of HD (Xifró et al., 2009). Some evidence suggests that the excitoxicity associated with overstimulation of the N-methyl-D-aspartate receptor (NMDAR) is associated with the pathogenesis of HD (Milnerwood et al., 2010, and references cited therein). Milnerwood et al. (2010) cite evidence that synaptic NMDAR transmission drives neuroprotective 342 A.J.L. Cooper and J.P. Blass gene expression, whereas extrasynaptic gene expression promotes cell death. The authors suggest that elevated extrasynaptic NMDAR activity may contribute to the neurodegeneration of HD. 4.7 Defects in Axonal Transport Several authors have provided evidence that pathological-length Q n repeats promote aberrant protein i nteractions that cause defects in axonal transport (Gunawardena and Goldstein, 2005; Smith et al., 2009; Schweitzer et al., 2009; Wu et al., 2010). Gunawardena and Goldstein (2005) have aptly described the phenomenon as “deadly traffic jams along the neuronal highway.” These jams would be particularly troublesome in long-narrow caliber axons. 4.8 Integration of Mechanisms Any theory that attempts to unify all the competing mechanisms that have been proposed to account for the toxicity of expanded Q n domains must account for the following observations: (1) Q n -expansion diseases typically become manifest in adulthood, and (2) different brain regions are selectively vulnerable in the various Q n -expansion diseases despite widespread expression of mutated protein throughout the brain and body. Evidently, proteins containing pathological-length Q n expan- sions exhibit normal (or near normal) biological functions early in life. Typically, only in adulthood does a pathological gain in function become prominent. This sit- uation contrasts dramatically with the A n -expansion diseases, where pathology is evident at birth, and disruption of normal protein function is present even in utero (Section 5). It is now becoming clear that differences in disease phenotype among the dif- ferent Q n -expansion diseases are not only dictated by the length of n, but also in part by the intrinsic function of the disease-causing mutation (Gatchel and Zoghbi, 2005). For example, modifications outside the Q n domain, such as phosphorylation of ataxin-1 at a crucial serine residue (Emamian et al., 2003) and SUMOylation of Htt (Steffan et al., 2004) are important determinants of toxicity. Moreover, Boat (brother of ataxin-1) was shown to interact with ataxin-1 at multiple sites, and altered expression of Boat in Purkinje cells may contribute to the neurodegeneration in SCA1 (Mizutani et al., 2005). Differences in susceptibility to TG-catalyzed cross- linking among the various mutated Q n -containing proteins may also contribute to the selectivity. A summary of many of the pathological mechanisms postulated to occur in Q n - expansion diseases and how they might be interrelated is shown in Fig. 2 (Steps 1–8). The Q n expansion causes aberrant protein conformation, which in turn leads to altered Ca 2+ homeostasis (1), mitochondrial dysfunction (2), altered energy metabolism (3), and excitoxicity/oxidative stress (4). The protein containing the Trinucleotide-Expansion Diseases 343 Fig. 2 Proposed pathological responses in polyglutamine (Q n )-expansion diseases. Pathological responses include (1) altered calcium homeostasis, (2) mitochondrial dysfunction, (3) altered energy metabolism, (4) excitoxicity, (5) altered transciptio/translation, (6) stalling of axonal trans- port, (7) proteasome malfunction, and (8) removal of essential proteins/factors in insoluble protein aggregates (formed via non-covalent and/or covalent cross linking). For a more detailed discus- sion of the various pathological responses see the text. K, lysine residue; Q, glutamine residue; Ub, ubiquitin. The figure is adapted from Gatchel and Zoghbi (2005) but with considerable modification 344 A.J.L. Cooper and J.P. Blass Q n expansion is subjected to proteolytic cleavage. The fragment containing the Q n expansion may enter the nucleus where it interferes with transcriptional regu- lation. This interference can lead to altered RNA metabolism and altered protein synthesis (5). The intact protein containing the Q n expansion, or a protein frag- ment containing the Q n expansion, may interact noncovalently with other proteins to form aggregates. Aggregate formation may also occur via TG-catalyzed cross- linking. Insoluble aggregates per se may be relatively harmless, but may indirectly be harmful by sequestering essential proteins. Soluble aggregates may cause stalling of axonal transport (6), clogging of the proteasome machinery (7), and excessive removal of essential proteins (8). Note that the magnitude of the various patholog- ical responses may vary among the different Q n -expansion diseases, as a result, in part, of the properties of the part of the protein that does not contain the Q n expan- sion. This variability might explain in part the selective vulnerability associated with the various Q n -expansion diseases. 4.9 Therapeutic Strategies Given the wide range of toxic mechanisms postulated to occur in Q n -expansion dis- eases, it is perhaps not surprising that a wide range of strategies to treat such diseases has been considered. For recent reviews see for example, Bauer and Nukina (2009), Ross and Shoulson (2009), and Spindler et al. (2009). As discussed by Ross and Shoulson (2009) possible strategies for treatment of HD include (a) use of anti- sense oligonucleotides/SiRNA to target the mutant Htt, (b) alteration of the Htt protein posttranslationally (by, e.g., phosphorylation, acetylation, SUMOylation, proteolytic cleavage), (c) bolstering chaperones and the proteasome machinery as a defenses against abnormal proteins, (d) countering abnormal transcription with histone deacetylase inhibitors or by stimulation of relevant gene products such as BDNF, and (e) enhancing energy metabolism/mitochondrial function with creatine and Coenzyme Q10. Presumably such approaches may be generally useful in other Q n -expansion diseases in adddition to HD. In respect to the last-mentioned strategy, Coenzyme Q10 has proved effective in mouse models of HD. Some human studies have been conducted with Coenzyme Q10 but more studies are needed (Spindler et al., 2009). 5 Diseases Due to a Coding Trinucleotide Expansion—Polyalanine (A n )-Expansion Diseases 5.1 General Description Much of the following discussion on A n -expansion diseases is from excellent reviews by Brown and Brown (2004) and Messaed and Rouleau (2009). Nine dis- eases are currently known to be associated with an expansion of an A n domain in the affected protein (Table 3). Synpolydactyly type II (SPD), cleidocranial dysplasia . and Windle, 2001). Proteins/polypeptides containing Q n domains in vitro form hydrogen bonds that link the glutaminyl side chains of the Q n domain to the peptide backbone in an adjacent protein/polypeptide resulting. neurotoxicity in an excitotoxic/pro-oxidant model of HD in rats (intrastriatal injection of quinolinate) (Maldonado et al., 2010) Calcineurin is a calcium-dependent serine/threonine phosphatase involved in. calcium-dependent protein cross-linking between glu- taminyl (Q) and lysyl (K) residues. Often, but not always, such cross-linked proteins are insoluble. In HD mice lacking TG 2 (the major TG in brain; also