There is conflicting evidence that the development o f identifiable aggregates or neuronal intranuclear inclusions is either necessary or sufficient to cause cell death. In addition, it remains unclear whether nuclear localisation is necessary for the inclusions to exert their toxic effect. It may be that intranuclear inclusions (NI) are toxic to neurones. N I appear before neuronal loss and before expression o f the phenotype in a mouse model o f HD
(Davies et al.^ 1997). They may be a correlate o f pathogenesis, since their density is
correlated to the expansion size (Becher et al., 1998). However, they are also present in
epithelial cells of a drosophila model of SCA3 in which there is no neuronal death or
phenotype (Warrick et al.^ 1998), and have been detected in non-affected neuronal tissues in
(Sathasivam et al.^ 1999) indicating that their presence is not sufficient to cause death and/or a phenotype.
It has been shown in a cellular model of H D that co-expression o f a dominant-negative mutant form o f a ubiquitin-conjugating enzyme with mutant huntingtin suppressed
huntingtin agrégation yet accelerated huntingtin repeat-dependent cell death (Saudou et al^
1998). Similarly, mutant-huntingtin-induced cell death was dissociated from aggregate
formation using two caspase inhibitors in immortalised striatal cells (Kim et al.^ 1999).
A controversial (Pemtz, 1999) study showed, that visible aggregates are not a prerequisite
for pathogenesis in a SCAl mouse model (Klement et al.^ 1998). A single amino acid
substitution that dismpted the nuclear localisation signal o f ataxin-1, normally a
predominandy nuclear protein in Purkinje cells, resulted in mutant ataxin-1 being localised to the cytoplasm. This blocked the induction o f ataxia and Purkinje cell pathology. In a second series o f transgenic mice, it was found that an ataxin-1 construct with an expanded polyglutamine tract, but with the previously characterised self-association region deleted, was still able to induce Purkinje cell pathology and ataxia in the absence o f ataxin-1 aggregates. Hence while nuclear localisation o f ataxin-1 seems critical for pathogenesis,
aggregation does not appear to be required for the induction o f a polyglutamine disease in
vitro.
It may be that protein aggregation precedes the development o f identifiable inclusions, and may be one important step in the process o f cell death. Some have proposed that
inclusions may represent a protective response to toxicity rather than being the toxic agent themselves (Sisodia, 1998).
A fascinating recent finding is that blockade o f expanded human H D transgene expression in symptomatic mice leads to a disappearance o f inclusions and an amelioration o f the
behavioural phenotype (Yamamoto et al,^ 2000). The authors suggest that a continuous
“influx o f the mutant protein” is required to maintain inclusions and symptoms, raising the possibility that HD may be reversible.
How might aggregates affect nuclear function? The proteasome degradation pathway has
been implicated in polyglutamine pathogenesis; numerous studies (Chai et al.^ 1999b;
Cummings et al.., 1998; Stenoien et al., 1999) have shown the presence o f the 20S
disease pathogenesis. It is possible that the normal activity o f the proteasome complex is thus disrupted such that normal regulation o f protein levels is altered. When proteasome activity is inhibited in SCA3-transfected cells, the formation o f mutant ataxin-3 aggregates
is enhanced (Chai et al.^ 1999b).
Further evidence that protein mis-folding is critical to the pathogenesis o f the polyglutamine disorders comes from observations that molecular chaperones which mediate the correct folding, assembly and degradation o f proteins, localise to aggregates
formed by ataxin-1 (Cummings et al^ 1998) and the androgen receptor (Stenoien et al.^
1999). The Heat Shock Proteins (HSPs) are a family o f chaperones inducible by heat and other stressors that serve essential functions under stress and non-stress conditions. Expression o f the Hsp40 chaperone, HJD-2, suppressed aggregation by both proteins in
non-neuronal cells. Chai et al. (Chai et al.^ 1999a)) have recently shown that Hsp40 and
Hsp70 localise to intranuclear aggregates in SCA3 disease tissue and cells expressing mutant ataxin-3, and that over-expression o f Hsp40 chaperones can suppress polyglutamine aggregation and that this suppression correlates with a decrease in neurotoxicity.
O f potentially great significance are the findings o f a recent study which has shown that in a transgenic mouse model o f SCAl where a Purkinje-cell specific promoter is used to drive
the expression o f mutant ataxin-1 (Lin et al^ 2000), some genes such as prenylcysteine
carboxymethyltransferase (PCCMT), type 1 ER inositol triphosphate receptor (IP3R1), and inositol polyphosphate 5-phosphatase (INPP5A), an ER calcium pump (SERCA2),
calcium ion channel TRP3 and the glutamate transporter EAAT4 are down-regulated. These transcripts encode proteins with effects on cellular systems known to be involved in neuronal degeneration, those o f calcium and glutamate metabolism. These changes
occurred before the onset o f Purkinje cell degeneration. As the mice age, after the
development o f clinical and pathological signs, another gene, alpha-1 antichymotrypsin, an acute phase protein, was up-regulated. These changes only occurred in mice expressing the expanded ataxin-1 in the nucleus; no change was seen in control mice expressing either wild-type ataxin-1 or an expanded ataxin-1 that lacked a nuclear localisation domain and was confined to the cytoplasm. The authors confirmed that three o f the gene products down-regulated in mutant mice (PCCMT, SERCA2 and IP3R1) were also under-expressed in the brain o f an early-onset SCAl patient. Alphal-antichymotrypsin was also up-
altered gene expression can only be speculative, but a direct transcriptional effect or an effect on chromatin structure, nuclear architecture or transcription factor stability can be hypothesised. The possibility exists that these changes merely represent so-called
‘bystander phenomena’ and are not directly causative, but perhaps a real effect on cytoplasmic calcium concentration or glutamate neurotransmission is the key step in the pathological process.