Alzheimer's Disease (AD), which is clinically characterized by a gradual deterioration of cognitive function, is the most common form of dementia (Brook meyer et al. 1998). It affects approximately 7-10% of individuals over the age of 65, and as many as 40% of individuals over the age of 80. These demographics are expected to grow, as average life expectancy continues to increase, and a growing percentage of the population is above the age of 65 years (Sisodia 1999).
Pathologically, AD is characterized by three forms of neurodegeneration: intra neuronal neurofibrillary tangles, dystrophic neurons, and extracellular amyloid deposits in cerebral vessels and neuritic plaques (Selkoe et al. 1986). Cerebral amyloid deposits consist primarily of b-amyloid peptide (Ab), which can aggregate spontaneously to form fibrils that deposit in the extra-cellular tissue spaces and cerebral vasculature. This peptide ranges in size from 39-43 amino acids (the most common form being Ab 1-42) and is generated from the proteolysis of amyloid precursor protein (APP; Gleaner and Wong 1984). APP is a large integral membrane protein that exists in numerous different isoforms (Gandy 1994); APP 695 is most selectively expressed in neurons and is the isoform primarily found in the human brain (Wisniewski et al.
1997). It has been reported that Ab in the brain is constitutively formed during normal cellular metabolism (Haas et al. 1992) and may play an important physiological role throughout life. Potential functions have been suggested, including growth factor (Beeson et al. 1994), mitogen (Schubert et al. 1989), or neurotoxin (Hardy et al.
1992). The amyloid hypothesis states that the accumulation of Ab peptide is the driving force behind AD pathogenesis and that other qualities of AD result from an imbalance of Ab clearance and production (Hardy and Selkoe 2002). The toxic effects of high concentrations of soluble Ab peptide have been demonstrated for certain protein conformations (Lorenzo and Yank ner 1994). However, it is not clearly understood whether it is Ab peptides, or Ab aggregates, that are responsible for neuronal death in AD (Huang et al.
2000). It has also been proposed that Ab deposition may be part of a neuroprotective mechanism, rather than a neurotoxic one (Wisniewski et al. 1997). Regardless, it is clear that A^a peptide appearance and deposition are both closely linked with AD.
Any relationship linking particular environmental conditions to the increased appearance of this protein may carry profound implications for human health. It is likely that environmental factors do play a role in some forms of AD. Familial AD, which has been associated with specific genetic mutations, develops around the age of 60 and accounts for less than 5% of all individuals with AD (Ku kull and Bowen 2002). The remaining 'sporadic' cases of AD develop later on in life (generally past the age of 70 years). These cases may involve some type of genetic predisposition to AD development, but are also the result of external factors.
Because APP 695 and Ab peptide are normal nervous system constituents, it seems likely that factors that potentiate or enhance the aggregation of Ab into insoluble fibrilla r protein deposits could play an important role in sporadic AD. One group of potential factors are metal ions, in particular Zn 2+, Al 3+, and Cu 2+. These ions have been demonstrated to potentiate b-amyloid formation; it has been shown that: these ions potentiate Ab aggregation in vitro; ion levels are elevated in Ab deposits; metal ion chela tors can solubilize Ab aggregates; AD-related abnormalities in metal ion metabolism do exist (Huang et al. 2000).
Zn 2+ is particularly interesting due to its elevated levels in amyloid deposits (Huang et al. 2000), its altered metabolism in AD (Corrigan et al. 1993), and its ability to bind to APP and Ab peptide (Bush et al 1994). In order to extensively study the effects of these metal ions on the appearance of AD pathology, a good animal model is essential. Though rodents are widely accepted models for biomedical research, there are several reasons why they may not be the ideal subjects for AD studies. APP is abundant in the neurons of laboratory rats (Beeson et al.
1994), but these animals do not develop extracellular Ab plaques with age (Games et al. 1992). Extracellular Ab plaques can be generated in transgenic mice, however, these deposits may not be comparable to their human equivalent (Games et al. 1995). Attempts to potentiate Ab deposits using metal ions have been unsuccessful, most likely because rats possess competent defence mechanisms to deal with such toxic insults.
These animals also have relatively short life spans (averaging approximately two years) in which amyloidosis may develop. Fish have been used extensively in the past to study the effects of toxic chemicals, due to the ease in which the test substances can be administered, in addition to their relatively decreased ability to handle toxic insults, and longer life spans. In regards to AD studies specifically, several reports have demonstrated the ability to immunohistochemical ly stain kokanee salmon (Oncorhynchus nera kennerlyi) brain tissue using rabbit antibodies to human APP 695 and Ab 1-42 (Maldonado et al. 2000, Maldonado et al. 2002). This species of fish reproduce only once, and die soon after spawning.
During their long spawning migration, internal and external tissue degeneration is observed that resembles the degenerative changes observed in other aging vertebrates, including humans. Maldonado et al. (2000) first reported the presence of intracellular APP and intra- and extra-cellular Ab 1-42 in numerous specific brain regions. Areas exhibiting APP and Ab presence were similar, and included regions involved in olfaction, vision, and muscular coordination. The suprachiastmatic nucleus, which is responsible for aging and daily rhythms in vertebrates, was also positively stained. In a later report, Maldonado et al.
(2002) expanded on these findings, and showed that APP and Ab appearance both increase with increasing salmon age.