Alzheimer's disease drug discovery: in vivo screening using Caenorhabditis elegans as a model for β-amyloid peptide-induced toxicity

Alzheimer's disease (AD) is a complex human neurodegenerative disease. Currently the therapeutics for AD only treats the symptoms. While numbers of excellent studies have used mammalian models to discover new compounds, the time and effort involved with screening large numbers of candidates is prohibitive. Cultured mammalian neurons are often used to perform high-throughput screens (HTS); however, cell culture lacks the organismal complexity involved in AD. To address these issues several researchers are turning to the roundworm, Caenorhabditis elegans. C. elegans has numerous models of both Tau and Ab induced toxicity, the two prime components observed to correlate with AD pathology. These models have led to the discovery of numerous AD modulating candidates. Further, the ease of performing RNA interference for any gene in the C. elegans genome allows for identification of proteins involved in the mechanism of drug action. These attributes make C. elegans well positioned to aid in the discovery of new AD therapies.

Invertebrate models of Alzheimer's disease

The intensely studied model organisms Caenorhabditis elegans and Drosophila melanogaster have been employed to study a number of neurodegenerative diseases, including Alzheimer's disease (AD). Although worms and flies are phylogenetically distant from humans, results of both classic genetic analyses and transgenic manipulation of these invertebrates suggest they are valid models for at least some aspects of AD. This review describes the rationale for AD-relevant studies in worms and flies and discusses both what has been learned from these studies and what may be discovered in the future.

Transgenic invertebrate models of age-associated neurodegenerative diseases

Transgenic Drosophila melanogaster and Caenorhabditis elegans strains have been engineered to express human proteins associated with neurodegenerative diseases. These model systems include transgenic animals expressing beta-amyloid peptide (Alzheimer's disease), polyglutamine repeat proteins (Huntington's disease, Spinocerebellar ataxia), and alpha-synuclein (Parkinson's disease). In most of these invertebrate models, some aspects of the human diseases are reproduced. Although expression of all these proteins in transgenic mice has been instructive, the invertebrate models offer experimental advantages (e.g. forward genetic screens) that can potentially address some of the outstanding questions regarding the cellular processes underlying these diseases. This review considers what has been learned from these invertebrate models, and speculates what further insight may be gained from them.

Visualization of fibrillar amyloid deposits in living, transgenic Caenorhabditis elegans animals using the sensitive amyloid dye, X-34

Transgenic Caenorhabditis elegans animals can be engineered to express high levels of the human beta amyloid peptide (Abeta). Histochemistry of fixed tissue from these animals reveals deposits reactive with the amyloid-specific dyes Congo Red and thioflavin S (Fay et al., J. Neurochem 71:1616, 1998). Here we show by immuno-electron microscopy that these animals contain intracellular immunoreactive deposits with classic amyloid fibrillar ultrastructure. These deposits can be visualized in living animals using the newly developed, intensively fluorescent, amyloid-specific dye X-34. This in vivo staining allows monitoring of amyloid deposition in individual animals over time. The specificity of this staining is demonstrated by examining transgenic animals expressing high levels of a non-fibrillar beta peptide variant, the beta single-chain dimer. These animals have deposits immunoreactive with anti-beta antibodies, but do not have X-34 deposits or deposits with a fibrillar ultrastructure. X-34 can also be used in vivo to visualize putative amyloid deposits resulting from accumulation of human transthyretin, another amyloidic protein. In vivo amyloid staining with X-34 may be a useful tool for monitoring anti-amyloidic treatments in real time or screening for genetic alterations that affect amyloid formation.

Direct observation of stress response in Caenorhabditis elegans using a reporter transgene

Transgenic Caenorhabditis elegans expressing jellyfish Green Fluorescent Protein under the control of the promoter for the inducible small heat shock protein gene hsp-16-2 have been constructed. Transgene expression parallels that of the endogenous hsp-16 gene, and, therefore, allows direct visualization, localization, and quantitation of hsp-16 expression in living animals. In addition to the expected upregulation by heat shock, we show that a variety of stresses, including exposure to superoxide-generating redox-cycling quinones and the expression of the human beta amyloid peptide, specifically induce the reporter transgene. The quinone induction is suppressed by coincubation with L-ascorbate. The ability to directly observe the stress response in living animals significantly simplifies the identification of both exogenous treatments and genetic alterations that modulate stress response, and possibly life span, in C. elegans.

In vitro and in vivo oxidative stress associated with Alzheimer's amyloid beta-peptide (1-42)

The amyloid beta-peptide (A beta)-associated free radical oxidative stress model for neuronal death in Alzheimer's disease (AD) brain predicts that neuronal protein oxidation is a consequence of A beta-associated free radicals [8]. In this study we have used both in vitro and in vivo models of beta-amyloid (A beta) toxicity to detect free radical induced oxidative stress by the measure of protein carbonyl levels. These model systems employed cultured hippocampal neurons exposed to exogenous synthetic A beta(1-42) and transgenic Caenorhabditis elegans (C. elegans) animals expressing A beta(1-42). We also investigated the importance of the A beta(1-42) Met35 residue for free radical formation in peptide solution and for peptide-induced protein oxidation and neuronal toxicity in these model systems. A beta(1-42) in solution yielded an EPR spectrum, suggesting that free radicals are associated with this peptide; however, neither the reverse [A beta(42-1)] nor methionine-substituted peptide [A beta(1-42)Met35Nlc] gave significant EPR spectra, suggesting the importance of the methionine residue in free radical formation. A beta(1-42) addition to cultured hippocampal neurons led to both neurotoxicity (30.1% cell death, p < 0.001) and increased protein oxidation (158% of controls, p < 0.001). and both of those effects were not observed with reverse or Met35Nle substituted peptides. C. elegans transgenic animals expressing human A beta(1-42) also had significantly increased in vivo protein carbonyls (176% of control animals, p < 0.001), consistent with our model. In contrast, transgenic animals with a Met35cys substitution in A beta(1-42) showed no increased protein carbonyls in vivo, in support of the hypothesis that methionine is important in A beta-associated free radical oxidative stress. These results are discussed with reference to the A beta-associated free radical oxidative stress model of neurotoxicity in AD brain.

In vivo aggregation of beta-amyloid peptide variants

Transgenic Caenorhabditis elegans animals have been engineered to express wild-type and single-amino acid variants of a long form of human beta-amyloid peptide (A beta 1-42). These animals express high levels (approximately 300 ng of A beta/mg of total protein) of apparently full-length peptide, as determined by quantitative immunoblot. Expression of wild-type A beta in these animals leads to rapid production of amyloid deposits reactive with Congo red and thioflavin S. This model system has been used to examine the effect of Leu17Pro, Leu17Val, Ala30Pro, Met35Cys, and Met35Leu substitutions on the in vivo production of amyloid deposits. We find that the Leu17Pro and Met35Cys substitutions completely block the formation of thioflavin S-reactive deposits, implicating these as key residues for in vivo amyloid formation. We have also constructed transgenic strains expressing a novel A beta variant, the single-chain dimer. Animals expressing high levels of this variant also fail to produce thioflavin S-reactive deposits.

Biochemical Analysis of Caenorhabditis elegans Surface Mutants

A collection of Caenorhabditis elegans mutants that show ectopic surface lectin binding (Srf mutants) was analyzed to determine the biochemical basis for this phenotype. This analysis involved selective removal or labeling of surface components, specific labeling of surface glycans, and fractionation of total protein with subsequent detection of wheat germ agglutinin (WGA) binding proteins. Wild-type and mutant nematodes showed no differences in their profiles of extractable surface glycoproteins or total WGA-binding proteins, suggesting that the ectopic lectin binding does not result from the novel expression of surface glycans. Instead, these results support a model in which ectopic lectin binding results from an unmasking of glycosylated components present in the insoluble cuticle matrix of wild-type animals. To explain the multiple internal defects found in some surface mutants, we propose that these mutants have a basic defect in protein processing. This defect would interfere with the expression of the postulated masking protein(s), as well as other proteins required for normal development.

Expression of human beta-amyloid peptide in transgenic Caenorhabditis elegans

Transgenic Caenorhabditis elegans nematodes have been engineered to express potentially amyloidic human proteins. These animals contain constructs in which the muscle-specific unc-54 promoter/enhancer of C. elegans drives the expression of the appropriate coding regions derived from human cDNA clones. Animals containing constructs expressing the 42-amino acid beta-amyloid peptide (derived from human amyloid precursor protein cDNA) produce muscle-specific deposits immunoreactive with anti-beta-amyloid polyclonal and monoclonal antibodies. A subset of these deposits also bind the amyloid-specific dye thioflavin S, indicating that these deposits have the tinctural characteristics of classic amyloid. Co-expression of beta-peptide and transthyretin, a protein implicated in preventing the formation of insoluble beta-amyloid, leads to a dramatic reduction in the number of dye-reactive deposits. These results suggest that this invertebrate model may be useful for in vivo investigation of factors that modulate amyloid formation.

Tissue treatment for whole mount internal lectin staining in the nematodes Caenorhabditis elegans, Panagrolaimus superbus and Acrobeloides maximus

Four different fixation schemes, using ten fluorescent-labelled lectins, were investigated for whole mount internal staining of three rhabditid nematodes: Caenorhabditis elegans, Panagrolaimus superbus and Acrobeloides maximus. Acetone-only fixation was found to give strong and reproducible staining, which could be prevented either by periodate treatment of the organisms or by specific inhibitory sugars of the lectins under investigation. Whereas the use of either phosphate or TRIS buffers had no effect on the staining pattern or the fluorescence intensity, the incubation time as well as the incubation temperature affected the staining reaction. The best results were obtained upon overnight incubation at 4 degrees C: the lectin staining could be inhibited in all cases, except for the intestinal brush border of C. elegans by the lectin of Lens culinaris.

Characterization of Caenorhabditis elegans lectin-binding mutants

We have identified 45 mutants of Caenorhabditis elegans that show ectopic surface binding of the lectins wheat germ agglutinin (WGA) and soybean agglutinin (SBA). These mutations are all recessive and define six genes: srf-2, srf-3, srf-4, srf-5, srf-8 and srf-9. Mutations in these genes fall into two phenotypic classes: srf-2, -3, -5 mutants are grossly wild-type, except for their lectin-binding phenotype; srf-4, -8, -9 mutants have a suite of defects, including uncoordinated movement, abnormal egg laying, and defective copulatory bursae morphogenesis. Characterization of these pleiotropic mutants at the cellular level reveals defects in the migration of the gonadal distal tip cell and in axon morphology. Unexpectedly, the pleiotropic mutations also interact with mutations in the lin-12 gene, which encodes a putative cell surface receptor involved in the control of cell fate. We propose that the underlying defect in the pleiotropic mutations may be in the general processing or secretion of extracellular proteins.

Mutant expression of male copulatory bursa surface markers in Caenorhabditis elegans

In a search for molecular markers of male tail morphogenesis in C. elegans, we have detected two surface markers that are specifically observed in the copulatory bursa of adult males and the vulva of adult hermaphrodites. These markers are defined by binding of a monoclonal antibody (Ab117) and the lectin wheat germ agglutinin (WGA) to live intact animals. Expression of these markers is dependent on sex, stage and anterior-posterior position in the animal. Four of ten mutants with specific defects in bursal development show altered expression of one or both markers. Because the WGA marker can be expressed in intersexual animals with very little bursal development, posterior surface expression of this marker can serve as an indication of subtle masculinization of hermaphrodites. The timing of expression of these markers is not affected by heterochronic mutations that cause larval animals to express adult cuticles or adult animals to express larval cuticles, indicating that marker expression can be uncoupled from general cuticle development. Mutant lin-22 males, which have an anterior-to-posterior transformation of cell fates in the lateral hypodermis, ectopically express both markers in a manner consistent with a ‘posteriorization’ of positional information in these animals. These markers should be useful for the isolation and characterization of mutants defective in bursal and vulval development, sex determination and expression of anterior-posterior positional information.

Isolation and characterization of a nematode transposable element from Panagrellus redivivus

We have isolated a transposable element, designated PAT-1, from the free-living nematode Panagrellus redivivus. P. redivivus strain C15 was found to have a high spontaneous mutation frequency compared to the standard Caenorhabditis elegans laboratory strain N2. To characterize the genetic lesions occurring in spontaneous C15 mutants, we molecularly cloned the homolog of the C. elegans unc-22 gene from wild-type P. redivivus and two strains carrying spontaneous mutations in this gene. One of these mutations resulted from the insertion of a 4.8-kilobase segment of repetitive DNA. This repetitive element (PAT-1) varies in copy number (10-50 copies) and location in different P. redivivus strains and is absent from C. elegans. The element could be useful as a transformation vector for C. elegans. Our approach is a general one that could be used to isolate additional nematode transposons from other species.

Genotypic exclusion: a novel relationship between the ribitol-arabitol and galactitol genes of E. coli

Genetic studies indicate that the E. coli C chromosomal genes which are responsible for catabolism of the pentitol sugars, ribitol and D-arabitol, are not present in the closely related E. coli K12 strains (Reiner 1975). Molecular studies of these tightly linked genes reveal that they are surrounded by 1.4 kilobase inverted repeats of imperfect homology (Link and Reiner 1982). Here we report that E. coli C lacks genes for catabolism of the hexitol sugar galactitol, genes which are present in E. coli K12. Furthermore, the ribitol-arabitol and galactitol genes, which show no mutual homology, are mutually exclusive when exchanged (by homologous recombination) between E. coli C and K12. Physical characterization of lambda specialized transducing phages carrying the ribitol-arabitol or galactitol genes demonstrates that this exclusion results because these genes have identical locations in their respective chromosomes. This novel type of allelic relationship between nonhomologous genes has not been previously described in prokaryotes. Analysis of the catabolic capabilities of a collection of natural E. coli strains suggests that this exclusion relationship extends to strains in the natural E. coli population. We suggest an insertion/deletion model to account for the origins of this unusual gene arrangement.