The urate oxidase gene of Drosophila pseudoobscura and Drosophila melanogaster: evolutionary changes of sequence and regulation

LMNA variants cause cytoplasmic distribution of nuclear pore proteins in Drosophila and human muscle

Mutations in the human LMNA gene, encoding A-type lamins, give rise to laminopathies, which include several types of muscular dystrophy. Here, heterozygous sequence variants in LMNA, which result in single amino-acid substitutions, were identified in patients exhibiting muscle weakness. To assess whether the substitutions altered lamin function, we performed in vivo analyses using a Drosophila model. Stocks were generated that expressed mutant forms of the Drosophila A-type lamin modeled after each variant. Larvae were used for motility assays and histochemical staining of the body-wall muscle. In parallel, immunohistochemical analyses were performed on human muscle biopsy samples from the patients. In control flies, muscle-specific expression of the wild-type A-type lamin had no apparent affect. In contrast, expression of the mutant A-type lamins caused dominant larval muscle defects and semi-lethality at the pupal stage. Histochemical staining of larval body wall muscle revealed that the mutant A-type lamin, B-type lamins, the Sad1p, UNC-84 domain protein Klaroid and nuclear pore complex proteins were mislocalized to the cytoplasm. In addition, cytoplasmic actin filaments were disorganized, suggesting links between the nuclear lamina and the cytoskeleton were disrupted. Muscle biopsies from the patients showed dystrophic histopathology and architectural abnormalities similar to the Drosophila larvae, including cytoplasmic distribution of nuclear envelope proteins. These data provide evidence that the Drosophila model can be used to assess the function of novel LMNA mutations and support the idea that loss of cellular compartmentalization of nuclear proteins contributes to muscle disease pathogenesis.

A comparative study of Drosophila and human A-type lamins

Nuclear intermediate filament proteins, called lamins, form a meshwork that lines the inner surface of the nuclear envelope. Lamins contain three domains: an N-terminal head, a central rod and a C-terminal tail domain possessing an Ig-fold structural motif. Lamins are classified as either A- or B-type based on structure and expression pattern. The Drosophila genome possesses two genes encoding lamins, Lamin C and lamin Dm₀, which have been designated A- and B-type, respectively, based on their expression profile and structural features. In humans, mutations in the gene encoding A-type lamins are associated with a spectrum of predominantly tissue-specific diseases known as laminopathies. Linking the disease phenotypes to cellular functions of lamins has been a major challenge. Drosophila is being used as a model system to identify the roles of lamins in development. Towards this end, we performed a comparative study of Drosophila and human A-type lamins. Analysis of transgenic flies showed that human lamins localize predictably within the Drosophila nucleus. Consistent with this finding, yeast two-hybrid data demonstrated conservation of partner-protein interactions. Drosophila lacking A-type lamin show nuclear envelope defects similar to those observed with human laminopathies. Expression of mutant forms of the A-type Drosophila lamin modeled after human disease-causing amino acid substitutions revealed an essential role for the N-terminal head and the Ig-fold in larval muscle tissue. This tissue-restricted sensitivity suggests a conserved role for lamins in muscle biology. In conclusion, we show that (1) localization of A-type lamins and protein-partner interactions are conserved between Drosophila and humans, (2) loss of the Drosophila A-type lamin causes nuclear defects and (3) muscle tissue is sensitive to the expression of mutant forms of A-type lamin modeled after those causing disease in humans. These studies provide new insights on the role of lamins in nuclear biology and support Drosophila as a model for studies of human laminopathies involving muscle dysfunction.

Molecular genetic analysis of the nested Drosophila melanogaster lamin C gene

Lamins are intermediate filaments that line the inner surface of the nuclear envelope, providing structural support and making contacts with chromatin. There are two types of lamins, A- and B-types, which differ in structure and expression. Drosophila possesses both lamin types, encoded by the LamC (A-type) and lamin Dm0 (B-type) genes. LamC is nested within an intron of the essential gene ttv. We demonstrate that null mutations in LamC are lethal, and expression of a wild-type LamC transgene rescues lethality of LamC but not ttv mutants. Mutations in the human A-type lamin gene lead to diseases called laminopathies. To determine if Drosophila might serve as a useful model to study lamin biology and disease mechanisms, we generated transgenic flies expressing mutant LamC proteins modeled after human disease-causing lamins. These transgenic animals display a nuclear lamin aggregation phenotype remarkably similar to that observed when human mutant A-type lamins are expressed in mammalian cells. LamC aggregates also cause disorganization of lamin Dm0, indicating interdependence of both lamin types for proper lamina assembly. Taken together, these data provide the first detailed genetic analysis of the LamC gene and support using Drosophila as a model to study the role of lamins in disease.

Mechanisms of urea tolerance in urea-adapted populations ofDrosophila melanogaster

When behavioral avoidance cannot prevent an animal from being exposed to novel environmental toxins, physiological mechanisms must cope with the toxin and its effects. We are investigating the basis of urea tolerance in populations of Drosophila melanogaster that have been selected to survive and develop in food containing 300mmoll−1 urea. Previous research has demonstrated that the urea-selected larvae have lower levels of urea in their body than control larvae reared under the same conditions. The current series of experiments focuses on three possible ways of reducing urea levels in the body: urea metabolism, increased urea excretion and decreased urea uptake from the environment. We tested for urea metabolism directly, by assaying for activity of two urea-metabolizing enzymes, and indirectly, by looking for reduced urea content of their medium. To measure urea excretion rates in whole animals, we reared control and urea-selected larvae on urea-containing food (urea food), switched them to normal food and monitored the loss of urea from their hemolymph. We measured urea uptake by rearing control and selected larvae on normal food, switching them to urea food and monitoring the rate of urea appearance in the hemolymph. We found no evidence for urea metabolism by either direct or indirect methods. Control larvae excreted urea at a higher rate than selected, probably because they contained more urea than the selected larvae and thus had a greater gradient for urea loss. The rate of urea uptake in selected larvae was 2mmoll−1h−1 slower than the rate in control larvae, a difference that could account for the measured differences in body urea levels. Thus the selected larvae appear to have adapted to urea exposure primarily by decreasing the ability of urea to enter their body in the first place. The mechanism responsible for this reduction in uptake is uncertain.

Variation in sex-, stage- and tissue-specific expression of the amylase genes in Drosophila ananassae

Expression of the amylase multigene family of Drosophila ananassae was investigated in third-instar larvae and adults. A developmental differentiation was found between the Amy1-2 and Amy3-4 gene clusters, the former being preferentially expressed in larvae, the latter in adults. During adult life, we observed a decrease in Amy1-2 expression in males of certain strans. We have raised some arguments for the existence of trans-active regulators, acting as repressors of Amy1-2 in adults. The putative repressors might exhibit a geographical polymorphism, with a fixed active form in Pacific regions and a polymorphic pattern in Africa, thus increasing the diversity observed in adult amylase phenotypes. A clear differentiation between the two gene clusters was also found in tissue-specific activity along the third-instar larval midgut. In the anterior midgut, only Amy1-2 is active, while both gene groups are expressed in the posterior midgut, with an additional subzonation within it.

Sequence and evolution of the Drosophila pseudoobscura glycerol-3-phosphate dehydrogenase locus

The Gpdh genomic region has been cloned and sequenced in Drosophila pseudoobscura. A total of 6.8 kb of sequence was obtained, encompassing all eight exons of the gene. The exons have been aligned with the sequence from D. melanogaster, and the rates of synonymous and nonsynonymous substitution have been compared to those of other genes sequenced in these two species. Gpdh has the lowest rate of nonsynonymous substitution yet seen in genes sequenced in both D. pseudoobscura and D. melanogaster. No insertion/deletion events were observed, and the overall architecture of the gene (i.e., intron sites, etc.) is conserved. An interesting amino acid reversal was noted between the D. melanogaster Fast allele and the D. pseudoobscura gene.

Adenine phosphoribosyltransferase genes in two Drosophila species: dosage compensation, a nuclear matrix attachment site, and a novel intron position

The Aprt locus of Drosophila encodes the structural gene for the purine salvage enzyme adenine phosphoribosyltransferase. Aprt is autosomal and enzyme activity is gene-dose-dependent in Drosophila melanogaster. However, Aprt is X-linked and dosage compensated in Drosophila pseudoobscura, as shown here. The Aprt genes of both Drosophila species contain a DNA sequence associated with nuclear matrix attachment sites and these Aprt sequences specifically bind to nuclear matrix in vitro. Putative promoter sequences positioned upstream of the predicted transcriptional start site in the two Aprt genes have a similar structure of direct repeats with an overlapping dyad symmetry, but the DNA sequence of these motifs is not conserved between the two species. Biological features in mutants of Aprt as well as natural variants suggest that dosage compensation of this gene in Drosophila pseudoobscura is due to a general control mechanism on X-linked genes rather than a gene-specific mechanism.

Nucleotide divergence of the rp49 gene region between Drosophila melanogaster and two species of the Obscura group of Drosophila

A 2.1-kb SstI fragment including the rp49 gene and the 3' end of the delta-serendipity gene has been cloned and sequenced in Drosophila pseudoobscura. rp49 maps at region 62 on the tip of chromosome II of this species. Both the coding and flanking regions have been aligned and compared with those of D. subobscura. There is no evidence for heterogeneity in the rate of silent substitution between the rp49 coding region and the rate of substitutions in flanking regions, the overall silent divergence per site being 0.19. Noncoding regions also differ between both species by different insertions/deletions, some of which are related to repeated sequences. The rp49 region of D. pseudoobscura shows a strong codon bias similar to those of D. subobscura and D. melanogaster. Comparison of the rates of silent (Ks) and nonsilent (Ka) substitutions of the rp49 gene and other genes completely sequenced in D. pseudoobscura and D. melanogaster confirms previous results indicating that rp49 is evolving slowly both at silent and nonsilent sites. According to the data for the rp49 region, D. pseudoobscura and D. subobscura lineages would have diverged some 9 Myr ago, if one assumes a divergence time of 30 Myr for the melanogaster and obscura groups.