Department of Biology, University of Winnipeg, Winnipeg, Manitoba, Canada
Correspondence: E-mail: a.civetta{at}uwinnipeg.ca.
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Abstract |
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Key Words: ADAM gene family reproductive proteins rapid evolution positive selection mouse-human comparisons
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Introduction |
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In mammals, sperm proteins in general (Wyckoff, Wang, and Wu 2000; Torgerson, Kulathinal, and Singh 2002) and those with a role in binding to the zona pellucida (Swanson, Nielsen, and Yang 2003) show signs of interspecific sequence divergence driven by positive selection. In these studies, sequence analysis was done by comparing rates of nonsynonymous and synonymous substitutions among species from a pool of mammalian genes that have a common site of expression (testes and/or sperm) to others. In such a diverse pool of reproductive genes, other characteristics may contribute to the patterns of interspecific divergence experienced by these proteins. Focusing the analysis on a specific gene family can minimize problems that arise from comparing genes with a wide variety of functions and evolutionary history.
The ADAM (a disintegrin and metalloprotease) family is a multigene family comprised of members with a wide variety of functions such as myogenesis, neurogenesis, cell-cycle control and spermatogenesis. Different domains can be identified within ADAM proteins and these domains usually have a defined function in at least one member of the gene family; however, ADAM proteins are characterized as a group by the presence of at least a metalloprotease and a disintegrin domain (Primakoff and Myles 2000). Not all members of the ADAM protein family retain a metalloprotease domain in the mature protein. The presence of a zinc-binding catalytic consensus sequence within the metalloprotease domain defines proteolytic ADAM proteins capable of cleaving substrates, while some inactive proteases lack the catalytic consensus sequence or have a mutated version of it (Schlöndorff and Blobel 1999; Primakoff and Myles 2000). The disintegrin domain of ADAM proteins provides adhesive properties for those ADAMs having an integrin-binding motif at the tip of a flexible protein loop (Primakoff and Myles 2000).
Little is known of the evolutionary relationship among ADAM genes, although there is some evidence to suggest that ADAM genes expressed in sperm and ADAM gene domains that function in sperm-egg interactions may be more diverged between species than other ADAM genes (Civetta 2003). There is heterogeneity in evolutionary rates within fertilin (ADAM 1) and fertilin ß (ADAM 2) with the prodomain and the metalloprotease domains more conserved than other regions, which was suggested to be a result of their roles in spermatogenesis or sperm capacitance, and positive selection detected in the disintegrin and cysteine-rich adhesion domains (Civetta 2003).
ADAMs are a group of widely expressed genes with potential roles in cell adhesion. Members of the ADAM family may also function as proteases based on whether a metalloprotease domain is present and active in the protein (Primakoff and Myles 2000). To date, 32 ADAM genes have been identified in the mouse, of which 19 have confirmed metalloprotease activity as mature proteins and are expressed either in reproductive tissue (testis or epididimys; n = 10) or somatic tissue (n = 9) (Primakoff and Myles 2000; Brachvogel et al. 2002; Gunn et al. 2002; Choi et al. 2003). At least 12 ADAM genes have been suggested to function in cell adhesion based on the presence of predictive amino acid motifs and/or in vitro binding assays to potential integrin ligands (Sagane et al. 1999; Evans 2002; Frayne, Hurd, and Hall 2002; Choi et al. 2003; table 1). Although few receptors have been confirmed, the disintegrin domain is implicated as the principal region mediating binding between ADAM ligands and integrins (Primakoff and Myles 2000).
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Materials and Methods |
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Amino acid sequence alignments were obtained using the ClustalW program within the DAMBE software package and used to generate nucleotide alignments (Xia 2000). An ADAM gene phylogeny for the 17 mouse and human sequences was constructed using distance (Neighbor-Joining), maximum parsimony (MP), and maximum likelihood (ML) methods (Mega 2.1; PAUP*4.0) (Swafford 1992; Kumar et al. 2001). The most parsimonious consensus tree was generated under the Tree Bisection-Reconnection (TBR) branch-swapping algorithm. The MP tree was evaluated using maximum likelihood methods under the heuristic model with 25 random-addition replicates and a transition/ transversion ratio = 2:1. The reliability of the inferred tree was assessed by bootstrapping with 1,000 replicates, and bootstrap values with scores greater than 80% were retained. For the maximum likelihood tree reconstruction, we used nucleotide substitution models that consider only transition/transversion bias (F84 model) or both transition/ transversion bias and transition rate heterogeneity (Tn93 model). The log likelihood ratio test (LR = 2 [InLTn93InLF84]) (Nei and Kumar 2000, p. 70) was used to assess which model best fit the underlying data.
Proportion of nonsynonymous substitutions per nonsynonymous site (dN) and synonymous substitutions per synonymous site (dS) between mouse and human were calculated for each ADAM gene using the modified Nei-Gojobori Jukes Cantor method (Nei and Gojobori 1986; Nei and Kumar 2000, pp. 5760), which takes into account deviations from a transition/transversion (R) = 1. We compared dN/dS ratios by grouping ADAM genes based on phylogenetic clustering, protease activity, potential ligand binding, and tissue of expression. Statistical comparisons of average dN/dS ratios among groups were tested by approximate randomization analysis with 25,000 permutations (Manly 1991).
Variation in Selective Pressures Across Lineages
For eight of the seventeen ADAM genes used to reconstruct the gene family phylogeny, sequences are available from GenBank for at least rat (Rattus novergicus) and/or macaque (Macaca fascicularis) (accession numbers available upon request). Tree topologies were generated using groups of three species, for a total of four possible combinations: human/rat/macaque, human/rat/mouse, macaque/rat/mouse, and human/mouse/macaque. We used tree topologies obtained from the clustal alignment to test for variation in selective pressures measured as dN/dS per codon () across lineages using the CODEML software package in PAML (Yang 1997). Likelihood estimates for each tree topology under the assumption of a unique
ratio for all tree branches (Model 0) was compared to estimates from a free-ratio model (Model 1, variable
among branches). This was achieved by changing the parameters for model to options 0 or 1 in the CODEML control file while keeping the NSsite option that allows for variation in
across codons fixed at one ratio value (M0). A likelihood ratio test was calculated by multiplying twice the difference between log likelihood scores (
) obtained under each model and comparing the result against a
2 distribution with degrees of freedom equal to the difference in the number of parameters estimated in each model (Nielsen and Yang 1998; Yang et al. 2000). To maximize statistical power the analysis was repeated with ADAM genes for which four or more different mammalian species sequences were available (accession numbers available upon request).
Variation in Selective Pressure Across Codon Sites
We used the CODEML software package in PAML to detect specific codon sites under positive selection among sites experiencing variable selective pressures by comparing likelihood estimates of a tree topology under a model that assumes a distribution of values restricted between 0 and 1 (M7, no positive selection) with an alternative model that allows for
values greater than 1 (M8 model). For this purpose, the parameter for model in the CODEML control file was set to 0 while changing the NSsite option to 7 (M7) or 8 (M8). Comparing
estimates between M7 and M8 depicts the existence of codons under positive selection (Yang et al. 2000). If the log-likelihood test suggests the presence of sites under positive selection, then these sites can be identified by using a Bayesian method to estimate posterior probabilities (P) that particular sites have experienced a
value greater than 1. Positively selected sites identified using this approach were assigned to different protein domains previously identified by using the Motif scan tool available in the PROSITE database of protein families and domains (http://hits.isb-sib.ch/cgi-bin/PFSCAN).
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Results |
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Phylogenetic analysis identified three ancestral gene clusters based on bootstrap estimates: cluster 1 is comprised exclusively of nonproteolytic genes expressed in the brain, and clusters 2 and 3 are a mix of proteolytic and nonproteolytic, somatic, and sex-related genes (fig. 1; see also online Supplementary Material). dN/dS ratio estimates between mouse and human varied across genes (one-sample t-test: t16 = 7.23, P < 0.001; table 1). When dN/dS estimates are grouped on the basis of their phylogenetic clustering, proteolytic activity, adhesion, or tissue of expression, results show a significantly higher average dN/dS for ADAM genes expressed in reproductive tissue versus nonsexual tissues (D6,11 = 0.193; P = 0.001; table 2). Interestingly, phylogenetic cluster 1 comprised exclusively of nonproteolytic genes expressed in the brain shows the lowest dN/dS ratio estimate (fig. 1 and table 2).
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Variation in Selective Pressures Across Lineages
Only four ADAM genes (ADAM 2, 17, 18, and 28) show evidence of significant variation in selective pressures across branches in the tree (table 3). Higher values seem to be found within primate branches (fig. 2) and this is supported by a significantly higher
for either of the primate branches compared to the rodent tree branch in human, macaque, and mouse (or rat) three-species comparisons (paired t: t6 = 2.83, P = 0.030). This observation could be a consequence of true differences in selective pressures or an underestimation of the
ratio in the rodent tree branch associated with larger rodent populations (Yang and Nielsen 1998). It is also possible that failure to reject the one-ratio model for phylogenies in which rodent lineages outnumber primate lineages is due to lack of power (Anisimova, Bielawski, and Yang 2001).
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Discussion |
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Genes expressed in both the testis and somatic tissue showed positive selection of codon sites in the carboxy tail, situated on the transmembrane and cytoplasmic segment of the ADAM gene. The role of the carboxy tail in ADAM proteins is poorly understood, although it may be important for localization of the sperm head prior to sperm-egg adhesion (Hunnicut, Koppel, and Myles 1997; Myles and Primakoff 1997; Cowan et al. 2001) or in signal transduction (Evans, Schultz, and Kopf 1998; Kang, Cao, and Zolkiewska 2000). There is also a potential adhesion role identified for the carboxy tail region of ADAMs 17 and 28 that may involve binding of proline-rich regions on the carboxy tail to SH3-domain-containing proteins (Howard, Maciewicz, and Blobel 2003) or of ADAM 17 to mitotic arrest deficient 2 (MAD2), a component of the mitotic machinery (Nelson, Schlöndorff, and Blobel 1999), although these functions remain speculative.
Among ADAMs showing signs of positive selection, we found the highest proportion of positively selected codon sites in the disintegrin/cysteine-rich region of genes expressed in the testis (ADAMs 2 and 32) (fig. 3). Similar results were found for ADAM 1, a heterodimer of ADAM 2 that plays a direct role in sperm-egg adhesion (Civetta 2003). ADAM 2 is supposed to be implicated in sperm-egg adhesion as a ligand of integrins located on the surface of the egg (Evans, Kopf, and Schultz 1997). Such adhesive function is mediated by sites located in the disintegrin domain of ADAM 2 (Zhu et al. 2000). Mice with an ADAM 2 knockout are sterile; in in vitro assays the null sperm is defective in binding to the egg zona pellucida and is rarely found in the oviduct of females despite normal motility. ADAM 2 null males also show a reduced ability to fuse the egg membrane, suggesting that ADAM 2 is required but not essential in sperm-egg membrane binding (Cho et al. 1998). ADAM 32 shares a close sequence homology to ADAM 2, including a sequence similar to the conserved binding motif of the disintegrin loop, suggesting a potential role in gamete adhesion (Choi et al. 2003).
ADAM genes with the potential for mediating cell-cell adhesion within endogenous tissues and/or with a role in embryonic development were found to be under purifying selection. Included in this category are ADAMs 11, 22, and 23, potential ligands of integrins in neural brain tissue (Sagane et al. 1999); ADAMs 9, 12, and 15, genes with adhesion roles in fibroblasts, myoblasts and haemopoietic cells (Nath et al.1999, 2000; Eto et al. 2000); and ADAMs 7 and 18, genes expressed in the epididymis with roles in sperm development (Cornwall and Hsia 1997; Frayne, Hurd, and Hall 2002).
Our study is the first to examine the evolution of ADAM genes in the context of a disintegrin multigene family. The ADAM proteins are thought to have evolved as a multidomain structure from a proteolytic ancestor and for which pressure on the disintegrin adhesion domain is the suggested mechanism promoting functional diversification (Moura-da-Silva, Theakston, and Crampton 1996). This process is similar to selection for antigen recognition sites of major histocompatability loci (MHC) (Hughes and Nei 1988; Hughes, Ota, and Nei 1990) and heterogeneity at binding sites within the antifreeze protein (AFP) multigene family (Swanson and Aquadro 2002).
Evidence for rapid evolution of reproductive and immune system genes has led to speculation that genes binding to "foreign" substances may be generally under positive selection (e.g., Wyckoff, Wang, and Wu 2000). Our results show that positive Darwinian selection is restricted to the adhesion domains of ADAM genes involved in sperm-egg plasma membrane adhesion, as opposed to binding domains of all ADAM genes. This is best illustrated by ADAM 28, for which we found no evidence of positive selection at adhesion sites, despite the potential binding role inferred by recognition of the disintegrin domain by integrin 4ß1 on human lymphocytes (Bridges et al. 2002).
Our study highlights the faster evolution of reproductive genes as a group and the adaptive diversification of sites within binding domains of sperm-egg adhesion ADAMs. Further functional studies are needed to determine the mechanism and significance of the actual changes driven by positive selection.
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Acknowledgements |
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Footnotes |
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Stephen Palumbi, Associate Editor
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