Evolutionary divergence and functions of the ADAM and ADAMTSgene families

ADAM subfamily A

ADAM11, ADAM22 and ADAM23 make up ADAM subfamily A (Figure 2, top). All three proteins lack critical residues found within the peptidase domain and are considered proteolytically inactive. ADAM22 transcribes at least five variants, which encode five different protein isoforms; little is known about the precise function of ADAM22 and its variants. The function of the remaining two genes in this subfamily also remains relatively unexplored. Interestingly, the expression of all three ADAM proteins is predominantly confined to the central and peripheral nervous systems, hence, they are believed to play an important role in neuronal function and development [21, 22]. Studies have indicated non-redundant roles for ADAM11, ADAM22 and ADAM23 in the brain, based on the phenotypes displayed in knockout mice. Mice lacking functional Adam11 exhibit decreased pain sensation and display learning deficiencies [23]. Adam22(-/-) knockout mice exhibit hypermyelination of peripheral nerves and die prior to weaning [24]. Disruption of the mouse Adam23 gene results in ataxia and premature death [25].

ADAM subfamily B

This group contains four genes: ADAM2, ADAM18, ADAM32 and ADAM9 (Figure 2, top). The first three of these genes in this subfamily lack peptidase activity and are associated with sperm development and fertilisation. Adam2 in mice is involved in spermatogenesis and fertility, but the exact role in humans has yet to be determined. Little is known about the function of ADAM18; however, it is expressed in human sperm, indicating that it may also be involved in spermatogenesis [26]. ADAM32 is predominantly expressed in the testis and may also participate in sperm development or possibly fertilisation [27]. ADAM9 is the only gene in this subfamily that contains potential SH3 binding sites in its CT. It is also the only member that has α-secretase activity and can metabolise Alzheimer's amyloid precursor protein (AAP) towards the non-amyloidogenic pathway. This potentially protects the brain from plaque formation during Alzheimer's disease. Polymorphisms within the ADAM9 promoter region are associated with protection against sporadic Alzheimer's disease [28].

ADAM subfamily C

Subgroup C contains four genes (Figure 2, top), including ADAM20, ADAM21, ADAM29 and ADAM30. All of the enzymes exhibit protease activity, with the exception of ADAM29. ADAM29 encodes four splice variants that differ in the composition of the 5'-untranslated region (UTR); nothing is known about the function of ADAM29 isoforms. ADAM29 is highly expressed in human testis and is believed to play a role during spermatogenesis [29]. ADAM20 and ADAM21 share 50 per cent protein identity. Both enzymes are pre-dominantly expressed in the testis and possibly function in sperm-egg cell fusion, based on homology to mouse Adam1 [30]. In rodents, Adam21 was reported to facilitate neurogenesis and neuronal plasticity [31]. ADAM30 is also highly expressed in the testis [32].

ADAM subfamily D

ADAM subfamily D contains ADAM7, ADAM28 and ADAMDEC1 (Figure 2, top). The three genes make up a cluster on human chromosome 8p21 and are believed to be the result of gene duplication events [33]. The genes also share a high degree of nucleotide and of amino acid homology. ADAM7 protein does not contain the conserved peptidase motif required for catalytic activity. By contrast, ADAM28 and ADAMDEC1 are considered to be functional proteases. ADAMDEC1, or decysin, is unique among ADAM members, in that it completely lacks a CR domain and contains only a portion of the DIS domain [33]. The gene is therefore referred to as being 'ADAM-like' and structurally distinct from other ADAM family members. The loss of downstream exons in ADAMDEC1 may be the result of an incomplete gene duplication process. ADAMDEC1 encodes three transcripts; however, two of the three transcripts are candidates for nonsense-mediated decay and might not be protein-coding.

The ADAM28 gene transcribes two variants. The longer transcript encodes a 775-amino acid protein. The shorter variant contains an alternative 3' coding region and 3'-UTR, resulting in a much shorter protein. The shorter COOH-terminus lacks both the TM and cytosolic domains and, similar to ADAMTS proteins, is secreted into the ECM. ADAM28 is highly expressed in the epididymis and, like many other ADAM genes, is believed to participate in sperm development [4]. Recently, ADAM28 was found to mediate epithelial-mesenchymal signalling during mammalian tooth development [34]. ADAM7 has also been associated with sperm development [35]. The function of ADAMDEC1 is not completely understood; however, expression was identified in activated mouse and human dendritic cells, indicating that it might participate in modulating the immune response [36].

ADAM subfamily E

Subfamily E includes ADAM8, ADAM12, ADAM15, ADAM19 and ADAM33 (Figure 2, top). All of these genes code for ADAM proteins that are catalytically active. ADAM12, ADAM19 and ADAM33 each encode two variants, and ADAM15 encodes six. The ADAM12 and ADAM19 variants differ in the composition of their COOH-termini. Similar to ADAM28, the longer ADAM12 transcript is the classical membrane-anchored form of the protein. The shorter transcript lacks a TM domain and is secreted into the ECM. It is unclear if a similar situation exists between the ADAM19 variants. The second ADAM33 splice variant lacks an internal exon and encodes a shorter protein. ADAM15 also encodes a number of variant transcripts. The exact function of each variant has yet to be determined, but the isoforms have been shown to have differential effects on cell adhesion and migration [37].

There are a number of proline-rich motifs (PXXP) located within the CT region of ADAM12, ADAM15, ADAM19 and ADAM33 [13]. PXXP motifs are known to bind to SH3 domains, indicating intracellular protein-protein interactions. Interestingly, ADAM15 variants differ in the number of PXXP motifs found in the CT; this suggests that the variants may interact with different cytosolic proteins. A positive association has been found between asthma and ADAM33 polymorphisms in several populations; however, the exact role of these polymorphisms remains unknown [38, 39].

ADAM8 shares the least homology (Figure 2, top) to the other subfamily E members and encodes a single transcript. Studies of asthma in mice showed a strong association between the Adam8 protein, also called CD185, and disease progression [40]. ADAM8 is predominantly expressed in haematopoietic cells and is upregulated during inflammation, which may explain the role it plays in asthma [41].

ADAM subfamily F

The final ADAM gene subfamily contains only two genes, ADAM10 and ADAM17. As depicted in the human ADAM dendrogram (Figure 2, top), these two proteins are distinctly more evolutionarily separated from the rest of the ADAM gene family. ADAM10 and ADAM17 are the only ADAM proteins that do not contain EGF-like domains. Both proteins bind zinc and are catalytically active against a number of protein substrates. The two proteins also contain potential SH3-binding sites located within the CT region [13]. ADAM10 and ADAM17 are both considered as sheddases (also called secretases) and cleave the extracellular domain of membrane-bound proteins, resulting in the release of cytokines, growth factors, adhesion molecules and other enzymes.

To date, ADAM17 is among the best-studied genes in the ADAM family. Also known as TNF convertase, ADAM17 cleaves TNF to its active form, which then modulates immune and pro-inflammatory responses. In addition to TNF, ADAM17 is the major sheddase for epiregulin, amphiregulin and heparin-binding EGF-like growth factors [42]. Recently, the oxidative stress-induced oxidation of ADAM17 was found to enhance peptidase activity; redox sensitivity was attributed to thiol-disulphide bonds formed between the DIS and CR domains found in the extracellular domain, suggesting that many ADAM and ADAMTS proteins might be redox regulated [43]. ADAM17 also cleaves membrane-tethered sonic hedgehog (ShhNp), possibly implicating ADAM proteins in the control of cell growth and patterning during development [44]. In addition, ADAM17 is upregulated in inflammatory bowel diseases [45].

ADAM proteins are linked to gene regulation by way of regulated RIP, in which proteolytic cleavage of a transmembrane protein produces signals that are subsequently targeted to the nucleus. An example is the cleavage of Notch by ADAM10. A number of ADAM proteins, including ADAM10 and ADAM17, are upregulated during tumour cell invasion into surrounding tissues [46]. ADAM10 is the major protease responsible for the release of EGF and betacellulin. ADAM10 was also found to contribute to TNF shedding in cells that lack ADAM17, indicating a functional overlap between the two genes, despite sharing only 25 per cent amino acid identity [47]. Both ADAM10 and ADAM17 also cleave AAP. This cleavage of AAP prevents the formation of the pathogenic amyloid-β fragment responsible for Alzheimer's disease; many studies have therefore begun to focus on this therapeutic potential.

ADAMTS subfamily A

The seven members of subfamily A include ADAMTS1, ADAMTS4, ADAMTS5, ADAMTS8, ADAMTS9, ADAMTS15 and ADAMTS20 (Figure 2, bottom). Members within this subgroup contain the widest range of TS repeats. ADAMTS4 contains only a single TS domain, whereas ADAMTS9 and ADAMTS20 contain 15 repeats [48].

ADAMTS9 and ADAMTS20 contain a gonadal (GON) domain (PF08685), indicating a possible role in gonadal development [49]. The proteins in this subgroup all cleave the major cartilage proteoglycan, aggrecan, and are often referred to as aggrecanases. ADAMTS1, ADAMTS4 and ADAMTS9 also cleave the related proteoglycan, versican [50]. Due to the importance of aggrecan and versican in cartilaginous tissue, many of these ADAMTS proteins have been implicated in the pathogenesis of arthritic disease [51, 52].

ADAMTS subfamily B

ADAMTS subclass B (Figure 2, bottom) is the largest subclass and contains ADAMTS6, ADAMTS7, ADAMTS10, ADAMTS12, ADAMTS16, ADAMTS17, ADAMTS18 and ADAMTS19. The eight members in this group contain between five and eight TS repeats [48]. ADAMTS7 and ADAMTS12 also contain a mucin domain (PF01456), located within the cluster of TS repeats in the COOH-terminus. The domain is predicted to be heavily O-glycosylated, suggesting that the proteins may act as proteoglycans within certain tissues [53]. ADAMTS10, ADAMTS12, ADAMTS17 and ADAMTS19 contain an additional protease-and-lacunin (PLAC) domain (PF08686) [48]. Other ECM proteins containing this domain are involved in tissue development and remodelling [54].

There is little information available related to the function of ADAMTS6. Both ADAMTS7 and ADAMTS12 cleave cartilage oligomeric matrix protein (COMP) and are upregulated in patients with rheumatoid arthritis, indicating that the genes encoding these proteins may play a significant part in the pathogenic progression of this disease [50]. Mutations in the ADAMTS10 gene are responsible for the autosomal recessive form of Weill-Marchesani syndrome, which is a rare connective tissue disorder [49]. The main physiological substrate for ADAMTS10 still remains unknown. ADAMTS16 was recently found to cleave α₂-macroglobulin (A2M). In addition, A2M is expressed in parietal granulosa cells during follicular development [56]. The functions and preferred substrates for ADAMTS17, ADAMTS18 and ADAM19TS remain unknown.

ADAMTS subfamily C

Subfamily C contains ADAMTS2, ADAMTS3 and ADAMTS14 (Figure 2, bottom). ADAMTS2 and ADAMTS14 both encode two transcript variants. The shorter ADAMTS2 variant lacks a number of TS repeats found in the COOH-terminal region of the protein. The ADAMTS14 variant lacks a small region of coding sequence, resulting in the exclusion of three amino acids. All three members of this subgroup contain a PLAC domain within the COOH-terminus, similar to those found in subfamily B. Furthermore, each of the ADAMTS2, ADAMTS3 and ADAMTS14 proteins contains a total of four TS repeats [48]. All members of this group are involved in the NH₂-terminal cleavage of procollagen to collagen and are also referred to as procollagen N-proteinases. Mutations in the ADAMTS2 gene cause Ehlers-Danlos syndrome type VIIC, which is characterised by extreme skin fragility. Moreover, Adamts3 and Adamts14 are unable to compensate for Adamts2 deficiency in Adamts2(-/-) knockout mice, indicating that the proteins may have distinct biological roles in ECM formation and maintenance [58].

ADAMTS subfamily D

The genes that comprise subfamily D contain significant structural differences, when compared with genes in the other ADAMTS subfamilies. This subfamily contains ADAMTS13 (Figure 2, bottom), as well as the five ADAMTSL genes. The ADAMTS13 gene contains 29 exons and encodes at least three transcriptional variants. One transcript lacks an internal coding region that results in the removal of 56 amino acids from the COOH-terminus, when compared with the longest form. Another variant lacks an additional upstream coding segment that codes for 31 amino acids. The PD of ADAMTS13 is significantly truncated, compared with the other ADAMTS subfamily genes. Moreover, ADAMTS13 is the only gene in the ADAMTS gene family to contain a CUB domain (PF00431) [58]. The CUB (Complement subcomponents C1r/C1s, (sea urchin epidermal growth factor1 [UEGF]) and bone morphogenetic protein-1 [BMP1])) domain is an extracellular motif consisting of ~110 residues which is found in a wide range of developmentally regulated proteins [59]. ADAMTS13 was originally identified as the protease responsible for cleaving the von Willebrand factor, which is required for normal blood clotting. Mutations in the ADAMTS13 gene are the cause of hereditary thrombotic thrombocytopenic purpura (TTP). Moreover, sporadic TTP is an autoimmune disease resulting from ADAMTS13 inhibition by antibodies.

The bulk of subfamily D is made up of the five proteolytically inactive ADAMTSL genes (Figure 2, bottom). ADAMTSL proteins lack many NH₂-terminal features, including the PD, MP and DIS domains. They contain the characteristic TS repeats, as well as the SD and CR domains. Most ADAMTSL proteins also feature a PLAC domain, with the exception of ADAMTSL5. Interestingly, both ADAMTSL1 and ADAMTSL3 are predicted to contain immunoglobulin-like (PF00047) and immunoglobulin I-set domains (PF07679), which are found in a number of cell adhesion molecules. ADAMTSL1 can encode either 1,762 or 525 residue proteins; the shorter isoform utilises an alternative 3' exon containing a premature stop codon, which results in subsequent COOH-terminus truncation.

ADAMTSL1 is N-glycosylated and secreted into the ECM [60]. ADAMTSL2 also encodes a secreted glycoprotein [61]. Two ADAMTSL2 transcripts have been identified, which vary in their 5'-UTR sequences. Mutations in the ADAMTSL2 gene result in dysregulation of TGF-β signalling and are associated with geleophysic dysplasia (happy facial expression, short stature and limb abnormalities).

Little is known about the function of ADAMTSL3. Like the previously mentioned ADAMTSL proteins, ADAMTSL3 is a secreted glycoprotein; it may play a role in mediating cell-cell interactions or in the assembly of extracellular matrices [62]. ADAMTSL3 mutations are also commonly associated with colorectal cancers [63]. The ADAMTSL4 gene is closely linked to ADAM15 and may have common ancestry; ADAMTSL4 is believed to originate from the interrupted inversion of a common familial ADAMTS gene [64]. Mutations in the ADAMTSL4 gene were recently found to cause ectopia lentis, an autosomal recessive disorder characterised by lens displacement [65]. The function of ADAMTSL5 has yet to be determined.