Background
ADAM (A Disintegrin And Metalloprotease) is a family of membrane-spanning multi-domain proteins containing a metalloproteinase-like domain and a disintegrin-like domain. Currently, more than 30 ADAMs have been identified in mammals. Their biological activities implicate ADAMs in fertilization, myogenesis and neurogenesis by proteolysis and adhesion. Some types of ADAM are catalytically active metalloproteases and shed the extracellular domains of membrane-bound growth factors or receptors [1,2]. For example, ADAM17 (TACE) has been shown to cleave several substrates, including tumour necrosis factor alpha[3,4], heparin-binding epidermal growth factor-like growth factor [6,7] and transforming growth factor alpha [8]. Studies of ADAM17-null mice have revealed that ADAM17 is critical in embryogenesis and plays an essential role in the supply of growth factors [6,8]. ADAMs are also involved in cell-cell or cell-matrix adhesion through their interaction with integrins or syndecans. More than 10 ADAMs have been shown to support integrin-mediated cell adhesion in vitro [9]. It has been reported that ADAM2-null and ADAM3-null male mutants are infertile and their spermatozoa lack egg-binding abilities [10-12]. Both ADAM2 and ADAM3 are not metalloproteases because they lack catalytic site sequences in their metalloprotease domain. These studies clearly showed that non-proteinase members of ADAMs also have significant roles in vivo.
We have reported the findings of ADAM11, ADAM22 and ADAM23 genes and their restricted expression in the human and murine nervous systems [13-15]. Sequence analysis suggests that they are not metalloproteases, since they all lack a catalytic motif. It has been reported that ADAM23 protein is localised to the cell surface [16], interacts with alpha-v beta-3 integrin heterodimer [17] and the disruption of ADAM23 gene in the mouse results in premature death associated with ataxia and tremor [18]. Although the cause of death in this mouse is unknown, for these phenotypes, impaired cell-cell or cell-matrix interactions in the nervous system caused by loss of ADAM23 may be responsible. ADAM22 and ADAM23 share highly homologous sequences in their extracellular domains. Especially, it is evident in their putative integrin binding loop sequences, CR(E/D)AVN(E/D)CD, which is located centre of the disintegrin domain. These findings led us to hypothesize that ADAM22 is an integrin binder and plays an important role in the nervous system, as does ADAM23. To determine the physiological functions of ADAM22, we generated and analysed Adam22 gene-targeted mice.