4803 Commentary Introduction Named after three prototypical members, cysteine-rich protein 61 (C yr61; also known as CCN1), connective tissue growth factor (C TGF; also known as CCN2) and nephroblastoma overexpressed protein (N ov; also known as CCN3), the CCN family comprises six secreted proteins grouped together on the basis of a similar predicted modular secondary structure (Fig. 1) (Bork, 1993; Perbal, 2004) 1 . CCN proteins comprise four modules: an insulin-like growth factor binding protein (IGFBP) domain (module I), a Von Willebrand factor domain (module II), a thrombospondin-homology domain (module III), and a cysteine knot, heparin-binding domain (module IV) (Bork, 1993; Perbal, 2004). Befitting secreted proteins, each also possesses a signal sequence (Lechner et al., 2000; Chen, Y. et al., 2001) (Fig. 1). Between modules II and III is the ‘hinge region’, which is susceptible to proteinase cleavage. Indeed, in biological fluids, CCN2 can be found as fragments, including N-terminal and C-terminal halves cleaved in the ‘hinge region’, as well as the individual 10-12 kDa C-terminal heparin-binding domain (module IV) (Brigstock et al., 1997). These variations in structure may have a direct effect on CCN function. The three prototypical members of the CCN family were originally identified ~15 years ago (O’Brien et al., 1990; Bradham et al., 1991; Joliot et al., 1992). Since their discovery, >800 papers on CCN2, ~200 papers on CCN1 and ~100 papers on CCN3 have been published. Substantially fewer papers have examined CCN4, CCN5 and CCN6. In part, progress in this field has been hampered by a lack of unrestricted, readily commercially available reagents, such as ‘gold standard’ recombinant material and neutralizing antibodies. Consequently in vitro studies have used a variety of different protein sources, purification procedures and antibodies. Indeed, a major difficulty in the CCN field has been the purification of active proteins, presumably because of the presence of repeated cysteine residues, which require the use of mammalian expression systems such as baculovirus or stable cell lines. Initially, it was believed that these proteins were classical growth factors, and that simple application of recombinant material to cells was sufficient to recapitulate the entire range of CCN-dependent activities. The collective work of many laboratories, and especially the recent development of transgenic and knockout mice, has resulted in a greater appreciation of the range and complexity of CCN action. Indeed, it is now established that the CCN proteins are not growth factors and thus should not be referred to as such. This fact was a principal driving force within the CCN community The CCN family is a group of six secreted proteins that specifically associate with the extracellular matrix. Structurally, CCN proteins are modular, containing up to four distinct functional domains. CCN family members are induced by growth factors and cytokines such as TGF and endothelin 1 and cellular stress such as hypoxia, and are overexpressed in pathological conditions that affect connective tissues, including scarring, fibrosis and cancer. Although CCN family members were discovered over a decade ago, the precise biological role, mechanism of action and physiological function of these proteins has remained elusive until recently, when several key mechanistic insights into the CCN family emerged. The CCNs have been shown to have key roles as matricellular proteins, serving as adaptor molecules connecting the cell surface and extracellular matrix (ECM). Although they appear not to have specific high-affinity receptors, they signal through integrins and proteoglycans. Furthermore, in addition to having inherent adhesive abilities that modulate focal adhesions and control cell attachment and migration, they execute their functions by modulating the activity of a variety of different growth factors, such as TGF. CCN proteins not only regulate crucial biological processes including cell differentiation, proliferation, adhesion, migration, apoptosis, ECM production, chondrogenesis and angiogenesis, but also have more sinister roles promoting conditions such as fibrogenesis. Key words: CCN1, CCN3, Connective tissue growth factor, Integrins, Signal transduction, CCN2 Summary All in the CCN family: essential matricellular signaling modulators emerge from the bunker Andrew Leask 1, * and David J. Abraham 2 1 CIHR Group in Skeletal Development and Remodeling, Division of Oral Biology, and Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, University of Western Ontario, Dental Sciences Building, London, ON N6A 5C1, Canada 2 Centre for Rheumatology and Connective Tissue Diseases, Royal Free and University College Medical School, University College London (Royal Free Campus), Rowland Hill Street, London, NW3 2PF, UK *Author for correspondence (e-mail: Andrew.Leask@schulich.uwo.ca) Accepted 20 September 2006 Journal of Cell Science 119, 4803-4810 Published by The Company of Biologists 2006 doi:10.1242/jcs.03270 1 Although each of the six members was initially given their own distinct name, the meeting in 2000 of the International CCN Society in St Malo, France, unified the nomenclature to the CCN family members 1-6 in order to reflect their structural similarity (Brigstock et al., 2003). Journal of Cell Science