CD47: a new target in cardiovascular therapy

Abstract

CD47, originally named integrin-associated protein, is a receptor for thrombospondin-1. A number of important roles for CD47 have been defined in regulating the migration, proliferation, and survival of vascular cells, and in regulation of innate and adaptive immunity. The recent discovery that thrombospondin-1 acts via CD47 to inhibit nitric oxide signaling throughout the vascular system has given new importance and perhaps a unifying mechanism of action to these enigmatic proteins. Here we trace the development of this exciting new paradigm for CD47 function in vascular physiology.

FIGURE 1. Models of CD47 and TSP1

A: CD47 (integrin-associated protein) is an atypical member of the Ig superfamily with a single IgV type domain connected to a multiple membrane spanning segment rather than a single transmembrane segment. It is also an atypical G protein-coupled receptor (GPCR) having only 5 transmembrane segments instead of seven. The C-terminal cytoplasmic tail of CD47 is alternatively spliced, giving rise to 4 isoforms that are expressed differentially in various tissues . In addition to its role in TSP1 signaling, CD47 binds the N-terminal or membrane distal, IgV domain of SIRPαand SIRPγ. In some systems, it has been shown that CD47 couples to and signals via heterotrimeric Gi . B: A domain model of TSP1 (after 89). The N-terminal (N) domain binds sulfated glycosaminoglycans and glycolipids and contains binding sites for four β1 integrins, calreticulin and LRP1 (LDL receptor-related protein1) , , . The N-domain is followed by a segment containing the trimer-forming cysteines and heptad repeats, a von Willebrand C domain, 3 TSRs (TSP type 1 repeat domains or properdin-like repeats) containing CD36 and additional β1 integrin binding sites, 3 EGF-like domains and a highly repetitive and extensible calcium-binding domain . At the C terminus is the β-sheet rich G domain , . The RGD site in the last of the calcium-binding repeats is cryptic in the fully calcium loaded protein, but may become available when TSP1 is bound to other cellular or matrix receptors. The C-terminal G domain contains the CD47 agonist peptide 4N1, which is routinely used as 4N1K (kRFYVVMWKk, lowercase k = non-native lysine).

Figure 2. Model of CD47-dependent regulation of NO signaling

CD47 can associate with integrins (here αvβ3) and CD36 in the plasma membrane and is coupled to heterotrimeric Gi through which at least some effects on NO signaling appear to be mediated. Known and potential ligands of CD36 and CD47 are indicated. It is not yet known if TSPs 2–5 or any SIRPs can impact NO signaling. CD36 associates with Src kinases, activation of which may impact CD47 signaling. CD36 also takes up free fatty acids (FFAs), among them myristate, which via N-myristoylation of src kinases, can promote eNOS activation. Binding of TSP1 to CD36 blocks FFA uptake thus inhibiting eNOS and subsequent NO-dependent responses . CD47 can inhibit the effect of NO, generated by NOS or supplied via a chemical donor, on sGC (soluble guanylyl cyclase) and the effect of 8Br-cGMP on cGK-I (cyclic GMP kinase I); these effects may be mediated by activation of heterotrimeric Gi.

FIGURE 3. Improved healing of skin flaps and grafts in TSP1- and CD47-null mice

A: Appearance of random cutaneous McFarlane flaps created on the dorsum of CD36, CD47 and TSP1 null mice after 3 days of healing. The hinge of skin left intact is at the bottom of each panel shown. CD36 null flaps, like WT flaps (not shown), routinely undergo necrosis in the distal portion (ca 40 to 50%) of flap, while CD47 and TSP1 null flaps heal nearly completely (90 to 100%). (from 65). B: Appearance of full thickness skin grafts 3 days after surgery. The host was a WT mouse (1^st and 2^nd panels) or a TSP1-null mouse (3^rd panel) and the graft was from a WT donor (1^st and 3^rd panels) or a TSP1-null donor (middle panel). While a TSP1-null graft does much better than a WT graft on WT hosts, a WT graft completely heals on a TSP1-null host. (from 66).