ITIM receptors: more than just inhibitors of platelet activation

Abstract

Since their discovery, immunoreceptor tyrosine-based inhibition motif (ITIM)-containing receptors have been shown to inhibit signaling from immunoreceptor tyrosine-based activation motif (ITAM)-containing receptors in almost all hematopoietic cells, including platelets. However, a growing body of evidence has emerged demonstrating that this is an oversimplification, and that ITIM-containing receptors are versatile regulators of platelet signal transduction, with functions beyond inhibiting ITAM-mediated platelet activation. PECAM-1 was the first ITIM-containing receptor identified in platelets and appeared to conform to the established model of ITIM-mediated attenuation of ITAM-driven activation. PECAM-1 was therefore widely accepted as a major negative regulator of platelet activation and thrombosis for many years, but more recent findings suggest a more complex role for this receptor, including the facilitation of α_IIbβ₃-mediated platelet functions. Since the identification of PECAM-1, several other ITIM-containing platelet receptors have been discovered. These include G6b-B, a critical regulator of platelet reactivity and production, and the noncanonical ITIM-containing receptor TREM-like transcript-1, which is localized to α-granules in resting platelets, binds fibrinogen, and acts as a positive regulator of platelet activation. Despite structural similarities and shared binding partners, including the Src homology 2 domain-containing protein-tyrosine phosphatases Shp1 and Shp2, knockout and transgenic mouse models have revealed distinct phenotypes and nonredundant functions for each ITIM-containing receptor in the context of platelet homeostasis. These roles are likely influenced by receptor density, compartmentalization, and as-yet unknown binding partners. In this review, we discuss the diverse repertoire of ITIM-containing receptors in platelets, highlighting intriguing new functions, controversies, and future areas of investigation.

Figure 1.

Inhibition of platelet activation by cyclic nucleotide generation. PGI₂ and NO activation of cyclic nucleotide generation provide generalized inhibition of platelet activation. AC, adenylate cyclase; ATP, adenosine triphosphate; cAMP, cyclic adenosine monophosphate; cGMP, cyclic guanosine monophosphate; Gs, guanine nucleotide-binding protein stimulatory; GTP, guanosine triphosphate; PKA, protein kinase A; PKG, protein kinase G; sGC, soluble guanlyate cyclase. Professional illustration by Patrick Lane, ScEYEnce Studios.

Figure 2.

Classical inhibitory function of ITIM-containing receptors. The inhibition of ITAM-containing receptor signaling through the recruitment of the Src homology 2 (SH2) domain-containing protein-tyrosine phosphatases Shp1 and Shp2, or SH2 domain-containing inositol 5’-phosphatase 1 Ship1. Btk, Bruton's tyrosine kinase; DAG, diacylglycerol; ER, endoplasmic reticulum; IP₃-R, inositol trisphosphate receptor; P, phosphate; PI3-K, phosphoinositide 3-kinase. Professional illustration by Patrick Lane, ScEYEnce Studios.

Figure 3.

Platelet ITIM-containing receptors. The main structural features are shown, including the extracellular IgC₂-like and IgV-like domains and the main intracellular signaling motifs, namely ITIMs (consensus sequence I/V/LxYxxL/V), ITSMs (consensus sequence TxYxxV/I), and PRRs (consensus sequence PxxP) along with nonconsensus ITIM/ITSM-like tyrosine residues. All receptors have been described in platelets except for LAIR-1, which is only found in megakaryocytes. Residues are numbered according to mature mouse peptide sequences, after cleavage of the signal peptide. Professional illustration by Patrick Lane, ScEYEnce Studios.

Figure 4.

Classical and putative functions of ITIM-containing receptors. Increasing evidence is implicating ITIM-containing receptors as more than just inhibitors of ITAM-containing receptors, particularly as positive regulators of integrin-mediated functions. Their potential role in both regulating cytoskeletal remodeling and G protein–coupled receptor (GPCR) signaling warrants additional investigation. Professional illustration by Patrick Lane, ScEYEnce Studios.

Figure 5.

Established and putative isoforms of human G6b. The main structural features are shown, including the IgV domain, ITIM, ITSM, and PRR. G6b-A and -B contain transmembrane regions and are therefore represented as surface receptors. G6b-C, -D, and -E, identified in transcriptome analysis but not as expressed protein, are predicted to be secreted because they lack the transmembrane domain. Residues are numbered according to mature human peptide sequences, after cleavage of the signal peptide. Professional illustration by Patrick Lane, ScEYEnce Studios.