A TREM family member, TLT-1, is found exclusively in the α-granules of megakaryocytes and platelets

Abstract

The triggering receptors expressed on myeloid cells (TREMs) have drawn considerable attention due to their ability to activate multiple cell types within the innate immune system, including neutrophils, monocyte/macrophages, and dendritic cells, via their association with DAP12. TLT-1 (TREM-like transcript-1) lies within the TREM gene cluster and contains the characteristic single V-set immunoglobulin (Ig) domain of the family, but its longer cytoplasmic tail is composed of both a proline-rich region and an immune receptor tyrosine-based inhibitory motif, the latter known to be used for interactions with protein tyrosine phosphatases. Here we report that TLT-1 is expressed exclusively in platelets and megakaryocytes (MKs) and that TLT-1 expression is up-regulated dramatically upon platelet activation. Consistent with this observation, confocal microscopy demonstrates that TLT-1 is prepackaged, along with CD62P, into both MK and platelet ␣- IntroductionThe triggering receptors expressed on myeloid cells (TREMs) are involved in the activation of various cell types of the innate immune system, including monocytes, macrophages, microglia, and neutrophils. [1][2][3][4][5][6] The family is characterized by a single V-set immunoglobulin (Ig) domain, a short cytoplasmic tail, and a charged residue in the transmembrane domain, enabling them to bind the DAP12 signaling chain. 2,3 To date, 3 activating TREMs have been characterized forming a loose cluster occupying a 150 kb region of human chromosome 6 and murine chromosome 17. 3,5,[7][8][9] TREMs play important roles in the regulation of both innate and adaptive immunity. TREM-1 is broadly expressed on neutrophils and monocytes, is up-regulated during bacterial infection, and administration of TREM-1/Ig-Fc fusion protein significantly decreases mortality in mice challenged with lipopolysaccharide (LPS). 2,3,10 TREM-2 is expressed on monocytes, macrophages, and osteoclasts, and its expression is up-regulated during dendritic cell development in vitro. 1,11 In fact, engagement of TREM-2 on immature dendritic cells (DCs) leads to DC maturation, identifying TREMs as important proteins in the transition from an innate to an adaptive immune response. 1 In addition, TREM-2 is associated with polycystic lipomembranous osteodysplasia with sclerosing leukoencephalopathy (PLOSL, also known as Nasu-Hakola disease [NHD]). 11-14 NHD patients harbor mutations in either TREM-2 or its signaling chain, DAP12, and suffer from systemic bone cysts and psychotic symptoms that rapidly progress to presenile dementia in the third or fourth decade of life. Patients die from NHD usually in their fifties, often due to chest infections. Recently, TREM-3 was characterized and shown to activate murine macrophages via its association with DAP12. 5 Given the well-established role of TREMs in the biology of various myeloid lineages, several laboratories have sought to identify regulatory receptors within the TREM locus. 6-8 Our studies identified TREM-like transcript-1 (TLT-1) as a gene within the TREM l...

Trem-Like Transcript 2 Is Expressed on Cells of the Myeloid/Granuloid and B Lymphoid Lineage and Is Up-Regulated in Response to Inflammation

“…By virtue of the interaction with DAP-12, TREM molecules, as well as the related NKp44 receptor, are able to initiate tyrosine-based signaling leading to cellular activation (1,2,22). Other receptors encoded within the TREM gene cluster such as TLT1 lack the conserved transmembrane lysine residue, but contain within their own cytoplasmic domain tyrosinebased signaling motifs (11). In the case of TLT1, there is an ITIM, which mediates recruitment of SHP1 and SHP2 (11,12).…”

“…Other receptors encoded within the TREM gene cluster such as TLT1 lack the conserved transmembrane lysine residue, but contain within their own cytoplasmic domain tyrosinebased signaling motifs (11). In the case of TLT1, there is an ITIM, which mediates recruitment of SHP1 and SHP2 (11,12).…”

“…The cytoplasmic tail of TLT-1, however, contains an ITIM allowing for the interaction with Src homology region 2 domain-containing phosphatase 1 (SHP1), suggesting an inhibitory role for this receptor (10). Interestingly, TLT1 is expressed exclusively in megakaryocytes and platelets (10,11). TLT1 is stored in the ␣-granules of platelets and is quickly redistributed to their surface upon activation.…”

Megakaryocyte-specific deletion of the protein-tyrosine phosphatases Shp1 and Shp2 causes abnormal megakaryocyte development, platelet production, and function

“…[11][12][13][14] Five ITIM-containing receptors have been identified in megakaryocytes and/or platelets to date, namely PECAM-1, carcinoembryonic antigen-related cell adhesion molecule 1, TREM-like transcript-1, leukocyte-associated immunoglobulin-like receptor-1, and G6b-B, all of which interact with Shp1 and Shp2 upon phosphorylation. [15][16][17][18][19] Unique among these is G6b-B, which is constitutively phosphorylated by SFKs and associated with Shp1 and Shp2 (supplemental Figure 1C). 20,21 Thus, G6b-B is thought to maintain active Shp1 and Shp2 at the plasma membrane, where they inhibit signaling from various receptors.…”

CEACAM1 negatively regulates platelet-collagen interactions and thrombus growth in vitro and in vivo

“…Several reports suggest that platelets contain other Ig-ITIM superfamily members, including G6B and TREM (triggering receptors expressed on myeloid cells)-like transcript-1 (TLT-1), that may attenuate platelet function. 10,11 In immunologic systems, SHP-1 proteintyrosine phosphatase is classified as a negative regulator, while in collagen-GPVI platelet responses, it has been proposed to act as a positive regulator. 12 Given this controversy, it is important that we study respective Ig-ITIM superfamily members that are linked to distinct protein-tyrosine phosphatases such as SHP-1 in platelets to determine their relative importance in regulating platelet thrombus formation.…”