Mast Cell–Derived Heparin Proteoglycans As a Model for a Local Antithrombotic

Abstract

Mast cell?derived heparin proteoglycans (HEP-PG) reside in vascular tissue and serve as a local antithrombotic. Heparin, as used clinically, is isolated from its protein backbone from porcine and bovine gut mucosa. The isolated heparin is an anticoagulant; however, when bound to a protein carrier the heparin conjugates will become antiplatelet agents by inhibiting collagen-induced platelet aggregation and procoagulant activity, as distinct from soluble heparin. HEP-PG, whether soluble or immobilized to a surface, inhibit platelet deposition on the collagen surface in flowing blood. Mimics of HEP-PG, can be tailored to molecules carrying both antiplatelet and anticoagulant (APAC) properties. These molecules can target the vascular injury site and take residence there. Inhibition of thrombus growth using these APACs under these conditions has been demonstrated in several animal models. Although efficacious for antiplatelet and anticoagulant effects, the bleeding time is shorter with APACs than with unfractionated heparin, suggestive of beneficial efficacy/safety ratio. These strategies may be utilized in drug development, where many vascular injury-related problems can be tackled locally.

Heparan sulfates are critical regulators of the inhibitory megakaryocyte-platelet receptor G6b-B

“…To determine the effect of G6b-B clustering in solution, we took advantage of APAC, which mimics naturally occurring macromolecular heparin proteoglycans and harboring a higher GAG density than perlecan. Similar to previous reports (Kauhanen, Kovanen, & Lassila, 2000; Lassila & Jouppila, 2014; Lassila et al, 1997), we found that APAC inhibited platelet activation via the ITAM-containing GPVI-FcR γ-chain receptor complex, but also towards the hemi-ITAM-containing receptor CLEC-2. Thus, by increasing the clustering capacity of heparin to a multivalent form, an inhibitory effect on platelet function was achieved in solution.…”

“…APAC consists of unfractionated heparin covalently coupled to a human albumin core, providing a high local density of heparin molecules. In contrast to single-chain heparin, APAC dose-dependently inhibited collagen-induced platelet aggregation (Figure 10), with an almost complete block observed at 0.5 μM, as previously described (Lassila & Jouppila, 2014).…”

“…Our findings demonstrate that G6b-B binds the HS chains of perlecan, as well as to heparin, eliciting functional responses in MKs and platelets. Moreover, we also show that a cross-linked, semisynthetic form of heparin, called anti-platelet anti-coagulant (APAC) (Lassila & Jouppila, 2014), beyond inhibiting collagen-mediated platelet aggregation, induces robust phosphorylation and downstream signaling of G6b-B. Collectively, these results reveal that HSs regulate G6b-B signaling and function, providing a novel mechanism by which MK and platelet function is regulated.…”

“…To overcome this limitations we took advantage of the multivalent semisynthetic heparin proteoglycan mimetic APAC (Lassila & Jouppila, 2014; Lassila et al, 1997) in this assay. APAC consists of unfractionated heparin covalently coupled to a human albumin core, providing a high local density of heparin molecules.…”

Mast cells as a unique hematopoietic lineage and cell system: From Paul Ehrlich's visions to precision medicine concepts

“…Several biochemical and functional studies in humans and other species suggest that MCs represent the major source of heparin 9 , 154 , 158 , 221 . It has also been reported that MC-deficient Kit W/Wv mice develop more severe thromboembolic events after India ink injection compared to their normal littermates, and that injection of heparin can rescue these mice from fatal thromboembolic events 222 .…”