Hepatic Lectin Receptors in Glycoprotein Homeostasis

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DESCRIPTION (provided by applicant): The classical studies of Gilbert Ashwell and Anatol Morell over 40 years ago reported the identification and isolation of the first cellular receptor ad the first mammalian lectin. Their findings revealed that the asialoglycoprotein receptor of hepatocytes, now termed the Ashwell-Morell receptor (AMR), binds and internalizes exogenously administered circulating glycoproteins that have been remodeled by neuraminidase (sialidase) activity. This remodeling of glycoproteins by neuraminidase removes sialic acid from the termini of the attached glycan chains, unmasking the underlying galactose. The resulting exposed galactose linkages contribute to the formation of AMR ligands. The high capacity and rapid kinetics of AMR-mediated clearance of such asialoglycoproteins suggests that this function has evolved to eliminate potentially deleterious blood components from circulation. However, in the decades since the discovery of the AMR, endogenous ligands remained undefined and no physiological purpose for the AMR was identified. Recently, the Principal Investigator's laboratory identified platelets as expressing endogenous ligands of the AMR. This discovery included the finding that hepatocytes cause thrombocytopenia by AMR-dependent platelet clearance in the course of sepsis induced by Streptococcal pneumoniae (SPN). In addition, AMR-dependent thrombocytopenia was linked to platelet de-sialylation by the bacterium's NanA neuraminidase. Research proposed herein will determine the contribution of thrombocytopenia to the protection afforded by AMR function in moderating the coagulopathy and lethality of SPN sepsis in the mouse. Using AMR-dependent and AMR-independent platelet clearance mechanisms, recombinant NanA neuraminidase and anti-CD41 treatment will similarly establish thrombocytopenia in comparing the outcomes on coagulopathy and mortality during SPN sepsis. Moreover, preliminary data indicates substantial therapeutic activity of NanA and anti-CD41 upon their administration during the course of SPN sepsis. Therefore these studies will be simultaneously characterizing a novel and potentially effective future therapeutic approach to treat sepsis. These studies will further resolve the role of the AMR in the therapeutic activities of recombinant NanA and anti-CD41 administration during SPN sepsis. The AMR-platelet receptor-ligand interaction has been hypothesized to require the highly sialylated platelet GpIba protein. Therefore, research proposed herein will test this hypothesis among GpIb?-deficient platelets, and will further determine whether thrombocytopenia due to GpIb? deficiency in also protective in the course of SPN sepsis. The reason why the AMR is highly conserved among mammals and evolved an ability to rapidly induce thrombocytopenia suggests that platelets may become pathogenic in some circumstances, such as the induction of Granzyme-B expression among platelets produced during sepsis. Granzyme-B causes cell death and tissue damage leading to lethality in sepsis. Therefore this proposal will test whether the protective effect of AMR-dependent thrombocytopenia can be ascribed to the elimination of platelets expressing Granzyme-B. PUBLIC HEALTH RELEVANCE: Hepatic lectins that include the Ashwel-Morell receptor (AMR) bind and eliminate from blood circulation components remodeled by neuraminidase activity. An acute elevation of neuraminidase activity occurs in the blood during sepsis, thereby inducing AMR-mediated clearance as a means to diminish coagulopathy and the rate of mortality. The research proposed herein will define the contribution of platelet clearance to this protective response, reveal the role of platelet Gp1b? as an AMR ligand, and determine whether AMR- mediated clearance of Granzyme B-expressing cytotoxic platelets is the basis of AMR protection during sepsis.