In multiple sclerosis (MS), demyelinated CNS lesions fail to sufficiently remyelinate, despite the presence of oligodendrocyte precursor cells (OPCs) capable of differentiating into mature oligodendrocytes. MS lesions contain damaged myelin debris that can inhibit OPC maturation and hinder repair. rHIgM22 is an experimental human recombinant IgM antibody that promotes remyelination in animal models and is being examined in patients with MS. rHIgM22 binds to CNS myelin and partially rescues OPC process outgrowth on myelin. Since rHIgM22 does not affect OPC process outgrowth in vitro on permissive substrate, we examined the possibility that it acts by enhancing phagocytic clearance of myelin debris by microglia. In this study, we tested if rHIgM22 binding could tag myelin for microglial phagocytosis. A mouse microglial cell line and primary rat microglia were treated with myelin and rHIgM22 and assayed for myelin phagocytosis. We found that: 1) rHIgM22 stimulates myelin phagocytosis in a dose-dependent manner; 2) rHIgM22-mediated myelin phagocytosis requires actin polymerization; and 3) rHIgM22-stimulation of myelin phagocytosis requires activity of rHIgM22 Fc domain and activation of Complement Receptor 3. Since myelin inhibits OPC differentiation, stimulation of phagocytic clearance of damaged myelin may be an important means by which rHIgM22 promotes remyelination.

Neuregulins control processes of both tissue development and homeostasis. Glial growth factor 2 (GGF2; Cimaglermin alfa [USAN]), a Type II isoform of the neuregulin 1 (Nrg1) gene, is a ligand selective for the ErbB3 and ErbB4 members of the EGFR‐family of receptor tyrosine kinases. GGF2 has been shown to be cardioprotective in both in vitro and in vivo injury models and is undergoing clinical testing for the treatment of chronic heart failure. The goal of this work was to define the roles of ErbB3 and ErbB4 receptors in GGF2‐mediated responses in a variety of cell types. Cell lines representing cardiac (HL1), liver (HepG2) and breast (MCF7) tissues were evaluated for ErbB receptor expression via qPCR and then specific receptor subtypes were targeted for knockdown of expression using siRNA. At times corresponding to maximal reduction of ErbB receptor protein levels, cells were stimulated with a range of GGF2 concentrations and evaluated for early Akt phosphorylation (pAkt) responses as well as longer term responses related to cell phenotype. Results from these studies indicate ErbB2, 3 and 4 all contribute to GGF2‐mediated pAkt formation in MCF7 and HL1 cells. However, in HepG2 cells, knockdown of ErbB3, but not ErbB2, attenuated GGF2‐dependent pAkt formation. Interestingly, reduction of ErbB4, but not ErbB3, reduced the cytoprotective effect of GGF2 in doxorubicin treated HL1 cells. Additional studies assessing other ErbB‐mediated signaling responses such as proliferation, maintenance of mitochondrial membrane potential and liver‐specific endpoints are being performed. Our results provide insight on the roles that ErbB receptors play in mediating GGF2 actions across a range of target cell types.