Osteopontin: Correlation with phagocytosis by brain macrophages in a rat model of stroke

Abstract

Osteopontin (OPN) is an adhesive glycoprotein linked to a variety of pathophysiological processes. We investigated whether OPN might act as an opsonin in the diseased brain by studying the postischemic expression and localization of OPN mRNA and protein in a rat model of ischemic stroke. In addition, we characterized the subcellular localization of OPN protein in the ischemic brain core. Induction of OPN mRNA occurred in activated microglia/macrophages in the ischemic core on days 3-7 after reperfusion and this was sustained up to day 28, at least. OPN protein was synthesized and secreted by brain macrophages, which first surrounded damaged striatal white matter tracts and then infiltrated into them. Punctate OPN-immunoreactive profiles were scattered throughout the infarction core except in white matter bundles. Electron microscopy showed the localization of OPN protein along the membranes lining what appeared to be the debris of dead neurons. These were located in the extracellular space and within the cytoplasm of brain macrophages, indicating that the OPN protein accumulated selectively on the surface of dead cells, most of which were phagocytosed subsequently by brain macrophages. However, no significant induction of OPN occurred in degenerating striatal white matter tracts or in brain macrophage-engulfed axonic or myelin debris. These data suggest that OPN secreted by brain macrophages in this rat model of stroke might be involved in the phagocytosis of fragmented cell debris and possibly not in the phagocytosis of axonic or myelin debris.

Osteopontin directly modulates cytokine expression of primary microglia and increases their survival

“…In contrast, we found that the proliferation of primary rat microglia was unaffected by OPN, as measured by both absolute cells numbers as well as by RT-qPCR for Ki67, suggesting that those effects depend on cell type, species, and potentially also culture conditions. Likewise, OPN was reported to enhance the phagocytic activity of porcine microglia (Tambuyzer, Casteleyn, 2012) and rat macrophages/microglia after experimental stroke in vivo (Shin et al , 2011), while we found no influence of OPN on phagocytic activity of primary rat microglia in vitro. While OPN is a known chemoattractant for macrophages (Lund et al , 2013), it did not exert a chemotactic effect on our primary rat microglia.…”

Osteopontin expression in acute immune response mediates hippocampal synaptogenesis and adaptive outcome following cortical brain injury

“…Since OPN is elevated under conditions of inflammation (Patarca et al, 1989; Hwang et al, 1994; Weber and Cantor, 1996; Chabas et al, 2001), and can direct the migration of macrophages and microglia to sites of brain injury (Ellison et al, 1998; Shin et al, 2011; Hashimoto et al, 2003), it is possible that manipulation of inflammation will affect OPN role in these processes during early synaptogenesis. To test this possibility, we used the tricyclic antibiotic minocycline (delivered acutely at 30 min and 6 h post-UEC lesion) to attenuate inflammatory response, as well as reduce microglial activation (Yrjanheikki et al, 1998; Yrjanheikki et al, 1999).…”

“…OPN fragments can also stimulate microglial activation, shifting the cells into a pro-inflammatory M1 state, as well as induce macrophage proliferation to facilitate phagocytosis (Tambuyzer et al, 2012). OPN producing macrophages participate in debris clearance after stroke (Shin et al, 2011) and CNS demyelination (Zhao et al, 2008). Acutely, OPN can stimulate reactivity of astrocytes at sites of deafferentation, enhancing MMP targeting of ECM proteins like agrin, phosphacan and N-cadherin, whose lysis permits deconstruction of injured synapses prior their reorganization.…”

Sustained Expression of Osteopontin Is Closely Associated with Calcium Deposits in the Rat Hippocampus After Transient Forebrain Ischemia

“…We recently suggested that OPN is involved in phagocytosis of fragmented cell debris by macrophages in response to cerebral ischemia (Shin et al 2011) and that calcium precipitation in cell debris facilitates local binding of OPN, leading to OPN-mediated phagocytosis (Shin et al 2012). This proposal was supported by the present finding that most OPN deposits were encompassed by brain macrophages at 12 weeks, the latest time point examined (Fig.…”