Selenoprotein K Is a Novel Target of m-Calpain, and Cleavage Is Regulated by Toll-like Receptor-induced Calpastatin in Macrophages

Abstract

Background: Selenoprotein K is important for calcium-dependent activation of immune cells. Results: Selenoprotein K is cleaved by m-calpain in resting macrophages, but Toll-like receptor activation induces calpastatin generating full-length, functional selenoprotein K. Conclusion: Proteolytic modulation of selenoprotein K is important for macrophage activation. Significance: New roles are defined for the calpain/calpastatin system and selenoprotein K during macrophage activation and inflammation.

Selenoprotein K Binds Multiprotein Complexes and Is Involved in the Regulation of Endoplasmic Reticulum Homeostasis

“…Among eukaryotic selenoproteins, SelK is one of the most widespread selenoproteins (29), but despite this evolutionary significance (29,30) and ubiquitous expression in mammalian tissues (27,31), the biochemical function of SelK is unknown. No light has been shed on the molecular basis of SelK function by the recent findings that (i) SelK is an important protein for promoting effective Ca 2ϩ flux during immune cell activation (28); (ii) SelK is a target of calpains, proteases that modulate apoptosis, proliferation, and migration and are involved in regulation of inflammation and immune response (32); and (iii) SelK is regulated by ER stress, and its knockdown aggravates cell death and apoptosis induced by ER stress agents (3), as well as other reports on SelK (27,34).…”

Stimulation of Unprimed Macrophages with Immune Complexes Triggers a Low Output of Nitric Oxide by Calcium-dependent Neuronal Nitric-oxide Synthase

“…Our laboratory recently identified selenoprotein K (SelK) as an endoplasmic reticulum membrane protein important for efficient Ca 2ϩ flux during the activation of immune cells (21,22). Macrophages from SelK KO mice were impaired in Fc␥R-mediated Ca 2ϩ flux, which enables the use of these macrophages to delineate signaling and effector functions that depend on Ca 2ϩ flux.…”

Calpains promote neutrophil recruitment and bacterial clearance in an acute bacterial peritonitis model

“…Next, we tested for effects of Capns1 deletion on calpain substrate levels in PMac cell lysates. SelK was recently identified as a substrate of calpain-2 downstream of TLR's in macrophages, neutrophils, and dendritic cells (DCs), but not in lymphoid cells [13]. Indeed, we observed increased levels of SelK in KO PC lysates compared with WT, along with mostly full length SelK in KO compared with primarily cleaved SelK in WT PC lysates (Fig.…”

“…Calpains also promote TNF-α-induced neutrophil arrest and ROS production in vitro [18]. In macrophages, calpain activation leads to cleavage of Selenoprotein K (SelK) [13], a transmembrane endoplasmic reticulum protein implicated in regulating Ca ++ flux, ROS production, and cell motility [19]. Calpain activation also allows Group B Streptococcus to evade killing by inducing caspase-independent apoptosis, disruption of the cytoskeleton, and phagosome escape in macrophages [20,21].…”

“…Calpastatin is an endogenous inhibitor of calpains that binds and obstructs the active site of the protease domain [11,12]. Calpastatin is upregulated in macrophages following activation of TLRs, providing a negative feedback mechanism to limit calpain activity likely invoked by calcium flux in activated innate immune cells [13]. Studies using calpain inhibitors, gene silencing, or gene knockout approaches have led to identification of a vast array of putative calpain substrates linked to regulation of cell growth, survival, and motility (reviewed in [14]).…”