Lifeguard Inhibits Fas Ligand-mediated Endoplasmic Reticulum-Calcium Release Mandatory for Apoptosis in Type II Apoptotic Cells

Abstract

Death receptors are members of the tumor necrosis factor receptor superfamily involved in the extrinsic apoptotic pathway. Lifeguard (LFG) is a death receptor antagonist mainly expressed in the nervous system that specifically blocks Fas ligand (FasL)-induced apoptosis. To investigate its mechanism of action, we studied its subcellular localization and its interaction with members of the Bcl-2 family proteins. We performed an analysis of LFG subcellular localization in murine cortical neurons and found that LFG localizes mainly to the ER and Golgi. We confirmed these results with subcellular fractionation experiments. Moreover, we show by coimmunoprecipitation experiments that LFG interacts with Bcl-X L and Bcl-2, but not with Bax or Bak, and this interaction likely occurs in the endoplasmic reticulum. We further investigated the relationship between LFG and Bcl-X L in the inhibition of apoptosis and found that LFG protects only type II apoptotic cells from FasLinduced death in a Bcl-X L dependent manner. The observation that LFG itself is not located in mitochondria raises the question as to whether LFG in the ER participates in FasL-induced death. Indeed, we investigated the degree of calcium mobilization after FasL stimulation and found that LFG inhibits calcium release from the ER, a process that correlates with LFG blockage of cytochrome c release to the cytosol and caspase activation. On the basis of our observations, we propose that there is a required step in the induction of type II apoptotic cell death that involves calcium mobilization from the ER and that this step is modulated by LFG.

The two sides of Faim2 -- modulation of cell death and regeneration

“…We found more neurogenesis (BrdU positive neurons) in the hippocampus 8 weeks after pneumococcal infection, improved learning after meningitis, and more pronounced sprouting 90 days after MPTP intoxication 3,4 . These findings are consistent with the now established role of Fas/CD95 for regenerative processes, including proliferation and differentiation of neuronal precursors and neurite outgrowth 30,24,31 . It is tempting to speculate that facilitating these regenerative effects of Fas/CD95 signaling is the "purpose" of the Faim2 downregulation observed in disease states.…”

“…Faim2 has been shown to physically interact with Fas and prevent downstream activation of caspases 7,8,24 . Still, it is unclear whether the interaction of Faim2 with Fas prevents Fas trimerization, ligand binding, trafficking, or formation of the "death inducing signaling complex" with FADD and the initiator caspase 8.…”

Hypoxia regulates TRAIL sensitivity of colorectal cancer cells through mitochondrial autophagy

“…Efficient release of apoptotic proteins from the intermembrane space of mitochondria depends on a stimulus-triggered increase in cytosolic Ca 2+ [15]. In CD95 signaling, CD95L-induced Ca 2+ efflux from the endoplasmic reticulum is important for mitochondrial permeabilization and CD95-mediated apoptosis [16, 17]. We analyzed TRAIL-induced changes in cytosolic Ca 2+ under hypoxic and normoxic conditions (Figure 3A).…”

Knockdown of the putative Lifeguard homologue CG3814 in neurons of Drosophila melanogaster

“…This appears to occur through its regulation by PI3K. Another mechanism through which LFG regulates apoptosis is via interaction with Bcl-X L and Bcl-2 at the ER to inhibit calcium release (Urresti et al, 2016). This interaction with Bcl-X L is contrary to previous findings where LFG was shown to interact with Bax (Reimers et al, 2006).…”

“…The CG3814-induced phenotypes may occur through a mechanism that does not involve interaction with pro-survival Bcl-2 proteins at the ER membrane (Urresti et al, 2016), to regulate the release of calcium from the ER. Therefore, knockdown of CG3814/ LFG in the neurons appears to result in neuronal degeneration and death.…”