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Heparan sulfates are long and complex polysaccharides that mediate a large number of biological processes at the cell surface. Here, the authors provide structural and functional insights into the human EXT1-EXT2 complex that carries out the polymerization of heparan sulfate chains.

Bacillus thuringiensis israelensis (Bti) produces the naturally-crystalline proteinaceous toxin Cyt1Aa that is toxic to mosquito larvae. Here the authors grow recombinant nanocrystals of the Cyt1Aa protoxin in vivo and use serial femtosecond crystallography to determine its structure at different redox and pH conditions and by combining their structural data with further biochemical, toxicological and biophysical analyses provide mechanistic insights into the Cyt1Aa bioactivation cascade.

Natural killer (NK) cells eliminate infected and tumorigenic cells through delivery of granzymes via perforin pores or by activation of caspases via death receptors. In order to understand how NK cells combine different cell death mechanisms, it is important to quantify target cell responses on a single cell level. However, currently existing reporters do not allow the measurement of several protease activities inside the same cell. Here, we present a strategy for the comparison of two different proteases at a time inside individual target cells upon engagement by NK cells. We developed single-fluorescent protein reporters containing the RIEAD or the VGPD cleavage site for the measurement of granzyme B activity. We show that these two granzyme B reporters can be applied in combination with caspase-8 or caspase-3 reporters. While we did not find that caspase-8 was activated by granzyme B, our method revealed that caspase-3 activity follows granzyme B activity with a delay of about 6 min. Finally, we illustrate the comparison of several different reporters for granzyme A, M, K, and H. The approach presented here is a valuable means for the investigation of the temporal evolution of cell death mediated by cytotoxic lymphocytes.

Summary: The death receptor CD95 is expressed in every cancer cell, thus providing a promising tool to target cancer. Activation of CD95 can, however, lead to apoptosis or proliferation. Yet the molecular determinants of CD95’s mode of action remain unclear. Here, we identify an optimal distance between CD95Ligand molecules that enables specific clustering of receptor-ligand pairs, leading to efficient CD95 activation. Surprisingly, efficient CD95 activation leads to apoptosis in cancer cells in vitro and increased tumor growth in vivo. We show that allowing a 3D aggregation of cancer cells in vitro switches the apoptotic response to proliferation. Indeed, we demonstrate that the absence or presence of cell-cell contacts dictates the cell response to CD95. Cell contacts increase global levels of phosphorylated tyrosines, including CD95’s tyrosine. A tyrosine-to-alanine CD95 mutant blocks proliferation in cells in contact. Our study sheds light into the regulatory mechanism of CD95 activation that can be further explored for anti-cancer therapies. : Gülcüler Balta et al. show that CD95 receptor activation is determined through the presentation of its ligand at a certain intermolecular distance. The type of signaling triggered by CD95 is, however, decided by the cellular environment. CD95 triggers survival in cancer cells in contact with other cells and death in isolated ones. Keywords: CD95, CD95 ligand, death receptors, apoptosis, survival, cell-cell contact, cancer, tyrosine kinase, supported membrane

Abstract This study explores the dilemma in cellular signaling that triggering of CD95 (Fas/APO‐1) in some situations results in cell death and in others leads to the activation of NF‐κB. We established an integrated kinetic mathematical model for CD95‐mediated apoptotic and NF‐κB signaling. Systematic model reduction resulted in a surprisingly simple model well approximating experimentally observed dynamics. The model postulates a new link between c‐FLIPL cleavage in the death‐inducing signaling complex (DISC) and the NF‐κB pathway. We validated experimentally that CD95 stimulation resulted in an interaction of p43‐FLIP with the IKK complex followed by its activation. Furthermore, we showed that the apoptotic and NF‐κB pathways diverge already at the DISC. Model and experimental analysis of DISC formation showed that a subtle balance of c‐FLIPL and procaspase‐8 determines life/death decisions in a nonlinear manner. We present an integrated model describing the complex dynamics of CD95‐mediated apoptosis and NF‐κB signaling.