Drilling into RIP1 biology: what compounds are in your toolkit?

Targeting Cell Death Pathways for Therapeutic Intervention in Kidney Diseases

Precise regulation of cell death and survival is essential for proper maintenance of organismal homeostasis, development, and the immune system. Deregulated cell death can lead to developmental defects, neuropathies, infections, and cancer. Kidney diseases, especially acute pathologies linked to ischemia-reperfusion injury, are among illnesses that profoundly are affected by improper regulation or execution of cell death pathways. Attempts to develop medicines for kidney diseases have been impacted by the complexity of these pathologies given the heterogeneous patient population and diverse etiologies. By analyzing cell death pathways activated in kidney diseases, we attempt to differentiate their importance for these pathologies with a goal of identifying those that have more profound impact and the best therapeutic potential. Although classic apoptosis still might be important, regulated necrosis pathways including necroptosis, ferroptosis, parthanatos, and mitochondrial permeability transition-associated cell death play a significantly role in kidney diseases, especially in acute kidney pathologies. Although targeting receptor-interacting protein 1 kinase appears to be the best therapeutic strategy, combination with inhibitors of other cell death pathways is likely to bring superior benefit and possible cure to patients suffering from kidney diseases.

Use of RIP1 Kinase Small-Molecule Inhibitors in Studying Necroptosis

RIP1 kinase plays a key role in regulating signaling pathways downstream of a number of innate immune receptors such as TNFRI and TLRs. The discovery of Necrostatin-1 (Nec-1) as a small-molecule inhibitor of RIP1 kinase has been very instrumental in defining the necroptotic and other signalling pathways regulated by RIP1, but certain characteristics of Nec-1 limits its utility in experimental systems. Next generation RIP1 kinase inhibitors have been identified and the use of these tool inhibitors along with Nec-1 has revealed that RIP1 is emerging as a key driver of inflammation and tissue injury in the pathogenesis of various diseases. Further studying the role of RIP1 to carefully unravel the complex biology requires the selection of the correct tool small-molecule inhibitors. In addition, it is important to consider the proper application of current tool inhibitors and understand the current limitiations. Here we will discuss key parameters that need to be considered when selecting and applying tool inhibitors to novel biological assays and systems. General protocols to explore the in vitro and in vivo potency, cellular selectivity, and pharmacokinetic properties of current small-molecule inhibitors of RIP1 kinase are provided.

Necroptosis Promotes Staphylococcus aureus Clearance by Inhibiting Excessive Inflammatory Signaling

Staphylococcus aureus triggers inflammation through inflammasome activation and recruitment of neutrophils, responses that are critical for pathogen clearance but are associated with substantial tissue damage. We postulated that necroptosis, cell death mediated by the RIPK1/RIPK3/MLKL pathway, would function to limit pathological inflammation. In models of skin infection or sepsis, Mlkl-/- mice had high bacterial loads, an inability to limit interleukin-1b (IL-1b) production, and excessive inflammation. Similarly, mice treated with RIPK1 or RIPK3 inhibitors had increased bacterial loads in a model of sepsis. Ripk3-/- mice exhibited increased staphylococcal clearance and decreased inflammation in skin and systemic infection, due to direct effects of RIPK3 on IL-1b activation and apoptosis. In contrast to Casp1/4-/- mice with defective S. aureus killing, the poor outcomes of Mlkl-/- mice could not be attributed to impaired phagocytic function. We conclude that necroptotic cell death limits the pathological inflammation induced by S. aureus.