Co-expression of recombinant RIPK3:MLKL complexes using the baculovirus-insect cell system

Chemometrics and analytical blank on the at-line monitoring of Zika-VLP production using near-infrared spectroscopy

The Zika disease caused by the Zika virus was declared a Public Health Emergency by the World Health Union (WHO), with microcephaly as the most critical consequence. Aiming to reduce the spread of the virus, biopharmaceutical organizations invest in vaccine research and production, based on multiple platforms. A crescent vaccine production approach is based on virus-like particles (VLP), for not having genetic material in its composition, hypoallergenic and non-mutant character. For bioprocess, it is essential to have means of real-time monitoring, which can be assessed using process analysis techniques such as Near-infrared (NIR) spectroscopy, that can be combined with chemometric methods, like Partial-Least Squares (PLS) and Artificial Neural Networks (ANN) for prediction of biochemical variables. This work proposes a biochemical Zika VLP upstream production at-line monitoring model using NIR spectroscopy comparing sampling conditions (with or without cells), analytical blank (air, ultrapure water), and spectra pre-processing approaches. Seven experiments in a benchtop bioreactor using recombinant baculovirus/Sf9 insect cell platform in serum-free medium were performed to obtain biochemical and spectral data for chemometrics modeling (PLS and ANN), composed by a random data split (80 % calibration, 20 % validation) for cross-validation of the PLS models and 70 % training, 15 % testing, 15 % validation for ANN. The best models generated in the present work presented an average absolute error of 1.59 × 105 cell/mL for density of viable cells, 2.37 % for cell viability, 0.25 g/L for glucose, 0.007 g/L for lactate, 0.138 g/L for glutamine, 0.18 g/L for glutamate, 0,003 g/L for ammonium, and 0.014 g/L for potassium.

Large-scale genomic investigation of pediatric cholestasis reveals a novel hepatorenal ciliopathy caused by PSKH1 mutations

PURPOSE

Pediatric cholestasis is the phenotypic expression of clinically and genetically heterogeneous disorders of bile acid synthesis and flow. Although a growing number of monogenic causes of pediatric cholestasis have been identified, the majority of cases remain undiagnosed molecularly.

METHODS

In a cohort of 299 pediatric participants (279 families) with intrahepatic cholestasis, we performed exome sequencing as a first-tier diagnostic test.

RESULTS

A likely causal variant was identified in 135 families (48.56%). These comprise 135 families that harbor variants spanning 37 genes with established or tentative links to cholestasis. In addition, we propose a novel candidate gene (PSKH1) (HGNC:9529) in 4 families. PSKH1 was particularly compelling because of strong linkage in 3 consanguineous families who shared a novel hepatorenal ciliopathy phenotype. Two of the 4 families shared a founder homozygous variant, whereas the third and fourth had different homozygous variants in PSKH1. PSKH1 encodes a putative protein serine kinase of unknown function. Patient fibroblasts displayed abnormal cilia that are long and show abnormal transport. A homozygous Pskh1 mutant mouse faithfully recapitulated the human phenotype and displayed abnormally long cilia. The phenotype could be rationalized by the loss of catalytic activity observed for each recombinant PSKH1 variant using in vitro kinase assays.

CONCLUSION

Our results support the use of genomics in the workup of pediatric cholestasis and reveal PSKH1-related hepatorenal ciliopathy as a novel candidate monogenic form.

Phosphorylation-dependent pseudokinase domain dimerization drives full-length MLKL oligomerization

The necroptosis pathway is a lytic, pro-inflammatory mode of cell death that is widely implicated in human disease, including renal, pulmonary, gut and skin inflammatory pathologies. The precise mechanism of the terminal steps in the pathway, where the RIPK3 kinase phosphorylates and triggers a conformation change and oligomerization of the terminal pathway effector, MLKL, are only emerging. Here, we structurally identify RIPK3-mediated phosphorylation of the human MLKL activation loop as a cue for MLKL pseudokinase domain dimerization. MLKL pseudokinase domain dimerization subsequently drives formation of elongated homotetramers. Negative stain electron microscopy and modelling support nucleation of the MLKL tetramer assembly by a central coiled coil formed by the extended, ~80 Å brace helix that connects the pseudokinase and executioner four-helix bundle domains. Mutational data assert MLKL tetramerization as an essential prerequisite step to enable the release and reorganization of four-helix bundle domains for membrane permeabilization and cell death.

A common human MLKL polymorphism confers resistance to negative regulation by phosphorylation

Across the globe, 2-3% of humans carry the p.Ser132Pro single nucleotide polymorphism in MLKL, the terminal effector protein of the inflammatory form of programmed cell death, necroptosis. Here we show that this substitution confers a gain in necroptotic function in human cells, with more rapid accumulation of activated MLKLS132P in biological membranes and MLKLS132P overriding pharmacological and endogenous inhibition of MLKL. In mouse cells, the equivalent Mlkl S131P mutation confers a gene dosage dependent reduction in sensitivity to TNF-induced necroptosis in both hematopoietic and non-hematopoietic cells, but enhanced sensitivity to IFN-β induced death in non-hematopoietic cells. In vivo, MlklS131P homozygosity reduces the capacity to clear Salmonella from major organs and retards recovery of hematopoietic stem cells. Thus, by dysregulating necroptosis, the S131P substitution impairs the return to homeostasis after systemic challenge. Present day carriers of the MLKL S132P polymorphism may be the key to understanding how MLKL and necroptosis modulate the progression of complex polygenic human disease.

Human RIPK3 C-lobe phosphorylation is essential for necroptotic signaling

Necroptosis is a caspase-independent, pro-inflammatory mode of programmed cell death which relies on the activation of the terminal effector, MLKL, by the upstream protein kinase RIPK3. To mediate necroptosis, RIPK3 must stably interact with, and phosphorylate the pseudokinase domain of MLKL, although the precise molecular cues that provoke RIPK3 necroptotic signaling are incompletely understood. The recent finding that RIPK3 S227 phosphorylation and the occurrence of a stable RIPK3:MLKL complex in human cells prior to exposure to a necroptosis stimulus raises the possibility that additional, as-yet-unidentified phosphorylation events activate RIPK3 upon initiation of necroptosis signaling. Here, we sought to identify phosphorylation sites of RIPK3 and dissect their regulatory functions. Phosphoproteomics identified 21 phosphorylation sites in HT29 cells overexpressing human RIPK3. By comparing cells expressing wild-type and kinase-inactive D142N RIPK3, autophosphorylation sites and substrates of other cellular kinases were distinguished. Of these 21 phosphosites, mutational analyses identified only pT224 and pS227 as crucial, synergistic sites for stable interaction with MLKL to promote necroptosis, while the recently reported activation loop phosphorylation at S164/T165 negatively regulate the kinase activity of RIPK3. Despite being able to phosphorylate MLKL to a similar or higher extent than wild-type RIPK3, mutation of T224, S227, or the RHIM in RIPK3 attenuated necroptosis. This finding highlights the stable recruitment of human MLKL by RIPK3 to the necrosome as an essential checkpoint in necroptosis signaling, which is independent from and precedes the phosphorylation of MLKL.