Arabidopsis PHYTOALEXIN DEFICIENT 4 promotes the maturation and nuclear accumulation of immune-related cysteine protease RD19

Arabidopsis PHYTOALEXIN DEFICIENT 4 (PAD4) has an essential role in pathogen resistance as a heterodimer with ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1). Here we investigated an additional PAD4 role in which it associates with and promotes the maturation of the immune-related cysteine protease RESPONSIVE TO DEHYDRATION 19 (RD19). We found that RD19 and its paralog RD19c promoted EDS1- and PAD4-mediated effector-triggered immunity to an avirulent Pseudomonas syringae strain, DC3000, expressing the effector AvrRps4 and basal immunity against the fungal pathogen Golovinomyces cichoracearum. Overexpression of RD19, but not RD19 protease-inactive catalytic mutants, in Arabidopsis transgenic lines caused EDS1- and PAD4-dependent autoimmunity and enhanced pathogen resistance. In these lines, RD19 maturation to a pro-form required its catalytic residues, suggesting that RD19 undergoes auto-processing. In transient assays, PAD4 interacted preferentially with the RD19 pro-protease and promoted its nuclear accumulation in leaf cells. Our results lead us to propose a model for PAD4-stimulated defense potentiation. PAD4 promotes maturation and nuclear accumulation of processed RD19, and RD19 then stimulates EDS1-PAD4 dimer activity to confer pathogen resistance. This study highlights potentially important additional PAD4 functions that eventually converge on canonical EDS1-PAD4 dimer signaling in plant immunity.

TIR-catalyzed ADP-ribosylation reactions produce signaling molecules for plant immunity

Plant pathogen-activated immune signaling by nucleotide-binding leucine-rich repeat (NLR) receptors with an N-terminal Toll/Interleukin-1 receptor (TIR) domain converges on Enhanced Disease Susceptibility 1 (EDS1) and its direct partners Phytoalexin Deficient 4 (PAD4) or Senescence-Associated Gene 101 (SAG101). TIR-encoded NADases produce signaling molecules to promote exclusive EDS1-PAD4 and EDS1-SAG101 interactions with helper NLR sub-classes. Here we show that TIR-containing proteins catalyze adenosine diphosphate (ADP)-ribosylation of adenosine triphosphate (ATP) and ADP ribose (ADPR) via ADPR polymerase-like and NADase activity, forming ADP-ribosylated ATP (ADPr-ATP) and ADPr-ADPR (di-ADPR), respectively. Specific binding of ADPr-ATP or di-ADPR allosterically promotes EDS1-SAG101 interaction with helper NLR N requirement gene 1A (NRG1A) in vitro and in planta . Our data reveal an enzymatic activity of TIRs that enables specific activation of the EDS1-SAG101-NRG1 immunity branch.

Identification and receptor mechanism of TIR-catalyzed small molecules in plant immunity

Plant nucleotide-binding leucine-rich repeat-containing (NLR) receptors with an N-terminal Toll/interleukin-1 receptor (TIR) domain sense pathogen effectors to enable TIR-encoded NADase activity for immune signaling. TIR-NLR signaling requires helper NLRs N requirement gene 1 (NRG1) and Activated Disease Resistance 1 (ADR1), and Enhanced Disease Susceptibility 1 (EDS1) that forms a heterodimer with each of its paralogs Phytoalexin Deficient 4 (PAD4) and Senescence-Associated Gene101 (SAG101). Here, we show that TIR-containing proteins catalyze production of 2'-(5′'-phosphoribosyl)-5′-adenosine mono-/di-phosphate (pRib-AMP/ADP) in vitro and in planta . Biochemical and structural data demonstrate that EDS1-PAD4 is a receptor complex for pRib-AMP/ADP, which allosterically promote EDS1-PAD4 interaction with ADR1-L1 but not NRG1A. Our study identifies TIR-catalyzed pRib-AMP/ADP as a missing link in TIR signaling via EDS1-PAD4 and as likely second messengers for plant immunity.

A new biochemistry connecting pathogen detection to induced defense in plants

Plant cell surface and intracellular immune receptors recognizing pathogen attack utilize the same defense machineries to mobilize resistance. New genetic, protein structural and biochemical information on receptor activation and signaling is transforming understanding of how their shared defense network operates. We discuss the biochemical activities of two classes of intracellular nucleotide-binding/leucine-rich repeat (NLR) receptor - one forming a Ca²⁺ channel, the other an NADase enzyme - which define engagement of enhanced disease susceptibility 1 (EDS1)-family heterodimers and cofunctioning helper NLRs (RNLs) to connect receptor systems and amplify defenses. Toll-interleukin-1 receptor (TIR) domain NLR receptors and TIR-domain proteins, with a capacity to produce NAD+-derived small molecules, require EDS1 dimers and RNLs for defense induction. The TIR-driven EDS1/RNL modules emerge as central elements in Ca²⁺ -based immunity signaling initiated by receptors outside and inside host cells.

PMR4-dependent cell wall depositions are a consequence but not the cause of temperature-induced autoimmunity

Plants possess a well-balanced immune system that is required for defense against pathogen infections. In autoimmune mutants or necrotic crosses, an intrinsic temperature-dependent imbalance leads to constitutive immune activation, resulting in severe damage or even death of plants. Recently, cell wall depositions were described as one of the symptoms following induction of the autoimmune phenotype in Arabidopsis saul1-1 mutants. However, the regulation and function of these depositions remained unclear. Here, we show that cell wall depositions, containing lignin and callose, were a common autoimmune feature and were deposited in proportion to the severity of the autoimmune phenotype at reduced ambient temperatures. When plants were exposed to reduced temperature for periods insufficient to induce an autoimmune phenotype, the cell wall depositions were not present. After low temperature intervals, sufficient to induce autoimmune responses, cell wall depositions correlated with a point of no return in saul1-1 autoimmunity. Although cell wall depositions were largely abolished in saul1-1 pmr4-1 double mutants lacking SAUL1 and the callose synthase gene GSL5/PMR4, their phenotype remained unchanged compared to that of the saul1-1 single mutant. Our data showed that cell wall depositions generally occur in autoimmunity, but appear not to be the cause of autoimmune phenotypes.

Discovery and pharmacological characterization of a novel potent inhibitor of diacylglycerol-sensitive TRPC cation channels

BACKGROUND AND PURPOSE

The cation channel transient receptor potential canonical (TRPC) 6 has been associated with several pathologies including focal segmental glomerulosclerosis, pulmonary hypertension and ischaemia reperfusion-induced lung oedema. We set out to discover novel inhibitors of TRPC6 channels and investigate the therapeutic potential of these agents.

EXPERIMENTAL APPROACH

A library of potential TRPC channel inhibitors was designed and synthesized. Activity of the compounds was assessed by measuring intracellular Ca(2+) levels. The lead compound SAR7334 was further characterized by whole-cell patch-clamp techniques. The effects of SAR7334 on acute hypoxic pulmonary vasoconstriction (HPV) and systemic BP were investigated.

KEY RESULTS

SAR7334 inhibited TRPC6, TRPC3 and TRPC7-mediated Ca(2+) influx into cells with IC50 s of 9.5, 282 and 226 nM, whereas TRPC4 and TRPC5-mediated Ca(2+) entry was not affected. Patch-clamp experiments confirmed that the compound blocked TRPC6 currents with an IC50 of 7.9 nM. Furthermore, SAR7334 suppressed TRPC6-dependent acute HPV in isolated perfused lungs from mice. Pharmacokinetic studies of SAR7334 demonstrated that the compound was suitable for chronic oral administration. In an initial short-term study, SAR7334 did not change mean arterial pressure in spontaneously hypertensive rats (SHR).

CONCLUSIONS AND IMPLICATIONS

Our results confirm the role of TRPC6 channels in hypoxic pulmonary vasoregulation and indicate that these channels are unlikely to play a major role in BP regulation in SHR. SAR7334 is a novel, highly potent and bioavailable inhibitor of TRPC6 channels that opens new opportunities for the investigation of TRPC channel function in vivo.

Discovery and evaluation of piperidinyl carboxylic acid derivatives as potent alpha(4)beta(1) integrin antagonists

Piperidinyl carboxylic acid-based derivatives were prepared as antagonists of the leukocyte cell adhesion process that is mediated through the interaction of the alpha(4)beta(1) integrin (VLA-4, very late antigen 4) and the vascular cell adhesion molecule 1 (VCAM-1). Compounds 2a-h inhibited the adhesion in a cell-based assay in the low and sub micromolar range, a pharmacokinetic study of 2d is reported.

Biosynthesis of a neo-epi-verrucosane diterpene in the liverwort Fossombronia alaskana. A retrobiosynthetic NMR study

The biosynthesis of the diterpene 8alpha-acetoxy-13alpha-hydroxy-5-oxo-13-epi- neoverrucosane in the arctic liverwort Fossombronia alaskana was studied by incorporation experiments using [1-(13)C]- and [U-(13)C(6)]glucose as precursors. The (13)C-labeling patterns of acetyl-CoA, pyruvate, and phosphoenolpyruvate in intermediary metabolism were reconstructed from the (13)C NMR data of biosynthetic amino acids (leucine, alanine, phenylalanine) and were used to predict hypothetical labeling patterns for isopentenyl pyrophosphate formed via the mevalonate pathway and the deoxyxylulose pathway. The labeling patterns observed for the neoverrucosane diterpene were consistent with the intermediate formation of geranyllinaloyl pyrophosphate assembled from dimethylallyl pyrophosphate and three molecules of isopentenyl pyrophosphate generated predominantly or entirely via 1-deoxyxylulose 5-phosphate. The experimental data can be integrated into a detailed biosynthetic scheme involving a 1,5-hydride shift. The postulated involvement of the 1,5-hydride shift was confirmed by an incorporation experiment with [6,6-(2)H(2)]glucose.

Structure of a stereoregular phosphorothioate DNA/RNA duplex

In this work, we present the first NMR solution structure of a DNA/RNA hybrid containing stereoregular Rp-phosphorothioate modifications of all DNA backbone linkages. The complex of the enzymatically synthesized phosphorothioate DNA octamer (all-Rp)-d(GCGTCAGG) and its complementary RNA r(CCUGACGC) was found to adopt an overall conformation within the A-form family. Most helical parameters and the sugar puckers of the DNA strand assume values intermediate between A- and B-form. The close structural similarity with the unmodified DNA/RNA hybrid of the same sequence may explain why both the natural and the sulfur-substituted complex can be recognized and digested by ribonuclease H.