Tale of cAMP as a second messenger in auxin signaling and beyond

Nitric oxide and cytokinin cross-talk and their role in plant hypoxia response

Nitric oxide (NO) and cytokinins (CKs) are known for their crucial contributions to plant development, growth, senescence, and stress response. Despite the importance of both signals in stress responses, their interaction remains largely unexplored. The interplay between NO and CKs emerges as particularly significant not only regarding plant growth and development but also in addressing plant stress response, particularly in the context of extreme weather events leading to yield loss. In this review, we summarize NO and CKs metabolism and signaling. Additionally, we emphasize the crosstalk between NO and CKs, underscoring its potential impact on stress response, with a focus on hypoxia tolerance. Finally, we address the most urgent questions that demand answers and offer recommendations for future research endeavors.

Protein degradation in auxin response

The signaling molecule auxin sits at the nexus of plant biology where it coordinates essentially all growth and developmental processes. Auxin molecules are transported throughout plant tissues and are capable of evoking highly specific physiological responses by inducing various molecular pathways. In many of these pathways, proteolysis plays a crucial role for correct physiological responses. This review provides a chronology of the discovery and characterization of the auxin receptor, which is a fascinating example of separate research trajectories ultimately converging on the discovery of a core auxin signaling hub that relies on degradation of a family of transcriptional inhibitor proteins-the Aux/IAAs. Beyond describing the "classical" proteolysis-driven auxin response system, we explore more recent examples of the interconnection of proteolytic systems, which target a range of other auxin signaling proteins, and auxin response. By highlighting these emerging concepts, we provide potential future directions to further investigate the role of protein degradation within the framework of auxin response.

Moonlighting Crypto-Enzymes and Domains as Ancient and Versatile Signaling Devices

Increasing numbers of reports have revealed novel catalytically active cryptic guanylate cyclases (GCs) and adenylate cyclases (ACs) operating within complex proteins in prokaryotes and eukaryotes. Here we review the structural and functional aspects of some of these cyclases and provide examples that illustrate their roles in the regulation of the intramolecular functions of complex proteins, such as the phytosulfokine receptor (PSKR), and reassess their contribution to signal generation and tuning. Another multidomain protein, Arabidopsis thaliana K+ uptake permease (AtKUP5), also harbors multiple catalytically active sites including an N-terminal AC and C-terminal phosphodiesterase (PDE) with an abscisic acid-binding site. We argue that this architecture may enable the fine-tuning and/or sensing of K+ flux and integrate hormone responses to cAMP homeostasis. We also discuss how searches with motifs based on conserved amino acids in catalytic centers led to the discovery of GCs and ACs and propose how this approach can be applied to discover hitherto masked active sites in bacterial, fungal, and animal proteomes. Finally, we show that motif searches are a promising approach to discover ancient biological functions such as hormone or gas binding.

A SNF1-related protein kinase regulatory subunit functions as a molecular tuner

Metabolic processes in prokaryotic and eukaryotic organisms are often modulated by kinases which are in turn, dependent on Ca2+ and the cyclic mononucleotides cAMP and cGMP. It has been established that some proteins have both kinase and cyclase activities and that active cyclases can be embedded within the kinase domains. Here, we identified phosphodiesterase (PDE) sites, enzymes that hydrolyse cAMP and cGMP, to AMP and GMP, respectively, in some of these proteins in addition to their kinase/cyclase twin-architecture. As an example, we tested the Arabidopsis thaliana KINγ, a subunit of the SnRK2 kinase, to demonstrate that all three enzymatic centres, adenylate cyclase (AC), guanylate cyclase (GC) and PDE, are catalytically active, capable of generating and hydrolysing cAMP and cGMP. These data imply that the signal output of the KINγ subunit modulates SnRK2, consequently affecting the downstream kinome. Finally, we propose a model where a single protein subunit, KINγ, is capable of regulating cyclic mononucleotide homeostasis, thereby tuning stimulus specific signal output.

Cyclic nucleotides - the rise of a family

Cyclic nucleotides 3',5'-cAMP and 3',5'-cGMP are now established signaling components of the plant cell while their 2',3' positional isomers are increasingly recognized as such. 3',5'-cAMP/cGMP is generated by adenylate cyclases (ACs) or guanylate cyclases (GCs) from ATP or GTP, respectively, whereas 2',3'-cAMP/cGMP is produced through the hydrolysis of double-stranded DNA or RNA by synthetases. Recent evidence suggests that the cyclic nucleotide generating and inactivating enzymes moonlight in proteins with diverse domain architecture operating as molecular tuners to enable dynamic and compartmentalized regulation of cellular signals. Further characterization of such moonlighting enzymes and extending the studies to noncanonical cyclic nucleotides promises new insights into the complex regulatory networks that underlie plant development and responses, thus offering exciting opportunities for crop improvement.

TIMP2 protects against sepsis-associated acute kidney injury by cAMP/NLRP3 axis-mediated pyroptosis

The tissue inhibitor of metalloproteinases 2 (TIMP2) has emerged as a promising biomarker for predicting the risk of sepsis-associated acute kidney injury (SA-AKI). However, its exact role in SA-AKI and the underlying mechanism remains unclear. In this study, we investigated the impact of kidney tubule-specific Timp2 knockout mice on kidney injury and inflammation. Our findings demonstrated that Timp2-knockout mice exhibited more severe kidney injury than wild-type mice, along with elevated levels of pyroptosis markers NOD-like receptor protein 3 (NLRP3), Caspase1, and gasdermin D (GSDMD) in the early stage of SA-AKI. Conversely, the expression of exogenous TIMP2 in TIMP2-knockout mice still protected against kidney damage and inflammation. In in vitro experiments, using recombinant TIMP2 protein, TIMP2 knockdown demonstrated that exogenous TIMP2 inhibited pyroptosis of renal tubular cells stimulated by lipopolysaccharide (LPS). Mechanistically, TIMP2 promoted the ubiquitination and autophagy-dependent degradation of NLRP3 by increasing intracellular cyclic adenosine monophosphate (cAMP), which mediated NLRP3 degradation through recruiting the E3 ligase MARCH7, attenuating downstream pyroptosis, and thus alleviating primary tubular cell damage. These results revealed the renoprotective role of extracellular TIMP2 in SA-AKI by attenuating tubular pyroptosis, and suggested that exogenous administration of TIMP2 could be a promising therapeutic intervention for SA-AKI treatment.NEW & NOTEWORTHY Tissue inhibitor of metalloproteinase 2 (TIMP-2) has been found to be the best biomarker for predicting the risk of sepsis-associated acute kidney injury (SA-AKI). However, its role and the underlying mechanism in SA-AKI remain elusive. The authors demonstrated in this study using kidney tubule-specific knockout mice model of SA-AKI and primary renal tubule cells stimulated with lipopolysaccharide (LPS) that extracellular TIMP-2 promoted NOD-like receptor protein 3 (NLRP3) ubiquitination and autophagy-dependent degradation by increasing intracellular cyclic adenosine monophosphate (cAMP), thus attenuated pyroptosis and alleviated renal damage.