Phos-tag SDS-PAGE resolves agonist- and isoform-specific activation patterns for PKD2 and PKD3 in cardiomyocytes and cardiac fibroblasts

Effect of deletion of the protein kinase PRKD1 on development of the mouse embryonic heart

Congenital heart disease (CHD) is the most common congenital anomaly, with an overall incidence of approximately 1% in the United Kingdom. Exome sequencing in large CHD cohorts has been performed to provide insights into the genetic aetiology of CHD. This includes a study of 1891 probands by our group in collaboration with others, which identified three novel genes-CDK13, PRKD1, and CHD4, in patients with syndromic CHD. PRKD1 encodes a serine/threonine protein kinase, which is important in a variety of fundamental cellular functions. Individuals with a heterozygous mutation in PRKD1 may have facial dysmorphism, ectodermal dysplasia and may have CHDs such as pulmonary stenosis, atrioventricular septal defects, coarctation of the aorta and bicuspid aortic valve. To obtain a greater appreciation for the role that this essential protein kinase plays in cardiogenesis and CHD, we have analysed a Prkd1 transgenic mouse model (Prkd1em1) carrying deletion of exon 2, causing loss of function. High-resolution episcopic microscopy affords detailed morphological 3D analysis of the developing heart and provides evidence for an essential role of Prkd1 in both normal cardiac development and CHD. We show that homozygous deletion of Prkd1 is associated with complex forms of CHD such as atrioventricular septal defects, and bicuspid aortic and pulmonary valves, and is lethal. Even in heterozygotes, cardiac differences occur. However, given that 97% of Prkd1 heterozygous mice display normal heart development, it is likely that one normal allele is sufficient, with the defects seen most likely to represent sporadic events. Moreover, mRNA and protein expression levels were investigated by RT-qPCR and western immunoblotting, respectively. A significant reduction in Prkd1 mRNA levels was seen in homozygotes, but not heterozygotes, compared to WT littermates. While a trend towards lower PRKD1 protein expression was seen in the heterozygotes, the difference was only significant in the homozygotes. There was no compensation by the related Prkd2 and Prkd3 at transcript level, as evidenced by RT-qPCR. Overall, we demonstrate a vital role of Prkd1 in heart development and the aetiology of CHD.

Small molecule inhibitor CRT0066101 inhibits cytokine storm syndrome in a mouse model of lung injury

Pneumonia is an acute inflammation of the lungs induced by pathogenic microorganisms, immune damage, physical and chemical factors, and other factors, and the latest outbreak of novel coronavirus pneumonia is also an acute lung injury (ALI) induced by viral infection. However, there are currently no effective treatments for inflammatory cytokine storms in patients with ALI/acute respiratory distress syndrome (ARDS). Protein kinase D (PKD) is a highly active kinase that has been shown to be associated with the production of inflammatory cytokines. Therefore, small-molecule compounds that inhibit PKD may be potential drugs for the treatment of ALI/ARDS. In the present study, we evaluated the ability of the small-molecule inhibitor CRT0066101 to attenuate lipopolysaccharide (LPS)-induced inflammatory cytokine production through in vitro cell experiments and a mouse pneumonia model. We found that CRT0066101 significantly reduced the protein and mRNA levels of LPS-induced cytokines (e.g., IL-6, TNF-α, and IL-1β). CRT0066101 inhibited MyD88 and TLR4 expression and reduced NF-κB, ERK, and JNK phosphorylation. CRT0066101 also reduced NLRP3 activation, inhibited the assembly of the inflammasome complex, and attenuated inflammatory cell infiltration and lung tissue damage. Taken together, our data indicate that CRT0066101 exerts anti-inflammatory effects on LPS-induced inflammation through the TLR4/MyD88 signaling pathway, suggesting that CRT0066101 may have therapeutic value in acute lung injury and other MyD88-dependent inflammatory diseases.

DNA-dependent protein kinase catalytic subunit (DNA-PKcs) drives chronic kidney disease progression in male mice

Kidney injury initiates epithelial dedifferentiation and myofibroblast activation during the progression of chronic kidney disease. Herein, we find that the expression of DNA-PKcs is significantly increased in the kidney tissues of both chronic kidney disease patients and male mice induced by unilateral ureteral obstruction and unilateral ischemia-reperfusion injury. In vivo, knockout of DNA-PKcs or treatment with its specific inhibitor NU7441 hampers the development of chronic kidney disease in male mice. In vitro, DNA-PKcs deficiency preserves epithelial cell phenotype and inhibits fibroblast activation induced by transforming growth factor-beta 1. Additionally, our results show that TAF7, as a possible substrate of DNA-PKcs, enhances mTORC1 activation by upregulating RAPTOR expression, which subsequently promotes metabolic reprogramming in injured epithelial cells and myofibroblasts. Taken together, DNA-PKcs can be inhibited to correct metabolic reprogramming via the TAF7/mTORC1 signaling in chronic kidney disease, and serve as a potential target for treating chronic kidney disease.

Decoding the Cardiac Actions of Protein Kinase D Isoforms

Protein kinase D (PKD) consists of a family of three structurally related enzymes that play key roles in a wide range of biological functions that contribute to the evolution of cardiac hypertrophy and heart failure. PKD1 (the founding member of this enzyme family) has been implicated in the phosphorylation of substrates that regulate cardiac hypertrophy, contraction, and susceptibility to ischemia/reperfusion injury, and de novo PRKD1 (protein kinase D1 gene) mutations have been identified in patients with syndromic congenital heart disease. However, cardiomyocytes coexpress all three PKDs. Although stimulus-specific activation patterns for PKD1, PKD2, and PKD3 have been identified in cardiomyocytes, progress toward identifying PKD isoform-specific functions in the heart have been hampered by significant gaps in our understanding of the molecular mechanisms that regulate PKD activity. This review incorporates recent conceptual breakthroughs in our understanding of various alternative mechanisms for PKD activation, with an emphasis on recent evidence that PKDs activate certain effector responses as dimers, to consider the role of PKD isoforms in signaling pathways that drive cardiac hypertrophy and ischemia/reperfusion injury. The focus is on whether the recently identified activation mechanisms that enhance the signaling repertoire of PKD family enzymes provide novel therapeutic strategies to target PKD enzymes and prevent or slow the evolution of cardiac injury and pathological cardiac remodeling. SIGNIFICANCE STATEMENT: PKD isoforms regulate a large number of fundamental biological processes, but the understanding of the biological actions of individual PKDs (based upon studies using adenoviral overexpression or gene-silencing methods) remains incomplete. This review focuses on dimerization, a recently identified mechanism for PKD activation, and the notion that this mechanism provides a strategy to develop novel PKD-targeted pharmaceuticals that restrict proliferation, invasion, or angiogenesis in cancer and prevent or slow the evolution of cardiac injury and pathological cardiac remodeling.

Mn2+ -Phos-Tag Polyacrylamide for the Quantification of Protein Phosphorylation Levels

This paper provides a guideline for optimizing and utilizing Mn²⁺ Phos-tag gel technology to separate phosphorylated proteins from their unphosphorylated counterparts. It provides key insights into methods for careful sample preparation and experimental directions for determining the appropriate Phos-tag gel compositions and electrophoresis and western blotting conditions. This protocol has been used to successfully resolve proteins extracted from cardiac and skeletal muscles. The guidelines can be extended for optimizing protocols to resolve proteins from other cells or tissue sources. With this, phosphoproteomics and the elucidation of underlying mechanisms of disease progression can be accelerated. © 2021 The Authors. Current Protocols published by Wiley Periodicals LLC.

Novel Autosomal Recessive Splice-Altering Variant in PRKD1 Is Associated with Congenital Heart Disease

Congenital heart defects (CHDs) are the most common types of birth defects, and global incidence of CHDs is on the rise. Despite the prevalence of CHDs, the genetic determinants of the defects are still in the process of being identified. Herein, we report a consanguineous Saudi family with three CHD affected daughters. We used whole exome sequencing (WES) to investigate the genetic cause of CHDs in the affected daughters. We found that all affected individuals were homozygous for a novel splice-altering variant (NM_001330069.1: c.265-1G>T) of PRKD1, which encodes a calcium/calmodulin-dependent protein kinase in the heart. The homozygous variant was found in the affected patients with Pulmonary Stenosis (PS), Truncus Arteriosis (TA), and Atrial Septal Defect (ASD). Based on the family’s pedigree, the variant acts in an autosomal recessive manner, which makes it the second autosomal recessive variant of PRKD1 to be identified with a link to CHDs, while all other previously described variants act dominantly. Interestingly, the father of the affected daughters was also homozygous for the variant, though he was asymptomatic of CHDs himself. Since both of his sisters had CHDs as well, this raises the possibility that the novel PRKD1 variant may undergo autosomal recessive inheritance mode with gender limitation. This finding confirms that CHD can be associated with both dominant and recessive mutations of the PRKD1 gene, and it provides a new insight to genotype–phenotype association between PRKD1 and CHDs. To our knowledge, this is the first report of this specific PRKD1 mutation associated with CHDs.

PKD3 promotes metastasis and growth of oral squamous cell carcinoma through positive feedback regulation with PD-L1 and activation of ERK-STAT1/3-EMT signalling

Oral squamous cell carcinoma (OSCC) has a high incidence of metastasis. Tumour immunotherapy targeting PD-L1 or PD-1 has been revolutionary; however, only a few patients with OSCC respond to this treatment. Therefore, it is essential to gain insights into the molecular mechanisms underlying the growth and metastasis of OSCC. In this study, we analysed the expression levels of protein kinase D3 (PKD3) and PD-L1 and their correlation with the expression of mesenchymal and epithelial markers. We found that the expression of PKD3 and PD-L1 in OSCC cells and tissues was significantly increased, which correlated positively with that of mesenchymal markers but negatively with that of epithelial markers. Silencing PKD3 significantly inhibited the growth, metastasis and invasion of OSCC cells, while its overexpression promoted these processes. Our further analyses revealed that there was positive feedback regulation between PKD3 and PD-L1, which could drive EMT of OSCC cells via the ERK/STAT1/3 pathway, thereby promoting tumour growth and metastasis. Furthermore, silencing PKD3 significantly inhibited the expression of PD-L1, and lymph node metastasis of OSCC was investigated with a mouse footpad xenograft model. Thus, our findings provide a theoretical basis for targeting PKD3 as an alternative method to block EMT for regulating PD-L1 expression and inhibiting OSCC growth and metastasis.

Protein kinase D3 regulates the expression of the immunosuppressive protein, PD‑L1, through STAT1/STAT3 signaling

Oral squamous cell carcinoma (OSCC) is capable of constructing a favorable immune escape environment through interactions of cells with cells and of cells with the environment. Programmed death ligand-1 (PD-L1) is a well-recognized inhibitor of anti-tumor immunity that plays an important role in tumor immune escape. However, the molecular mechanisms regulating PD-L1 expression are not yet fully understood. In this study, to investigate the role of protein kinase D3 (PKD3) in the regulation of PD-L1 expression, the expression and correlation of PKD3 and PD-L1 were first analyzed by the immunostaining of human OSCC tissue sections, cell experiments and TCGA gene expression databases. The expression levels of PKD3 and PD-L1 were found to be significantly higher in OSCC cells than in normal tissues or cells. In addition, the expression levels of PKD3 and PD-L1 were found to be significantly positively correlated. Subsequently, it was found that the levsel of PD-L1 expression decreased following the silencing of PKD3 and that the ability of interferon (IFN)-γ to induce PD-L1 expression was also decreased in OSCC. The opposite phenomenon occurred following the overexpression of PKD3. It was also found that the phosphorylation of signal transducer and activator of transcription (STAT)1/STAT3 was reduced by the knockdown of PKD3 in OSCC. Moreover, the expression level of PD-L1 was decreased after the use of siRNA to knockdown STAT1 or STAT3. On the whole, the findings of this study confirm that PKD3 regulates the expression of PD-L1 induced by IFN-γ by regulating the phosphorylation of STAT1/STAT3. These findings broaden the understanding of the biological function of PKD3, suggesting that PKD is a potential therapeutic target for OSCC.

Effect of Sphingosine-1-Phosphate on Intracellular Free Ca²⁺ in Cat Esophageal Smooth Muscle Cells

A comprehensive collection of proteins senses local changes in intracellular Ca²⁺ concentrations ([Ca²⁺]_i) and transduces these signals into responses to agonists. In the present study, we examined the effect of sphingosine-1-phosphate (S1P) on modulation of intracellular Ca²⁺ concentrations in cat esophageal smooth muscle cells. To measure [Ca²⁺]_i levels in cat esophageal smooth muscle cells, we used a fluorescence microscopy with the Fura-2 loading method. S1P produced a concentration-dependent increase in [Ca²⁺]_i in the cells. Pretreatment with EGTA, an extracellular Ca²⁺ chelator, decreased the S1P-induced increase in [Ca²⁺]_i, and an L-type Ca²⁺-channel blocker, nimodipine, decreased the effect of S1P. This indicates that Ca²⁺ influx may be required for muscle contraction by S1P. When stimulated with thapsigargin, an intracellular calcium chelator, or 2-Aminoethoxydiphenyl borate (2-APB), an InsP₃ receptor blocker, the S1P-evoked increase in [Ca²⁺]_i was significantly decreased. Treatment with pertussis toxin (PTX), an inhibitor of G_i-protein, suppressed the increase in [Ca²⁺]_i evoked by S1P. These results suggest that the S1P-induced increase in [Ca²⁺]_i in cat esophageal smooth muscle cells occurs upon the activation of phospholipase C and subsequent release of Ca²⁺ from the InsP₃-sensitive Ca²⁺ pool in the sarcoplasmic reticulum. These results suggest that S1P utilized extracellular Ca²⁺ via the L type Ca²⁺ channel, which was dependent on activation of the S1P₄ receptor coupled to PTX-sensitive G_i protein, via phospholipase C-mediated Ca²⁺ release from the InsP₃-sensitive Ca²⁺ pool in cat esophageal smooth muscle cells.

Acute and Chronic Effects of Protein Kinase-D Signaling on Cardiac Energy Metabolism

Protein kinase-D (PKD) is increasingly recognized as a key regulatory signaling hub in cardiac glucose uptake and also a major player in the development of hypertrophy. Glucose is one of the predominant energy substrates for the heart to support contraction. However, a cardiac substrate switch toward glucose over-usage is associated with the development of cardiac hypertrophy. Hence, regulation of PKD activity must be strictly coordinated. This review provides mechanistic insights into the acute and chronic regulatory functions of PKD signaling in the healthy and hypertrophied heart. First an overview of the activation pathways of PKD1, the most abundant isoform in the heart, is provided. Then the various regulatory roles of the PKD isoforms in the heart in relation to cardiac glucose and fatty acid metabolism, contraction, morphology, function, and the development of cardiac hypertrophy are described. Finally, these findings are integrated and the possibility of targeting this kinase as a novel strategy to combat cardiac diseases is discussed.

Function and Regulation of Protein Kinase D in Oxidative Stress: A Tale of Isoforms

Oxidative stress is a condition that arises when cells are faced with levels of reactive oxygen species (ROS) that destabilize the homeostatic redox balance. High levels of ROS can cause damage to macromolecules including DNA, lipids, and proteins, eventually resulting in cell death. Moderate levels of ROS however serve as signaling molecules that can drive and potentiate several cellular phenotypes. Increased levels of ROS are associated with a number of diseases including neurological disorders and cancer. In cancer, increased ROS levels can contribute to cancer cell survival and proliferation via the activation of several signaling pathways. One of the downstream effectors of increased ROS is the protein kinase D (PKD) family of kinases. In this review, we will discuss the regulation and function of this family of ROS-activated kinases and describe their unique isoform-specific features, in terms of both kinase regulation and signaling output.

Differential regulation of PKD isoforms in oxidative stress conditions through phosphorylation of a conserved Tyr in the P+1 loop

Protein kinases are essential molecules in life and their crucial function requires tight regulation. Many kinases are regulated via phosphorylation within their activation loop. This loop is embedded in the activation segment, which additionally contains the Mg²⁺ binding loop and a P + 1 loop that is important in substrate binding. In this report, we identify Abl-mediated phosphorylation of a highly conserved Tyr residue in the P + 1 loop of protein kinase D2 (PKD2) during oxidative stress. Remarkably, we observed that the three human PKD isoforms display very different degrees of P + 1 loop Tyr phosphorylation and we identify one of the molecular determinants for this divergence. This is paralleled by a different activation mechanism of PKD1 and PKD2 during oxidative stress. Tyr phosphorylation in the P + 1 loop of PKD2 increases turnover for Syntide-2, while substrate specificity and the role of PKD2 in NF-κB signaling remain unaffected. Importantly, Tyr to Phe substitution renders the kinase inactive, jeopardizing its use as a non-phosphorylatable mutant. Since large-scale proteomics studies identified P + 1 loop Tyr phosphorylation in more than 70 Ser/Thr kinases in multiple conditions, our results do not only demonstrate differential regulation/function of PKD isoforms under oxidative stress, but also have implications for kinase regulation in general.

Emergency Spatiotemporal Shift: The Response of Protein Kinase D to Stress Signals in the Cardiovascular System

Protein Kinase D isoforms (PKD 1-3) are key mediators of neurohormonal, oxidative, and metabolic stress signals. PKDs impact a wide variety of signaling pathways and cellular functions including actin dynamics, vesicle trafficking, cell motility, survival, contractility, energy substrate utilization, and gene transcription. PKD activity is also increasingly linked to cancer, immune regulation, pain modulation, memory, angiogenesis, and cardiovascular disease. This increasing complexity and diversity of PKD function, highlights the importance of tight spatiotemporal control of the kinase via protein–protein interactions, post-translational modifications or targeting via scaffolding proteins. In this review, we focus on the spatiotemporal regulation and effects of PKD signaling in response to neurohormonal, oxidant and metabolic signals that have implications for myocardial disease. Precise targeting of these mechanisms will be crucial in the design of PKD-based therapeutic strategies.