Protein kinase D regulates basolateral membrane protein exit from trans-Golgi network

Protein kinase D2 modulates hepatic insulin sensitivity in male mice

OBJECTIVES

Protein kinase D (PKD) family is emerging as relevant regulator of metabolic homeostasis. However, the precise role of PKD2 in modulating hepatic insulin signaling has not been fully elucidated and it is the aim of this study.

METHODS

PKD inhibition was analyzed for insulin signaling in mouse and human hepatocytes. PKD2 was overexpressed in Huh7 hepatocytes and mouse liver, and insulin responses were evaluated. Mice with hepatocyte-specific PKD2 depletion (PKD2ΔHep) and PKD2fl/fl mice were fed a chow (CHD) or high fat diet (HFD) and glucose homeostasis and lipid metabolism were investigated.

RESULTS

PKD2 silencing enhanced insulin signaling in hepatocytes, an effect also found in primary hepatocytes from PKD2ΔHep mice. Conversely, a constitutively active PKD2 mutant reduced insulin-stimulated AKT phosphorylation. A more in-depth analysis revealed reduced IRS1 serine phosphorylation under basal conditions and increased IRS1 tyrosine phosphorylation in PKD2ΔHep primary hepatocytes upon insulin stimulation and, importantly PKD co-immunoprecipitates with IRS1. In vivo constitutively active PKD2 overexpression resulted in a moderate impairment of glucose homeostasis and reduced insulin signaling in the liver. On the contrary, HFD-fed PKD2ΔHep male mice displayed improved glucose and pyruvate tolerance, as well as higher peripheral insulin tolerance and enhanced hepatic insulin signaling compared to control PKD2fl/fl mice. Despite of a remodeling of hepatic lipid metabolism in HFD-fed PKD2ΔHep mice, similar steatosis grade was found in both genotypes.

CONCLUSIONS

Results herein have unveiled an unknown role of PKD2 in the control of insulin signaling in the liver at the level of IRS1 and point PKD2 as a therapeutic target for hepatic insulin resistance.

Calcium flow at ER-TGN contact sites facilitates secretory cargo export

Ca2+ influx into the trans-Golgi Network (TGN) promotes secretory cargo sorting by the Ca2+-ATPase SPCA1 and the luminal Ca2+ binding protein Cab45. Cab45 oligomerizes upon local Ca2+ influx, and Cab45 oligomers sequester and separate soluble secretory cargo from the bulk flow of proteins in the TGN. However, how this Ca2+ flux into the lumen of the TGN is achieved remains mysterious, as the cytosol has a nanomolar steady-state Ca2+ concentration. The TGN forms membrane contact sites (MCS) with the Endoplasmic Reticulum (ER), allowing protein-mediated exchange of molecular species such as lipids. Here, we show that the TGN export of secretory proteins requires the integrity of ER-TGN MCS and inositol 3 phosphate receptor (IP3R)-dependent Ca2+ fluxes in the MCS, suggesting Ca2+ transfer between these organelles. Using an MCS-targeted Ca2+ FRET sensor module, we measure the Ca2+ flow in these sites in real time. These data show that ER-TGN MCS facilitates the Ca2+ transfer required for Ca2+-dependent cargo sorting and export from the TGN, thus solving a fundamental question in cell biology.

Sorting of secretory proteins at the trans-Golgi network by human TGN46

Secretory proteins are sorted at the trans-Golgi network (TGN) for export into specific transport carriers. However, the molecular players involved in this fundamental process remain largely elusive. Here, we identified the human transmembrane protein TGN46 as a receptor for the export of secretory cargo protein PAUF in CARTS - a class of protein kinase D-dependent TGN-to-plasma membrane carriers. We show that TGN46 is necessary for cargo sorting and loading into nascent carriers at the TGN. By combining quantitative fluorescence microscopy and mutagenesis approaches, we further discovered that the lumenal domain of TGN46 encodes for its cargo sorting function. In summary, our results define a cellular function of TGN46 in sorting secretory proteins for export from the TGN.

Regulation of the apico-basolateral trafficking polarity of the homologous copper-ATPases ATP7A and ATP7B

The homologous P-type copper-ATPases (Cu-ATPases) ATP7A and ATP7B are the key regulators of copper homeostasis in mammalian cells. In polarized epithelia, upon copper treatment, ATP7A and ATP7B traffic from the trans-Golgi network (TGN) to basolateral and apical membranes, respectively. We characterized the sorting pathways of Cu-ATPases between TGN and the plasma membrane and identified the machinery involved. ATP7A and ATP7B reside on distinct domains of TGN in limiting copper conditions, and in high copper, ATP7A traffics to basolateral membrane, whereas ATP7B traverses common recycling, apical sorting and apical recycling endosomes en route to apical membrane. Mass spectrometry identified regulatory partners of ATP7A and ATP7B that include the adaptor protein-1 complex. Upon knocking out pan-AP-1, sorting of both Cu-ATPases is disrupted. ATP7A loses its trafficking polarity and localizes on both apical and basolateral surfaces in high copper. By contrast, ATP7B loses TGN retention but retained its trafficking polarity to the apical domain, which became copper independent. Using isoform-specific knockouts, we found that the AP-1A complex provides directionality and TGN retention for both Cu-ATPases, whereas the AP-1B complex governs copper-independent trafficking of ATP7B solely. Trafficking phenotypes of Wilson disease-causing ATP7B mutants that disrupts putative ATP7B-AP1 interaction further substantiates the role of AP-1 in apical sorting of ATP7B.

Protein kinase D drives the secretion of invasion mediators in triple-negative breast cancer cell lines

The protein kinase D (PKD) family members regulate the fission of cargo vesicles at the Golgi complex and play a pro-oncogenic role in triple-negative breast cancer (TNBC). Whether PKD facilitates the secretion of tumor-promoting factors in TNBC, however, is still unknown. Using the pharmacological inhibition of PKD activity and siRNA-mediated depletion of PKD2 and PKD3, we identified the PKD-dependent secretome of the TNBC cell lines MDA-MB-231 and MDA-MB-468. Mass spectrometry-based proteomics and antibody-based assays revealed a significant downregulation of extracellular matrix related proteins and pro-invasive factors such as LIF, MMP-1, MMP-13, IL-11, M-CSF and GM-CSF in PKD-perturbed cells. Notably, secretion of these proteins in MDA-MB-231 cells was predominantly controlled by PKD2 and enhanced spheroid invasion. Consistently, PKD-dependent secretion of pro-invasive factors was more pronounced in metastatic TNBC cell lines. Our study thus uncovers a novel role of PKD2 in releasing a pro-invasive secretome.

Gβγ signaling regulates microtubule-dependent control of Golgi integrity

Gβγ subunits regulate several non-canonical functions at distinct intracellular organelles. Previous studies have shown that Gβγ signaling at the Golgi is necessary to mediate vesicular protein transport function and to regulate mitotic Golgi fragmentation. Disruption of Golgi structure also occurs in response to microtubule depolymerizing agents, such as nocodazole. In this study, we use siRNA against Gβ1/2 or specific Gγ subunits to deplete their expression, and show that their knockdown causes a significant reduction in nocodazole-induced Golgi fragmentation. We establish that knockdown of Gβγ or inhibition of Gβγ with gallein resulted in decreased activation of protein kinase D (PKD) in response to nocodazole treatment. We demonstrate that restricting the amount of free Gβγ available for signaling by either inhibiting Gαi activation using pertussis toxin or by knockdown of the non-GPCR GEF, Girdin/GIV protein, results in a substantial decrease in nocodazole-induced Golgi fragmentation and PKD phosphorylation. Our results also indicate that depletion of Gβγ or inhibition with gallein or pertussis toxin significantly reduces the microtubule disruption-dependent Golgi fragmentation phenotype observed in cells transfected with mutant SOD1, a major causative protein in familial amyotrophic lateral sclerosis (ALS). These results provide compelling evidence that Gβγ signaling is critical for the regulation of Golgi integrity.

Membrane trafficking in breast cancer progression: protein kinase D comes into play

Protein kinase D (PKD) is a serine/threonine kinase family that controls important cellular functions, most notably playing a key role in the secretory pathway at the trans-Golgi network. Aberrant expression of PKD isoforms has been found mainly in breast cancer, where it promotes various cellular processes such as growth, invasion, survival and stem cell maintenance. In this review, we discuss the isoform-specific functions of PKD in breast cancer progression, with a particular focus on how the PKD controlled cellular processes might be linked to deregulated membrane trafficking and secretion. We further highlight the challenges of a therapeutic approach targeting PKD to prevent breast cancer progression.

Potential role for protein kinase D inhibitors in prostate cancer

Protein kinase D (PrKD), a novel serine-threonine kinase, belongs to a family of calcium calmodulin kinases that consists of three isoforms: PrKD1, PrKD2, and PrKD3. The PrKD isoforms play a major role in pathologic processes such as cardiac hypertrophy and cancer progression. The charter member of the family, PrKD1, is the most extensively studied isoform. PrKD play a dual role as both a proto-oncogene and a tumor suppressor depending on the cellular context. The duplicity of PrKD can be highlighted in advanced prostate cancer (PCa) where expression of PrKD1 is suppressed whereas the expressions of PrKD2 and PrKD3 are upregulated to aid in cancer progression. As understanding of the PrKD signaling pathways has been better elucidated, interest has been garnered in the development of PrKD inhibitors. The broad-spectrum kinase inhibitor staurosporine acts as a potent PrKD inhibitor and is the most well-known; however, several other novel and more specific PrKD inhibitors have been developed over the last two decades. While there is tremendous potential for PrKD inhibitors to be used in a clinical setting, none has progressed beyond preclinical trials due to a variety of challenges. In this review, we focus on PrKD signaling in PCa and the potential role of PrKD inhibitors therein, and explore the possible clinical outcomes based on known function and expression of PrKD isoforms at different stages of PCa.

Lipid Alterations in Glioma: A Systematic Review

Gliomas are highly lethal tumours characterised by heterogeneous molecular features, producing various metabolic phenotypes leading to therapeutic resistance. Lipid metabolism reprogramming is predominant and has contributed to the metabolic plasticity in glioma. This systematic review aims to discover lipids alteration and their biological roles in glioma and the identification of potential lipids biomarker. This systematic review was conducted using the preferred reporting items for systematic reviews and meta-analyses (PRISMA) guidelines. Extensive research articles search for the last 10 years, from 2011 to 2021, were conducted using four electronic databases, including PubMed, Web of Science, CINAHL and ScienceDirect. A total of 158 research articles were included in this study. All studies reported significant lipid alteration between glioma and control groups, impacting glioma cell growth, proliferation, drug resistance, patients' survival and metastasis. Different lipids demonstrated different biological roles, either beneficial or detrimental effects on glioma. Notably, prostaglandin (PGE2), triacylglycerol (TG), phosphatidylcholine (PC), and sphingosine-1-phosphate play significant roles in glioma development. Conversely, the most prominent anti-carcinogenic lipids include docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and vitamin D3 have been reported to have detrimental effects on glioma cells. Furthermore, high lipid signals were detected at 0.9 and 1.3 ppm in high-grade glioma relative to low-grade glioma. This evidence shows that lipid metabolisms were significantly dysregulated in glioma. Concurrent with this knowledge, the discovery of specific lipid classes altered in glioma will accelerate the development of potential lipid biomarkers and enhance future glioma therapeutics.

Circulating microRNA sequencing revealed miRNome patterns in hematology and oncology patients aiding the prognosis of invasive aspergillosis

Invasive aspergillosis (IA) may occur as a serious complication of hematological malignancy. Delays in antifungal therapy can lead to an invasive disease resulting in high mortality. Currently, there are no well-established blood circulating microRNA biomarkers or laboratory tests which can be used to diagnose IA. Therefore, we aimed to define dysregulated miRNAs in hematology and oncology (HO) patients to identify biomarkers predisposing disease. We performed an in-depth analysis of high-throughput small transcriptome sequencing data obtained from the whole blood samples of our study cohort of 50 participants including 26 high-risk HO patients and 24 controls. By integrating in silico bioinformatic analyses of small noncoding RNA data, 57 miRNAs exhibiting significant expression differences (P < 0.05) were identified between IA-infected patients and non-IA HO patients. Among these, we found 36 differentially expressed miRNAs (DEMs) irrespective of HO malignancy. Of the top ranked DEMs, we found 14 significantly deregulated miRNAs, whose expression levels were successfully quantified by qRT-PCR. MiRNA target prediction revealed the involvement of IA related miRNAs in the biological pathways of tumorigenesis, the cell cycle, the immune response, cell differentiation and apoptosis.

Addressing the role of PKD3 in the T cell compartment with knockout mice

BACKGROUND

The Protein kinase D3 (PKD3) has been implicated in signal transduction downstream of the T cell receptor (TCR). However, its role for the activation of primary T lymphocytes has not been elucidated so far.

METHODS

Expression of PKD isoforms in primary murine T cells was determined by RT-PCR and SDS-Page. A germline PKD3-knockout mouse line was analyzed for its immune response to OVA/alum intraperitoneal immunization. Phenotyping of the T cell compartment ex vivo as well as upon stimulation in vitro was performed by flow cytometry. Additionally, cytokine expression was assessed by flow cytometry, RT-PCR and Luminex technology.

RESULTS

PKD expression in T cells is modulated by TCR stimulation, leading to a rapid down-regulation on mRNA and on protein level. PKD3-deficient mice respond to immunization with enhanced T follicular helper cell generation. Furthermore, peripheral PKD3-deficient CD4+ T cells express more interleukin-2 than wild type CD4+ T cells upon TCR stimulation ex vivo. However, purified naïve CD4+ T cells do not differ in their phenotype upon differentiation in vitro from wild type T cells. Moreover, we observed a shift towards an effector/memory phenotype of splenic T cells at steady state, which might explain the contradictory results obtained with pan-T cells ex vivo and naïve-sorted T cells.

CONCLUSION

While PKD3-deficiency in vivo in mice leads to a skewing of the T cell compartment towards a more activated phenotype, this kinase seems to be dispensable for naïve CD4+ T cell differentiation in vitro. Video Abstract.

PKD-dependent PARP12-catalyzed mono-ADP-ribosylation of Golgin-97 is required for E-cadherin transport from Golgi to plasma membrane

Adenosine diphosphate (ADP)-ribosylation is a posttranslational modification involved in key regulatory events catalyzed by ADP-ribosyltransferases (ARTs). Substrate identification and localization of the mono-ADP-ribosyltransferase PARP12 at the trans-Golgi network (TGN) hinted at the involvement of ARTs in intracellular traffic. We find that Golgin-97, a TGN protein required for the formation and transport of a specific class of basolateral cargoes (e.g., E-cadherin and vesicular stomatitis virus G protein [VSVG]), is a PARP12 substrate. PARP12 targets an acidic cluster in the Golgin-97 coiled-coil domain essential for function. Its mutation or PARP12 depletion, delays E-cadherin and VSVG export and leads to a defect in carrier fission, hence in transport, with consequent accumulation of cargoes in a trans-Golgi/Rab11-positive intermediate compartment. In contrast, PARP12 does not control the Golgin-245-dependent traffic of cargoes such as tumor necrosis factor alpha (TNFα). Thus, the transport of different basolateral proteins to the plasma membrane is differentially regulated by Golgin-97 mono-ADP-ribosylation by PARP12. This identifies a selective regulatory mechanism acting on the transport of Golgin-97- vs. Golgin-245-dependent cargoes. Of note, PARP12 enzymatic activity, and consequently Golgin-97 mono-ADP-ribosylation, depends on the activation of protein kinase D (PKD) at the TGN during traffic. PARP12 is directly phosphorylated by PKD, and this is essential to stimulate PARP12 catalytic activity. PARP12 is therefore a component of the PKD-driven regulatory cascade that selectively controls a major branch of the basolateral transport pathway. We propose that through this mechanism, PARP12 contributes to the maintenance of E-cadherin-mediated cell polarity and cell-cell junctions.

Advances in Analyzing the Breast Cancer Lipidome and Its Relevance to Disease Progression and Treatment

Abnormal lipid metabolism is common in breast cancer with the three main subtypes, hormone receptor (HR) positive, human epidermal growth factor 2 (HER2) positive, and triple negative, showing common and distinct lipid dependencies. A growing body of studies identify altered lipid metabolism as impacting breast cancer cell growth and survival, plasticity, drug resistance, and metastasis. Lipids are a class of nonpolar or polar (amphipathic) biomolecules that can be produced in cells via de novo synthesis or acquired from the microenvironment. The three main functions of cellular lipids are as essential components of membranes, signaling molecules, and nutrient storage. The use of mass spectrometry-based lipidomics to analyze the global cellular lipidome has become more prevalent in breast cancer research. In this review, we discuss current lipidomic methodologies, highlight recent breast cancer lipidomic studies and how these findings connect to disease progression and therapeutic development, and the potential use of lipidomics as a diagnostic tool in breast cancer. A better understanding of the breast cancer lipidome and how it changes during drug resistance and tumor progression will allow informed development of diagnostics and novel targeted therapies.

Constitutive activation of S1P receptors at the trans-Golgi network is required for surface transport carrier formation

The importance of the G-protein βγ subunits in the regulation of cargo transport from the trans-Golgi network (TGN) to the plasma membrane (PM) is well accepted; however, the molecular mechanism underlying the G-protein activation at the TGN remains unclear. We show here that sphingosine 1-phosphate (S1P) receptors at the PM were trafficked to the TGN in response to a surface transport cargo, temperature-sensitive vesicular stomatitis virus glycoprotein tagged with green fluorescent protein accumulation in the Golgi. The receptor internalization occurred in an S1P-independent manner but required phosphorylation by G-protein receptor kinase 2 and β-arrestin association before internalization. Continuously activated S1P receptors in a manner dependent on S1P at the TGN kept transmitting G-protein signals including the βγ subunits supply necessary for transport carrier formation at the TGN destined for the PM.

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S1P receptors traffic from the PM to Golgi in a surface cargo-dependent manner

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S1PR trafficking follows GRK2-dependent phosphorylation and β-arrestin binding

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S1PRs at the Golgi are continuously activated by S1P while sending G-protein signals

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S1PR/Gβγ signals at the Golgi are indispensable for surface transport carrier formation

Gβγ regulates mitotic Golgi fragmentation and G2/M cell cycle progression

Heterotrimeric G proteins (αβγ) function at the cytoplasmic surface of a cell’s plasma membrane to transduce extracellular signals into cellular responses. However, numerous studies indicate that G proteins also play noncanonical roles at unique intracellular locations. Previous work has established that G protein βγ subunits (Gβγ) regulate a signaling pathway on the cytoplasmic surface of Golgi membranes that controls the exit of select protein cargo. Now, we demonstrate a novel role for Gβγ in regulating mitotic Golgi fragmentation, a key checkpoint of the cell cycle that occurs in the late G2 phase. We show that small interfering RNA–mediated depletion of Gβ1 and Gβ2 in synchronized cells causes a decrease in the number of cells with fragmented Golgi in late G2 and a delay of entry into mitosis and progression through G2/M. We also demonstrate that during G2/M Gβγ acts upstream of protein kinase D and regulates the phosphorylation of the Golgi structural protein GRASP55. Expression of Golgi-targeted GRK2ct, a Gβγ-sequestering protein used to inhibit Gβγ signaling, also causes a decrease in Golgi fragmentation and a delay in mitotic progression. These results highlight a novel role for Gβγ in regulation of Golgi structure.

The PKD-Dependent Biogenesis of TGN-to-Plasma Membrane Transport Carriers

Membrane trafficking is essential for processing and transport of proteins and lipids and to establish cell compartmentation and tissue organization. Cells respond to their needs and control the quantity and quality of protein secretion accordingly. In this review, we focus on a particular membrane trafficking route from the trans-Golgi network (TGN) to the cell surface: protein kinase D (PKD)-dependent pathway for constitutive secretion mediated by carriers of the TGN to the cell surface (CARTS). Recent findings highlight the importance of lipid signaling by organelle membrane contact sites (MCSs) in this pathway. Finally, we discuss our current understanding of multiple signaling pathways for membrane trafficking regulation mediated by PKD, G protein-coupled receptors (GPCRs), growth factors, metabolites, and mechanosensors.

A PKD-MFF signaling axis couples mitochondrial fission to mitotic progression

Mitochondria are highly dynamic organelles subjected to fission and fusion events. During mitosis, mitochondrial fission ensures equal distribution of mitochondria to daughter cells. If and how this process can actively drive mitotic progression remains largely unknown. Here, we discover a pathway linking mitochondrial fission to mitotic progression in mammalian cells. The mitochondrial fission factor (MFF), the main mitochondrial receptor for the Dynamin-related protein 1 (DRP1), is directly phosphorylated by Protein Kinase D (PKD) specifically during mitosis. PKD-dependent MFF phosphorylation is required and sufficient for mitochondrial fission in mitotic but not in interphasic cells. Phosphorylation of MFF is crucial for chromosome segregation and promotes cell survival by inhibiting adaptation of the mitotic checkpoint. Thus, PKD/MFF-dependent mitochondrial fission is critical for the maintenance of genome integrity during cell division.

The oncogenic role of protein kinase D3 in cancer

Protein kinase D3 (PRKD3), a serine/threonine kinase, belongs to protein kinase D family, which contains three members: PRKD1, PRKD2, and PRKD3. PRKD3 is activated by many stimuli including phorbol esters, and G-protein-coupled receptor agonists. PRKD3 promotes cancer cell proliferation, growth, migration, and invasion in various tumor types including colorectal, gastric, hepatic, prostate, and breast cancer. Accumulating data supports that PRKD3 is a promising therapeutic target for treatment of cancer. This review discusses the functions and mechanisms of PRKD3 in promoting tumorigenesis and tumor progression of various tumor types as well as the latest developments of small-molecule inhibitors selection for PRKD/PRKD3.

Deletion of Protein Kinase D3 Promotes Liver Fibrosis in Mice

BACKGROUND AND AIMS

Liver fibrosis (LF) is a central pathological process that occurs in most types of chronic liver diseases. Advanced LF causes cirrhosis, hepatocellular carcinoma, and liver failure. However, the exact molecular mechanisms underlying the initiation and progression of LF remain largely unknown.

APPROACH AND RESULTS

This study was designed to investigate the role of protein kinase D3 (PKD3; gene name Prkd3) in the regulation of liver homeostasis. We generated global Prkd3 knockout (Prkd3^-/- ) mice and myeloid-cell-specific Prkd3 knockout (Prkd3^∆LysM ) mice, and we found that both Prkd3^-/- mice and Prkd3^∆LysM mice displayed spontaneous LF. PKD3 deficiency also aggravated CCl₄ -induced LF. PKD3 is highly expressed in hepatic macrophages (HMs), and PKD3 deficiency skewed macrophage polarization toward a profibrotic phenotype. Activated profibrotic macrophages produced transforming growth factor beta that, in turn, activates hepatic stellate cells to become matrix-producing myofibroblasts. Moreover, PKD3 deficiency decreased the phosphatase activity of SH2-containing protein tyrosine phosphatase-1 (a bona-fide PKD3 substrate), resulting in sustained signal transducer and activator of transcription 6 activation in macrophages. In addition, we observed that PKD3 expression in HMs was down-regulated in cirrhotic human liver tissues.

CONCLUSIONS

PKD3 deletion in mice drives LF through the profibrotic macrophage activation.

Low Rho activity in hepatocytes prevents apical from basolateral cargo separation during trans-Golgi network to surface transport

Hepatocytes, the main epithelial cells of the liver, organize their polarized membrane domains differently from ductal epithelia. They also differ in their biosynthetic delivery of single-membrane-spanning and glycophosphatidylinositol-anchored proteins to the apical domain. While ductal epithelia target apical proteins to varying degrees from the trans-Golgi network (TGN) to the apical surface directly, hepatocytes target them first to the basolateral domain, from where they undergo basolateral-to-apical transcytosis. How TGN-to-surface transport differs in both scenarios is unknown. Here, we report that the basolateral detour of a hepatocyte apical protein is due, in part, to low RhoA activity at the TGN, which prevents its segregation from basolateral transport carriers. Activating Rho in hepatocytic cells, which switches their polarity from hepatocytic to ductal, also led to apical-basolateral cargo segregation at the TGN as is typical for ductal cells, affirming a central role for Rho-signaling in different aspects of the hepatocytic polarity phenotype. Nevertheless, Rho-induced cargo segregation was not sufficient to target the apical protein directly; thus, failure to recruit apical targeting machinery also contributes to its indirect itinerary.

Integrin β3 Deficiency Results in Hypertriglyceridemia via Disrupting LPL (Lipoprotein Lipase) Secretion

OBJECTIVE

Integrin β3 is implicated in numerous biological processes such as its relevance to blood triglyceride, yet whether β3 deficiency affects this metabolic process remains unknown. Approach and Results: We showed that the Chinese patients with β3-deficient Glanzmann thrombasthenia had a 2-fold higher serum triglyceride level together with a lower serum LPL (lipoprotein lipase) level than those with an αIIb deficiency or healthy subjects. The β3 knockout mice recapitulated these phenotypic features. The elevated plasma triglyceride level was due to impaired LPL-mediated triglyceride clearance caused by a disrupted LPL secretion. Further analysis revealed that β3 directly bound LPL via a juxtamembrane TIH (threonine isoleucine histidine)^720-722 motif in its cytoplasmic domain and functioned as an adaptor protein by interacting with LPL and PKD (protein kinase D) to form the PKD/β3/LPL complex that is required for β3-mediated LPL secretion. Furthermore, the impaired triglyceride clearance in β3 knockout mice could be corrected by adeno-associated virus serotype 9 (AAV9)-mediated delivery of wild-type but not TIH^720-722-mutated β3 genes.

CONCLUSIONS

This study reveals a hypertriglyceridemia in both β3-deficient Chinese patients and mice and provides novel insights into the molecular mechanisms of the significant roles of β3 in LPL secretion and triglyceride metabolism, drawing attention to the metabolic consequences in patients with β3-deficient Glanzmann thrombasthenia.

It Takes Two to Tango: Activation of Protein Kinase D by Dimerization

The recent discovery and structure determination of a novel ubiquitin-like dimerization domain in protein kinase D (PKD) has significant implications for its activation. PKD is a serine/threonine kinase activated by the lipid second messenger diacylglycerol (DAG). It is an essential and highly conserved protein that is implicated in plasma membrane directed trafficking processes from the trans-Golgi network. However, many open questions surround its mechanism of activation, its localization, and its role in the biogenesis of cargo transport carriers. In reviewing this field, the focus is primarily on the mechanisms that control the activation of PKD at precise locations in the cell. In light of the new structural findings, the understanding of the mechanisms underlying PKD activation is critically evaluated, with particular emphasis on the role of dimerization in PKD autophosphorylation, and the provenance and recognition of the DAG that activates PKD.

Protein kinase D2 regulates epithelial sodium channel activity and aldosterone non-genomic responses in renal cortical collecting duct cells

Protein kinase D2 (PKD2) is a serine/threonine protein kinase which plays an important role in vesicle fission at the trans-Golgi network (TGN) to coordinate subcellular trafficking with gene expression. We found that in the rat kidney, PKD2 is specifically expressed in collecting duct principal cells predominantly at the apical membrane and with lower basal expression in cytosolic compartments. When rats were maintained on a Na⁺ depleted diet (<0.87 mmol Na⁺/kg) to increase plasma aldosterone levels, PKD2 became internalized to a cytoplasmic compartment. Treatment of murine M1 cortical collecting duct (M1-CCD) cells with aldosterone (10 nM) promoted PKD2 co-localization with the trans-Golgi network within 30 min. PKD2 underwent autophosphorylation at Ser876 within 10 min of aldosterone treatment and remained phosphorylated (active) for at least 24 h. A stable PKD2 shRNA knock-down (PKD2 KD) M1-CCD cell line was developed to study the role of PKD2 in epithelial Na⁺ channel (ENaC) trafficking and transepithelial Na⁺ transport (SCC) in epithelial monolayers grown in Ussing chambers. The PKD2 KD cells developed transepithelial resistance with kinetics equivalent to wild-type cells, however the transepithelial voltage and Na⁺ current were significantly elevated in PKD2 knock-down CCD epithelia. The higher basal SCC was due to increased ENaC activity. Aldosterone treatment for 24 h resulted in a decline in ENaC activity in the PKD2 KD cells as opposed to the increase observed in the wild-type cells. The paradoxical inhibition of SCC by aldosterone in PKD2 KD epithelium was attributed to a reduction in ENaC current and lower membrane abundance of ENaC, demonstrating that PKD2 plays a critical tonic role in ENaC trafficking and channel subunit stability. The rapid activation of PKD2 by aldosterone is synergistic with the transcriptional activity of MR and contributes to increased ENaC activity.

Concerted regulation of actin polymerization during constitutive secretion by cortactin and PKD2

Constitutive secretion from the trans-Golgi-network (TGN) is facilitated by a concerted regulation of vesicle biogenesis and fission processes. The protein kinase D family (PKD) has been previously described to enhance vesicle fission by modifying the lipid environment. PKD also phosphorylates the actin regulatory protein cortactin at S298 to impair synergistic actin polymerization. We here report additional functions for PKD2 (also known as PRKD2) and cortactin in the regulation of actin polymerization during the fission of transport carriers from the TGN. Phosphorylation of cortactin at S298 impairs the interaction between WIP (also known as WIPF1) and cortactin. WIP stabilizes the autoinhibited conformation of N-WASP (also known as WASL). This leads to an inhibition of synergistic Arp2/3-complex-dependent actin polymerization at the TGN. PKD2 activity at the TGN is controlled by active CDC42-GTP which directly activates N-WASP, inhibits PKD2 and shifts the balance to non-S298-phosphorylated cortactin, which can in turn sequester WIP from N-WASP. Consequently, synergistic actin polymerization at the TGN and constitutive secretion are enhanced.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Protein kinase D and Gβγ mediate sustained nociceptive signaling by biased agonists of protease-activated receptor-2

Proteases sustain hyperexcitability and pain by cleaving protease-activated receptor-2 (PAR₂) on nociceptors through distinct mechanisms. Whereas trypsin induces PAR₂ coupling to Gα_q, Gα_s, and β-arrestins, cathepsin-S (CS) and neutrophil elastase (NE) cleave PAR₂ at distinct sites and activate it by biased mechanisms that induce coupling to Gα_s, but not to Gα_q or β-arrestins. Because proteases activate PAR₂ by irreversible cleavage, and activated PAR₂ is degraded in lysosomes, sustained extracellular protease-mediated signaling requires mobilization of intact PAR₂ from the Golgi apparatus or de novo synthesis of new receptors by incompletely understood mechanisms. We found here that trypsin, CS, and NE stimulate PAR₂-dependent activation of protein kinase D (PKD) in the Golgi of HEK293 cells, in which PKD regulates protein trafficking. The proteases stimulated translocation of the PKD activator Gβγ to the Golgi, coinciding with PAR₂ mobilization from the Golgi. Proteases also induced translocation of a photoconverted PAR₂-Kaede fusion protein from the Golgi to the plasma membrane of KNRK cells. After incubation of HEK293 cells and dorsal root ganglia neurons with CS, NE, or trypsin, PAR₂ responsiveness initially declined, consistent with PAR₂ cleavage and desensitization, and then gradually recovered. Inhibitors of PKD, Gβγ, and protein translation inhibited recovery of PAR₂ responsiveness. PKD and Gβγ inhibitors also attenuated protease-evoked mechanical allodynia in mice. We conclude that proteases that activate PAR₂ by canonical and biased mechanisms stimulate PKD in the Golgi; PAR₂ mobilization and de novo synthesis repopulate the cell surface with intact receptors and sustain nociceptive signaling by extracellular proteases.

The Golgi apparatus in neurorestoration

The central role of the Golgi apparatus in critical cellular processes such as the transport, processing, and sorting of proteins and lipids has placed it at the forefront of cell science. Golgi apparatus dysfunction caused by primary defects within the Golgi or pharmacological and oxidative stress has been implicated in a wide range of neurodegenerative diseases. In addition to participating in disease progression, the Golgi apparatus plays pivotal roles in angiogenesis, neurogenesis, and synaptogenesis, thereby promoting neurological recovery. In this review, we focus on the functions of the Golgi apparatus and its mediated events during neurorestoration.

Protein kinase D1 conditional null mice show minimal bone loss following ovariectomy

We previously found that 3- and 6-month-old male mice with conditional ablation of protein kinase D1 (PRKD1) in osteoprogenitor cells (expressing Osterix) exhibited reduced bone mass. Others have demonstrated similar effects in young female PRKD1-deficient mice. Here we examined the bone resorptive response of adult female floxed control and conditional knockout (cKO) mice undergoing sham surgery or ovariectomy (OVX). Femoral and tibial bone mineral density (BMD) values were significantly reduced upon OVX in control, but not cKO, females compared to the respective sham-operated mice. Micro-CT analysis showed that OVX significantly increased trabecular number and decreased trabecular spacing in cKO but not control mice. Finally, in control mice serum levels of a marker of bone resorption (pyridinoline crosslinks) and the osteoclast activator RANKL significantly increased upon OVX; however, no such OVX-induced increase was observed in cKO mice. Our results suggest the potential importance of PRKD1 in response to estrogen loss in bone.

Aldosterone-induced protein kinase signalling and the control of electrolyte balance

Aldosterone acts through the mineralocorticoid receptor (MR) to modulate gene expression in target tissues. In the kidney, the principal action of aldosterone is to promote sodium conservation in the distal nephron and so indirectly enhance water conservation under conditions of hypotension. Over the last twenty years the rapid activation of protein kinase signalling cascades by aldosterone has been described in various tissues. This review describes the integration of rapid protein kinase D signalling responses with the non-genomic actions of aldosterone and transcriptional effects of MR activation.

Protein kinase D2: a versatile player in cancer biology

Protein kinase D2 (PKD2) is a serine/threonine kinase that belongs to the PKD family of calcium-calmodulin kinases, which comprises three isoforms: PKD1, PKD2, and PKD3. PKD2 is activated by many stimuli including growth factors, phorbol esters, and G-protein-coupled receptor agonists. PKD2 participation to uncontrolled growth, survival, neovascularization, metastasis, and invasion has been documented in various tumor types including pancreatic, colorectal, gastric, hepatic, lung, prostate, and breast cancer, as well as glioma multiforme and leukemia. This review discusses the versatile functions of PKD2 from the perspective of cancer hallmarks as described by Hanahan and Weinberg. The PKD2 status, signaling pathways affected in different tumor types and the molecular mechanisms that lead to tumorigenesis and tumor progression are presented. The latest developments of small-molecule inhibitors selective for PKD/PKD2, as well as the need for further chemotherapies that prevent, slow down, or eliminate tumors are also discussed in this review.

Kidins220 and tumour development: Insights into a complexity of cross-talk among signalling pathways (Review)

The mechanistic complexes of kinase D-interacting substrate of 220 kDa/ankyrin repeat-rich membrane spanning (Kidins220/ARMS) bind and integrate a variety of cellular cues to mediate neuronal activities such as neuronal differentiation, survival, and cytoskeleton remodelling by interacting with a variety of binding partners. Accumulated evidence has also indicated its role in the regulation of vascular development. Mice with Kidins220 knockdown phenotypically present with cardiovascular abnormalities. Kidins220 also contributes to immunomodulation in combination with B cells and T cells. Moreover, emerging evidence has revealed that this protein regulates many crucial cellular processes and thus has been implicated in an increasing number of malignancies. Here, we review recent advances in our understanding of Kidins220 and its role in cancer development. Further investigation is warranted to shed light on the role played by Kidins220 in the dynamic arrangement of the cytoskeleton and epithelial–mesenchymal transition, and its implication in tumourigenesis and cancer progression.

In Vivo Imaging of Diacylglycerol at the Cytoplasmic Leaflet of Plant Membranes

Diacylglycerol (DAG) is an important intermediate in lipid biosynthesis and plays key roles in cell signaling, either as a second messenger itself or as a precursor of phosphatidic acid. Methods to identify distinct DAG pools have proven difficult because biochemical fractionation affects the pools, and concentrations are limiting. Here, we validate the use of a genetically encoded DAG biosensor in living plant cells. The sensor is composed of a fusion between yellow fluorescent protein and the C1a domain of protein kinase C (YFP-C1aPKC) that specifically binds DAG, and was stably expressed in suspension-cultured tobacco BY-2 cells and whole Arabidopsis thaliana plants. Confocal imaging revealed that the majority of the YFP-C1aPKC fluorescence did not locate to membranes but was present in the cytosol and nucleus. Treatment with short-chain DAG or PMA (phorbol-12-myristate-13-acetate), a phorbol ester that binds the C1a domain of PKC, caused the recruitment of the biosensor to the plasma membrane. These results indicate that the biosensor works and that the basal DAG concentration in the cytoplasmic leaflet of membranes (i.e. accessible to the biosensor) is in general too low, and confirms that the known pools in plastids, the endoplasmic reticulum and mitochondria are located at the luminal face of these compartments (i.e. inaccessible to the biosensor). Nevertheless, detailed further analysis of different cells and tissues discovered four novel DAG pools, namely at: (i) the trans-Golgi network; (ii) the cell plate during cytokinesis; (iii) the plasma membrane of root epidermal cells in the transition zone, and (iv) the apex of growing root hairs. The results provide new insights into the spatiotemporal dynamics of DAG in plants and offer a new tool to monitor this in vivo.

Inducible Inhibition of Gβγ Reveals Localization-dependent Functions at the Plasma Membrane and Golgi

Heterotrimeric G proteins signal at a variety of endomembrane locations, in addition to their canonical function at the cytoplasmic surface of the plasma membrane (PM), where they are activated by cell surface G protein-coupled receptors. Here we focus on βγ signaling at the Golgi, where βγ activates a signaling cascade, ultimately resulting in vesicle fission from the trans-Golgi network (TGN). To develop a novel molecular tool for inhibiting endogenous βγ in a spatial-temporal manner, we take advantage of a lipid association mutant of the widely used βγ inhibitor GRK2ct (GRK2ct-KERE) and the FRB/FKBP heterodimerization system. We show that GRK2ct-KERE cannot inhibit βγ function when expressed in cells, but recruitment to a specific membrane location recovers the ability of GRK2ct-KERE to inhibit βγ signaling. PM-recruited GRK2ct-KERE inhibits lysophosphatidic acid-induced phosphorylation of Akt, whereas Golgi-recruited GRK2ct-KERE inhibits cargo transport from the TGN to the PM. Moreover, we show that Golgi-recruited GRK2ct-KERE inhibits model basolaterally targeted but not apically targeted cargo delivery, for both PM-destined and secretory cargo, providing the first evidence of selectivity in terms of cargo transport regulated by βγ. Last, we show that Golgi fragmentation induced by ilimaquinone and nocodazole is blocked by βγ inhibition, demonstrating that βγ is a key regulator of multiple pathways that impact Golgi morphology. Thus, we have developed a new molecular tool, recruitable GRK2ct-KERE, to modulate βγ signaling at specific subcellular locations, and we demonstrate novel cargo selectivity for βγ regulation of TGN to PM transport and a novel role for βγ in mediating Golgi fragmentation.

A Novel Conserved Domain Mediates Dimerization of Protein Kinase D (PKD) Isoforms: DIMERIZATION IS ESSENTIAL FOR PKD-DEPENDENT REGULATION OF SECRETION AND INNATE IMMUNITY

Protein kinase D (PKD) isoforms are protein kinase C effectors in signaling pathways regulated by diacylglycerol. Important physiological processes (including secretion, immune responses, motility, and transcription) are placed under diacylglycerol control by the distinctive substrate specificity and subcellular distribution of PKDs. Potentially, broadly co-expressed PKD polypeptides may interact to generate homo- or heteromultimeric regulatory complexes. However, the frequency, molecular basis, regulatory significance, and physiological relevance of stable PKD-PKD interactions are largely unknown. Here, we demonstrate that mammalian PKDs 1-3 and the prototypical Caenorhabditis elegans PKD, DKF-2A, are exclusively (homo- or hetero-) dimers in cell extracts and intact cells. We discovered and characterized a novel, highly conserved N-terminal domain, comprising 92 amino acids, which mediates dimerization of PKD1, PKD2, and PKD3 monomers. A similar domain directs DKF-2A homodimerization. Dimerization occurred independently of properties of the regulatory and kinase domains of PKDs. Disruption of PKD dimerization abrogates secretion of PAUF, a protein carried in small trans-Golgi network-derived vesicles. In addition, disruption of DKF-2A homodimerization in C. elegans intestine impaired and degraded the immune defense of the intact animal against an ingested bacterial pathogen. Finally, dimerization was indispensable for the strong, dominant negative effect of catalytically inactive PKDs. Overall, the structural integrity and function of the novel dimerization domain are essential for PKD-mediated regulation of a key aspect of cell physiology, secretion, and innate immunity in vivo.

Protein Interaction Network of Human Protein Kinase D2 Revealed by Chemical Cross-Linking/Mass Spectrometry

We investigated the interaction network of human PKD2 in the cytosol and in Golgi-enriched subcellular protein fractions by an affinity enrichment strategy combined with chemical cross-linking/mass spectrometry (MS). Analysis of the subproteomes revealed the presence of distinct proteins in the cytosolic and Golgi fractions. The covalent fixation of transient or weak interactors by chemical cross-linking allowed capturing interaction partners that might otherwise disappear during conventional pull-down experiments. In total, 31 interaction partners were identified for PKD2, including glycogen synthase kinase-3 beta (GSK3B), 14-3-3 protein gamma (YWHAG), and the alpha isoform of 55 kDa regulatory subunit B of protein phosphatase 2A (PPP2R2A). Remarkably, the entire seven-subunit Arp2/3 complex (ARPC1B, ARPC2, ARPC3, ARPC4, ARPC5, ACTR3, ACTR2) as well as ARPC1A and ARPC5L, which are putative substitutes of ARPC1B and ARPC5, were identified. We provide evidence of a direct protein-protein interaction between PKD2 and Arp2/3. Our findings will pave the way for further structural and functional studies of PKD2 complexes, especially the PKD2/Arp2/3 interaction, to elucidate the role of PKD2 for transport processes at the trans-Golgi network. Data are available via ProteomeXchange with identifiers PXD003909 (enrichment from cytosolic fractions), PXD003913 (enrichment from Golgi fractions), and PXD003917 (subcellular fractionation).

Protein Kinase D and Gβγ Subunits Mediate Agonist-evoked Translocation of Protease-activated Receptor-2 from the Golgi Apparatus to the Plasma Membrane

Agonist-evoked endocytosis of G protein-coupled receptors has been extensively studied. The mechanisms by which agonists stimulate mobilization and plasma membrane translocation of G protein-coupled receptors from intracellular stores are unexplored. Protease-activated receptor-2 (PAR2) traffics to lysosomes, and sustained protease signaling requires mobilization and plasma membrane trafficking of PAR2 from Golgi stores. We evaluated the contribution of protein kinase D (PKD) and Gβγ to this process. In HEK293 and KNRK cells, the PAR2 agonists trypsin and 2-furoyl-LIGRLO-NH2 activated PKD in the Golgi apparatus, where PKD regulates protein trafficking. PAR2 activation induced translocation of Gβγ, a PKD activator, to the Golgi apparatus, determined by bioluminescence resonance energy transfer between Gγ-Venus and giantin-Rluc8. Inhibitors of PKD (CRT0066101) and Gβγ (gallein) prevented PAR2-stimulated activation of PKD. CRT0066101, PKD1 siRNA, and gallein all inhibited recovery of PAR2-evoked Ca(2+) signaling. PAR2 with a photoconvertible Kaede tag was expressed in KNRK cells to examine receptor translocation from the Golgi apparatus to the plasma membrane. Irradiation of the Golgi region (405 nm) induced green-red photo-conversion of PAR2-Kaede. Trypsin depleted PAR2-Kaede from the Golgi apparatus and repleted PAR2-Kaede at the plasma membrane. CRT0066101 inhibited PAR2-Kaede translocation to the plasma membrane. CRT0066101 also inhibited sustained protease signaling to colonocytes and nociceptive neurons that naturally express PAR2 and mediate protease-evoked inflammation and nociception. Our results reveal a major role for PKD and Gβγ in agonist-evoked mobilization of intracellular PAR2 stores that is required for sustained signaling by extracellular proteases.

Protein Kinase D1 Signaling in Angiogenic Gene Expression and VEGF-Mediated Angiogenesis

Protein kinase D 1 (PKD-1) is a signaling kinase important in fundamental cell functions including migration, proliferation, and differentiation. PKD-1 is also a key regulator of gene expression and angiogenesis that is essential for cardiovascular development and tumor progression. Further understanding molecular aspects of PKD-1 signaling in the regulation of angiogenesis may have translational implications in obesity, cardiovascular disease, and cancer. The author will summarize and provide the insights into molecular mechanisms by which PKD-1 regulates transcriptional expression of angiogenic genes, focusing on the transcriptional regulation of CD36 by PKD-1-FoxO1 signaling axis along with the potential implications of this axis in arterial differentiation and morphogenesis. He will also discuss a new concept of dynamic balance between proangiogenic and antiangiogenic signaling in determining angiogenic switch, and stress how PKD-1 signaling regulates VEGF signaling-mediated angiogenesis.

Differential vesicular sorting of AMPA and GABAA receptors

In mature neurons AMPA receptors cluster at excitatory synapses primarily on dendritic spines, whereas GABAA receptors cluster at inhibitory synapses mainly on the soma and dendritic shafts. The molecular mechanisms underlying the precise sorting of these receptors remain unclear. By directly studying the constitutive exocytic vesicles of AMPA and GABAA receptors in vitro and in vivo, we demonstrate that they are initially sorted into different vesicles in the Golgi apparatus and inserted into distinct domains of the plasma membrane. These insertions are dependent on distinct Rab GTPases and SNARE complexes. The insertion of AMPA receptors requires SNAP25-syntaxin1A/B-VAMP2 complexes, whereas insertion of GABAA receptors relies on SNAP23-syntaxin1A/B-VAMP2 complexes. These SNARE complexes affect surface targeting of AMPA or GABAA receptors and synaptic transmission. Our studies reveal vesicular sorting mechanisms controlling the constitutive exocytosis of AMPA and GABAA receptors, which are critical for the regulation of excitatory and inhibitory responses in neurons.

Mechanisms and Regulation of the Mitotic Inheritance of the Golgi Complex

In mammalian cells, the Golgi complex is structured in the form of a continuous membranous system composed of stacks connected by tubular bridges: the "Golgi ribbon." At the onset of mitosis, the Golgi complex undergoes a multi-step fragmentation process that is required for its correct partition into the dividing cells. Importantly, inhibition of Golgi disassembly results in cell-cycle arrest at the G2 stage, which indicates that accurate inheritance of the Golgi complex is monitored by a "Golgi mitotic checkpoint." Moreover, mitotic Golgi disassembly correlates with the release of a set of Golgi-localized proteins that acquire specific functions during mitosis, such as mitotic spindle formation and regulation of the spindle checkpoint. Most of these events are regulated by small GTPases of the Arf and Rab families. Here, we review recent studies that are revealing the fundamental mechanisms, the molecular players, and the biological significance of mitotic inheritance of the Golgi complex in mammalian cells. We also briefly comment on how Golgi partitioning is coordinated with mitotic progression.

Protein Kinase D2 Assembles a Multiprotein Complex at the Trans-Golgi Network to Regulate Matrix Metalloproteinase Secretion

Vesicle formation and fission are tightly regulated at the trans-Golgi network (TGN) during constitutive secretion. Two major protein families regulate these processes: members of the adenosyl-ribosylation factor family of small G-proteins (ARFs) and the protein kinase D (PKD) family of serine/threonine kinases. The functional relationship between these two key regulators of protein transport from the TGN so far is elusive. We here demonstrate the assembly of a novel functional protein complex at the TGN and its key members: cytosolic PKD2 binds ARF-like GTPase (ARL1) and shuttles ARL1 to the TGN. ARL1, in turn, localizes Arfaptin2 to the TGN. At the TGN, where PKD2 interacts with active ARF1, PKD2, and ARL1 are required for the assembly of a complex comprising of ARF1 and Arfaptin2 leading to secretion of matrix metalloproteinase-2 and -7. In conclusion, our data indicate that PKD2 is a core factor in the formation of this multiprotein complex at the TGN that controls constitutive secretion of matrix metalloproteinase cargo.

Functional and therapeutic significance of protein kinase D enzymes in invasive breast cancer

The protein kinase D (PKD) family members, PKD1, PKD2 and PKD3 constitute a family of serine/threonine kinases that are essential regulators of cell migration, proliferation and protein transport. Multiple types of cancers are characterized by aberrant expression of PKD isoforms. In breast cancer PKD isoforms exhibit distinct expression patterns and regulate various oncogenic processes. In highly invasive breast cancer, the leading cause of cancer-associated deaths in females, the loss of PKD1 is thought to promote invasion and metastasis, while PKD2 and upregulated PKD3 have been shown to be positive regulators of proliferation, chemoresistance and metastasis. In this review, we examine the differential expression pattern, mechanisms of regulation and contributions made by each PKD isoform to the development and maintenance of invasive breast cancer. In addition, we discuss the potential therapeutic approaches for targeting PKD in this disease.

DAG tales: the multiple faces of diacylglycerol--stereochemistry, metabolism, and signaling

The neutral lipids diacylglycerols (DAGs) are involved in a plethora of metabolic pathways. They function as components of cellular membranes, as building blocks for glycero(phospho)lipids, and as lipid second messengers. Considering their central role in multiple metabolic processes and signaling pathways, cellular DAG levels require a tight regulation to ensure a constant and controlled availability. Interestingly, DAG species are versatile in their chemical structure. Besides the different fatty acid species esterified to the glycerol backbone, DAGs can occur in three different stereo/regioisoforms, each with unique biological properties. Recent scientific advances have revealed that DAG metabolizing enzymes generate and distinguish different DAG isoforms, and that only one DAG isoform holds signaling properties. Herein, we review the current knowledge of DAG stereochemistry and their impact on cellular metabolism and signaling. Further, we describe intracellular DAG turnover and its stereochemistry in a 3-pool model to illustrate the spatial and stereochemical separation and hereby the diversity of cellular DAG metabolism.

A time frame permissive for Protein Kinase D2 activity to direct angiogenesis in mouse embryonic stem cells

The protein kinase D isoenzymes PKD1/2/3 are prominent downstream targets of PKCs (Protein Kinase Cs) and phospholipase D in various biological systems. Recently, we identified PKD isoforms as novel mediators of tumour cell-endothelial cell communication, tumour cell motility and metastasis. Although PKD isoforms have been implicated in physiological/tumour angiogenesis, a role of PKDs during embryonic development, vasculogenesis and angiogenesis still remains elusive. We investigated the role of PKDs in germ layer segregation and subsequent vasculogenesis and angiogenesis using mouse embryonic stem cells (ESCs). We show that mouse ESCs predominantly express PKD2 followed by PKD3 while PKD1 displays negligible levels. Furthermore, we demonstrate that PKD2 is specifically phosphorylated/activated at the time of germ layer segregation. Time-restricted PKD2-activation limits mesendoderm formation and subsequent cardiovasculogenesis during early differentiation while leading to branching angiogenesis during late differentiation. In line, PKD2 loss-of-function analyses showed induction of mesendodermal differentiation in expense of the neuroectodermal germ layer. Our in vivo findings demonstrate that embryoid bodies transplanted on chicken chorioallantoic membrane induced an angiogenic response indicating that timed overexpression of PKD2 from day 4 onwards leads to augmented angiogenesis in differentiating ESCs. Taken together, our results describe novel and time-dependent facets of PKD2 during early cell fate determination.

A RhoA Signaling Pathway Regulates Dendritic Golgi Outpost Formation

The neuronal Golgi apparatus (GA) localizes to the perinuclear region and dendrites as tubulo-vesicular structures designated Golgi outposts (GOPs). Current evidence suggests that GOPs shape dendrite morphology and serve as platforms for the local delivery of synaptic receptors. However, the mechanisms underlying GOP formation remain a mystery. Using live-cell imaging and confocal microscopy in cultured hippocampal neurons, we now show that GOPs destined to major "apical" dendrites are generated from the somatic GA by a sequence of events involving: (1) generation of a GA-derived tubule; (2) tubule elongation and deployment into the dendrite; (3) tubule fission; and (4) transport and condensation of the fissioned tubule. A RhoA-Rock signaling pathway involving LIMK1, PKD1, slingshot, cofilin, and dynamin regulates polarized GOP formation by controlling the tubule fission. Our observations identify a mechanism underlying polarized GOP biogenesis and provide new insights regarding involvement of RhoA in dendritic development and polarization.

Identification, localization, and quantification of neuronal cell membrane receptors with plasmonic probes: role of protein kinase D1 in their distribution

Detecting, imaging, and being able to localize the distribution of several cell membrane receptors on a single neuron are very important topics in neuroscience research. In the present work, the distribution of metabotropic glutamate receptor 1a (mGluR1a) density on neuron cells on subcellular length scales is determined by evaluating the role played by protein kinase D1 (PKD1) in the trafficking of membrane proteins, comparing the distribution of mGluR1a in experiments performed in endogenous PKD1 expression with those in the presence of kinase-inactive protein kinase D1 (PKD1-kd). The localization, distribution, and density of cell surface mGluR1a were evaluated using 90 nm diameter Au nanoparticle (NP) probes specifically functionalized with a high-affinity and multivalent labeling function, which allows not only imaging NPs where this receptor is present but also quantifying by optical means the NP density. This is so because the NP generates a density (ρ)-dependent SERS response that facilitated a spatial mapping of the mGluR1a density distribution on subcellular length scales (dendrites and axons) in an optical microscope. The measured ρ values were found to be significantly higher on dendrites than on axons for endogenous PKD1, while an increase of ρ on axons was observed when PKD1 is altered. The spatial distribution of the NP immunolabels through scanning electron microscopy (SEM) confirmed the results obtained by fluorescence bright-field analysis and dark-field spectroscopy and provided additional structural details. In addition, it is shown using electrodynamic simulations that SERS spectroscopy could be a very sensitive tool for the spatial mapping of cell membrane receptors on subcellular length scales, as SERS signals are almost linearly dependent on NP density and therefore give indirect information on the distribution of cell membrane proteins. This result is important since the calibration of the ρ-dependent near-field enhancement of the Au immunolabels through correlation of SERS and SEM paves the way toward quantitative immunolabeling studies of cell membrane proteins involved in neuron polarity. From the molecular biology point of view, this study shows that in cultured hippocampal pyramidal cells mGluR1a is predominantly transported to dendrites and excluded from axons. Expression of kinase-inactive protein kinase D1 (PKD1-kd) dramatically and selectively alters the intracellular trafficking and membrane delivery of mGluR1a-containing vesicles.

Ongoing activation of sphingosine 1-phosphate receptors mediates maturation of exosomal multivesicular endosomes

During late endosome maturation, cargo molecules are sorted into intralumenal vesicles (ILVs) of multivesicular endosomes (MVEs), and are either delivered to lysosomes for degradation or fused with the plasma membranes for exosome release. The mechanism underlying formation of exosomal ILVs and cargo sorting into ILVs destined for exosome release is still unclear. Here we show that inhibitory G protein (Gi)-coupled sphingosine 1-phosphate (S1P) receptors regulate exosomal MVE maturation. Gi-coupled S1P receptors on MVEs are constitutively activated through a constant supply of S1P via autocrine activation within organelles. We also found that the continuous activation of Gi-coupled S1P receptors on MVEs is essential for cargo sorting into ILVs destined for exosome release. Our results reveal a mechanism underlying ESCRT-independent maturation of exosomal MVEs.

JMY is involved in anterograde vesicle trafficking from the trans-Golgi network

Junction-mediating and regulatory protein (JMY) was originally identified as a transcriptional co-factor in the p53-response to DNA damage. Aside from this nuclear function, recent years have uncovered an additional function of JMY, namely in cytoskeleton remodelling and actin assembly. The C-terminus of JMY comprises a canonical VCA-module, the sequence signature of Arp2/3 complex activators. Furthermore, tandem repeats of 3 WH2 (V, or more recently also W) domains render JMY capable of Arp2/3 independent actin assembly. The motility promoting cytoplasmic function of JMY is abrogated upon DNA-damage and nuclear translocation of JMY. To address the precise cellular function of JMY in cellular actin rearrangements, we have searched for potential new interaction partners by mass spectrometry. We identified several candidates and correlated their localization with the subcellular dynamics of JMY. JMY is localized to dynamic vesiculo-tubular structures throughout the cytoplasm, which are decorated with actin and Arp2/3 complex. Moreover, JMY partially colocalizes and interacts with VAP-A, which is involved in vesicle-based transport processes. Finally, overexpression of JMY results in Golgi dispersal by loss from the trans-site and affects VSV-G transport. These analyses, together with biochemical experiments, indicate that JMY drives vesicular trafficking in the trans-Golgi region and at ER-membrane contact sites (MCS), distinct from other Arp2/3 activators involved in vesicle transport processes such as the related WHAMM or WASH.

Organization and execution of the epithelial polarity programme

Epithelial cells require apical-basal plasma membrane polarity to carry out crucial vectorial transport functions and cytoplasmic polarity to generate different cell progenies for tissue morphogenesis. The establishment and maintenance of a polarized epithelial cell with apical, basolateral and ciliary surface domains is guided by an epithelial polarity programme (EPP) that is controlled by a network of protein and lipid regulators. The EPP is organized in response to extracellular cues and is executed through the establishment of an apical-basal axis, intercellular junctions, epithelial-specific cytoskeletal rearrangements and a polarized trafficking machinery. Recent studies have provided insight into the interactions of the EPP with the polarized trafficking machinery and how these regulate epithelial polarization and depolarization.