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

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.

Non-canonical Golgi-compartmentalized Gβγ signaling: mechanisms, functions, and therapeutic targets

G protein Gβγ subunits are key mediators of G protein-coupled receptor (GPCR) signaling under physiological and pathological conditions; their inhibitors have been tested for the treatment of human disease. Conventional wisdom is that the Gβγ complex is activated and subsequently exerts its functions at the plasma membrane (PM). Recent studies have revealed non-canonical activation of Gβγ at intracellular organelles, where the Golgi apparatus is a major locale, via translocation or local activation. Golgi-localized Gβγ activates specific signaling cascades and regulates fundamental cell processes such as membrane trafficking, proliferation, and migration. More recent studies have shown that inhibiting Golgi-compartmentalized Gβγ signaling attenuates cardiomyocyte hypertrophy and prostate tumorigenesis, indicating new therapeutic targets. We review novel activation mechanisms and non-canonical functions of Gβγ at the Golgi, and discuss potential therapeutic interventions by targeting Golgi-biased Gβγ-directed signaling.

The olfactory receptor OR51E2 activates ERK1/2 through the Golgi-localized Gβγ-PI3Kγ-ARF1 pathway in prostate cancer cells

The olfactory receptor OR51E2 is ectopically expressed in prostate tissues and regulates prostate cancer progression, but its function and regulation in oncogenic mitogen-activate protein kinase (MAPK) activation are poorly defined. Here we demonstrate that β-ionone, an OR51E2 agonist, dose-dependently activates extracellular signal-regulated kinases 1 and 2 (ERK1/2) in prostate cancer cells, with an EC50 value of approximate 20 μM and an efficiency comparable to other receptor agonists. We also find that CRISPR-Cas9-mediated knockout of Golgi-translocating Gγ9 subunit, phosphoinositide 3-kinase γ (PI3Kγ) and the small GTPase ADP-ribosylation factor 1 (ARF1), as well as pharmacological inhibition of Gβγ, PI3Kγ and Golgi-localized ARF1, each abolishes ERK1/2 activation by β-ionone. We further show that β-ionone significantly promotes ARF1 translocation to the Golgi and activates ARF1 that can be inhibited by Gγ9 and PI3Kγ depletion. Collectively, our data demonstrate that OR51E2 activates ERK1/2 through the Gβγ-PI3Kγ-ARF1 pathway that occurs spatially at the Golgi, and also provide important insights into MAPK hyper-activation in prostate cancer.

Characterization of lipoprotein lipase storage vesicles in 3T3-L1 adipocytes

Lipoprotein lipase (LPL) is a secreted triglyceride lipase involved in the clearance of very-low-density lipoproteins and chylomicrons from circulation. LPL is expressed primarily in adipose and muscle tissues and transported to the capillary lumen. LPL secretion is regulated by insulin in adipose tissue; however, few studies have examined the regulatory and trafficking steps involved in secretion. Here, we describe the intracellular localization and insulin-dependent trafficking of LPL in 3T3-L1 adipocytes. We compared LPL trafficking to the better characterized trafficking pathways taken by leptin and GLUT4 (also known as SLC2A4). We show that the LPL trafficking pathway shares some characteristics of these other pathways, but that LPL subcellular localization and trafficking are distinct from those of GLUT4 and leptin. LPL secretion occurs slowly in response to insulin and rapidly in response to the Ca2+ ionophore ionomycin. This regulated trafficking is dependent on Golgi protein kinase D and the ADP-ribosylation factor GTPase ARF1. Together, these data give support to a new trafficking pathway for soluble cargo that is active in adipocytes.

Mice expressing fluorescent PAR2 reveal that endocytosis mediates colonic inflammation and pain

G protein-coupled receptors (GPCRs) regulate many pathophysiological processes and are major therapeutic targets. The impact of disease on the subcellular distribution and function of GPCRs is poorly understood. We investigated trafficking and signaling of protease-activated receptor 2 (PAR₂) in colitis. To localize PAR₂ and assess redistribution during disease, we generated knockin mice expressing PAR₂ fused to monomeric ultrastable green fluorescent protein (muGFP). PAR₂-muGFP signaled and trafficked normally. PAR₂ messenger RNA was detected at similar levels in Par₂-mugfp and wild-type mice. Immunostaining with a GFP antibody and RNAScope in situ hybridization using F2rl1 (PAR₂) and Gfp probes revealed that PAR₂-muGFP was expressed in epithelial cells of the small and large intestine and in subsets of enteric and dorsal root ganglia neurons. In healthy mice, PAR₂-muGFP was prominently localized to the basolateral membrane of colonocytes. In mice with colitis, PAR₂-muGFP was depleted from the plasma membrane of colonocytes and redistributed to early endosomes, consistent with generation of proinflammatory proteases that activate PAR₂ PAR₂ agonists stimulated endocytosis of PAR₂ and recruitment of Gα_q, Gα_i, and β-arrestin to early endosomes of T84 colon carcinoma cells. PAR₂ agonists increased paracellular permeability of colonic epithelial cells, induced colonic inflammation and hyperalgesia in mice, and stimulated proinflammatory cytokine release from segments of human colon. Knockdown of dynamin-2 (Dnm2), the major colonocyte isoform, and Dnm inhibition attenuated PAR₂ endocytosis, signaling complex assembly and colonic inflammation and hyperalgesia. Thus, PAR₂ endocytosis sustains protease-evoked inflammation and nociception and PAR₂ in endosomes is a potential therapeutic target for colitis.

Protease-Activated Receptors - Key Regulators of Inflammatory Bowel Diseases Progression

The pathogenesis and course of inflammatory bowel diseases are related to both immune system disorders and dysfunction of colon permeability. Moreover, co-existing diseases in patients with Crohn's disease and ulcerative colitis are identified. Currently, there are some therapeutic strategies that affect the function of cytokine/s causing inflammation in the intestinal wall. However, additional approaches which target other components of inflammatory bowel diseases pathogenesis are still needed. Accumulating evidence suggests that proteases and protease-activated receptors seem to be responsible for colitis progression. Experimental and observational studies showed alteration of protease-activated receptors expression in the colon of patients with Crohn's disease and ulcerative colitis. Furthermore, it was suggested that the expression of protease-activated receptors correlated with inflammatory bowel diseases activity. Moreover, regulation of protease-activated receptors seems to be responsible for the modulation of colitis and clinical manifestation of inflammatory bowel diseases. In this review, we present the current state of knowledge about the contribution of protease-activated receptors to Crohn's disease and ulcerative colitis and its implications for diagnosis and treatment.

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.

Gβγ translocation to the Golgi apparatus activates ARF1 to spatiotemporally regulate G protein-coupled receptor signaling to MAPK

After activation of G protein-coupled receptors, G protein βγ dimers may translocate from the plasma membrane to the Golgi apparatus (GA). We recently report that this translocation activates extracellular signal-regulated protein kinases 1 and 2 (ERK1/2) via PI3Kγ; however, how Gβγ-PI3Kγ activates the ERK1/2 pathway is unclear. Here, we demonstrate that chemokine receptor CXCR4 activates ADP-ribosylation factor 1 (ARF1), a small GTPase important for vesicle-mediated membrane trafficking. This activation is blocked by CRISPR-Cas9-mediated knockout of the GA-translocating Gγ9 subunit. Inducible targeting of different Gβγ dimers to the GA can directly activate ARF1. CXCR4 activation and constitutive Gβγ recruitment to the GA also enhance ARF1 translocation to the GA. We further demonstrate that pharmacological inhibition and CRISPR-Cas9-mediated knockout of PI3Kγ markedly inhibit CXCR4-mediated and Gβγ translocation-mediated ARF1 activation. We also show that depletion of ARF1 by siRNA and CRISPR-Cas9 and inhibition of GA-localized ARF1 activation abolish ERK1/2 activation by CXCR4 and Gβγ translocation to the GA and suppress prostate cancer PC3 cell migration and invasion. Collectively, our data reveal a novel function for Gβγ translocation to the GA to activate ARF1 and identify GA-localized ARF1 as an effector acting downstream of Gβγ-PI3Kγ to spatiotemporally regulate G protein-coupled receptor signaling to mitogen-activated protein kinases.

G protein βγ translocation to the Golgi apparatus activates MAPK via p110γ-p101 heterodimers

The Golgi apparatus (GA) is a cellular organelle that plays a critical role in the processing of proteins for secretion. Activation of G protein-coupled receptors at the plasma membrane (PM) induces the translocation of G protein βγ dimers to the GA. However, the functional significance of this translocation is largely unknown. Here, we study PM-GA translocation of all 12 Gγ subunits in response to chemokine receptor CXCR4 activation and demonstrate that Gγ9 is a unique Golgi-translocating Gγ subunit. CRISPR-Cas9-mediated knockout of Gγ9 abolishes activation of extracellular signal-regulated kinase 1 and 2 (ERK1/2), two members of the mitogen-activated protein kinase family, by CXCR4. We show that chemically induced recruitment to the GA of Gβγ dimers containing different Gγ subunits activates ERK1/2, whereas recruitment to the PM is ineffective. We also demonstrate that pharmacological inhibition of phosphoinositide 3-kinase γ (PI3Kγ) and depletion of its subunits p110γ and p101 abrogate ERK1/2 activation by CXCR4 and Gβγ recruitment to the GA. Knockout of either Gγ9 or PI3Kγ significantly suppresses prostate cancer PC3 cell migration, invasion, and metastasis. Collectively, our data demonstrate a novel function for Gβγ translocation to the GA, via activating PI3Kγ heterodimers p110γ-p101, to spatiotemporally regulate mitogen-activated protein kinase activation by G protein-coupled receptors and ultimately control tumor progression.

Regulation of Osteoclast Differentiation at Multiple Stages by Protein Kinase D Family Kinases

Balanced osteoclast and osteoblast activity is necessary for skeletal health, whereas unbalanced osteoclast activity causes bone loss in many skeletal conditions. A better understanding of pathways that regulate osteoclast differentiation and activity is necessary for the development of new therapies to better manage bone resorption. The roles of Protein Kinase D (PKD) family of serine/threonine kinases in osteoclasts have not been well characterized. In this study we use immunofluorescence analysis to reveal that PKD2 and PKD3, the isoforms expressed in osteoclasts, are found in the nucleus and cytoplasm, the mitotic spindle and midbody, and in association with the actin belt. We show that PKD inhibitors CRT0066101 and CID755673 inhibit several distinct aspects of osteoclast formation. Treating bone marrow macrophages with lower doses of the PKD inhibitors had little effect on M-CSF + RANKL-dependent induction into committed osteoclast precursors, but inhibited their motility and subsequent differentiation into multinucleated mature osteoclasts, whereas higher doses of the PKD inhibitors induced apoptosis of the preosteoclasts. Treating post-fusion multinucleated osteoclasts with the inhibitors disrupted the osteoclast actin belts and impaired their resorptive activity. In conclusion, these data implicate PKD kinases as positive regulators of osteoclasts, which are essential for multiple distinct processes throughout their formation and function.

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.

Phytochrome B and AGB1 Coordinately Regulate Photomorphogenesis by Antagonistically Modulating PIF3 Stability in Arabidopsis

Phytochrome B (phyB), the primary red light photoreceptor, promotes photomorphogenesis in Arabidopsis by interacting with the basic helix-loop-helix transcriptional factor PIF3 and inducing its phosphorylation and degradation. Heterotrimeric G proteins are known to regulate various developmental processes in plants and animals. In Arabidopsis, the G-protein β subunit AGB1 is known to repress photomorphogenesis. However, whether and how phyB and AGB1 coordinately regulate photomorphogenesis are largely unknown. Here we show that phyB physically interacts with AGB1 in a red light-dependent manner and that AGB1 interacts directly with PIF3. Moreover, we demonstrate that the AGB1-PIF3 interaction inhibits the association of PIF3 with phyB, leading to reduced phosphorylation and degradation of PIF3, whereas the phyB-AGB1 interaction represses the association of PIF3 with AGB1, resulting in enhanced phosphorylation and degradation of PIF3. Our results suggest that phyB and AGB1 antagonistically regulate PIF3 stability by dynamically interacting with each other and PIF3. This dynamic mechanism may allow plants to balance phyB and G-protein signaling to optimize photomorphogenesis.

Targeting G protein-coupled receptor signalling by blocking G proteins

G protein-coupled receptors (GPCRs) are the largest class of drug targets, largely owing to their druggability, diversity and physiological efficacy. Many drugs selectively target specific subtypes of GPCRs, but high specificity for individual GPCRs may not be desirable in complex multifactorial disease states in which multiple receptors may be involved. One approach is to target G protein subunits rather than the GPCRs directly. This approach has the potential to achieve broad efficacy by blocking pathways shared by multiple GPCRs. Additionally, because many GPCRs couple to multiple G protein signalling pathways, blocking specific G protein subunits can 'bias' GPCR signals by inhibiting only a subset of these signals. Molecules that target G protein α or βγ-subunits have been developed and show strong efficacy in multiple preclinical disease models and biased inhibition of G protein signalling. In this Review, we discuss the development and characterization of G protein α and βγ-subunit ligands and the preclinical evidence that this exciting new approach has potential for therapeutic efficacy in a number of indications, such as pain, thrombosis, asthma and heart failure.

Protease-activated receptor-2 in endosomes signals persistent pain of irritable bowel syndrome

Once activated at the surface of cells, G protein-coupled receptors (GPCRs) redistribute to endosomes, where they can continue to signal. Whether GPCRs in endosomes generate signals that contribute to human disease is unknown. We evaluated endosomal signaling of protease-activated receptor-2 (PAR2), which has been proposed to mediate pain in patients with irritable bowel syndrome (IBS). Trypsin, elastase, and cathepsin S, which are activated in the colonic mucosa of patients with IBS and in experimental animals with colitis, caused persistent PAR2-dependent hyperexcitability of nociceptors, sensitization of colonic afferent neurons to mechanical stimuli, and somatic mechanical allodynia. Inhibitors of clathrin- and dynamin-dependent endocytosis and of mitogen-activated protein kinase kinase-1 prevented trypsin-induced hyperexcitability, sensitization, and allodynia. However, they did not affect elastase- or cathepsin S-induced hyperexcitability, sensitization, or allodynia. Trypsin stimulated endocytosis of PAR2, which signaled from endosomes to activate extracellular signal-regulated kinase. Elastase and cathepsin S did not stimulate endocytosis of PAR2, which signaled from the plasma membrane to activate adenylyl cyclase. Biopsies of colonic mucosa from IBS patients released proteases that induced persistent PAR2-dependent hyperexcitability of nociceptors, and PAR2 association with β-arrestins, which mediate endocytosis. Conjugation to cholestanol promoted delivery and retention of antagonists in endosomes containing PAR2 A cholestanol-conjugated PAR2 antagonist prevented persistent trypsin- and IBS protease-induced hyperexcitability of nociceptors. The results reveal that PAR2 signaling from endosomes underlies the persistent hyperexcitability of nociceptors that mediates chronic pain of IBS. Endosomally targeted PAR2 antagonists are potential therapies for IBS pain. GPCRs in endosomes transmit signals that contribute to human diseases.

A Rho signaling network links microtubules to PKD controlled carrier transport to focal adhesions

Protein kinase D (PKD) is a family of serine/threonine kinases that is required for the structural integrity and function of the Golgi complex. Despite its importance in the regulation of Golgi function, the molecular mechanisms regulating PKD activity are still incompletely understood. Using the genetically encoded PKD activity reporter G-PKDrep we now uncover a Rho signaling network comprising GEF-H1, the RhoGAP DLC3, and the Rho effector PLCε that regulate the activation of PKD at trans-Golgi membranes. We further show that this molecular network coordinates the formation of TGN-derived Rab6-positive transport carriers delivering cargo for localized exocytosis at focal adhesions.

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.

Gβγ subunits-Different spaces, different faces

Gβγ subunits play key roles in modulation of canonical effectors in G protein-coupled receptor (GPCR)-dependent signalling at the cell surface. However, a number of recent studies of Gβγ function have revealed that they regulate a large number of molecules at distinct subcellular sites. These novel, non-canonical Gβγ roles have reshaped our understanding of how important Gβγ signalling is compared to our original notion of Gβγ subunits as simple negative regulators of Gα subunits. Gβγ dimers have now been identified as regulators of transcription, anterograde and retrograde trafficking and modulators of second messenger molecule generation in intracellular organelles. Here, we review some recent advances in our understanding of these novel non-canonical roles of Gβγ.