Functional redundancy of protein kinase D1 and protein kinase D2 in neuronal polarity

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.

Getting smart - Deciphering the neuronal functions of protein kinase D

Protein kinase D (PKD) is a family of serine/threonine kinases that play important roles in various signalling pathways in cells, including neuronal cells. In the nervous system, PKD has been shown to be involved in learning and memory formation by regulating neurotransmitter release, neurite outgrowth and dendrite development, synapse formation and synaptic plasticity. In addition, PKD has been implicated in pain perception or neuroprotection during oxidative stress. Dysregulation of PKD expression and activity has been linked to several neurological disorders, including autism and epilepsy. In this review, we summarize the current knowledge on the function of the PKD family members in neuronal cells, including the spatial regulation of their downstream signalling pathways. We will further discuss the potential role of PKD in the pathogenesis of neurological disorders.

Molecular mechanisms of polarized transport to the apical plasma membrane

Cell polarity is essential for cellular function. Directional transport within a cell is called polarized transport, and it plays an important role in cell polarity. In this review, we will introduce the molecular mechanisms of polarized transport, particularly apical transport, and its physiological importance.

Rab6-Mediated Polarized Transport of Synaptic Vesicle Precursors Is Essential for the Establishment of Neuronal Polarity and Brain Formation

Neurons are highly polarized cells that are composed of a single axon and multiple dendrites. Axon-dendrite polarity is essential for proper tissue formation and brain functions. Intracellular protein transport plays an important role in the establishment of neuronal polarity. However, the regulatory mechanism of polarized transport remains unclear. Here, we show that Rab6, a small GTPase that acts on the regulation of intracellular vesicular trafficking, plays key roles in neuronal polarization and brain development. Central nervous system-specific Rab6a/b double knock-out (Rab6 DKO) mice of both sexes exhibit severe dysplasia of the neocortex and the cerebellum. In the Rab6 DKO neocortex, impaired axonal extension of neurons results in hypoplasia of the intermediate zone. In vitro, deletion of Rab6a and Rab6b in cultured neurons from both sexes causes the abnormal accumulation of synaptic vesicle precursors (SVPs) adjacent to the Golgi apparatus, which leads to defects in axonal extension and the loss of axon-dendrite polarity. Moreover, Rab6 DKO causes significant expansion of lysosomes in the soma in neurons. Overall, our results reveal that Rab6-mediated polarized transport of SVPs is crucial for neuronal polarization and subsequent brain formation.

Role of protein kinase D1 in vasoconstriction and haemodynamics in rats

AIMS

Protein kinase D (PKD), once considered an effector of protein kinase C (PKC), now plays many pathophysiological roles in various tissues. However, little is known about role of PKD in vascular function. We investigated the role of PKD in contraction of rat aorta and human aortic smooth muscle cells (HASMCs) and in haemodynamics in rats.

METHODS AND RESULTS

Isometric tension of rat aortic was measured to examine norepinephrine-induced contraction in the presence of PKD, PKC and Rho-kinase inhibitors. Phosphorylation of PKD1, myosin targeting subunit-1 (MYPT1), myosin light chain (MLC), CPI-17 and heat-shock protein 27 (HSP27), and actin polymerization were measured in the aorta. Phosphorylation of MYPT1 and MLC was also measured in HASMCs knocked down with specific siRNAs of PKD 1, 2 and 3. Intracellular calcium concentrations and cell shortening were measured in HASMCs. Norepinephrine-induced aortic contraction was accompanied by increased phosphorylation of PKD1, MYPT1 and MLC and actin polymerization, all of which were attenuated with PKD inhibitor CRT0066101. PKD1 phosphorylation was not inhibited by PKC inhibitor, chelerythrine or Rho kinase inhibitor, fasudil. In HASMCs, the phosphorylation of MYPT1 and MLC was attenuated by PKD1, but not PKD2, 3 knockdown. In HASMCs, CRT0066101 inhibited norepinephrine-induced cell shortening without affecting calcium concentration. Administration of CRT0066101 decreased systemic vascular resistance and blood pressure without affecting cardiac output in rats.

CONCLUSIONS

PKD1 may play roles in aorta contraction and haemodynamics via phosphorylation of MYPT1 and actin polymerization in a calcium-independent manner.

Protein kinase D2 confers neuroprotection by promoting AKT and CREB activation in ischemic stroke

Ischemic stroke, constituting 80-90% of all strokes, is a leading cause of death and long-term disability in adults. There is an urgent need to discover new targets and therapies for this devastating condition. Protein kinase D (PKD), as a key target of diacylglycerol involved in ischemic responses, has not been well studied in ischemic stroke, particularly PKD2. In this study, we found that PKD2 expression and activity were significantly upregulated in the ipsilateral side of the brain after transient focal cerebral ischemia, which coincides with the upregulation of PKD2 in primary neurons in response to in vitro ischemia, implying a potential role of PKD2 in neuronal survival in ischemic stroke. Using kinase-dead PKD2 knock-in (PKD2-KI) mice, we examined whether loss of PKD2 activity affected stroke outcomes in mice subjected to 1 h of transient middle cerebral artery occlusion (tMCAO) and 24 h of reperfusion. Our data demonstrated that PKD2-KI mice exhibited larger infarction volumes and worsened neurological scores, indicative of increased brain injury, as compared to the wild-type (WT) mice, confirming a neuroprotective role of PKD2 in ischemia/reperfusion (I/R) injury. Mouse primary neurons obtained from PKD2-KI mice also exhibited increased cell death as compared to the WT neurons when subjected to in vitro ischemia. We have further identified AKT and CREB as two main signaling nodes through which PKD2 regulates neuronal survival during I/R injury. In summary, PKD2 confers neuroprotection in ischemic stroke by promoting AKT and CREB activation and targeted activation of PKD2 may benefit neuronal survival in ischemic stroke.

Small-Molecule Inhibitor Targeting Protein Kinase D: A Potential Therapeutic Strategy

The protein kinase D (PKD) family is a family of serine-threonine kinases that are members of the calcium/calmodulin-dependent kinase (CaMK) superfamily. PKDs have been increasingly implicated in multiple pivotal cellular processes and pathological conditions. PKD dysregulation is associated with several diseases, including cancer, inflammation, and obesity. Over the past few years, small-molecule inhibitors have emerged as alternative targeted therapy with fewer adverse side effects than currently available chemotherapy, and these specifically targeted inhibitors limit non-specific toxicities. The successful development of PKD inhibitors would significantly suppress the growth and proliferation of various cancers and inhibit the progression of other diseases. Various PKD inhibitors have been studied in the preclinical setting. In this context, we summarize the PKD inhibitors under investigation and their application for different kinds of diseases.

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.

Autism-associated protein kinase D2 regulates embryonic cortical neuron development

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder, characterized by impaired social interaction, repetitive behavior and restricted interests. Although the molecular etiology of ASD remains largely unknown, recent studies have suggested that de novo mutations are significantly involved in the risk of ASD. We and others recently identified spontaneous de novo mutations in PKD2, a protein kinase D family member, in sporadic ASD cases. However, the biological significance of the de novo PKD2 mutations and the role of PKD2 in brain development remain unclear. Here, we performed functional analysis of PKD2 in cortical neuron development using in utero electroporation. PKD2 is highly expressed in cortical neural stem cells in the developing cortex and regulates cortical neuron development, including the neuronal differentiation of neural stem cells and migration of newborn neurons. Importantly, we determined that the ASD-associated de novo mutations impair the kinase activity of PKD2, suggesting that the de novo PKD2 mutations can be a risk factor for the disease by loss of function of PKD2. Our current findings provide novel insight into the molecular and cellular pathogenesis of ASD.

PKD1 Promotes Functional Synapse Formation Coordinated with N-Cadherin in Hippocampus

Functional synapse formation is critical for the wiring of neural circuits in the developing brain. The cell adhesion molecule N-cadherin plays important roles in target recognition and synaptogenesis. However, the molecular mechanisms that regulate the localization of N-cadherin and the subsequent effects remain poorly understood. Here, we show that protein kinase D1 (PKD1) directly binds to N-cadherin at amino acid residues 836-871 and phosphorylates it at Ser 869, 871, and 872, thereby increasing the surface localization of N-cadherin and promoting functional synapse formation in primary cultured hippocampal neurons obtained from embryonic day 18 rat embryos of either sex. Intriguingly, neuronal activity enhances the interactions between N-cadherin and PKD1, which are critical for the activity-dependent growth of dendritic spines. Accordingly, either disruption the binding between N-cadherin and PKD1 or preventing the phosphorylation of N-cadherin by PKD1 in the hippocampal CA1 region of male rat leads to the reduction in synapse number and impairment of LTP. Together, this study demonstrates a novel mechanism of PKD1 regulating the surface localization of N-cadherin and suggests that the PKD1-N-cadherin interaction is critical for synapse formation and function.SIGNIFICANCE STATEMENT Defects in synapse formation and function lead to various neurological diseases, although the mechanisms underlying the regulation of synapse development are far from clear. Our results suggest that protein kinase D1 (PKD1) functions upstream of N-cadherin, a classical synaptic adhesion molecule, to promote functional synapse formation. Notably, we identified a crucial binding fragment to PKD1 at C terminus of N-cadherin, and this fragment also contains PKD1 phosphorylation sites. Through this interaction, PKD1 enhances the stability of N-cadherin on cell membrane and promotes synapse morphogenesis and synaptic plasticity in an activity-dependent manner. Our study reveals the role of PKD1 and the potential downstream mechanism in synapse development, and contributes to the research for neurodevelopment and the therapy for neurological diseases.