Protein kinase D regulates metabolism and growth by controlling secretion of insulin like peptide

Distinct molecular pathways govern presynaptic homeostatic plasticity

Presynaptic homeostatic plasticity (PHP) stabilizes synaptic transmission by counteracting impaired neurotransmitter receptor function through neurotransmitter release potentiation. PHP is thought to be triggered by impaired receptor function and to involve a stereotypic signaling pathway. However, here we demonstrate that different receptor perturbations that similarly reduce synaptic transmission result in different responses at the Drosophila neuromuscular junction. While receptor inhibition by the glutamate receptor (GluR) antagonist γ-D-glutamylglycine (γDGG) is not compensated by PHP, the GluR inhibitors Philanthotoxin-433 (PhTx) and Gyki-53655 (Gyki) induce compensatory PHP. Intriguingly, PHP triggered by PhTx and Gyki involve separable signaling pathways, including inhibition of distinct GluR subtypes, differential modulation of the active-zone scaffold Bruchpilot, and short-term plasticity. Moreover, while PHP upon Gyki treatment does not require genes promoting PhTx-induced PHP, it involves presynaptic protein kinase D. Thus, synapses not only respond differentially to similar activity impairments, but achieve homeostatic compensation via distinct mechanisms, highlighting the diversity of homeostatic signaling.

Genetic interactions between Protein Kinase D and Lobe mutants during eye development of Drosophila melanogaster

Background

In Drosophila, the development of the fly eye involves the activity of several, interconnected pathways that first define the presumptive eye field within the eye anlagen, followed by establishment of the dorso-ventral boundary, and the regulation of growth and apoptosis. In Lobe (L) mutant flies, parts of the eye or even the complete eye are absent because the eye field has not been properly defined. Manifold genetic interactions indicate that L influences the activity of several signalling pathways, resulting in a conversion of eye tissue into epidermis, and in the induction of apoptosis. As information on the molecular nature of the L mutation is lacking, the underlying molecular mechanisms are still an enigma.

Results

We have identified Protein Kinase D (PKD) as a strong modifier of the L mutant phenotype. PKD belongs to the PKC/CAMK class of Ser/Thr kinases that have been involved in diverse cellular processes including stress resistance and growth. Despite the many roles of PKD, Drosophila PKD null mutants are without apparent phenotype apart from sensitivity to oxidative stress. Here we report an involvement of PKD in eye development in the sensitized genetic background of Lobe. Absence of PKD strongly enhanced the dominant eye defects of heterozygous L² flies, and decreased their viability. Moreover, eye-specific overexpression of an activated isoform of PKD considerably ameliorated the dominant L² phenotype. This genetic interaction was not allele specific but similarly seen with three additional, weaker L alleles (L¹, L⁵, L^G), demonstrating its specificity.

Conclusions

We propose that PKD-mediated phosphorylation is involved in underlying processes causing the L phenotype, i.e. in the regulation of growth, the epidermal transformation of eye tissue and apoptosis, respectively.

Protein Kinase D Is Dispensable for Development and Survival of Drosophila melanogaster

Members of the Protein Kinase D (PKD) family are involved in numerous cellular processes in mammals, including cell survival after oxidative stress, polarized transport of Golgi vesicles, as well as cell migration and invasion. PKD proteins belong to the PKC/CAMK class of serine/threonine kinases, and transmit diacylglycerol-regulated signals. Whereas three PKD isoforms are known in mammals, Drosophila melanogaster contains a single PKD homolog. Previous analyses using overexpression and RNAi studies indicated likewise multi-facetted roles for Drosophila PKD, including the regulation of secretory transport and actin-cytoskeletal dynamics. Recently, involvement in growth regulation has been proposed based on the hypomorphic dPKDH allele. We have generated PKD null alleles that are homozygous viable without apparent phenotype. They largely match control flies regarding fertility, developmental timing and weight. Males, but not females, are slightly shorter lived and starvation sensitive. Furthermore, migration of pole cells in embryos and border cells in oocytes appears normal. PKD mutants tolerate heat, cold and osmotic stress like the control but are sensitive to oxidative stress, conforming to the described role for mammalian PKDs. A candidate screen to identify functionally redundant kinases uncovered genetic interactions of PKD with Pkcδ, sqa and Drak mutants, further supporting the role of PKD in oxidative stress response, and suggesting its involvement in starvation induced autophagy and regulation of cytoskeletal dynamics. Overall, PKD appears dispensable for fly development and survival presumably due to redundancy, but influences environmental responses.

Maintenance of Rhodopsin levels in Drosophila photoreceptor and phototransduction requires Protein Kinase D

During Drosophila phototransduction, the G protein coupled receptor (GPCR) Rhodopsin (Rh1) transduces photon absorption into electrical signal via G-protein coupled activation of phospholipase C (PLC). Rh1 levels in the plasma membrane are critical for normal sensitivity to light. In this study, we report that Protein Kinase D (dPKD) regulates Rh1 homeostasis in adult photoreceptors. Although eye development and retinal structure are unaffected in the dPKD hypomorph (dPKD^H), it exhibited elevated levels of Rh1. Surprisingly, despite having elevated levels of Rh1, no defect was observed in the electrical response to light in these flies. By contrast the levels of another transmembrane protein of the photoreceptor plasma membrane, Transient receptor potential (TRP) was not altered in dPKD^H. Our results indicate that dPKD is dispensable for eye development but is required for maintaining Rh1 levels in adult photoreceptors.