Summary The mammalian Phospholipase D MitoPLD facilitates mitochondrial fusion by generating the signaling lipid phosphatidic acid (PA). The Drosophila MitoPLD homolog Zucchini (Zuc), a proposed cytoplasmic nuclease, is required for piRNA generation, a critical event in germline development. We show that Zuc localizes to mitochondria and has MitoPLD-like activity. Conversely, MitoPLD−/− mice exhibit the meiotic arrest, DNA damage, and male sterility characteristic of mice lacking piRNAs. The primary function of MitoPLD appears to be the generation of mitochondrial-surface PA. This PA in turn recruits the phosphatase Lipin 1, which converts PA to diacylglycerol and promotes mitochondrial fission, suggesting a mechanism for mitochondrial morphology homeostasis. MitoPLD and Lipin 1 have opposing effects on mitochondria length and on intermitochondrial cement (nuage), a structure found between aggregated mitochondria that is implicated in piRNA generation. We propose that mitochondrial-surface PA generated by MitoPLD / Zuc recruits or activates nuage components critical for piRNA production.

Analyses of viral protein-protein interactions are an important step to understand viral protein functions and their underlying molecular mechanisms. In this study, we adopted a mammalian two-hybrid system to screen the genome-wide intraviral protein-protein interactions of SARS coronavirus (SARS-CoV) and therefrom revealed a number of novel interactions which could be partly confirmed by in vitro biochemical assays. Three pairs of the interactions identified were detected in both directions: non-structural protein (nsp) 10 and nsp14, nsp10 and nsp16, and nsp7 and nsp8. The interactions between the multifunctional nsp10 and nsp14 or nsp16, which are the unique proteins found in the members of Nidovirales with large RNA genomes including coronaviruses and toroviruses, may have important implication for the mechanisms of replication/transcription complex assembly and functions of these viruses. Using a SARS-CoV replicon expressing a luciferase reporter under the control of a transcription regulating sequence, it has been shown that several viral proteins (N, X and SUD domains of nsp3, and nsp12) provided in trans stimulated the replicon reporter activity, indicating that these proteins may regulate coronavirus replication and transcription. Collectively, our findings provide a basis and platform for further characterization of the functions and mechanisms of coronavirus proteins.

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Liver fatty acid binding protein (FABP1, L-FABP) has high affinity for and enhances uptake of arachidonic acid (ARA, C20:4, n-6) which, when esterified to phospholipids, is the requisite precursor for synthesis of endocannabinoids (EC) such as arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG). The brain derives most of its ARA from plasma, taking up ARA and transporting it intracellularly via cytosolic fatty acid binding proteins (FABPs 3,5, and 7) localized within the brain. In contrast, the much more prevalent cytosolic FABP1 is not detectable in the brain but is instead highly expressed in the liver. Therefore, the possibility that FABP1 outside the central nervous system may regulate brain AEA and 2-AG was examined in wild-type (WT) and FABP1 null (LKO) male mice. LKO increased brain levels of AA-containing EC (AEA, 2-AG), correlating with increased free and total ARA in brain and serum. LKO also increased brain levels of non-ARA that contain potentiating endocannabinoids (EC*) such as OEA, PEA, 2-OG, and 2-PG. Concomitantly, LKO decreased serum total ARA-containing EC, but not non-ARA endocannabinoids. LKO did not elicit these changes in the brain EC and EC* due to compensatory upregulation of brain protein levels of enzymes in EC synthesis (NAPEPLD, DAGLα) or cytosolic EC chaperone proteins (FABPs 3, 5, 7, SCP-2, HSP70), or cannabinoid receptors (CB1, TRVP1). These data show for the first time that the non-CNS fatty acid binding protein FABP1 markedly affected brain levels of both ARA-containing endocannabinoids (AEA, 2-AG) as well as their non-ARA potentiating endocannabinoids.

Phospholipase D (PLD), a superfamily of signaling enzymes that most commonly generate the lipid second messenger Phosphatidic Acid (PA), is found in diverse organisms from bacteria to man and functions in multiple cellular pathways. Since the early 1980’s when mammalian PLD activities were first described, most of the important insights concerning PLD function have been gained from studies on cellular models. Reports on physiological and pathophysiological roles for members of the mammalian PLD superfamily are now starting to emerge through from genetic models. In this review, we summarize recent findings on PLD functions in these model systems, highlighting newly appreciated connections of the superfamily to cancer, neuronal pathophysiology, cardiovascular topics, spermatogenesis, and infectious disease.