The quantitatively minor phospholipid phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P2] fulfils many cellular functions in the plasma membrane (PM), whereas its major synthetic precursor, phosphatidylinositol 4-phosphate (PI4P), has no assigned PM roles apart from PI(4,5)P2 synthesis. We used a combination of pharmacological and chemical genetic approaches to probe the function of PM PI4P, which was not required for the synthesis or functions of PI(4,5)P2. However, depletion of both lipids was required to prevent PM targeting of proteins that interact with acidic lipids, or activation of the transient receptor potential vanilloid 1 cation channel. Therefore, PI4P contributes to the pool of polyanionic lipids that define plasma membrane identity, and to some functions previously attributed specifically to PI(4,5)P2 that may be fulfilled by a more general polyanionic lipid requirement.

Characterization of a new biosensor for PtdIns4P reveals a wider cellular distribution for the polyphosphoinositide than the Golgi localization reported previously, including pools in both the plasma membrane and late endosomes/lysosomes.

PtdIns4P is the major precursor for the synthesis of the multifunctional plasma membrane lipid, PtdIns(4,5)P2. Yet PtdIns4P also functions as a regulatory lipid in its own right, particularly at the Golgi apparatus. In the present study we define specific conditions that enable preservation of several organellar membranes for the immunocytochemical detection of PtdIns4P. We report distinct pools of this lipid in both Golgi and plasma membranes, which are synthesized by different PI4K (phosphatidylinositol 4-kinase) activities, and also the presence of PtdIns4P in cytoplasmic vesicles, which are not readily identifiable as PI4K containing trafficking intermediates. In addition, we present evidence that the majority of PtdIns4P resides in the plasma membrane, where it is metabolically distinct from the steady-state plasma membrane pool of PtdIns(4,5)P2.

Polyphosphoinositides (PPIn) are an important family of phospholipids located on the cytoplasmic leaflet of eukaryotic cell membranes. Collectively, they are critical for the regulation many aspects of membrane homeostasis and signaling, with notable relevance to human physiology and disease. This regulation is achieved through the selective interaction of these lipids with hundreds of cellular proteins, and thus the capability to study these localized interactions is crucial to understanding their functions. In this review, we discuss current knowledge of the principle types of PPIn-protein interactions, focusing on specific lipid-binding domains. We then discuss how these domains have been re-tasked by biologists as molecular probes for these lipids in living cells. Finally, we describe how the knowledge gained with these probes, when combined with other techniques, has led to the current view of the lipids’ localization and function in eukaryotes, focusing mainly on animal cells.

Sohn et al. show that plasma membrane PI(4,5)P2 controls the level of its precursor, PI4P, by regulating PI4P/PS exchange activity of ORP5/8. This control is achieved via regulation of ORP5/8 interaction with the plasma membrane by both of these phosphoinositides.

Lysosomal distribution is linked to the role of lysosomes in many cellular functions, including autophagosome degradation, cholesterol homeostasis, antigen presentation, and cell invasion. Alterations in lysosomal positioning contribute to different human pathologies, such as cancer, neurodegeneration, and lysosomal storage diseases. Here we report the identification of a novel mechanism of lysosomal trafficking regulation. We found that the lysosomal transmembrane protein TMEM55B recruits JIP4 to the lysosomal surface, inducing dynein-dependent transport of lysosomes toward the microtubules minus-end. TMEM55B overexpression causes lysosomes to collapse into the cell center, whereas depletion of either TMEM55B or JIP4 results in dispersion toward the cell periphery. TMEM55B levels are transcriptionally upregulated following TFEB and TFE3 activation by starvation or cholesterol-induced lysosomal stress. TMEM55B or JIP4 depletion abolishes starvation-induced retrograde lysosomal transport and prevents autophagosome–lysosome fusion. Overall our data suggest that the TFEB/TMEM55B/JIP4 pathway coordinates lysosome movement in response to a variety of stress conditions.

Background: PI4KA is a critical host factor for replication of hepatitis C virus in liver and a potential therapeutic target. Results: PI4KA inhibitors prevent the maintenance of PtdIns(4,5)P 2 pools during strong PLC activation. Conclusion: PI4KA plays a critical role in maintaining plasma membrane phosphoinositide pools. Significance: Safe pharmacological targeting of PI4KA is not feasible.

Transient Receptor Potential Vanilloid 1 (TRPV1) is a polymodal, Ca 2ϩ-permeable cation channel crucial to regulation of nociceptor responsiveness. Sensitization of TRPV1 by G-protein coupled receptor (GPCR) agonists to its endogenous activators, such as low pH and noxious heat, is a key factor in hyperalgesia during tissue injury as well as pathological pain syndromes. Conversely, chronic pharmacological activation of TRPV1 by capsaicin leads to calcium influx-induced adaptation of the channel. Paradoxically, both conditions entail activation of phospholipase C (PLC) enzymes, which hydrolyze phosphoinositides. We found that in sensory neurons PLC␤ activation by bradykinin led to a moderate decrease in phosphatidylinositol-4,5-bisphosphate (PI(4,5)P 2 ), but no sustained change in the levels of its precursor PI(4)P. Preventing this selective decrease in PI(4,5)P 2 inhibited TRPV1 sensitization, while selectively decreasing PI(4,5)P 2 independently of PLC potentiated the sensitizing effect of protein kinase C (PKC) on the channel, thereby inducing increased TRPV1 responsiveness. Maximal pharmacological TRPV1 stimulation led to a robust decrease of both PI(4,5)P 2 and its precursor PI(4)P in sensory neurons. Attenuating the decrease of either lipid significantly reduced desensitization, and simultaneous reduction of PI(4,5)P 2 and PI(4)P independently of PLC inhibited TRPV1. We found that, on the mRNA level, the dominant highly Ca 2ϩ -sensitive PLC isoform in dorsal root ganglia is PLC␦4. Capsaicin-induced desensitization of TRPV1 currents was significantly reduced, whereas capsaicininduced nerve impulses in the skin-nerve preparation increased in mice lacking this isoform. We propose a comprehensive model in which differential changes in phosphoinositide levels mediated by distinct PLC isoforms result in opposing changes in TRPV1 activity.