Eleonora Zakharian

Gating of Transient Receptor Potential Melastatin 8 (TRPM8) Channels Activated by Cold and Chemical Agonists in Planar Lipid Bilayers

The transient receptor potential melastatin 8 (TRPM8) ion channel is a major sensor of environmental cold temperatures. It is activated by cold and chemical agonists, such as menthol and icilin. The activation of these channels both by cold and cooling agents requires the presence of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. The mechanism of TRPM8 activation by physical and chemical factors is unknown, and the involvement of cellular signaling pathways has been considered. Here we have characterized the gating mechanism of the rat TRPM8 reconstituted in planar lipid bilayers and its activation by different stimuli. In this system, the influence of cellular signaling pathways can be excluded. We found that TRPM8 activated by cold exhibits steep temperature dependence [temperature coefficient (Q 10) of ∼40], and the channel openings are accompanied by large changes in entropy and enthalpy, suggesting a substantial conformation change. TRPM8 channel behavior upon menthol and icilin activation was distinguishable, and the effect of icilin depended on the presence of calcium on the intracellular side of the protein. Here we also demonstrate that PI(4,5)P2 is the prime factor that impacts the gating of TRPM8 and that other phosphoinositides are less efficient in supporting channel activity. Menthol increases the potency of PI(4,5)P2 to activate the channels and increases binding of phosphoinositides to the full-length channel protein. Our data demonstrate conclusively that TRPM8 is gated by cold and its chemical agonists directly, and that dependence of its gating on PI(4,5)P2 is a result of direct specific interactions with the lipid.

Inorganic polyphosphate modulates TRPM8 channels

Polyphosphate (polyP) is an inorganic polymer built of tens to hundreds of phosphates, linked by high-energy phosphoanhydride bonds. PolyP forms complexes and modulates activities of many proteins including ion channels. Here we investigated the role of polyP in the function of the transient receptor potential melastatin 8 (TRPM8) channel. Using whole-cell patch-clamp and fluorescent calcium measurements we demonstrate that enzymatic breakdown of polyP by exopolyphosphatase (scPPX1) inhibits channel activity in human embryonic kidney and F-11 neuronal cells expressing TRPM8. We demonstrate that the TRPM8 channel protein is associated with polyP. Furthermore, addition of scPPX1 altered the voltage-dependence and blocked the activity of the purified TRPM8 channels reconstituted into planar lipid bilayers, where the activity of the channel was initiated by cold and menthol in the presence of phosphatidylinositol 4,5-biphosphate (PtdIns(4,5)P2). The biochemical analysis of the TRPM8 protein also uncovered the presence of poly-(R)-3-hydroxybutyrate (PHB), which is frequently associated with polyP. We conclude that the TRPM8 protein forms a stable complex with polyP and its presence is essential for normal channel activity.

Promiscuous activation of transient receptor potential vanilloid 1 (TRPV1) channels by negatively charged intracellular lipids: the key role of endogenous phosphoinositides in maintaining channel activity.

Background: Regulation of TRPV1 by phosphoinositides is controversial. Results: ATP reactivates TRPV1 after run-down in excised inside-out patches by generating phosphoinositides; many other negatively charged lipids may also support TRPV1 activity. Conclusion: Despite its promiscuous activation, phosphoinositides are the key endogenous cofactors for TRPV1 activity. Significance: Our data may reconcile discordant data obtained in different experimental settings. The regulation of the heat- and capsaicin-activated transient receptor potential vanilloid 1 (TRPV1) channels by phosphoinositides is controversial. Data in cellular systems support the dependence of TRPV1 activity on phosphoinositides. The purified TRPV1, however, was recently shown to be fully functional in artificial liposomes in the absence of phosphoinositides. Here, we show that several other negatively charged phospholipids, including phosphatidylglycerol, can also support TRPV1 activity in excised patches at high concentrations. When we incorporated TRPV1 into planar lipid bilayers consisting of neutral lipids, capsaicin-induced activity depended on phosphatidylinositol 4,5-bisphosphate. We also found that TRPV1 activity in excised patches ran down and that MgATP reactivated the channel. Inhibition of phosphatidylinositol 4-kinases or enzymatic removal of phosphatidylinositol abolished this effect of MgATP, suggesting that it activated TRPV1 by generating endogenous phosphoinositides. We conclude that endogenous phosphoinositides are positive cofactors for TRPV1 activity. Our data highlight the importance of specificity in lipid regulation of ion channels and may reconcile discordant data obtained in various experimental settings.

The TRPM8 Protein Is a Testosterone Receptor II. FUNCTIONAL EVIDENCE FOR AN IONOTROPIC EFFECT OF TESTOSTERONE ON TRPM8

Background: TRPM8 channels are highly expressed in prostate tissues, where the role of this cold receptor is not well understood. Results: Testosterone activates TRPM8 in various cellular systems and in the planar lipid bilayers. Conclusion: TRPM8 is an ionotropic testosterone receptor. Significance: TRPM8 channels may be implicated in various physiological processes regulated by androgens. Testosterone is a key steroid hormone in the development of male reproductive tissues and the regulation of the central nervous system. The rapid signaling mechanism induced by testosterone affects numerous behavioral traits, including sexual drive, aggressiveness, and fear conditioning. However, the currently identified testosterone receptor(s) is not believed to underlie the fast signaling, suggesting an orphan pathway. Here we report that an ion channel from the transient receptor potential family, TRPM8, commonly known as the cold and menthol receptor is the major component of testosterone-induced rapid actions. Using cultured and primary cell lines along with the purified TRPM8 protein, we demonstrate that testosterone directly activates TRPM8 channel at low picomolar range. Specifically, testosterone induced TRPM8 responses in primary human prostate cells, PC3 prostate cancer cells, dorsal root ganglion neurons, and hippocampal neurons. Picomolar concentrations of testosterone resulted in full openings of the purified TRPM8 channel in planar lipid bilayers. Furthermore, acute applications of testosterone on human skin elicited a cooling sensation. Our data conclusively demonstrate that testosterone is an endogenous and highly potent agonist of TRPM8, suggesting a role of TRPM8 channels well beyond their well established function in somatosensory neurons. This discovery may further imply TRPM8 channel function in testosterone-dependent behavioral traits.

The TRPM8 Protein Is a Testosterone Receptor I. BIOCHEMICAL EVIDENCE FOR DIRECT TRPM8-TESTOSTERONE INTERACTIONS

Background: TRPM8 channels are highly expressed in prostate tissues, where the role of this cold receptor is not well understood. Results: Testosterone directly interacts with the TRPM8 protein. Conclusion: TRPM8 is a testosterone receptor. Significance: TRPM8 channels may be implicated in various physiological processes regulated by androgens. The transient receptor potential ion channel of the melastatin subfamily, TRPM8, is a major cold receptor in the peripheral nervous system. Along with the sensory neurons, the TRPM8 protein is highly expressed in the prostate epithelial cells, and this expression is regulated by androgens. Here we investigated the expression and intracellular localization of the TRPM8 channel in relationship to androgens. We performed experiments using human prostate tissues obtained from healthy individuals and patients with prostate cancer at various stages of the disease as well as in cultured cells. Using an immunohistochemistry approach, we detected an intensive colocalization pattern of the TRPM8 protein with endogenous androgens in all tissues tested, suggesting possible interactions. Co-immunoprecipitation experiments performed using cultured prostate epithelial cells, prostate cancer cells, and HEK-293 cells stably expressing TRPM8 further confirmed direct binding of the steroid hormone, testosterone, to the TRPM8 protein. Applications of picomolar concentrations of testosterone to the primary human prostate cells, endogenously expressing TRPM8, elicited Ca2+ responses and channel currents, and those were inhibited in the presence of TRPM8 antagonist, N-(2-aminoethyl)-N-(4-(benzyloxy)-3-methoxybenzyl)thiophene-2-carboxamide hydrochloride. These results indicate that the TRPM8 channel is physically associated with testosterone and suggest that, in addition to a genomic role, testosterone plays a role in direct regulation of the TRPM8 channel function.

Intracellular ATP supports TRPV6 activity via lipid kinases and the generation of PtdIns(4,5)P2

Transient receptor potential vanilloid 6 (TRPV6) channels play an important role in Ca2+ absorption in the intestines. Both phosphatidylinositol 4,5‐bisphosphate [PtdIns(4,5)P2] and cytoplasmic ATP have been proposed to be important for maintaining TRPV6 activity. To evaluate whether PtdIns(4,5)P2 and ATP affect channel activity directly or indirectly, we have used a dual approach, examining channel activity in excised patches and planar lipid bilayers. In excised inside‐out patch‐clamp measurements, ATP reactivated the human TRPV6 channels after current rundown only in the presence of Mg2+. The effect of MgATP was inhibited by 3 structurally different compounds that inhibit type III phosphatidylinositol 4‐kinases (PI4Ks). PtdIns(4,5)P2 also activated TRPV6 in excised patches, while its precursor PtdIns(4)P had only minimal effect. These data demonstrate that MgATP provides substrate for lipid kinases, allowing the resynthesis of PtdIns(4,5)P2. To determine whether PtdIns(4,5)P2 is a direct activator of TRPV6, we purified and reconstituted the channel protein in planar lipid bilayers. The reconstituted channel showed high activity in the presence of PtdIns(4,5)P2, while PtdIns(4)P induced only minimal activity. Our data establish PtdIns(4,5)P2 as a direct activator of TRPV6 and demonstrate that intracellular ATP regulates the channel indirectly as a substrate for type III PI4Ks.—Zakharian, E., Cao, C., Rohacs, T. Intracellular ATP supports TRPV6 activity via lipid kinases and the generation of PtdIns(4,5)P2. FASEB J. 25, 3915–3928 (2011). www.fasebj.org

Interplay between calmodulin and phosphatidylinositol 4,5-bisphosphate in Ca2+-induced inactivation of Transient Receptor Potential Vanilloid 6 channels

Background: A large number of calmodulin-binding sites have been proposed in TRPV6. Results: We have identified the site that is responsible for inhibition of TRPV6 by calmodulin in excised inside-out patch clamp experiments. Conclusion: Calmodulin and PI(4,5)P2 antagonistically regulate TRPV6, but not through direct competition. Significance: This study provides mechanistic insight into Ca2+-induced inactivation of TRPV6. The epithelial Ca2+ channel transient receptor potential vanilloid 6 (TRPV6) undergoes Ca2+-induced inactivation that protects the cell from toxic Ca2+ overload and may also limit intestinal Ca2+ transport. To dissect the roles of individual signaling pathways in this phenomenon, we studied the effects of Ca2+, calmodulin (CaM), and phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) in excised inside-out patches. The activity of TRPV6 strictly depended on the presence of PI(4,5)P2, and Ca2+-CaM inhibited the channel at physiologically relevant concentrations. Ca2+ alone also inhibited TRPV6 at high concentrations (IC50 = ∼20 μm). A double mutation in the distal C-terminal CaM-binding site of TRPV6 (W695A/R699E) essentially eliminated inhibition by CaM in excised patches. In whole cell patch clamp experiments, this mutation reduced but did not eliminate Ca2+-induced inactivation. Providing excess PI(4,5)P2 reduced the inhibition by CaM in excised patches and in planar lipid bilayers, but PI(4,5)P2 did not inhibit binding of CaM to the C terminus of the channel. Overall, our data show a complex interplay between CaM and PI(4,5)P2 and show that Ca2+, CaM, and the depletion of PI(4,5)P2 all contribute to inactivation of TRPV6.

Role of polyhydroxybutyrate in mitochondrial calcium uptake

Polyhydroxybutyrate (PHB) is a biological polymer which belongs to the class of polyesters and is ubiquitously present in all living organisms. Mammalian mitochondrial membranes contain PHB consisting of up to 120 hydroxybutyrate residues. Roles played by PHB in mammalian mitochondria remain obscure. It was previously demonstrated that PHB of the size similar to one found in mitochondria mediates calcium transport in lipid bilayer membranes. We hypothesized that the presence of PHB in mitochondrial membrane might play a significant role in mitochondrial calcium transport. To test this, we investigated how the induction of PHB hydrolysis affects mitochondrial calcium transport. Mitochondrial PHB was altered enzymatically by targeted expression of bacterial PHB hydrolyzing enzyme (PhaZ7) in mitochondria of mammalian cultured cells. The expression of PhaZ7 induced changes in mitochondrial metabolism resulting in decreased mitochondrial membrane potential in HepG2 but not in U87 and HeLa cells. Furthermore, it significantly inhibited mitochondrial calcium uptake in intact HepG2, U87 and HeLa cells stimulated by the ATP or by the application of increased concentrations of calcium to the digitonin permeabilized cells. Calcium uptake in PhaZ7 expressing cells was restored by mimicking calcium uniporter properties with natural electrogenic calcium ionophore - ferutinin. We propose that PHB is a previously unrecognized important component of the mitochondrial calcium uptake system.

Regulation of the temperature-dependent activation of transient receptor potential vanilloid 1 (TRPV1) by phospholipids in planar lipid bilayers

Background: Regulation of the temperature-dependent activity of TRPV1 channels with phosphoinositides is not well understood. Results: Phosphoinositides directly facilitate heat activation of TRPV1 reconstituted in neutral planar lipid bilayers. Different anionic lipids evoke TRPV1 channel activity with distinct open probability and conductance states. Conclusion: Phospholipids are required for heat-induced activation of TRPV1. Significance: Phosphoinositides are essential regulators of TRPV1 gating. TRPV1 (transient receptor potential vanilloid 1) proteins are heat-activated nonselective cation channels. TRPV1 channels are polymodal in their function and exhibit multifaceted regulation with various molecular compounds. In this regard, phosphoinositides, particularly phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 4-phosphate, are important channel regulators. However, their effects on TRPV1 channel activity have not been conclusively determined. To characterize temperature-induced activation of TRPV1 in the presence of different phospholipids, we purified the TRPV1 protein from HEK-293 cells and incorporated it into planar lipid bilayers. In the presence of 2.5 μm phosphatidylinositol 4,5-bisphosphate, TRPV1 channels demonstrated rapid activation at 33–39 °C and achieved full channel opening at 42 °C. At this temperature range, TRPV1 heat activation exhibited steep temperature dependence (temperature coefficient (Q10) of 18), and the channel openings were accompanied by large changes in entropy and enthalpy, suggesting a substantial conformation change. At a similar temperature range, another phosphoinositide, phosphatidylinositol 4-phosphate, also potentiated heat activation of TRPV1, but with much lower efficiency. Negatively charged phosphatidylglycerol could also induce heat activation of TRPV1 channels, although with a small-conductance state. Our data demonstrate that phospholipids, specifically phosphoinositides, are important regulators of TRPV1 and are required for heat-induced channel activity.

RTX Toxin Plays a Key Role in Kingella kingae Virulence in an Infant Rat Model

ABSTRACT Kingella kingae is a human oral bacterium that can cause diseases of the skeletal system in children and infective endocarditis in children and adults. K. kingae produces a toxin of the RTX group, RtxA. To investigate the role of RtxA in disease pathogenesis in vivo, K. kingae strain PYKK081 and its isogenic RtxA-deficient strain KKNB100 were tested for their virulence and pathological consequences upon intraperitoneal injections in 7-day-postnatal (PN 7) rats. At the doses above 8.0 × 106 cells/animal, PYKK081 was able to cause a fatal illness, resulting in rapid weight loss, bacteremia, and abdominal necrotic lesion formation. Significant histopathology was observed in thymus, spleen, and bone marrow. Strain KKNB100 was less toxic to animals. Neither weight loss, bacteremia, nor histopathological changes were evident. Animals injected with KKNB100 exhibited a significantly elevated circulating white blood cell (WBC) count, whereas animals injected with PYKK081 had a WBC count that resembled that of the uninfected control. This observation parallels the subtleties associated with clinical presentation of K. kingae disease in humans and suggests that the toxin contributes to WBC depletion. Thus, our results demonstrate that RtxA is a key K. kingae virulence factor. Furthermore, our findings suggest that the PN 7 rat can serve as a useful model for understanding disease caused by K. kingae and for elucidating diagnostic parameters in human patients.