Table of Contents

    Topic review

    TPC1 in plants

    View times: 219


    TPC1 in plants is localized in the vacuolar membrane. Its activity is strictly regulated by several factors emphasizing its complex structure and function. The physiological role of TPC1 is under debate. The TPC1 hyperactive version fou2 (carring D454N mutation) is characterized by an overproduction of jasmonate acid (JA), however the tpc1-2 knockout mutant has no pronounced phenotype. The intriguing concept of Ca2+-induced Ca2+ release was assigned to Vicia faba TPC1 in 1994 by Ward and Schroeder, however it has still not been confirmed for the model plant Arabidopsis thaliana.

    1. TPC (two-pore channel) 

    TPC1 is a non-selective, cation channel belonging to the superfamily of voltage-gated ion channels. TPC1 consists of 12 transmembrane domains (S1–S12) subdivided into two shaker-like segments connected via a cytoplasmic linker region carrying two EF-hand motifs. Each shaker-like segment consists of six transmembrane domains (S1–S6) including the voltage-sensing S4 domain and the ion-conducting pore domain between S5 and S6 [1].

    TPCs probably originate from a gene-duplication event of single-domain NaV channels and in contrast to TRPML channels, are widely found in terrestrial (e.g., Arabidopsis thaliana) and marine plants (e.g., Klebsormidium nitens[1][2]. All plants harbor at least one TPC gene, already present in the genome of charophytic algae that appeared on earth around 793 million years ago [1][3][4]. TPC1 activity was first shown by Hedrich and Neher in barley mesophyll vacuoles [5]. Upon activation, plant TPC1 provides an ion-conducting pathway for various cations, mainly K+ and Na+ [6]. Plant TPC1 (or SV, slow vacuolar channel as it was originally named) is modulated by several factors, underpinning its complex regulation. Beside voltage, TPC1 is regulated by calmodulin [7] , 14-3-3 proteins [8], kinases and phosphatases [9][10], pH [6][11], redox state [12], and Mg2+ and Ca2+ [5][13]. In addition, natural polyamines (e.g., spermidine [14][15]) and heavy metals [16] have been reported to inhibit ion passage through open TPC1 channels in red beet and radish.

    2. TPC1 function 

    Since loss of TPC1 function does not drastically impair plant growth [17], its physiological role is a matter of debate. However, roots of seedlings exposed to salt treatment show reduced growth in the TPC1 knockout tpc1-2 mutant compared to WT plants [18]. In contrast, TPC1 overexpression increases salt tolerance [18]. Interestingly, salt-triggered propagating Ca2+ signals in the root were attenuated in tpc1-2 mutants, but increased in TPC1 overexpression lines [18]. Furthermore, it was shown that systemic Ca2+ signals, generated upon wounding, were gone upon loss of TPC1 function. This observation pointed to a role of TPC1 in systemic Ca2+ signaling [19]. The Arabidopsis thaliana TPC1 fou2 variant (fatty acid oxygenation upregulated 2) point mutation D454N leads to an increased production of the stress hormone jasmonate, even under non-stressed conditions. The fou2 plants exhibit a strong growth retardation phenotype [20][21], probably originating from the increased vacuolar K+ release due to TPC1 hyperactivity [21]. It is important to note that a TPC1-independent pathway of jasmonate signaling has also been postulated [22]. Since TPC1 participates likely indirectly in the generation/modulation of the Ca2+ wave, it seems to be reasonable to suggest a supreme trigger, regulating Ca2+ and K+ fluxes [22]. Vacuolar membrane depolarization may be one of the missing early triggers for jasmonate production. Furthermore, TPC1 is a prerequisite for vacuole membrane excitability [23], thus triggering of vacuolar membrane depolarization in local spots may be an elaborate way to encode more complicated information in long- and short-distance signaling pathways in plants.

    3. Calcium Induced Calcium Release theory

    The concept of Ca2+-induced Ca2+ release (CICR), initially proposed by Fabiato et al. (1985) in the animal field, was adapted by Ward and Schroeder (1994) to plant research [24][25][26]. Based on patch-clamp measurements, they postulated cytosolic Ca2+ signals generated by TPC1 in Vicia faba guard cell vacuoles. However, the ionic composition used in this study was far away from the physiological concentration for Ca2+ and K+. By applying non-physiological ionic conditions, TPC1 channel-mediated Ca2+ currents were also recorded in other species, but only in the opposite direction, from cytosol to vacuole [27][28][29]. Furthermore, an inhibitory effect of vacuolar Ca2+ was postulated [30], likely attributable to the highly conserved vacuolar Ca2+-binding motifs of TPC1 [1][31]. To solve the above long-lasting debate, structural models for the different species will be helpful. Of note, the gain-of-function Arabidopsis thaliana TPC1 channel variant (fou2) shows increased vacuolar Ca2+ and slightly lower resting cytosolic Ca2+ levels compared to WT, which would be difficult to reconcile with TPC1 releasing Ca2+ under physiological conditions [11][22]. In sum, the contribution of plant TPC1 to global as well as local Ca2+ signals remains debated and needs to be further evaluated. A similar complex debate exists in the field of mammalian TPC research, where the role of TPCs in endo-lysosomal Ca2+ release remains likewise controversially discussed [32][33][34][35][36][37][38].

    The entry is from 10.3390/ijms21114181


    1. Hedrich, R.; Mueller, T.D.; Becker, D.; Marten, I. Structure and Function of TPC1 Vacuole SV Channel Gains Shape. Mol. Plant 2018, 11, 764–775.
    2. Carpaneto, A.; Cantu’, A.M.; Busch, H.; Gambale, F. Ion channels in the vacuoles of the seagrass Posidonia oceanica. FEBS Lett. 1997, 412, 236–240.
    3. Yoon, H.S.; Hackett, J.D.; Ciniglia, C.; Pinto, G.; Bhattacharya, D. A molecular timeline for the origin of photosynthetic eukaryotes. Mol. Biol. Evol. 2004, 21, 809–818.
    4. Wickett, N.J.; Mirarab, S.; Nguyen,N.;Warnow, T.; Carpenter, E.; Matasci, N.; Ayyampalayam, S.; Barker, M.S.; Burleigh, J.G.; Gitzendanner, M.A.; et al. Phylotranscriptomic analysis of the origin and early diversification of land plants. Proc. Natl. Acad. Sci. USA 2014, 111, E4859–E4868.
    5. Hedrich, R.; Neher, E. Cytoplasmic calcium regulates voltage-dependent ion channels in plant vacuoles. Nature 1987, 329, 833–836.
    6. Schulz-Lessdorf, B.; Hedrich, R. Protons and calcium modulate SV-type channels in the vacuolar-lysosomal compartment—Channel interaction with calmodulin inhibitors. Planta 1995, 197, 655–671.
    7. Weiser, T.; Blum, W.; Bentrup, F.W. Calmodulin regulates the Ca2+-dependent slow-vacuolar ion channel in the tonoplast of Chenopodium rubrum suspension cells. Planta 1991, 185, 440–442.
    8. Latz, A.; Becker, D.; Hekman, M.; Müller, T.; Beyhl, D.; Marten, I.; Eing, C.; Fischer, A.; Dunkel, M.; Bertl, A.; et al. TPK1, a Ca2+-regulated Arabidopsis vacuole two-pore K(+) channel is activated by 14-3-3 proteins. Plant J. 2007, 52, 449–459.
    9. Allen, G.J.; Sanders, D. Calcineurin, a Type 2B Protein Phosphatase, Modulates the Ca2+-Permeable Slow Vacuolar Ion Channel of Stomatal Guard Cells. Plant Cell 1995, 7, 1473–1483.
    10. Bethke, P.C.; Jones, R.L. Ca2+-Calmodulin Modulates Ion Channel Activity in Storage Protein Vacuoles of Barley Aleurone Cells. Plant Cell 1994, 6, 277–285.
    11. Beyhl, D.; Hörtensteiner, S.; Martinoia, E.; Farmer, E.E.; Fromm, J.; Marten, I.; Hedrich, R. The fou2 mutation in the major vacuolar cation channel TPC1 confers tolerance to inhibitory luminal calcium. Plant J. 2009, 58, 715–723.
    12. Carpaneto, A.; Cantù, A.M.; Gambale, F. Redox agents regulate ion channel activity in vacuoles from higher plant cells. FEBS Lett. 1999, 442, 129–132.
    13. Carpaneto, A.; Cantù, A.M.; Gambale, F. Effects of cytoplasmic Mg2+ on slowly activating channels in isolated vacuoles of Beta vulgaris. Planta 2001, 213, 457–468.
    14. Dobrovinskaya, O.R.; Muñiz, J.; Pottosin, I.I. Asymmetric block of the plant vacuolar Ca2+-permeable channel by organic cations. Eur. Biophys. J. 1999, 28, 552–563.
    15. Dobrovinskaya, O.R.; Muñiz, J.; Pottosin, I.I. Inhibition of vacuolar ion channels by polyamines. J. Membr. Biol. 1999, 167, 127–140.
    16. Carpaneto, A. Nickel inhibits the slowly activating channels of radish vacuoles. Eur. Biophys. J. 2003, 32, 60–66.
    17. Peiter, E.; Maathuis, F.J.M.; Mills, L.N.; Knight, H.; Pelloux, J.; Hetherington, A.M.; Sanders, D. The vacuolar Ca2+-activated channel TPC1 regulates germination and stomatal movement. Nature 2005, 434, 404–408.
    18. Choi, W.-G.; Toyota, M.; Kim, S.-H.; Hilleary, R.; Gilroy, S. Salt stress-induced Ca2+ waves are associated with rapid, long-distance root-to-shoot signaling in plants. Proc. Natl. Acad. Sci. USA 2014, 111, 6497–6502.
    19. Kiep, V.; Vadassery, J.; Lattke, J.; Maaß, J.-P.; Boland, W.; Peiter, E.; Mithöfer, A. Systemic cytosolic Ca2+ elevation is activated upon wounding and herbivory in Arabidopsis. New Phytol. 2015, 207, 996–1004.
    20. Bonaventure, G.; Gfeller, A.; Rodríguez, V.M.; Armand, F.; Farmer, E.E. The fou2 gain-of-function allele and the wild-type allele of Two Pore Channel 1 contribute to different extents or by different mechanisms to defense gene expression in Arabidopsis. Plant Cell Physiol. 2007, 48, 1775–1789.
    21. Bonaventure, G.; Gfeller, A.; Proebsting, W.M.; Hörtensteiner, S.; Chételat, A.; Martinoia, E.; Farmer, E.E. A gain-of-function allele of TPC1 activates oxylipin biogenesis after leaf wounding in Arabidopsis. Plant J. 2007, 49, 889–898.
    22. Lenglet, A.; Jaślan, D.; Toyota, M.; Mueller, M.; Müller, T.; Schönknecht, G.; Marten, I.; Gilroy, S.; Hedrich, R.; Farmer, E.E. Control of basal jasmonate signalling and defence through modulation of intracellular cation flux capacity. New Phytol. 2017, 216, 1161–1169.
    23. Jaślan, D.; Dreyer, I.; Lu, J.; O’Malley, R.; Dindas, J.; Marten, I.; Hedrich, R. Voltage-dependent gating of SV channel TPC1 confers vacuole excitability. Nat. Commun. 2019, 10, 2659.
    24. Fabiato, A. Simulated calcium current can both cause calcium loading in and trigger calcium release from the sarcoplasmic reticulum of a skinned canine cardiac Purkinje cell. J. Gen. Physiol. 1985, 85, 291–320.
    25. Fabiato, A. Time and calcium dependence of activation and inactivation of calcium-induced release of calcium from the sarcoplasmic reticulum of a skinned canine cardiac Purkinje cell. J. Gen. Physiol. 1985, 85, 247–289.
    26. Ward, J.M.; Schroeder, J.I. Calcium-Activated K+ Channels and Calcium-Induced Calcium Release by Slow Vacuolar Ion Channels in Guard Cell Vacuoles Implicated in the Control of Stomatal Closure. Plant Cell 1994, 6, 669–683.
    27. Ivashikina, N.; Hedrich, R. K+ currents through SV-type vacuolar channels are sensitive to elevated luminal sodium levels. Plant J. 2005, 41, 606–614.
    28. Gradogna, A.; Scholz-Starke, J.; Gutla, P.V.K.; Carpaneto, A. Fluorescence combined with excised patch: Measuring calcium currents in plant cation channels. Plant J. 2009, 58, 175–182.
    29. Rienmüller, F.; Beyhl, D.; Lautner, S.; Fromm, J.; Al-Rasheid, K.A.S.; Ache, P.; Farmer, E.E.; Marten, I.; Hedrich, R. Guard cell-specific calcium sensitivity of high density and activity SV/TPC1 channels. Plant Cell Physiol. 2010, 51, 1548–1554.
    30. Pottosin, I.I.; Tikhonova, L.I.; Hedrich, R.; Schönknecht, G. Slowly activating vacuolar channels can not mediate Ca2+-induced Ca2+ release. Plant J. 1997, 12, 1387–1398.
    31. Dadacz-Narloch, B.; Beyhl, D.; Larisch, C.; López-Sanjurjo, E.J.; Reski, R.; Kuchitsu, K.; Müller, T.D.; Becker, D.; Schönknecht, G.; Hedrich, R. A novel calcium binding site in the slow vacuolar cation channel TPC1 senses luminal calcium levels. Plant Cell 2011, 23, 2696–2707.
    32. Calcraft, P.J.; Ruas, M.; Pan, Z.; Cheng, X.; Arredouani, A.; Hao, X.; Tang, J.; Rietdorf, K.; Teboul, L.;Chuang, K.-T.; et al. NAADP mobilizes calcium from acidic organelles through two-pore channels. Nature 2009, 459, 596–600.
    33. Cang, C.; Zhou, Y.; Navarro, B.; Seo, Y.-J.; Aranda, K.; Shi, L.; Battaglia-Hsu, S.; Nissim, I.; Clapham, D.E.; Ren, D. mTOR regulates lysosomal ATP-sensitive two-pore Na(+) channels to adapt to metabolic state. Cell 2013, 152, 778–790.
    34. Wang, X.; Zhang, X.; Dong, X.-P.; Samie, M.; Li, X.; Cheng, X.; Goschka, A.; Shen, D.; Zhou, Y.; Harlow, J.; et al. TPC proteins are phosphoinositide- activated sodium-selective ion channels in endosomes and lysosomes. Cell 2012, 151, 372–383.
    35. Ruas, M.; Galione, A.; Parrington, J. Two-Pore Channels: Lessons from Mutant Mouse Models. Messenger (Los Angel) 2015, 4, 4–22.
    36. Grimm, C.; Holdt, L.M.; Chen, C.-C.; Hassan, S.; Müller, C.; Jörs, S.; Cuny, H.; Kissing, S.; Schröder, B.; Butz, E.; et al. High susceptibility to fatty liver disease in two-pore channel 2-deficient mice. Nat. Commun. 2014, 5, 4699.
    37. Zhang, X.; Chen, W.; Li, P.; Calvo, R.; Southall, N.; Hu, X.; Bryant-Genevier, M.; Feng, X.; Geng, Q.; Gao, C.; et al. Agonist-specific voltage-dependent gating of lysosomal two-pore Na+ channels. eLife 2019, 8.
    38. Gerndt, S.; Chen, C.-C.; Chao, Y.-K.; Yuan, Y.; Burgstaller, S.; Scotto Rosato, A.; Krogsaeter, E.; Urban, N.; Jacob, K.; Nguyen, O.N.P.; et al. Agonist mediated switching of ion selectivity in TPC2 differentially promotes lysosomal function. eLife 2020, 9.
    1. Please check and comment entries here.