Implication of voltage-gated potassium channels in neoplastic cell proliferation

Voltage-gated potassium channels (Kv) are the largest group of ion channels. Kv are involved in controlling the resting potential and action potential duration in the heart and brain. Additionally, these proteins participate in cell cycle progression as well as in several other important features in mammalian cell physiology, such as activation, differentiation, apoptosis, and cell volume control. Therefore, Kv remarkably participate in the cell function by balancing responses. The implication of Kv in physiological and pathophysiological cell growth is the subject of study, as Kv are proposed as therapeutic targets for tumor regression. Though it is widely accepted that Kv channels control proliferation by allowing cell cycle progression, their role is controversial. Kv expression is altered in many cancers, and their participation, as well as their use as tumor markers, is worthy of effort. There is an ever-growing list of Kv that remodel during tumorigenesis. This review focuses on the actual knowledge of Kv channel expression and their relationship with neoplastic proliferation. In this work, we provide an update of what is currently known about these proteins, thereby paving the way for a more precise understanding of the participation of Kv during cancer development

Caveolin interaction governs Kv1.3 lipid raft targeting

The spatial localization of ion channels at the cell surface is crucial for their functional role. Many channels localize in lipid raft microdomains, which are enriched in cholesterol and sphingolipids. Caveolae, specific lipid rafts which concentrate caveolins, harbor signaling molecules and their targets becoming signaling platforms crucial in cell physiology. However, the molecular mechanisms involved in such spatial localization are under debate. Kv1.3 localizes in lipid rafts and participates in the immunological response. We sought to elucidate the mechanisms of Kv1.3 surface targeting, which govern leukocyte physiology. Kv1 channels share a putative caveolin-binding domain located at the intracellular N-terminal of the channel. This motif, lying close to the S1 transmembrane segment, is situated near the T1 tetramerization domain and the determinants involved in the Kvβ subunit association. The highly hydrophobic domain (FQRQVWLLF) interacts with caveolin 1 targeting Kv1.3 to caveolar rafts. However, subtle variations of this cluster, putative ancillary associations and different structural conformations can impair the caveolin recognition, thereby altering channel's spatial localization. Our results identify a caveolin-binding domain in Kv1 channels and highlight the mechanisms that govern the regulation of channel surface localization during cellular processes

Functional implications of Kv1.3 spatial organization

Kv1.3 is a voltage-gated potassium channel mainly expressed at the plasma membrane and at the inner mitochondrial membrane of different cell types. A dual role for the channel has been suggested: Plasma membrane channel is linked to proliferation and activation while mitochondrial channel controls apoptosis. In this thesis, we analyse in depth the duality of Kv1.3 function. We show that Kv1.3 orchestrates cellular physiology in a way even more complex than expected. The thesis is structured in two parts: Plasma membrane Kv1.3 and Mitochondrial Kv1.3. Each part contains three contributions addressed to the understanding of Kv1.3 traffic and physiological implications of its localization. In the first part (Plasma membrane Kv1.3), we describe the caveolin-binding domain (CBD) of Kv1.3 as the determinant for the lipid raft targeting of the channel in cells expressing caveolin (Contribution I). The caveolar localization of the channel is required for proper insulin signalling in adipocytes. In these cells, Kv1.3 controls glucose uptake and insulin resistance. Kv1.3 is a target of Insulin Receptor activation via its localization at caveolae (Contribution II). Finally, we focus on the mechanisms of membrane arrangement of Kv1.3 in systems with limited Caveolin expression, such as lymphocytes, where the expression of Kv1.3 at the plasma membrane is also crucial. In this context, Kv1.3 palmitoylation resulted to be important for the Kv1.3 organization at the immunological synapse. This is the first time that Kv1.3 lipidation is reported and that Kv1.3 is visualized at the immunological synapse platform. Thus, our work expands our knowledge to define better therapeutic strategies in autoimmune diseases, where Kv1.3 is aberrantly expressed at the immunological synapse (Contribution III). In the second part (Mitochondrial Kv1.3), we first identify caveolin interaction as regulator of Kv1.3 pro-apoptotic activity. Kv1.3/Caveolin axis is an important regulator of cell apoptosis, with important consequences for chemotherapy resistance (Contribution IV). We also describe, for the first time, the mechanisms of Kv1.3 mitochondrial targeting, which involve an unconventional TIM/TOM pathway. We identify cytosolic chaperones as key regulators of Kv1.3 mitochondrial import and show how transmembrane domains cooperate to mediate such process (Contribution V). Finally, we show, for the first time, that mitochondrial Kv1.3 can also regulate proliferation through the control of mitochondrial fusion/fission equilibrium. This is a novel finding because reveals a novel function for mitochondrial Kv1.3 beyond apoptosis. At the same time, puzzle out Kv1.3 as an important component of the mitochondrial fusion/fission machinery (ContributionVI). In conclusion, this thesis includes novel and relevant findings about Kv1.3 function linked to its localization in the cell.Kv1.3 és un canal de potassi depenent de voltatge expressat, principalment, a la membrana plasmàtica i a la membrana mitocondrial interna. La localització dual del canal a membrana i a mitocondri s’ha associat amb la seva funció antagònica, on el canal regula proliferació i apoptosi, respectivament. En el context d’aquesta tesi hem estudiat els mecanismes que regulen l’arribada del canal al seu destí, i les implicacions funcionals de la seva presència en aquests compartiments subcel·lulars. La tesi conté 6 contribucions que inclouen els següents resultats: En primer lloc, es descriu la interacció del Kv1.3 amb la Caveolina com a mecanisme principal de tràfic a estructures de membrana lípid raft, on el canal exerceix la seva funció. En segon lloc, es demostra que la localització del canal a les caveoles és necessària per a la seva participació en la cascada de senyalització per insulina en adipòcits, gràcies a la qual el canal regula la fisiologia adipocitària i la captació de glucosa dependent d’insulina. En la tercera contribució, descrivim la palmitoilació de Kv1.3 com a mecanisme de regulació de la distribució del canal a la sinapsi immunològica en limfòcits T. Durant aquest fenomen, Kv1.3 té un paper clau per a l’activació del limfòcits i aberracions en la seva localització i activitat s’han relacionat en el desenvolupament de malalties autoimmunitàries. A la contribució quarta, descrivim la modulació de la caveolina sobre l’activitat pro-apoptòtica del canal a mitocondris. A la cinquena contribució demostrem que el canal segueix un mecanisme no convencional d’import a mitocondris i que els dominis transmembrana defineix motius estructurals per tal de promoure aquest procés. Finalment, amb la última contribució, demostrem que el canal està implicat en la regulació de l’equilibri de fusió/fissió mitocondrial, amb importants conseqüències per al cicle cel·lular

The potassium channel odyssey: Mechanisms of traffic and membrane arrangement

Ion channels are transmembrane proteins that conduct specific ions across biological membranes. Ion channels are present at the onset of many cellular processes, and their malfunction triggers severe pathologies. Potassium channels (KChs) share a highly conserved signature that is necessary to conduct K+ through the pore region. To be functional, KChs require an exquisite regulation of their subcellular location and abundance. A wide repertoire of signatures facilitates the proper targeting of the channel, fine-tuning the balance that determines traffic and location. These signature motifs can be part of the secondary or tertiary structure of the protein and are spread throughout the entire sequence. Furthermore, the association of the pore-forming subunits with different ancillary proteins forms functional complexes. These partners can modulate traffic and activity by adding their own signatures as well as by exposing or masking the existing ones. Post-translational modifications (PTMs) add a further dimension to traffic regulation. Therefore, the fate of a KCh is not fully dependent on a gene sequence but on the balance of many other factors regulating traffic. In this review, we assemble recent evidence contributing to our understanding of the spatial expression of KChs in mammalian cells. We compile specific signatures, PTMs, and associations that govern the destination of a functional chann