Differential effects of G- and F-actin on the plasma membrane calcium pump activity

Tubulin Regulates Plasma Membrane Ca2+-ATPase Activity in a Lipid Environment-dependent Manner

Ca2+ plays a crucial role in cell signaling, cytosolic Ca2+ can change up to 10,000-fold in concentration due to the action of Ca2+-ATPases, including PMCA, SERCA and SCR. The regulation and balance of these enzymes are essential to maintain cytosolic Ca2+ homeostasis. Our laboratory has discovered a novel PMCA regulatory system, involving acetylated tubulin alone or in combination with membrane lipids. This regulation controls cytosolic Ca2+ levels and influences cellular properties such as erythrocyte rheology. This review summarizes the findings on the regulatory mechanism of PMCA activity by acetylated tubulin in combination with lipids. The combination of tubulin cytoskeleton and membrane lipids suggests a novel regulatory system for PMCA, which consequently affects cytosolic Ca2+ content, depending on cytoskeletal and plasma membrane dynamics. Understanding the interaction between acetylated tubulin, lipids and PMCA activity provides new insights into Ca2+ signaling and cell function. Further research may shed light on potential therapeutic targets for diseases related to Ca2+ dysregulation. This discovery contributes to a broader understanding of cellular processes and offers opportunities to develop innovative approaches to treat Ca2+-related disorders. By elucidating the complex regulatory mechanisms of Ca2+ homeostasis, we advance our understanding of cell biology and its implications for human health.

The interdependent transport of yeast vacuole Ca2+ and H+ and the role of phosphatidylinositol 3,5-bisphosphate

Yeast vacuoles are acidified by the v-type H+-ATPase (V-ATPase) that is comprised of the membrane embedded VO complex and the soluble cytoplasmic V1 complex. The assembly of the V1-VO holoenzyme on the vacuole is stabilized in part through interactions between the VO a-subunit ortholog Vph1 and the lipid phosphatidylinositol 3,5-bisphosphate (PI(3,5)P2). PI(3,5)P2 also affects vacuolar Ca2+ release through the channel Yvc1 and uptake through the Ca2+ pump Pmc1. Here, we asked if H+ and Ca2+ transport activities were connected through PI(3,5)P2. We found that overproduction of PI(3,5)P2 by the hyperactive fab1T2250A mutant augmented vacuole acidification, whereas the kinase-inactive fab1EEE mutant attenuated the formation of a H+ gradient. Separately, we tested the effects of excess Ca2+ on vacuole acidification. Adding micromolar Ca2+ blocked vacuole acidification, whereas chelating Ca2+ accelerated acidification. The effect of adding Ca2+ on acidification was eliminated when the Ca2+/H+ antiporter Vcx1 was absent, indicating that the vacuolar H+ gradient can collapse during Ca2+ stress through Vcx1 activity. This, however, was independent of PI(3,5)P2, suggesting that PI(3,5)P2 plays a role in submicromolar Ca2+ flux but not under Ca2+ shock. To see if the link between Ca2+ and H+ transport was bidirectional, we examined Ca2+ transport when vacuole acidification was inhibited. We found that Ca2+ transport was inhibited by halting V-ATPase activity with Bafilomycin or neutralizing vacuolar pH with chloroquine. Together, these data show that Ca2+ transport and V-ATPase efficacy are connected but not necessarily through PI(3,5)P2.

Intensive stretch-activated CRT-PMCA1 feedback loop promoted apoptosis of myoblasts through Ca2+ overloading

Mechanical stretch exerted pro-apoptotic effect on myoblasts, the mechanism of which is currently unknown. Intracellular Ca²⁺ accumulation has been implicated in stretch-induced apoptosis. calreticulin (CRT) and plasma membrane Ca²⁺ transporting ATPase 1 (PMCA1) are two critical components of Ca²⁺ signaling system participating in intracellular Ca²⁺ homeostasis. In this study, we explored the contribution of CRT and PMCA1 in mediating stretch-induced Ca²⁺ accumulation and apoptosis of myoblasts. Stretching stimuli elevated level of CRT while inhibited activity of PMCA1. Moreover, there were bidirectional regulations between CRT and PMCA1, which formed the positive feedback loop leading to continuous increment of CRT level and repression of PMCA1 activity, in stretched myoblasts. Specifically, increased CRT level inhibited PMCA1 activity via suppressing Calmodulin (CaM), while reduced PMCA1 activity promoted CRT expression through activating p38MAPK pathway. Thus, the CRT-CaM-PMCA1 and PMCA1-p38MAPK-CRT pathways constituted a close cycle comprising CRT, PMCA1, CaM and p38MAPK. Inhibition of both CaM and p38MAPK affected the other three factors in stretched myoblasts. Circulation of the vicious cycle resulted in escalated Ca²⁺ overloading in myoblasts under continuous stretching stimuli. CRT knock-down, PMCA1 overexpression, and p38MAPK inhibition all attenuated the raised intracellular Ca²⁺ level and ameliorated myoblast apoptosis in the stretching environment. Conversely, CRT overexpression, PMCA1 knock-down, and CaM inhibition all aggravated stretch-induced Ca²⁺ overloading and myoblast apoptosis. A positive feedback loop between CRT and PMCA1 was activated in stretched myoblasts, which contributed to intracellular Ca²⁺ accumulation and resultant myoblast apoptosis.

Signaling Enzymes and Ion Channels Being Modulated by the Actin Cytoskeleton at the Plasma Membrane

A cell should deal with the changing external environment or the neighboring cells. Inevitably, the cell surface receives and transduces a number of signals to produce apt responses. Typically, cell surface receptors are activated, and during this process, the subplasmalemmal actin cytoskeleton is often rearranged. An intriguing point is that some signaling enzymes and ion channels are physically associated with the actin cytoskeleton, raising the possibility that the subtle changes of the local actin cytoskeleton can, in turn, modulate the activities of these proteins. In this study, we reviewed the early and new experimental evidence supporting the notion of actin-regulated enzyme and ion channel activities in various cell types including the cells of immune response, neurons, oocytes, hepatocytes, and epithelial cells, with a special emphasis on the Ca²⁺ signaling pathway that depends on the synthesis of inositol 1,4,5-trisphosphate. Some of the features that are commonly found in diverse cells from a wide spectrum of the animal species suggest that fine-tuning of the activities of the enzymes and ion channels by the actin cytoskeleton may be an important strategy to inhibit or enhance the function of these signaling proteins.

Parasite-Mediated Remodeling of the Host Microfilament Cytoskeleton Enables Rapid Egress of Trypanosoma cruzi following Membrane Rupture

Chagas’ disease arises as a direct consequence of the lytic cycle of Trypanosoma cruzi in the mammalian host. While invasion is well studied for this pathogen, study of egress has been largely neglected. Here, we provide the first description of T. cruzi egress documenting a coordinated mechanism by which T. cruzi engineers its escape from host cells in which it has proliferated and which is essential for maintenance of infection and pathogenesis. Our results indicate that this parasite egress is a sudden event involving coordinated remodeling of host cell cytoskeleton and subsequent rupture of host cell plasma membrane. We document that host cells maintain plasma membrane integrity until immediately prior to parasite release and report the sequential transformation of the host cell’s actin cytoskeleton from normal meshwork in noninfected cells to spheroidal cages—a process initiated shortly after amastigogenesis. Quantification revealed gradual reduction in F-actin over the course of infection, and using cytoskeletal preparations and electron microscopy, we were able to observe disruption of the F-actin proximal to intracellular trypomastigotes. Finally, Western blotting experiments suggest actin degradation driven by parasite proteases, suggesting that degradation of cytoskeleton is a principal component controlling the initiation of egress. Our results provide the first description of the cellular mechanism that regulates the lytic component of the T. cruzi lytic cycle. We show graphically how it is possible to preserve the envelope of host cell plasma membrane during intracellular proliferation of the parasite and how, in cells packed with amastigotes, differentiation into trypomastigotes may trigger sudden egress.

Charge-dependent interactions of monomeric and filamentous actin with lipid bilayers

The cytoskeletal protein actin polymerizes into filaments that are essential for the mechanical stability of mammalian cells. In vitro experiments showed that direct interactions between actin filaments and lipid bilayers are possible and that the net charge of the bilayer as well as the presence of divalent ions in the buffer play an important role. In vivo, colocalization of actin filaments and divalent ions are suppressed, and cells rely on linker proteins to connect the plasma membrane to the actin network. Little is known, however, about why this is the case and what microscopic interactions are important. A deeper understanding is highly beneficial, first, to obtain understanding in the biological design of cells and, second, as a possible basis for the building of artificial cortices for the stabilization of synthetic cells. Here, we report the results of coarse-grained molecular dynamics simulations of monomeric and filamentous actin in the vicinity of differently charged lipid bilayers. We observe that charges on the lipid head groups strongly determine the ability of actin to adsorb to the bilayer. The inclusion of divalent ions leads to a reversal of the binding affinity. Our in silico results are validated experimentally by reconstitution assays with actin on lipid bilayer membranes and provide a molecular-level understanding of the actin-membrane interaction.

Natural flavonoids inhibit the plasma membrane Ca2+-ATPase

Research on flavonoids from plant sources has recently sparked increasing interest because of their beneficial health properties. Different studies have shown that flavonoids change the intracellular Ca²⁺ homeostasis linked to alterations in the function of mitochondria, Ca²⁺ channels and Ca²⁺ pumps. These findings hint at plasma membrane Ca²⁺-ATPase (PMCA) involvement, as it transports Ca²⁺ actively to the extracellular medium coupled to ATP hydrolysis, thus maintaining ion cellular homeostasis. The present study aims to investigate the effect of several natural flavonoids on PMCA both in isolated protein systems and in living cells, and to establish the relationship between flavonoid structure and inhibitory activity on PMCA. Our results show that natural flavonoids inhibited purified and membranous PMCA with different effectiveness: quercetin and gossypin were the most potent and their inhibition mechanisms seem to be different, as quercetin does not prevent ATP binding whereas gossypin does. Moreover, PMCA activity was inhibited in human embryonic kidney cells which transiently overexpress PMCA, suggesting that the effects observed on isolated systems could occur in a complex structure like a living cell. In conclusion, this work reveals a novel molecular mechanism through which flavonoids inhibit PMCA, which leads to Ca²⁺ homeostasis and signaling alterations in the cell.

Regulation of the Plasma Membrane Calcium ATPases by the actin cytoskeleton

Associations between the cortical cytoskeleton and the components of the plasma membrane are no longer considered to be merely of structural and mechanical nature but are nowadays recognized as dynamic interactions that modulate a plethora of cellular responses. Reorganization of actin filaments upon diverse stimuli - among which is the rise in cytosolic Ca²⁺ - is involved in cell motility and adhesion, phagocytosis, cytokinesis, and secretion. Actin dynamics also participates in the regulation of ion transport across the membranes where it not only plays a key role in the delivery and stabilization of channels and transporters in the plasma membrane but also in the regulation of their activity. The recently described functional interaction between actin and the Plasma Membrane Ca²⁺-ATPase (PMCA) represents a novel regulatory mechanism of the pump at the time that unveils a new pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca²⁺ homeostasis. In this review, we summarize the current knowledge on the interaction between the cortical actin cytoskeleton and the PMCA and discuss the possible mechanisms that may explain the pump's modulation.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

Sphingomyelin-induced inhibition of the plasma membrane calcium ATPase causes neurodegeneration in type A Niemann-Pick disease

Niemann-Pick disease type A (NPA) is a rare lysosomal storage disorder characterized by severe neurological alterations that leads to death in childhood. Loss-of-function mutations in the acid sphingomyelinase (ASM) gene cause NPA, and result in the accumulation of sphingomyelin (SM) in lysosomes and plasma membrane of neurons. Using ASM knockout (ASMko) mice as a NPA disease model, we investigated how high SM levels contribute to neural pathology in NPA. We found high levels of oxidative stress both in neurons from these mice and a NPA patient. Impaired activity of the plasma membrane calcium ATPase (PMCA) increases intracellular calcium. SM induces PMCA decreased activity, which causes oxidative stress. Incubating ASMko-cultured neurons in the histone deacetylase inhibitor, SAHA, restores PMCA activity and calcium homeostasis and, consequently, reduces the increased levels of oxidative stress. No recovery occurs when PMCA activity is pharmacologically impaired or genetically inhibited in vitro. Oral administration of SAHA prevents oxidative stress and neurodegeneration, and improves behavioral performance in ASMko mice. These results demonstrate a critical role for plasma membrane SM in neuronal calcium regulation. Thus, we identify changes in PMCA-triggered calcium homeostasis as an upstream mediator for NPA pathology. These findings can stimulate new approaches for pharmacological remediation in a disease with no current clinical treatments.

Regulation of Cell Calcium and Role of Plasma Membrane Calcium ATPases

The plasma membrane Ca²⁺ ATPase (PMCA pump) is a member of the superfamily of P-type pumps. It has 10 transmembrane helices and 2 cytosolic loops, one of which contains the catalytic center. Its most distinctive feature is a C-terminal tail that contains most of the regulatory sites including that for calmodulin. The pump is also regulated by acidic phospholipids, kinases, a dimerization process, and numerous protein interactors. In mammals, four genes code for the four basic isoforms. Isoform complexity is increased by alternative splicing of primary transcripts. Pumps 2 and 3 are expressed preferentially in the nervous system. The pumps coexist with more powerful systems that clear Ca²⁺ from the bulk cytosol: their role is thus the regulation of Ca²⁺ in selected subplasma membrane microdomains, where a number of important Ca²⁺-dependent enzymes interact with them. Malfunctions of the pump lead to disease phenotypes that affect the nervous system preferentially.

Cofilin recruits F-actin to SPCA1 and promotes Ca2+-mediated secretory cargo sorting

The cofilin CFL-1 recruits actin to the P-type calcium ATPase SPCA1 at the trans-Golgi network, thereby activating the ATPase, promoting Ca²⁺ influx, and driving secretory cargo sorting.

The actin filament severing protein cofilin-1 (CFL-1) is required for actin and P-type ATPase secretory pathway calcium ATPase (SPCA)-dependent sorting of secretory proteins at the trans-Golgi network (TGN). How these proteins interact and activate the pump to facilitate cargo sorting, however, is not known. We used purified proteins to assess interaction of the cytoplasmic domains of SPCA1 with actin and CFL-1. A 132–amino acid portion of the SPCA1 phosphorylation domain (P-domain) interacted with actin in a CFL-1–dependent manner. This domain, coupled to nickel nitrilotriacetic acid (Ni-NTA) agarose beads, specifically recruited F-actin in the presence of CFL-1 and, when expressed in HeLa cells, inhibited Ca²⁺ entry into the TGN and secretory cargo sorting. Mutagenesis of four amino acids in SPCA1 that represent the CFL-1 binding site also affected Ca²⁺ import into the TGN and secretory cargo sorting. Altogether, our findings reveal the mechanism of CFL-1–dependent recruitment of actin to SPCA1 and the significance of this interaction for Ca²⁺ influx and secretory cargo sorting.

The essence of insect metamorphosis and aging: electrical rewiring of cells driven by the principles of juvenile hormone-dependent Ca(2+)-homeostasis

In holometabolous insects the fall to zero of the titer of Juvenile Hormone ends its still poorly understood "status quo" mode of action in larvae. Concurrently it initiates metamorphosis of which the programmed cell death of all internal tissues that actively secrete proteins, such as the fat body, midgut, salivary glands, prothoracic glands, etc. is the most drastic aspect. These tissues have a very well developed rough endoplasmic reticulum, a known storage site of intracellular Ca(2+). A persistent high [Ca(2+)]i is toxic, lethal and causal to apoptosis. Metamorphosis becomes a logical phenomenon if analyzed from: (1) the causal link between calcium toxicity and apoptosis; (2) the largely overlooked fact that at least some isoforms of Ca(2+)-ATPases have a binding site for farnesol-like endogenous sesquiterpenoids (FRS). The Ca(2+)-ATPase blocker thapsigargin, like JH a sesquiterpenoid derivative, illustrates how absence of JH might work. The Ca(2+)-homeostasis system is concurrently extremely well conserved in evolution and highly variable, enabling tissue-, developmental-, and species specificity. As long as JH succeeds in keeping [Ca(2+)]i low by keeping the Ca(2+)-ATPases pumping, it acts as "the status quo" hormone. When it disappears, its various inhibitory effects are lifted. The electrical wiring system of cells, in particular in the regenerating tissues, is subject to change during metamorphosis. The possibility is discussed that in vertebrates an endogenous farnesol-like sesquiterpenoid, probably farnesol itself, acts as a functional, but hitherto completely overlooked Juvenile anti-aging "Inbrome", a novel concept in signaling.

The plasma membrane calcium pump: new ways to look at an old enzyme

The three-dimensional structure of the PMCA pump has not been solved, but its basic mechanistic properties are known to repeat those of the other Ca(2+) pumps. However, the pump also has unique properties. They concern essentially its numerous regulatory mechanisms, the most important of which is the autoinhibition by its C-terminal tail. Other regulatory mechanisms involve protein kinases and the phospholipids of the membrane in which the pump is embedded. Permanent activation of the pump, e.g. by calmodulin, is physiologically as harmful to cells as its absence. The concept is now emerging that the global control of cell Ca(2+) may not be the main function of the pump; in some cell types, it could even be irrelevant. The main pump role would be the regulation of Ca(2+) in cell microdomains in which the pump co-segregates with partners that modulate the Ca(2+) message and transduce it to important cell functions.

Conformational changes produced by ATP binding to the plasma membrane calcium pump

The aim of this work was to study the plasma membrane calcium pump (PMCA) reaction cycle by characterizing conformational changes associated with calcium, ATP, and vanadate binding to purified PMCA. This was accomplished by studying the exposure of PMCA to surrounding phospholipids by measuring the incorporation of the photoactivatable phosphatidylcholine analog 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine to the protein. ATP could bind to the different vanadate-bound states of the enzyme either in the presence or in the absence of Ca(2+) with high apparent affinity. Conformational movements of the ATP binding domain were determined using the fluorescent analog 2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate. To assess the conformational behavior of the Ca(2+) binding domain, we also studied the occlusion of Ca(2+), both in the presence and in the absence of ATP and with or without vanadate. Results show the existence of occluded species in the presence of vanadate and/or ATP. This allowed the development of a model that describes the transport of Ca(2+) and its relation with ATP hydrolysis. This is the first approach that uses a conformational study to describe the PMCA P-type ATPase reaction cycle, adding important features to the classical E1-E2 model devised using kinetics methodology only.

Plasma membrane calcium ATPase activity is regulated by actin oligomers through direct interaction

As recently described by our group, plasma membrane calcium ATPase (PMCA) activity can be regulated by the actin cytoskeleton. In this study, we characterize the interaction of purified G-actin with isolated PMCA and examine the effect of G-actin during the first polymerization steps. As measured by surface plasmon resonance, G-actin directly interacts with PMCA with an apparent 1:1 stoichiometry in the presence of Ca(2+) with an apparent affinity in the micromolar range. As assessed by the photoactivatable probe 1-O-hexadecanoyl-2-O-[9-[[[2-[(125)I]iodo-4-(trifluoromethyl-3H-diazirin-3-yl)benzyl]oxy]carbonyl]nonanoyl]-sn-glycero-3-phosphocholine, the association of PMCA to actin produced a shift in the distribution of the conformers of the pump toward a calmodulin-activated conformation. G-actin stimulates Ca(2+)-ATPase activity of the enzyme when incubated under polymerizing conditions, displaying a cooperative behavior. The increase in the Ca(2+)-ATPase activity was related to an increase in the apparent affinity for Ca(2+) and an increase in the phosphoenzyme levels at steady state. Although surface plasmon resonance experiments revealed only one binding site for G-actin, results clearly indicate that more than one molecule of G-actin was needed for a regulatory effect on the pump. Polymerization studies showed that the experimental conditions are compatible with the presence of actin in the first stages of assembly. Altogether, these observations suggest that the stimulatory effect is exerted by short oligomers of actin. The functional interaction between actin oligomers and PMCA represents a novel regulatory pathway by which the cortical actin cytoskeleton participates in the regulation of cytosolic Ca(2+) homeostasis.