On the mechanism of ATP-driven Ca2+ transport by the calcium ATPase of sarcoplasmic reticulum

α-Synuclein Overexpression Induces Lysosomal Dysfunction and Autophagy Impairment in Human Neuroblastoma SH-SY5Y

Although the etiology of Parkinson's disease (PD) is multifactorial, it has been linked to abnormal accumulation of α-synuclein (α-syn) in dopaminergic neurons, which could lead to dysfunctions on intracellular organelles, with potential neurodegeneration. Patients with familial early-onset PD frequently present mutation in the α-syn gene (SNCA), which encodes mutant α-syn forms, such as A30P and A53T, which potentially regulate Ca2+ unbalance. Here we investigated the effects of overexpression of wild-type α-syn (WT) and the mutant forms A30P and A53T, on modulation of lysosomal Ca2+ stores and further autophagy activation. We found that in α-syn-overexpressing cells, there was a decrease in Ca2+ released from endoplasmic reticulum (ER) which is related to the increase in lysosomal Ca2+ release, coupled to lysosomal pH alkalization. Interestingly, α-syn-overexpressing cells showed lower LAMP1 levels, and a disruption of lysosomal morphology and distribution, affecting autophagy. Interestingly, all these effects were more evident with A53T mutant isoform when compared to A30P and WT α-syn types, indicating that the pathogenic phenotype for PD is potentially related to impairment of α-syn degradation. Taken together, these events directly impact PD-related dysfunctions, being considered possible molecular targets for neuroprotection.

The regulation of thapsigargin-sensitive sarcoendoplasmic reticulum Ca2+-ATPase activity in estivation

Estivation (aerobic dormancy) is characterized by sustained metabolic rate depression, which is crucial to survival in the face of unfavorable environmental conditions and enables the preservation of endogenous fuel reserves. Ion pumping is one of the most energetically taxing physiological processes in cells, and ion motive ATPases are likely loci to be differentially regulated in models of metabolic arrest. We proposed that the sarcoendoplasmic reticulum (SER) calcium-ATPase (SERCA) would be deactivated in the estivating desert snail Otala lactea, potentially contributing to the overall suppression of metabolism. SERCA kinetic parameters [decreased maximal velocities, increased substrate K (m) values, increased Arrhenius activation energy (E (a))] were indicative of a less active enzyme in the estivated state. Interestingly, the less active SERCA population in dormant snails featured greater kinetic (K (m) Mg.ATP versus temperature) and conformational (resistance to urea denaturation) stability than that in active snails. Western blotting confirmed that SERCA protein content did not change during estivation. In light of this observation, we proposed that estivation-dependent changes in SERCA activity was due to changes in SERCA phosphorylation state. In vitro studies promoting specific kinase or phosphatase action indicated that decreased SERCA activity in estivation was linked with endogenous kinase activity whereas reactivation of SERCA was facilitated by endogenous protein phosphatases (PP).

Pharmacological modulation of sarcoplasmic reticulum function in smooth muscle

The sarco/endoplasmic reticulum (SR/ER) is the primary storage and release site of intracellular calcium (Ca2+) in many excitable cells. The SR is a tubular network, which in smooth muscle (SM) cells distributes close to cellular periphery (superficial SR) and in deeper aspects of the cell (deep SR). Recent attention has focused on the regulation of cell function by the superficial SR, which can act as a buffer and also as a regulator of membrane channels and transporters. Ca2+ is released from the SR via two types of ionic channels [ryanodine- and inositol 1,4,5-trisphosphate-gated], whereas accumulation from thecytoplasm occurs exclusively by an energy-dependent sarco-endoplasmic reticulum Ca2+-ATPase pump (SERCA). Within the SR, Ca2+ is bound to various storage proteins. Emerging evidence also suggests that the perinuclear portion of the SR may play an important role in nuclear transcription. In this review, we detail the pharmacology of agents that alter the functions of Ca2+ release channels and of SERCA. We describe their use and selectivity and indicate the concentrations used in investigating various SM preparations. Important aspects of cell regulation and excitation-contractile activity coupling in SM have been uncovered through the use of such activators and inhibitors of processes that determine SR function. Likewise, they were instrumental in the recent finding of an interaction of the SR with other cellular organelles such as mitochondria. Thus, an appreciation of the pharmacology and selectivity of agents that interfere with SR function in SM has greatly assisted in unveiling the multifaceted nature of the SR.

A coarse-grained normal mode approach for macromolecules: an efficient implementation and application to Ca(2+)-ATPase

A block normal mode (BNM) algorithm, originally proposed by Tama et al., (Proteins Struct. Func. Genet. 41:1-7, 2000) was implemented into the simulation program CHARMM. The BNM approach projects the hessian matrix into local translation/rotation basis vectors and, therefore, dramatically reduces the size of the matrix involved in diagonalization. In the current work, by constructing the atomic hessian elements required in the projection operation on the fly, the memory requirement for the BNM approach has been significantly reduced from that of standard normal mode analysis and previous implementation of BNM. As a result, low frequency modes, which are of interest in large-scale conformational changes of large proteins or protein-nucleic acid complexes, can be readily obtained. Comparison of the BNM results with standard normal mode analysis for a number of small proteins and nucleic acids indicates that many properties dominated by low frequency motions are well reproduced by BNM; these include atomic fluctuations, the displacement covariance matrix, vibrational entropies, and involvement coefficients for conformational transitions. Preliminary application to a fairly large system, Ca(2+)-ATPase (994 residues), is described as an example. The structural flexibility of the cytoplasmic domains (especially domain N), correlated motions among residues on domain interfaces and displacement patterns for the transmembrane helices observed in the BNM results are discussed in relation to the function of Ca(2+)-ATPase. The current implementation of the BNM approach has paved the way for developing efficient sampling algorithms with molecular dynamics or Monte Carlo for studying long-time scale dynamics of macromolecules.

The luminal Ca2+ transient controls Ca2+ release/re-uptake of sarcoplasmic reticulum

Our recent study (Saiki, Y., and Ikemoto, N., Biochemistry 38, 3112-3119, 1999) suggests that Ca2+ release and re-uptake of the released Ca2+ are coordinated. The following results suggest that the coordination is mediated by the luminal Ca2+ ([Ca2+]lum) transient. Upon inducing the release of the passively loaded Ca2+ from the SR with polylysine, the luminal Ca2+ ([Ca2+]lum) first increased then decreased ([Ca2+]lum transient). The activity of the SR Ca2+ ATPase was monitored at different times after inducing Ca2+ release. The phosphoenzyme (EP) formation as determined by the MANT-fluorescence increased concurrently with the initial rapid increase in the [Ca2+]lum. EP decay (pumping turnover) was accelerated concurrently with a decrease of the [Ca2+]lum. The results suggest that the [Ca2+]lum transient serves as a mediator for the acceleration of the Ca2+ re-uptake occurring soon after the induction of Ca2+ release.

Guanylyl cyclase activity associated with putative bifunctional integral membrane proteins in Plasmodium falciparum

We report here that guanylyl cyclase activity is associated with two large integral membrane proteins (PfGCalpha and PfGCbeta) in the human malaria parasite Plasmodium falciparum. Unusually, the proteins appear to be bifunctional; their amino-terminal regions have strong similarity with P-type ATPases, and the sequence and structure of the carboxyl-terminal regions conform to that of G protein-dependent adenylyl cyclases, with two sets of six transmembrane sequences, each followed by a catalytic domain (C1 and C2). However, amino acids that are enzymatically important and present in the C2 domain of mammalian adenylyl cyclases are located in the C1 domain of the P. falciparum proteins and vice versa. In addition, certain key residues in these domains are more characteristic of guanylyl cyclases. Consistent with this, guanylyl cyclase activity was obtained following expression of the catalytic domains of PfGCbeta in Escherichia coli. In P. falciparum, expression of both genes was detectable in the sexual but not the asexual blood stages of the life cycle, and PfGCalpha was localized to the parasite/parasitophorous vacuole membrane region of gametocytes. The profound structural differences identified between mammalian and parasite guanylyl cyclases suggest that aspects of this signaling pathway may be mechanistically distinct.

From chemistry to biochemistry to catalysis to movement

Mechanisms of chemical reaction can often be predicted by determining the dependence of the lifetime of reaction intermediates on the structure of the reactants. When there is no lifetime in the presence of another reactant or catalyst the reaction proceeds through an enforced concerted mechanism. Noncovalent binding interactions between enzymes and their substrates provide a major contribution to catalysis by decreasing entropy and by destabilizing the ground state relative to the transition state, as well as by covalent and noncovalent chemical interactions with the substrate. Movement in biological systems, such as muscle contraction and the active transport of ions, is generally brought about through a series of alternating chemical and vectorial steps that involve a series of changes in the specificity for catalysis of the chemical and vectorial reactions. These changes divide the overall reaction into segments so that neither the chemical nor the vectorial reaction will be completed unless the other is also completed.

The structure and interactions of Ca(2+)-ATPase

Electron crystallographic studies on membrane crystals of Ca(2+)-ATPase reveal different patterns of ATPase-ATPase interactions depending on enzyme conformation. Physiologically relevant changes in Ca2+ concentration and membrane potential affect these interactions. Ca2+ induced difference FTIR spectra of Ca(2+)-ATPase triggered by photolysis of caged Ca2+ are consistent with changes in secondary structure and carboxylate groups upon Ca2+ binding; the changes are reversed during ATP hydrolysis suggesting that a phosphorylated enzyme form of low Ca2+ affinity is the dominant intermediate during Ca2+ transport. A two-channel model of Ca2+ translocation is proposed involving the membrane-spanning helices M2-M5 and M4, M5, M6 and M8 respectively, with separate but interacting Ca2+ binding sites.

Calcium pump kinetics determined in single erythrocyte ghosts by microphotolysis and confocal imaging

The activity of the plasma membrane calcium pump was measured in single cells. Human red blood cell ghosts were loaded with a fluorescent calcium indicator and either caged calcium and ATP (protocol A) or caged ATP and calcium (protocol B). In a suitably modified laser scanning microscope either calcium or ATP were released by a short UV light pulse. The time-dependent fluorescence intensity of the calcium indicator was then followed in single ghosts by repetitive confocal imaging. The fluorescence intensity was converted into calcium concentration, which in turn was used to derive the kinetic parameters of the calcium pump, the Michaelis-Menten constant Km, and the maximal transport rate vmax. Km and vmax values derived in this manner were 24 +/- 14 microM and 1.0 +/- 0.6 microM/(ghost s) for protocol A, and 4 +/- 3 microM and 1.0 +/- 0.6 microM/(ghost s) for protocol B, respectively. The difference between A and B is presumably caused by calmodulin, which is inactive in the experiments with protocol A. The possibilities to extend the new method to living nucleus-containing cells transiently transfected with mutants of the plasma membrane calcium pump are discussed.

The application of vectorial coupling theory to the calcium pump

Models for the calcium pump of the E1E2 type appear to be inconsistent with new evidence for the binding of internal and external calcium ions, simultaneously, to the free pump. The models are shown here to be incomplete but not necessarily wrong; they omit the required modulation of the mobility, specificity, and enzyme activity of the pump, which is brought about through substrate-controlled conformational changes. A reaction scheme based on the E1E2 model but incorporating the conformational equilibria in question is shown to account for a variety of experimental findings, including those at odds with the simple model: (i) binding of lumenal Ca2+ to the free as well as the phosphorylated pump; (ii) uncoupled exit of lumenal Ca2+ at high concentrations; (iii) the absence of any effect of lumenal Ca2+ on the binding of external Ca2+; (iv) uncoupled ATPase activity in dimethyl sulfoxide.