Correlation between lipid fluidity and tryptic susceptibility of Ca2+-ATPase in sarcoplasmic reticulum membranes

Evolving concepts on the age-related changes in "muscle quality"

The deterioration of skeletal muscle with advancing age has long been anecdotally recognized and has been of scientific interest for more than 150 years. Over the past several decades, the scientific and medical communities have recognized that skeletal muscle dysfunction (e.g., muscle weakness, poor muscle coordination, etc.) is a debilitating and life-threatening condition in the elderly. For example, the age-associated loss of muscle strength is highly associated with both mortality and physical disability. It is well-accepted that voluntary muscle force production is not solely dependent upon muscle size, but rather results from a combination of neurologic and skeletal muscle factors, and that biologic properties of both of these systems are altered with aging. Accordingly, numerous scientists and clinicians have used the term "muscle quality" to describe the relationship between voluntary muscle strength and muscle size. In this review article, we discuss the age-associated changes in the neuromuscular system-starting at the level of the brain and proceeding down to the subcellular level of individual muscle fibers-that are potentially influential in the etiology of dynapenia (age-related loss of muscle strength and power).

Augmentation of α1-Adrenoceptor-Mediated Contraction by Warming Without Increased Phosphorylation of Myosin in Rat Caudal Arterial Smooth Muscle

We previously reported the relationship between alpha1-adrenoceptor-mediated contraction and phosphorylation of 20-kDa myosin light chain (LC20) in de-endothelialized rat caudal arterial smooth muscle at room temperature (Mita M, Walsh MP. Biochem J. 1997;327:669-674). We now describe the effect of increasing the temperature to 37 degrees C on this relationship. The EC50 value (76.6 +/- 18.2 nM) for cirazoline (alpha1-adrenergic agonist)-induced contraction of the strips at room temperature (23 degrees C) was significantly greater than that (14.5 +/- 1.9 nM) at 37 degrees C. The initial rate of the contraction to a sub-maximal concentration of cirazoline (0.3 microM) was similar at the two temperatures. However, cirazoline-induced maximal force at 37 degrees C was approximately 1.8 times that at room temperature. LC20 phosphorylation in response to cirazoline at room temperature and 37 degrees C closely matched the time courses of contraction, but values were not significantly different at the two temperatures: resting phosphorylation levels were 0.09 +/- 0.04 mol P(i)/mol LC20 at 37 degrees C and 0.22 +/- 0.06 mol P(i)/mol LC20 at room temperature; maximal cirazoline-stimulated LC20 phosphorylation levels were 0.58 +/- 0.08 mol P(i)/mol LC20 at room temperature and 0.49 +/- 0.05 mol P(i)/mol LC20 at 37 degrees C. We conclude, therefore, that the enhanced cirazoline-induced contraction at 37 degrees C is not due to increased LC20 phosphorylation.

Partitioning of exogenous delta-tocopherol between the triacylglycerol and membrane lipid fractions of chicken muscle

The partitioning of exogenous delta-tocopherol, added dissolved in ethanol, between the neutral triacylglycerols and membranes of chicken leg muscles was investigated. The two lipid fractions were separated using differential ultracentrifugation techniques. Triacylglycerols were obtained after high-speed centrifugation of the minced muscle at 130000 g for 30 min. Membranes were collected from a muscle-buffer homogenate (pH 7.5) between 10000 g for 20 min and 130000 g for 30 min. The triacylglycerols collected represented from 15 to 80% of the total triacylglycerols of the minced muscle, the yields increasing with increasing muscle triacylglycerol content. The phospholipids in the isolated membrane fraction represented from 20 to 35% of the total phospholipids of the muscle. At low muscle total lipid contents (3-5%), the added delta-tocopherol was present in approximately the same concentration in both muscle lipid fractions. At higher total lipid contents, achieved by adding exogenous triacylglycerols, the delta-tocopherol concentration in the membranes increased relative to that in the triacylglycerols.

Length-dependent modulation of smooth muscle activation: effects of agonist, cytochalasin, and temperature

We tested the hypothesis that mechanical strain modulates agonist sensitivity of smooth muscle by measuring myosin phosphorylation and contractile force in bovine tracheal smooth muscle activated by various concentrations of the muscarinic receptor agonist carbachol and at various muscle lengths. Increasing carbachol concentration by 10,000-fold did not restore myosin phosphorylation levels at shorter muscle lengths to the level at optimal length (Lo). Maximum levels of myosin phosphorylation induced by carbachol at 0.6, 0.8, and 1.0 Lo were similar but became lower at <0.6 Lo. Cytochalasin D significantly attenuated carbachol-induced contraction by 54%. In addition, cytochalasin D treatment induced a parallel downward shift in the length-myosin phosphorylation relation. Lowering temperature from 37 to 23 degrees C did not significantly change the length dependencies of carbachol-induced active force and myosin phosphorylation. These results have led us to conclude that 1) agonist sensitivity and maximum level of activation (as measured by myosin phosphorylation) are targets of length-dependent modulation, 2) actin filaments involved in contraction and length-dependent modulation are distinct in sensitivity to cytochalasin D, and 3) length-dependent modulation is relatively temperature insensitive.

Protein-lipid interactions and Torpedo californica nicotinic acetylcholine receptor function. 2. Membrane fluidity and ligand-mediated alteration in the accessibility of gamma subunit cysteine residues to cholesterol

Fluorescence-quenching and energy-transfer measurements were carried out to further characterize lipid-protein interactions involving the nicotinic acetylcholine receptor (AChR) from Torpedo californica in reconstituted membranes. To assess the fluidity of the receptor microenvironment, cis- and trans-parinaric acids were used to take advantage of the preferential partitioning behavior of the trans isomer for the gel phase. A relatively higher extent of energy transfer from the intrinsic tryptophan fluorescence of AChR in dielaidoylphosphatidylcholine bilayers to cis-parinaric acid in both the gel and the fluid phase suggests that the AChR is surrounded by a relatively fluid annulus of lipids. The ability of AChR to accommodate and interact with specific lipids such as cholesterol and fatty acids in the vicinity of pyrene-labeled cysteine residues in the membranous domain and/or the membrane-water interface region of the gamma subunit was assessed. Pyrene-labeled AChR prepared in (6,7-dibromostearoyl)phosphatidylcholine showed a 25% decrease in fluorescence as sites accessible to phospholipids were occupied; subsequent addition of dibromocholesterol hemisuccinate (DiBrCHS) caused further quenching by about 25%. This result is consistent with the presence of sites accessible to cholesterol, but not accessible to phospholipids, in the vicinity of the cysteine-bound pyrene in the membranous domain of the AChR. Quenching by DiBrCHS was sensitive to the presence of an AChR activator (carbamylcholine) but not a competitive antagonist (alpha-bungarotoxin). The Stern-Volmer quenching constant was 0.123 in the absence of added ligands and 0.167 and 0.134 in the presence of carbamylcholine and alpha-bungarotoxin, respectively, corresponding to accessibilities of 65%, 90%, and 70%.

Uncoupling of occlusion from ATP hydrolysis activity in sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase

The uncoupling of Ca2+ transport from ATP hydrolysis in the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase by trypsin digestion was re-investigated by comparing ATPase activity with the ability of the enzyme to occlude Eu3+ (a transport parameter) after various tryptic digests. With this method, re-examination of uncoupling by tryptic digest of the ATPase revealed that TD2 cleavage (Arg-198) had no effect on either occlusion or ATPase activity. Digestion past TD2 in the presence of 5 mM Ca2+ and at 25 degrees C resulted in the loss of about 70% of the ATPase activity, but no loss of occlusion. Digestion past TD2 in the presence of 5 mM Ca2+, 3 mM ATP, and at 25 degrees C resulted in a partially uncoupled enzyme complex which retained about 50% of the ATPase activity, but completely lost the ability to occlude Eu3+. Digest past TD2 in the presence of 5 mM Ca2+ and 3 mM AMP-PNP (a non-hydrolyzable ATP analog) at 25 degrees C resulted in no loss of occlusion, thus revealing the absolute requirement of ATP during the digest to eliminate occlusion. From these findings we conclude that uncoupling of Ca2+ transport from ATPase activity is possible by tryptic digestion of the (Ca2+ + Mg2+)-ATPase. Interestingly, only after phosphorylation of the enzyme do the susceptible bond(s) which lead to the loss of occlusion become exposed to trypsin.

Effect of the biochemical state of the Ca-ATPase protein of scallop sarcoplasmic reticulum on its interaction with trans-parinaric acid

The polyene fluorescent probe trans-parinaric acid (tPA) was used to compare lipid-protein interactions in the scallop fragmented sarcoplasmic reticulum (FSR) between biochemical states where the Ca-ATPase molecules were arranged differently in the membrane and had different tertiary conformations. The state of the bulk lipid phase was examined over the temperature range -3 to +32 degrees C by exciting the tPA directly at 320 nm. The state of the system close to the Ca-ATPase protein was followed over the same temperature range by indirectly exciting the tPA through resonance energy transfer from the Ca-ATPase protein, with approximately one twenty-fifth the quantum yield of the directly excited probe. Raising the tPA/lipid ratio in the membrane to high levels (approx. 1:9), caused the quantum yield of indirectly excited tPA to reach a maximum, which may reflect saturation of the annular lipid phase with the probe, or contribution to the fluorescence from indirectly excited tPA bound directly to the protein. In the presence of 0.1 M KCl, a thermal perturbation was observed at approx. 7 degrees C using indirect excitation when the Ca(2+)-binding sites on the Ca-ATPase were occupied, and the subunits were disorganized. This transition was not detected in the presence of 0.1 M KCl and EGTA, when the Ca(2+)-binding sites were empty, and the Ca-ATPase subunits were organized in dimeric arrays. The transition seen with the E1(Ca2+)2 form of the membrane may involve an event at the protein/lipid interface, or a change in the environment of tPA bound to the Ca-ATPase. The temperature at which the perturbation occurs is close to that of a discontinuity in the Arrhenius plot of the Ca-ATPase enzyme activity determined in the presence of 0.1 M KCl (Kalabokis, V.N. and Hardwicke, P.M.D. (1988) J. Biol. Chem. 263, 15184-15188). No perturbation was observed in the bulk properties of the lipid component of the membrane in either the E1(Ca2+)2 or E2 states.

Unsaturated aminophospholipids are preferentially retained by the fast skeletal muscle CaATPase during detergent solubilization. Evidence for a specific association between aminophospholipids and the calcium pump protein

When fast twitch skeletal muscle vesicles (SR) and purified calcium pump protein are stripped with the nonionic detergent C12E8 (octaethylene glycol dodecyl ether), not all the membrane phospholipids are removed from the calcium pump protein. Maximal extraction produces a remnant of 6-8 mol of phospholipid/mole of calcium ATPase (CaATPase). In contrast to native SR and the prestripped purified CaATPase, the remaining phospholipid is markedly enriched in phosphatidylethanolamine (PE) and phosphatidylserine (PS) in both preparations; the remaining lipid is also enriched in phospholipid that is predominantly unsaturated. In addition, virtually all of the associated PE is plasmalogenic (96% as opposed to 63% in the native SR). The amino-specific cross-linking reagent DFDNB (1,5-difluoro-2,4-dinitrobenzene sulfonic acid) and the amino binding reagent TNBS (2,4,6-trinitrobenzene sulfonic acid) were utilized to identify the monolayer of the native preparation where these phospholipids reside, and to determine which phospholipids are closely associated with the calcium pump protein following detergent treatment. These studies demonstrate that PE and PS are closely associated with the pump protein, PE residing almost exclusively in the outer monolayer of SR, while PS resides in the inner monolayer. Nonspecific phospholipid exchange protein was shown to be capable of exchanging phospholipids from donor vesicles into those phospholipids associated with the CaATPase; stripping of lipid-exchanged vesicles with C12E8 exhibited the same specificity with regard to head-group species (i.e., PE is markedly enriched in the extracted protein associated fraction). The results suggest that specific protein-lipid interactions exist, favoring the association of plasmalogenic aminophospholipids with the calcium pump protein.

Lipid-protein interactions and the function of the Ca2+-ATPase of sarcoplasmic reticulum

Regardless of the nature of the protein constituents of membranes, the molecular arrangement of lipids interacting with them must satisfy hydrophobic, ionic, and steric requirements. Biological membranes have a great diversity of lipid constituents, and this diversity might have functional roles. It has been proposed, for example, that the hydrophobic regions of membrane proteins are stabilized in the membrane through interactions with lipids able to adopt configurations other than the bilayer structure. Progress in understanding at the molecular level how lipid-protein interactions control the properties of membrane proteins has been hindered by the lack of information concerning the structure of the hydrophobic regions of membrane proteins. Nevertheless, there are many examples in the literature describing how changes in the lipid environment affect physical and biochemical properties of membrane proteins. From these studies, discussed in this review, an overall picture of how lipids and proteins interact in membranes is beginning to emerge.