Measurement of nucleotide exchange rate constants in single rabbit soleus myofibrils during shortening and lengthening using a fluorescent ATP analog

Enhanced small interfering RNA delivery into cells by exploiting the additive effect between photo-sensitive peptides and targeting ligands

OBJECTIVES

To enhance the targeting delivery efficiency of small interfering RNA (siRNA) to tumour cells, a novel multifunctional liposome (PSP/NGR-L) comodified with photo-sensitive cell-penetrating peptides (PSP) and asparagine-glycine-arginine peptide (NGR) was constructed and investigated.

METHODS

PSP was conjugated to 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-maleimide(polyethylene glycol)-2000 (DSPE-PEG2000 -MAL) to generate DSPE-PEG2000 -PSP and used to form PSP/NGR-L, the features of the liposomes were determined. HT-1080 and MCF-7 cells were used for cellular uptake tests, and the cellular uptake pathways were identified. Intracellular trafficking and endosomal escape were also evaluated. In-vitro siRNA transfection evaluations were carried out in HT-1080 cells.

KEY FINDINGS

The encapsulation efficiencies of liposomes were about 80%, and the mean particle sizes were around 100 nm. The targeting specificity of PSP/NGR-L was significantly enhanced via NGR navigation and ultraviolet (UV) light illumination. The internalization of PSP/NGR-L in HT-1080 cells was mediated by more than one cellular uptake mechanisms. The constructed nanocarrier could escape from the endosome to produce its effects in the cellular cytoplasm with the help of UV illumination. PSP/NGR-L could down-regulate expression of c-myc and augmented cell apoptosis in HT-1080 cells.

CONCLUSIONS

The application of combined PSP and NGR modifications may be a new approach for the selectively targeted delivery of siRNA to cancer cells.

A photo-responsive peptide- and asparagine-glycine-arginine (NGR) peptide-mediated liposomal delivery system

The conjugation of tunable peptides or materials with nanocarriers represents a promising approach for drug delivery to tumor cells. In this study, we report the development of a novel liposomal carrier system that exploits the cell surface binding synergism between photo-sensitive peptides (PSPs) and targeting ligands. The positive charges of the lysine residues on the cell-penetrating peptides (CPPs) were temporarily caged by the photolabile-protective groups (PG), thereby forming a PSP. Furthermore, this PSP enhances specific uptake into cancer cells after rapidly uncaging the PG via near-infrared (NIR) light illumination. In the circulatory system, the cell penetrability of PSP was hindered. In contrast, the asparagine-glycine-arginine (NGR) peptide moieties, selectively bind to CD13-positive tumors, were attached to the nanocarrier to facilitate the active accumulation of this liposomal carrier in tumor tissue. The dual-modified liposomes (PSP/NGR-L) were prepared by emulsification method, and the concentrations of DSPE-PEG₂₀₀₀-psCPP and DSPE-PEG₅₀₀₀-NGR in the liposomes were chosen to be 4% and 1% (molar ratio), respectively. The mean particle size of the PSP/NGR-L was about 95 nm, and the drug entrapment efficiency was more than 90%. Cellular uptake results demonstrated that the proposed PSP/NGR-L had an enhancement of cancer cell recognition and specific uptake. Furthermore, the PSP/NGR-L demonstrated a stronger antitumor efficacy in the HT-1080 tumor model in nude mice with the aid of NIR illumination.

Single turnovers of fluorescent ATP bound to bipolar myosin filament during actin filaments sliding

The nucleotide turnover rates of bipolar myosin thick filament along which actin filament slides were measured by the displacement of prebound fluorescent ATP analog 2'(3')-O-[N-[2-[(Cy3)]amindo]ethyl] carbamoyl]-adenosine 5' triphosphate (Cy3-EDA-ATP) upon flash photolysis of caged ATP. The fluorescence of the thick filament where actin filament slides decayed with two exponential processes. The slower rate constant was the same as that without actin filament. Along bipolar myosin thick filament, actin filaments slide at a fast speed towards the central bare zone (forward), but more slowly away from the bare zone (backward). The displacement rate constant of fluorescent ATP from the myosin filament where actin filament moved forward was 5.0 s(-1), whereas the rate constant where the actin filament slid backward was 1.7 s(-1). These findings suggest that the slow ADP release rate is responsible for the slow backward sliding movement.

Light induced drug delivery into cancer cells

Cell-penetrating peptides (CPPs) can be used for intracellular delivery of a broad variety of cargoes, including various nanoparticulate pharmaceutical carriers. However, the cationic nature of all CPP sequences, and thus lack of cell specificity, limits their in vivo use for drug delivery applications. Here, we have devised and tested a strategy for site-specific delivery of dyes and drugs into cancer cells by using polymers bearing a light activated caged CPP (cCPP). The positive charge of Lys residues on the minimum sequence of the CPP penetratin ((52)RRMKWKK(58)) was masked with photo-cleavable groups to minimize non-specific adsorption and cellular uptake. Once illuminated by UV light, these protecting groups were cleaved, the positively charged CPP regained its activity and facilitated rapid intracellular delivery of the polymer-dye or polymer-drug conjugates into cancer cells. We have found that a 10-min light illumination time was sufficient to enhance the penetration of the polymer-CPP conjugates bearing the proapoptotic peptide, (D)(KLAKLAK)(2), into 80% of the target cells, and to promote a 'switch' like cytotoxic activity resulting a shift from 100% to 10% in cell viability after 2 h. This report provides an example for tumor targeting by means of light activation of cell-penetrating peptides for intracellular drug delivery.

Insights into the kinetics of Ca2+-regulated contraction and relaxation from myofibril studies

Muscle contraction results from force-generating interactions between myosin cross-bridges on the thick filament and actin on the thin filament. The force-generating interactions are regulated by Ca(2+) via specialised proteins of the thin filament. It is controversial how the contractile and regulatory systems dynamically interact to determine the time course of muscle contraction and relaxation. Whereas kinetics of Ca(2+)-induced thin-filament regulation is often investigated with isolated proteins, force kinetics is usually studied in muscle fibres. The gap between studies on isolated proteins and structured fibres is now bridged by recent techniques that analyse the chemical and mechanical kinetics of small components of a muscle fibre, subcellular myofibrils isolated from skeletal and cardiac muscle. Formed of serially arranged repeating units called sarcomeres, myofibrils have a complete fully structured ensemble of contractile and Ca(2+) regulatory proteins. The small diameter of myofibrils (few micrometres) facilitates analysis of the kinetics of sarcomere contraction and relaxation induced by rapid changes of [ATP] or [Ca(2+)]. Among the processes studied on myofibrils are: (1) the Ca(2+)-regulated switch on/off of the troponin complex, (2) the chemical steps in the cross-bridge adenosine triphosphatase cycle, (3) the mechanics of force generation and (4) the length dynamics of individual sarcomeres. These studies give new insights into the kinetics of thin-filament regulation and of cross-bridge turnover, how cross-bridges transform chemical energy into mechanical work, and suggest that the cross-bridge ensembles of each half-sarcomere cooperate with each other across the half-sarcomere borders. Additionally, we now have a better understanding of muscle relaxation and its impairment in certain muscle diseases.

A dominant role of cardiac molecular motors in the intrinsic regulation of ventricular ejection and relaxation

Molecular motors housed in myosins of the thick filament react with thin-filament actins and promote force and shortening in the sarcomeres. However, other actions of these motors sustain sarcomeric activation by cooperative feedback mechanisms in which the actin-myosin interaction promotes thin-filament activation. Mechanical feedback also affects the actin-myosin interaction. We discuss current concepts of how these relatively under-appreciated actions of molecular motors are responsible for modulation of the ejection time and isovolumic relaxation in the beating heart.

E22K mutation of RLC that causes familial hypertrophic cardiomyopathy in heterozygous mouse myocardium: effect on cross-bridge kinetics

Familial hypertrophic cardiomyopathy is a disease characterized by left ventricular and/or septal hypertrophy and myofibrillar disarray. It is caused by mutations in sarcomeric proteins, including the ventricular isoform of myosin regulatory light chain (RLC). The E22K mutation is located in the RLC Ca(2+)-binding site. We have studied transgenic (Tg) mouse cardiac myofibrils during single-turnover contraction to examine the influence of E22K mutation on 1) dissociation time (tau(1)) of myosin heads from thin filaments, 2) rebinding time (tau(2)) of the cross bridges to actin, and 3) dissociation time (tau(3)) of ADP from the active site of myosin. tau(1) was determined from the increase in the rate of rotation of actin monomer to which a cross bridge was bound. tau(2) was determined from the rate of anisotropy change of the recombinant essential light chain of myosin labeled with rhodamine exchanged for native light chain (LC1) in the cardiac myofibrils. tau(3) was determined from anisotropy of muscle preloaded with a stoichiometric amount of fluorescent ADP. Cross bridges were induced to undergo a single detachment-attachment cycle by a precise delivery of stoichiometric ATP from a caged precursor. The times were measured in Tg-mutated (Tg-m) heart myofibrils overexpressing the E22K mutation of human cardiac RLC. Tg wild-type (Tg-wt) and non-Tg muscles acted as controls. tau(1) was statistically greater in Tg-m than in controls. tau(2) was shorter in Tg-m than in non-Tg, but the same as in Tg-wt. tau(3) was the same in Tg-m and controls. To determine whether the difference in tau(1) was due to intrinsic difference in myosin, we estimated binding of Tg-m and Tg-wt myosin to fluorescently labeled actin by measuring fluorescent lifetime and time-resolved anisotropy. No difference in binding was observed. These results suggest that the E22K mutation has no effect on mechanical properties of cross bridges. The slight increase in tau(1) was probably caused by myofibrillar disarray. The decrease in tau(2) of Tg hearts was probably caused by replacement of the mouse RLC for the human isoform in the Tg mice.

Simultaneous measurement of rotations of myosin, actin and ADP in a contracting skeletal muscle fiber

The rotation of myosin heads and actin were measured simultaneously with an indicator of the enzymatic activity of myosin. To minimize complications due to averaging of signals from many molecules, the signal was measured in a small population residing in a femtoliter volume of a muscle fiber. The onset of rotation was synchronized by a sudden release of caged ATP. The orientation of cross-bridges was measured by anisotropy of recombinant fluorescent regulatory light chains exchanged with native regulatory light chains. The orientation of actin was measured by anisotropy of phalloidin added to actin filaments. The enzymatic activity of myosin was measured by dissociation of fluorescent ADP from the active site. The onset of all three events occurred at the same time. This suggests that in contracting muscle, actin does not move independently of myosin and that ATP hydrolysis is strongly coupled to the rotation of cross-bridges.

Correlation between mechanical and enzymatic events in contracting skeletal muscle fiber

The conventional hypothesis of muscle contraction postulates that the interaction between actin and myosin involves tight coupling between the power stroke and hydrolysis of ATP. However, some in vitro experiments suggested that hydrolysis of a single molecule of ATP caused multiple mechanical cycles. To test whether the tight coupling is present in contracting muscle, we simultaneously followed mechanical and enzymatic events in a small population of cross-bridges of glycerinated rabbit psoas fibers. Such small population behaves as a single cross-bridge when muscle contraction is initiated by a sudden release of caged ATP. Mechanical events were measured by changes of orientation of probes bound to the regulatory domain of myosin. Enzymatic events were simultaneously measured from the same cross-bridge population by the release of fluorescent ADP from the active site. If the conventional view were true, ADP desorption would occur simultaneously with dissociation of cross-bridges from thin filaments and would be followed by cross-bridge rebinding to thin filaments. Such sequence of events was indeed observed in contracting muscle fibers, suggesting that mechanical and enzymatic events are tightly coupled in vivo.

At physiological temperatures the ATPase rates of shortening soleus and psoas myofibrils are similar

We obtained the temperature dependences of the adenosine triphosphatase (ATPase) activities (calcium-activated and relaxed) of myofibrils from a slow muscle, which we compared with those from a fast muscle. We chose rabbit soleus and psoas because their myosin heavy chains are almost pure: isoforms I and IIX, respectively. The Arrhenius plots of the ATPases are linear (4-35 degrees C) with energies of activation for soleus myofibrils 155 kJ mol(-1) (activated) and 78 kJ mol(-1) (relaxed). With psoas myofibrils, the energies of activation were 71 kJ mol(-1) (activated) and 60 kJ mol(-1) (relaxed). When extrapolated to 42 degrees C the ATPase rates of the two types of myofibril were identical: 50 s(-1) (activated) and 0.23 s(-1) (relaxed). Whereas with psoas myofibrils the K(m) for adenosine triphosphate (activated ATPase) is relatively insensitive to temperature, that for soleus myofibrils increased from 0.3 microM at 4 degrees C to 66.5 microM at 35 degrees C. Our results illustrate the importance of temperature when comparing the mechanochemical coupling in different types of muscle. We discuss the problem of how to reconcile the similarity of the myofibrillar ATPase rates at physiological temperatures with their different mechanical properties.

Synthesis and application of caged peptides and proteins

Caged compounds have covalently attached groups that are rapidly cleaved upon exposure to UV light. Attachment of photolabile groups makes the molecule inert until photolysis releases it in its bioactive form. When caged compounds are applied to the experimental system in advance, the concentration jump of biologically active substances can be brought about immediately in a limited area upon irradiation with pulsed and focused UV light. Therefore, caged compounds of low molecular weight, which are commercially available, have been used effectively to study the mechanisms of temporal biological phenomena, such as muscle contraction, intracellular signaling, and neurotransmission. Because many proteins and peptides play important roles in these phenomena, their caged derivatives should serve as powerful tools to clarify complex biological systems. To prepare caged proteins and peptides, several groups have improved upon a chemical modification method, as well as developed two new methods: (1) nonsense codon suppression and (2) solid-phase peptide synthesis. In this review, we summarize recent advances made in the design, preparation, and application of caged peptides and proteins.

Photolytic release of MgADP reduces rigor force in smooth muscle

Photolytic release of MgADP (25-300 microM) from caged ADP in permeabilized tonic (rabbit femoral artery-Rfa) and phasic (rabbit bladder-Rbl) smooth muscle in high-tension rigor state, in the absence of Ca(2+), caused an exponential decline (approximately 1.5% in Rfa and approximately 6% in Rbl) of rigor force, with the rate proportional to the liberated [MgADP]. The apparent second-order rate constant of MgADP binding was estimated as approximately 1.0 x 10(6) M(-1) s(-1) for both smooth muscles. In control experiments, designed to test the specificity of MgADP, photolysis of caged ADP in the absence of Mg(2+) did not decrease rigor force in either smooth muscle, but rigor force decreased after photolytic release of Mg(2+) in the presence of ADP. The effects of photolysis of caged ADP were similar in smooth muscles containing thiophosphorylated or non-phosphorylated regulatory myosin light chains. Stretching or releasing (within range of 0.1-1.2% of initial Ca(2+)-activated force) did not affect the rate or relative amplitude of the force decrease. The effect of additions of MgADP to rigor cross-bridges could result from rotation of the lever arm of smooth muscle myosin, but this need not imply that ADP-release is a significant force-producing step of the physiological cross-bridge cycle.