Protein-based nano-vaccines against SARS-CoV-2: Current design strategies and advances of candidate vaccines.

The outbreak of coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has shaken the global health system. Various nanotechnology-based strategies for vaccine development have played pivotal roles in fighting against SARS-CoV-2. Among them, the safe and effective protein-based nanoparticle (NP) platforms display a highly repetitive array of foreign antigens on their surface, which is urgent for improving the immunogenicity of vaccines. These platforms greatly improved antigen uptake by antigen presenting cells (APCs), lymph node trafficking, and B cell activation, due to the optimal size, multivalence, and versatility of NPs. In this review, we summarize the advances of protein-based NP platforms, strategies of antigen attachment, and the current progress of clinical and preclinical trials in the development of SARS-CoV-2 vaccines based on protein-based NP platforms. Importantly, the lessons learnt and design approaches developed for these NP platforms against SARS-CoV-2 also provide insights into the development of protein-based NP strategies for preventing other epidemic diseases.

Immunoinformatic Analysis of T- and B-Cell Epitopes for SARS-CoV-2 Vaccine Design.

Currently, there is limited knowledge about the immunological profiles of Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2). We used computer-based immunoinformatic analysis and the newly resolved 3-dimensional (3D) structures of the SARS-CoV-2 S trimeric protein, together with analyses of the immunogenic profiles of SARS-CoV, to anticipate potential B-cell and T-cell epitopes of the SARS-CoV-2 S protein for vaccine design, particularly for peptide-driven vaccine design and serological diagnosis. Nine conserved linear B-cell epitopes and multiple discontinuous B-cell epitopes composed of 69 residues on the surface of the SARS-CoV-2 trimeric S protein were predicted to be highly antigenic. We found that the SARS-CoV-2 S protein has a different antigenic profile than that of the SARS-CoV S protein due to the variations in their primary and 3D structures. Importantly, SARS-CoV-2 may exploit an immune evasion mechanism through two point mutations in the critical and conserved linear neutralization epitope (overlap with fusion peptide) around a sparsely glycosylated area. These mutations lead to a significant decrease in the antigenicity of this epitope in the SARS-CoV-2 S protein. In addition, 62 T-cell epitopes in the SARS-CoV-2 S protein were predicted in our study. The structure-based immunoinformatic analysis for the SARS-CoV-2 S protein in this study may improve vaccine design, diagnosis, and immunotherapy against the pandemic of COVID-19.

Mechanistic role of movement and strain sensitivity in muscle contraction.

Tension generation can be studied by applying step perturbations to contracting muscle fibers and subdividing the mechanical response into exponential phases. The de novo tension-generating isomerization is associated with one of these phases. Earlier work has shown that a temperature jump perturbs the equilibrium constant directly to increase tension. Here, we show that a length jump functions quite differently. A step release (relative movement of thick and thin filaments) appears to release a steric constraint on an ensemble of noncompetent postphosphate release actomyosin cross-bridges, enabling them to generate tension, a concentration jump in effect. Structural studies [Taylor KA, et al. (1999) Tomographic 3D reconstruction of quick-frozen, Ca(2+)-activated contracting insect flight muscle. Cell 99:421-431] that map to these kinetics indicate that both catalytic and lever arm domains of noncompetent myosin heads change angle on actin, whereas lever arm movement alone mediates the power stroke. Together, these kinetic and structural observations show a 13-nm overall interaction distance of myosin with actin, including a final 4- to 6-nm power stroke when the catalytic domain is fixed on actin. Raising fiber temperature with both perturbation techniques accelerates the forward, but slows the reverse rate constant of tension generation, kinetics akin to the unfolding/folding of small proteins. Decreasing strain, however, causes both forward and reverse rate constants to increase. Despite these changes in rate, the equilibrium constant is strain-insensitive. Activation enthalpy and entropy data show this invariance to be the result of enthalpy-entropy compensation. Reaction amplitudes confirm a strain-invariant equilibrium constant and thus a strain-insensitive ratio of pretension- to tension-generating states as work is done.

Adaptive postprocessing techniques for myocardial tissue tracking with displacement-encoded MR imaging.

UNLABELLED: The purpose of this study was to prospectively assess the effects of two adaptive postprocessing techniques on the evaluation of myocardial function with displacement-encoded magnetic resonance (MR) imaging, including sensitivity for abnormal wall motion, with two-dimensional echocardiography as the reference standard. Sixteen patients (11 men, five women; age range, 26-74 years) and 12 volunteers (six men, six women; age range, 29-53 years) underwent breath-hold MR imaging. Institutional review board approval and informed consent were obtained. Adaptive phase-unwrapping and spatial filtering techniques were compared with conventional phase-unwrapping and spatial filtering techniques. Use of the adaptive techniques led to a reduced rate of failure with the phase-unwrapping technique from 18.9% to 0.6% (P < .001), resulted in lower variability of segmental strain measurements among healthy volunteers (P < .001 to P = .02), and increased the sensitivity of quantitative detection of abnormal segments in patients from 82.5% to 87.7% (P = .034). The adaptive techniques improved the semiautomated postprocessing of displacement-encoded cardiac images and increased the sensitivity of detection of abnormal wall motion in patients. SUPPLEMENTAL MATERIAL: http://radiology.rsnajnls.org/cgi/content/full/246/1/229/DC1.

Mechanism of tension generation in muscle: an analysis of the forward and reverse rate constants.

Tension generation in muscle occurs during the attached phase of the ATP-powered cyclic interaction of myosin heads with thin filaments. The transient nature of tension-generating intermediates and the complexity of the mechanochemical cross-bridge cycle have impeded a quantitative description of tension generation. Recent experiments performed under special conditions yielded a sigmoidal dependence of fiber tension on temperature--a unique case that simplifies the system to a two-state transition. We have applied this two-state analysis to kinetic data obtained from biexponential laser temperature-jump tension transients. Here we present the forward and reverse rate constants for de novo tension generation derived from analysis of the kinetics of the fast laser temperature-jump phase tau(2) (equivalent of the length-jump phase 2(slow)). The slow phase tau(3) is temperature-independent indicating coupling to rather than a direct role in, de novo tension generation. Increasing temperature accelerates the forward, and slows the reverse, rate constant for the creation of the tension-generating state. Arrhenius behavior of the forward and anti-Arrhenius behavior of the reverse rate constant is a kinetic signature of multistate multipathway protein-folding reactions. We conclude that locally unfolded tertiary and/or secondary structure of the actomyosin cross-bridge mediates the power stroke.

Adult murine skeletal muscle contains cells that can differentiate into beating cardiomyocytes in vitro.

It has long been held as scientific fact that soon after birth, cardiomyocytes cease dividing, thus explaining the limited restoration of cardiac function after a heart attack. Recent demonstrations of cardiac myocyte differentiation observed in vitro or after in vivo transplantation of adult stem cells from blood, fat, skeletal muscle, or heart have challenged this view. Analysis of these studies has been complicated by the large disparity in the magnitude of effects seen by different groups and obscured by the recently appreciated process of in vivo stem-cell fusion. We now show a novel population of nonsatellite cells in adult murine skeletal muscle that progress under standard primary cell-culture conditions to autonomously beating cardiomyocytes. Their differentiation into beating cardiomyocytes is characterized here by video microscopy, confocal-detected calcium transients, electron microscopy, immunofluorescent cardiac-specific markers, and single-cell patch recordings of cardiac action potentials. Within 2 d after tail-vein injection of these marked cells into a mouse model of acute infarction, the marked cells are visible in the heart. By 6 d they begin to differentiate without fusing to recipient cardiac cells. Three months later, the tagged cells are visible as striated heart muscle restricted to the region of the cardiac infarct.

Kinetic effects of fiber type on the two subcomponents of the Huxley-Simmons phase 2 in muscle.

The Huxley-Simmons phase 2 controls the kinetics of the first stages of tension recovery after a step-change in fiber length and is considered intimately associated with tension generation. It had been shown that phase 2 is comprised of two distinct unrelated phases. This is confirmed here by showing that the properties of phase 2(fast) are independent of fiber type, whereas those of phase 2(slow) are fiber type dependent. Phase 2(fast) has a rate of 1000-2000 s(-1) and is temperature insensitive (Q(10) approximately 1.16) in fast, medium, and slow speed fibers. Regardless of fiber type and temperature, the amplitude of phase 2(fast) is half (approximately 0.46) that of phase 1 (fiber instantaneous stiffness). Consequently, fiber compliance (cross-bridge and thick/thin filament) appears to be the common source of both phase 1 elasticity and phase 2(fast) viscoelasticity. In fast fibers, stiffness increases in direct proportion to tension from an extrapolated positive origin at zero tension. The simplest explanation is that tension generation can be approximated by two-state transition from attached preforce generating (moderate stiffness) to attached force generating (high stiffness) states. Phase 2(slow) is quite different, progressively slowing in concert with fiber type. An interesting interpretation of the amplitude and rate data is that reverse coupling of phase 2(slow) back to P(i) release and ATP hydrolysis appears absent in fast fibers, detectable in medium speed fibers, and marked in slow fibers contracting isometrically. Contracting slow and heart muscles stretched under load could employ this enhanced reversibility of the cross-bridge cycle as a mechanism to conserve energy.

Sensing stretch is fundamental.

Stretch induces changes in cardiomyocyte biology that are implicated in heart failure, but the mechanism by which stretch is sensed and signals are transduced is unknown. New understanding of the Z disc elements of contractile units are beginning to elucidate the mechanism of stretch sensing and its relation to cardiac adaptation and disease.

Kinetic effects of myosin regulatory light chain phosphorylation on skeletal muscle contraction.

Kinetic analysis of contracting fast and slow rabbit muscle fibers in the presence of the tension inhibitor 2,3-butanedione monoxime suggests that regulatory light chain (RLC) phosphorylation up-regulates the flux of weakly attached cross-bridges entering the contractile cycle by increasing the actin-catalyzed release of phosphate from myosin. This step appears to be separate from earlier Ca(2+) regulated steps. Small step-stretches of single skinned fibers were used to study the effect of phosphorylation on fiber mechanics. Subdivision of the resultant tension transients into the Huxley-Simmons phases 1, 2(fast), 2(slow), 3, and 4 reveals that phosphorylation reduces the normalized amplitude of the delayed rise in tension (stretch activation response) by decreasing the amplitudes of phase 3 and, to a lesser extent, phase 2(slow). In slow fibers, the RLC P1 isoform phosphorylates at least 4-fold faster than the P2 isoform, complicating the role of RLC phosphorylation in heart and slow muscle. We discuss the functional relevance of the regulation of stretch activation by RLC phosphorylation for cardiac and other oscillating muscles and speculate how the interaction of the two heads of myosin could account for the inverse effect of Ca(2+) levels on isometric tension and rate of force redevelopment (k(TR)).