Erythrocyte spectrin maintains its segmental motions on oxidation: a spin-label EPR study

A Highly Ordered Nitroxide Side Chain for Distance Mapping and Monitoring Slow Structural Fluctuations in Proteins

Site-directed spin labeling electron paramagnetic resonance (SDSL-EPR) is an established tool for exploring protein structure and dynamics. Although nitroxide side chains attached to a single cysteine via a disulfide linkage are commonly employed in SDSL-EPR, their internal flexibility complicates applications to monitor slow internal motions in proteins and to structure determination by distance mapping. Moreover, the labile disulfide linkage prohibits the use of reducing agents often needed for protein stability. To enable the application of SDSL-EPR to the measurement of slow internal dynamics, new spin labels with hindered internal motion are desired. Here, we introduce a highly ordered nitroxide side chain, designated R9, attached at a single cysteine residue via a non-reducible thioether linkage. The reaction to introduce R9 is highly selective for solvent-exposed cysteine residues. Structures of R9 at two helical sites in T4 Lysozyme were determined by X-ray crystallography and the mobility in helical sequences was characterized by EPR spectral lineshape analysis, Saturation Transfer EPR, and Saturation Recovery EPR. In addition, interspin distance measurements between pairs of R9 residues are reported. Collectively, all data indicate that R9 will be useful for monitoring slow internal structural fluctuations, and applications to distance mapping via dipolar spectroscopy and relaxation enhancement methods are anticipated.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00723-023-01618-8.

Differential dielectroscopic data on the relation of erythrocyte membrane skeleton to erythrocyte deformability and flicker

Two dielectric relaxations, βsp (1.5 MHz) and γ1sp (7 MHz), have been detected on spectrin-based membrane skeleton (MS) of red blood cells (RBCs) using the plot of admittance changes at the spectrin denaturation temperature (Ivanov and Paarvanova in Bioelectrochemistry 110: 59-68, 2016, Electrochim Acta 317: 289-300, 2019a). In this study, we treated RBCs and RBC ghost membranes with agents that make membranes rigid and suppress membrane flicker, and studied the effect on βsp and γ1sp relaxations. Diamide (diazene dicarboxylic acid bis-(N,N-dimethylamide)) (up to 0.85 mM), taurine mustard (tris(2-chloroethyl)amine) (up to 2 mM), known to specifically cross-link and stiffen spectrin, and glutaraldehyde (up to 0.044%) all inhibited the relaxations in RBC ghost membranes. Similar inhibition was obtained resealing RBC ghost membranes with 2,3-diphosphoglicerate (up to 15 mM), binding WGA (wheat germ agglutinin) (up to 0.025 mg/ml) to exofacial aspect of RBCs, incubating RBCs in hypotonic (200 mOsm) and hypertonic (600-900 mOsm) media and depleting RBCs of ATP. By contrast, concanavalin A (1 mg/ml) and DIDS (4,4'-diiso-thiocyanato stilbene-2,2'-disulfonic acid) (75 μM, pH 8.2), both known to bind specifically band 3 integral protein of RBCs without effect on RBC membrane rigidity, did not affect the relaxations. We conclude there might be a relation between the strength of dielectric relaxations on MS spectrin and the deformability and flicker of RBC membrane.

Dielectric relaxations on erythrocyte membrane as revealed by spectrin denaturation

We studied the effect of spectrin denaturation at 49.5°C (TA) on the dielectric relaxations and related changes in the complex impedance, Z*, complex capacitance, C*, and dielectric loss curve of suspensions containing human erythrocytes, erythrocyte ghost membranes (EMs) and Triton-X-100 residues of EMs. The loss curve prior to, minus the loss curve after TA, resulted in a bell-shaped peak at 1.5MHz. The changes in the real and imaginary components of Z* and C* at TA, i.e., ΔZre, ΔZim, ΔCre and ΔCim, calculated in the same way, strongly varied with frequency. Between 1.0 and 12MHz the -ΔZim vs ΔZre, and ΔCim vs ΔCre plots depicted semicircles with critical frequencies, fcr, at 2.5MHz expressing recently reported relaxation of spectrin dipoles. Between 0.02 and 1.0MHz the -ΔZim vs ΔZre plot exhibited another relaxation whose fcr mirrored that of beta relaxation. This relaxation was absent on Triton-X-shells, while on erythrocytes and EMs it was inhibited by selective dissociation of either attachment sites between spectrin and bilayer. Considering above findings and inaccessibility of cytosole to outside field at such frequencies, the latter relaxation was assumed originating from a piezoelectric effect on the highly deformable spectrin filaments.

The relationship between red blood cell deformability metrics and perfusion of an artificial microvascular network

The ability of red blood cells (RBC) to undergo a wide range of deformations while traversing the microvasculature is crucial for adequate perfusion. Interpretation of RBC deformability measurements performed in vitro in the context of microvascular perfusion has been notoriously difficult. This study compares the measurements of RBC deformability performed using micropore filtration and ektacytometry with the RBC ability to perfuse an artificial microvascular network (AMVN). Human RBCs were collected from healthy consenting volunteers, leukoreduced, washed and exposed to graded concentrations (0-0.08%) of glutaraldehyde (a non-specific protein cross-linker) and diamide (a spectrin-specific protein cross-linker) to impair the deformability of RBCs. Samples comprising cells with two different levels of deformability were created by adding non-deformable RBCs (hardened by exposure to 0.08% glutaraldehyde) to the sample of normal healthy RBCs. Ektacytometry indicated a nearly linear decline in RBC deformability with increasing glutaraldehyde concentration. Micropore filtration showed a significant reduction only for concentrations of glutaraldehyde higher than 0.04%. Neither micropore filtration nor ektacytometry measurements could accurately predict the AMVN perfusion. Treatment with diamide reduced RBC deformability as indicated by ektacytometry, but had no significant effect on either micropore filtration or the AMVN perfusion. Both micropore filtration and ektacytometry showed a linear decline in effective RBC deformability with increasing fraction of non-deformable RBCs in the sample. The corresponding decline in the AMVN perfusion plateaued above 50%, reflecting the innate ability of blood flow in the microvasculature to bypass occluded capillaries. Our results suggest that in vitro measurements of RBC deformability performed using either micropore filtration or ektacytometry may not represent the ability of same RBCs to perfuse microvascular networks. Further development of biomimetic tools for measuring RBC deformability (e.g. the AMVN) could enable a more functionally relevant testing of RBC mechanical properties.

Cellular uptake, intracellular trafficking, and cytotoxicity of nanomaterials

The interactions of nanoparticles with the soft surfaces of biological systems like cells play key roles in executing their biomedical functions and in toxicity. The discovery or design of new biomedical functions, or the prediction of the toxicological consequences of nanoparticles in vivo, first require knowledge of the interplay processes of the nanoparticles with the target cells. This article focusses on the cellular uptake, location and translocation, and any biological consequences, such as cytotoxicity, of the most widely studied and used nanoparticles, such as carbon-based nanoparticles, metallic nanoparticles, and quantum dots. The relevance of the size and shape, composition, charge, and surface chemistry of the nanoparticles in cells is considered. The intracellular uptake pathways of the nanoparticles and the cellular responses, with potential signaling pathways activated by nanoparticle interactions, are also discussed.

Fullerene nanoparticles selectively enter oxidation-damaged cerebral microvessel endothelial cells and inhibit JNK-related apoptosis

There is a dearth in fundamental cellular-level understanding of how nanoparticles interact with the cells of the blood brain barrier (BBB), particularly under the oxidative environment. The apoptosis of cerebral microvessel endothelial cells (CMECs) induced by oxidative stress injury plays a key role in the dysfunction of BBB. By use of CMECs as an in vitro BBB model, we show for the first time that C(60)(C(COOH)(2))(2) nanoparticles can selectively enter oxidized CMECs rather than normal cells, and maintain CMECs integrity by attenuating H(2)O(2)-induced F-actin depolymerization via the observation of several state-of-the art microscopic techniques. Additionally, we have found that C(60)(C(COOH)(2))(2) nanoparticles greatly inhibit the apoptosis of CMECs induced by H(2)O(2), which is related to their modulation of the JNK pathway. C(60)(C(COOH)(2))(2) nanoparticles can regulate several downstream signaling events related to the JNK pathway, including reduction of JNK phosphorylation, activation of activator protein 1 (AP-1) and caspase-3, and inhibition of polyADP-ribose polymerase (PARP) cleavage and mitochondrial cytochrome c release. Our results indicate that C(60)(C(COOH)(2))(2) nanoparticles possess a novel ability of selectively entering oxidation-damaged cerebral endothelial cells rather than normal endothelial cells and then protecting them from apoptosis.