The allosteric mechanism induced by protein kinase A (PKA) phosphorylation of dematin (band 4.9)

Signaling mechanisms in red blood cells: A view through the protein phosphorylation and deformability

Intracellular signaling mechanisms in red blood cells (RBCs) involve various protein kinases and phosphatases and enable rapid adaptive responses to hypoxia, metabolic requirements, oxidative stress, or shear stress by regulating the physiological properties of the cell. Protein phosphorylation is a ubiquitous mechanism for intracellular signal transduction, volume regulation, and cytoskeletal organization in RBCs. Spectrin-based cytoskeleton connects integral membrane proteins, band 3 and glycophorin C to junctional proteins, ankyrin and Protein 4.1. Phosphorylation leads to a conformational change in the protein structure, weakening the interactions between proteins in the cytoskeletal network that confers a more flexible nature for the RBC membrane. The structural organization of the membrane and the cytoskeleton determines RBC deformability that allows cells to change their ability to deform under shear stress to pass through narrow capillaries. The shear stress sensing mechanisms and oxygenation-deoxygenation transitions regulate cell volume and mechanical properties of the membrane through the activation of ion transporters and specific phosphorylation events mediated by signal transduction. In this review, we summarize the roles of Protein kinase C, cAMP-Protein kinase A, cGMP-nitric oxide, RhoGTPase, and MAP/ERK pathways in the modulation of RBC deformability in both healthy and disease states. We emphasize that targeting signaling elements may be a therapeutic strategy for the treatment of hemoglobinopathies or channelopathies. We expect the present review will provide additional insights into RBC responses to shear stress and hypoxia via signaling mechanisms and shed light on the current and novel treatment options for pathophysiological conditions.

A preliminary study of phosphodiesterases and adenylyl cyclase signaling pathway on red blood cell deformability of sickle cell patients

Sickle cell disease (SCD) is an inherited hemoglobinopathy characterized by chronic anemia, intravascular hemolysis, and the occurrence of vaso-occlusive crises due to the mechanical obstruction of the microcirculation by poorly deformable red blood cells (RBCs). RBC deformability is a key factor in the pathogenesis of SCD, and is affected by various factors. In this study, we investigated the effects of adenylyl cyclase (AC) signaling pathway modulation and different phosphodiesterase (PDE) modulatory molecules on the deformability and mechanical stress responses of RBC from SCD patients (HbSS genotype) by applying 5 Pa shear stress with an ektacytometer (LORRCA). We evaluated RBC deformability before and after the application of shear stress. AC stimulation with Forskolin had distinct effects on RBC deformability depending on the application of 5 Pa shear stress. RBC deformability was increased by Forskolin before shear stress application but decreased after 5 Pa shear stress. AC inhibition with SQ22536 and protein kinase A (PKA) inhibition with H89 increased RBC deformability before and after the shear stress application. Non-selective PDE inhibition with Pentoxifylline increased RBC deformability. However, modulation of the different PDE types had distinct effects on RBC deformability, with PDE1 inhibition by Vinpocetine increasing deformability while PDE4 inhibition by Rolipram decreased RBC deformability after the shear stress application. The effects of the drugs varied greatly between patients suggesting some could benefit from one drug while others not. Developing drugs targeting the AC signaling pathway could have clinical applications for SCD, but more researches with larger patient cohorts are needed to identify the differences in the responses of sickle RBCs.

Dematin Regulates Calcium Mobilization, Thrombosis, and Early Akt Activation in Platelets

The complex intrinsic and extrinsic pathways contributing to platelet activation profoundly impact hemostasis and thrombosis. Detailed cellular mechanisms that regulate calcium mobilization, Akt activation, and integrin signaling in platelets remain incompletely understood. Dematin is a broadly expressed actin binding and bundling cytoskeletal adaptor protein regulated by phosphorylation via cAMP-dependent protein kinase. Here, we report the development of a conditional mouse model specifically lacking dematin in platelets. Using the new mouse model termed PDKO, we provide direct evidence that dematin is a major regulator of calcium mobilization, and its genetic deletion inhibits the early phase of Akt activation in response to collagen and thrombin agonists in platelets. The aberrant platelet shape change, clot retraction, and in vivo thrombosis observed in PDKO mice will enable future characterization of dematin-mediated integrin activation mechanisms in thrombogenic as well as nonvascular pathologies.

Structural basis of membrane skeleton organization in red blood cells

The spectrin-based membrane skeleton is a ubiquitous membrane-associated two-dimensional cytoskeleton underneath the lipid membrane of metazoan cells. Mutations of skeleton proteins impair the mechanical strength and functions of the membrane, leading to several different types of human diseases. Here, we report the cryo-EM structures of the native spectrin-actin junctional complex (from porcine erythrocytes), which is a specialized short F-actin acting as the central organizational unit of the membrane skeleton. While an α-/β-adducin hetero-tetramer binds to the barbed end of F-actin as a flexible cap, tropomodulin and SH3BGRL2 together create an absolute cap at the pointed end. The junctional complex is strengthened by ring-like structures of dematin in the middle actin layers and by patterned periodic interactions with tropomyosin over its entire length. This work serves as a structural framework for understanding the assembly and dynamics of membrane skeleton and offers insights into mechanisms of various ubiquitous F-actin-binding factors in other F-actin systems.

Dematin inhibits glioblastoma malignancy through RhoA-mediated CDKs downregulation and cytoskeleton remodeling

Glioblastoma multiforme (GBM) is well known as a highly aggressive brain tumor subtype. Here, we show that overexpression (OE) of dematin actin-binding protein (DMTN) inhibits GBM proliferation and invasion by affecting cell cycle regulation and actin remodeling, respectively. RT-qPCR, western blotting, and immunohistochemical (IHC) staining demonstrated a significant reduction in DMTN expression in gliomas, especially in high-grade gliomas (HGG) compared with normal brains, which correlates with worse survival in HGG patients. Functional studies revealed inhibitory effects of DMTN on tumor proliferation and migratory capacities. The attenuation in tumor proliferative ability upon DMTN OE was accompanied by RhoA suppression and CDK1, CDK2, CDK4, and cyclin D1 downregulation, while RhoA rescue restored the proliferative phenotype. Meanwhile, overexpression of DMTN produced profoundly disorganized stress fibers, which led to impaired tumor invasion. Furthermore, DMTN overexpression produced substantial suppression of tumor growth upon subcutaneous and intracranial implantation in mice, and this was accompanied by significantly reduced vinculin expression and Ki67 positivity. Taken together, these findings demonstrate the role of DMTN in regulating GBM cell proliferation, actin cytoskeleton, and cell morphology and identify DMTN as a vital tumor suppressor in GBM progression.

Sortase-mediated segmental labeling: A method for segmental assignment of intrinsically disordered regions in proteins

A significant number of proteins possess sizable intrinsically disordered regions (IDRs). Due to the dynamic nature of IDRs, NMR spectroscopy is often the tool of choice for characterizing these segments. However, the application of NMR to IDRs is often hindered by their instability, spectral overlap and resonance assignment difficulties. Notably, these challenges increase considerably with the size of the IDR. In response to these issues, here we report the use of sortase-mediated ligation (SML) for segmental isotopic labeling of IDR-containing samples. Specifically, we have developed a ligation strategy involving a key segment of the large IDR and adjacent folded headpiece domain comprising the C-terminus of A. thaliana villin 4 (AtVLN4). This procedure significantly reduces the complexity of NMR spectra and enables group identification of signals arising from the labeled IDR fragment, a process we refer to as segmental assignment. The validity of our segmental assignment approach is corroborated by backbone residue-specific assignment of the IDR using a minimal set of standard heteronuclear NMR methods. Using segmental assignment, we further demonstrate that the IDR region adjacent to the headpiece exhibits nonuniform spectral alterations in response to temperature. Subsequent residue-specific characterization revealed two segments within the IDR that responded to temperature in markedly different ways. Overall, this study represents an important step toward the selective labeling and probing of target segments within much larger IDR contexts. Additionally, the approach described offers significant savings in NMR recording time, a valuable advantage for the study of unstable IDRs, their binding interfaces, and functional mechanisms.

abLIM1 constructs non-erythroid cortical actin networks to prevent mechanical tension-induced blebbing

The cell cortex is a layer of cytoskeletal networks underneath the plasma membrane, formed by filamentous actin (F-actin) and cortex proteins including spectrin, adducin, and myosin. It provides cells with proper stiffness, elasticity, and surface tension to allow morphogenesis, division, and migration. Although its architecture and formation have been widely studied in red blood cells, they are poorly understood in non-erythrocytes due to structural complexity and versatile functions. In this study, we identify the actin-binding protein abLIM1 as a novel non-erythroid cell-specific cortex organizer. Endogenous abLIM1 colocalized with cortical βII spectrin but upon overexpression redistributed to thick cortical actin bundles. abLIM1 associated with major cortex proteins such as spectrins and adducin in vivo. Depletion of abLIM1 by RNAi induced prominent blebbing during membrane protrusions of spreading or migrating RPE1 cells and impaired migration efficiency. Reducing cortical tensions by culturing the cells to confluency or inhibiting myosin activity repressed the blebbing phenotype. abLIM1-depleted RPE1 or U2OS cells lacked the dense interwoven cortical actin meshwork observed in control cells but were abundant in long cortical actin bundles along the long axis of the cells. In-vitro assays indicated that abLIM1 was able to crosslink and bundle F-actin to induce dense F-actin network formation. Therefore, abLIM1 governs the formation of dense interconnected cortical actin meshwork in non-erythroid cells to prevent mechanical tension-induced blebbing during cellular activities such as spreading and migration.

Gene disruption of dematin causes precipitous loss of erythrocyte membrane stability and severe hemolytic anemia

Dematin is a relatively low abundance actin binding and bundling protein associated with the spectrin-actin junctions of mature erythrocytes. Primary structure of dematin includes a loosely folded core domain and a compact headpiece domain that was originally identified in villin. Dematin's actin binding properties are regulated by phosphorylation of its headpiece domain by cyclic adenosine monophosphate-dependent protein kinase. Here, we used a novel gene disruption strategy to generate the whole body dematin gene knockout mouse model (FLKO). FLKO mice, while born at a normal Mendelian ratio, developed severe anemia and exhibited profound aberrations of erythrocyte morphology and membrane stability. Having no apparent effect on primitive erythropoiesis, FLKO mice show significant enhancement of erythroblast enucleation during definitive erythropoiesis. Using membrane protein analysis, domain mapping, electron microscopy, and dynamic deformability measurements, we investigated the mechanism of membrane instability in FLKO erythrocytes. Although many membrane and cytoskeletal proteins remained at their normal levels, the major peripheral membrane proteins spectrin, adducin, and actin were greatly reduced in FLKO erythrocytes. Our results demonstrate that dematin plays a critical role in maintaining the fundamental properties of the membrane cytoskeleton complex.

Plasmodiumfalciparum infection induces dynamic changes in the erythrocyte phospho-proteome

The phosphorylation status of red blood cell proteins is strongly altered during the infection by the malaria parasite Plasmodium falciparum. We identify the key phosphorylation events that occur in the erythrocyte membrane and cytoskeleton during infection, by a comparative analysis of global phospho-proteome screens between infected (obtained at schizont stage) and uninfected RBCs. The meta-analysis of reported mass spectrometry studies revealed a novel compendium of 495 phosphorylation sites in 182 human proteins with regulatory roles in red cell morphology and stability, with about 25% of these sites specific to infected cells. A phosphorylation motif analysis detected 7 unique motifs that were largely mapped to kinase consensus sequences of casein kinase II and of protein kinase A/protein kinase C. This analysis highlighted prominent roles for PKA/PKC involving 78 phosphorylation sites. We then compared the phosphorylation status of PKA (PKC) specific sites in adducin, dematin, Band 3 and GLUT-1 in uninfected RBC stimulated or not by cAMP to their phosphorylation status in iRBC. We showed cAMP-induced phosphorylation of adducin S59 by immunoblotting and we were able to demonstrate parasite-induced phosphorylation for adducin S726, Band 3 and GLUT-1, corroborating the protein phosphorylation status in our erythrocyte phosphorylation site compendium.

Anatomy of the red cell membrane skeleton: unanswered questions

The red cell membrane skeleton is a pseudohexagonal meshwork of spectrin, actin, protein 4.1R, ankyrin, and actin-associated proteins that laminates the inner membrane surface and attaches to the overlying lipid bilayer via band 3-containing multiprotein complexes at the ankyrin- and actin-binding ends of spectrin. The membrane skeleton strengthens the lipid bilayer and endows the membrane with the durability and flexibility to survive in the circulation. In the 36 years since the first primitive model of the red cell skeleton was proposed, many additional proteins have been discovered, and their structures and interactions have been defined. However, almost nothing is known of the skeleton's physiology, and myriad questions about its structure remain, including questions concerning the structure of spectrin in situ, the way spectrin and other proteins bind to actin, how the membrane is assembled, the dynamics of the skeleton when the membrane is deformed or perturbed by parasites, the role lipids play, and variations in membrane structure in unique regions like lipid rafts. This knowledge is important because the red cell membrane skeleton is the model for spectrin-based membrane skeletons in all cells, and because defects in the red cell membrane skeleton underlie multiple hemolytic anemias.

cAMP-Signalling Regulates Gametocyte-Infected Erythrocyte Deformability Required for Malaria Parasite Transmission

Blocking Plasmodium falciparum transmission to mosquitoes has been designated a strategic objective in the global agenda of malaria elimination. Transmission is ensured by gametocyte-infected erythrocytes (GIE) that sequester in the bone marrow and at maturation are released into peripheral blood from where they are taken up during a mosquito blood meal. Release into the blood circulation is accompanied by an increase in GIE deformability that allows them to pass through the spleen. Here, we used a microsphere matrix to mimic splenic filtration and investigated the role of cAMP-signalling in regulating GIE deformability. We demonstrated that mature GIE deformability is dependent on reduced cAMP-signalling and on increased phosphodiesterase expression in stage V gametocytes, and that parasite cAMP-dependent kinase activity contributes to the stiffness of immature gametocytes. Importantly, pharmacological agents that raise cAMP levels in transmissible stage V gametocytes render them less deformable and hence less likely to circulate through the spleen. Therefore, phosphodiesterase inhibitors that raise cAMP levels in P. falciparum infected erythrocytes, such as sildenafil, represent new candidate drugs to block transmission of malaria parasites.

Malaria transmission is ensured by deformable mature gametocyte-infected erythrocytes being taken up when a mosquito bites. Non-deformable immature gametocyte stages are sequestered in the bone marrow, as their lack of deformability would lead to their splenic clearance. In the present study, we apply nano-filtration technology to mimic splenic retention and demonstrate that deformability of transmissible mature stage V gametocytes is regulated by parasite cyclic AMP-dependent kinase signalling. Importantly, when we used drugs to raise cAMP levels we render transmissible mature gametocytes as stiff as non-transmissible gametocytes. In contrast, when we inhibit the cAMP-dependent kinase we render immature gametocytes more deformable. Thus, by two different approaches we confirm that the drop in cAMP levels in mature gametocytes leads to an increase in their deformability and hence more likely to circulate through the spleen. Our molecular observations have the potential to be translated into therapies for blocking malaria transmission by demonstrating that raising cAMP levels with sildenafil also known as “Viagra” renders mature gametocytes rigid. These findings provide the proof of principle that deformability of circulating gametocytes is targetable by pharmacological agents and as such, it provides a novel approach to prevent the spread of parasites. PDE inhibitors therefore represent novel drug leads potentially capable of blocking transmission and improving the worldwide fight to eliminate malaria from the human population.

Phosphorylated and unphosphorylated serine 13 of CDC37 stabilize distinct interactions between its client and HSP90 binding domains

Folding and maturation of most protein kinases require chaperone assistance. In higher eukaryotes, CDC37 is the predominant cochaperone that facilitates the transfer of kinase clients to HSP90. Kinase recognition is thought to occur through the N-terminal domain, which has, thus far, eluded structure determination. Client processing also requires the phosphorylation of the N-terminal tail at Ser13 by protein kinase CK2 (casein kinase 2). How phosphorylation alters the molecular properties of CDC37 is not understood. We show that the phosphorylation at Ser13 induces a large shift toward a more compact structure, based on ANS fluorescence, while modestly increasing secondary structure. Moreover, this transition requires interactions of the N-terminal domain and the remainder of CDC37. The stabilizing property of the phosphorylation event can be recreated in trans by a (phospho-Ser13) peptide derived from the N-terminal tail. However, the phosphorylation-induced transition is not dependent on the transferred phosphate group but rather the loss of serine-like properties at position 13. The complete absence of the N-terminal tail results in reduced secondary structure and unresponsiveness to subsequent addition of peptides. The N-terminal tail may therefore serve as an intramolecular chaperone that ensures that CDC37 assumes one of two readily interconvertible states in a manner that impacts the interaction of the client binding N-domain and the MC-domains, involved in dimerization and HSP90 binding.

Digested disorder: Quarterly intrinsic disorder digest (January/February/March, 2013)

The current literature on intrinsically disordered proteins is blooming. A simple PubMed search for “intrinsically disordered protein OR natively unfolded protein” returns about 1,800 hits (as of June 17, 2013), with many papers published quite recently. To keep interested readers up to speed with this literature, we are starting a “Digested Disorder” project, which will encompass a series of reader’s digest type of publications aiming at the objective representation of the research papers and reviews on intrinsically disordered proteins. The only two criteria for inclusion in this digest are the publication date (a paper should be published within the covered time frame) and topic (a paper should be dedicated to any aspect of protein intrinsic disorder). The current digest covers papers published during the period of January, February and March of 2013. The papers are grouped hierarchically by topics they cover, and for each of the included paper a short description is given on its major findings.

The human erythrocyte plasma membrane: a Rosetta Stone for decoding membrane-cytoskeleton structure

The mammalian erythrocyte, or red blood cell (RBC), is a unique experiment of nature: a cell with no intracellular organelles, nucleus or transcellular cytoskeleton, and a plasma membrane with uniform structure across its entire surface. By virtue of these specialized properties, the RBC membrane has provided a template for discovery of the fundamental actin filament network machine of the membrane skeleton, now known to confer mechanical resilience, anchor membrane proteins, and organize membrane domains in all cells. This chapter provides a historical perspective and critical analysis of the biochemistry, structure, and physiological functions of this actin filament network in RBCs. The core units of this network are nodes of ~35-37 nm-long actin filaments, interconnected by long strands of (α1β1)₂-spectrin tetramers, forming a 2D isotropic lattice with quasi-hexagonal symmetry. Actin filament length and stability is critical for network formation, relying upon filament capping at both ends: tropomodulin-1 at pointed ends and αβ-adducin at barbed ends. Tropomodulin-1 capping is essential for precise filament lengths, and is enhanced by tropomyosin, which binds along the short actin filaments. αβ-adducin capping recruits spectrins to sites near barbed ends, promoting network formation. Accessory proteins, 4.1R and dematin, also promote spectrin binding to actin and, with αβ-adducin, link to membrane proteins, targeting actin nodes to the membrane. Dissection of the molecular organization within the RBC membrane skeleton is one of the paramount achievements of cell biological research in the past century. Future studies will reveal the structure and dynamics of actin filament capping, mechanisms of precise length regulation, and spectrin-actin lattice symmetry.