Molecular insight into the effect of lipid bilayer environments on thrombospondin-1 and calreticulin interactions

Calreticulin accelerates corneal wound closure and mitigates fibrosis: Potential therapeutic applications

The processes involved in regeneration of cutaneous compared to corneal tissues involve different intrinsic mechanisms. Importantly, cutaneous wounds involve healing by angiogenesis but vascularization of the cornea obscures vision. Previous studies showed that topically applied calreticulin (CALR) healed full-thickness excisional animal wounds by a tissue regenerative process markedly enhancing repair without evoking angiogenesis. In the current study, the application of CALR in a rabbit corneal injury model: (1) accelerated full wound closure by 3 days (2) accelerated delayed healing caused by corticosteroids, routinely used to prevent post-injury inflammation, by 6 days and (3) healed wounds without vascularization or fibrosis/hazing. In vitro, CALR stimulated proliferation of human corneal epithelial cells (CE) and corneal stromal cells (keratocytes) by 1.5-fold and 1.4-fold, respectively and induced migration of CE cells and keratocytes, by 72% and 85% compared to controls of 44% and 59%, respectively. As a marker of decreased fibrosis, CALR treated corneal wounds showed decreased immunostaining for α-smooth muscle actin (α-SMA) by keratocytes and following CALR treatment in vitro, decreased the levels of TGF-β2 in human CE cells and α-SMA in keratocytes. CALR has the potential to be a novel therapeutic both, to accelerate corneal healing from various injuries and in conjunction with corticosteroids.

Thrombospondin-1 Signaling Through the Calreticulin/LDL Receptor Related Protein 1 Axis: Functions and Possible Roles in Glaucoma

Thrombospondin-1 (TSP-1) is a matricellular extracellular matrix protein. Matricellular proteins are components of the extracellular matrix (ECM) that regulate key cellular functions and impact ECM organization, but which lack direct primary structural roles in the ECM. TSP-1 expression is upregulated in response to injury, hypoxia, growth factor stimulation, inflammation, glucose, and by reactive oxygen species. Relevant to glaucoma, TSP-1 is also a mechanosensitive molecule upregulated by mechanical stretch. TSP-1 expression is increased in ocular remodeling in glaucoma in both the trabecular meshwork and in the optic nerve head. The exact roles of TSP-1 in glaucoma remain to be defined, however. It plays important roles in cell behavior and in ECM remodeling during wound healing, fibrosis, angiogenesis, and in tumorigenesis and metastasis. At the cellular level, TSP-1 can modulate cell adhesion and migration, protease activity, growth factor activity, anoikis resistance, apoptosis, and collagen secretion and matrix assembly and cross-linking. These multiple functions and macromolecular and receptor interactions have been ascribed to specific domains of the TSP-1 molecule. In this review, we will focus on the cell regulatory activities of the TSP-1 N-terminal domain (NTD) sequence that binds to cell surface calreticulin (Calr) and which regulates cell functions via signaling through Calr complexed with LDL receptor related protein 1 (LRP1). We will describe TSP-1 actions mediated through the Calr/LRP1 complex in regulating focal adhesion disassembly and cytoskeletal reorganization, cell motility, anoikis resistance, and induction of collagen secretion and matrix deposition. Finally, we will consider the relevance of these TSP-1 functions to the pathologic remodeling of the ECM in glaucoma.

Vitamin D3 and its hydroxyderivatives as promising drugs against COVID-19: a computational study

The epidemiologic correlation between the poor prognosis of SARS-CoV-2 infection and vitamin D deficiency has been observed worldwide, however, their molecular mechanisms are not fully understood. In this study, we used combined molecular docking, molecular dynamics simulations and binding free energy analyses to investigate the potentials of vitamin D3 and its hydroxyderivatives as TMPRSS2 inhibitor and to inhibit the SARS-CoV-2 receptor binding domain (RBD) binding to angiotensin-converting enzyme 2 (ACE2), as well as to unveil molecular and structural basis of 1,25(OH)₂D3 capability to inhibit ACE2 and SARS-CoV-2 RBD interactions. The results show that vitamin D3 and its hydroxyderivatives are favorable to bind active site of TMPRSS2 and the binding site(s) between ACE2 and SARS-CoV2-RBD, which indicate that vitamin D3 and its biologically active hydroxyderivatives can serve as TMPRSS2 inhibitor and can inhibit ACE2 binding of SARS-CoV-2 RBD to prevent SARS-CoV-2 entry. Interaction of 1,25(OH)₂D3 with SARS-CoV-2 RBD and ACE2 resulted in the conformation and dynamical motion changes of the binding surfaces between SARS-CoV-2 RBD and ACE2 to interrupt the binding of SARS-CoV-2 RBD with ACE2. The interaction of 1,25(OH)₂D3 with TMPRSS2 also caused the conformational and dynamical motion changes of TMPRSS2, which could affect TMPRSS2 to prime SARS-CoV-2 spike proteins. Our results propose that vitamin D3 and its biologically active hydroxyderivatives are promising drugs or adjuvants in the treatment of COVID-19. Communicated by Ramaswamy H. Sarma.

Role of thrombospondin‑1 and thrombospondin‑2 in cardiovascular diseases (Review)

Thrombospondin (TSP)-1 and TSP-2 are matricellular proteins in the extracellular matrix (ECM), which serve a significant role in the pathological processes of various cardiovascular diseases (CVDs). The multiple effects of TSP-1 and TSP-2 are due to their ability to interact with various ligands, such as structural components of the ECM, cytokines, cellular receptors, growth factors, proteases and other stromal cell proteins. TSP-1 and TSP-2 regulate the structure and activity of the aforementioned ligands by interacting directly or indirectly with them, thereby regulating the activity of different types of cells in response to environmental stimuli. The pathological processes of numerous CVDs are associated with the degradation and remodeling of ECM components, and with cell migration, dysfunction and apoptosis, which may be regulated by TSP-1 and TSP-2 through different mechanisms. Therefore, investigating the role of TSP-1 and TSP-2 in different CVDs and the potential signaling pathways they are associated with may provide a new perspective on potential therapies for the treatment of CVDs. In the present review, the current understanding of the roles TSP-1 and TSP-2 serve in various CVDs were summarized. In addition, the interacting ligands and the potential pathways associated with these thrombospondins in CVDs are also discussed.

Molecular insights into the effect of an apoptotic raft-like bilayer on the conformation and dynamics of calreticulin

Cell surface calreticulin (CRT) can mediate apoptotic cells removal by binding and activating LDL receptor-related protein (LRP1). Phosphatidylserine (PS) lipids in the inner leaflet of the cell membrane are externalized and become exposed in cholesterol (CHOL)-rich membrane raft-like microdomain during apoptosis and co-localized with cell surface CRT. How the apoptotic raft-like membrane microdomain affects the structure and dynamics of CRT, further affecting CRT binding with LRP1 to signal apoptotic-cell clearance, remains unknown. In this study, we investigate the interactions between CRT and raft-like bilayers with or without POPS lipids with molecular dynamics simulations. In addition, the effect of an apoptotic raft bilayer on the binding between CRT and thrombospondin-1 (TSP1), a ligand of CRT on the cell surface to signal focal adhesion disassembly, was also investigated. Results of single CRT interactions with raft-like bilayers show that PS lipids in apoptotic raft-like bilayer increased the interactions between CRT and lipid bilayer, which enhanced the conformational stability and increased dynamical motion of CRT. The microscopic and mesoscopic properties of apoptotic raft-like bilayer were altered by the binding of CRT with lipid bilayer. Results of CRT-TSP1 complex interactions with raft-like bilayers show that the binding free energy between TSP1 and CRT was reduced in apoptotic raft-like bilayer environment. This study provides molecular and structural insight into the effect of an apoptotic raft-like bilayer on the conformation and dynamics of CRT, which could enrich our understanding of CRT-mediated apoptotic-cell clearance and focal adhesion disassembly.

Characterization of Lipid-Protein Interactions and Lipid-Mediated Modulation of Membrane Protein Function through Molecular Simulation

The cellular membrane constitutes one of the most fundamental compartments of a living cell, where key processes such as selective transport of material and exchange of information between the cell and its environment are mediated by proteins that are closely associated with the membrane. The heterogeneity of lipid composition of biological membranes and the effect of lipid molecules on the structure, dynamics, and function of membrane proteins are now widely recognized. Characterization of these functionally important lipid-protein interactions with experimental techniques is however still prohibitively challenging. Molecular dynamics (MD) simulations offer a powerful complementary approach with sufficient temporal and spatial resolutions to gain atomic-level structural information and energetics on lipid-protein interactions. In this review, we aim to provide a broad survey of MD simulations focusing on exploring lipid-protein interactions and characterizing lipid-modulated protein structure and dynamics that have been successful in providing novel insight into the mechanism of membrane protein function.

Lipid composition and macromolecular crowding effects on CYP2J2-mediated drug metabolism in nanodiscs

Lipid composition and macromolecular crowding are key external effectors of protein activity and stability whose role varies between different proteins. Therefore, it is imperative to study their effects on individual protein function. CYP2J2 is a membrane-bound cytochrome P450 in the heart involved in the metabolism of fatty acids and xenobiotics. In order to facilitate this metabolism, cytochrome P450 reductase (CPR), transfers electrons to CYP2J2 from NADPH. Herein, we use nanodiscs to show that lipid composition of the membrane bilayer affects substrate metabolism of the CYP2J2-CPR nanodisc (ND) system. Differential effects on both NADPH oxidation and substrate metabolism by CYP2J2-CPR are dependent on the lipid composition. For instance, sphingomyelin containing nanodiscs produced more secondary substrate metabolites than discs of other lipid compositions, implying a possible conformational change leading to processive metabolism. Furthermore, we demonstrate that macromolecular crowding plays a role in the lipid-solubilized CYP2J2-CPR system by increasing the K_m and decreasing the V_max , and effect that is size-dependent. Crowding also affects the CYP2J2-CPR-ND system by decreasing both the K_m and V_max for Dextran-based macromolecular crowding agents, implying an increase in substrate affinity but a lack of metabolism. Finally, protein denaturation studies show that crowding agents destabilize CYP2J2, while the multidomain protein CPR is stabilized. Overall, these studies are the first report on the role of the surrounding lipid environment and macromolecular crowding in modulating enzymatic function of CYP2J2-CPR membrane protein system.

Modeling the structural and dynamical changes of the epithelial calcium channel TRPV5 caused by the A563T variation based on the structure of TRPV6

TRPV5, transient receptor potential cation channel vanilloid subfamily member 5, is an epithelial Ca²⁺ channel that plays a key role in the active Ca²⁺ reabsorption process in the kidney. A single nucleotide polymorphism (SNP) rs4252499 in the TRPV5 gene results in an A563T variation in the sixth transmembrane (TM) domain of TRPV5. Our previous study indicated that this variation increases the Ca²⁺ transport function of TRPV5. To understand the molecular mechanism, a model of TRPV5 was established based on the newly deposited structure of TRPV6 that has 83.1% amino acid identity with TRPV5 in the modeled region. Computational simulations were performed to study the structural and dynamical differences between the TRPV5 variants with A563 and T563. Consistent with the TRPV1-based simulation, the results indicate that the A563T variation increases the contacts between residues 563 and V540, which is one residue away from the key residue D542 in the Ca²⁺-selective filter. The variation enhanced the stability of the secondary structure of the pore region, decreased the fluctuation of residues around residue 563, and reduced correlated and anti-correlated motion between monomers. Furthermore, the variation increases the pore radius at the selective filter. These findings were confirmed using simulations based on the recently determined structure of rabbit TRPV5. The simulation results provide an explanation for the observation of enhanced Ca²⁺ influx in TRPV5 caused by the A563T variation. The A563T variation is an interesting example of how a residue distant from the Ca²⁺-selective filter influences the Ca²⁺ transport function of the TRPV5 channel. Communicated by Ramaswamy H. Sarma.

Multiscale simulation of the interaction of calreticulin-thrombospondin-1 complex with a model membrane microdomain

Cell surface calreticulin (CRT) binding to thrombospondin-1 (TSP1), regulates cell adhesion, migration, anoikis resistance, and collagen production. Due to the essential role of membrane microdomains in CRT-mediated focal adhesion disassembly, we previously studied the effect of raft-like bilayers on TSP1-CRT interactions with all-atom molecular dynamics (AAMD) simulations. However, the simulated systems of protein on the surface of the bilayer(s) in the explicit solvent are too large for long timescale AAMD simulations due to computational expense. In this study, we adopted a multiscale modeling approach of combining AAMD, coarse-grained molecule dynamics (CGMD), and reversed AAMD (REV AAMD) simulations to investigate the interactions of single CRT or of the TSP1-CRT complex with a membrane microdomain at microsecond timescale. Results showed that CRT conformational stabilization by binding of TSP1 in AAMD simulation was undetectable in CGMD simulation, but it was recovered in REV AAMD simulation. Similarly, interactions of the CRT N-domain and TSP1 with the membrane microdomain were lost in CGMD simulations but they were re-gained in the REV AAMD simulations. There was the higher coordination of the CRT P-domain in the TSP1-CRT complex with the lipid components of membrane microdomain compared to that of single CRT, which could directly affect the conformation of CRT and further mediate CRT recruitment of LDL receptor-related protein for signaling events. This study provides structural and molecular insights into TSP1-CRT interactions in a membrane microdomain environment and demonstrates the feasibility of using multiscale simulations to investigate the interactions between protein and membrane microdomains at a long timescale.

Molecular insights into the specific recognition between the RNA binding domain qRRM2 of hnRNP F and G-tract RNA: A molecular dynamics study

Heterogeneous nuclear ribonucleoprotein F (hnRNP F) controls the expression of various genes through regulating the alternative splicing of pre-mRNAs in the nucleus. It uses three quasi-RNA recognition motifs (qRRMs) to recognize G-tract RNA which contains at least three consecutive guanines. The structures containing qRRMs of hnRNP F in complex with G-tract RNA have been determined by nuclear magnetic resonance (NMR) spectroscopy, shedding light on the recognition mechanism of qRRMs with G-tract RNA. However, knowledge of the recognition details is still lacking. To investigate how qRRMs specifically bind with G-tract RNA and how the mutations of any guanine to an adenine in the G-tract affect the binding, molecular dynamics simulations with binding free energy analysis were performed based on the NMR structure of qRRM2 in complex with G-tract RNA. Simulation results demonstrate that qRRM2 binds strongly with G-tract RNA, but any mutation of the G-tract leads to a drastic reduction of the binding free energy. Further comparisons of the energetic components reveal that van der Waals and non-polar interactions play essential roles in the binding between qRRM2 and G-tract RNA, but the interactions are weakened by the effect of RNA mutations. Structural and dynamical analyses indicate that when qRRM2 binds with G-tract RNA, both qRRM2 and G-tract maintain stabilized structures and dynamics; however, the stability is disrupted by the mutations of the G-tract. These results provide novel insights into the recognition mechanism of qRRM2 with G-tract RNA that are not elucidated by the NMR technique.

The L530R variation associated with recurrent kidney stones impairs the structure and function of TRPV5

TRPV5 is a Ca²⁺-selective channel that plays a key role in the reabsorption of Ca²⁺ ions in the kidney. Recently, a rare L530R variation (rs757494578) of TRPV5 was found to be associated with recurrent kidney stones in a founder population. However, it was unclear to what extent this variation alters the structure and function of TRPV5. To evaluate the function and expression of the TRPV5 variant, Ca²⁺ uptake in Xenopus oocytes and western blot analysis were performed. The L530R variation abolished the Ca²⁺ uptake activity of TRPV5 in Xenopus oocytes. The variant protein was expressed with drastic reduction in complex glycosylation. To assess the structural effects of this L530R variation, TRPV5 was modeled based on the crystal structure of TRPV6 and molecular dynamics simulations were carried out. Simulation results showed that the L530R variation disrupts the hydrophobic interaction between L530 and L502, damaging the secondary structure of transmembrane domain 5. The variation also alters its interaction with membrane lipid molecules. Compared to the electroneutral L530, the positively charged R530 residue shifts the surface electrostatic potential towards positive. R530 is attracted to the negatively charged phosphate group rather than the hydrophobic carbon atoms of membrane lipids. This shifts the pore helix where R530 is located and the D542 residue in the Ca²⁺-selective filter towards the surface of the membrane. These alterations may lead to misfolding of TRPV5, reduction in translocation of the channel to the plasma membrane and/or impaired Ca²⁺ transport function of the channel, and ultimately disrupt TRPV5-mediated Ca²⁺ reabsorption.

Deprotonation states of the two active site water molecules regulate the binding of protein phosphatase 5 with its substrate: A molecular dynamics study

Protein phosphatase 5 (PP5), mainly localized in human brain, can dephosphorylate tau protein whose high level of phosphorylation is related to Alzheimer's disease. Similar to other protein phosphatases, PP5 has a conserved motif in the catalytic domain that contains two binding sites for manganese (Mn²⁺ ) ions. Structural data indicate that two active site water molecules, one bridging the two Mn²⁺ ions and the other terminally coordinated with one of the Mn²⁺ ions (Mn1), are involved in catalysis. Recently, a density functional theory study revealed that the two water molecules can be both deprotonated to keep a neutral active site for catalysis. The theoretical study gives us an insight into the catalytic mechanism of PP5, but the knowledge of how the deprotonation states of the two water molecules affect the binding of PP5 with its substrate is still lacking. To approach this problem, molecular dynamics simulations were performed to model the four possible deprotonation states. Through structural, dynamical and energetic analyses, the results demonstrate that the deprotonation states of the two water molecules affect the structure of the active site including the distance between the two Mn²⁺ ions and their coordination, impact the interaction energy of residues R275, R400 and H304 which directly interact with the substrate phosphoserine, and mediate the dynamics of helix αJ which is involved in regulation of the enzyme's activity. Furthermore, the deprotonation state that is preferable for PP5 binding of its substrate has been identified. These findings could provide new design strategy for PP5 inhibitor.

High serum thrombospondin-1 concentration is associated with slower abdominal aortic aneurysm growth and deficiency of thrombospondin-1 promotes angiotensin II induced aortic aneurysm in mice

Abdominal aortic aneurysm (AAA) is a common age-related vascular disease characterized by progressive weakening and dilatation of the aortic wall. Thrombospondin-1 (TSP-1; gene Thbs1) is a member of the matricellular protein family important in the control of extracellular matrix (ECM) remodelling. In the present study, the association of serum TSP-1 concentration with AAA progression was assessed in 276 men that underwent repeated ultrasound for a median 5.5 years. AAA growth was negatively correlated with serum TSP-1 concentration (Spearman's rho -0.129, P=0.033). Men with TSP-1 in the highest quartile had a reduced likelihood of AAA growth greater than median during follow-up (OR: 0.40; 95% confidence interval (CI): 0.19-0.84, P=0.016, adjusted for other risk factors). Immunohistochemical staining for TSP-1 was reduced in AAA body tissues compared with the relatively normal AAA neck. To further assess the role of TSP-1 in AAA initiation and progression, combined TSP-1 and apolipoprotein deficient (Thbs1-/-ApoE-/-, n=20) and control mice (ApoE-/-, n=20) were infused subcutaneously with angiotensin II (AngII) for 28 days. Following AngII infusion, Thbs1-/- ApoE-/- mice had larger AAAs by ultrasound (P=0.024) and ex vivo morphometry measurement (P=0.006). The Thbs1-/-ApoE-/- mice also showed increased elastin filament degradation along with elevated systemic levels and aortic expression of matrix metalloproteinase (MMP)-9. Suprarenal aortic segments and vascular smooth muscle cells (VSMCs) isolated from Thbs1-/-ApoE-/- mice showed reduced collagen 3A1 gene expression. Furthermore, Thbs1-/-ApoE-/- mice had reduced aortic expression of low-density lipoprotein (LDL) receptor-related protein 1. Collectively, findings from the present study suggest that TSP-1 deficiency promotes maladaptive remodelling of the ECM leading to accelerated AAA progression.

Activation mechanisms of αVβ3 integrin by binding to fibronectin: A computational study

Integrin αVβ3 plays an important role in regulating cellular activities and in human diseases. Although the structure of αVβ3 has been studied by crystallography and electron microscopy, the detailed activation mechanism of integrin αVβ3 induced by fibronectin remains unclear. In this study, we investigated the conformational and dynamical motion changes of Mn2+ -bound integrin αVβ3 by binding to fibronectin with molecular dynamics simulations. Results showed that fibronectin binding to integrin αVβ3 caused the changes of the conformational flexibility of αVβ3 domains, the essential mode of motion for the domains of αV subunit and β3 subunit and the degrees of correlated motion of residues between the domains of αV subunit and β3 subunit of integrin αVβ3. The angle of Propeller domain with respect to the Calf-2 domain of αV subunit and the angle of Hybrid domain with respect to βA domain of β3 subunit significantly increased when integrin αVβ3 was bound to fibronectin. These changes could result in the conformational change tendency of αVβ3 from a bend conformation to an extended conformation and lead to the open swing of Hybrid domain relative to βA domain of β3 subunit, which have demonstrated their importance for αVβ3 activation. Fibronectin binding to integrin αVβ3 significantly decreased the relative position of α1 helix to βA domain and that to metal ion-dependent adhesion site, stabilized Mn2+ ions binding in integrin αVβ3 and changed fibronectin conformation, which are important for αVβ3 activation. Results from this study provide important molecular insight into the "outside-in" activation mechanism of integrin αVβ3 by binding to fibronectin.

Phosphorylation of KLHL3 at serine 433 impairs its interaction with the acidic motif of WNK4: a molecular dynamics study

Interaction between the acidic motif (AM) of protein kinase WNK4 and the Kelch domain of KLHL3 are involved in the pathogenesis of pseudohypoaldosteronism type II, a hereditary form of hypertension. This interaction is disrupted by some disease-causing mutations in either WNK4 or KLHL3, or by angiotensin II- and insulin-induced phosphorylation of KLHL3 at serine 433, which is also a site frequently mutated in patients. However, the mechanism by which this phosphorylation disrupts the interaction is unclear. In this study, we approached this problem using molecular dynamics simulation with structural, dynamical and energetic analyses. Results from independent simulations indicate that when S433 was phosphorylated, the electrostatic potential became more negative in the AM binding site of KLHL3 and therefore was unfavorable for binding with the negatively charged AM. In addition, the intermolecular hydrogen bond network that kept the AM stable in the binding site of KLHL3 was disrupted, and the forces for the hydrophobic interactions between the AM of WNK4 and KLHL3 were also reduced. As a result, the weakened interactions were no longer capable of holding the AM of WNK4 at its binding site in KLHL3. In conclusion, phosphorylation of KLHL3 at S433 disrupts the hydrogen bonds, hydrophobic and electrostatic interactions between the Kelch domain of KLHL3 and the AM of WNK4. This study provides a key molecular understanding of the KLHL3-mediated regulation of WNK4, which is an integrative regulator of electrolyte homeostasis and blood pressure regulation in the kidney. Significances Statement: WNK4 is an integrative regulator of electrolyte homeostasis, which is important in the blood pressure regulation by the kidney. Interaction between WNK4 and KLHL3 is a key physiological process that is impaired in a hereditary form of hypertension. This study provides substantial new insights into the role of phosphorylation of KLHL3 in regulating the interaction with WNK4, and therefore advances our understanding of molecular pathogenesis of hypertension and the mechanism of blood pressure regulation.

The activities of LDL Receptor-related Protein-1 (LRP1) compartmentalize into distinct plasma membrane microdomains

LDL Receptor-related Protein-1 (LRP1) is an endocytic receptor for diverse ligands. In neurons and neuron-like cells, ligand-binding to LRP1 initiates cell-signaling. Herein, we show that in PC12 and N2a neuron-like cells, LRP1 distributes into lipid rafts and non-raft plasma membrane fractions. When lipid rafts were disrupted, using methyl-β-cyclodextrin or fumonisin B1, activation of Src family kinases and ERK1/2 by the LRP1 ligands, tissue-type plasminogen activator and activated α₂-macroglobulin, was blocked. Biological consequences of activated LRP1 signaling, including neurite outgrowth and cell growth, also were blocked. The effects of lipid raft disruption on ERK1/2 activation and neurite outgrowth, in response to LRP1 ligands, were reproduced in experiments with cerebellar granule neurons in primary culture. Because the reagents used to disrupt lipid rafts may have effects on the composition of the plasma membrane outside lipid rafts, we studied the effects of these reagents on LRP1 activities unrelated to cell-signaling. Lipid raft disruption did not affect the total ligand binding capacity of LRP1, the affinity of LRP1 for its ligands, or its endocytic activity. These results demonstrate that well described activities of LRP1 require localization of this receptor to distinct plasma membrane microdomains.

Structural Insight for Roles of DR5 Death Domain Mutations on Oligomerization of DR5 Death Domain-FADD Complex in the Death-Inducing Signaling Complex Formation: A Computational Study

Death receptor 5 (DR5)-induced apoptosis that prioritizes the death of tumor cells has been proposed as one of the promising cancer therapies. In this process, oligomerized DR5 death domain (DD) binding to Fas-associated death domain (FADD) leads to FADD activating caspase-8, which marks the formation of the death-inducing signaling complex (DISC) that initiates apoptosis. DR5 DD mutations found in cancer cells have been suggested to play an important pathological role, the mechanism through which those mutants prevent the DR5-activated DISC formation is not clear yet. This study sought to provide structural and molecular insight for the roles of four selected DR5 DD mutations (E355K, E367K, K415N, and L363F) in the oligomerization of DR5 DD-FADD complex during the DISC formation. Results from the molecular dynamics simulations show that the simulated mutants induce conformational, dynamical motions and interactions changes in the DR5 DD-FADD tetramer complex, including changes in a protein's backbone flexibility, less exposure of FADD DED's caspase-8 binding site, reduced H-bonding and hydrophobic contacts at the DR5 DD-FADD DD binding, altered distribution of the electrostatic potentials and correlated motions of residues, and reduced binding affinity of DR5 DD binding to FADD. This study provides structural and molecular insight for the influence of DR5 DD mutations on oligomerization of DR5 DD-FADD complex, which is expected to foster understanding of the DR5 DD mutants' resistance mechanism against DR5-activated DISC formation.

Molecular Modeling of the Structural and Dynamical Changes in Calcium Channel TRPV5 Induced by the African-Specific A563T Variation

Transient receptor potential cation channels, vanilloid subfamily, member 5 (TRPV5) plays a key role in active Ca(2+) reabsorption in the kidney. Variations in TRPV5 occur at high frequency in African populations and may contribute to their higher efficiency of Ca(2+) reabsorption. One of the African specific variations, A563T, exhibits increased Ca(2+) transport ability. However, it is unclear how this variation influences the channel pore. On the basis of the structure of TRPV1, a TRPV5 model was generated to simulate the structural and dynamical changes induced by the A563T variation. On the basis of this model, amino acid residue 563 interacts with V540, which is one residue away from the key residue, D542, involved in Ca(2+) selectivity and Mg(2+) blockade. The A563T variation increases secondary structure stability and reduces dynamical motion of D542. In addition, the A563T variation alters the electrostatic potential of the outer surface of the pore. Differences in contact between selective filter residues and residue 563 and in electrostatic potential between the two TRPV5 variants were also observed in another model derived from an alternative alignment in the selective filters between TRPV5 and TRPV1. These findings indicate that the A563T variation induces structural, dynamical, and electrostatic changes in the TRPV5 pore, providing structural insight into the functional alterations associated with the A563T variation.

Store-operated calcium entry: unveiling the calcium handling signalplex

Store-operated Ca(2+) entry (SOCE) is an important mechanism for Ca(2+) influx in non-excitable cells, also present in excitable cells. The activation of store-operated channels (SOCs) is finely regulated by the filling state of the intracellular agonist-sensitive Ca(2+) compartments, and both, the mechanism of sensing the Ca(2+) stores and the nature and functional properties of the SOCs, have been a matter of intense investigation and debate. The identification of STIM1 as the endoplasmic reticulum Ca(2+) sensor and both Orai1, as the pore-forming subunit of the channels mediating the Ca(2+)-selective store-operated current, and the members of the TRPC subfamily of proteins, as the channels mediating the cation-permeable SOCs, has shed new light on the underlying events. This review summarizes the initial hypothesis and the current advances on the mechanism of activation of SOCE.