Membrane physical properties influence transmembrane helix formation

Neutron spin echo shows pHLIP is capable of retarding membrane thickness fluctuations

Cell membranes are responsible for a range of biological processes that require interactions between lipids and proteins. While the effects of lipids on proteins are becoming better understood, our knowledge of how protein conformational changes influence membrane dynamics remains rudimentary. Here, we performed experiments and computer simulations to study the dynamic response of a lipid membrane to changes in the conformational state of pH-low insertion peptide (pHLIP), which transitions from a surface-associated (SA) state at neutral or basic pH to a transmembrane (TM) α-helix under acidic conditions. Our results show that TM-pHLIP significantly slows down membrane thickness fluctuations due to an increase in effective membrane viscosity. Our findings suggest a possible membrane regulatory mechanism, where the TM helix affects lipid chain conformations, and subsequently alters membrane fluctuations and viscosity.

Cholesterol inhibits assembly and activation of the EphA2 receptor

The receptor tyrosine kinase EphA2 drives cancer malignancy by facilitating metastasis. EphA2 can be found in different self-assembly states: as a monomer, dimer, and oligomer. However, our understanding remains limited regarding which EphA2 state is responsible for driving pro-metastatic signaling. To address this limitation, we have developed SiMPull-POP, a single-molecule method for accurate quantification of membrane protein self-assembly. Our experiments revealed that a reduction of plasma membrane cholesterol strongly promoted EphA2 self-assembly. Indeed, low cholesterol caused a similar effect to the EphA2 ligand ephrinA1-Fc. These results indicate that cholesterol inhibits EphA2 assembly. Phosphorylation studies in different cell lines revealed that low cholesterol increased phospho-serine levels, the signature of oncogenic signaling. Investigation of the mechanism that cholesterol uses to inhibit the assembly and activity of EphA2 indicate an in-trans effect, where EphA2 is phosphorylated by protein kinase A downstream of beta-adrenergic receptor activity, which cholesterol also inhibits. Our study not only provides new mechanistic insights on EphA2 oncogenic function, but also suggests that cholesterol acts as a molecular safeguard mechanism that prevents uncontrolled self-assembly and activation of EphA2.

A novel approach to pH-Responsive targeted cancer Therapy: Inhibition of FaDu cancer cell proliferation with a pH low insertion Peptide-Conjugated DGAT1 inhibitor

Targeting enzymes involved in lipid metabolism is increasingly recognized as a promising anticancer strategy. Efficient inhibition of diacylglycerol O-transferase 1 (DGAT1) can block fatty acid (FA) storage. This, in turn, triggers an increase in free polyunsaturated FA concentration, leading to peroxidation and ferroptosis. In this study, we report the development of a pH-sensitive peptide (pHLIP)-drug conjugate designed to selectively deliver DGAT1 inhibitors to cancer cells nested within the acidic microenvironment of tumors. We utilized two previously established pHLIP sequences for coupling with drugs. The study of DGAT1 conjugates in large unilamellar vesicles (LUVs) of different compositions did not reveal enhanced pH-dependent insertion compared to POPC LUVs. However, using in vitro 3D tumor spheroids, significant antiproliferative effects were observed upon exposure to pHLIP-T863 (DGAT1 inhibitor) conjugates, surpassing the inhibitory activity of T863 alone. In conclusion, our study provides the first evidence that pHLIP-based conjugates with DGAT1 inhibitors have the potential to specifically target the acidic compartment of tumors. Moreover, it sheds light on the limitations of LUV models in capturing the pH-dependency of such conjugates.

Aiming the magic bullet: targeted delivery of imaging and therapeutic agents to solid tumors by pHLIP peptides

The family of pH (Low) Insertion Peptides (pHLIP) comprises a tumor-agnostic technology that uses the low pH (or high acidity) at the surfaces of cells within the tumor microenvironment (TME) as a targeted biomarker. pHLIPs can be used for extracellular and intracellular delivery of a variety of imaging and therapeutic payloads. Unlike therapeutic delivery targeted to specific receptors on the surfaces of particular cells, pHLIP targets cancer, stromal and some immune cells all at once. Since the TME exhibits complex cellular crosstalk interactions, simultaneous targeting and delivery to different cell types leads to a significant synergistic effect for many agents. pHLIPs can also be positioned on the surfaces of various nanoparticles (NPs) for the targeted intracellular delivery of encapsulated payloads. The pHLIP technology is currently advancing in pre-clinical and clinical applications for tumor imaging and treatment.

D-to-E and T19V Variants of the pH-Low Insertion Peptide and Their Doxorubicin Conjugates Interact with Membrane at Higher pH Ranges Than WT

To improve targeted cargo delivery to cancer cells, pH-Low Insertion Peptide (pHLIP) variants were developed to interact with the membrane at pH values higher than those of the WT. The Asp-to-Glu variants aim to increase side chain pKa without disturbing the sequence of protonations that underpin membrane insertion. The Thr19 variants represent efforts to perturb the critical Pro20 residue. To study the effect of cargo on pHLIP insertion, doxorubicin (Dox), a fluorescent antineoplastic drug, was conjugated to selected variants near the inserting C-terminus. Variants and conjugates were characterized on a POPC membrane using Trp and Dox fluorescence methods to define the entire pH range of insertion (pHinitial-pHfinal). Compared to WT with a pHi-pHf range of 6.7-5.6, D25E-D31E-D33E, D14E-D25E-D31E-D33E, and T19V-D25E variants demonstrated higher pHi-pHf ranges of 7.3-6.1, 7.3-6.3, and 8.2-5.4, respectively. The addition of Dox expanded the pHi-pHf range, mainly by shifting pHi to higher pH values (e.g., WT pHLIP-Dox has a pHi-pHf range of 7.7-5.2). Despite the low Hill coefficient observed for the conjugates, D14E-D25E-D31E-D33E pHLIP-Dox completed insertion by a pHf of 5.7. However, the Dox cargo remained in the hydrophobic membrane interior after pHLIP insertion, which may impede drug release. Finally, a logistic function can describe pHLIP insertion as a peripheral-to-TM (start-to-finish) two-state transition; wherever possible, we discuss data deviating from such sigmoidal fitting in support of the idea that pH-specific intermediate states distinct from the initial peripheral state and the final TM state exist at intervening pH values.

Promoting the activity of a receptor tyrosine phosphatase with a novel pH-responsive transmembrane agonist inhibits cancer-associated phenotypes

Cell signaling by receptor protein tyrosine kinases (RTKs) is tightly controlled by the counterbalancing actions of receptor protein tyrosine phosphatases (RPTPs). Due to their role in attenuating the signal-initiating potency of RTKs, RPTPs have long been viewed as therapeutic targets. However, the development of activators of RPTPs has remained limited. We previously reported that the homodimerization of a representative member of the RPTP family (protein tyrosine phosphatase receptor J or PTPRJ) is regulated by specific transmembrane (TM) residues. Disrupting this interaction by single point mutations promotes PTPRJ access to its RTK substrates (e.g., EGFR and FLT3), reduces RTK's phosphorylation and downstream signaling, and ultimately antagonizes RTK-driven cell phenotypes. Here, we designed and tested a series of first-in-class pH-responsive TM peptide agonists of PTPRJ that are soluble in aqueous solution but insert as a helical TM domain in lipid membranes when the pH is lowered to match that of the acidic microenvironment of tumors. The most promising peptide reduced EGFR's phosphorylation and inhibited cancer cell EGFR-driven migration and proliferation, similar to the PTPRJ's TM point mutations. Developing tumor-selective and TM-targeting peptide binders of critical RPTPs could afford a potentially transformative approach to studying RPTP's selectivity mechanism without requiring less specific inhibitors and represent a novel class of therapeutics against RTK-driven cancers.

Increasing the Realism of in Silico pHLIP Peptide Models with a Novel pH Gradient CpHMD Method

The pH-low insertion peptides (pHLIP) are pH-dependent membrane inserting peptides, whose function depends on the cell microenvironment acidity. Several peptide variants have been designed to improve upon the wt-sequence, particularly the state transition kinetics and the selectivity for tumor pH. The variant 3 (Var3) peptide is a 27 residue long peptide, with a key titrating residue (Asp-13) that, despite showing a modest performance in liposomes (pK^ins ∼ 5.0), excelled in tumor cell experiments. To help rationalize these results, we focused on the pH gradient in the cell membrane, which is one of the crucial properties that are not present in liposomes. We extended our CpHMD-L method and its pH replica-exchange (pHRE) implementation to include a pH gradient and mimic the pHLIP-membrane microenvironment in a cell where the internal pH is fixed (pH 7.2) and the external pH is allowed to change. We showed that, by properly modeling the pH-gradient, we can correctly predict the experimentally observed loss and gain of performance in tumor cells experiments by the wt and Var3 sequences, respectively. In sum, the pH gradient implementation allowed for more accurate and realistic pK_a estimations and was a pivotal step in bridging the in silico data and the in vivo cell experiments.

Multiplexed Imaging Reveals the Spatial Relationship of the Extracellular Acidity-Targeting pHLIP with Necrosis, Hypoxia, and the Integrin-Targeting cRGD Peptide

pH (low) insertion peptides (pHLIPs) have been developed for cancer imaging and therapy targeting the acidic extracellular microenvironment. However, the characteristics of intratumoral distribution (ITD) of pHLIPs are not yet fully understood. This study aimed to reveal the details of the ITD of pHLIPs and their spatial relationship with other tumor features of concern. The fluorescent dye-labeled pHLIPs were intravenously administered to subcutaneous xenograft mouse models of U87MG and IGR-OV1 expressing αVβ3 integrins (using large necrotic tumors). The αVβ3 integrin-targeting Cy5.5-RAFT-c(-RGDfK-)4 was used as a reference. In vivo and ex vivo fluorescence imaging, whole-tumor section imaging, fluorescence microscopy, and multiplexed fluorescence colocalization analysis were performed. The ITD of fluorescent dye-labeled pHLIPs was heterogeneous, having a high degree of colocalization with necrosis. A direct one-to-one comparison of highly magnified images revealed the cellular localization of pHLIP in pyknotic, karyorrhexis, and karyolytic necrotic cells. pHLIP and hypoxia were spatially contiguous but not overlapping cellularly. The hypoxic region was found between the ITDs of pHLIP and the cRGD peptide and the Ki-67 proliferative activity remained detectable in the pHLIP-accumulated regions. The results provide a better understanding of the characteristics of ITD of pHLIPs, leading to new insights into the theranostic applications of pHLIPs.

Ca2+ -dependent interactions between lipids and the tumor-targeting peptide pHLIP

Cancerous tissues undergo extensive changes to their cellular environments that differentiate them from healthy tissues. These changes include changes in extracellular pH and Ca2+ concentrations, and the exposure of phosphatidylserine (PS) to the extracellular environment, which can modulate the interaction of peptides and proteins with the plasma membrane. Deciphering the molecular mechanisms of such interactions is critical for advancing the knowledge-based design of cancer-targeting molecular tools, such as pH-low insertion peptide (pHLIP). Here, we explore the effects of PS, Ca2+ , and peptide protonation state on the interactions of pHLIP with lipid membranes. Cellular studies demonstrate that exposed PS on the plasma membrane promotes pHLIP targeting. The magnitude of this effect is dependent on extracellular Ca2+ concentration, indicating that divalent cations play an important role in pHLIP targeting in vivo. The targeting mechanism is further explored with a combination of fluorescence and circular dichroism experiments in model membranes and microsecond-timescale all-atom molecular dynamics simulations. Our results demonstrate that Ca2+ is engaged in coupling peptide-lipid interactions in the unprotonated transmembrane conformation of pHLIP. The simulations reveal that while the pH-induced insertion leads to a strong depletion of PS around pHLIP, the Ca2+ -induced insertion has the opposite effect. Thus, extracellular levels of Ca2+ are crucial to linking cellular changes in membrane lipid composition with the selective targeting and insertion of pHLIP. The characterized Ca2+ -dependent coupling between pHLIP sidechains and PS provides atomistic insights into the general mechanism for lipid-coupled regulation of protein-membrane insertion by divalent cations.

Targeting cancer cells in acidosis with conjugates between the carnitine palmitoyltransferase 1 inhibitor etomoxir and pH (low) Insertion Peptides

Targeting enzymes involved in tumor metabolism is a promising way to tackle cancer progression. The inhibition of carnitine palmitoyltransferase 1 (CPT1) by etomoxir (Eto) efficiently slows down the growth of various cancers. Unfortunately, the clinical use of this drug was abandoned because of hepatotoxic effects. We report the development of pH-sensitive peptide (pHLIP)-drug conjugate to deliver Eto selectively to cancer cells exposed to acidic microenvironmental conditions. A newly designed sequence for the pHLIP peptide, named pHLIPd, was compared with a previously published reference pHLIP peptide, named pHLIPr. We showed that the conjugate between pHLIPd and Eto has a better pH-dependent insertion and structuration than the pHLIPr-based conjugate inside POPC vesicles. We observed antiproliferative effects when applied on acid-adapted cancer cells, reaching a larger inhibitory activity than Eto alone. In conclusion, this study brings the first evidence that pHLIP-based conjugates with a CPT1 inhibitor has the potential to specifically target the tumor acidic compartment and exert anticancer effects while sparing healthy tissues.

pH-induced insertion of pHLIP into a lipid bilayer: In-situ SEIRAS characterization of a folding intermediate at neutral pH

The pH low insertion peptide (pHLIP) is a pH-sensitive cell penetrating peptide that transforms from an unstructured coil on the membrane surface at pH > 7, to a transmembrane (TM) α-helix at pH < 5. By exploiting this unique property, pHLIP attracts interest as a potential tool for drug delivery and visualisation of acidic tissues produced by various maladies such as cancer, inflammation, hypoxia etc. Even though the structures of initial and end states of pHLIP insertion have been widely accepted, the intermediate structures in between these two states are less clear. Here, we have applied in situ Surface-Enhanced Infrared Absorption spectroscopy to examine the pH-induced insertion and folding processes of pHLIP into a solid-supported lipid bilayer. We show that formation of partially helical structure already takes place at pH only slightly below 7.0, but with the helical axis parallel to the membrane surface. The peptide starts to reorientate its helix from horizontal to vertical direction, accompanied by the insertion into the TM region at pH < 6.2. Further insertion into the TM region of the peptide results in an increase of inherent α-helical structure and complete secondary structure formation at pH 5.3. Analysis of the changes of the carboxylate vibrational bands upon pH titration shows two distinctive groups of aspartates and glutamates with pK_a values of 4.5 and 6.3, respectively. Comparison to the amide bands of the peptide backbone suggests that the latter Asp/Glu groups are directly involved in the conformational changes of pHLIP in the respective intermediate states.

Molecular packing of lipid membranes and action mechanisms of membrane-active peptides

Biomembranes are involved in diverse cellular activities. How membranes and proteins interact in the activities might hinge on the former's physical characteristics, which in turn are influenced by packing of lipid molecules. Yet, the validity of this understanding and its mechanism are unclear. By varying chain saturation of membranes, we explored correlations between lipid packing and peptide-mediated membrane disruption for the antimicrobial peptide, melittin, and amyloidogenic peptide, β-amyloid (1-42). Remarkably, reducing molecular packing flexibility enhanced the membrane disruption, possibly due to a shift from membrane perforation to micellization. A theoretical analysis suggested the energetic basis of this shift. This mechanistically shows that a peptide's mechanism might be dictated not only by its intrinsic properties but also by physical characteristics of membranes.

The interaction of steroids with phospholipid bilayers and membranes

Steroids are critical for various physiological processes and used to treat inflammatory conditions. Steroids act by two distinct pathways. The genomic pathway is initiated by the steroid binding to nuclear receptors while the non-genomic pathway involves plasma membrane receptors. It has been proposed that steroids might also act in a more indirect mechanism by altering biophysical properties of membranes. Yet, little is known about the effect of steroids on membranes, and steroid-membrane interactions are complex and challenging to characterise. The focus of this review is to outline what is currently known about the interactions of steroids with phospholipid bilayers and illustrate the complexity of these systems using cortisone and progesterone as the main examples. The combined findings from current work demonstrate that the hydrophobicity and planarity of the steroid core does not provide a consensus for steroid-membrane interactions. Even small differences in the substituents on the steroid core can result in significant changes in steroid-membrane interactions. Furthermore, steroid-induced changes in phospholipid bilayer properties are often dependent on steroid concentration and lipid composition. This complexity means that currently there is insufficient information to establish a reliable structure-activity relationship to describe the effect of steroids on membrane properties. Future work should address the challenge of connecting the findings from studying the effect of steroids on phospholipid bilayers to cell membranes. Insights from steroid-membrane interactions will benefit our understanding of normal physiology and assist drug development.

Roles of key residues and lipid dynamics reveal pHLIP-membrane interactions at intermediate pH

The pH-low insertion peptide (pHLIP) and its analogs sense the microenvironmental pH variations in tumorous cells and serve as useful anticancer drug deliveries. The pHLIP binds peripherally to membranes and adopts random coil conformation at the physiological pH. The peptide switches from random coil to α-helical conformation and inserts unidirectionally into membrane bilayers when pH drops below a critical transition value that has been routinely determined by the Trp fluorescence spectroscopy. Recent high-resolution studies using solid-state NMR spectroscopy revealed the presence of thermodynamically stable intermediate states of membrane-associated pHLIP around the fluorescence-based transition pH-value. However, the molecular structural features and their mechanistic roles of these intermediate states in the pH-driven membrane insertion process of pHLIP remain largely unknown. This work utilizes solid-state NMR spectroscopy to explore 1) the mechanistic roles of key proline and arginine residues within the pHLIP sequence at intermediate pH-values, and 2) the changes in lipid dynamics at intermediate pH-values in multiple types of model bilayers with anionic phospholipid and/or cholesterol. Our results demonstrate several molecular structural and dynamics changes at around the transition pH-values, including the isomerization of proline-threonine backbone configuration, breaking of arginine-aspartic acid salt bridge and the formation of arginine-lipid interactions, and a universal decreasing of dynamics in lipid headgroups and alkyl chains. Overall, the outcomes provide important insights on the molecular interactions between pHLIP and membrane bilayers at intermediate pH-values and, therefore, prompt the understanding of pH-driven membrane insertion process of this anticancer drug-delivering peptide.

Intramolecular interactions play key role in stabilization of pHLIP at acidic conditions

The pH-Low Insertion Peptide (pHLIP) is a membrane-active peptide that spontaneously folds into a transmembrane α-helix upon acidification. This activity enables pHLIP to potentially act as a vector for drugs related to diseases characterized by acidosis such as cancer or heart ischemia. Presently, due to aggregation-based effects, formulations of pHLIP are only viable at near-μM concentrations. In addition, since most of pHLIP's measurable qualities involve a membrane, probing the details of pHLIP in the interstitial region is difficult. In attempts to shed light on these issues, we performed constant pH molecular dynamics simulations on pHLIP as well as P20G, a variant with increased helicity, in solution at 0 and 150 mM NaCl over a broad range of pHs. In general, the addition of ions reduced the effective pK_a of the acidic residues in pHLIP. P20G exhibits a higher helicity than pHLIP in general and is more compact than pHLIP at pH values under 4. In terms of charge effects, sodium cations localized predominantly to the C-terminus of the peptide with a high density of acidic residues. Additionally, the salt bridge between R11 and D14 is by far the most favored and particularly so with pHLIP at 150 mM NaCl. We expect that this approach will be a valuable tool to screen variants of pHLIP for favorable properties in solution, an aspect of pHLIP design that to this point has largely been neglected.

Single-molecule fluorescence vistas of how lipids regulate membrane proteins

The study of membrane proteins is undergoing a golden era, and we are gaining unprecedented knowledge on how this key group of proteins works. However, we still have only a basic understanding of how the chemical composition and the physical properties of lipid bilayers control the activity of membrane proteins. Single-molecule (SM) fluorescence methods can resolve sample heterogeneity, allowing to discriminate between the different molecular populations that biological systems often adopt. This short review highlights relevant examples of how SM fluorescence methodologies can illuminate the different ways in which lipids regulate the activity of membrane proteins. These studies are not limited to lipid molecules acting as ligands, but also consider how the physical properties of the bilayer can be determining factors on how membrane proteins function.

pH-triggered pore-forming peptides with strong composition-dependent membrane selectivity

Peptides that self-assemble into nanometer-sized pores in lipid bilayers could have utility in a variety of biotechnological and clinical applications if we can understand their physical chemical properties and learn to control their membrane selectivity. To empower such control, we have used synthetic molecular evolution to identify the pH-dependent delivery peptides, a family of peptides that assemble into macromolecule-sized pores in membranes at low peptide concentration but only at pH < ∼6. Further advancements will also require better selectivity for specific membranes. Here, we determine the effect of anionic headgroups and bilayer thickness on the mechanism of action of the pH-dependent delivery peptides by measuring binding, secondary structure, and macromolecular poration. The peptide pHD15 partitions and folds equally well into zwitterionic and anionic membranes but is less potent at pore formation in phosphatidylserine-containing membranes. The peptide also binds and folds similarly in membranes of various thicknesses, but its ability to release macromolecules changes dramatically. It causes potent macromolecular poration in vesicles made from phosphatidylcholine with 14 carbon acyl chains, but macromolecular poration decreases sharply with increasing bilayer thickness and does not occur at any peptide concentration in fluid bilayers made from phosphatidylcholine lipids with 20-carbon acyl chains. The effects of headgroup and bilayer thickness on macromolecular poration cannot be accounted for by the amount of peptide bound but instead reflect an inherent selectivity of the peptide for inserting into the membrane-spanning pore state. Molecular dynamics simulations suggest that the effect of thickness is due to hydrophobic match/mismatch between the membrane-spanning peptide and the bilayer hydrocarbon. This remarkable degree of selectivity based on headgroup and especially bilayer thickness is unusual and suggests ways that pore-forming peptides with exquisite selectivity for specific membranes can be designed or evolved.

Examination of pH dependency and orientation differences of membrane spanning alpha helices carrying a single or pair of buried histidine residues

We have employed the peptide framework of GWALP23 (acetyl-GGALWLALALALALALALWLAGA-amide) to examine the orientation, dynamics and pH dependence of peptides having buried single or pairs of histidine residues. When residue L8 is substituted to yield GWALP23-H⁸, acetyl-GGALWLAH⁸ALALALALALWLAGA-amide, the deuterium NMR spectra of ²H-labeled core alanine residues reveal a helix that occupies a single transmembrane orientation in DLPC, or in DMPC at low pH, yet shows multiple states at higher pH or in bilayers of DOPC. Moreover, a single histidine at position 8 or 16 in the GWALP23 framework is sensitive to pH. Titration points are observed near pH 3.5 for the deprotonation of H8 in lipid bilayers of DLPC or DMPC, and for H16 in DOPC. When residues L8 and L16 both are substituted to yield GWALP23-H^8,16, the ²H NMR spectra show, interestingly, no titration dependence from pH 2-8, yet bilayer thickness-dependent orientation differences. The helix with H8 and H16 is found to adopt a transmembrane orientation in thin bilayers of DLPC, a combination of transmembrane and surface orientations in DMPC, and then a complete transition to a surface bound orientation in the thicker DPoPC and DOPC lipid bilayers. In the surface orientations, alanine A7 no longer fits within the core helix. These results along with previous studies with different locations of histidine residues suggest that lipid hydrophobic thickness is a first determinant and pH a second determinant for the helical orientation, along with possible side-chain snorkeling, when the His residues are incorporated into the hydrophobic region of a lipid membrane-associated helix.

Effect of DMSO on the Mechanical and Structural Properties of Model and Biological Membranes

Dimethyl sulfoxide (DMSO) is widely used in a number of biological and biotechnological applications, mainly because of its effects on the cell plasma membrane, but the molecular origins of this action are yet to be fully clarified. In this work, we used two- and three-component synthetic membranes (liposomes) and the plasma membrane of human erythrocytes to investigate the effect of DMSO when added to the membrane-solvating environment. Fourier transform infrared spectroscopy and thermal fluctuation spectroscopy revealed significant differences in the response of the two types of liposome systems to DMSO in terms of the bilayer thermotropic behavior, available free volume of the bilayer, its excess surface area, and bending elasticity. DMSO also alters the mechanical properties of the erythrocyte membrane in a concentration-dependent manner and is capable of increasing membrane permeability to ATP at even relatively low concentrations (3% v/v and above). Taken in its entirety, these results show that DMSO is likely to have a differential effect on heterogeneous biological membranes, depending on their local composition and structure, and could affect membrane-hosted biological functions.

Insertion of Bacteriorhodopsin Helix C Variants into Biological Membranes

A peptide corresponding to bacteriorhodopsin (bR) helix C, later named pHLIP, inserts across lipid bilayers as a monomeric α-helix at acidic pH, but is an unstructured surface-bound monomer at neutral pH. As a result of such pH-responsiveness, pHLIP targets acidic tumors and has been used as a vehicle for imaging and drug-delivery cargoes. To gain insights about the insertion of bR helix C into biological membranes, we replaced two key aspartic residues that control the topological transition from the aqueous phase into a lipid bilayer. Here, we used an in vitro transcription-translation system to study the translocon-mediated insertion of helix C-derived segments into rough microsomes. Our data provide the first quantitative biological understanding of this effect. Interestingly, replacing the aspartic residues by glutamic residues does not significantly alters the insertion propensity, while replacement by alanines promotes a transmembrane orientation. These results are consistent with mutational data obtained in synthetic liposomes by manipulating pH conditions. Our findings support the notion that the translocon facilitates topogenesis under physiological pH conditions.

Tumor-Targeted, Cytoplasmic Delivery of Large, Polar Molecules Using a pH-Low Insertion Peptide

Tumor-targeted drug delivery systems offer not only the advantage of an enhanced therapeutic index, but also the possibility of overcoming the limitations that have largely restricted drug design to small, hydrophobic, "drug-like" molecules. Here, we explore the ability of a tumor-targeted delivery system centered on the use of a pH-low insertion peptide (pHLIP) to directly deliver moderately polar, multi-kDa molecules into tumor cells. A pHLIP is a short, pH-responsive peptide capable of inserting across a cell membrane to form a transmembrane helix at acidic pH. pHLIPs target the acidic tumor microenvironment with high specificity, and a drug attached to the inserting end of a pHLIP can be translocated across the cell membrane during the insertion process. We investigate the ability of wildtype pHLIP to deliver peptide nucleic acid (PNA) cargoes of varying sizes across lipid membranes. We find that pHLIP effectively delivers PNAs up to ∼7 kDa into cells in a pH-dependent manner. In addition, pHLIP retains its tumor-targeting capabilities when linked to cargoes of this size, although the amount delivered is reduced for PNA cargoes greater than ∼6 kDa. As drug-like molecules are traditionally restricted to sizes of ∼500 Da, this constitutes an order-of-magnitude expansion in the size range of deliverable drug candidates.

Differential sensitivity of pHLIP to ester and ether lipids

pH (low) insertion peptide (pHLIP) is a polypeptide from the third transmembrane helix of bacteriorhodopsin. The pH-dependent membrane insertion of pHLIP has been conveniently exploited for translocation of cargo molecules and as a novel imaging agent in cancer biology due to low extracellular pH in cancer tissues. Although the application of pHLIP for imaging tumor and targeted drug delivery is well studied, literature on pHLIP-membrane interaction is relatively less studied. Keeping this in mind, we explored the differential interaction of pHLIP with ester and ether lipid membranes utilizing fluorescence and CD spectroscopy. We report, for the first time, higher binding affinity of pHLIP toward ether lipid relative to ester lipid membranes. There results gain relevance since Halobacterium halobium (source of bacteriorhodopsin) is enriched with ether lipids. In addition, we monitored the difference in microenvironment around pHLIP tryptophans utilizing red edge excitation shift and observed increased motional restriction of water molecules in the interfacial region in ether lipid membranes. These changes were accompanied with increase in helicity of pHLIP in ether lipid relative to ester lipid membranes. Our results assume further relevance since ether lipids are upregulated in cancer cells and have emerged as potential biomarkers of various diseases including cancer.

The conical shape of DIM lipids promotes Mycobacterium tuberculosis infection of macrophages

Phthiocerol dimycocerosate (DIM) is a major virulence factor of the pathogen Mycobacterium tuberculosis (Mtb). While this lipid promotes the entry of Mtb into macrophages, which occurs via phagocytosis, its molecular mechanism of action is unknown. Here, we combined biophysical, cell biology, and modeling approaches to reveal the molecular mechanism of DIM action on macrophage membranes leading to the first step of Mtb infection. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry showed that DIM molecules are transferred from the Mtb envelope to macrophage membranes during infection. Multiscale molecular modeling and ³¹P-NMR experiments revealed that DIM adopts a conical shape in membranes and aggregates in the stalks formed between 2 opposing lipid bilayers. Infection of macrophages pretreated with lipids of various shapes uncovered a general role for conical lipids in promoting phagocytosis. Taken together, these results reveal how the molecular shape of a mycobacterial lipid can modulate the biological response of macrophages.

Hydrophobic matching of HIV-1 Vpu transmembrane helix-helix interactions is optimized for subcellular location

The HIV-1 accessory protein Vpu mediates the downregulation of several host cell proteins, an activity that is critical for viral replication in vivo. As the first step in directing cell-surface proteins to internal cellular compartments, and in many cases degradation, Vpu binds a subset of its target proteins through their transmembrane domains. Each of the known targets of Vpu are synthesized in the ER, and must traverse the different membrane environments found along the secretory pathway, thus it is important to consider how membrane composition might influence the interactions between Vpu and its targets. We have used Förster resonance energy transfer (FRET) to measure the oligomerization of Vpu with the transmembrane domains of target proteins in model membranes of varying lipid composition. Our data show that both lipid bilayer thickness and acyl chain order can significantly influence monomer-oligomer equilibria within the Vpu-target system. Changes in oligomerization levels were found to be non-specific with no single Vpu-target interaction being favored under any condition. Our analysis of the influence of the membrane environment on the strength of helix-helix interactions between Vpu and its targets in vitro suggests that the strength of Vpu-target interactions in vivo will be partially dependent on the membrane environment found in specific membrane compartments.

Ions Modulate Key Interactions between pHLIP and Lipid Membranes

The pH-low insertion peptide (pHLIP) is used for targeted delivery of drug cargoes to acidic tissues such as tumors. The extracellular acidosis found in solid tumors triggers pHLIP to transition from a membrane-adsorbed state to fold into a transmembrane α-helix. Different factors influence the acidity required for pHLIP to insert into lipid membranes. One of them is the lipid headgroup composition, which defines the electrostatic profile of the membrane. However, the molecular interactions that drive the adsorption of pHLIP to the bilayer surface are poorly understood. In this study, we combine biophysical experiments and all-atom molecular dynamics simulations to understand the role played by electrostatics in the interaction between pHLIP and a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer. We observed that the solution ionic strength affects the structure of pHLIP at the membrane surface as well as the acidity needed for different steps in the membrane insertion process. In particular, our simulations revealed that an increase in ionic strength affected both pHLIP and the bilayer; the coordination of sodium ions with the C-terminus of pHLIP led to localized changes in helicity, whereas the coordination of sodium ions with the phosphate moiety of the phosphocholine headgroups had a condensing effect on our model bilayer. These results are relevant to our understanding of environmental influences on the ability of pHLIP to adsorb to the cell membrane and are useful in our fundamental understanding of the absorption of pH-responsive peptides and cell-penetrating peptides.

Cardiolipin-Dependent Properties of Model Mitochondrial Membranes from Molecular Simulations

Cardiolipin is an anionic lipid found in the mitochondrial membranes of eukaryotes ranging from unicellular microorganisms to metazoans. This unique lipid contributes to various mitochondrial functions, including metabolism, mitochondrial membrane fusion and/or fission dynamics, and apoptosis. However, differences in cardiolipin content between the two mitochondrial membranes, as well as dynamic fluctuations in cardiolipin content in response to stimuli and cellular signaling events, raise questions about how cardiolipin concentration affects mitochondrial membrane structure and dynamics. Although cardiolipin’s structural and dynamic roles have been extensively studied in binary mixtures with other phospholipids, the biophysical properties of cardiolipin in higher number lipid mixtures are still not well resolved. Here, we used molecular dynamics simulations to investigate the cardiolipin-dependent properties of ternary lipid bilayer systems that mimic the major components of mitochondrial membranes. We found that changes to cardiolipin concentration only resulted in minor changes to bilayer structural features but that the lipid diffusion was significantly affected by those alterations. We also found that cardiolipin position along the bilayer surfaces correlated to negative curvature deflections, consistent with the induction of negative curvature stress in the membrane monolayers. This work contributes to a foundational understanding of the role of cardiolipin in altering the properties in ternary lipid mixtures composed of the major mitochondrial phospholipids, providing much-needed insights to help understand how cardiolipin concentration modulates the biophysical properties of mitochondrial membranes.

Bilayer Thickness and Curvature Influence Binding and Insertion of a pHLIP Peptide

The physical properties of lipid bilayers, such as curvature and fluidity, can affect the interactions of polypeptides with membranes, influencing biological events. Additionally, given the growing interest in peptide-based therapeutics, understanding the influence of membrane properties on membrane-associated peptides has potential utility. pH low insertion peptides (pHLIPs) are a family of water-soluble peptides that can insert across cell membranes in a pH-dependent manner, enabling the use of pH to follow peptide-lipid interactions. Here we study pHLIP interactions with liposomes varying in size and composition, to determine the influence of several key membrane physical properties. We find that pHLIP binding to bilayer surfaces at neutral pH is governed by the ease of access to the membrane's hydrophobic core, which can be facilitated by membrane curvature, thickness, and the cholesterol content of the membrane. After surface binding, if the pH is lowered, the kinetics of pHLIP folding to form a helix and subsequent insertion across the membrane depends on the fluidity and energetic dynamics of the membrane. We showed that pHLIP is capable of forming a helix across lipid bilayers of different thicknesses at low pH. However, the kinetics of the slow phase of insertion corresponding to the translocation of C-terminal end of the peptide across lipid bilayer, vary approximately twofold, and correlate with bilayer thickness and fluidity. Although these influences are not large, local curvature variations in membranes of different fluidity could selectively influence surface binding in mixed cell populations.

Reactions at Biomembrane Interfaces

Membranes surrounding the biological cell and its internal compartments host proteins that catalyze chemical reactions essential for the functioning of the cell. Rather than being a passive structural matrix that holds membrane-embedded proteins in place, the membrane can largely shape the conformational energy landscape of membrane proteins and impact the energetics of their chemical reaction. Here, we highlight the challenges in understanding how lipids impact the conformational energy landscape of macromolecular membrane complexes whose functioning involves chemical reactions including proton transfer. We review here advances in our understanding of how chemical reactions occur at membrane interfaces gleaned with both theoretical and experimental advances using simple protein systems as guides. Our perspective is that of bridging experiments with theory to understand general physicochemical principles of membrane reactions, with a long term goal of furthering our understanding of the role of the lipids on the functioning of complex macromolecular assemblies at the membrane interface.

Phosphatidylserine Asymmetry Promotes the Membrane Insertion of a Transmembrane Helix

The plasma membrane (PM) contains an asymmetric distribution of lipids between the inner and outer bilayer leaflets. A lipid of special interest in eukaryotic membranes is the negatively charged phosphatidylserine (PS). In healthy cells, PS is actively sequestered to the inner leaflet of the PM, but PS redistributes to the outer leaflet when the cell is damaged or at the onset of apoptosis. However, the influence of PS asymmetry on membrane protein structure and folding are poorly understood. The pH low insertion peptide (pHLIP) adsorbs to the membrane surface at a neutral pH, but it inserts into the membrane at an acidic pH. We have previously observed that in symmetric vesicles, PS affects the membrane insertion of pHLIP by lowering the pH midpoint of insertion. Here, we studied the effect of PS asymmetry on the membrane interaction of pHLIP. We developed a modified protocol to create asymmetric vesicles containing PS and employed Annexin V labeled with an Alexa Fluor 568 fluorophore as a new probe to quantify PS asymmetry. We observed that the membrane insertion of pHLIP was promoted by the asymmetric distribution of negatively charged PS, which causes a surface charge difference between bilayer leaflets. Our results indicate that lipid asymmetry can modulate the formation of an α-helix on the membrane. A corollary is that model studies using symmetric bilayers to mimic the PM may fail to capture important aspects of protein-membrane interactions.

Co-administration of 20(S)-protopanaxatriol (g-PPT) and EGFR-TKI overcomes EGFR-TKI resistance by decreasing SCD1 induced lipid accumulation in non-small cell lung cancer

BACKGROUND

Non-small cell lung cancer (NSCLC) patients with sensitive epidermal growth factor receptor (EGFR) mutations are successfully treated with EGFR tyrosine kinase inhibitors (EGFR-TKIs); however, resistance to treatment inevitably occurs. Given lipid metabolic reprogramming is widely known as a hallmark of cancer and intimately linked with EGFR-stimulated cancer growth. Activation of EGFR signal pathway increased monounsaturated fatty acids (MUFA) and lipid metabolism key enzyme Stearoyl-CoA Desaturase 1 (SCD1) expression. However the correlation between EGFR-TKI resistance and lipid metabolism remains to be determined.

METHODS

In this study the differences in lipid synthesis between paired TKI-sensitive and TKI-resistant patient tissues and NSCLC cell lines were explored. Oleic acid (OA, a kind of MUFA, the SCD1 enzymatic product) was used to simulate a high lipid metabolic environment and detected the affection on the cytotoxic effect of TKIs (Gefitinib and osimertinib) in cell lines with EGFR-activating mutations. (20S)-Protopanaxatriol (g-PPT), an aglycone of ginsenosides, has been reported to be an effective lipid metabolism inhibitor, was used to inhibit lipid metabolism. Additionally, synergism in cytotoxic effects and signal pathway activation were evaluated using CCK-8 assays, Western blotting, flow cytometry, Edu assays, plate clone formation assays and immunofluorescence. Furthermore, two xenograft mouse models were used to verify the in vitro results.

RESULTS

Gefitinib-resistant cells have higher lipid droplet content and SCD1 expression than Gefitinib-sensitive cells in both NSCLC cell lines and patient tissues. Additionally oleic acid (OA, a kind of MUFA, the SCD1 enzymatic product) abrogates the cytotoxic effect of both Gefitinib and osimertinib in cell lines with EGFR-activating mutations. As a reported effective lipid metabolism inhibitor, g-PPT significantly inhibited the expression of SCD1 in lung adenocarcinoma cells, and then down-regulated the content of intracellular lipid droplets. Combined treatment with Gefitinib and g-PPT reverses the resistance to Gefitinib and inhibits the activation of p-EGFR and the downstream signaling pathways.

CONCLUSIONS

Our findings uncover a link between lipid metabolic reprogramming and EGFR-TKI resistance, confirmed that combination target both EGFR and abnormal lipid metabolism maybe a promising therapy for EGFR-TKI resistance and highlighting the possibility of monitoring lipid accumulation in tumors for predicting drug resistance.

Diblock copolymers enhance folding of a mechanosensitive membrane protein during cell-free expression

Membrane protein folding is a critical step that underlies proper cellular function as well as the design of technologies like vesicle-based biosensors and artificial cells. Membrane composition is known to play a role in membrane protein folding; however, the specific mechanical properties of membranes that govern protein folding remain unclear. Using a highly elastic nonnatural amphiphile, we highlight the importance of a membrane mechanical property, membrane elasticity, on the spontaneous insertion and folding of a model α-helical membrane protein. Through this study, we gain a deeper understanding of cellular membrane protein folding and offer a potential approach to improve the production of membrane proteins through optimizing the mechanical properties of synthetic scaffolds present in cell-free reactions.

The expression and integration of membrane proteins into vesicle membranes is a critical step in the design of cell-mimetic biosensors, bioreactors, and artificial cells. While membrane proteins have been integrated into a variety of nonnatural membranes, the effects of the chemical and physical properties of these vesicle membranes on protein behavior remain largely unknown. Nonnatural amphiphiles, such as diblock copolymers, provide an interface that can be synthetically controlled to better investigate this relationship. Here, we focus on the initial step in a membrane protein’s life cycle: expression and folding. We observe improvements in both the folding and overall production of a model mechanosensitive channel protein, the mechanosensitive channel of large conductance, during cell-free reactions when vesicles containing diblock copolymers are present. By systematically tuning the membrane composition of vesicles through incorporation of a poly(ethylene oxide)-b-poly(butadiene) diblock copolymer, we show that membrane protein folding and production can be improved over that observed in traditional lipid vesicles. We then reproduce this effect with an alternate membrane-elasticizing molecule, C₁₂E₈. Our results suggest that global membrane physical properties, specifically available membrane surface area and the membrane area expansion modulus, significantly influence the folding and yield of a membrane protein. Furthermore, our results set the stage for explorations into how nonnatural membrane amphiphiles can be used to both study and enhance the production of biological membrane proteins.

Recent Development of pH-Responsive Polymers for Cancer Nanomedicine

Cancer remains a leading cause of death worldwide with more than 10 million new cases every year. Tumor-targeted nanomedicines have shown substantial improvements of the therapeutic index of anticancer agents, addressing the deficiencies of conventional chemotherapy, and have had a tremendous growth over past several decades. Due to the pathophysiological characteristics that almost all tumor tissues have lower pH in comparison to normal healthy tissues, among various tumor-targeted nanomaterials, pH-responsive polymeric materials have been one of the most prevalent approaches for cancer diagnosis and treatment. In this review, we summarized the types of pH-responsive polymers, describing their chemical structures and pH-response mechanisms; we illustrated the structure-property relationships of pH-responsive polymers and introduced the approaches to regulating their pH-responsive behaviors; we also highlighted the most representative applications of pH-responsive polymers in cancer imaging and therapy. This review article aims to provide general guidelines for the rational design of more effective pH-responsive nanomaterials for cancer diagnosis and treatment.

Constitutive boost of a K+ channel via inherent bilayer tension and a unique tension-dependent modality

Molecular mechanisms underlying channel-membrane interplay have been extensively studied. Cholesterol, as a major component of the cell membrane, participates either in specific binding to channels or via modification of membrane physical features. Here, we examined the action of various sterols (cholesterol, epicholesterol, etc.) on a prototypical potassium channel (KcsA). Single-channel current recordings of the KcsA channel were performed in a water-in-oil droplet bilayer (contact bubble bilayer) with a mixed phospholipid composition (azolectin). Upon membrane perfusion of sterols, the activated gate at acidic pH closed immediately, irrespective of the sterol species. During perfusion, we found that the contacting bubbles changed their shapes, indicating alterations in membrane physical features. Absolute bilayer tension was measured according to the principle of surface chemistry, and inherent bilayer tension was ∼5 mN/m. All tested sterols decreased the tension, and the nonspecific sterol action to the channel was likely mediated by the bilayer tension. Purely mechanical manipulation that reduced bilayer tension also closed the gate, whereas the resting channel at neutral pH never activated upon increased tension. Thus, rather than conventional stretch activation, the channel, once ready to activate by acidic pH, changes the open probability through the action of bilayer tension. This constitutes a channel regulating modality by two successive stimuli. In the contact bubble bilayer, inherent bilayer tension was high, and the channel remained boosted. In the cell membrane, resting tension is low, and it is anticipated that the ready-to-activate channel remains closed until bilayer tension reaches a few millinewton/meter during physiological and pathological cellular activities.

Interactions Between Pseudomonas Immunotoxins and the Plasma Membrane: Implications for CAT-8015 Immunotoxin Therapy

Acute Lymphoblastic Leukemia (ALL) remains the most frequent cause of cancer-related mortality in children and novel therapies are needed for the treatment of relapsed/refractory childhood ALL. One approach is the targeting of ALL blasts with the Pseudomonas immunotoxin CAT-8015. Although CAT-8015 has potent anti-leukemia activity, with a 32% objective response rate in a phase 1 study of childhood ALL, haemolytic-uremic syndrome (HUS) and vascular leak syndrome (VLS), major dose-limiting toxicities, have limited the use of this therapeutic approach in children. Investigations into the pathogenesis of CAT-8015-induced HUS/VLS are hindered by the lack of an adequate model system that replicates clinical manifestations, but damage to vascular endothelial cells (ECs) and blood cells are believed to be major initiating factors in both syndromes. Since there is little evidence that murine models replicate human HUS/VLS, and CAT-8015-induced HUS/VLS predominantly affects children, we developed human models and used novel methodologies to investigate CAT-8015 interactions with red blood cells (RBCs) from pediatric ALL patients and ECs of excised human mesenteric arteries. We provide evidence that CAT-8015 directly interacts with RBCs, mediated by Pseudomonas toxin. We also show correlation between the electrical properties of the RBC membrane and RBC susceptibility to CAT-8015-induced lysis, which may have clinical implication. Finally, we provide evidence that CAT-8015 is directly cytototoxic to ECs of excised human mesenteric arteries. In conclusion, the human models we developed constitutes the first, and very important, step in understanding the origins of HUS/VLS in immunotoxin therapy and will allow further investigations of HUS/VLS pathogenesis.

Electrostatic lock in the transport cycle of the multidrug resistance transporter EmrE

EmrE is a small, homodimeric membrane transporter that exploits the established electrochemical proton gradient across the Escherichia coli inner membrane to export toxic polyaromatic cations, prototypical of the wider small-multidrug resistance transporter family. While prior studies have established many fundamental aspects of the specificity and rate of substrate transport in EmrE, low resolution of available structures has hampered identification of the transport coupling mechanism. Here we present a complete, refined atomic structure of EmrE optimized against available cryo-electron microscopy (cryo-EM) data to delineate the critical interactions by which EmrE regulates its conformation during the transport process. With the model, we conduct molecular dynamics simulations of the transporter in explicit membranes to probe EmrE dynamics under different substrate loading and conformational states, representing different intermediates in the transport cycle. The refined model is stable under extended simulation. The water dynamics in simulation indicate that the hydrogen-bonding networks around a pair of solvent-exposed glutamate residues (E14) depend on the loading state of EmrE. One specific hydrogen bond from a tyrosine (Y60) on one monomer to a glutamate (E14) on the opposite monomer is especially critical, as it locks the protein conformation when the glutamate is deprotonated. The hydrogen bond provided by Y60 lowers the [Formula: see text] of one glutamate relative to the other, suggesting both glutamates should be protonated for the hydrogen bond to break and a substrate-free transition to take place. These findings establish the molecular mechanism for the coupling between proton transfer reactions and protein conformation in this proton-coupled secondary transporter.

Cooperativity Principles in Self-Assembled Nanomedicine

Nanomedicine is a discipline that applies nanoscience and nanotechnology principles to the prevention, diagnosis, and treatment of human diseases. Self-assembly of molecular components is becoming a common strategy in the design and syntheses of nanomaterials for biomedical applications. In both natural and synthetic self-assembled nanostructures, molecular cooperativity is emerging as an important hallmark. In many cases, interplay of many types of noncovalent interactions leads to dynamic nanosystems with emergent properties where the whole is bigger than the sum of the parts. In this review, we provide a comprehensive analysis of the cooperativity principles in multiple self-assembled nanostructures. We discuss the molecular origin and quantitative modeling of cooperative behaviors. In selected systems, we describe the examples on how to leverage molecular cooperativity to design nanomedicine with improved diagnostic precision and therapeutic efficacy in medicine.

Protonation Enhances the Inherent Helix-Forming Propensity of pHLIP

Cell-penetrating peptides (CPPs) can be potentially used in targeted delivery of therapeutic cargoes. However, their conformation in solution is poorly understood. We employed molecular dynamics simulations to probe the structural fluctuations of an anionic CPP, pH Low Insertion Peptide (pHLIP), in solution to determine the effects of modifications to selected residues on the structure of pHLIP. Two types of modifications were tested: (1) protonation of aspartic acid residues and (2) point mutations known to affect the acid sensitivity of pHLIP. pHLIP samples conformations ranging from coil to helix to sheet, and modifications to pHLIP lead to subtle shifts in the balance between these conformations. In some instances, pHLIP is as likely to form a helical conformation as it is to form an unstructured coil. Understanding the behavior of pHLIP in solution is necessary for determining optimal conditions for administration of pHLIP and design of promising pHLIP variants.

Stabilization and structural analysis of a membrane-associated hIAPP aggregation intermediate

Membrane-assisted amyloid formation is implicated in human diseases, and many of the aggregating species accelerate amyloid formation and induce cell death. While structures of membrane-associated intermediates would provide tremendous insights into the pathology and aid in the design of compounds to potentially treat the diseases, it has not been feasible to overcome the challenges posed by the cell membrane. Here, we use NMR experimental constraints to solve the structure of a type-2 diabetes related human islet amyloid polypeptide intermediate stabilized in nanodiscs. ROSETTA and MD simulations resulted in a unique β-strand structure distinct from the conventional amyloid β-hairpin and revealed that the nucleating NFGAIL region remains flexible and accessible within this isolated intermediate, suggesting a mechanism by which membrane-associated aggregation may be propagated. The ability of nanodiscs to trap amyloid intermediates as demonstrated could become one of the most powerful approaches to dissect the complicated misfolding pathways of protein aggregation.

Comparison of lipid-dependent bilayer insertion of pHLIP and its P20G variant

The ability of the pH-Low Insertion Peptide (pHLIP) to insert into lipid membranes in a transbilayer conformation makes it an important tool for targeting acidic diseased tissues. pHLIP can also serve as a model template for thermodynamic studies of membrane insertion. We use intrinsic fluorescence and circular dichroism spectroscopy to examine the effect of replacing pHLIP's central proline on the pH-triggered lipid-dependent conformational switching of the peptide. We find that the P20G variant (pHLIP-P20G) has a higher helical propensity than the native pHLIP (pHLIP-WT), in both water:organic solvent mixtures and in the presence of lipid bilayers. Spectral shifts of tryptophan fluorescence reveal that with both pHLIP-WT and pHLIP-P20G, the deeply penetrating interfacial form (traditionally called State II) is populated only in pure phosphocholine bilayers. The presence of either anionic lipids or phosphatidylethanolamine leads to a much shallower penetration of the peptide (referred to here as State II^S, for "shallow"). This novel state can be differentiated from soluble state by a reduction in accessibility of tryptophans to acrylamide and by FRET to vesicles doped with Dansyl-PE, but not by a spectral shift in fluorescence emission. FRET experiments indicate free energies for interfacial partitioning range from 6.2 to 6.8kcal/mol and are marginally more favorable for pHLIP-P20G. The effective pKa for the insertion of both peptides depends on the lipid composition, but is always higher for pHLIP-P20G than for pHLIP-WT by approximately one pH unit, which corresponds to a difference of 1.3kcal/mol in free energy of protonation favoring insertion of pHLIP-P20G.

Transmembrane Segment XI of the Na+/H+ Antiporter of S. pombe is a Critical Part of the Ion Translocation Pore

The Na⁺/H⁺ exchanger of the plasma membrane of S. pombe (SpNHE1) removes intracellular sodium in exchange for an extracellular proton. We examined the structure and functional role of amino acids 360–393 of putative transmembrane (TM) segment XI of SpNHE1. Structural analysis suggested that it had a helical propensity over amino acids 360–368, an extended region from 369–378 and was helical over amino acids 379–386. TM XI was sensitive to side chain alterations. Mutation of eight amino acids to alanine resulted in loss of one or both of LiCl or NaCl tolerance when re-introduced into SpNHE1 deficient S. pombe. Mutation of seven other amino acids had minor effects. Analysis of structure and functional mutations suggested that Glu³⁶¹ may be involved in cation coordination on the cytoplasmic face of the protein with a negative charge in this position being important. His³⁶⁷, Ile³⁷¹ and Gly³⁷² were important in function. Ile³⁷¹ may have important hydrophobic interactions with other residues and Gly³⁷² may be important in maintaining an extended conformation. Several residues from Val³⁷⁷ to Leu³⁸⁴ are important in function possibly involved in hydrophobic interactions with other amino acids. We suggest that TM XI forms part of the ion translocation core of this Na⁺/H⁺ exchanger.

Determination of the Membrane Translocation pK of the pH-Low Insertion Peptide

The pH-low insertion peptide (pHLIP) is a leading peptide technology to target the extracellular acidosis that characterizes solid tumors. The pHLIP binds to lipid membranes, and responds to acidification by undergoing a coupled folding/membrane insertion process. In the final transmembrane state, the C terminus of pHLIP gets exposed to the cytoplasm of the target cell, providing a means to translocate membrane-impermeable drug cargoes across the plasma membrane of cancer cells. There exists a need to develop improved pHLIP variants to target tumors with greater efficiency. Characterization of such variants typically relies on determining the pK parameter, the pH midpoint of peptide insertion into the lipid bilayer. Here we report that the value of the pK can be strongly dependent on the method used for its determination. Membrane insertion of pHLIP involves at least four intermediate states, which are believed to be linked to the staggered titration of key acidic residues. We propose that some spectroscopic methods are influenced more heavily by specific membrane folding intermediates, and as a result yield different pK values. To address this potential problem, we have devised an assay to independently monitor the environment of the two termini of pHLIP. This approach provides insights into the conformation pHLIP adopts immediately before the establishment of the transmembrane configuration. Additionally, our data indicate that the membrane translocation of the C terminus of pHLIP, the folding step more directly relevant to drug delivery, occurs at more acidic pH values than previously considered. Consequently, such a pK difference could have substantial ramifications for assessing the translocation of drug cargoes conjugated to pHLIP.

Applications of pHLIP Technology for Cancer Imaging and Therapy

Acidity is a biomarker of cancer that is not subject to the blunting clonal selection effects that reduce the efficacy of other biomarker technologies, such as antibody targeting. The pH (low) insertion peptides (pHLIP^®s) provide new opportunities for targeting acidic tissues. Through the physical mechanism of membrane-associated folding, pHLIPs are triggered by the acidic microenvironment to insert and span the membranes of tumor cells. The pHLIP platform can be applied to imaging acidic tissues, delivering cell-permeable and impermeable molecules to the cytoplasm, and promoting the cellular uptake of nanoparticles. Since acidosis is a hallmark of tumor development, progression, and aggressiveness, the pHLIP technology may prove useful in targeting cancer cells and metastases for tumor diagnosis, imaging, and therapy.

Therapeutic Efficacy of a Family of pHLIP-MMAF Conjugates in Cancer Cells and Mouse Models

The targeting of therapeutics specifically to diseased tissue is crucial for the development of successful cancer treatments. The approach here is based on the pH(low) insertion peptide (pHLIP) for the delivery of a potent mitotic inhibitor monomethyl auristatin F (MMAF). We investigated six pHLIP variants conjugated to MMAF to compare their efficacy in vitro against cultured cancer cells. While all pHLIP-MMAF conjugates exhibit potent pH- and concentration-dependent killing, their cytotoxicity profiles are remarkably different. We also show that the lead conjugate exhibits significant therapeutic efficacy in mouse models without overt toxicities. This study confirms pHLIP-monomethyl auristatin conjugates as possible new therapeutic options for cancer treatment and supports their further development.

Highly Hydrophilic Segments Attached to Hydrophobic Peptides Translocate Rapidly across Membranes

Hydrophilic segments attached to transmembrane helices often cross membranes. In an increasing number of cases, it has become apparent that this occurs in a biologically relevant post-translational event. In this study, we investigate whether juxta-membrane (JM) hydrophilic sequences attached to hydrophobic helices are able to rapidly cross lipid bilayers via their ability or inability to block hydrophobic helix interconversion between a transmembrane (TM) and non-TM membrane-associated state. Interconversion was triggered by changing the protonation state of an Asp residue in the hydrophobic core of the peptides, and peptide configuration was monitored by the fluorescence of a Trp residue at the center of the hydrophobic sequence. In POPC vesicles, conversion of the TM to non-TM state at high pH and the non-TM to TM state at low pH was rapid (seconds or less) for KK, KKNN, and the KKNNNNNN flanking sequences on both N- and C-termini and the KLFAGHQ sequence that flanks the spontaneously TM-inserting 3A protein of polio virus. In vesicles composed of 6:4 (mol/mol) POPC/cholesterol, interconversion was still rapid, with the exception of the peptide flanked by KKNNNNNN sequences, for which the half time of interconversion slowed to minutes. This behavior suggests that, at least in membranes with low levels of cholesterol, movement of hydrophilic JM segments (and analogous hydrophobic loops in multipass TM proteins) across membranes may be more facile than previously thought. This may have important biological implications.

Red Blood Cell Susceptibility to Pneumolysin: CORRELATION WITH MEMBRANE BIOCHEMICAL AND PHYSICAL PROPERTIES

This study investigated the effect of the biochemical and biophysical properties of the plasma membrane as well as membrane morphology on the susceptibility of human red blood cells to the cholesterol-dependent cytolysin pneumolysin, a key virulence factor of Streptococcus pneumoniae, using single cell studies. We show a correlation between the physical properties of the membrane (bending rigidity and surface and dipole electrostatic potentials) and the susceptibility of red blood cells to pneumolysin-induced hemolysis. We demonstrate that biochemical modifications of the membrane induced by oxidative stress, lipid scrambling, and artificial cell aging modulate the cell response to the toxin. We provide evidence that the diversity of response to pneumolysin in diabetic red blood cells correlates with levels of glycated hemoglobin and that the mechanical properties of the red blood cell plasma membrane are altered in diabetes. Finally, we show that diabetic red blood cells are more resistant to pneumolysin and the related toxin perfringolysin O relative to healthy red blood cells. Taken together, these studies indicate that the diversity of cell response to pneumolysin within a population of human red blood cells is influenced by the biophysical and biochemical status of the plasma membrane and the chemical and/or oxidative stress pre-history of the cell.

Identification of Novel Raft Marker Protein, FlotP in Bacillus anthracis

Lipid rafts are dynamic, nanoscale assemblies of specific proteins and lipids, distributed heterogeneously on eukaryotic membrane. Flotillin-1, a conserved eukaryotic raft marker protein (RMP) harbor SPFH (Stomatin, Prohibitin, Flotillin, and HflK/C) and oligomerization domains to regulate various cellular processes through its interactions with other signaling or transport proteins. Rafts were thought to be absent in prokaryotes hitherto, but recent report of its presence and significance in physiology of Bacillus subtilis prompted us to investigate the same in pathogenic bacteria (PB) also. In prokaryotes, proteins of SPFH2a subfamily show highest identity to SPFH domain of Flotillin-1. Moreover, bacterial genome organization revealed that Flotillin homolog harboring SPFH2a domain exists in an operon with an upstream gene containing NFeD domain. Here, presence of RMP in PB was initially investigated in silico by analyzing the presence of SPFH2a, oligomerization domains in the concerned gene and NfeD domain in the adjacent upstream gene. After investigating 300 PB, four were found to harbor RMP. Among them, domains of Bas0525 (FlotP) of Bacillus anthracis (BA) showed highest identity with characteristic domains of RMP. Considering the global threat of BA as the bioterror agent, it was selected as a model for further in vitro characterization of rafts in PB. In silico and in vitro analysis showed significant similarity of FlotP with numerous attributes of Flotillin-1. Its punctate distribution on membrane with exclusive localization in detergent resistant membrane fraction; strongly favors presence of raft with RMP FlotP in BA. Furthermore, significant effect of Zaragozic acid (ZA), a raft associated lipid biosynthesis inhibitor, on several patho-physiological attributes of BA such as growth, morphology, membrane rigidity etc., were also observed. Specifically, a considerable decrease in membrane rigidity, strongly recommended presence of an unknown raft associated lipid molecule on membrane of BA. In addition, treatment with ZA decreased secretion of anthrax toxins and FlotP expression, suggesting potential role of raft in pathogenesis and physiology of BA. Thus, the present study not only suggest the existence and role of raft like entity in pathophysiology of BA but also its possible use for the development of novel drugs or vaccines against anthrax.