Engineering a pH-Sensitive Liposomal MRI Agent by Modification of a Bacterial Channel

Mechanosensitive channel MscL gating transitions coupling with constriction point shift

The mechanosensitive channel of large conductance (MscL) acts as an "emergency release valve" that protects bacterial cells from acute hypoosmotic stress, and it serves as a paradigm for studying the mechanism underlying the transduction of mechanical forces. MscL gating is proposed to initiate with an expansion without opening, followed by subsequent pore opening via a number of intermediate substates, and ends in a full opening. However, the details of gating process are still largely unknown. Using in vivo viability assay, single channel patch clamp recording, cysteine cross-linking, and tryptophan fluorescence quenching approach, we identified and characterized MscL mutants with different occupancies of constriction region in the pore domain. The results demonstrated the shifts of constriction point along the gating pathway towards cytoplasic side from residue G26, though G22, to L19 upon gating, indicating the closed-expanded transitions coupling of the expansion of tightly packed hydrophobic constriction region to conduct the initial ion permeation in response to the membrane tension. Furthermore, these transitions were regulated by the hydrophobic and lipidic interaction with the constricting "hot spots". Our data reveal a new resolution of the transitions from the closed to the opening substate of MscL, providing insights into the gating mechanisms of MscL.

Designing Smart Iron Oxide Nanoparticles for MR Imaging of Tumors

Iron oxide nanoparticles (IONPs) possess unique magnetism and good biocompatibility, and they have been widely applied as contrast agents (CAs) for magnetic resonance imaging (MRI). Traditional CAs typically show a fixed enhanced signal, thus exhibiting the limitations of low sensitivity and a lack of specificity. Nowadays, the progress of stimulus-responsive IONPs allows alteration of the relaxation signal in response to internal stimuli of the tumor, or external stimuli, thus providing an opportunity to overcome those limitations. This review summarizes the current status of smart IONPs as tumor imaging MRI CAs that exhibit responsiveness to endogenous stimuli, such as pH, hypoxia, glutathione, and enzymes, or exogenous stimuli, such as magnets, light, and so on. We discuss the challenges and future opportunities for IONPs as MRI CAs and comprehensively illustrate the applications of these stimuli-responsive IONPs. This review will help provide guidance for designing IONPs as MRI CAs and further promote the reasonable design of magnetic nanoparticles and achieve early and accurate tumor detection.

Approaches for the modulation of mechanosensitive MscL channel pores

MscL was the first mechanosensitive ion channel identified in bacteria. The channel opens its large pore when the turgor pressure of the cytoplasm increases close to the lytic limit of the cellular membrane. Despite their ubiquity across organisms, their importance in biological processes, and the likelihood that they are one of the oldest mechanisms of sensory activation in cells, the exact molecular mechanism by which these channels sense changes in lateral tension is not fully understood. Modulation of the channel has been key to understanding important aspects of the structure and function of MscL, but a lack of molecular triggers of these channels hindered early developments in the field. Initial attempts to activate mechanosensitive channels and stabilize functionally relevant expanded or open states relied on mutations and associated post-translational modifications that were often cysteine reactive. These sulfhydryl reagents positioned at key residues have allowed the engineering of MscL channels for biotechnological purposes. Other studies have modulated MscL by altering membrane properties, such as lipid composition and physical properties. More recently, a variety of structurally distinct agonists have been shown bind to MscL directly, close to a transmembrane pocket that has been shown to have an important role in channel mechanical gating. These agonists have the potential to be developed further into antimicrobial therapies that target MscL, by considering the structural landscape and properties of these pockets.

Mechanosensitive channel MscL induces non-apoptotic cell death and its suppression of tumor growth by ultrasound

Mechanosensitive channel of large conductance (MscL) is the most thoroughly studied mechanosensitive channel in prokaryotes. Owing to its small molecular weight, clear mechanical gating mechanism, and nanopore forming ability upon opening, accumulating studies are implemented in regulating cell function by activating mechanosensitive channel of large conductance in mammalian cells. This study aimed to investigate the potentials of mechanosensitive channel of large conductance as a nanomedicine and a mechano-inducer in non-small cell lung cancer (NSCLC) A549 cells from the view of molecular pathways and acoustics. The stable cytoplasmic vacuolization model about NSCLC A549 cells was established via the targeted expression of modified mechanosensitive channel of large conductance channels in different subcellular organelles. Subsequent morphological changes in cellular component and expression levels of cell death markers are analyzed by confocal imaging and western blots. The permeability of mitochondrial inner membrane (MIM) exhibited a vital role in cytoplasmic vacuolization formation. Furthermore, mechanosensitive channel of large conductance channel can be activated by low intensity focused ultrasound (LIFU) in A549 cells, and the suppression of A549 tumors in vivo was achieved by LIFU with sound pressure as low as 0.053 MPa. These findings provide insights into the mechanisms underlying non-apoptotic cell death, and validate the nanochannel-based non-invasive ultrasonic strategy for cancer therapy.

Probing nitric oxide signaling using molecular MRI

Wide field measurements of nitric oxide (NO) signaling could help understand and diagnose the many physiological processes in which NO plays a key role. Magnetic resonance imaging (MRI) can support particularly powerful approaches for this purpose if equipped with molecular probes sensitized to NO and NO-associated targets. In this review, we discuss the development of MRI-detectable probes that could enable studies of nitrergic signaling in animals and potentially human subjects. Major families of probes include contrast agents designed to capture and report integrated NO levels directly, as well as molecules that respond to or emulate the activity of nitric oxide synthase enzymes. For each group, we outline the relevant molecular mechanisms and discuss results that have been obtained in vitro and in animals. The most promising in vivo data described to date have been acquired using NO capture-based relaxation agents and using engineered nitric oxide synthases that provide hemodynamic readouts of NO signaling pathway activation. These advances establish a beachhead for ongoing efforts to improve the sensitivity, specificity, and clinical applicability of NO-related molecular MRI technology.

Recent Advances in Gene Therapy for Cancer Theranostics

There is great interest in developing gene therapies for many disease indications, including cancer. However, successful delivery of nucleic acids to tumor cells is a major challenge, and in vivo efficacy is difficult to predict. Cancer theranostics is an approach combining anti-tumor therapy with imaging or diagnostic capabilities, with the goal of monitoring successful delivery and efficacy of a therapeutic agent in a tumor. Successful theranostics must maintain a high degree of anticancer targeting and efficacy while incorporating high-contrast imaging agents that are nontoxic and compatible with clinical imaging modalities. This review highlights recent advancements in theranostic strategies, including imaging technologies and genetic engineering approaches. Graphical Abstract.

A native cell membrane nanoparticles system allows for high-quality functional proteoliposome reconstitution

Proteoliposomes mimic the cell membrane environment allowing for structural and functional membrane protein analyses as well as antigen presenting and drug delivery devices. To make proteoliposomes, purified functional membrane proteins are required. Detergents have traditionally been used for the first step in this process However, they can irreversibly denature or render membrane proteins unstable, and the necessary removal of detergents after reconstitution can decrease proteoliposome yields. The recently developed native cell membrane nanoparticles (NCMN) system has provided a variety of detergent-free alternatives for membrane protein preparation for structural biology research. Here we attempt to employ the MCMN system for the functional reconstitution of channels into proteoliposomes. NCMN polymers NCMNP1–1 and NCMNP7–1, members of a NCMN polymer library that have been successful in extraction and affinity purification of a number of intrinsic membrane proteins, were selected for the purification and subsequent reconstitution of three bacterial channels: KcsA and the mechanosensitive channels of large and small conductance (MscL and MscS). We found that channels in NCMN particles, which appeared to be remarkably stable when stored at 4 °C, can be reconstituted into bilayers by simply incubating with lipids. We show that the resulting proteoliposomes can be patched for electrophysiological studies or used for the generation of liposome-based nanodevices. In sum, the findings demonstrate that the NCMN system is a simple and robust membrane protein extraction and reconstitution approach for making high-quality functional proteoliposomes that could significantly impact membrane protein research and the development of nanodevices.

Iron(III)-tCDTA derivatives as MRI contrast agents: Increased T1 relaxivities at higher magnetic field strength and pH sensing

PURPOSE

Low molecular weight iron(III) complex-based contrast agents (IBCA) including iron(III) trans-cyclohexane diamine tetraacetic acid [Fe(tCDTA)]^- could serve as alternatives to gadolinium-based contrast agents in MRI. In search for IBCA with enhanced properties, we synthesized derivatives of [Fe(tCDTA)]^- and compared their contrast effects.

METHODS

Trans-cyclohexane diamine tetraacetic acid (tCDTA) was chemically modified in 2 steps: first the monoanhydride of Trans-cyclohexane diamine tetraacetic acid was generated, and then it was coupled to amines in the second step. After purification, the chelators were analyzed by high-performance liquid chromatography, mass spectrometry, and NMR spectrometry. The chelators were complexed with iron(III), and the relaxivities of the complexes were measured at 0.94, 1.5, 3, and 7 Tesla. Kinetic stabilities of the complexes were analyzed spectrophotometrically and the redox properties by cyclic voltammetry.

RESULTS

Using ethylenediamine (en) and trans-1,4-diaminocyclohexane, we generated monomers and dimers of tCDTA: en-tCDTA, en-tCDTA-dimer, trans-1,4-diaminocyclohexane-tCDTA, and trans-1,4-diaminocyclohexane-tCDTA-dimer. The iron(III) complexes of these derivatives had similarly high stabilities as [Fe(tCDTA)]^- . The iron(III) complexes of the trans-1,4-diaminocyclohexane derivatives had higher T₁ relaxivities than [Fe(tCDTA)]^- that increased with increasing magnetic field strengths and were highest at 6.8 L·mmol^-1 ·s^-1 per molecule for the dimer. Remarkably, the relaxivity of [Fe(en-tCDTA)]⁺ had a threefold increase from neutral pH toward pH6.

CONCLUSION

Four iron(III) complexes with similar stability in comparison to [Fe(tCDTA)]^- were synthesized. The relaxivities of trans-1,4-diaminocyclohexane-tCDTA and trans-1,4-diaminocyclohexane-tCDTA-dimer complexes were in the same range as gadolinium-based contrast agents at 3 Tesla. The [Fe(en-tCDTA)]⁺ complex is a pH sensor at weakly acidic pH levels, which are typical for various cancer types.

Inducible intracellular membranes: molecular aspects and emerging applications

Membrane remodeling and phospholipid biosynthesis are normally tightly regulated to maintain the shape and function of cells. Indeed, different physiological mechanisms ensure a precise coordination between de novo phospholipid biosynthesis and modulation of membrane morphology. Interestingly, the overproduction of certain membrane proteins hijack these regulation networks, leading to the formation of impressive intracellular membrane structures in both prokaryotic and eukaryotic cells. The proteins triggering an abnormal accumulation of membrane structures inside the cells (or membrane proliferation) share two major common features: (1) they promote the formation of highly curved membrane domains and (2) they lead to an enrichment in anionic, cone-shaped phospholipids (cardiolipin or phosphatidic acid) in the newly formed membranes. Taking into account the available examples of membrane proliferation upon protein overproduction, together with the latest biochemical, biophysical and structural data, we explore the relationship between protein synthesis and membrane biogenesis. We propose a mechanism for the formation of these non-physiological intracellular membranes that shares similarities with natural inner membrane structures found in α-proteobacteria, mitochondria and some viruses-infected cells, pointing towards a conserved feature through evolution. We hope that the information discussed in this review will give a better grasp of the biophysical mechanisms behind physiological and induced intracellular membrane proliferation, and inspire new applications, either for academia (high-yield membrane protein production and nanovesicle production) or industry (biofuel production and vaccine preparation).

Non-apoptotic cell death induced by opening the large conductance mechanosensitive channel MscL in hepatocellular carcinoma HepG2 cells

Most anticancer therapies trigger apoptosis to eliminate malignant cells. However, the majority of malignant cancer cells are resistant to apoptosis due to genetic mutations or heterogeneity. Here, we report that opening the pore of the bacterial large conductance mechanosensitivity channel (MscL) provides a novel approach of inducing non-apoptotic cell death. The gain-of-function mutant V23A-MscL and chemically responsive mutant G26C-MscL can be functionally expressed in hepatocellular carcinoma HepG2 cells. V23A-MscL spontaneously opens, and G26C-MscL also responds to its chemical activator MTSET. Opening of the MscL channel causes increased intracellular Ca²⁺ concentration and suppressed cell growth and viability. MTSET-activated G26C channels induce necrosis, while V23A-MscL expression leads to cytoplasmic vacuolization cell death in HepG2 cells and suppresses tumor growth in a mouse model. We propose that MscL may act as a nanovalve through which intracellular homeostasis suffers a disruption and results in malignant tumor cell damage, leading to a new strategy for cancer therapy.

A Nanostrategy for Efficient Imaging-Guided Antitumor Therapy through a Stimuli-Responsive Branched Polymeric Prodrug

A stimuli-responsive polymeric prodrug-based nanotheranostic system with imaging agents (cyanine5.5 and gadolinium-chelates) and a therapeutic agent paclitaxel (PTX) is prepared via polymerization and conjugating chemistry. The branched polymeric PTX-Gd-based nanoparticles (BP-PTX-Gd NPs) demonstrate excellent biocompatibility, and high stability under physiological conditions, but they stimuli-responsively degrade and release PTX rapidly in a tumor microenvironment. The in vitro behavior of NPs labeled with fluorescent dyes is effectively monitored, and the NPs display high cytotoxicity to 4T1 cells similar to free PTX by impairing the function of microtubules, downregulating anti-apoptotic protein Bcl-2, and upregulating the expression of Bax, cleaved caspase-3, cleaved caspase-9, cleaved-PARP, and p53 proteins. Great improvement in magnetic resonance imaging (MRI) is demonstrated by these NPs, and MRI accurately maps the temporal change profile of the tumor volume after injection of NPs and the tumor treatment process is also closely correlated with the T 1 values measured from MRI, demonstrating the capability of providing real-time feedback to the chemotherapeutic treatment effectiveness. The imaging-guided chemotherapy to the 4T1 tumor in the mice model achieves an excellent anti-tumor effect. This stimuli-responsive polymeric nano-agent opens a new door for efficient breast cancer treatment under the guidance of fluorescence/MRI.

Life with Bacterial Mechanosensitive Channels, from Discovery to Physiology to Pharmacological Target

General principles in biology have often been elucidated from the study of bacteria. This is true for the bacterial mechanosensitive channel of large conductance, MscL, the channel highlighted in this review. This channel functions as a last-ditch emergency release valve discharging cytoplasmic solutes upon decreases in osmotic environment. Opening the largest gated pore, MscL passes molecules up to 30 Å in diameter; exaggerated conformational changes yield advantages for study, including in vivo assays. MscL contains structural/functional themes that recur in higher organisms and help elucidate how other, structurally more complex, channels function. These features of MscL include (i) the ability to directly sense, and respond to, biophysical changes in the membrane, (ii) an α helix ("slide helix") or series of charges ("knot in a rope") at the cytoplasmic membrane boundary to guide transmembrane movements, and (iii) important subunit interfaces that, when disrupted, appear to cause the channel to gate inappropriately. MscL may also have medical applications: the modality of the MscL channel can be changed, suggesting its use as a triggered nanovalve in nanodevices, including those for drug targeting. In addition, recent studies have shown that the antibiotic streptomycin opens MscL and uses it as one of the primary paths to the cytoplasm. Moreover, the recent identification and study of novel specific agonist compounds demonstrate that the channel is a valid drug target. Such compounds may serve as novel-acting antibiotics and adjuvants, a way of permeabilizing the bacterial cell membrane and, thus, increasing the potency of commonly used antibiotics.

An agonist of the MscL channel affects multiple bacterial species and increases membrane permeability and potency of common antibiotics

The bacterial MscL channel normally functions as an emergency release valve discharging cytoplasmic solutes upon osmotic stress. The channel opens and passes molecules up to 30 Å and its pore is the largest of any gated channel. Opening the MscL pore inappropriately is detrimental to the bacterial cell, suggesting MscL as a potential novel drug target. A small-molecule compound, 011A, has been shown to increase sensitivity of the Escherichia coli MscL channel, slow growth, and even decrease viability of quiescent cultures. The mscL gene is highly conserved and found in the vast majority of bacterial species, including pathogens. Here, we test the hypothesis that 011A can influence the growth and viability of other bacterial species, specifically Staphylococcus aureus and Mycobacterium smegmatis, in a MscL-dependent manner. Furthermore, we demonstrate that the 011A compound can increase potency of other antibiotics, presumably by permeabilizing the membrane and allowing easier access of the antibiotic into the cytoplasm. Thus, MscL activators have potential as novel broad-spectrum antibiotics or adjuvants that work with antibiotics to selectively allow passage across bacterial membranes.

Atherosclerosis Treatment with Stimuli-Responsive Nanoagents: Recent Advances and Future Perspectives

Atherosclerosis is the root of approximately one-third of global mortalities. Nanotechnology exhibits splendid prospects to combat atherosclerosis at the molecular level by engineering smart nanoagents with versatile functionalizations. Significant advances in nanoengineering enable nanoagents to autonomously navigate in the bloodstream, escape from biological barriers, and assemble with their nanocohort at the targeted lesion. The assembly of nanoagents with endogenous and exogenous stimuli breaks down their shells, facilitates intracellular delivery, releases their cargo to kill the corrupt cells, and gives imaging reports. All these improvements pave the way toward personalized medicine for atherosclerosis. This review systematically summarizes the recent advances in stimuli-responsive nanoagents for atherosclerosis management and its progress in clinical trials.

Effects of Low Intensity Focused Ultrasound on Liposomes Containing Channel proteins

The ability to reversibly and non-invasively modulate region-specific brain activity in vivo suggests Low Intensity Focused Ultrasound (LIFU) as potential therapeutics for neurological dysfunctions such as epilepsy and Parkinson's disease. While in vivo studies provide evidence of the bioeffects of LIFU on neuronal activity, they merely hint at potential mechanisms but do not fully explain how this technology achieves these effects. One potential hypothesis is that LIFU produces local membrane depolarization by mechanically perturbing the neuronal cell membrane, or activating channels or other proteins embedded in the membrane. Proteins that sense mechanical perturbations of the membrane, such as those gated by membrane tension, are prime candidates for activating in response to LIFU and thus leading to the neurological responses that have been measured. Here we use the bacterial mechanosensitive channel MscL, which has been purified and reconstituted in liposomes, to determine how LIFU may affect the activation of this membrane-tension gated channel. Two bacterial voltage-gated channels, KvAP and NaK2K F92A channels were also studied. Surprisingly, the results suggest that ultrasound modulation and membrane perturbation does not induce channel gating, but rather induces pore formation at the membrane protein-lipid interface. However, in vesicles with high MscL mechanosensitive channel concentrations, apparent decreases in pore formation are observed, suggesting that this membrane-tension-sensitive protein may serve to increase the elasticity of the membrane, presumably because of expansion of the channel in the plane of the membrane independent of channel gating.