Mechanism underlying liquid-to-solid phase transition in fused in sarcoma liquid droplets.

The RNA-binding protein fused in sarcoma (FUS) forms ribonucleoprotein granules via liquid-liquid phase separation (LLPS) in the cytoplasm. The phase separation of FUS accelerates aberrant liquid-solid phase separation and leads to the onset of familial amyotrophic lateral sclerosis (ALS). We previously found that FUS forms two types of liquid condensates in equilibrium, specifically LP-LLPS (i.e., normal type) and HP-LLPS (i.e., aberrant type), each with different partial molar volumes. However, it is unclear how liquid condensates are converted to the pathogenic solid phase. Here, we report a mechanism underlying the aberrant liquid-to-solid phase transition of FUS liquid condensates and the inhibition of this transition with small molecules. We found that the liquid condensate formed via HP-LLPS had greatly reduced dynamics, which is a common feature of aged wild-type FUS droplets and the droplet-like assembly of the ALS patient-type FUS variant. The longer FUS remained on the HP-LLPS, the harder it was to transform it into a mixed state (i.e., one-phase). These results indicate that liquid-to-solid phase transition, namely the aging of droplets, is accelerated with HP-LLPS. Interestingly, arginine suppressed the aging of droplets and HP-LLPS formation more strongly than LP-LLPS formation. These data indicate that the formation of HP-LLPS via the one-phase state or LP-LLPS is a pathway leading to irreversible solid aggregates. Dopamine and pyrocatechol also suppressed HP-LLPS formation. Our data highlight the potential of HP-LLPS to be used as a therapeutic target and arginine as a plausible drug candidate for ALS-causing FUS.

How internal cavities destabilize a protein.

Although many proteins possess a distinct folded structure lying at a minimum in a funneled free energy landscape, thermal energy causes any protein to continuously access lowly populated excited states. The existence of excited states is an integral part of biological function. Although transitions into the excited states may lead to protein misfolding and aggregation, little structural information is currently available for them. Here, we show how NMR spectroscopy, coupled with pressure perturbation, brings these elusive species to light. As pressure acts to favor states with lower partial molar volume, NMR follows the ensuing change in the equilibrium spectroscopically, with residue-specific resolution. For T4 lysozyme L99A, relaxation dispersion NMR was used to follow the increase in population of a previously identified "invisible" folded state with pressure, as this is driven by the reduction in cavity volume by the flipping-in of a surface aromatic group. Furthermore, multiple partly disordered excited states were detected at equilibrium using pressure-dependent H/D exchange NMR spectroscopy. Here, unfolding reduced partial molar volume by the removal of empty internal cavities and packing imperfections through subglobal and global unfolding. A close correspondence was found for the distinct pressure sensitivities of various parts of the protein and the amount of internal cavity volume that was lost in each unfolding event. The free energies and populations of excited states allowed us to determine the energetic penalty of empty internal protein cavities to be 36 cal⋅Å-3.

Pressure-Jump Kinetics of Liquid-Liquid Phase Separation: Comparison of Two Different Condensed Phases of the RNA-Binding Protein, Fused in Sarcoma.

The RNA-binding protein fused in sarcoma (FUS) undergoes liquid-liquid phase separation (LLPS) both in vivo and in vitro. Self-assembled liquid droplets of FUS transform into reversible hydrogels and into more irreversible and toxic aggregates. Although LLPS can be a precursor of irreversible aggregates, a generic method to study kinetics of the formation of LLPS has not been developed. Here, we demonstrated the pressure-jump kinetics of phase transition between the 1-phase state and FUS-LLPS states observed at low pressure (<2 kbar, LP-LLPS) and high pressure (>2 kbar, HP-LLPS) using high-pressure UV/vis spectroscopy. Absorbance (turbidity) changes were reproduced repeatedly using pressure cycles. The Johnson-Mehl-Avrami-Kolmogorov theory was used to understand droplet formation occurring via nucleation and growth. The Avrami exponent n, representing the dimensionality of growing droplets, and the reaction rate constant k were calculated. The HP-LLPS formation rate was ∼2-fold slower than that of LP-LLPS. The Avrami exponent obtained for both LLPS states could be explained by diffusion-limited growth. Nucleation and growth rates decreased during LP-LLPS formation (n = 0.51), and the nucleation rate decreased with a constant growth rate in HP-LLPS formation (n = 1.4). The HP-LLPS vanishing rate was ∼20-fold slower than that of LP-LLPS. This difference in vanishing rates indicates a stronger intermolecular interaction in HP-LLPS than in LP-LLPS, which might promote transformation into irreversible aggregates in the droplets. Further, direct transition from HP-LLPS to LP-LLPS was observed. This indicates that interconversion between LP-LLPS and HP-LLPS occurs in equilibrium. Formation of reversible liquid droplets, followed by phase transition into another liquid phase, could thus be part of the physiological maturation process of FUS-LLPS.

Dynamic aspects of pressure and temperature-stabilized intermediates of outer surface protein A.

Structural characterization of alternatively folded and partially disordered protein conformations remains challenging. Outer surface protein A (OspA) is a pivotal protein in Borrelia infection, which is the etiological agent of Lyme disease. OspA exists in equilibrium with intermediate conformations, in which the central and the C-terminal regions of the protein have lower stabilities than the N-terminal. Here, we characterize pressure- and temperature-stabilized intermediates of OspA by nuclear magnetic resonance spectroscopy combined with paramagnetic relaxation enhancement (PRE). We found that although the C-terminal region of the intermediate was partially disordered, it retains weak specific contact with the N-terminal region, owing to a twist of the central ß-sheet and increased flexibility in the polypeptide chain. The disordered C-terminal region of the pressure-stabilized intermediate was more compact than that of the temperature-stabilized form. Further, molecular dynamics simulation demonstrated that temperature-induced disordering of the ß-sheet was initiated at the C-terminal region and continued through to the central region. An ensemble of simulation snapshots qualitatively described the PRE data from the intermediate and indicated that the intermediate structures of OspA may expose tick receptor-binding sites more readily than does the basic folded conformation.

Gene delivery to cone photoreceptors by subretinal injection of rAAV2/6 in the mouse retina.

Adeno-associated virus (AAV) has been studied as a safe delivery tool for gene therapy of retinal blinding diseases such as Leber's congenital amaurosis (LCA). The tropism of recombinant AAV (rAAV) including its specificity and efficiency in targeting retinal cell types has been studied with native or engineered capsids, along with specific promoters. However, one of the rAAV serotypes, rAAV2/6, has not been well-studied based on a report of low infection efficiency in the retina. We investigated the tropism of several rAAVs by subretinal injection in the adult mouse and found that rAAV2/6 predominantly infected cone photoreceptors including the main spectral type. Our data suggest that subretinal injection with rAAV2/6 may provide both an efficacious and specific means of gene delivery to cone photoreceptors in murine retinas.

Water-Protein Interactions Coupled with Protein Conformational Transition.

Conformational fluctuations of proteins are crucially important for their functions. However, changes in the location and dynamics of hydrated water in many proteins accompanied by the conformational transition have not been fully understood. Here, we used phase-modulated clean chemical exchange NMR approach to investigate pressure-induced changes in water-to-amide proton exchange occurring at sub-second time scale. With the transition of ubiquitin from its native conformation (N1) to an alternative conformation (N2) at 250 MPa, proton exchange rates of residues 32-35, 40-41, and 71, which are located at the C-terminal side of the protein, were significantly increased. These observations can be explained by the destabilization of the hydrogen bonds in the backbone and partial exposure of those amide groups to solvent in N2. We conclude that phase-modulated clean chemical exchange NMR approach coupled with pressure perturbation will be a useful tool for investigations of more open and hydrated protein structures.

Analysis of O2-binding Sites in Proteins Using Gas-Pressure NMR Spectroscopy: Outer Surface Protein A.

Internal cavities in proteins produce conformational fluctuations and enable the binding of small ligands. Here, we report a NMR analysis of O2-binding sites by O2-induced paramagnetic relaxation enhancements (PREs) on amide groups of proteins in solution. Outer surface protein A contains a nonglobular single-layer ß-sheet that connects the N- and C-terminal globular domains. Several cavities have been observed in both domains of the crystallized protein structure. The receptor-binding sites are occluded and line the largest cavity of the C-terminal domain. We observed significant O2-induced PREs for amide protons located around the largest cavity and at the central ß-sheet. We suggested three potential O2-accessible sites in the protein based on the 1/r6 distance dependence of the PRE. Two sites were in or close to the largest cavity and the third site was in the surface crevice of the central ß-sheet. These results provide, to our knowledge, the first evidence of ligand binding to the surface crevice and cavity of the protein in solution. Because O2 generally binds more specifically to hydrophobic rather than hydrophilic cavities within a protein, the results also indicated that the receptor-binding sites lining the largest cavity were in the hydrophobic environment in the ground-state conformation. Molecular dynamics simulations permitted the visualization of the rotational and translational motions of O2 within the largest cavity, egress of O2 from the cavity, and ingress of O2 in the surface crevice of the ß-sheet. These molecular dynamics simulation results qualitatively explained the O2-induced changes in NMR observations. Exploring cavities that are sufficiently dynamic to enable access by small molecules can be a useful strategy for the design of stable proteins and their ligands.

Interactions Controlling the Slow Dynamic Conformational Motions of Ubiquitin.

Rational mutation of proteins based on their structural and dynamic characteristics is a useful strategy for amplifying specific fluctuations in proteins. Here, we show the effects of mutation on the conformational fluctuations and thermodynamic stability of ubiquitin. In particular, we focus on the salt bridge between K11 and E34 and the hydrogen bond between I36 and Q41, which are predicted to control the fluctuation between the basic folded state, N1, and the alternatively folded state, N2, of the protein, using high-pressure NMR spectroscopy. The E34A mutation, which disrupts the salt bridge, did not alter picosecond-to-nanosecond, microsecond-to-millisecond dynamic motions, and stability of the protein, while the Q41N mutation, which destabilizes the hydrogen bond, specifically amplified the N1-N2 conformational fluctuation and decreased stability. Based on the observed thermodynamic stabilities of the various conformational states, we showed that in the Q41N mutant, the N1 state is more significantly destabilized than the N2 state, resulting in an increase in the relative population of N2. Identifying the interactions controlling specific motions of a protein will facilitate molecular design to achieve functional dynamics beyond native state dynamics.

Aberrant increase of NMR signal in hydrogen exchange experiments. Observation and explanation.

Hydrogen exchange (HX) NMR spectroscopy is widely used for monitoring structure, stability and dynamics of proteins at the level of individual residues. The stochastic replacement of protons by deuterons typically leads to an exponential decrease of the NMR signals. However, an unusual signal increase was observed in HX of several amides for T4 lysozyme L99A. This effect can be attributed to peak sharpening as a result of reduced dipolar relaxation from proximal amide protons that experience more rapid hydrogen/deuterium (H/D) exchange. The behavior was specifically observed at the termini of secondary structure elements, where large differences in protection against H/D exchange are observed. This effect is expected to be more widespread in NMR HX studies, and is important for the accurate determination of protection factors.

Modeling (15)N NMR chemical shift changes in protein backbone with pressure.

Nitrogen chemical shift is a useful parameter for determining the backbone three-dimensional structure of proteins. Empirical models for fast calculation of N chemical shift are improving their reliability, but there are subtle effects that cannot be easily interpreted. Among these, the effects of slight changes in hydrogen bonds, both intramolecular and with water molecules in the solvent, are particularly difficult to predict. On the other hand, these hydrogen bonds are sensitive to changes in protein environment. In this work, the change of N chemical shift with pressure for backbone segments in the protein ubiquitin is correlated with the change in the population of hydrogen bonds involving the backbone amide group. The different extent of interaction of protein backbone with the water molecules in the solvent is put in evidence.

High-Pressure NMR Spectroscopy Reveals Functional Sub-states of Ubiquitin and Ubiquitin-Like Proteins.

High-pressure nuclear magnetic resonance (NMR) spectroscopy has revealed that ubiquitin has at least two high-energy states--an alternatively folded state N2 and a locally disordered state I--between the basic folded state N1 and totally unfolded U state. The high-energy states are conserved among ubiquitin-like post-translational modifiers, ubiquitin, NEDD8, and SUMO-2, showing the E1-E2-E3 cascade reaction. It is quite intriguing that structurally similar high-energy states are evolutionally conserved in the ubiquitin-like modifiers, and the thermodynamic stabilities vary among the proteins. To investigate atomic details of the high-energy states, a Q41N mutant of ubiquitin was created as a structural model of N2, which is 71% populated even at atmospheric pressure. The convergent structure of the "pure" N2 state was obtained by nuclear Overhauser effect (NOE)-based structural analysis of the Q41N mutant at 2.5 kbar, where the N2 state is 97% populated. The N2 state of ubiquitin is closely similar to the conformation of the protein bound to the ubiquitin-activating enzyme E1. The recognition of E1 by ubiquitin is best explained by conformational selection rather than by induced-fit motion.

Cavity as a source of conformational fluctuation and high-energy state: high-pressure NMR study of a cavity-enlarged mutant of T4 lysozyme.

Although the structure, function, conformational dynamics, and controlled thermodynamics of proteins are manifested by their corresponding amino acid sequences, the natural rules for molecular design and their corresponding interplay remain obscure. In this study, we focused on the role of internal cavities of proteins in conformational dynamics. We investigated the pressure-induced responses from the cavity-enlarged L99A mutant of T4 lysozyme, using high-pressure NMR spectroscopy. The signal intensities of the methyl groups in the (1)H/(13)C heteronuclear single quantum correlation spectra, particularly those around the enlarged cavity, decreased with the increasing pressure, and disappeared at 200 MPa, without the appearance of new resonances, thus indicating the presence of heterogeneous conformations around the cavity within the ground state ensemble. Above 200 MPa, the signal intensities of >20 methyl groups gradually decreased with the increasing pressure, without the appearance of new resonances. Interestingly, these residues closely matched those sensing a large conformational change between the ground- and high-energy states, at atmospheric pressure. (13)C and (1)H NMR line-shape simulations showed that the pressure-induced loss in the peak intensity could be explained by the increase in the high-energy state population. In this high-energy state, the aromatic side chain of F114 gets flipped into the enlarged cavity. The accommodation of the phenylalanine ring into the efficiently packed cavity may decrease the partial molar volume of the high-energy state, relative to the ground state. We suggest that the enlarged cavity is involved in the conformational transition to high-energy states and in the volume fluctuation of the ground state.

Close identity between alternatively folded state N2 of ubiquitin and the conformation of the protein bound to the ubiquitin-activating enzyme.

We present the nuclear Overhauser effect-based structure determination of the Q41N variant of ubiquitin at 2500 bar, where the alternatively folded N2 state is 97% populated. This allows us to characterize the structure of the "pure" N2 state of ubiquitin. The N2 state shows a substantial change in the orientation of strand ß5 compared to that of the normal folded N1 state, which matches the changes seen upon binding of ubiquitin to ubiquitin-activating enzyme E1. The recognition of E1 by ubiquitin is therefore best explained by conformational selection rather than induced-fit motion.

Aberrant assembly of RNA recognition motif 1 links to pathogenic conversion of TAR DNA-binding protein of 43 kDa (TDP-43).

Aggregation of TAR DNA-binding protein of 43 kDa (TDP-43) is a pathological signature of amyotrophic lateral sclerosis (ALS). Although accumulating evidence suggests the involvement of RNA recognition motifs (RRMs) in TDP-43 proteinopathy, it remains unclear how native TDP-43 is converted to pathogenic forms. To elucidate the role of homeostasis of RRM1 structure in ALS pathogenesis, conformations of RRM1 under high pressure were monitored by NMR. We first found that RRM1 was prone to aggregation and had three regions showing stable chemical shifts during misfolding. Moreover, mass spectrometric analysis of aggregated RRM1 revealed that one of the regions was located on protease-resistant ß-strands containing two cysteines (Cys-173 and Cys-175), indicating that this region served as a core assembly interface in RRM1 aggregation. Although a fraction of RRM1 aggregates comprised disulfide-bonded oligomers, the substitution of cysteine(s) to serine(s) (C/S) resulted in unexpected acceleration of amyloid fibrils of RRM1 and disulfide-independent aggregate formation of full-length TDP-43. Notably, TDP-43 aggregates with RRM1-C/S required the C terminus, and replicated cytopathologies of ALS, including mislocalization, impaired RNA splicing, ubiquitination, phosphorylation, and motor neuron toxicity. Furthermore, RRM1-C/S accentuated inclusions of familial ALS-linked TDP-43 mutants in the C terminus. The relevance of RRM1-C/S-induced TDP-43 aggregates in ALS pathogenesis was verified by immunolabeling of inclusions of ALS patients and cultured cells overexpressing the RRM1-C/S TDP-43 with antibody targeting misfolding-relevant regions. Our results indicate that cysteines in RRM1 crucially govern the conformation of TDP-43, and aberrant self-assembly of RRM1 at amyloidogenic regions contributes to pathogenic conversion of TDP-43 in ALS.

Solvent environments significantly affect the enzymatic function of Escherichia coli dihydrofolate reductase: comparison of wild-type protein and active-site mutant D27E.

To investigate the contribution of solvent environments to the enzymatic function of Escherichia coli dihydrofolate reductase (DHFR), the salt-, pH-, and pressure-dependence of the enzymatic function of the wild-type protein were compared with those of the active-site mutant D27E in relation to their structure and stability. The salt concentration-dependence of enzymatic activity indicated that inorganic cations bound to and inhibited the activity of wild-type DHFR at neutral pH. The BaCl2 concentration-dependence of the (1)H-(15)N HSQC spectra of the wild-type DHFR-folate binary complex showed that the cation-binding site was located adjacent to the Met20 loop. The insensitivity of the D27E mutant to univalent cations, the decreased optimal pH for its enzymatic activity, and the increased Km and Kd values for its substrate dihydrofolate suggested that the substrate-binding cleft of the mutant was slightly opened to expose the active-site side chain to the solvent. The marginally increased fluorescence intensity and decreased volume change due to unfolding of the mutant also supported this structural change or the modified cavity and hydration. Surprisingly, the enzymatic activity of the mutant increased with pressurization up to 250MPa together with negative activation volumes of -4.0 or -4.8mL/mol, depending on the solvent system, while that of the wild-type was decreased and had positive activation volumes of 6.1 or 7.7mL/mol. These results clearly indicate that the insertion of a single methylene at the active site could substantially change the enzymatic reaction mechanism of DHFR, and solvent environments play important roles in the function of this enzyme.

An aberrant fused in sarcoma liquid droplet of amyotrophic lateral sclerosis pathological variant, R495X, accelerates liquid-solid phase transition.

Intracellular aggregation of fused in sarcoma (FUS) is associated with the pathogenesis of familial amyotrophic lateral sclerosis (ALS). Under stress, FUS forms liquid droplets via liquid-liquid phase separation (LLPS). Two types of wild-type FUS LLPS exist in equilibrium: low-pressure LLPS (LP-LLPS) and high-pressure LLPS (HP-LLPS); the former dominates below 2 kbar and the latter over 2 kbar. Although several disease-type FUS variants have been identified, the molecular mechanism underlying accelerated cytoplasmic granule formation in ALS patients remains poorly understood. Herein, we report the reversible formation of the two LLPS states and the irreversible liquid-solid transition, namely droplet aging, of the ALS patient-type FUS variant R495X using fluorescence microscopy and ultraviolet-visible absorption spectroscopy combined with perturbations in pressure and temperature. Liquid-to-solid phase transition was accelerated in the HP-LLPS of R495X than in the wild-type variant; arginine slowed the aging of droplets at atmospheric conditions by inhibiting the formation of HP-LLPS more selectively compared to that of LP-LLPS. Our findings provide new insight into the mechanism by which R495X readily forms cytoplasmic aggregates. Targeting the aberrantly formed liquid droplets (the HP-LLPS state) of proteins with minimal impact on physiological functions could be a novel therapeutic strategy for LLPS-mediated protein diseases.

Solution structure of the Q41N variant of ubiquitin as a model for the alternatively folded N2 state of ubiquitin.

It is becoming increasingly clear that proteins transiently populate high-energy excited states as a necessary requirement for function. Here, we demonstrate that rational mutation based on the characteristics of the structure and dynamics of proteins obtained from pressure experiments is a new strategy for amplifying particular fluctuations in proteins. We have previously shown that ubiquitin populates a high-energy conformer, N2, at high pressures. Here, we show that the Q41N mutation favors N2: high-pressure nuclear magnetic resonance (NMR) shows that N2 is ∼70% populated in Q41N but only ∼20% populated in the wild type at ambient pressure. This allows us to characterize the structure of N2, in which α1-helix, the following loop, ß3-strand, and ß5-strand change their orientations relative to the remaining regions. Conformational fluctuation on the microsecond time scale, characterized by (15)N spin relaxation NMR analysis, is markedly increased for these regions of the mutant. The N2 conformers produced by high pressure and by the Q41N mutation are quite similar in both structure and dynamics. The conformational change to produce N2 is proposed to be a novel dynamic feature beyond the known recognition dynamics of the protein. Indeed, it is orthogonal to that seen when proteins containing a ubiquitin-interacting motif bind at the hydrophobic patch of ubiquitin but matches changes seen on binding to the E2 conjugating enzyme. More generally, structural and dynamic effects of hydrodynamic pressure are shown to be useful for characterizing functionally important intermediates.

Exploring the folding energy landscape with pressure.

The unique role of pressure in protein folding studies is emphasized. Variable-pressure NMR experiments carried out under equilibrium conditions give unique opportunities to explore the energy landscape for protein folding. Intermediate conformers that may appear transiently in the kinetic folding experiments may be stably trapped under pressure, allowing examination of their conformations in site-specific detail with modern NMR spectroscopy. The intimate relationship between the kinetic folding experiment and the equilibrium pressure experiment is described with examples from ubiquitin and hen lysozyme.

[EXPERIENCES OF CONTACT LASER VAPORIZATION OF THE PROSTATE (CVP) FOR TREATING BENIGN PROSTATIC HYPERPLASIA IN THE LOCAL HOSPITAL].

(Objective) We started contact laser vaporization of the prostate (CVP) for treating benign prostatic hyperplasia at our hospital in July 2019. Forty-five patients were treated with CVP from July 2019 to April 2021. (Methods) Patients were assessed preoperatively and at one and three months after CVP treatment by using the International Prostate Symptom Score (IPSS), quality of life index (QOL index), peak urinary flow rate (Qmax), and postvoid residual urine volume (PVR). (Results) IPSS, QOL index, Qmax, and PVR significantly improved three months after CVP (p<0.05). Regarding adverse events, five patients developed early external urinary meatus strictures, two had postoperative bleeding, and three had temporary urinary retention. (Conclusions) In our hospital, elderly patients and patients who cannot discontinue an antithrombotic drug were treated by CVP for benign prostatic hyperplasia relatively safely.