Characterization of high-molecular-weight nonnative aggregates and aggregation kinetics by size exclusion chromatography with inline multi-angle laser light scattering

Doxorubicin as a Drug Repurposing for Disruption of α-Chymotrypsinogen-A Aggregates

Protein conformation is affected by interaction of several small molecules resulting either stabilization or disruption depending on the nature of the molecules. In our earlier communication, Hg2+ was known to disrupt the native structure of α-Cgn A leading to aggregation (Ansari, N.K., Rais, A. & Naeem, A. Methotrexate for Drug Repurposing as an Anti-Aggregatory Agent to Mercuric Treated α-Chymotrypsinogen-A. Protein J (2024). https://doi.org/10.1007/s10930-024-10187-z ). Accumulation of β-rich aggregates in the living system is found to be linked with copious number of disorders. Here, we have investigated the effect of varying concentration of doxorubicin (DOX) i.e. 0-100 µM on the preformed aggregates of α-Cgn A upon incubation with 120 µM Hg2+. The decrease in the intrinsic fluorescence and enzyme activity with respect to increase in the Hg2+ concentration substantiate the formation of aggregates. The DOX showed the dose dependent decrease in the ThT fluorescence, turbidity and RLS measurements endorsing the dissolution of aggregates which were consistent with red shift in ANS, confirming the breakdown of aggregates. The α-Cgn A has 30% α-helical content which decreases to 3% in presence of Hg2+. DOX increased the α-helicity to 28% confirming its anti-aggregatory potential. The SEM validates the formation of aggregates with Hg2+ and their dissolution upon incubation with the DOX. Hemolysis assay checked the cytotoxicity of α-Cgn A aggregates. Docking revealed that the DOX interacted Lys203, Cys201, Cys136, Ser159, Leu10, Trp207, Val137 and Thr134 of α-Cgn A through hydrophobic interactions and Gly133, Thr135 and Lys202 forms hydrogen bonds.

A guide to studying protein aggregation

Disrupted protein folding or decreased protein stability can lead to the accumulation of (partially) un- or misfolded proteins, which ultimately cause the formation of protein aggregates. Much of the interest in protein aggregation is associated with its involvement in a wide range of human diseases and the challenges it poses for large-scale biopharmaceutical manufacturing and formulation of therapeutic proteins and peptides. On the other hand, protein aggregates can also be functional, as observed in nature, which triggered its use in the development of biomaterials or therapeutics as well as for the improvement of food characteristics. Thus, unmasking the various steps involved in protein aggregation is critical to obtain a better understanding of the underlying mechanism of amyloid formation. This knowledge will allow a more tailored development of diagnostic methods and treatments for amyloid-associated diseases, as well as applications in the fields of new (bio)materials, food technology and therapeutics. However, the complex and dynamic nature of the aggregation process makes the study of protein aggregation challenging. To provide guidance on how to analyse protein aggregation, in this review we summarize the most commonly investigated aspects of protein aggregation with some popular corresponding methods.

Computational models for studying physical instabilities in high concentration biotherapeutic formulations

Computational prediction of the behavior of concentrated protein solutions is particularly advantageous in early development stages of biotherapeutics when material availability is limited and a large set of formulation conditions needs to be explored. This review provides an overview of the different computational paradigms that have been successfully used in modeling undesirable physical behaviors of protein solutions with a particular emphasis on high-concentration drug formulations. This includes models ranging from all-atom simulations, coarse-grained representations to macro-scale mathematical descriptions used to study physical instability phenomena of protein solutions such as aggregation, elevated viscosity, and phase separation. These models are compared and summarized in the context of the physical processes and their underlying assumptions and limitations. A detailed analysis is also given for identifying protein interaction processes that are explicitly or implicitly considered in the different modeling approaches and particularly their relations to various formulation parameters. Lastly, many of the shortcomings of existing computational models are discussed, providing perspectives and possible directions toward an efficient computational framework for designing effective protein formulations.

A strategy for the synthesis of low-molecular-weight welan gum by eliminating capsule form of Sphingomonas strains

Welan gum is widely used in food, concrete additives, and oil recovery. Here we changed the capsule form of Sphingomonas strains by knocked out the sortase gene (srtW). The obtained welan gum was mainly composed of mannose, glucose, rhamnose, and glucuronic acid at a molar ratio of 4.0:5.8:1.6:1, respectively. Meanwhile, the molecular weight of welan gum decreased sharply (about 68 kDa). Moreover, the low molecular weight (LMW) welan gum was characterized by FT-IR and NMR spectroscopy. The rheological results revealed that the LMW welan gum solution is a pseudoplastic fluid with a lower apparent viscosity. Furthermore, the oscillation test illustrated stable dynamic viscoelasticity within the temperature range of 5-68 °C and frequency range of 0.01-15 rad/s. To the best of our knowledge, this is the first report of LMW welan gum production and characterization. These results provide references for LMW welan gum applications, and likely applicable for other biopolymers production.

Effect of Azide Preservative on Thermomechanical Aggregation of Purified Reference Protein Materials

Protein aggregation can affect the quality of protein-based therapeutics. Attempting to unravel factors influencing protein aggregation involves systematic studies. These studies often include sodium azide or similar preservatives in the aggregation buffer. This work shows effects of azide on aggregation of two highly purified reference proteins, both a bovine serum albumin (BSA) as well as a monoclonal antibody (NISTmAb). The proteins were aggregated by thermomechanical stress, consisting of simultaneous heating of the solution with gentle agitation. Protein aggregates were characterized by asymmetric flow field flow fractionation (AF⁴) with light scattering measurements along with quantification by UV spectroscopy, revealing strong time-dependent generation of aggregated protein and an increase in aggregate molar mass. Gel electrophoresis was used to probe the reversibility of the aggregation and demonstrated complete reversibility for the NISTmAb, but not so for the BSA. Kinetic fitting to a commonly implemented nucleated polymerization model was also employed to provide mechanistic details into the kinetic process. The model suggests that the aggregation of the NISTmAb proceeds via nucleated growth and aggregate-aggregate condensation in a way that is dependent on the concentration (and presence) of the azide anion. This work overall implicates azide preservatives as having demonstrable effects on thermomechanical stress and aggregation of proteins undergoing systematic aggregation and stability studies.

Macromolecular crowding stabilises native structure of α-chymotrypsinogen-A against hexafluoropropanol-induced aggregates

Cell interior is extremely congested with tightly packed biological macromolecules that exerts macromolecular crowding effect, influencing biophysical properties of proteins. To have a deeper insight into it we studied consequences of crowding on aggregation susceptibility and structural stability of α-chymotrypsinogen-A, pro-enzyme of serine protease family, upon addition of co-solvent reported to exert stress on polypeptides crafting favourable conditions for aggregation. Hexafluoropropan-2-ol (HFIP), a fluorinated alcohol caused structural disruption at 5% v/v unveiled by reduced intrinsic intensity and blue shifted ANS spectra. Significantly enhanced, red-shifted ThT and Congo red spectra sustained conformational changes concomitant with aggregation. FTIR and CD results confirmed transition of native structure to non-native extended, cross-linked beta-sheets. Transmission electron micrographs visibly exhibited incidence of amorphous aggregates. Macromolecular crowding, typically mimicked by concentrated solutions of dextran 70, was noticeably witnessed to defend conformational stability under denaturing condition. The native structure was retained maximally in presence of 100 mg/ml followed by 200 and 300 mg/ml dextran indicating concentration dependent deceleration of aggregate formation. It can be established that explicit consideration of crowding effects using relevant range of inert crowding agents must be a requisite for presumptions on intracellular conformational behaviour of proteins deduced from in vitro experiments.

Bacterial inclusion bodies are industrially exploitable amyloids

Understanding the structure, functionalities and biology of functional amyloids is an issue of emerging interest. Inclusion bodies, namely protein clusters formed in recombinant bacteria during protein production processes, have emerged as unanticipated, highly tunable models for the scrutiny of the physiology and architecture of functional amyloids. Based on an amyloidal skeleton combined with varying amounts of native or native-like protein forms, bacterial inclusion bodies exhibit an unusual arrangement that confers mechanical stability, biological activity and conditional protein release, being thus exploitable as versatile biomaterials. The applicability of inclusion bodies in biotechnology as enriched sources of protein and reusable catalysts, and in biomedicine as biocompatible topographies, nanopills or mimetics of endocrine secretory granules has been largely validated. Beyond these uses, the dissection of how recombinant bacteria manage the aggregation of functional protein species into structures of highly variable complexity offers insights about unsuspected connections between protein quality (conformational status compatible with functionality) and cell physiology.

On the Reproducibility of Early-Stage Thermally Induced and Contact-Stir-Induced Protein Aggregation

Is aggregation kinetics for a protein under given conditions reproducible? Is aggregation inherently deterministic, stochastic, or even chaotic? Because protein aggregation in ex vivo formulations is complex, with many origins and manifestations, the question of aggregation reproducibility for a given protein, formulation, and stressor is of both fundamental and practical significance. This work concerns temperature-induced and contact-stir-induced aggregation of bovine serum albumin (BSA) and a monoclonal antibody (mAbX). It assesses reproducibility via early-stage aggregation rates (ARs) from light scattering. "Global stressors" affect the entire protein population, for example, temperature. "Local stressors" affect only a partial population at a given instant, for example, stirring. The instrumental error distribution (IED) allows stochasticity to be identified for AR distributions (ARDs) broader than IED. For ARD at the limit of the IED, the behavior is "minimally stochastic" or "operationally deterministic." A stochastic index is defined in terms of the ratio of the standard deviation (SD) of log(AR) data and the SD of IED. Thermal aggregation was operationally deterministic for BSA and mAbX, although significant lot-to-lot variations for BSA were found. ARD from contact-stir-stress was stochastic for BSA and mAb. Despite this, log(AR) decreases logarithmically with rpm. These trends may hold for other global and local stressors.

Parallel chromatography and in situ scattering to interrogate competing protein aggregation pathways

Protein aggregation can follow different pathways, and these can result in different net aggregation rates and kinetic profiles. α-chymotypsinogen A (aCgn) was used as a model system to quantitatively and qualitatively assess an approach that combines ex situ size-exclusion chromatography (SEC) with in situ laser scattering (LS) to monitor aggregation vs. time. Aggregation was monitored for a series of temperatures and initial dimer (ID) levels for starting conditions that were primarily (> 97%) monomer, and under initial-rate conditions (limited to low monomer conversion-less than 20% monomer mass loss), as these conditions are of most to interest to many pharmaceutical and biotechnology applications. SEC results show that modest decreases of ID levels can greatly reduce monomer loss rates, but do not affect the effective activation energy for aggregation. The normalized aggregation rates determined from LS were typically ∼ 1 order of magnitude higher than the corresponding rates from SEC. Furthermore, LS signals vs. time became variable and highly nonlinear with decreasing ID level, temperature, and/or total protein concentration. Temperature-cycling LS experiments showed this corresponded to conditions where dimer/oligomer "seeding" was suppressed, and high levels of reversible oligomers ("prenuclei") were formed prior to "nucleation" and growth of stable aggregates. In those conditions, aggregation rates inferred from LS and SEC are greatly different, as the techniques monitor different stages of the aggregation process. Overall, the results illustrate an approach for interrogating non-native protein aggregation pathways, and potential pitfalls if one relies on a single method to monitor aggregation-this holds more generally than the particular methods here.

Structural elucidation and immunostimulatory activity of polysaccharide isolated by subcritical water extraction from Cordyceps militaris

Water-soluble polysaccharides were obtained from Cordyceps militaris (C. militaris) (CMP) by subcritical water extraction (SWE). Two polysaccharides fractions, CMP-W1 and CMP-S1, were isolated from CMP using DEAE-52 cellulose and Sephadex G-150 column chromatography. The structural characteristics of CMP-W1 and CMP-S1 were investigated. The results showed that the molecular weight of CMP-W1 and CMP-S1 are 3.66×105Da and 4.60×105Da, respectively, and both of them were heteropolysaccharides composed of d-mannose, d-glucose, d-galactose with the molar ratios of 2.84:1:1.29 and 2.05:1:1.09, respectively. FT-IR spectra analysis suggested that CMP-W1 and CMP-S1 belonged to pyranose form sugar and protein free. For immunostimulatory activity assay in vitro, CMP-W1 and CMP-S1 significantly promoted lymphatic spleen cell proliferation of mice. Therefore, the polysaccharides obtained from C. militaris by SWE can be used as potential natural immunostimulant in functional foods or medicine.

Modulating non-native aggregation and electrostatic protein-protein interactions with computationally designed single-point mutations

Non-native protein aggregation is a ubiquitous challenge in the production, storage and administration of protein-based biotherapeutics. This study focuses on altering electrostatic protein-protein interactions as a strategy to modulate aggregation propensity in terms of temperature-dependent aggregation rates, using single-charge variants of human γ-D crystallin. Molecular models were combined to predict amino acid substitutions that would modulate protein-protein interactions with minimal effects on conformational stability. Experimental protein-protein interactions were quantified by the Kirkwood-Buff integrals (G22) from laser scattering, and G22 showed semi-quantitative agreement with model predictions. Experimental initial-rates for aggregation showed that increased (decreased) repulsive interactions led to significantly increased (decreased) aggregation resistance, even based solely on single-point mutations. However, in the case of a particular amino acid (E17), the aggregation mechanism was altered by substitution with R or K, and this greatly mitigated improvements in aggregation resistance. The results illustrate that predictions based on native protein-protein interactions can provide a useful design target for engineering aggregation resistance; however, this approach needs to be balanced with consideration of how mutations can impact aggregation mechanisms.

Weak protein interactions and pH- and temperature-dependent aggregation of human Fc1

The Fc (fragment crystallizable) is a common structural region in immunoglobulin gamma (IgG) proteins, IgG-based multi-specific platforms, and Fc-fusion platform technologies. Changes in conformational stability, protein-protein interactions, and aggregation of NS0-produced human Fc1 were quantified experimentally as a function of pH (4 to 6) and temperature (30 to 77 °C), using a combination of differential scanning calorimetry, laser light scattering, size-exclusion chromatography, and capillary electrophoresis. The Fc1 was O-glycosylated at position 3 (threonine), and confirmed to correspond to the intact IgG1 by comparison with Fc1 produced by cleavage of the parent IgG1. Changing the pH caused large effects for thermal unfolding transitions, but it caused surprisingly smaller effects for electrostatic protein-protein interactions. The aggregation behavior was qualitatively similar across different solution conditions, with soluble dimers and larger oligomers formed in most cases. Aggregation rates spanned approximately 5 orders of magnitude and could be divided into 2 regimes: (i) Arrhenius, unfolding-limited aggregation at temperatures near or above the midpoint-unfolding temperature of the CH2 domain; (ii) a non-Arrhenius regime at lower temperatures, presumably as a result of the temperature dependence of the unfolding enthalpy for the CH2 domain. The non-Arrhenius regime was most pronounced for lower temperatures. Together with the weak protein-protein repulsions, these highlight challenges that are expected for maintaining long-term stability of biotechnology products that are based on human Fc constructs.

Heterogeneous glycoform separation by process chromatography: I: Monomer purification and characterization

Fc fusion proteins with high and low sialylation were purified and separated by preparative ion-exchange and hydrophobic interaction chromatography. Heterogeneity in sialylation and glycosylation led to variation in surface charge and hydrophobicity, and resulted in multiple distinct glycoform populations in response to various purification conditions. Monomer with high sialic acid content has higher surface charge and adsorbs stronger to ion-exchange resin, while the less sialylated monomer interacts more favorably with hydrophobic resin. Extensive biophysical characterization was carried out for purified monomers at different level of sialylation. In general, different monomeric glycoforms have different surface charge and hydrophobicity, different thermal stability, and different aggregation propensity. The surface charge corresponds well with sialic acid content, as evidenced by electrophoresis, N-link domain analysis, and zeta potential results. The sialylation also contributes to minor modification of protein size, molecular mass and tertiary structure. Notably, fluorescence emission spectra and thermal transition became less distinguishable when the monomers containing low and high sialic acid were prepared in high ionic strength solution. Such finding reiterates the fact that the electrostatic forces, which are largely dependent on sialic acid content of protein, plays a dominant role in many intra- and inter-molecular interactions. Overall, the characterization data agreed well with separation behaviors and provided valuable insight to control of glycoform profile in purification process.

Characterization of small protein aggregates and oligomers using size exclusion chromatography with online detection by native electrospray ionization mass spectrometry

Self-association of proteins is important in a variety of processes ranging from acquisition of native quaternary structure (where the association is tightly controlled and proceeds in a highly ordered fashion) to aggregation and amyloidosis. The latter is frequently accompanied (or indeed triggered) by the loss of the native structure, but a clear understanding of the complex relationship between conformational changes and protein self-association/aggregation remains elusive due to the great difficulty in characterizing these complex and frequently heterogeneous species. In this study, size exclusion chromatography (SEC) was used in combination with online detection by native electrospray ionization mass spectrometry (ESI MS) to characterize a commercial protein sample (serum albumin) that forms small aggregates. Although noncovalent dimers and trimers of this protein are readily detected by native ESI MS alone, combination of SEC and ESI MS allows a distinction to be made between the oligomers present in solution and those formed during the ESI process (artifacts of ESI MS). Additionally, native ESI MS detection allows a partial loss of conformation integrity to be detected across all albumin species present in solution. Finally, ESI MS detection allows these analyses to be carried out readily even in the presence of other abundant proteins coeluting with albumin. Native ESI MS as an online detection method for SEC also enables meaningful characterization of species representing different quaternary organization of a recombinant glycoprotein human arylsulfatase A even when their rapid interconversion prevents their separation on the SEC time scale.

High-throughput thermal stability analysis of a monoclonal antibody by attenuated total reflection FT-IR spectroscopic imaging

The use of biotherapeutics, such as monoclonal antibodies, has markedly increased in recent years. It is thus essential that biotherapeutic production pipelines are as efficient as possible. For the production process, one of the major concerns is the propensity of a biotherapeutic antibody to aggregate. In addition to reducing bioactive material recovery, protein aggregation can have major effects on drug potency and cause highly undesirable immunological effects. It is thus essential to identify processing conditions which maximize recovery while avoiding aggregation. Heat resistance is a proxy for long-term aggregation propensity. Thermal stability assays are routinely performed using various spectroscopic and scattering detection methods. Here, we evaluated the potential of macro attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopic imaging as a novel method for the high-throughput thermal stability assay of a monoclonal antibody. This chemically specific visualization method has the distinct advantage of being able to discriminate between monomeric and aggregated protein. Attenuated total reflection is particularly suitable for selectively probing the bottom of vessels, where precipitated aggregates accumulate. With focal plane array detection, we tested 12 different buffer conditions simultaneously to assess the effect of pH and ionic strength on protein thermal stability. Applying the Finke model to our imaging kinetics allowed us to determine the rate constants of nucleation and autocatalytic growth. This analysis demonstrated the greater stability of our immunoglobulin at higher pH and moderate ionic strength, revealing the key role of electrostatic interactions. The high-throughput approach presented here has significant potential for analyzing the stability of biotherapeutics as well as any other biological molecules prone to aggregation.

Reduction of the C191-C220 disulfide of α-chymotrypsinogen A reduces nucleation barriers for aggregation

Proper disulfide formation can be essential for the conformational stability of natively folded proteins. For proteins that must unfold in order to aggregate, disruption of native disulfides may therefore promote aggregation. This study characterizes differences in the aggregation process for wild-type (WT) α-chymostrypsinogen A (aCgn) and the same molecule with one of its native disulfides (C191-C220) reduced to free thiols (aCgnSH) at acidic pH, where WT aCgn forms semi-flexible amyloid polymers. Loss of the disulfide leads to no discernable differences in folded monomer secondary or tertiary structure based on circular dichroism (CD) or intrinsic fluorescence (FL), and causes a small decrease in the free energy change upon unfolding. After unfolding-mediated aggregation, the resulting amyloid morphology and structure are similar or indistinguishable for aCgn and aCgnSH by CD, FL, ThT binding, multi-angle laser light scattering, and transmission electron microscopy. Aggregates of aCgn and aCgnSH are also able to cross-seed with monomers of the other species. However, aggregates of aCgnSH are more resistive than aCgn aggregates to urea-mediated dissociation, suggesting some degree of structural differences in the aggregated species that was not resolvable in detail without higher resolution methods. Mechanistic analyses of aggregation kinetics indicate that the initiation or nucleation of new aggregates from aCgnSH involves a mono-molecular rate limiting step, possibly the unfolding step. In contrast, that for aCgn involves an oligomeric intermediate, suggesting native disulfide linkages help to hinder non-native protein aggregation by providing conformational barriers to key nucleation event(s).

Aggregates of α-chymotrypsinogen anneal to access more stable states

Non-native protein aggregates present a variety of problems in fundamental and applied biochemistry and biotechnology, from quality and safety issues in pharmaceutical development to their association with a number of chronic diseases. The aggregated, often amyloid, protein state is often considered to be more thermodynamically and kinetically stable than (partially) unfolded or folded monomers except under highly denaturing conditions. However, evolution of the structure and stability of aggregated states has received much less attention. Here it is shown that under mildly-denaturing conditions (elevated temperature or [urea]), where the native monomer (N) is slightly favored compared to the unfolded state (U), α-chymotrypsinogen A (aCgn) non-native aggregates undergo a structural relaxation or annealing process to reach even more stable states. The annealed aggregates are more resistant to dissociation than aggregates that do not undergo this relaxation process. Aggregates without annealing dissociate via linear chain depolymerization, and annealing is accelerated under conditions that promote slow dissociation (partially denaturing conditions). This is consistent with a free energy landscape with multiple barriers and local minima that allows for a kinetic competition between aggregate dissociation and structural relaxation to more stable aggregate states. This highlights added complexities for protein refolding or aggregate dissociation processes, and may explain why it is often difficult to completely recover monomeric protein from aggregates.

Production, preliminary characterization, and bioactivities of exopolysaccharides from Pleurotus geesteranus 5(#)

The optimal culture conditions of exopolysaccharides (EPS) production in submerged culture medium by Pleurotus geesteranus 5(#) were determined using an orthogonal matrix method. The optimal defined medium (per liter) was 60.0 g maltose, 5.0 g tryptone, 1 mM NaCl, 5 mM KH(2)PO(4), and initial pH 6.0 at 28 °C. In the optimal culture medium, the maximum EPS production was 16.97 g/L in a shake flask. Two groups of EPSs (designated as Fr-I and Fr-II) were obtained from the culture filtrates by size exclusion chromatography (SEC), and their molecular characteristics were examined by a multiangle laser-light scattering (MALLS) and refractive index (RI) detector system. The approximate weight-average molar masses of the Fr-I and Fr-II of EPS were determined to be 3.263 × 10(4) and 5.738 × 10(3) g/mol, respectively. The low values of polydispersity ratio (1.176 and 1.124 for Fr-I and Fr-II, respectively) of EPSs mean that these EPS molecules exist much less dispersed in aqueous solution without forming large aggregates. Furthermore, the experiments in vitro indicated that P. geesteranus 5(#) EPS exhibit high antitumor and antioxidative effects.

Characterization of Fc-fusion protein aggregates derived from extracellular domain disulfide bond rearrangements

Aggregation of protein biotherapeutics has consequences for decreasing production and has been implicated in immunogenicity. The mechanisms of protein aggregation vary depending on the protein and the expression system utilized, making it difficult to elucidate the conditions that promote their formation. Nonnative aggregation of recombinant immunoglobulin G protein therapeutics from mammalian expression systems has been extensively studied. To better understand the mechanisms behind aggregation of glycosylated fusion proteins produced in Chinese hamster ovarian cells, we have examined the high-molecular-weight (HMW) species of activin receptor-like kinase 1 Fc fusion protein. Size-exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicate that two populations of aggregate exist: (1) nondisulfide-linked, higher-order aggregates and (2) disulfide-linked oligomers. The largest aggregated species have increased nonnative structure, whereas the smallest aggregated species maintain structure similar to monomer. The HMW species display decreased levels of O-linked glycosylation, higher occupancy of high-mannose N-linked oligosaccharide structures, and overall less sialylation as their size increases. Disulfide-linked aggregate species were found to associate through the extracellular domain. N-linked glycosylation on the extracellular domain (ECD) appears to discourage disulfide-linked aggregation. Elucidation of the specific mechanisms behind disulfide-linked aggregate formation may assist in designing processes that limit aggregate formation in cell culture, with implications for increased production.

Antigen generation and display in therapeutic antibody drug discovery -- a neglected but critical player

Disease intervention by targeting a critical pathway molecule through a blocking antibody or interference by therapeutic proteins is currently en vogue. Generation of blocking antibodies or therapeutic proteins inevitably requires the production of recombinant proteins or cell-based immunogens. Thus, one could call the antigen molecule the neglected player in antibody drug discovery. The variety of methods available for making recombinant proteins or recombinant cell lines that present the target on the cell surface is extensive. These need to be addressed in conjunction with biochemical and biophysical quality criteria and the experimental application intended. Fundamentally, successful production and isolation of monoclonal antibodies requires optimized antigen preparation and presentation to the immune host. This review summarizes the most important aspects of antigen generation and display, enabling logical decision making to give rise to potent high-affinity antibodies.

Relating particle formation to salt- and pH-dependent phase separation of non-native aggregates of alpha-chymotrypsinogen A

Visible and subvisible particle formation during the storage of protein solutions is of increasing concern for pharmaceutical products. Previous work (Li Y, Ogunnaike BA, Roberts CJ. 2010. J Pharm Sci 99:645-662) showed that the model protein, alpha-chymotrypsinogen A (aCgn), forms non-native aggregates under accelerated (heated) conditions, but the size and morphology of the resulting aggregates depended sensitively on pH and NaCl. Here, it is shown that aggregates created as high-molecular-weight soluble aggregates undergo a pH- and salt-dependent reversible phase transition to a condensed or insoluble phase of suspended microparticles, whereas monomers remain completely soluble in the same regime. The location of the phase boundary is quantitatively consistent with the different regimes of kinetic behavior observed previously for aCgn. This suggests that the while kinetics is important for controlling the rates of monomer loss during non-native aggregation, it may be possible to tune solution thermodynamics and phase behavior to suppress otherwise soluble aggregates from propagating to form visible or large subvisible particles. Interestingly, the aggregate phase boundary is sensitive to the identity of salt anions in solution, highlighting the importance of electrostatics and preferential salt interactions in mediating aggregate condensation and particle formation.

Thermodynamics of amyloid dissociation provide insights into aggregate stability regimes

Amyloid aggregates have been hypothesized as a global low free energy state for proteins at finite concentrations. Near its midpoint unfolding temperature, α-chymotrypsinogen A (aCgn) spontaneously forms amyloid polymers, indicating the free energy of aggregates (A) is significantly lower than that for unfolded (U) and native (N) monomers at those particular conditions. The relative thermodynamic stability of A, U, and N states was estimated semi-quantitatively as a function of temperature (T) and [urea] via a combination of calorimetry, urea-assisted unfolding and dissociation, aggregation kinetics, and changes in solvent-exposed surface area, combined with thermodynamic integration and a linear transfer free energy model. The results at first suggest that N is more thermodynamically stable than A at sufficiently low T and [urea], but this may be convoluted with kinetic effects. Interestingly, the kinetic stability of aggregates highlights that the practical measure of stability may be the free energy barrier(s) between A and U, as U serves as a key intermediate between N and A states.