Structural and functional consequences of age-related isomerization in α-crystallins

Stability of Protein Pharmaceuticals: Recent Advances

There have been significant advances in the formulation and stabilization of proteins in the liquid state over the past years since our previous review. Our mechanistic understanding of protein-excipient interactions has increased, allowing one to develop formulations in a more rational fashion. The field has moved towards more complex and challenging formulations, such as high concentration formulations to allow for subcutaneous administration and co-formulation. While much of the published work has focused on mAbs, the principles appear to apply to any therapeutic protein, although mAbs clearly have some distinctive features. In this review, we first discuss chemical degradation reactions. This is followed by a section on physical instability issues. Then, more specific topics are addressed: instability induced by interactions with interfaces, predictive methods for physical stability and interplay between chemical and physical instability. The final parts are devoted to discussions how all the above impacts (co-)formulation strategies, in particular for high protein concentration solutions.'

Quantification of serine residue stereoinversion in a short peptide by reversed-phase high-performance liquid chromatography: analysis of mechanisms promoting serine stereoinversion

Stereoinversion of Ser residues within proteins, which has been identified in long-lived proteins, influences protein function. To quantify the stereoinversion of Ser residues, we investigated the potential adaptation of our direct peptide analytical method originally established for analyzing the isomerization of asparaginyl/aspartyl residues. Peptide pairs containing L-Ser or D-Ser residues with lengths of four or five residues were synthesized. Separation conditions for these peptide pairs were systematically examined by precisely adjusting the pH of the elution solvent using reverse-phase high-performance liquid chromatography (HPLC). Optimal separation conditions were successfully developed for all peptide pairs, enabling the direct quantification of Ser residue stereoinversion through a single HPLC run. Subsequently, the degree of Ser stereoinversion within the model peptide, Gly-Ser-Gly-Tyr, was determined using the method established in this study. Surprisingly, the stereoinversion of Ser residues occurred only when the absolute configurations of Ser and Tyr residues of the peptide differed from each other, whereas no stereoinversion was observed when their absolute configurations were identical. The experiments using peptides similar to the model peptide reveal that both the N-terminal amino group and the hydroxyl group of the C-terminal Tyr residue are involved in the stereoinversion of the Ser residue. By applying a simple method to quantify the stereoinversion of Ser residues, valuable insights into the mechanisms governing these stereoinversions were obtained.

Probing effects of site-specific aspartic acid isomerization on structure and stability of GB1 through chemical protein synthesis

Chemical modifications of long-lived proteins, such as isomerization and epimerization, have been evoked as prime triggers for protein-damage related diseases. Deamidation of Asn residues, which results in formation of a mixture of l- and d-Asp and isoAsp via an intermediate aspartyl succinimide, can result in the disruption of cellular proteostasis and toxic protein depositions. In contrast to extensive data on the biological prevalence and functional implications of aspartyl succinimide formation, much less is known about the impact of the resulting altered backbone composition on properties of individual proteins at a molecular level. Here, we report the total chemical synthesis, biophysical characterization, and NMR structural analysis of a series of variants of the B1 domain of protein G from Streptococcal bacteria (GB1) in which all possible Asp isomers as well as an aspartyl succinimide were individually incorporated at a defined position in a solvent-exposed loop. Subtle local structural effects were observed; however, these were accompanied by notable differences in thermodynamic folded stability. Surprisingly, the noncanonical backbone connectivity of d-isoAsp led to a variant that exhibited enhanced stability relative to the natural protein.

Resolving D-Amino Acid Containing Peptides Using Ion Mobility-Mass Spectrometry: Challenges and Recent Developments

Peptides and proteins having D-amino acids in their sequences are now believed to be widespread among different living organisms. Their significance is attributed to the diverse functions of these molecules, such as having a certain pathological implication or enhancing biological activity. Indeed, some peptide molecules with D-amino acids in their structure have already found their way to clinical use such as the antibacterial gramicidin and the antidiabetic nateglinide. Ion mobility mass spectrometry (IM-MS) added an additional dimension of separation as it depends on ions mobility in the space, which is dependent on their shapes, and the shape depends on the orientation of atoms. Thus, D-amino acids containing peptides (DAACPs) will have different mobility and collision cross-section values than those with L-amino acids. Eventually, this will lead to baseline separation of the two peptides. Additionally, ion mobility can precisely locate the position of D-amino acids by analyzing the difference in the arrival times of the fragment ions. The importance of DAACPs, as well as the difficulties in discovering them, were addressed in this review. Similarly, we emphasized how recent developments in IM-MS have improved their detection and analysis. Consequently, the LC-IM-MS/MS platform appears to be promising in isomeric mixture analysis.

Teicoplanin aglycone media and carboxypeptidase Y: Tools for finding low-abundance D-amino acids and epimeric peptides

D-amino acids and epimeric peptides/proteins can play crucial biological roles and adversely affect protein folding and oligopeptide aggregation in age-related pathologies in humans. This has ignited interest in free D-amino acids as well as those incorporated in peptides/proteins and their effects in humans. However, such stereoisomeric analytes are often elusive and in low abundance with few existing methodologies capable of scouting for and identifying them. In this work, we examine the feasibility of using teicoplanin aglycone, a macrocyclic antibiotic, which has been reported to strongly retain D-amino acids and peptides with a D-amino acid on the C-terminus, for use as a solid phase extraction (SPE) medium. The HPLC retention factors of L-/D-amino acids and C-terminus modified D-amino acid-containing peptides and their L-amino acid exclusive counterparts on teicoplanin aglycone are presented. Retention curve differences between amino acids and peptides highlight regions of solvent composition that can be utilized for their separation. This approach is particularly useful when coupled with enzymatic hydrolysis via carboxypeptidase Y to eliminate all L-amino acid exclusive peptides. The remaining peptides with carboxy-terminal D-amino acids are then more easily concentrated and identified.

Influence of Asp Isomerization on Trypsin and Trypsin-like Proteolysis

Long-lived proteins (LLPs), although less common than their short-lived counterparts, are increasingly recognized to play important roles in age-related diseases such as Alzheimer's. In particular, spontaneous chemical modifications can accrue over time that serve as both indicators of and contributors to disrupted autophagy. For example, isomerization in LLPs is common and occurs in the absence of protein turnover while simultaneously interfering with the protein turnover by impeding proteolysis. In addition to the biological implications this creates, isomerization may also interfere with its own analysis. To clarify, bottom-up proteomics experiments rely on protein digestion by proteases, most commonly trypsin, but the extent to which isomerization might interfere with trypsin digestion is unknown. Here, we use a combination of liquid chromatography and mass spectrometry to examine the effect of isomerization on proteolysis by trypsin and chymotrypsin. Isomerized aspartic acid and serine residues (which represent the most common sites of isomerization in LLPs) were placed at various locations relative to the preferred protease cleavage point to evaluate the influence on digestion efficiency. Trypsin was found to be relatively tolerant of isomerization, except when present at the residue immediately C-terminal to Arg/Lys. For chymotrypsin, the influence of isomerization on digestion was less predictable, resulting in long-range interference for some isomer/peptide combinations. Given the trypsin- and chymotrypsin-like behaviors of the 20S proteasome, and to further establish the biological relevance of isomerization in LLPs, substrates with isomerized sites were also tested against proteasomal degradation. Significant disruption of 20S proteolysis was observed, suggesting that if LLPs persist long enough to isomerize, it will be difficult for the cells to digest them.

PIMT-Mediated Labeling of l-Isoaspartic Acid with Tris Facilitates Identification of Isomerization Sites in Long-Lived Proteins

Isomerization of individual residues in long-lived proteins (LLPs) is a subject of growing interest in connection with many age-related human diseases. When isomerization occurs in LLPs, it can lead to deleterious changes in protein structure, function, and proteolytic degradation. Herein, we present a novel labeling technique for rapid identification of l-isoAsp using the enzyme protein l-isoaspartyl methyltransferase (PIMT) and Tris. The succinimide intermediate formed during reaction of l-isoAsp-containing peptides with PIMT and S-adenosyl methionine (SAM) is reactive with Tris base and results in a Tris-modified aspartic acid residue with a mass shift of +103 Da. Tris-modified aspartic acid exhibits prominent and repeated neutral loss of water when subjected to collisional activation. In addition, another dissociation pathway regenerates the original peptide following loss of a characteristic mass shift. Furthermore, it is demonstrated that Tris modification can be used to identify sites of isomerization in LLPs from biological samples such as the lens of the eye. This approach simplifies identification by labeling isomerization sites with a tag that causes a mass shift and provides characteristic loss during collisional activation.

Protein Aggregation and Cataract: Role of Age-Related Modifications and Mutations in α-Crystallins

* The article is published as a part of the Special Issue "Protein Misfolding and Aggregation in Cataract Disorders" (Vol. 87, No. 2). ** To whom correspondence should be addressed. Cataract is a major cause of blindness. Due to the lack of protein turnover, lens proteins accumulate age-related and environmental modifications that alter their native conformation, leading to the formation of aggregation-prone intermediates, as well as insoluble and light-scattering aggregates, thus compromising lens transparency. The lens protein, α-crystallin, is a molecular chaperone that prevents protein aggregation, thereby maintaining lens transparency. However, mutations or post-translational modifications, such as oxidation, deamidation, truncation and crosslinking, can render α-crystallins ineffective and lead to the disease exacerbation. Here, we describe such mutations and alterations, as well as their consequences. Age-related modifications in α-crystallins affect their structure, oligomerization, and chaperone function. Mutations in α-crystallins can lead to the aggregation/intracellular inclusions attributable to the perturbation of structure and oligomeric assembly and resulting in the rearrangement of aggregation-prone regions. Such rearrangements can lead to the exposure of hitherto buried aggregation-prone regions, thereby populating aggregation-prone state(s) and facilitating amorphous/amyloid aggregation and/or inappropriate interactions with cellular components. Investigations of the mutation-induced changes in the structure, oligomer assembly, aggregation mechanisms, and interactomes of α-crystallins will be useful in fighting protein aggregation-related diseases.

Understanding and controlling amyloid aggregation with chirality

Amyloid aggregation and human disease are inextricably linked. Examples include Alzheimer disease, Parkinson disease, and type II diabetes. While seminal advances on the mechanistic understanding of these diseases have been made over the last decades, controlling amyloid fibril formation still represents a challenge, and it is a subject of active research. In this regard, chiral modifications have increasingly been proved to offer a particularly well-suited approach toward accessing to previously unknown aggregation pathways and to provide with novel insights on the biological mechanisms of action of amyloidogenic peptides and proteins. Here, we summarize recent advances on how the use of mirror-image peptides/proteins and d-amino acid incorporations have helped modulate amyloid aggregation, offered new mechanistic tools to study cellular interactions, and allowed us to identify key positions within the peptide/protein sequence that influence amyloid fibril growth and toxicity.

Association of Alpha-Crystallin with Fiber Cell Plasma Membrane of the Eye Lens Accompanied by Light Scattering and Cataract Formation

α-crystallin is a major protein found in the mammalian eye lens that works as a molecular chaperone by preventing the aggregation of proteins and providing tolerance to stress in the eye lens. These functions of α-crystallin are significant for maintaining lens transparency. However, with age and cataract formation, the concentration of α-crystallin in the eye lens cytoplasm decreases with a corresponding increase in the membrane-bound α-crystallin, accompanied by increased light scattering. The purpose of this review is to summarize previous and recent findings of the role of the: (1) lens membrane components, i.e., the major phospholipids (PLs) and sphingolipids, cholesterol (Chol), cholesterol bilayer domains (CBDs), and the integral membrane proteins aquaporin-0 (AQP0; formally MIP26) and connexins, and (2) α-crystallin mutations and post-translational modifications (PTMs) in the association of α-crystallin to the eye lens's fiber cell plasma membrane, providing thorough insights into a molecular basis of such an association. Furthermore, this review highlights the current knowledge and need for further studies to understand the fundamental molecular processes involved in the association of α-crystallin to the lens membrane, potentially leading to new avenues for preventing cataract formation and progression.

α-Crystallins in the Vertebrate Eye Lens: Complex Oligomers and Molecular Chaperones

α-Crystallins are small heat-shock proteins that act as holdase chaperones. In humans, αA-crystallin is expressed only in the eye lens, while αB-crystallin is found in many tissues. α-Crystallins have a central domain flanked by flexible extensions and form dynamic, heterogeneous oligomers. Structural models show that both the C- and N-terminal extensions are important for controlling oligomerization through domain swapping. α-Crystallin prevents aggregation of damaged β- and γ-crystallins by binding to the client protein using a variety of binding modes. α-Crystallin chaperone activity can be compromised by mutation or posttranslational modifications, leading to protein aggregation and cataract. Because of their high solubility and their ability to form large, functional oligomers, α-crystallins are particularly amenable to structure determination by solid-state nuclear magnetic resonance (NMR) and solution NMR, as well as cryo-electron microscopy.

Chemical Properties Determine Solubility and Stability in βγ-Crystallins of the Eye Lens

βγ-Crystallins are the primary structural and refractive proteins found in the vertebrate eye lens. Because crystallins are not replaced after early eye development, their solubility and stability must be maintained for a lifetime, which is even more remarkable given the high protein concentration in the lens. Aggregation of crystallins caused by mutations or post-translational modifications can reduce crystallin protein stability and alter intermolecular interactions. Common post-translational modifications that can cause age-related cataracts include deamidation, oxidation, and tryptophan derivatization. Metal ion binding can also trigger reduced crystallin solubility through a variety of mechanisms. Interprotein interactions are critical to maintaining lens transparency: crystallins can undergo domain swapping, disulfide bonding, and liquid-liquid phase separation, all of which can cause opacity depending on the context. Important experimental techniques for assessing crystallin conformation in the absence of a high-resolution structure include dye-binding assays, circular dichroism, fluorescence, light scattering, and transition metal FRET.

Imbalances in the eye lens proteome are linked to cataract formation

The prevalent model for cataract formation in the eye lens posits that damaged crystallin proteins form light-scattering aggregates. The α-crystallins are thought to counteract this process as chaperones by sequestering misfolded crystallin proteins. In this scenario, chaperone pool depletion would result in lens opacification. Here we analyze lenses from different mouse strains that develop early-onset cataract due to point mutations in α-, β-, or γ-crystallin proteins. We find that these mutant crystallins are unstable in vitro; in the lens, their levels are substantially reduced, and they do not accumulate in the water-insoluble fraction. Instead, all the other crystallin proteins, including the α-crystallins, are found to precipitate. The changes in protein composition and spatial organization of the crystallins observed in the mutant lenses suggest that the imbalance in the lenticular proteome and altered crystallin interactions are the bases for cataract formation, rather than the aggregation propensity of the mutant crystallins.

Differentiation of peptide isomers by excited-state photodissociation and ion-molecule interactions

Solvochromatic effects are most frequently associated with solution-phase phenomena. However, in the gas phase, the absence of solvent leads to intramolecular solvation that can be driven by strong forces including hydrogen bonds and ion-dipole interactions. Here we examine whether isomerization of a single residue in a peptide results in structural changes sufficient to shift the absorption of light by an appended chromophore. By carrying out the experiments inside a mass spectrometer, we can easily monitor photodissociation yield as a readout for chromophore excitation. A series of peptides of different lengths, charge states, and position and identity of the isomerized residue were examined by excitation with both 266 and 213 nm light. The results reveal that differences in intramolecular solvation do lead to solvochromatic shifts in many cases. In addition, the primary product following photoexcitation is a radical. Ion-molecule reactions with this radical and adventitious oxygen were monitored and also found to vary as a function of isomeric state. In this case, differences in intramolecular solvation alter the availability of the reactive radical. Overall, the results reveal that small changes in a single amino acid can influence the overall structural ensemble sufficient to alter the efficiency of multiple gas-phase reactions.

The Aggregation of αB-Crystallin under Crowding Conditions Is Prevented by αA-Crystallin: Implications for α-Crystallin Stability and Lens Transparency

One of the most crowded biological environments is the eye lens which contains a high concentration of crystallin proteins. The molecular chaperones αB-crystallin (αBc) with its lens partner αA-crystallin (αAc) prevent deleterious crystallin aggregation and cataract formation. However, some forms of cataract are associated with structural alteration and dysfunction of αBc. While many studies have investigated the structure and function of αBc under dilute in vitro conditions, the effect of crowding on these aspects is not well understood despite its in vivo relevance. The structure and chaperone ability of αBc under conditions that mimic the crowded lens environment were investigated using the polysaccharide Ficoll 400 and bovine γ-crystallin as crowding agents and a variety of biophysical methods, principally contrast variation small-angle neutron scattering. Under crowding conditions, αBc unfolds, increases its size/oligomeric state, decreases its thermal stability and chaperone ability, and forms kinetically distinct amorphous and fibrillar aggregates. However, the presence of αAc stabilizes αBc against aggregation. These observations provide a rationale, at the molecular level, for the aggregation of αBc in the crowded lens, a process that exhibits structural and functional similarities to the aggregation of cataract-associated αBc mutants R120G and D109A under dilute conditions. Strategies that maintain or restore αBc stability, as αAc does, may provide therapeutic avenues for the treatment of cataract.

Two-dimensional identification and localization of isomers in crystallin peptides using TWIM-MS

Recent studies have illuminated connections between spontaneous chemical reactions that cause isomerization at specific protein residues and various age-related diseases including cataracts and Alzheimer's. These discoveries provide impetus for better analytical methods to detect and characterize isomerization in proteins, which will enable a more complete understanding of the underlying relationship between these modifications and biology. Herein we employ a two-dimensional approach for identification of peptides isomers that also includes pinpointing of the modified residue. Collision-induced dissociation is used to fragment ions in the first dimension, followed by separation of the fragments with travelling-wave ion mobility. By comparing data obtained from both isomers, differences in either fragment-ion intensities or arrival-time distributions can be used to identify isomeric forms and the specific site of modification within the peptides. Synthetic peptide standards with sequences derived from long-lived proteins in the eye lens and isomerization at serine, aspartic acid, and glutamic acid were examined. Although both dimensions are capable of isomer identification, ion mobility is much better at determining the site of modification. In general, separation of isomeric forms by ion mobility is possible but does not follow predictable trends dictated by sequence or fragment-ion length. In most cases, however, the site of isomerization can be precisely determined.

Recent progress in the analysis of protein deamidation using mass spectrometry

Deamidation is a nonenzymatic and spontaneous posttranslational modification (PTM) that introduces changes in both structure and charge of proteins, strongly associated with aging proteome instability and degenerative diseases. Deamidation is also a common PTM occurring in biopharmaceutical proteins, representing a major cause of degradation. Therefore, characterization of deamidation alongside its inter-related modifications, isomerization and racemization, is critically important to understand their roles in protein stability and diseases. Mass spectrometry (MS) has become an indispensable tool in site-specific identification of PTMs for proteomics and structural studies. In this review, we focus on the recent advances of MS analysis in protein deamidation. In particular, we provide an update on sample preparation, chromatographic separation, and MS technologies at multi-level scales, for accurate and reliable characterization of protein deamidation in both simple and complex biological samples, yielding important new insight on how deamidation together with isomerization and racemization occurs. These technological progresses will lead to a better understanding of how deamidation contributes to the pathology of aging and other degenerative diseases and the development of biopharmaceutical drugs.

Long-lived metabolic enzymes in the crystalline lens identified by pulse-labeling of mice and mass spectrometry

The lenticular fiber cells are comprised of extremely long-lived proteins while still maintaining an active biochemical state. Dysregulation of these activities has been implicated in diseases such as age-related cataracts. However, the lenticular protein dynamics underlying health and disease is unclear. We sought to measure the global protein turnover rates in the eye using nitrogen-15 labeling of mice and mass spectrometry. We measured the ¹⁴N/¹⁵N-peptide ratios of 248 lens proteins, including Crystallin, Aquaporin, Collagen and enzymes that catalyze glycolysis and oxidation/reduction reactions. Direct comparison of lens cortex versus nucleus revealed little or no ¹⁵N-protein contents in most nuclear proteins, while there were a broad range of ¹⁴N/¹⁵N ratios in cortex proteins. Unexpectedly, like Crystallins, many enzymes with relatively high abundance in nucleus were also exceedingly long-lived. The slow replacement of these enzymes in spite of young age of mice suggests their potential roles in age-related metabolic changes in the lens.

Isomerization as the secret Achilles' heel of long-lived proteins

Crystallin proteins, the dominant constituents of the eye lens, are prototypes of long-lived proteins. Such proteins can accumulate harmful modifications over their life span that render them prone to aggregation, which, in the case of lens crystallin, contributes to cataract formation. Lyon et al. now explore the structural and functional consequences of amino acid isomerization in α-crystallins using mass spectrometry, molecular dynamics simulations, and other strategies. Their results highlight the potential deleterious effects of these under-detected modifications on protein structural integrity and function.

Spatiotemporal changes in the human lens proteome: Critical insights into long-lived proteins

The ocular lens is a unique tissue that contains an age gradient of cells and proteins ranging from newly differentiated cells containing newly synthesized proteins to cells and proteins that are as old as the organism. Thus, the ocular lens is an excellent model for studying long-lived proteins (LLPs) and the effects of aging and post-translational modifications on protein structure and function. Given the architecture of the lens, with young fiber cells in the outer cortex and the oldest cells in the lens nucleus, spatially-resolved studies provide information on age-specific protein changes. In this review, experimental strategies and proteomic methods that have been used to examine age-related and cataract-specific changes to the human lens proteome are described. Measured spatio-temporal changes in the human lens proteome are summarized and reveal a highly consistent, time-dependent set of modifications observed in transparent human lenses. Such measurements have led to the discovery of cataract-specific modifications and the realization that many animal systems are unsuitable to study many of these modifications. Mechanisms of protein modifications such as deamidation, racemization, truncation, and protein-protein crosslinking are presented and the implications of such mechanisms for other long-lived proteins in other tissues are discussed in the context of age-related neurological diseases. A comprehensive understanding of LLP modifications will enhance our ability to develop new therapies for the delay, prevention or reversal of age-related diseases.

Analysis of Glutamine Deamidation: Products, Pathways, and Kinetics

Spontaneous chemical modifications play an important role in human disease and aging at the molecular level. Deamidation and isomerization are known to be among the most prevalent chemical modifications in long-lived human proteins and are implicated in a growing list of human pathologies, but the relatively minor chemical change associated with these processes has presented a long standing analytical challenge. Although the adoption of high-resolution mass spectrometry has greatly aided the identification of deamidation sites in proteomic studies, isomerization (and the isomeric products of deamidation) remain exceptionally challenging to characterize. Herein, we present a liquid chromatography/mass spectrometry-based approach for rapidly characterizing the isomeric products of Gln deamidation using diagnostic fragments that are abundantly produced and capable of unambiguously identifying both Glu and isoGlu. Importantly, the informative fragment ions are produced through orthogonal fragmentation pathways, thereby enabling the simultaneous detection of both isomeric forms while retaining compatibility with shotgun proteomics. Furthermore, the diagnostic fragments associated with isoGlu pinpoint the location of the modified residue. The utility of this technique is demonstrated by characterizing the isomeric products generated during in vitro aging of a series of glutamine-containing peptides. Sequence-dependent product profiles are obtained, and the abundance of deamidation-linked racemization is examined. Finally, comparisons are made between Gln deamidation, which is relatively poorly understood, and asparagine deamidation, which has been more thoroughly studied.

Spontaneous Isomerization of Long-Lived Proteins Provides a Molecular Mechanism for the Lysosomal Failure Observed in Alzheimer's Disease

Proteinaceous aggregation is a well-known observable in Alzheimer's disease (AD), but failure and storage of lysosomal bodies within neurons is equally ubiquitous and actually precedes bulk accumulation of extracellular amyloid plaque. In fact, AD shares many similarities with certain lysosomal storage disorders though establishing a biochemical connection has proven difficult. Herein, we demonstrate that isomerization and epimerization, which are spontaneous chemical modifications that occur in long-lived proteins, prevent digestion by the proteases in the lysosome (namely, the cathepsins). For example, isomerization of aspartic acid into l-isoAsp prevents digestion of the N-terminal portion of Aβ by cathepsin L, one of the most aggressive lysosomal proteases. Similar results were obtained after examination of various target peptides with a full series of cathepsins, including endo-, amino-, and carboxy-peptidases. In all cases peptide fragments too long for transporter recognition or release from the lysosome persisted after treatment, providing a mechanism for eventual lysosomal storage and bridging the gap between AD and lysosomal storage disorders. Additional experiments with microglial cells confirmed that isomerization disrupts proteolysis in active lysosomes. These results are easily rationalized in terms of protease active sites, which are engineered to precisely orient the peptide backbone and cannot accommodate the backbone shift caused by isoaspartic acid or side chain dislocation resulting from epimerization. Although Aβ is known to be isomerized and epimerized in plaques present in AD brains, we further establish that the rates of modification for aspartic acid in positions 1 and 7 are fast and could accrue prior to plaque formation. Spontaneous chemistry can therefore provide modified substrates capable of inducing gradual lysosomal failure, which may play an important role in the cascade of events leading to the disrupted proteostasis, amyloid formation, and tauopathies associated with AD.

The l-isoaspartate modification within protein fragments in the aging lens can promote protein aggregation

Transparency in the lens is accomplished by the dense packing and short-range order interactions of the crystallin proteins in fiber cells lacking organelles. These features are accompanied by a lack of protein turnover, leaving lens proteins susceptible to a number of damaging modifications and aggregation. The loss of lens transparency is attributed in part to such aggregation during aging. Among the damaging post-translational modifications that accumulate in long-lived proteins, isomerization at aspartate residues has been shown to be extensive throughout the crystallins. In this study of the human lens, we localize the accumulation of l-isoaspartate within water-soluble protein extracts primarily to crystallin peptides in high-molecular weight aggregates and show with MS that these peptides are from a variety of crystallins. To investigate the consequences of aspartate isomerization, we investigated two αA crystallin peptides ⁵²LFRTVLDSGISEVR⁶⁵ and ⁸⁹VQDDFVEIH⁹⁸, identified within this study, with the l-isoaspartate modification introduced at Asp⁵⁸ and Asp⁹¹, respectively. Importantly, whereas both peptides modestly increase protein precipitation, the native ⁵²LFRTVLDSGISEVR⁶⁵ peptide shows higher aggregation propensity. In contrast, the introduction of l-isoaspartate within a previously identified anti-chaperone peptide from water-insoluble aggregates, αA crystallin ⁶⁶SDRDKFVIFL(isoAsp)VKHF⁸⁰, results in enhanced amyloid formation in vitro The modification of this peptide also increases aggregation of the lens chaperone αB crystallin. These findings may represent multiple pathways within the lens wherein the isomerization of aspartate residues in crystallin peptides differentially results in peptides associating with water-soluble or water-insoluble aggregates. Here the eye lens serves as a model for the cleavage and modification of long-lived proteins within other aging tissues.