Evolutionary and structural constraints influencing apolipoprotein A-I amyloid behavior.

Apolipoprotein A-I (apoA-I) has a key function in the reverse cholesterol transport. However, aggregation of apoA-I single point mutants can lead to hereditary amyloid pathology. Although several studies have tackled the biophysical and structural consequences introduced by these mutations, there is little information addressing the relationship between the evolutionary and structural features that contribute to the amyloid behavior of apoA-I. We combined evolutionary studies, in silico mutagenesis and molecular dynamics (MD) simulations to provide a comprehensive analysis of the conservation and pathogenic role of the aggregation-prone regions (APRs) present in apoA-I. Sequence analysis demonstrated that among the four amyloidogenic regions described for human apoA-I, only two (APR1 and APR4) are evolutionary conserved across different species of Sarcopterygii. Moreover, stability analysis carried out with the FoldX engine showed that APR1 contributes to the marginal stability of apoA-I. Structural properties of full-length apoA-I models suggest that aggregation is avoided by placing APRs into highly packed and rigid portions of its native fold. Compared to silent variants extracted from the gnomAD database, the thermodynamic and pathogenic impact of amyloid mutations showed evidence of a higher destabilizing effect. MD simulations of the amyloid variant G26R evidenced the partial unfolding of the alpha-helix bundle with the concomitant exposure of APR1 to the solvent, suggesting an insight into the early steps involved in its aggregation. Our findings highlight APR1 as a relevant component for apoA-I structural integrity and emphasize a destabilizing effect of amyloid variants that leads to the exposure of this region.

Understanding the role of apolipoproteinA-I in atherosclerosis. Post-translational modifications synergize dysfunction?

BACKGROUND: The identification of dysfunctional human apolipoprotein A-I (apoA-I) in atherosclerotic plaques suggests that protein structure and function may be hampered under a chronic pro inflammatory scenario. Moreover, the fact that natural mutants of this protein elicit severe cardiovascular diseases (CVD) strongly indicates that the native folding could shift due to the mutation, yielding a structure more prone to misfold or misfunction. To understand the events that determine the failure of apoA-I structural flexibility to fulfill its protective role, we took advantage of the study of a natural variant with a deletion of the residue lysine 107 (K107del) associated with atherosclerosis. METHODS: Biophysical approaches, such as electrophoresis, fluorescence and spectroscopy were used to characterize proteins structure and function, either in native conformation or under oxidation or intramolecular crosslinking. RESULTS: K107del structure was more flexible than the protein with the native sequence (Wt) but interactions with artificial membranes were preserved. Instead, structural restrictions by intramolecular crosslinking impaired the Wt and K107del lipid solubilization function. In addition, controlled oxidation decreased the yield of the native dimer conformation for both variants. CONCLUSIONS: We conclude that even though mutations may alter protein structure and spatial arrangement, the highly flexible conformation compensates the mild shift from the native folding. Instead, post translational apoA-I modifications (probably chronic and progressive) are required to raise a protein conformation with significant loss of function and increased aggregation tendency. GENERAL SIGNIFICANCE: The results learnt from this variant strength a close association between amyloidosis and atherosclerosis.

Dataset of the construction and characterization of stable biological nanoparticles.

This article shows the dataset of clearance assays and the reconstitution of stable biological nano-complexes using both detergent-assisted and spontaneous solubilization of phospholipids by the recombinant purified apolipoprotein A-I (apoA-I). Protein was intra-chain crosslinked in order to introduce steric constrains. Then, native and crosslinked protein function was evaluated by a data collection of dimiristoyl phosphatidyl choline (DMPC) micellization curves. Additionally, resulting particles from spontaneous or detergent-assisted lipid solubilization were characterized by transmission electron microscopy (TEM), size exclusion chromatography (SEC), and native polyacrylamide gel electrophoresis (PAGE). Here we set up an experimental design that may help study protein structure based on its function, since interaction with biological membranes and lipids is an intrinsic activity attributed to many proteins in circulation. In addition, by t-test analysis of collected-data, we examined the formation of lipoprotein particles by native and intra-chain crosslinked proteins under different conditions like temperature and time incubation. Thus, data shown here strengthen the usefulness of an easy, rapid, accessible and inexpensive approach to test protein flexibility related to its function.

Data regarding the sensibility to proteolysis of a natural apolipoprotein A-I mutant.

The article shows dataset of the proteolysis of a natural variant of apolipoprotein A-I (apoA-I) with a substitution of a leucine by and arginine in position 60 (L60R), in comparison with the protein with the native sequence (Wt). This information demonstrates the potential of in vitro partial proteolysis experiments as it may be applicable to different approaches in the biophysical field. We have analyzed by different electrophoresis techniques apoA-I variants, quantified the degree of proteolysis after staining and compared the proteolysis efficiency with the computed cleavage patterns. The data shown here clearly strengthen the usefulness of this approach to test protein flexibility, as it may be attained with enzymes which are not expected to modify in vivo this protein but have a well-known digestion pattern. In addition it is appropriate for evaluating protein catabolism, as it is exemplified here by the evidence with metalloproteinase 12 (MMP-12), which is a physiological protease that may elicit the pro-inflammatory processing of this variant within the lesions. We support the work "Structural analysis of a natural apolipoprotein A-I variant (L60R) associated with amyloidosis" (Gaddi, et al., 2020), gaining insights on protein folding from a characterization by proteolysis analysis [1].

Structural analysis of a natural apolipoprotein A-I variant (L60R) associated with amyloidosis.

The reason that determines the pathological deposition of human apolipoprotein A-I variants inducing organ failure has been under research since the early description of natural mutations in patients. To shed light into the events associated with protein aggregation, we studied the structural perturbations that may occur in the natural variant that shows a substitution of a Leucine by an Arginine in position 60 (L60R). Circular dichroism, intrinsic fluorescence measurements, and proteolysis analysis indicated that L60R was more unstable, more sensitive to cleavage and the N-terminus was more disorganized than the protein with the native sequence (Wt). A higher tendency to aggregate was also detected when L60R was incubated at physiological pH. In addition, the small structural rearrangement observed for the freshly folded variant led to the release of tumor necrosis factor-α and interleukin-1ß from a model of macrophages. However, the mutant preserved both its dimeric conformation and its lipid-binding capacity. Our results strongly suggest that the chronic disease may be a consequence of the native conformation loss which elicits the release of protein conformations that could be either cytotoxic or precursors of amyloid conformations.

Fibrillar conformation of an apolipoprotein A-I variant involved in amyloidosis and atherosclerosis.

BACKGROUND: Different protein conformations may be involved in the development of clinical manifestations associated with human amyloidosis. Although a fibrillar conformation is usually the signature of damage in the tissues of patients, it is not clear whether this species is per se the cause or the consequence of the disease. Hereditary amyloidosis due to variants of apolipoprotein A-I (apoA-I) with a substitution of a single amino acid is characterized by the presence of fibrillar protein within the lesions. Thus mutations result in increased protein aggregation. Here we set up to characterize the folding of a natural variant with a mutation leading to a deletion at position 107 (apoA-I Lys107-0). Patients carrying this variant show amyloidosis and severe atherosclerosis. METHODS: We oxidized this variant under controlled concentrations of hydrogen peroxide and analyzed the structure obtained after 30-day incubation by fluorescence, circular dichroism and microscopy approaches. Neutrophils activation was characterized by confocal microscopy. RESULTS: We obtained a high yield of well-defined stable fibrillar structures of apoA-I Lys107-0. In an in vitro neutrophils system, we were able to detect the induction of Neutrophils Extracellular Traps (NETs) when we incubated with oxidized apoA-I variants. This effect was exacerbated by the fibrillar structure of oxidized Lys 107-0. CONCLUSIONS: We conclude that a pro-inflammatory microenvironment could result in the formation of aggregation-prone species, which, in addition may induce a positive feed-back in the activation of an inflammatory response. GENERAL SIGNIFICANCE: These events may explain a close association between amyloidosis due to apoA-I Lys107-0 and atherosclerosis.