Aggregation Propensity of Serum Amyloid A1 in Captive Zoo and Free‐living Wild Animals

Abstract

AA Amyloidosis is the most common form of amyloid disease, occurring in humans, domestic animals, wildlife and captive zoo animals and is defined by the misfolding of serum amyloid A (SAA), an apolipoprotein of high-density lipoprotein (HDL). SAA is a positive acute phase protein that is synthesized by the liver during chronic states of inflammation or stress, depositing mainly in the liver, spleen and kidneys. AA amyloidosis poses the highest risk to zoo species within captivity given the diverse number of species susceptible to the disease, transmissibility of the disease, pre-disposing factors, and close housing conditions in zoo facilities. Susceptibility to AA amyloidosis may be dependent upon the propensity of SAA to aggregate in different species. Variation in the 104 amino acid sequence of the SAA protein may contribute to the likelihood that SAA will misfold under chronic conditions. While the proteolytic cleavage or removal of the C-terminal tail of SAA is known to result in formation of fibrils, the molecular mechanism of SAA aggregation is not completely understood. This study proposes to investigate areas of the SAA1 peptide that are likely to misfold and how mutation has led to evolutionary divergence and species-specific susceptibility to the disease. A bioinformatic analysis was performed to predict the aggregation score (Tango program). Homology and variance in each SAA1 peptide were demonstrated via sequence alignment prepared in BioEdit. A phylogenetic analysis prompted us to identify potential mutations for the design of a peptide library. Using bioinformatic (tango program) and thioflavin-T fluorescence assays, we studied the aggregation propensity of SAA1 fragments derived from amyloidogenic species and non-amyloidogenic species. Aggregation propensity in the first 1-25 amino acids was assessed via Tango to which SAA1 in humans, felids, mice and equids were predicted to be most susceptible to aggregation, while SAA1 in avian and reptile taxa were least likely to aggregate. A ThT experiment performed on fragments belonging to 8 different species demonstrated that fragments from felids, mice and humans are highly prone to aggregate and supportive of bioinformatic results. SAA1 fragment peptides derived from the rainbow trout and Chinese soft-shelled turtles exhibited a low propensity of aggregation. Mutation in the SAA1 peptide may contribute to the amyloidogenicity of the protein, which may help identify high-risk species and establish an evolutionary basis for the disease. Evaluation and identification of peptide residues that have a higher propensity to aggregate, in addition to the evaluation of conditions that may contribute to misfolding, will help direct the development of small molecule therapeutics that prevent AA amyloidosis.