The Toxicological Intersection between Allergen and Toxin: A Structural Comparison of the Cat Dander Allergenic Protein Fel d1 and the Slow Loris Brachial Gland Secretion Protein

Mapping Binding Domains of Viral and Allergenic Proteins with Dual-Cleavable Cross-Linking Technology

The dual-cleavable nature of the cross-linking technology (DUCCT) enhances the reliable identification of cross-linked peptides via mass spectrometry. The DUCCT approach uses a cross-linking agent that can be selectively cleaved by two different tandem mass spectrometry techniques: collision-induced dissociation (CID) and electron transfer dissociation (ETD). This results in distinct signatures in two independent mass spectra for the same cross-linked precursor, leading to unambiguous identification and the validation of the spectra. In this study, we expanded the application of the DUCCT cross-linker to evaluate the binding domains of a specific cat dander allergen, Fel d 1, which exists as the Fel d 1 A and B protein complex, and a viral spike protein from SARS-CoV-2, which invades host cells. To assess the cross-linked products obtained by DUCCT, we utilized a software tool called Cleave-XL, which effectively identified cross-linked sites using data from CID and ETD. Dual cleavable cross-linking studies identified cross-linked peptides in these complexes, which have been reported in bioinformatics analysis and proposed for immunotherapy using synthetic peptides. A benchmark study was also conducted using a commercial cross-linker disuccinimidyl suberate (DSS). Overall, we expect that DUCCT cross-linking technology will greatly facilitate the rapid screening of binding interfaces, thereby advancing structural biology and cell signaling investigations.

Genetic diversity of the major cat allergen, Fel d 1

Cat allergy affects ∼15% of the US population and can cause severe symptoms, including asthma. The major cat allergen, Fel d 1, drives IgE antibody responses. We conducted a comparative analysis of Fel d 1 genes, CH1 and CH2, and investigated structural features of Fel d 1 homologs across the family Felidae. The CH1 and CH2 domestic cat DNA references were used to identify homologous sequences in domestic and exotic cat genomes. Variability of these sequences within or across cat species was analyzed. Comprehensive alignments of Fel d 1 sequences and homologs from 276 domestic or exotic cats identified >100 unique, dissimilar substitutions in the protein sequences across Felidae. Selective pressure analyses of 37 exotic cat species revealed that Fel d 1 experienced positive selection, or greater variability over time, in CH1 and CH2. Linear regression of the mean pairwise identities of Fel d 1 DNA or protein sequences indicated that the genes largely reflected the evolution of Felidae. The Fel d 1 genes are highly variable (41 and 58% of the amino acid residues encoded by CH1 and CH2, respectively), suggesting that the biological function of Fel d 1, which is currently unknown, may vary among cat species and/or that Fel d 1 may be nonessential for cats. This is supported by Fel d 1 homology to nonessential proteins and recent evidence of healthy cats with CRISPR-edited CH2 genes. Fel d 1 variability could confer an evolutionary advantage for cats by allowing the allergen to bind different physiological ligands.

Evaluation of anti-Fel d 1 IgY ingredient for pet food on growth performance in kittens

Introduction

The domestic cat (Felis catus) is one of the most common pets. Worldwide, approximately one in five adults are sensitive to cat allergens. The major cat allergen is the secretoglobulin Fel d 1, which is primarily produced in the salivary and sebaceous glands. Chickens produce IgY antibodies, which are similar in structure to mammalian IgG. When chickens are exposed to Fel d 1, anti-Fel d 1-specific IgY (AFD1) is produced and is naturally concentrated in egg yolk. The aim of this study was to evaluate the tolerability, effects on growth and food consumption, and potential adverse effects of a chicken egg product ingredient containing AFD1 in kittens.

Methods

This was a blinded, controlled study. Twenty-seven (27) eight-week old kittens were randomly assigned to three feeding groups containing 0 ppm AFD1 (Group 0), 8 ppm AFD1 (Group 1), and 16 ppm AFD1 (Group 2) for 84 days. Veterinary exams and bloodwork were performed on Day 42 and Day 84, and body weight and body condition score (BCS) were monitored weekly.

Results

Throughout the study, there were no signs of nutritional deficiency or adverse clinical events in any of the subjects. Administration of a chicken egg product ingredient containing AFD1 in the diet (whether in coating or combination of coating and top dress) had no significant effect on body weight nor food consumption, and all subjects maintained a healthy Body Condition Score (BCS) throughout the study. Moreover, there were no biologically significant differences in the mean clinical chemistry and hematology parameters.

Discussion

This study demonstrated that a diet formulated to contain up to 16  ppm AFD1, included in the coating and the top-dress of dry kitten food, was well tolerated, promoted adequate growth, and exhibited no adverse effects.

Slowly Making Sense: A Review of the Two-Step Venom System within Slow (Nycticebus spp.) and Pygmy Lorises (Xanthonycticebus spp.)

Since the early 2000s, studies of the evolution of venom within animals have rapidly expanded, offering new revelations on the origins and development of venom within various species. The venomous mammals represent excellent opportunities to study venom evolution due to the varying functional usages, the unusual distribution of venom across unrelated mammals and the diverse variety of delivery systems. A group of mammals that excellently represents a combination of these traits are the slow (Nycticebus spp.) and pygmy lorises (Xanthonycticebus spp.) of south-east Asia, which possess the only confirmed two-step venom system. These taxa also present one of the most intriguing mixes of toxic symptoms (cytotoxicity and immunotoxicity) and functional usages (intraspecific competition and ectoparasitic defence) seen in extant animals. We still lack many pieces of the puzzle in understanding how this venom system works, why it evolved what is involved in the venom system and what triggers the toxic components to work. Here, we review available data building upon a decade of research on this topic, focusing especially on why and how this venom system may have evolved. We discuss that research now suggests that venom in slow lorises has a sophisticated set of multiple uses in both intraspecific competition and the potential to disrupt the immune system of targets; we suggest that an exudate diet reveals several toxic plants consumed by slow and pygmy lorises that could be sequestered into their venom and which may help heal venomous bite wounds; we provide the most up-to-date visual model of the brachial gland exudate secretion protein (BGEsp); and we discuss research on a complement component 1r (C1R) protein in saliva that may solve the mystery of what activates the toxicity of slow and pygmy loris venom. We conclude that the slow and pygmy lorises possess amongst the most complex venom system in extant animals, and while we have still a lot more to understand about their venom system, we are close to a breakthrough, particularly with current technological advances.

The Fast and the Furriest: Investigating the Rate of Selection on Mammalian Toxins

The evolution of venom and the selection pressures that act on toxins have been increasingly researched within toxinology in the last two decades, in part due to the exceptionally high rates of diversifying selection observed in animal toxins. In 2015, Sungar and Moran proposed the 'two-speed' model of toxin evolution linking evolutionary age of a group to the rates of selection acting on toxins but due to a lack of data, mammals were not included as less than 30 species of venomous mammal have been recorded, represented by elusive species which produce small amounts of venom. Due to advances in genomics and transcriptomics, the availability of toxin sequences from venomous mammals has been increasing. Using branch- and site-specific selection models, we present the rates of both episodic and pervasive selection acting upon venomous mammal toxins as a group for the first time. We identified seven toxin groups present within venomous mammals, representing Chiroptera, Eulipotyphla and Monotremata: KLK1, Plasminogen Activator, Desmallipins, PACAP, CRiSP, Kunitz Domain One and Kunitz Domain Two. All but one group (KLK1) was identified by our results to be evolving under both episodic and pervasive diversifying selection with four toxin groups having sites that were implicated in the fitness of the animal by TreeSAAP (Selection on Amino Acid Properties). Our results suggest that venomous mammal ecology, behaviour or genomic evolution are the main drivers of selection, although evolutionary age may still be a factor. Our conclusion from these results indicates that mammalian toxins are following the two-speed model of selection, evolving predominately under diversifying selection, fitting in with other younger venomous taxa like snakes and cone snails-with high amounts of accumulating mutations, leading to more novel adaptions in their toxins.

Evolutionary Biology and Gene Editing of Cat Allergen, Fel d 1

Allergy to domestic cat affects up to 15% of the population, and sensitization to cat allergen is associated with asthma. Despite the pervasiveness of cat allergic disease, current treatments have limited impact. Here, we present a bioinformatics analysis of the major cat allergen, Fel d 1, and demonstrate proof of principle for CRISPR gene editing of the allergen. Sequence and structural analyses of Fel d 1 from 50 domestic cats identified conserved coding regions in genes CH1 and CH2 suitable for CRISPR editing. Comparative analyses of Fel d 1 and orthologous sequences from eight exotic felid species determined relatively low-sequence identities for CH1 and CH2, and implied that the allergen may be nonessential for cats, given the apparent lack of evolutionary conservation. In vitro knockouts of domestic cat Fel d 1 using CRISPR-Cas9 yielded editing efficiencies of up to 55% and found no evidence of editing at predicted potential off-target sites. Taken together, our data indicate that Fel d 1 is both a rational and viable candidate for gene deletion, which may profoundly benefit cat allergy sufferers by removing the major allergen at the source.

New Frontiers: Precise Editing of Allergen Genes Using CRISPR

Genome engineering with clustered regularly interspaced short palindromic repeats (CRISPR) technology offers the unique potential for unequivocally deleting allergen genes at the source. Compared to prior gene editing approaches, CRISPR boasts substantial improvements in editing efficiency, throughput, and precision. CRISPR has demonstrated success in several clinical applications such as sickle cell disease and β-thalassemia, and preliminary knockout studies of allergenic proteins using CRISPR editing show promise. Given the advantages of CRISPR, as well as specific DNA targets in the allergen genes, CRISPR gene editing is a viable approach for tackling allergy, which may lead to significant disease improvement. This review will highlight recent applications of CRISPR editing of allergens, particularly cat allergen Fel d 1, and will discuss the advantages and limitations of this approach compared to existing treatment options.

Severe Anaphylactic Shock Following a Slow Loris Bite in a Patient with Cat Allergy

The slow loris monkey is one of the few venomous mammals. Its venom repels predators and can cause anaphylactic shock and even death in humans. The venom protein has been evaluated and has high sequence similarity to cat allergen; however, no studies involving subjects with cat allergy and who have been exposed to slow loris venom have been reported. We herein report the first case of severe anaphylactic shock following a slow loris bite in a patient with cat allergy.

Venom Use in Eulipotyphlans: An Evolutionary and Ecological Approach

Venomousness is a complex functional trait that has evolved independently many times in the animal kingdom, although it is rare among mammals. Intriguingly, most venomous mammal species belong to Eulipotyphla (solenodons, shrews). This fact may be linked to their high metabolic rate and a nearly continuous demand of nutritious food, and thus it relates the venom functions to facilitation of their efficient foraging. While mammalian venoms have been investigated using biochemical and molecular assays, studies of their ecological functions have been neglected for a long time. Therefore, we provide here an overview of what is currently known about eulipotyphlan venoms, followed by a discussion of how these venoms might have evolved under ecological pressures related to food acquisition, ecological interactions, and defense and protection. We delineate six mutually nonexclusive functions of venom (prey hunting, food hoarding, food digestion, reducing intra- and interspecific conflicts, avoidance of predation risk, weapons in intraspecific competition) and a number of different subfunctions for eulipotyphlans, among which some are so far only hypothetical while others have some empirical confirmation. The functions resulting from the need for food acquisition seem to be the most important for solenodons and especially for shrews. We also present several hypotheses explaining why, despite so many potentially beneficial functions, venomousness is rare even among eulipotyphlans. The tentativeness of many of the arguments presented in this review highlights our main conclusion, i.e., insights regarding the functions of eulipotyphlan venoms merit additional study.

Hello, kitty: could cat allergy be a form of intoxication?

Background

The relationship between slow loris (Nycticebus spp.) venom (BGE protein) and the major cat allergen (Fel d 1) from domestic cat (Felis catus) is known for about two decades. Along this time, evidence was accumulated regarding convergences between them, including their almost identical mode of action.

Methods

Large-scale database mining for Fel d 1 and BGE proteins in Felidae and Nycticebus spp., alignment, phylogeny proposition and molecular modelling, associated with directed literature review were assessed.

Results

Fel d 1 sequences for 28 non-domestic felids were identified, along with two additional loris BGE protein sequences. Dimer interfaces are less conserved among sequences, and the chain 1 shows more sequence similarity than chain 2. Post-translational modification similarities are highly probable.

Conclusions

Fel d 1 functions beyond allergy are discussed, considering the great conservation of felid orthologs of this protein. Reasons for toxicity being found only in domestic cats are proposed in the context of domestication. The combination of the literature review, genome-derived sequence data, and comparisons with the venomous primate slow loris may point to domestic cats as potentially poisonous mammals.