Functional amyloidogenesis in immunocytes from the colonial ascidian Botryllus schlosseri: Evolutionary perspective

Lessons on neurodegeneration and aging from the Lagoon of Venice: the marine invertebrate Botryllus schlosseri

In this work, the authors proposed a novel and interesting animal model for studying human neurodegenerative diseases, Botryllus schlosseri, a small invertebrate inhabiting temperate seas worldwide, which shares remarkable similarities with mammals in the expression of genes involved in pathological aging.

Multiple Forms of Neural Cell Death in the Cyclical Brain Degeneration of A Colonial Chordate

Human neuronal loss occurs through different cellular mechanisms, mainly studied in vitro. Here, we characterized neuronal death in B. schlosseri, a marine colonial tunicate that shares substantial genomic homology with mammals and has a life history in which controlled neurodegeneration happens simultaneously in the brains of adult zooids during a cyclical phase named takeover. Using an ultrastructural and transcriptomic approach, we described neuronal death forms in adult zooids before and during the takeover phase while comparing adult zooids in takeover with their buds where brains are refining their structure. At takeover, we found in neurons clear morphologic signs of apoptosis (i.e., chromatin condensation, lobed nuclei), necrosis (swollen cytoplasm) and autophagy (autophagosomes, autolysosomes and degradative multilamellar bodies). These results were confirmed by transcriptomic analyses that highlighted the specific genes involved in these cell death pathways. Moreover, the presence of tubulovesicular structures in the brain medulla alongside the over-expression of prion disease genes in late cycle suggested a cell-to-cell, prion-like propagation recalling the conformational disorders typical of some human neurodegenerative diseases. We suggest that improved understanding of how neuronal alterations are regulated in the repeated degeneration-regeneration program of B. schlosseri may yield mechanistic insights relevant to the study of human neurodegenerative diseases.

Botryllin, a Novel Antimicrobial Peptide from the Colonial Ascidian Botryllus schlosseri

By mining the transcriptome of the colonial ascidian Botryllus schlosseri, we identified a transcript for a novel styelin-like antimicrobial peptide, which we named botryllin. The gene is constitutively transcribed by circulating cytotoxic morula cells (MCs) as a pre-propeptide that is then cleaved to mature peptide. The synthetic peptide, obtained from in silico translation of the transcript, shows robust killing activity of bacterial and unicellular yeast cells, causing breakages of both the plasma membrane and the cell wall. Specific monoclonal antibodies were raised against the epitopes of the putative amino acid sequence of the propeptide and the mature peptide; in both cases, they label the MC granular content. Upon MC degranulation induced by the presence of nonself, the antibodies recognise the extracellular nets with entrapped bacteria nearby MC remains. The obtained results suggest that the botryllin gene carries the information for the synthesis of an AMP involved in the protection of B. schlosseri from invading foreign cells.

New putative phenol oxidase in ascidian blood cells

The phenol oxidase system is ancient and ubiquitously distributed in all living organisms. In various groups it serves for the biosynthesis of pigments and neurotransmitters (dopamine), defence reactions and tissue hardening. Ascidians belong to subphylum Tunicata, which is considered the closest living relative to Vertebrates. Two phenol oxidases previously described for ascidians are vertebrate-like and arthropod-like phenol oxidases. In our present study, we described a new ascidian protein, Tuphoxin, with putative phenol oxidase function, which bears no sequence similarity with two enzymes described previously. The closest related proteins to Tuphoxin are mollusc haemocyanins. Unlike haemocyanins, which are oxygen transporting plasma proteins, Tuphoxin is synthesised in ascidian blood cells and secreted in the extracellular matrix of the tunic—ascidian outer coverings. Single mature transcript coding for this phenol oxidase can give several protein products of different sizes. Thus limited proteolysis of the initial protein is suggested. A unique feature of Tuphoxins and their homologues among Tunicata is the presence of thrombospondin first type repeats (TSP1) domain in their sequence which is supposed to provide interaction with extracellular matrix. The finding of TSP1 in the structure of phenol oxidases is new and we consider this to be an innovation of Tunicata evolutionary lineage.

Searching for the Origin and the Differentiation of Haemocytes before and after Larval Settlement of the Colonial Ascidian Botryllus schlosseri: An Ultrastructural Viewpoint

The colonial ascidian Botryllus schlosseri possesses an innate immunity, which plays fundamental roles in its survival, adaptability, worldwide spread and ecological success. Three lines of differentiation pathways of circulating haemocytes are known to be present in the haemolymph, starting from undifferentiated haemoblasts: (i) the phagocytic line (hyaline amoebocytes and macrophage-like cells), (ii) the cytotoxic line (granular amoebocytes and morula cells) and (iii) the storage cell line (pigment cells and nephrocytes). Many questions remain about their origin, and thus, observations during various stages of development were undertaken in this study. Haemocytes were detected beginning from the early tailbud embryo stage. Haemoblasts were always present and morula cells were the first differentiated haemocytes detected. In both the next stage, just before hatching, and the swimming tadpole larva stage, hyaline amoebocytes and pigment cells were also recognisable. Some morula cells containing active phenoloxidase migrated from the haemolymph into the tunic after having crossed the epidermis, and this behaviour could be related to the preparation of a defensive function for spatial competition. During larval metamorphosis, macrophage-like cells appeared with their phagosomes positive to acid phosphatase activity and containing apoptotic cells from tail tissue degeneration. After metamorphosis, in the filter-feeding oozoid stage, nephrocytes involved in nitrogen catabolism finally appeared. In both the subendostylar sinus and the peripheral blind-sac vessels (ampullae), clusters of haemoblasts were recognisable, some of which showed incipient specialisations, considering the hypothesis of the presence of putative niches of haemolymph stem cells.

Immune properties of invertebrate phenoloxidases

Melanin production from different types of phenoloxidases (POs) confers immunity from a variety of pathogens ranging from viruses and microorganisms to parasites. The arthropod proPO expresses a variety of activities including cytokine, opsonin and microbiocidal activities independent of and even without melanin production. Proteolytic processing of proPO and its activating enzyme gives rise to several peptide fragments with a variety of separate activities in a process reminiscent of vertebrate complement system activation although proPO bears no sequence similarity to vertebrate complement factors. Pathogens influence proPO activation and thereby what types of immune effects that will be produced. An increasing number of specialised pathogens - from parasites to viruses - have been identified who can synthesise compounds specifically aimed at the proPO-system. In invertebrates outside the arthropods phylogenetically unrelated POs are participating in melanization reactions obviously aimed at intruders and/or aberrant tissues.

TIAMMAt: Leveraging biodiversity to revise protein domain models, evidence from innate immunity

Sequence annotation is fundamental for studying the evolution of protein families, particularly when working with nonmodel species. Given the rapid, ever-increasing number of species receiving high-quality genome sequencing, accurate domain modeling that is representative of species diversity is crucial for understanding protein family sequence evolution and their inferred function(s). Here, we describe a bioinformatic tool called Taxon-Informed Adjustment of Markov Model Attributes (TIAMMAt) which revises domain profile hidden Markov models (HMMs) by incorporating homologous domain sequences from underrepresented and nonmodel species. Using innate immunity pathways as a case study, we show that revising profile HMM parameters to directly account for variation in homologs among underrepresented species provides valuable insight into the evolution of protein families. Following adjustment by TIAMMAt, domain profile HMMs exhibit changes in their per-site amino acid state emission probabilities and insertion/deletion probabilities while maintaining the overall structure of the consensus sequence. Our results show that domain revision can heavily impact evolutionary interpretations for some families (i.e., NLR’s NACHT domain), whereas impact on other domains (e.g., rel homology domain and interferon regulatory factor domains) is minimal due to high levels of sequence conservation across the sampled phylogenetic depth (i.e., Metazoa). Importantly, TIAMMAt revises target domain models to reflect homologous sequence variation using the taxonomic distribution under consideration by the user. TIAMMAt’s flexibility to revise any subset of the Pfam database using a user-defined taxonomic pool will make it a valuable tool for future protein evolution studies, particularly when incorporating (or focusing) on nonmodel species.

Stress granules in Ciona robusta: First evidences of TIA-1-related nucleolysin and tristetraprolin gene expression under metal exposure

Stress granules are non-membranous cytoplasmic foci, composed of non-translating messenger ribonucleoproteins, translational initiation factors and other additional proteins. They represent a primary mechanism to rapidly modulate gene expression when cells are subjected to adverse environmental conditions. Very few works have been devoted to study the presence of the molecular components of stress granules in invertebrates. In this work, we characterized the transcript sequences for two important protein components of stress granules, TIA-1-related nucleolysin (TIAR) and tristetraprolin (TTP), in the solitary ascidian Ciona robusta, an invertebrate chordate, and carried out the first studies on their gene expression under stress conditions induced by metals (Cu, Zn and Cd). Data on mRNA expression levels, provided by qRT-PCR analyses, show a generalized decrease at the second day of metal-exposure for both tiar and ttp, suggesting that metal accumulation induces acute stress and the inhibition of the transcription for the two studied proteins. In-situ hybridization analyses demonstrate that TIAR and TTP antisense riboprobes recognize circulating granular amoebocytes in the hemolymph, in both blood lacunae and tunic. The results obtained in this work increase our knowledge on the evolution of anti-stress proteins in metazoans and emphasize the importance of the transcription of tiar and ttp, which represents an efficient physiological response allowing organisms to survive in the environment under stress conditions.

Conservation of Cell Communication Systems in Invertebrate Host-Defence Mechanisms: Possible Role in Immunity and Disease

Innate immunity is continuously revealing multiple and highly conserved host-defence mechanisms. Studies on mammalian immunocytes are showing different communication systems that may play a role in coordinating innate immune responses also in invertebrates. Extracellular traps (ETs) are an immune response by which cells release net-like material, including DNA, histones and proteins. ETs are thought to immobilise and kill microorganisms, but are also involved in inflammation and autoimmune disease. Immune cells are also known to communicate through extracellular vesicles secreted in the extracellular environment or exosomes, which can carry a variety of different signalling molecules. Tunnelling nanotubes (TNTs) represent a direct cell-to-cell communication over a long distance, that allow for bi- or uni-directional transfer of cellular components between cells. Their functional role in a number of physio-pathological processes, including immune responses and pathogen transfer, has been underlined. Although ETs, exosomes, and TNTs have been described in invertebrate species, their possible role in immune responses is not fully understood. In this work, available data on these communication systems are summarised, in an attempt to provide basic information for further studies on their relevance in invertebrate immunity and disease.

Amyloids in Site-Specific Autoimmune Reactions and Inflammatory Responses

Amyloid deposition is a histological hallmark of common human disorders including Alzheimer's disease (AD) and type 2 diabetes. Although some reports highlight that amyloid fibrils might activate the innate immunity system via pattern recognition receptors, here, we provide multiple lines of evidence for the protection by site-specific amyloid protein analogs and fibrils against autoimmune attacks: (1) strategies targeting clearance of the AD-related brain amyloid plaque induce high risk of deadly autoimmune destructions in subjects with cognitive dysfunction; (2) administration of amyloidogenic peptides with either full length or core hexapeptide structure consistently ameliorates signs of experimental autoimmune encephalomyelitis; (3) experimental autoimmune encephalomyelitis is exacerbated following genetic deletion of amyloid precursor proteins; (4) absence of islet amyloid coexists with T-cell-mediated insulitis in autoimmune diabetes and autoimmune polyendocrine syndrome; (5) use of islet amyloid polypeptide agonists rather than antagonists improves diabetes care; and (6) common suppressive signaling pathways by regulatory T cells are activated in both local and systemic amyloidosis. These findings indicate dual modulation activity mediated by amyloid protein monomers, oligomers, and fibrils to maintain immune homeostasis. The protection from autoimmune destruction by amyloid proteins offers a novel therapeutic approach to regenerative medicine for common degenerative diseases.

Study of the complement activation by amyloid aggregates of smooth muscle titin in vitro

The giant muscle protein, titin, is the third most abundant protein in muscle (after myosin and actin). It was shown previously that smooth muscle titin (SMT) with a molecular mass of 500 kDa can form in vitro amorphous amyloid aggregates in two conditions: in solution of low ionic strength (0.15 M Glycine-KOH, pH 7.0) (SMT(Gly) aggregates) and in solution with ionic strength in the physiological range (0.2 M KCl, 20 mM imidazole, pH 7.2-7.4) (SMT(KCl) aggregates). Such aggregation in vivo, which may play a pathological or functional role, is not excluded. In view of the fact that some pathological amyloids can activate the classical and alternative pathways of complement system, we investigated the binding of complement component C1q and C3b to smooth muscle titin amyloid aggregates. The binding of С1q and C3b to SMT aggregates was not observed with ELISA assay. Since SMT aggregates do not activate the complement system, they are hardly implicated in the inflammatory process caused by muscle damage in amyloidoses.Abbreviations: SMT: smooth muscle titin; SMT(KCl) aggregates: SMT aggregates in solution containing 0.2 M KCl, 10 mM imidazole, pH 7.0; SMT(Gly) aggregates: SMT aggregates in solution containing 0.15 M glycine-KOH, pH 7.2-7.4; MAC: membrane attack complex; DLS: dynamic light scattering; NHS: Normal Human Serum.

Modulation of Innate Immunity by Amyloidogenic Peptides

Amyloid formation contributes to the development of progressive metabolic and neurodegenerative diseases, while also serving functional roles in host defense. Emerging evidence suggests that as amyloidogenic peptides populate distinct aggregation states, they interact with different combinations of pattern recognition receptors (PRRs) to direct the phenotype and function of tissue-resident and infiltrating innate immune cells. We review recent evidence of innate immunomodulation by distinct forms of amyloidogenic peptides produced by mammals (humans, non-human primates), bacteria, and fungi, as well as the corresponding cell-surface and intracellular PRRs in these interactions, in human and mouse models. Our emerging understanding of peptide aggregate-innate immune cell interactions, and the factors regulating the balance between amyloid function and pathogenicity, might aid the development of anti-amyloid and immunomodulating therapies.