id: A0A8M1NAS7 gene_symbol: elnb product_type: PROTEIN status: DRAFT taxon: id: NCBITaxon:7955 label: Danio rerio description: >- elnb (elastin b, formerly eln2) is one of two zebrafish tropoelastin genes, co-orthologous with the single tetrapod ELN gene, that arose in the teleost-specific whole-genome duplication together with its paralog elna. It encodes a large (about 173 kDa, 59 exons), secreted, non-enzymatic tropoelastin that keeps the alternating hydrophobic and crosslinking domain arrangement of the elastin family but is highly divergent in sequence, with longer hydrophobic domains and lower overall hydrophobicity than Elna or tetrapod elastin. elnb is the earliest and predominant elastin of the bulbus arteriosus, the elastic, smooth-muscle-rich outflow chamber that is unique to teleosts; expression begins in the outflow tract at about 54 hpf to 3 dpf, and this bulbus-restricted pattern is conserved in medaka, stickleback and trout. The Elnb-rich matrix gives the bulbus arteriosus its uniquely low extracellular stiffness and its Windkessel elasticity. Loss of elnb, by morpholino or by germline nonsense mutation, causes bulbus arteriosus hypoplasia with less elastin, loss of the low-stiffness environment, and ectopic cardiomyocytes in place of smooth muscle; this phenotype is rescued by elnb but not by elna mRNA or by the single elastin gene of Polypterus. existing_annotations: - term: id: GO:0003674 label: molecular_function evidence_type: ND original_reference_id: GO_REF:0000015 qualifier: enables review: summary: >- Root-term ND placeholder recording that no molecular function had been curated for elnb. Elnb is now characterized as a divergent tropoelastin that sets the elasticity and low stiffness of the bulbus arteriosus matrix. action: REMOVE reason: >- Superseded by an informative molecular function (extracellular matrix constituent conferring elasticity), supported by mutant stiffness measurements. supported_by: - reference_id: PMID:41712646 supporting_text: Notably, this developmental decrease in extracellular stiffness in the BA was not observed in elnb mutant, suggesting that elnb contributes to the mechanical properties of the BA extracellular environment. nd_case: STALE - term: id: GO:0005201 label: extracellular matrix structural constituent evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: enables review: summary: >- InterPro2GO mapping from the tropoelastin signature (IPR003979). Correct: Elnb is a tropoelastin whose product makes up the elastic matrix of the bulbus arteriosus, and loss of elnb removes the low extracellular stiffness of that organ. The term is accurate but general; the specific activity of elastin is GO:0030023 extracellular matrix constituent conferring elasticity. action: MODIFY reason: >- Under-specific rather than wrong. Although Elnb is very divergent in sequence (about 18% identity to human ELN), it keeps the elastin domain arrangement, and direct measurements show it confers the compliant, low-stiffness matrix of the bulbus arteriosus. The same refinement is proposed for elna, so the pair is consistent at the molecular-function level; the copies differ in where the activity is deployed, not in its kind. proposed_replacement_terms: - id: GO:0030023 label: extracellular matrix constituent conferring elasticity supported_by: - reference_id: PMID:41712646 supporting_text: Notably, this developmental decrease in extracellular stiffness in the BA was not observed in elnb mutant, suggesting that elnb contributes to the mechanical properties of the BA extracellular environment. - reference_id: PMID:26783159 supporting_text: Elnb is likely a major contributor to BA elasticity - reference_id: PMID:16982180 supporting_text: All tropoelastins shared a predominant and characteristic alternating domain arrangement, as well as the fundamental crosslinking sequence motifs. supporting_entities: - InterPro:IPR003979 - term: id: GO:0008150 label: biological_process evidence_type: ND original_reference_id: GO_REF:0000015 qualifier: involved_in review: summary: >- Root-term ND placeholder. Germline and morpholino loss-of-function data now establish roles in elastic fiber assembly and bulbus arteriosus development (proposed below). action: REMOVE reason: >- Superseded by the proposed GO:0048251 elastic fiber assembly and GO:0003232 bulbus arteriosus development annotations. supported_by: - reference_id: PMID:41712646 supporting_text: Homozygous elnb mutants exhibited a more severe hypoplastic BA than elnb morphants and developed ectopic cardiomyocytes in the BA nd_case: STALE - term: id: GO:0031012 label: extracellular matrix evidence_type: IEA original_reference_id: GO_REF:0000002 qualifier: located_in supporting_entities: - InterPro:IPR003979 review: summary: >- InterPro2GO location. Correct: Elnb is secreted (signal peptide in UniProt) and its product forms the extracellular elastic matrix of the bulbus arteriosus. action: ACCEPT reason: >- Correct general location; the more specific elastic fiber location is proposed as NEW. supported_by: - reference_id: PMID:17112714 supporting_text: with the notable exception of the bulbus arteriosus in which eln2 expression and its gene product was predominant - reference_id: file:DANRE/elnb/elnb-deep-research-falcon.md supporting_text: Its principal molecular output is a compliant extracellular matrix. - term: id: GO:0071953 label: elastic fiber evidence_type: IDA original_reference_id: PMID:17112714 review: summary: >- Proposed location. The eln2 (elnb) gene product was detected in elastic tissues and is the predominant elastin of the bulbus arteriosus, whose smooth muscle is surrounded by elastin fibres. action: NEW reason: >- Elastin is the core of the elastic fiber; this is the specific location of the mature Elnb product and matches the term proposed for elna and the direct-assay annotations of mammalian elastin. supported_by: - reference_id: PMID:17112714 supporting_text: with the notable exception of the bulbus arteriosus in which eln2 expression and its gene product was predominant - reference_id: PMID:39460530 supporting_text: The smooth muscle expresses elnb, MLCK, Tagln, and Sox9, is producing NO, and is surrounded by elastin fibres. - term: id: GO:0048251 label: elastic fiber assembly evidence_type: IMP original_reference_id: PMID:26783159 review: summary: >- Proposed process. elnb morphants show decreased elastin accumulation in a hypoplastic bulbus arteriosus, and germline elnb nonsense mutants lack the low-stiffness elastic matrix that normally appears in the bulbus after elnb expression begins. action: NEW reason: >- Participation test: tropoelastin is the structural monomer that is assembled and crosslinked into the fiber core, so Elnb does part of the work of elastic fiber assembly (the structure case), not merely being required for it. Comparator check: mouse Eln carries the parent term GO:0085029 extracellular matrix assembly (IMP), and FBN1, FBLN5, EMILIN1 and MFAP4 carry GO:0048251. The same term is proposed for elna, whose loss depletes valve elastin. supported_by: - reference_id: PMID:26783159 supporting_text: The elnb morphants exhibited severe hypoplasia of the BA and decreased elastin accumulation, while elna morphants exhibited no obvious defect in BA morphology - reference_id: PMID:41712646 supporting_text: Notably, this developmental decrease in extracellular stiffness in the BA was not observed in elnb mutant, suggesting that elnb contributes to the mechanical properties of the BA extracellular environment. - term: id: GO:0003232 label: bulbus arteriosus development evidence_type: IMP original_reference_id: PMID:41712646 review: summary: >- Proposed process. A germline elnb nonsense allele (truncation to 81 residues) produces a hypoplastic bulbus arteriosus with ectopic cardiomyocytes, more severe than the morphant phenotype; the morphant phenotype is rescued by elnb mRNA but not by elna or Polypterus eln mRNA. Artificial stiffening of the bulbus matrix phenocopies loss of elnb. action: NEW reason: >- Participation test: Elnb is not a signal that acts on the bulbus from outside; it is the principal structural component of the organ's elastic matrix, and that matrix's low stiffness is what directs precursors to smooth muscle (stiffening alone is sufficient to induce ectopic cardiomyocytes). Elnb therefore contributes the structure the process depends on. Comparator check: human ELN carries GO:0003151 outflow tract morphogenesis (IMP), the tetrapod equivalent for an elastin in the same structural role, and the existing GO:0003232 annotations in zebrafish (pak1, dhfr, pbx4) are all IMP. The term is copy-specific: elna morphants have no bulbus morphology defect, so it is not proposed for elna. The more specific child GO:0003233 bulbus arteriosus morphogenesis was not chosen, because the defect is mainly in cell fate and elastin content rather than shape. supported_by: - reference_id: PMID:41712646 supporting_text: This elnb mutant carries a premature stop codon that results in a truncated 81-amino acid Elnb protein, compared to the full-length 2,041-amino acid protein in wild-type fish - reference_id: PMID:41712646 supporting_text: Homozygous elnb mutants exhibited a more severe hypoplastic BA than elnb morphants and developed ectopic cardiomyocytes in the BA - reference_id: PMID:26783159 supporting_text: Such ectopic cardiomyocyte formation in the BA was observed only in elnb morphants and not in elna morphants. - reference_id: PMID:26783159 supporting_text: injection of elnb MO and elna full-length mRNA did not rescue the elnb morphant phenotype core_functions: - description: >- Elnb is a divergent tropoelastin assembled into the elastic matrix of the teleost bulbus arteriosus, where it confers the organ's uniquely low extracellular stiffness and Windkessel elasticity. This compliant matrix is required for bulbus precursors to become smooth muscle rather than cardiomyocytes, so elnb is required for development of the bulbus arteriosus. Neither elna nor the single ancestral-type Polypterus eln can replace it. molecular_function: id: GO:0030023 label: extracellular matrix constituent conferring elasticity directly_involved_in: - id: GO:0048251 label: elastic fiber assembly - id: GO:0003232 label: bulbus arteriosus development locations: - id: GO:0071953 label: elastic fiber supported_by: - reference_id: PMID:41712646 supporting_text: Notably, this developmental decrease in extracellular stiffness in the BA was not observed in elnb mutant, suggesting that elnb contributes to the mechanical properties of the BA extracellular environment. - reference_id: PMID:26783159 supporting_text: both of these Polypterus eln mRNAs did not rescue the elnb morphant phenotype references: - id: GO_REF:0000002 title: Gene Ontology annotation through association of InterPro records with GO terms findings: [] - id: GO_REF:0000015 title: Use of the ND evidence code for Gene Ontology (GO) terms findings: [] - id: PMID:16982180 title: 'Sequences and domain structures of mammalian, avian, amphibian and teleost tropoelastins: Clues to the evolutionary history of elastins.' findings: - statement: Zebrafish expresses two tropoelastin genes with the shared tropoelastin domain arrangement. supporting_text: Unlike other species, two tropoelastin genes were expressed in zebrafish. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Abstract only; original description of both zebrafish tropoelastins. - id: PMID:17112714 title: Differential expression of two tropoelastin genes in zebrafish. findings: - statement: eln2 (elnb) is the predominant elastin of the bulbus arteriosus and is upregulated more strongly than eln1. supporting_text: While the temporal pattern of expression was similar for both genes, the upregulation of eln2 was much stronger than that of eln1. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Abstract only. eln2 = elnb (UniProt synonym). - id: PMID:17628459 title: 'Comparative genomics of elastin: Sequence analysis of a highly repetitive protein.' findings: - statement: Both zebrafish elastins show a block of putative exon duplications. supporting_text: exons 20-31 in both zebrafish elastins reference_review: relevance: LOW correctness: VERIFIED review_notes: Abstract only; sequence-evolution background. - id: PMID:26783159 title: Evolution of the fish heart by sub/neofunctionalization of an elastin gene. findings: - statement: elnb expression is restricted to the bulbus arteriosus in zebrafish, medaka and stickleback, while elna is broadly expressed. supporting_text: We found that elnb expression patterns were restricted to the BA, while elna was observed in various tissues in both medaka - statement: elnb morphants and F0 mutants form ectopic cardiomyocytes in the bulbus arteriosus; elna and Polypterus eln mRNA do not rescue. supporting_text: These results indicate that the function of elnb is distinct from those of elna and ancestral eln; neofunctionalization likely occurred between these two paralogues. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Full text. Knockdown by translation-blocking MO; CRISPR embryos were assessed by T7 endonuclease assay (likely F0 crispants). - id: PMID:35216218 title: Zebrafish as a Model to Study Vascular Elastic Fibers and Associated Pathologies. findings: - statement: Elnb is far more divergent from human ELN than Elna is. supporting_text: They share approximately 37% identity and 41% similarity, whereas for Elnb, around 18% identity and 21% similarity were calculated. reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: Review; sequence comparison and synteny summary. - id: PMID:39460530 title: Zebrafish arterial valve development occurs through direct differentiation of second heart field progenitors. findings: - statement: elnb is the earliest exclusive marker of the bulbus arteriosus smooth muscle. supporting_text: We have identified that elnb is the earliest, exclusive marker of the bulbus arteriosus reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: Full text; expression only (uses elnb as a marker). - id: PMID:41465165 title: Duplication, Divergence and Cardiac Expression of Tropoelastin in Jawed Fishes, Including Tetraploid Rainbow Trout (Oncorhynchus mykiss). findings: - statement: Teleost ElnB has lower hydrophobicity than other fish and tetrapod elastins, and trout elnb is enriched in the bulbus. supporting_text: the bulbar expression of elnb was 15 times higher than the ventricular levels in juvenile fish reference_review: relevance: MEDIUM correctness: VERIFIED review_notes: Full text; comparative genomics across 51 jawed fishes. - id: PMID:41712646 title: Extracellular stiffness regulates cell fate determination and drives the emergence of evolutionary novelty in the teleost heart. findings: - statement: A germline elnb nonsense mutant has a hypoplastic bulbus with ectopic cardiomyocytes and fails to develop the bulbus's low stiffness; stiffening alone phenocopies loss of elnb. supporting_text: Finally, we artificially increased the stiffness of the BA extracellular environment by applying local mechanical pressure using a glass capillary, and this manipulation led to the formation of ectopic cardiomyocytes in the BA, phenocopying the elnb KD or KO condition. reference_review: relevance: HIGH correctness: VERIFIED review_notes: Full text; stable germline PTC allele (exon 2), so transcriptional adaptation of elna was possible but not tested. - id: file:DANRE/elnb/elnb-deep-research-falcon.md title: Deep research report for elnb (Falcon) findings: [] reference_review: relevance: MEDIUM correctness: UNVERIFIED review_notes: Used as a literature pointer only; it does not cover the 2026 germline mutant paper. suggested_questions: - question: >- Which sequence features of Elnb (longer hydrophobic domains, hybrid domains, lower hydrophobicity) are responsible for the low stiffness of the bulbus matrix, and would a chimeric Elna carrying them rescue elnb loss? - question: >- Did the failure of elna and Polypterus eln mRNA to rescue reflect a true difference in protein function, or insufficient protein at the bulbus from ubiquitous mRNA injection? - question: >- Is elna upregulated in the germline elnb nonsense mutant (transcriptional adaptation), and does an elna;elnb double mutant have a stronger phenotype? suggested_experiments: - description: >- Knock elna coding sequence into the elnb locus (or express it from an elnb promoter) in elnb mutants, to test protein-level non-equivalence under the correct expression pattern. experiment_type: in vivo cross-rescue - description: >- Measure bulbus stiffness and cell fate in elna;elnb double mutants and in an RNA-less elnb allele. experiment_type: genetic loss-of-function