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.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003674 molecular_function | ND GO_REF:0000015 | REMOVE | 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. Reason: Superseded by an informative molecular function (extracellular matrix constituent conferring elasticity), supported by mutant stiffness measurements. Supporting Evidence: PMID:41712646 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. |
| GO:0005201 extracellular matrix structural constituent | IEA GO_REF:0000002 | MODIFY | 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. 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 replacements: extracellular matrix constituent conferring elasticity Supporting Evidence: PMID:41712646 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. PMID:26783159 Elnb is likely a major contributor to BA elasticity PMID:16982180 All tropoelastins shared a predominant and characteristic alternating domain arrangement, as well as the fundamental crosslinking sequence motifs. |
| GO:0008150 biological_process | ND GO_REF:0000015 | REMOVE | 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). Reason: Superseded by the proposed GO:0048251 elastic fiber assembly and GO:0003232 bulbus arteriosus development annotations. Supporting Evidence: PMID:41712646 Homozygous elnb mutants exhibited a more severe hypoplastic BA than elnb morphants and developed ectopic cardiomyocytes in the BA |
| GO:0031012 extracellular matrix | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO location. Correct: Elnb is secreted (signal peptide in UniProt) and its product forms the extracellular elastic matrix of the bulbus arteriosus. Reason: Correct general location; the more specific elastic fiber location is proposed as NEW. Supporting Evidence: PMID:17112714 with the notable exception of the bulbus arteriosus in which eln2 expression and its gene product was predominant file:DANRE/elnb/elnb-deep-research-falcon.md Its principal molecular output is a compliant extracellular matrix. |
| GO:0071953 elastic fiber | IDA PMID:17112714 Differential expression of two tropoelastin genes in zebrafi... | NEW | 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. 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. Supporting Evidence: PMID:17112714 with the notable exception of the bulbus arteriosus in which eln2 expression and its gene product was predominant PMID:39460530 The smooth muscle expresses elnb, MLCK, Tagln, and Sox9, is producing NO, and is surrounded by elastin fibres. |
| GO:0048251 elastic fiber assembly | IMP PMID:26783159 Evolution of the fish heart by sub/neofunctionalization of a... | NEW | 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. 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. Supporting Evidence: PMID:26783159 The elnb morphants exhibited severe hypoplasia of the BA and decreased elastin accumulation, while elna morphants exhibited no obvious defect in BA morphology PMID:41712646 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. |
| GO:0003232 bulbus arteriosus development | IMP PMID:41712646 Extracellular stiffness regulates cell fate determination an... | NEW | 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. 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. Supporting Evidence: PMID:41712646 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 PMID:41712646 Homozygous elnb mutants exhibited a more severe hypoplastic BA than elnb morphants and developed ectopic cardiomyocytes in the BA PMID:26783159 Such ectopic cardiomyocyte formation in the BA was observed only in elnb morphants and not in elna morphants. PMID:26783159 injection of elnb MO and elna full-length mRNA did not rescue the elnb morphant phenotype |
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Download this section (compressed HTML)Q: 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?
Q: 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?
Q: Is elna upregulated in the germline elnb nonsense mutant (transcriptional adaptation), and does an elna;elnb double mutant have a stronger phenotype?
Experiment: 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.
Type: in vivo cross-rescue
Experiment: Measure bulbus stiffness and cell fate in elna;elnb double mutants and in an RNA-less elnb allele.
Type: genetic loss-of-function
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