FBLN5

FBLN5 is a secreted extracellular matrix glycoprotein that serves as the central organizer of elastic fiber assembly and mediates integrin-dependent cell signaling in vascular, dermal, and pulmonary tissues. It deposits on fibrillin-1 microfibrils—guided by LTBP-2—to promote tropoelastin coacervation and tethers lysyl oxidase-like enzymes (LOXL1/2/4) for tropoelastin crosslinking into mature elastic fibers (PMID:17371835, PMID:17581631). Through its RGD motif, FBLN5 binds integrins (αvβ3, αvβ5, α9β1) to activate FAK-MAPK signaling, and its expression is induced by TGF-β via a Smad3-independent pathway and transcriptionally regulated by HIF1α downstream of mitochondrial ROS (PMID:10428823, PMID:12021267, PMID:33614230, PMID:37644092). Loss-of-function mutations in FBLN5 cause autosomal recessive cutis laxa, and a heterozygous FBLN5 mutation has been identified in autosomal dominant demyelinating neuropathy (PMID:12189163, PMID:28332470).

1. 1999 High

   Establishing that FBLN5 is a secreted ECM protein with an RGD-integrin cell-adhesion function resolved the question of how this novel fibulin family member mediates cell-matrix interactions.

   Evidence Recombinant FBLN5 protein expression and cell adhesion assays demonstrating RGD-dependent integrin binding

   PMID:10428823

   Open questions at the time

   • Specific integrin receptor subtypes not fully defined
   • Downstream intracellular signaling from integrin engagement not characterized
   • In vivo adhesion function not tested
2. 1999 Medium

   Demonstrating that FBLN5 is highly expressed in developing arteries and re-induced after vascular injury established it as a dynamically regulated vascular ECM component rather than a constitutive structural protein.

   Evidence In situ hybridization and subtraction hybridization in mouse vascular tissues

   PMID:10347091

   Open questions at the time

   • Functional consequence of re-induction after injury not tested
   • Transcriptional regulators of FBLN5 in vascular development unknown
3. 2002 High

   Linking TGF-β to FBLN5 induction via a Smad3-independent route and showing that FBLN5 amplifies ERK and p38 MAPK signaling revealed FBLN5 as both a target and modulator of growth factor pathways.

   Evidence Overexpression, dominant-negative constructs, and kinase assays in 3T3-L1 fibroblasts

   PMID:12021267

   Open questions at the time

   • Identity of the Smad3-independent TGF-β signaling intermediate unknown
   • Whether MAPK amplification requires integrin engagement not tested
4. 2002 Medium

   Identification of a homozygous S227P mutation causing autosomal recessive cutis laxa directly linked FBLN5 to human elastogenesis and elastic tissue disease.

   Evidence Genetic mapping and Sanger sequencing in affected families

   PMID:12189163

   Open questions at the time

   • Molecular mechanism by which S227P disrupts FBLN5 function not determined biochemically
   • No rescue experiment performed
5. 2003 Medium

   Demonstration that a tandem duplication adding extra EGF-like domains also causes cutis laxa established that the precise domain architecture of FBLN5 is essential for elastogenic function.

   Evidence mRNA and protein analysis of mutant FBLN5 from an affected individual

   PMID:12618961

   Open questions at the time

   • Whether the mutant protein acts in a dominant-negative fashion or is simply nonfunctional was not resolved
   • Elastogenesis assay with the duplicated protein not performed
6. 2007 High

   Reconstitution experiments established FBLN5 as the elastogenic organizer by showing it deposits on fibrillin-1 microfibrils, promotes tropoelastin coacervation, and tethers LOXL1/2/4 for crosslinking—resolving how elastic fiber assembly is coordinated.

   Evidence Recombinant FBLN5 elastogenesis assays, knockdown, co-localization and binding assays with LOXL enzymes in cultured cells

   PMID:17371835

   Open questions at the time

   • Structural basis of FBLN5–LOXL interaction not determined
   • Whether age-related truncated FBLN5 actively inhibits elastogenesis or is merely inactive not fully resolved
7. 2007 High

   Identifying LTBP-2 as a physical binding partner that targets FBLN5 onto fibrillin-1 microfibrils explained how FBLN5 is spatially directed during elastic fiber assembly.

   Evidence Co-immunoprecipitation, siRNA knockdown of LTBP-2, and recombinant rescue in human skin fibroblasts

   PMID:17581631

   Open questions at the time

   • Binding interface between LTBP-2 and FBLN5 not mapped
   • In vivo validation of LTBP-2 requirement for FBLN5 microfibrillar deposition not shown
8. 2017 Medium

   A heterozygous FBLN5 mutation causing autosomal dominant demyelinating neuropathy extended FBLN5's disease spectrum beyond elastic tissue disorders to peripheral nerve maintenance.

   Evidence Genetic sequencing with nerve biopsy and conduction studies in an affected family

   PMID:28332470

   Open questions at the time

   • Mechanism by which FBLN5 mutation leads to demyelination is unknown
   • Not independently replicated in additional families
   • No functional assay of mutant protein in nerve tissue
9. 2021 Medium

   Demonstrating that LOXL1–FBLN5–αvβ3 integrin forms a signaling axis activating FAK-MAPK in endothelial cells linked the elastogenic scaffold function of FBLN5 to intracellular pro-angiogenic signaling.

   Evidence Co-immunoprecipitation, RGD domain mutation, knockdown/overexpression in vascular endothelial cells with in vitro and in vivo functional assays

   PMID:33614230

   Open questions at the time

   • Whether FAK-MAPK activation is independent of elastic fiber context not resolved
   • Stoichiometry and direct vs. indirect nature of the ternary complex unclear
10. 2023 Medium

    Placement of FBLN5 as a transcriptional target of HIF1α downstream of mitochondrial ROS revealed an oxygen-sensing transcriptional input for FBLN5 regulation in cancer.

    Evidence Knockdown/overexpression of NDUFS1, Western blot, confocal microscopy in gastric cancer cells

    PMID:37644092

    Open questions at the time

    • Direct HIF1α binding to FBLN5 promoter not shown by ChIP
    • Relevance of this axis to normal elastic tissue homeostasis unknown

• The structural basis of FBLN5 interactions with tropoelastin, LOXL enzymes, and LTBP-2 remains unresolved, and the mechanism by which FBLN5 mutations cause demyelinating neuropathy is unknown.
• No high-resolution structure of FBLN5 or its complexes
• Mechanism of FBLN5 in peripheral nerve maintenance entirely uncharacterized
• Whether age-related truncation of FBLN5 is a regulated process or a degradation artifact is unresolved