Lior Zilberberg

Mutations in LTBP4 Cause a Syndrome of Impaired Pulmonary, Gastrointestinal, Genitourinary, Musculoskeletal, and Dermal Development

We report recessive mutations in the gene for the latent transforming growth factor-beta binding protein 4 (LTBP4) in four unrelated patients with a human syndrome disrupting pulmonary, gastrointestinal, urinary, musculoskeletal, craniofacial, and dermal development. All patients had severe respiratory distress, with cystic and atelectatic changes in the lungs complicated by tracheomalacia and diaphragmatic hernia. Three of the four patients died of respiratory failure. Cardiovascular lesions were mild, limited to pulmonary artery stenosis and patent foramen ovale. Gastrointestinal malformations included diverticulosis, enlargement, tortuosity, and stenosis at various levels of the intestinal tract. The urinary tract was affected by diverticulosis and hydronephrosis. Joint laxity and low muscle tone contributed to musculoskeletal problems compounded by postnatal growth delay. Craniofacial features included microretrognathia, flat midface, receding forehead, and wide fontanelles. All patients had cutis laxa. Four of the five identified LTBP4 mutations led to premature termination of translation and destabilization of the LTBP4 mRNA. Impaired synthesis and lack of deposition of LTBP4 into the extracellular matrix (ECM) caused increased transforming growth factor-beta (TGF-beta) activity in cultured fibroblasts and defective elastic fiber assembly in all tissues affected by the disease. These molecular defects were associated with blocked alveolarization and airway collapse in the lung. Our results show that coupling of TGF-beta signaling and ECM assembly is essential for proper development and is achieved in multiple human organ systems by multifunctional proteins such as LTBP4.

Latent TGF-β-binding proteins

The LTBPs (or latent transforming growth factor β binding proteins) are important components of the extracellular matrix (ECM) that interact with fibrillin microfibrils and have a number of different roles in microfibril biology. There are four LTBPs isoforms in the human genome (LTBP-1, -2, -3, and -4), all of which appear to associate with fibrillin and the biology of each isoform is reviewed here. The LTBPs were first identified as forming latent complexes with TGFβ by covalently binding the TGFβ propeptide (LAP) via disulfide bonds in the endoplasmic reticulum. LAP in turn is cleaved from the mature TGFβ precursor in the trans-golgi network but LAP and TGFβ remain strongly bound through non-covalent interactions. LAP, TGFβ, and LTBP together form the large latent complex (LLC). LTBPs were originally thought to primarily play a role in maintaining TGFβ latency and targeting the latent growth factor to the extracellular matrix (ECM), but it has also been shown that LTBP-1 participates in TGFβ activation by integrins and may also regulate activation by proteases and other factors. LTBP-3 appears to have a role in skeletal formation including tooth development. As well as having important functions in TGFβ regulation, TGFβ-independent activities have recently been identified for LTBP-2 and LTBP-4 in stabilizing microfibril bundles and regulating elastic fiber assembly.

Specificity of latent TGF-β binding protein (LTBP) incorporation into matrix: Role of fibrillins and fibronectin†

Fibrillin microfibrils are extracellular matrix structures with essential functions in the development and the organization of tissues including blood vessels, bone, limbs and the eye. Fibrillin‐1 and fibrillin‐2 form the core of fibrillin microfibrils, to which multiple proteins associate to form a highly organized structure. Defining the components of this structure and their interactions is crucial to understand the pathobiology of microfibrillopathies associated with mutations in fibrillins and in microfibril‐associated molecules. In this study, we have analyzed both in vitro and in vivo the role of fibrillin microfibrils in the matrix deposition of latent TGF‐β binding protein 1 (LTBP‐1), ‐3 and ‐4; the three LTBPs that form a complex with TGF‐β. In Fbn1−/− ascending aortas and lungs, LTBP‐3 and LTBP‐4 are not incorporated into a matrix lacking fibrillin‐1 microfibrils, whereas LTBP‐1 is still deposited. In addition, in cultures of Fbn1−/− smooth muscle cells or lung fibroblasts, LTBP‐3 and LTBP‐4 are not incorporated into a matrix lacking fibrillin‐1 microfibrils, whereas LTBP‐1 is still deposited. Fibrillin‐2 is not involved in the deposition of LTBP‐1 in Fbn1−/− extracellular matrix as cells deficient for both fibrillin‐1 and fibrillin‐2 still incorporate LTBP‐1 in their matrix. However, blocking the formation of the fibronectin network in Fbn1−/− cells abrogates the deposition of LTBP‐1. Together, these data indicate that LTBP‐3 and LTBP‐4 association with the matrix depends on fibrillin‐1 microfibrils, whereas LTBP‐1 association depends on a fibronectin network. J. Cell. Physiol. 227: 3828–3836, 2012.

Perturbation of transforming growth factor (TGF)-ß1 association with latent TGF-β binding protein yields inflammation and tumors

Transforming growth factor-β (TGF-β) activity is controlled at many levels including the conversion of the latent secreted form to its active state. TGF-β is often released as part of an inactive tripartite complex consisting of TGF-β, the TGF-β propeptide, and a molecule of latent TGF-β binding protein (LTBP). The interaction of TGF-β and its cleaved propeptide renders the growth factor latent, and the liberation of TGF-β from this state is crucial for signaling. To examine the contribution of LTBP to TGF-β function, we generated mice in which the cysteines that link the propeptide to LTBP were mutated to serines, thereby blocking covalent association. Tgfb1C33S/C33S mice had multiorgan inflammation, lack of skin Langerhans cells (LC), and a shortened lifespan, consistent with decreased TGF-β1 levels. However, the inflammatory response and decreased lifespan were not as severe as observed with Tgfb1−/− animals. Tgfb1C33S/C33S mice exhibited decreased levels of active TGF-β1, decreased TGF-β signaling, and tumors of the stomach, rectum, and anus. These data suggest that the association of LTBP with the latent TGF-β complex is important for proper TGF-β1 function and that Tgfb1C33S/C33S mice are hypomorphs for active TGF-β1. Moreover, although mechanisms exist to activate latent TGF-β1 in the absence of LTBP, these mechanisms are not as efficient as those that use the latent complex containing LTBP.

A rapid and sensitive bioassay to measure bone morphogenetic protein activity

BackgroundBone morphogenetic proteins (BMPs) are members of the TGF-beta superfamily and were originally identified as proteins that induce ectopic bone formation. BMPs were shown subsequently to be involved in several biological processes during development and in adult tissues through the regulation of the growth, differentiation and apoptosis of various cell types. An alkaline phosphatase (ALP)-based assay is the most widely used assay to evaluate BMP activity. However, the ALP assay is not rapid and not sensitive enough to measure BMP activity at physiological concentrations. In this paper, we describe a highly sensitive, rapid, and specific cell-based assay for the quantification of BMP activity.ResultsTwo cells lines, C2C12 and HepG2 were stably transfected with a reporter plasmid consisting of BMP-responsive elements from the Id1 promoter fused to a luciferase reporter gene. Exposure of cells containing this construct to BMPs induces the expression of luciferase, which can be quantified with a luminometer. The bioassay is specific for BMPs and can detect BMP-4 activity at a concentration as low as 3 pM. Related family members, such as TGF-beta1, TGF-beta2 and TGF-beta3, do not induce the reporter gene.ConclusionThe assay is rapid (less than 24 hours) and can be used, as described in this paper, to measure BMP activity in complex solutions and in cell culture in a simple and efficient way.

p38 MAPK Is an Early Determinant of Promiscuous Smad2/3 Signaling in the Aortas of Fibrillin-1 (Fbn1)-null Mice

Excessive transforming growth factor-β (TGF-β) signaling characterizes the progression of aortic aneurysm in mouse models of Marfan syndrome, a systemic disorder of the connective tissue that is caused by mutations in the gene encoding the extracellular matrix protein fibrillin-1. Fibrillin-1 mutations are believed to promote abnormal Smad2/3 signaling by impairing the sequestration of latent TGF-β complexes into the extracellular matrix. Here we report that promiscuous Smad2/3 signaling is the cell-autonomous phenotype of primary cultures of vascular smooth muscle cells (VSMC) explanted from the thoracic aortas of Fbn1 mutant mice with either neonatal onset or progressively severe aortic aneurysm. This cellular phenotype was characterized in VSMC isolated from Fbn1-null (mgN/mgN) mice, which recapitulate the most severe form of Marfan syndrome. We found that loss of fibrillin-1 deposition promotes the production of intracellular reactive oxygen species and abnormal accumulation of phosphorylated TGF-β-activated kinase 1 and p38 MAPK, in addition to increasing the levels of endogenous phospho-Smad2. We showed that improper Smad2/3 signaling in Fbn1-null VSMC is in part stimulated by phospho-p38 MAPK, which is in turn activated in response to signals other than those mediated by the kinase activity of the ALK5 receptor. Consistent with these cell culture data, in vivo analyses documented that phospho-p38 MAPK accumulates earlier than phospho-Smad2 in the aortic wall of mgN/mgN mice and that systemic inhibition of phospho-p38 MAPK activity lowers the levels of phospho-Smad2 in this tissue. Collectively, these findings indicate that improper activation of p38 MAPK is a precursor of constitutive Smad2/3 signaling in the aortic wall of a mouse model of neonatal lethal Marfan syndrome.

Latent TGF-β binding protein 4 promotes elastic fiber assembly by interacting with fibulin-5

Elastic fiber assembly requires deposition of elastin monomers onto microfibrils, the mechanism of which is incompletely understood. Here we show that latent TGF-β binding protein 4 (LTBP-4) potentiates formation of elastic fibers through interacting with fibulin-5, a tropoelastin-binding protein necessary for elastogenesis. Decreased expression of LTBP-4 in human dermal fibroblast cells by siRNA treatment abolished the linear deposition of fibulin-5 and tropoelastin on microfibrils. It is notable that the addition of recombinant LTBP-4 to cell culture medium promoted elastin deposition on microfibrils without changing the expression of elastic fiber components. This elastogenic property of LTBP-4 is independent of bound TGF-β because TGF-β–free recombinant LTBP-4 was as potent an elastogenic inducer as TGF-β–bound recombinant LTBP-4. Without LTBP-4, fibulin-5 and tropoelastin deposition was discontinuous and punctate in vitro and in vivo. These data suggest a unique function for LTBP-4 during elastic fibrogenesis, making it a potential therapeutic target for elastic fiber regeneration.

Long form of latent TGF-β binding protein 1 (Ltbp1L) regulates cardiac valve development

Transforming Growth Factor β (TGF‐β) is crucial for valve development and homeostasis. The long form of Latent TGF‐β binding protein 1 (LTBP1L) covalently binds all TGF‐β isoforms and regulates their bioavailability. Ltbp1L expression analysis during valvulogenesis revealed two patterns of Ltbp1L production: an early one (E9.5–11.5) associated with endothelial‐to‐mesenchymal transformation (EMT); and a late one (E12.5 to birth) contemporaneous with valve remodeling. Similarly, histological analysis of Ltbp1L−/− developing valves identified two different pathologies: generation of hypoplastic endocardial cushions in early valvulogenesis, followed by development of hyperplastic valves in late valvulogenesis. Ltbp1L promotes valve EMT, as Ltbp1L absence yields hypoplastic endocardial cushions in vivo and attenuated EMT in vitro. Ltbp1L−/− valve hyperplasia in late valvuogenesis represents a consequence of prolonged EMT. We demonstrate that Ltbp1L is a major regulator of Tgf‐β activity during valvulogenesis since its absence results in a perturbed Tgf‐β pathway that causes all Ltbp1L−/− valvular defects. Developmental Dynamics, 2011.

Function of latent TGFβ binding protein 4 and fibulin 5 in elastogenesis and lung development.

Mice deficient in Latent TGFβ Binding Protein 4 (Ltbp4) display a defect in lung septation and elastogenesis. The lung septation defect is normalized by genetically decreasing TGFβ2 levels. However, the elastic fiber assembly is not improved in Tgfb2‐/‐;Ltbp4S‐/‐ compared to Ltbp4S‐/‐ lungs. We found that decreased levels of TGFβ1 or TGFβ3 did not improve lung septation indicating that the TGFβ isoform elevated in Ltbp4S‐/‐ lungs is TGFβ2. Expression of a form of Ltbp4 that could not bind latent TGFβ did not affect lung phenotype indicating that normal lung development does not require the formation of LTBP4‐latent TGFβ complexes. Therefore, the change in TGFβ level in the lungs is not directly related to Ltbp4 deficiency but probably is a consequence of changes in the extracellular matrix. Interestingly, combination of the Ltbp4S‐/‐ mutation with a fibulin‐5 null mutant in Fbln5‐/‐;Ltbp4S‐/‐ mice improves the lung septation compared to Ltbp4S‐/‐ lungs. Large globular elastin aggregates characteristic for Ltbp4S‐/‐ lungs do not form in Fbln5‐/‐;Ltbp4S‐/‐ lungs and EM studies showed that elastic fibers in Fbln5‐/‐;Ltbp4S‐/‐ lungs resemble those found in Fbln5‐/‐ mice. These results are consistent with a role for TGFβ2 in lung septation and for Ltbp4 in regulating fibulin‐5 dependent elastic fiber assembly. J. Cell. Physiol. 230: 226–236, 2015.

Control of lung development by latent TGF-β binding proteins.

The latent TGF‐β binding proteins (LTBP‐1 ‐3, and ‐4) assist in the secretion and localization of latent TGF‐β molecules. Ltbp3−/−and Ltbp4S−/− mice have distinct phenotypes and only in the lungs does deficiency of either Ltbp‐3 or Ltbp‐4 cause developmental abnormalities. To determine if these two LTBPs have additional common functions, we generated mice deficient for both Ltbp‐3 and Ltbp‐4S. The only novel defect in Ltbp3−/−;Ltbp4S−/− mice was an early lethality compared to mice with single mutations. In addition lung abnormalities were exacerbated and the terminal air sac septation defect was more severe in Ltbp3−/−;Ltbp4S−/− mice than in Ltbp4S−/− mice. Decreased cellularity of Ltbp3−/−;Ltbp4S−/− lungs was correlated with higher rate of apoptosis in newborn lungs of Ltbp3−/−;Ltbp4S−/− animals compared to WT, Ltbp3−/−, and Ltbp4S−/− mice. No differences in the maturation of the major lung cell types were discerned between the single and double mutant mice. However, the distribution of type 2 cells and myofibroblasts was abnormal, and myofibroblast segregation in some areas might be an indication of early fibrosis. We also observed differences in ECM composition between Ltbp3−/−;Ltbp4S−/− and Ltbp4S−/− lungs after birth, reflected in decreased incorporation of fibrillin‐1 and ‐2 in Ltbp3−/−;Ltbp4S−/− matrix. The function of the lungs of Ltbp3−/−;Ltbp4S−/− mice after the first week of life was potentially further compromised by macrophage infiltration, as proteases secreted from macrophages might exacerbate developmental emphysema. Together these data indicate that LTBP‐3 and ‐4 perform partially overlapping functions only in the lungs. J. Cell. Physiol. 226: 1499–1509, 2011.