Eric J. Carlson

In Vivo Studies of Mutant Fibrillin-1 Microfibrils

In humans, mutations in fibrillin-1 result in a variety of genetic disorders with distinct clinical phenotypes. While most of the known mutations in fibrillin-1 cause Marfan syndrome, a number of other mutations lead to clinical features unrelated to Marfan syndrome. Pathogenesis of Marfan syndrome is currently thought to be driven by mechanisms due to haploinsufficiency of wild-type fibrillin-1. However, haploinsufficiency-driven mechanisms cannot explain the distinct phenotypes found in other fibrillinopathies. To test the hypothesis that mutations in fibrillin-1 cause disorders through primary effects on microfibril structure, two different mutations were generated in Fbn1 in mice. One mutation leads to a truncated fibrillin-1 molecule that is tagged with green fluorescent protein, allowing visualization of mutant fibrillin-1 incorporated into microfibrils. In heterozygosity, these mutant mice demonstrate progressive fragmentation of the aortic elastic lamellae and also display fragmentation of microfibrils in other tissues. Fibrillin-2 epitopes are also progressively revealed in these mice, suggesting that fibrillin-2 immunoreactivity can serve as a marker for microfibril degradation. In contrast, a second mutation (in-frame deletion of the first hybrid domain) in fibrillin-1 results in stable microfibrils, demonstrating that fibrillin-1 molecules are not required to be in perfect register for microfibril structure and function and that the first hybrid domain is dispensable for microfibril assembly. Taken together, these results suggest that perturbation of microfibril structure may underlie one of the major features of the Marfan syndrome: fragmentation of aortic elastic lamellae.

p97 functions as an auxiliary factor to facilitate TM domain extraction during CFTR ER-associated degradation

The AAA‐ATPase (ATPase associated with various cellular activities) p97 has been implicated in the degradation of misfolded and unassembled proteins in the endoplasmic reticulum (ERAD). To better understand its role in this process, we used a reconstituted cell‐free system to define the precise contribution of p97 in degrading immature forms of the polytopic, multi‐domain protein CFTR (cystic fibrosis transmembrane conductance regulator). Although p97 augmented both the rate and the extent of CFTR degradation, it was not obligatorily required for ERAD. Only a 50% decrease in degradation was observed in the complete absence of p97. Moreover, p97 specifically stimulated the degradation of CFTR transmembrane (TM) domains but had no effect on isolated cytosolic domains. Consistent with this, p97‐mediated extraction of intact TM domains was independent of proteolytic cleavage and influenced by TM segment hydrophobicity, indicating that the relative contribution of p97 is partially determined by substrate stability. Thus, we propose that p97 functions in ERAD as a nonessential but important ancillary component to the proteasome where it facilitates substrate presentation and increases the degradation rate and efficiency of stable (TM) domains.

Microenvironmental Regulation by Fibrillin-1

Fibrillin-1 is a ubiquitous extracellular matrix molecule that sequesters latent growth factor complexes. A role for fibrillin-1 in specifying tissue microenvironments has not been elucidated, even though the concept that fibrillin-1 provides extracellular control of growth factor signaling is currently appreciated. Mutations in FBN1 are mainly responsible for the Marfan syndrome (MFS), recognized by its pleiotropic clinical features including tall stature and arachnodactyly, aortic dilatation and dissection, and ectopia lentis. Each of the many different mutations in FBN1 known to cause MFS must lead to similar clinical features through common mechanisms, proceeding principally through the activation of TGFβ signaling. Here we show that a novel FBN1 mutation in a family with Weill-Marchesani syndrome (WMS) causes thick skin, short stature, and brachydactyly when replicated in mice. WMS mice confirm that this mutation does not cause MFS. The mutation deletes three domains in fibrillin-1, abolishing a binding site utilized by ADAMTSLIKE-2, -3, -6, and papilin. Our results place these ADAMTSLIKE proteins in a molecular pathway involving fibrillin-1 and ADAMTS-10. Investigations of microfibril ultrastructure in WMS humans and mice demonstrate that modulation of the fibrillin microfibril scaffold can influence local tissue microenvironments and link fibrillin-1 function to skin homeostasis and the regulation of dermal collagen production. Hence, pathogenetic mechanisms caused by dysregulated WMS microenvironments diverge from Marfan pathogenetic mechanisms, which lead to broad activation of TGFβ signaling in multiple tissues. We conclude that local tissue-specific microenvironments, affected in WMS, are maintained by a fibrillin-1 microfibril scaffold, modulated by ADAMTSLIKE proteins in concert with ADAMTS enzymes.

Interactions of the ectodomain of HFE with the transferrin receptor are critical for iron homeostasis in cells

Expression of wild type HFE reduces the ferritin levels of cells in culture. In this report we demonstrate that the predominant hereditary hemochromatosis mutation, C282Y2 HFE, does not reduce ferritin expression. However, the second mutation, H63D HFE, reduces ferritin expression to a level indistinguishable from cells expressing wild type HFE. Further, two HFE cytoplasmic domain mutations engineered to disrupt potential signal transduction, S335M and Y342C, were functionally indistinguishable from wild type HFE in this assay, as was soluble HFE. These results implicate a role for the interaction of HFE with the transferrin receptor in lowering cellular ferritin levels.

Abnormal Activation of BMP Signaling Causes Myopathy in Fbn2 Null Mice.

Fibrillins are large extracellular macromolecules that polymerize to form the backbone structure of connective tissue microfibrils. Mutations in the gene for fibrillin-1 cause the Marfan syndrome, while mutations in the gene for fibrillin-2 cause Congenital Contractural Arachnodactyly. Both are autosomal dominant disorders, and both disorders affect musculoskeletal tissues. Here we show that Fbn2 null mice (on a 129/Sv background) are born with reduced muscle mass, abnormal muscle histology, and signs of activated BMP signaling in skeletal muscle. A delay in Myosin Heavy Chain 8, a perinatal myosin, was found in Fbn2 null forelimb muscle tissue, consistent with the notion that muscle defects underlie forelimb contractures in these mice. In addition, white fat accumulated in the forelimbs during the early postnatal period. Adult Fbn2 null mice are already known to demonstrate persistent muscle weakness. Here we measured elevated creatine kinase levels in adult Fbn2 null mice, indicating ongoing cycles of muscle injury. On a C57Bl/6 background, Fbn2 null mice showed severe defects in musculature, leading to neonatal death from respiratory failure. These new findings demonstrate that loss of fibrillin-2 results in phenotypes similar to those found in congenital muscular dystrophies and that FBN2 should be considered as a candidate gene for recessive congenital muscular dystrophy. Both in vivo and in vitro evidence associated muscle abnormalities and accumulation of white fat in Fbn2 null mice with abnormally activated BMP signaling. Genetic rescue of reduced muscle mass and accumulation of white fat in Fbn2 null mice was accomplished by deleting a single allele of Bmp7. In contrast to other reports that activated BMP signaling leads to muscle hypertrophy, our findings demonstrate the exquisite sensitivity of BMP signaling to the fibrillin-2 extracellular environment during early postnatal muscle development. New evidence presented here suggests that fibrillin-2 can sequester BMP complexes in a latent state.

Thoracic Aortic Aneurysm Frequency and Dissection are Associated with Fibrillin-1 Fragment Concentrations in Circulation

Rationale: Mutations in fibrillin-1 are associated with thoracic aortic aneurysm (TAA) in Marfan syndrome. Genome-wide association studies also implicate fibrillin-1 in sporadic TAA. Fragmentation of the aortic elastic lamellae is characteristic of TAA. Objective: Immunoassays were generated to test whether circulating fragments of fibrillin-1, or other microfibril fragments, are associated with TAA and dissection. Methods and Results: Plasma samples were obtained from 1265 patients with aortic aneurysm or dissection and from 125 control subjects. Concentrations of fibrillin-1, fibrillin-2, and fibulin-4 were measured with novel immunoassays. One hundred and seventy-four patients (13%) had aneurysms with only abdominal aortic involvement (abdominal aortic aneurysm), and 1091 (86%) had TAA. Of those with TAA, 300 patients (27%) had chronic dissection and 109 (10%) had acute or subacute dissection. Associations of fragment concentrations with TAA (versus abdominal aortic aneurysm) or with dissection (versus no dissection) were estimated with odds ratios (OR) and 95% confidence intervals (CI) adjusted for age, sex, and smoking. Compared with controls, significantly higher percentages of aneurysm patients had detectable levels of fibrillin fragments. TAA was significantly more common (than abdominal aortic aneurysm) in the highest compared with lowest quartile of fibrillin-1 concentration (OR=2.9; 95% CI, 1.6–5.0). Relative to TAA without dissection, acute or subacute dissection (OR=2.9; 95% CI, 1.6–5.3), but not chronic dissection, was more frequent in the highest compared with lowest quartile of fibrillin-1 concentration. Neither TAA nor dissection was associated with fibrillin-2 or fibulin-4. Conclusions: Circulating fibrillin-1 fragments represent a new potential biomarker for TAA and acute aortic dissection.

Uncoupling proteasome peptidase and ATPase activities results in cytosolic release of an ER polytopic protein

The 26S proteasome is the primary protease responsible for degrading misfolded membrane proteins in the endoplasmic reticulum. Here we examine the specific role of β subunit function on polypeptide cleavage and membrane release of CFTR, a prototypical ER-associated degradation substrate with 12 transmembrane segments. In the presence of ATP, cytosol and fully active proteasomes, CFTR was rapidly degraded and released into the cytosol solely in the form of trichloroacetic acid (TCA)-soluble peptide fragments. Inhibition of proteasome β subunits markedly decreased CFTR degradation but surprisingly, had relatively minor effects on membrane extraction and release. As a result, large TCA-insoluble degradation intermediates derived from multiple CFTR domains accumulated in the cytosol where they remained stably bound to inhibited proteasomes. Production of TCA-insoluble fragments varied for different proteasome inhibitors and correlated inversely with the cumulative proteolytic activities of β1, β2 and β5 subunits. By contrast, ATPase inhibition decreased CFTR release but had no effect on the TCA solubility of the released fragments. Our results indicate that the physiologic balance between membrane extraction and peptide cleavage is maintained by excess proteolytic capacity of the 20S subunit. Active site inhibitors reduce this capacity, uncouple ATPase and peptidase activities, and generate cytosolic degradation intermediates by allowing the rate of unfolding to exceed the rate of polypeptide cleavage.

Reticulocyte Lysate as a Model System to Study Endoplasmic Reticulum Membrane Protein Degradation

Recent studies have revealed that rabbit reticulocyte lysate (RRL) efficiently reconstitutes endoplasmic reticulum-associated degradation (ERAD) of mutant and misfolded membrane proteins. When supplemented with canine pancreas microsomal membranes, the RRL system faithfully carries out ER targeting, translocation, glycosylation, and membrane integration events and therefore provides a ready source of 35S-labeled protein with defined transmembrane topology. These substrates can be rapidly isolated in native ER membranes which, when incubated in RRL lacking exogenous hemin, are degraded in an ATP-dependent manner by the ubiquitin-proteasome pathway. Because the newly translated protein is the only source of radiolabel, degradation can be followed to its end state by conversion into trichloroacetic acid (TCA)-soluble peptide fragments. A particularly useful aspect of this system is that both membrane-associated and cytosolic components are amenable to biochemical and pharmacological manipulation. Here we describe techniques for preparing translation- and degradation-competent RRL, affinity depletion, identification of cytosolic factors involved in degrading the cystic fibrosis transmembrane conductance regulator (CFTR), and reconstitution of ERAD by add-back of purified recombinant proteins. These techniques provide a powerful tool for dissecting components involved in ubiquitination, degradation, and in particular, extraction of transmembrane ERAD substrates.

Sex, pregnancy and aortic disease in Marfan syndrome

Background Sex-related differences as well as the adverse effect of pregnancy on aortic disease outcome are well-established phenomena in humans with Marfan syndrome (MFS). The underlying mechanisms of these observations are largely unknown. Objectives In an initial (pilot) step we aimed to confirm the differences between male and female MFS patients as well as between females with and without previous pregnancy. We then sought to evaluate whether these findings are recapitulated in a pre-clinical model and performed in-depth cardiovascular phenotyping of mutant male and both nulliparous and multiparous female Marfan mice. The effect of 17β-estradiol on fibrillin-1 protein synthesis was compared in vitro using human aortic smooth muscle cells and fibroblasts. Results Our small retrospective study of aortic dimensions in a cohort of 10 men and 20 women with MFS (10 pregnant and 10 non-pregnant) confirmed that aortic root growth was significantly increased in the pregnant group compared to the non-pregnant group (0.64mm/year vs. 0.12mm/year, p = 0.018). Male MFS patients had significantly larger aortic root diameters compared to the non-pregnant and pregnant females at baseline and follow-up (p = 0.002 and p = 0.007, respectively), but no significant increase in aortic root growth was observed compared to the females after follow-up (p = 0.559 and p = 0.352). In the GT-8/+ MFS mouse model, multiparous female Marfan mice showed increased aortic diameters when compared to nulliparous females. Aortic dilatation in multiparous females was comparable to Marfan male mice. Moreover, increased aortic diameters were associated with more severe fragmentation of the elastic lamellae. In addition, 17β-estradiol was found to promote fibrillin-1 production by human aortic smooth muscle cells. Conclusions Pregnancy-related changes influence aortic disease severity in otherwise protected female MFS mice and patients. There may be a role for estrogen in the female sex protective effect.

Correction: Sex, pregnancy and aortic disease in Marfan syndrome

[This corrects the article DOI: 10.1371/journal.pone.0181166.].