Fibrillin-rich microfibrils - structural and instructive determinants of mammalian development and physiology

Insights into elastic fiber fragmentation: Mechanisms and treatment of aortic aneurysm in Marfan syndrome

Marfan syndrome (MFS) is an autosomal dominant connective tissue disorder caused by mutations in fibrillin 1 (FBN1) gene. These mutations result in defects in the skeletal, ocular, and cardiovascular systems. Aortic aneurysm is the leading cause of premature mortality in untreated MFS patients. Elastic fiber fragmentation in the aortic vessel wall is a hallmark of MFS-associated aortic aneurysms. FBN1 mutations result in FBN1 fragments that also contribute to elastic fiber fragmentation. Although recent research has advanced our understanding of MFS, the contribution of elastic fiber fragmentation to the pathogenesis of aneurysm formation remains poorly understood. This review provides a comprehensive overview of the molecular mechanisms of elastic fiber fragmentation and its role in the pathogenesis of aortic aneurysm progression. Increased comprehension of elastic fragmentation has significant clinical implications for developing targeted interventions to block aneurysm progression, which would benefit not only individuals with Marfan syndrome but also other patients with aneurysms. Moreover, this review highlights an overlooked connection between inhibiting aneurysm and the restoration of elastic fibers in the vessel wall with various aneurysm inhibitors, including drugs and chemicals. Investigating the underlying molecular mechanisms could uncover innovative therapeutic strategies to inhibit elastin fragmentation and prevent the progression of aneurysms.

Perlecan in Pericellular Mechanosensory Cell-Matrix Communication, Extracellular Matrix Stabilisation and Mechanoregulation of Load-Bearing Connective Tissues

In this study, we review mechanoregulatory roles for perlecan in load-bearing connective tissues. Perlecan facilitates the co-acervation of tropoelastin and assembly of elastic microfibrils in translamellar cross-bridges which, together with fibrillin and elastin stabilise the extracellular matrix of the intervertebral disc annulus fibrosus. Pericellular perlecan interacts with collagen VI and XI to define and stabilize this matrix compartment which has a strategic position facilitating two-way cell-matrix communication between the cell and its wider extracellular matrix. Cues from the extracellular matrix are fed through this pericellular matrix back to the chondrocyte, allowing it to perceive and respond to subtle microenvironmental changes to regulate tissue homeostasis. Thus perlecan plays a key regulatory role in chondrocyte metabolism, and in chondrocyte differentiation. Perlecan acts as a transport proteoglycan carrying poorly soluble, lipid-modified proteins such as the Wnt or Hedgehog families facilitating the establishment of morphogen gradients that drive tissue morphogenesis. Cell surface perlecan on endothelial cells or osteocytes acts as a flow sensor in blood and the lacunar canalicular fluid providing feedback cues to smooth muscle cells regulating vascular tone and blood pressure, and the regulation of bone metabolism by osteocytes highlighting perlecan's multifaceted roles in load-bearing connective tissues.

Estrogen-related mechanisms in sex differences of hypertension and target organ damage

Hypertension (HTN) is a primary risk factor for cardiovascular (CV) events, target organ damage (TOD), premature death and disability worldwide. The pathophysiology of HTN is complex and influenced by many factors including biological sex. Studies show that the prevalence of HTN is higher among adults aged 60 and over, highlighting the increase of HTN after menopause in women. Estrogen (E2) plays an important role in the development of systemic HTN and TOD, exerting several modulatory effects. The influence of E2 leads to alterations in mechanisms regulating the sympathetic nervous system, renin-angiotensin-aldosterone system, body mass, oxidative stress, endothelial function and salt sensitivity; all associated with a crucial inflammatory state and influenced by genetic factors, ultimately resulting in cardiac, vascular and renal damage in HTN. In the present article, we discuss the role of E2 in mechanisms accounting for the development of HTN and TOD in a sex-specific manner. The identification of targets with therapeutic potential would contribute to the development of more efficient treatments according to individual needs.

Assessment of Bones Deficient in Fibrillin-1 Microfibrils Reveals Pronounced Sex Differences

Defects in the extracellular matrix protein fibrillin-1 that perturb transforming growth factor beta (TGFβ) bioavailability lead to Marfan syndrome (MFS). MFS is an autosomal-dominant disorder, which is associated with connective tissue and skeletal defects, among others. To date, it is unclear how biological sex impacts the structural and functional properties of bone in MFS. The aim of this study was to investigate the effects of sex on bone microarchitecture and mechanical properties in mice with deficient fibrillin-1, a model of human MFS. Bones of 11-week-old male and female Fbn1^mgR/mgR mice were investigated. Three-dimensional micro-computed tomography of femora and vertebrae revealed a lower ratio of trabecular bone volume to tissue volume, reduced trabecular number and thickness, and greater trabecular separation in females vs. males. Three-point bending of femora revealed significantly lower post-yield displacement and work-to-fracture in females vs. males. Mechanistically, we found higher Smad2 and ERK1/2 phosphorylation in females vs. males, demonstrating a greater activation of TGFβ signaling in females. In summary, the present findings show pronounced sex differences in the matrix and function of bones deficient in fibrillin-1 microfibrils. Consequently, sex-specific analysis of bone characteristics in patients with MFS may prove useful in improving the clinical management and life quality of these patients, through the development of sex-specific therapeutic approaches.

3-photon fluorescence imaging of sulforhodamine B-labeled elastic fibers in the mouse skin in vivo

Elastic fibers are key constituents of the skin. The commonly adopted optical technique for visualizing elastic fibers in the animal skin in vivo is 2-photon microscopy (2 PM) of autofluorescence, which typically suffers from low signal level. Here we demonstrate a new optical methodology to image elastic fibers in animal models in vivo: 3-photon microscopy (3 PM) excited at the 1700-nm window combining with preferential labeling of elastic fibers using sulforhodamine B (SRB). First, we demonstrate that intravenous injection of SRB can circumvent the skin barrier (encountered in topical application) and preferentially label elastic fibers, as verified by simultaneous 2 PM of both autofluorescence and SRB fluorescence from skin structures. Then through 3-photon excitation property characterization, we show that 3-photon fluorescence can be excited from SRB at the 1700-nm window, and 1600-nm excitation is most efficient according to our 3-photon action cross section measurement. Based on these results and using our developed 1600-nm femtosecond laser source, we finally demonstrate 3 PM of SRB-labeled elastic fibers through the whole dermis in the mouse skin in vivo, with only 3.7-mW optical power deposited on the skin surface. We expect our methodology will provide novel optical solution to elastic fiber research.

Molecular Mechanisms of Dermal Aging and Antiaging Approaches

The dermis is primarily composed of the extracellular matrix (ECM) and fibroblasts. During the aging process, the dermis undergoes significant changes. Collagen, which is a major component of ECM, becomes fragmented and coarsely distributed, and its total amount decreases. This is mainly due to increased activity of matrix metalloproteinases, and impaired transforming growth factor-β signaling induced by reactive oxygen species generated during aging. The reduction in the amount of collagen hinders the mechanical interaction between fibroblasts and the ECM, and consequently leads to the deterioration of fibroblast function and further decrease in the amount of dermal collagen. Other ECM components, including elastic fibers, glycosaminglycans (GAGs), and proteoglycans (PGs), also change during aging, ultimately leading to a reduction in the amount of functional components. Elastic fibers decrease in intrinsically aged skin, but accumulate abnormally in photoaged skin. The changes in the levels of GAGs and PGs are highly diverse, and previous studies have reported conflicting results. A reduction in the levels of functional dermal components results in the emergence of clinical aging features, such as wrinkles and reduced elasticity. Various antiaging approaches, including topicals, energy-based procedures, and dermal fillers, can restore the molecular features of dermal aging with clinical efficacy. This review summarizes the current understanding of skin aging at the molecular level, and associated treatments, to put some of the new antiaging technology that has emerged in this rapidly expanding field into molecular context.

Contribution of metabolic disease to bone fragility in MAGP1-deficient mice

Microfibril-associated glycoprotein-1 (MAGP1) is an extracellular matrix protein that interacts with fibrillin and is involved in regulating the bioavailability of signaling molecules such as TGFβ. Mice with germline MAGP1 deficiency (Mfap2-/-) develop increased adiposity, hyperglycemia, insulin resistance, bone marrow adipose tissue expansion, reduced cancellous bone mass, cortical bone thinning and bone fragility. The goal of this study was to assess whether the Mfap2-/- bone phenotypes were due to loss of MAGP1 locally or secondary to a change in whole body physiology (metabolic dysfunction). To do this, mice with conditional deletion of MAGP1 in the limb skeleton were generated by crossing MAGP1-flox mice (Mfap2lox/lox) with Prx1-Cre mice. Mfap2Prx-/- mice did not show any changes in peripheral adiposity, hyperglycemia or insulin sensitivity, but did have increased bone length and cancellous bone loss that was comparable to the germline Mfap2-/- knockout. Unlike the germline knockout, marrow adiposity, cortical bone thickness and bone strength in Mfap2Prx-/- mice were normal. These findings implicate systemic metabolic dysfunction in the development of bone fragility in germline Mfap2-/- mice. An unexpected finding of this study was the detection of MAGP1 protein in the Mfap2Prx-/- hematopoietic bone marrow, despite the absence of MAGP1 protein in osseous bone matrix and absent Mfap2 transcript expression at both sites. This suggests MAGP1 from a secondary site may accumulate in the bone marrow, but not be incorporated into the bone matrix, during times of regional MAGP1 depletion.

The TGF-β Signalling Network in Muscle Development, Adaptation and Disease

Skeletal muscle possesses remarkable ability to change its size and force-producing capacity in response to physiological stimuli. Impairment of the cellular processes that govern these attributes also affects muscle mass and function in pathological conditions. Myostatin, a member of the TGF-β family, has been identified as a key regulator of muscle development, and adaptation in adulthood. In muscle, myostatin binds to its type I (ALK4/5) and type II (ActRIIA/B) receptors to initiate Smad2/3 signalling and the regulation of target genes that co-ordinate the balance between protein synthesis and degradation. Interestingly, evidence is emerging that other TGF-β proteins act in concert with myostatin to regulate the growth and remodelling of skeletal muscle. Consequently, dysregulation of TGF-β proteins and their associated signalling components is increasingly being implicated in muscle wasting associated with chronic illness, ageing, and inactivity. The growing understanding of TGF-β biology in muscle, and its potential to advance the development of therapeutics for muscle-related conditions is reviewed here.

Characterization of metabolic health in mouse models of fibrillin-1 perturbation

Mutations in the microfibrillar protein fibrillin-1 or the absence of its binding partner microfibril-associated glycoprotein (MAGP1) lead to increased TGFβ signaling due to an inability to sequester latent or active forms of TGFβ, respectively. Mouse models of excess TGFβ signaling display increased adiposity and predisposition to type-2 diabetes. It is therefore interesting that individuals with Marfan syndrome, a disease in which fibrillin-1 mutation leads to aberrant TGFβ signaling, typically present with extreme fat hypoplasia. The goal of this project was to characterize multiple fibrillin-1 mutant mouse strains to understand how fibrillin-1 contributes to metabolic health. The results of this study demonstrate that fibrillin-1 contributes little to lipid storage and metabolic homeostasis, which is in contrast to the obesity and metabolic changes associated with MAGP1 deficiency. MAGP1 but not fibrillin-1 mutant mice had elevated TGFβ signaling in their adipose tissue, which is consistent with the difference in obesity phenotypes. However, fibrillin-1 mutant strains and MAGP1-deficient mice all exhibit increased bone length and reduced bone mineralization which are characteristic of Marfan syndrome. Our findings suggest that Marfan-associated adipocyte hypoplasia is likely not due to microfibril-associated changes in adipose tissue, and provide evidence that MAGP1 may function independently of fibrillin in some tissues.

MAGP1, the extracellular matrix, and metabolism

Adipose tissue and the extracellular matrix were once considered passive players in regulating physiological processes. Now, both entities are acknowledged for their capacity to engage signal transduction pathways, and for their involvement in maintaining normal tissue homeostasis. We recently published a series of studies that identified a novel mechanism whereby an extracellular matrix molecule, MAGP1 (microfibril associated glycoprotein 1), can regulate energy metabolism in adipose tissue. MAGP1 is a component of extracellular microfibrils and plays a supportive role in maintaining thermoregulation by indirectly regulating expression of the thermogenic uncoupling proteins (UCPs). The focus of this commentary is to draw attention to the role of the extracellular matrix in regulating the bioavailability of signaling molecules, like transforming growth factor β (TGFβ), and exemplify that a better understanding of the extracellular matrix's biological properties could unveil a new source of therapeutic targets for metabolic diseases.

The extracellular matrix protein MAGP1 supports thermogenesis and protects against obesity and diabetes through regulation of TGF-β

Microfibril-associated glycoprotein 1 (MAGP1) is a component of extracellular matrix microfibrils. Here we show that MAGP1 expression is significantly altered in obese humans, and inactivation of the MAGP1 gene (Mfap2(-/-)) in mice results in adipocyte hypertrophy and predisposition to metabolic dysfunction. Impaired thermoregulation was evident in Mfap2(-/-) mice prior to changes in adiposity, suggesting a causative role for MAGP1 in the increased adiposity and predisposition to diabetes. By 5 weeks of age, Mfap2(-/-) mice were maladaptive to cold challenge, uncoupling protein-1 expression was attenuated in the brown adipose tissue, and there was reduced browning of the subcutaneous white adipose tissue. Levels of transforming growth factor-β (TGF-β) activity were elevated in Mfap2(-/-) adipose tissue, and the treatment of Mfap2(-/-) mice with a TGF-β-neutralizing antibody improved their body temperature and prevented the increased adiposity phenotype. Together, these findings indicate that the regulation of TGF-β by MAGP1 is protective against the effects of metabolic stress, and its absence predisposes individuals to metabolic dysfunction.

Early fibrillin-1 assembly monitored through a modifiable recombinant cell approach

Fibrillin proteins constitute the backbone of extra-cellular macromolecular microfibrils. Mutations in fibrillins cause heritable connective tissue disorders, including Marfan syndrome, dominant Weill-Marchesani syndrome, and stiff skin syndrome. Fibronectin provides a critical scaffold for microfibril assembly in cell culture models. Full length recombinant fibrillin-1 was expressed by HEK 293 cells, which deposited the secreted protein in a punctate pattern on the cell surface. Cocultured fibroblasts consistently triggered assembly of recombinant fibrillin-1, which was dependent on a fibronectin network formed by the fibroblasts. Deposition of recombinant fibrillin-1 on fibronectin fibers occurred first in discrete packages that subsequently extended along fibronectin fibers. Mutant fibrillin-1 harboring either a cysteine 204 to serine mutation or a RGD to RGA mutation which prevents integrin binding, did not affect fibrillin-1 assembly. In conclusion, we developed a modifiable recombinant full-length fibrillin-1 assembly system that allows for rapid analysis of critical roles in fibrillin assembly and functionality. This system can be used to study the contributions of specific residues, domains, or regions of fibrillin-1 to the biogenesis and functionality of microfibrils. It provides also a method to evaluate disease-causing mutations, and to produce microfibril-containing matrices for tissue engineering applications, for example, in designing novel vascular grafts or stents.

Differential stage-dependent regulation of prostatic epithelial morphogenesis by Hedgehog signaling

Published studies of Hh (Hedgehog) signaling in the developing prostate have reported varying and discrepant effects on epithelial proliferation, ductal morphogenesis and growth. We report here that these differing observations accrue from stage-specific effects of Hh signaling in the developing prostate. Using in vitro organ cultures of the E16 UGS and P1 prostate, we show that ectopic Hh pathway activation stimulates epithelial proliferation prenatally, but inhibits epithelial proliferation postnatally. Extrapolating from previously published observations that Hh target gene expression is altered in the reactive stroma of prostate cancer, we examined and found discordant regulation of a subset of target genes by Hh signaling in the prenatal and postnatal prostate. Cell based studies and recombination assays show that these changes are not simply attributable to the age of the mesenchyme or the epithelium, but more likely reflect a complex regulation by the cellular microenvironment. To determine the in vivo relevance of these observations, we examined the effect of transgenic activation of Hh signaling on epithelial proliferation in the prenatal and postnatal prostate and confirmed the operation of stage-specific effects. These observations demonstrate stage-specific differences in the effect of Hh signaling on epithelial proliferation in the developing prostate and suggest that these are a product of complex interactions determined by the cellular microenvironment.

Comparative immunolocalisation of fibrillin-1 and perlecan in the human foetal, and HS-deficient hspg2 exon 3 null mutant mouse intervertebral disc

The aim of this study was to examine the comparative localisations of fibrillin-1 and perlecan in the foetal human, wild-type C57BL/6 and HS-deficient hspg2Δ³⁻/Δ³⁻ exon 3 null mouse intervertebral disc (IVD) using fluorescent laser scanning confocal microscopy. Fibrillin-1 fibrils were prominent components of the outer posterior and anterior annulus fibrosus (AF) of the foetal human IVD. Finer fibrillin-1 fibrils were evident in the inner AF where they displayed an arcade-type arrangement in the developing lamellae. Relatively short but distinct fibrillin-1 fibrils were evident in the central region of the IVD and presumptive cartilaginous endplate and defined the margins of the nuclear sheath in the developing nucleus pulposus (NP). Fibrillin-1 was also demonstrated in the AF of C57BL/6 wild-type mice but to a far lesser extent in the HS-deficient hspg2Δ³⁻/Δ³⁻ exon 3 null mouse. This suggested that the HS chains of perlecan may have contributed to fibrillin-1 assembly or its deposition in the IVD. The cell-matrix interconnections provided by the fibrillin fibrils visualised in this study may facilitate communication between disc cells and their local biomechanical microenvironment in mechanosensory processes which regulate tissue homeostasis. The ability of fibrillin-1 to sequester TGF-β a well-known anabolic growth factor in the IVD also suggests potential roles in disc development and/or remodelling.

Oophorectomy-induced bone loss is attenuated in MAGP1-deficient mice

Microfibril-associated glycoprotein-1 (MAGP1), together with the fibrillins, are constitutive components of vertebrate microfibrils. Mice deficient in MAGP1 (murine MAGP1 knockout animals (Mfap2(-/-)); MAGP1Δ) is appropriate develop progressive osteopenia and reduced whole bone strength, and have elevated numbers of osteoclasts lining the bone surface. Our previous studies suggested that the increased osteoclast population was associated with elevated levels of receptor activator of NF-κB ligand (RANKL), a positive regulator of osteoclast differentiation. To explore the relationship between RANKL expression and osteoclast differentiation in MAGP1 deficiency, oophorectomy (OVX) was used to stimulate RANKL expression in both WT and MAGP1Δ animals. Bone loss following OVX was monitored using whole body DEXA and in vivo µCT. While WT mice exhibited significant bone loss following OVX, percent bone loss was reduced in MAGP1Δ mice. Further, serum RANKL levels rose significantly in OVX WT mice, whereas, there was only a modest increase in RANKL following OVX in the mutant mice due to already high baseline levels. Elevated RANKL expression was normalized when cultured MAGP1Δ osteoblasts were treated with a neutralizing antibody targeting free TGFβ. These studies provide support for increased RANKL expression associated with MAGP1 deficiency and provide a link to altered TGF-β signaling as a possible causative signaling pathway regulating RANKL expression in MAGP1Δ osteoblasts.

Dissecting the fibrillin microfibril: structural insights into organization and function

Force-bearing tissues such as blood vessels, lungs, and ligaments depend on the properties of elasticity and flexibility. The 10 to 12 nm diameter fibrillin microfibrils play vital roles in maintaining the structural integrity of these highly dynamic tissues and in regulating extracellular growth factors. In humans, defective microfibril function results in several diseases affecting the skin, cardiovascular, skeletal, and ocular systems. Despite the discovery of fibrillin-1 having occurred more than two decades ago, the structure and organization of fibrillin monomers within the microfibrils are still controversial. Recent structural data have revealed strategies by which fibrillin is able to maintain its architecture in dynamic tissues without compromising its ability to interact with itself and other cell matrix components. This review summarizes our current knowledge of microfibril structure, from individual fibrillin domains and the calcium-dependent tuning of pairwise interdomain interactions to microfibril dynamics, and how this relates to microfibril function in health and disease.

Fibrillin-1 genotype and risk of prevalent hypertension: a study in two independent populations

OBJECTIVE

Mutations in the fibrillin-1 gene are the cause of Marfan syndrome. We wanted to investigate the relationship between a mutation in this gene and risk of prevalent hypertension.

METHODS

In a cross-sectional study, the effect of a G-A substitution in intron 27 in the fibrillin-1 gene (rs11856553) on risk of prevalent hypertension was studied in two large population-based studies: the Health 2006 study, consisting of 3193 women and men, age 18-69 years, and the MONICA10 study, consisting of 2408 women and men, age 41-72 years. In 1646 MONICA10 participants, blood pressure (BP) was also measured by 24-h ambulatory recordings.

RESULTS

Among the 3193 Health 2006 participants 23 had the G-A variant, and among the 2408 MONICA10 participants 18 had the G-A variant. In Health 2006, the odds ratio estimate (95% confidence intervals) for the G-A variant for risk of hypertension, defined as systolic (S) BP ≥ 140 mmHg or diastolic (D) BP ≥ 90 mmHg or on antihypertensive medicine, was 2.67 (1.14-6.18), p = 0.022. The corresponding figure for moderate to severe hypertension, defined as SBP ≥ 160 mmHg or DBP ≥ 100 mmHg, was 9.68 (4.24-22.12), p < 0.0001. In MONICA10, the odds ratio estimate (95% confidence intervals) for the G-A variant for risk of moderate to severe ambulatory hypertension, defined as 24-h mean SBP ≥ 150 mmHg or 24-h mean DBP ≥ 90 mmHg, was 5.73 (1.96-16.7), p = 0.0014.

CONCLUSION

The G-A substitution in the fibrillin-1 gene (rs11856553) is a rare genetic variant that is associated with an increased risk of prevalent hypertension, particularly of moderate to severe prevalent hypertension.

Colocalization in vivo and association in vitro of perlecan and elastin

We have colocalized elastin and fibrillin-1 with perlecan in extracellular matrix of tensional and weight-bearing connective tissues. Elastin and fibrillin-1 were identified as prominent components of paraspinal blood vessels, and posterior longitudinal ligament in the human fetal spine and outer annulus fibrosus of the fetal intervertebral disc. We also colocalized perlecan with a synovial elastic basal lamina, where the attached synovial cells were observed to produce perlecan. Elastin, fibrillin-1 and perlecan were co-localized in the intima and media of small blood vessels in the synovium and in human fetal paraspinal blood vessels. Elastic fibers were observed at the insertion point of the anterior cruciate ligament to bone in the ovine stifle joint where they colocalized with perlecan. Elastin has not previously been reported to be spatially associated with perlecan in these tissues. Interactions between the tropoelastin and perlecan heparan sulfate chains were demonstrated using quartz crystal microbalance with dissipation solid phase binding studies. Electrostatic interactions through the heparan sulfate chains of perlecan and core protein mediated the interactions with tropoelastin, and were both important in the coacervation of tropoelastin and deposition of elastin onto perlecan immobilized on the chip surface. This may help us to understand the interactions which are expected to occur in vivo between the tropoelastin and perlecan to facilitate the deposition of elastin and formation of elastic microfibrils in situ and would be consistent with the observed distributions of these components in a number of connective tissues.

Molecular aspects of skin ageing

Ageing of human skin may result from both the passage of time (intrinsic ageing) and from cumulative exposure to external influences (extrinsic ageing) such as ultraviolet radiation (UVR) which promote wrinkle formation and loss of tissue elasticity. Whilst both ageing processes are associated with phenotypic changes in cutaneous cells, the major functional manifestations of ageing occur as a consequence of structural and compositional remodeling of normally long-lived dermal extracellular matrix proteins. This review briefly considers the effects of ageing on dermal collagens and proteoglycans before focusing on the mechanisms, functional consequences and treatment of elastic fibre remodeling in ageing skin. The early stages of photoageing are characterised by the differential degradation of elastic fibre proteins and whilst the activity of extracellular matrix proteases is increased in photoexposed skin, the substrate specificity of these enzymes is low. We have recently shown however, that isolated fibrillin microfibrils are susceptible to direct degradation by physiologically attainable doses of UV-B radiation and that elastic fibre proteins as a group are highly enriched in UV-absorbing amino acid residues. Functionally, elastic fibre remodeling events may adversely impact on: the mechanical properties of tissues, the recruitment and activation of immune cells, the expression of matrix metalloproteinases and cytokine signaling (by perturbing fibrillin microfibril sequestration of TGFβ). Finally, newly developed topical interventions appear to be capable of regenerating elements of the elastic fibre system in ageing skin, whilst systemic treatments may potentially prevent the pathological tissue remodeling events which occur in response to elastic fibre degradation.

Microfibril-associated glycoprotein-1, an extracellular matrix regulator of bone remodeling

MAGP1 is an extracellular matrix protein that, in vertebrates, is a ubiquitous component of fibrillin-rich microfibrils. We previously reported that aged MAGP1-deficient mice (MAGP1Delta) develop lesions that are the consequence of spontaneous bone fracture. We now present a more defined bone phenotype found in MAGP1Delta mice. A longitudinal DEXA study demonstrated age-associated osteopenia in MAGP1Delta animals and muCT confirmed reduced bone mineral density in the trabecular and cortical bone. Further, MAGP1Delta mice have significantly less trabecular bone, the trabecular microarchitecture is more fragmented, and the diaphyseal cross-sectional area is significantly reduced. The remodeling defect seen in MAGP1Delta mice is likely not due to an osteoblast defect, because MAGP1Delta bone marrow stromal cells undergo osteoblastogenesis and form mineralized nodules. In vivo, MAGP1Delta mice exhibit normal osteoblast number, mineralized bone surface, and bone formation rate. Instead, our findings suggest increased bone resorption is responsible for the osteopenia. The number of osteoclasts derived from MAGP1Delta bone marrow macrophage cells is increased relative to the wild type, and osteoclast differentiation markers are expressed at earlier time points in MAGP1Delta cells. In vivo, MAGP1Delta mice have more osteoclasts lining the bone surface. RANKL (receptor activator of NF-kappaB ligand) expression is significantly higher in MAGP1Delta bone, and likely contributes to enhanced osteoclastogenesis. However, bone marrow macrophage cells from MAGP1Delta mice show a higher propensity than do wild-type cells to differentiate to osteoclasts in response to RANKL, suggesting that they are also primed to respond to osteoclast-promoting signals. Together, our findings suggest that MAGP1 is a regulator of bone remodeling, and its absence results in osteopenia associated with an increase in osteoclast number.

Marfan syndrome and schizophrenia: a case report and literature review

INTRODUCTION

Marfan Syndrome (MFS), a disease of microfibril dysfunction, has been associated with schizophrenia in multiple case reports.

CASE REPORT

We present one case and review the literature that suggests these conditions may share a common etiologic pathway.

DISCUSSION

A possible underlying mechanism of both schizophrenia and MFS is the abnormal expression of growth factors and signaling cascades.

CONCLUSION

MFS patients should be monitored for psychiatric symptoms and patients with signs of MFS should be referred for appropriate medical care. Also, by understanding shared mechanisms, we may develop better understanding and treatments.

Neuraminidase-1 is required for the normal assembly of elastic fibers

The assembly of elastic fibers in tissues that undergo repeated cycles of extension and recoil, such as the lungs and blood vessels, is dependent on the proper interaction and alignment of tropoelastin with a microfibrillar scaffold. Here, we describe in vivo histopathological effects of neuraminidase-1 (Neu1) deficiency on elastin assembly in the lungs and aorta of mice. These mice exhibited a tight-skin phenotype very similar to the Tsk mouse. Normal septation of Neu1-null mice did not occur in neonatal mice, resulting in enlarged alveoli that were maintained in adults. The abnormal development of elastic fibers was remarkable under electron microscopy and confirmed by the overlapping distribution of elastin, fibrillin-1, fibrillin-2, and fibulin-5 (Fib-5) by the light microscopy immunostainings. Fib-5 fibers appeared diffuse and unorganized around the alveolar walls and the apex of developing secondary septal crests. Fibrillin-2 deposition was also abnormal in neonatal and adult lungs. Dispersion of myofibroblasts appeared abnormal in developing lungs of Neu1-null mice, with a random distribution of myofibroblast around the alveolar walls, rather than concentrating at sites of elastin synthesis. The elastic lamellae in the aorta of the Neu1-null mice were thinner and separated by hypertrophic smooth muscle cells that were surrounded by an excess of the sialic acid-containing moieties. The concentration of elastin, as measure by desmosine levels, was significantly reduced in the aorta of Neu1-null mice. Message levels for tropoelastin and Fib-5 were normal, suggesting the elastic fiber defects in Neu1-null mice result from impaired extracellular assembly.