Novel Therapeutic Targets and/or Prognostic Markers in Marfan Syndrome [P01AR049698-07]

Marfan syndrome (MFS) is a congenital disorder of the connective tissue that is caused by mutations in fibrillin-1, the major structural component of extracellular microfibrils. We have originally hypothesized and subsequently demonstrated that flbrillin-1 mutations impair the sequestration of latent TGF beta complexes in the extracellular matrix with deleterious consequence to cell performance. This seminal discovery has led to the realization that TGF beta blockade is a productive new strategy to mitigate systemic manifestations in mouse models of MFS and perhaps in human patients. We now present data that implicate MAPKs and BMPs as additional contributors to organ-specific abnormalities in mouse models of MFS. These exciting new findings are consistent with the emerging role of microfibrils in the extracellular storage of several TGF beta superfamily members. They are also in line with the notion that TGF beta and BMPs can signal through MAPK-mediated pathways and that MAPKs can influence Smad activity in response to environmental stress and tissue injury. We therefore hypothesize that flbrillin-1 mutations trigger a series of matrix-dependent events that disrupt the physiological balance of TGF beta and BMP signaling in individual tissues, and that improper stimulation of p38 MAPK activity (through TGF beta and/or stress-response signals) exacerbates this disease-causing process Accordingly, the main focus of the present project is to evaluate the potential role of p38 and BMPs in the pathogenesis of aortic aneurysm and skeletal deformities in mouse models of progressively severe MFS. Specifically, we propose to: Aim 1: Interrogate the contribution of p38 activity to aortic aneurysm in Fbn1 mutant mice that are also defective in non-canonical TGF beta signaling or are systemically treated with inhibitors of p38 or Smad2/3 signaling. Aim 2: Evaluate the role of unbalanced TGF beta and BMP signaling in bone overgrowth using mice in which Fbnl deficiency is paired with loss of TGF beta or BMP type II receptors, or with over-expression of a BMP antagonist. The proposed studies will complement and be informed by parallel investigations on the contribution to aortic aneurysm of ERK1/2 signaling (Project 1), latent TGF beta activators (Project 2) and a structurally abnormal extracellular matrix (Project 3). Collectively, our efforts will shed new light on the complexity of the physiological roles that fibrillin-rich microfibrils play in organ formafion and function, in addition to providing evidence-based opportunities for therapeutic intervention in MFS and related disorders of the connective tissue.

Pathophysiology of Basement Membrane Zone Collagens [R01AR038648-25]

We seek to elucidate how collagen molecules participate in the assembly of higher order tissue structure and implicitly, how collagen dysfunctions contribute to the genesis and progression of connective tissue disorders.

Collagens are the most abundant and diverse components of the connective tissue and thus, the major structural determinants of organ formation and function. Stage and tissue-specific interactions among collagens, other matrix components, surrounding cells, and signaling molecules are responsible for the variety of forms and functions of the developing and mature connective tissue.

In spite of much progress, there is still a significant gap in our understanding of the mechanisms that translate the structural and cellular interactions of collagen molecules into the physiological properties of connective tissues. The lack of this basic science information in turn hampers our ability to model effective new therapies for a variety of human diseases in which organ function is severely or irreversibly impaired.

Based on past discoveries and exciting new data, we hypothesize that timely deposition of collagen XIX in selected basement membrane zones (BMZ) specifies their organization and instructs tissue differentiation. BMZ are morphologically defined entities that consist of BM scaffolds interposed between the cells and interstitial matrices. BMZ play both structural and instructive role in tissue formation, maintenance and remodeling. Mutations in BMZ-associated collagens result in clinical manifestations as diverse as skin blistering, cardiovascular dysfunctions, ocular degeneration, kidney failure, hemorrhagic stroke, and muscle degeneration.

In contrast to the cell-BM interface, information about the molecular interconnections between the BM and the underlying stroma is still primitive. Our genetic studies in mice have recently implicated collagen XIX as a potential new component of the molecular network that organizes the architecture of specialized BMZ. They have also suggested that assembly of a collagen XlX-rich matrix promotes cell differentiation.

It is our main objective to fully characterize the role of collagen XIX in organ physiology as the means to advance knowledge of the mechanisms that specify tissue architectures and functions. We therefore propose to characterize the structural and instructive roles of collagen XIX by studying the structural properties and molecular interactions that contribute to BMZ assembly and function, and tissue differentiation and growth.

Architectural Microfibrils in Bone Physiology [R01AR042044-17]

We seek to understand the contribution of extracellular microfibrils to bone physiology and implicitly, to elucidate the pathological underpinning of skeletal manifestations in Marfan syndrome (MFS) and congenital contractural arachnodactyly (CCA).

MFS and CCA are respectively caused by mutations in fibrillins 1 and 2, the major structural components of microfibrils. Extracellular microfibrils, alone or in association with elastin as elastic fibers, constitute the architectural scaffold of multiple organ systems, including the skeleton. Progress during the past funding cycle has revealed that disease progression in fibrillinopathies is accounted for in part by loss of tissue integrity and in part by dysregulated signaling events and abnormal cell performance. Preliminary studies suggest that fibrillin-1 and fibrillin-2 deficiencies affect bone formation and resorption to different extents by altering the balance of local signals that control maintenance of bone mass.

We therefore hypothesize that a causal relationship exists in the skeleton between the extent of the microfibril defect and the levels of dysregulated signaling and cellular responses. By analogy to the consequences of collagen I mutations on bone function, we also postulate that fibrillin mutations may negatively impact on the ability of these components of the architectural matrix to confer material and structural properties to skeletal tissue. The premise of the present application therefore rests on the innovative, evidence-based hypothesis that architectural microfibrils play two distinct roles in bone physiology — the role of regulators of signaling molecules to modulate bone formation, growth and turnover, and the role of a structural support for the matrix to impart bone strength.

Accordingly, we propose to utilize fibrillin mutant mice to:

  • Characterize the identity and nature of the cellular defects responsible for reduced bone mass in mice underexpressing fibrillin-1 or lacking fibrillin-2
  • Elucidate the differential roles of fibrillins 1 and 2 in cortical bone formation
  • Assess the impact of graded loss of fibrillin-rich microfibrils on matrix organization, material quality and biomechanical properties of bone and cartilage

The significance of the proposed studies is that an understanding of fibrillin function in the skeleton will shed new light on the ill-defined relationship between resident cells and the architectural matrix in this organ system. The long-term goal is to generate basic science information that will benefit the design of rational therapies for bone mineral replacement in patients affected with Marfan syndrome,and that improve our understanding of the nature of predisposing factors in osteoporosis.