Tang Lab 

Simon Tang, Ph.D

Assistant Professor

Department of Orthopaedics Washington University

tangs@wudosis.wustl.edu
BJC - Institute of Health 
11th floor - RM 11621         
Phone: (314) 268-2664 

Projects

The role of advanced glycation end-products in the degeneration of the intervertebral disc

Advanced glycation end-products (AGEs) are non-enzymatic sugar-based modifications of collagen that accumulate with aging, diabetes, and drug treatment, and increased AGEs impair the material properties of the extracellular matrix and contribute to skeletal fragility.  Although the role of AGEs in bone has been studied extensively, much less is known about their role in the intervertebral disc.  AGEs accumulates in the IVD with aging and progressive degeneration, and the increased AGEs may adversely affect the material properties, hydration status, and structural biomechanics of the intervertebral disc. Beyond the biomechanical effects, AGEs may play a signaling role in the inflammatory cascade that leads to abnormal matrix synthesis and early apoptosis in disc cells, consistent with observations in the degenerating disc.  The receptor-of-advanced-glycation-endproducts (r-AGEs) may be a potential mechanism that initiates the cellular degeneration of the IVD, leading to pain and matrix destruction.  Preliminary data from primary bovine disc cells also demonstrate a causative role between advanced glycation end-products, r-AGEs, and inflammatory cytokines. Future work will be focused on biologics and other targets for r-AGEs mediated inflammatory behavior as well as rescuing the effects of NEG-mediated mechanical changes in the disc.

 Figure 1: (A) MR imaging on intervertebral discs show that glycation significantly reduces the hydration of the disc tissues with greater effects on the nucleus pulposus than the annulus fibrosus (B). From Jazini et al 2011.


Structural changes of the spine during standing and its association to low back pain.

Low back pain (LBP) is a public health issue that afflicts a significant number of the U.S. population. The goal of this study is to examine the effects of mechanical strains of the IVD in the development of low back pain in a paradigm of standing utilizing the Open, Upright Magnetic Resonance Imaging System. The proposed studies are expected to identify the deformational changes of the loaded spine and how they are altered with LBP. The long-term objective is to define the multiscale clinical tools that relate clinical examination findings and the associated pathophysiology in LBP; and using these findings to guide clinical and rehabilitation strategies.


 

High-resolution Contrast-enhanced MicroCT imaging of Soft Tissues

Magnetic Resonance Imaging is commonly used to quantify intervertebral disc morphology changes in vivo for humans, but the low resolution of this modality limits its application to small rodent models, particularly for monitoring early degenerative changes. Histology has also been widely used to characterize the spatial distribution of proteins within the tissue, but it requires destructive preparation of tissues and only provides a 2D measurement. Micro-CT, which is based on x-ray attenuation of tissue structure, provides 3D quantification at a voxel resolution at micrometer level, but it provides poor detection of low attenuating unmineralized soft tissues.  We have developed a novel contrast-enhanced microCT technique in the lab for imaging rodent intervertebral discs capable of detecting nuanced structural and biochemical changes.  We are working to implement this technique for in vivo use.  

Figure 2: Contrast-enhanced microCT of the murine intervertebral disc showing progressive degeneration.  (Credit: Q. Wu)


 

The effects of nonenzymatic glycation on Proteome stability

The IVD is susceptible to nonenzymatic glycation (NEG) and advanced glycation endproduct (AGE) formation during aging, degeneration, and diabetes. The accumulation of AGEs in the IVD has adverse mechanical consequences that include a loss of viscous behavior and reduction in energy dissipation capability. At the molecular level, NEG negatively impacts protein structure and dynamics by altering protein stability and unfolding profiles. We hypothesize that in vitro NEG of IVD tissues would affect protein unfolding. Moreover, the Maillard chemical reaction that produces AGEs also carbonylates proteins. In avascular, low-turnover tissues such as the eye lens, articular cartilage, and the IVD, protein carbonylation contributes to protein misfolding and creates oxidative stress via reactive carbonyl species (RCS) formation, which can lead to increased reactive oxygen species (ROS) and lead to cell senescence and apoptosis. Consequently, an on-going area of investigation in the lab is the effect of NEG on IVD proteome stability and oxidative damage accumulation.  We specifically can quantify protein lability through two novel carbonyl and urea denaturation assays, quantified through continuous spectral analysis, to determine the relative contributions of NEG carbonylation in degenerating human IVD tissues.

Figure 3: Glycated tissues show a distinct profile in their carbonylation states and the resistance towards chaotropic stresses (Credit:  J.Liu).


 

Cell-Matrix Mechanobiologic interactions in Soft Tissues

Cells in load-bearing tissues exist in a constant feedback loop with the extracellular matrix (ECM) that regulates cell state, division, and senescence. External mechanical changes detected by mechanical receptors attached to the cell cytoskeleton regulate downstream biochemical pathways. Altered cytoskeleton organization corresponding to altered cell mechanical properties have been associated with changes in cell state during cancer, differentiation, and endothelial cell inflammation. Moreover, muscle cells show chemokine-induced cytoskeleton rearrangement regulating glucose uptake. Since diabetes induces chondrocyte senescence, apoptosis and morophological changes such as hypertrophy, the altered cell state may correspond to altered mechanical sensitivity that may lead to inflammatory, proteolytic, and reduced catabolic cell responses that may compromise tissue matrix integrity and increase the risk of damage and further degeneration. We plan continue to characterized cell state through the expression of inflammatory genes, cell susceptibility to mechanical damage, and strain-field analysis.

 Figure 1: Cells in different glycemic and AGEs environments exhibit altered morphologies and subsequent whole-cell stiffness as probed by atomic force microscopy (Credit: J.Liu).


 

 

Patient-specific methods for the in vivo diagnostic of fracture risk

DXA (Dual energy X-ray Absorptiometry), the clinical standard for bone fracture risk, while valuable and informative, is substantially limited in the prediction fracture and monitor drug therapies5. In addition to the investigation of fracture mechanisms, I will also develop patient-specific methods to assess fracture risk. The fracture resistance of bone is ultimately determined by bone’s ability to effectively dissipate mechanical energy, and mechanical testing is the most direct way to assess fracture resistance. Because the obvious destructive nature of traditional mechanical testing, as well as the scarcity of tissues and the invasiveness of biopsies to the patient, mechanical testing of bone tissue to determine its competence has never before been feasible in a clinical setting. In collaboration with industry partners, we will further investigate the predictive nature of minimally invasive microindentation towards whole bone fragility and stress fractures. This project will develop and extend measures of bone quality in the clinical assessment of fracture risk in conjunction with existing technologies. The results would be particularly applicable to age-related deterioration of bone quality, as well as the in vivo monitoring of alterations due to pharmaceutical treatments and therapies.

 Figure 5:MicroCT analyses of human bone enables us to understand the relationships between local mechanical behavior and the structure of the tissues such as porosity and local tissue mineralization (Credit: A. Abraham and A. Agarawalla).