Graduation Year
2022
Document Type
Dissertation
Degree
Ph.D.
Degree Name
Doctor of Philosophy (Ph.D.)
Degree Granting Department
Medicine
Major Professor
Lianchun Wang, M.D.
Committee Member
Jerome Breslin, Ph.D.
Committee Member
George E. Davis, M.D., Ph.D.
Committee Member
David Kang, Ph.D.
Keywords
Angiogenesis, Endothelial cell, Tau, Alzheimer’s disease
Abstract
My dissertation consists of two research projects. The first project investigated the roles of extracellular tau in the vasculature, including tau-induced angiogenesis and tau internalization, a topic related to tauopathy. The second project was to understand the regulatory functions of heparan sulfate proteoglycans in VEGFR1 signaling.
Tauopathies are a class of neurodegenerative diseases, including Alzheimer's disease, and are characterized by the appearance of intraneuronal tau inclusion in the brain and the patient's cognitive decline. Recent studies have emerged that soluble and aggregated tau also exists outside neurons in the central nervous system. Intriguingly, overexpressing tau in neurons in the mouse brain leads to increased cerebrovascular density and abnormal cerebrovascular morphology accompanied by disrupted cerebral blood flow and increased blood brain barrier permeability. In addition, the increased blood brain barrier permeability is normalized after the suppression of tau overexpression. These observations suggest that tau may regulate brain vasculature, supported by the observation that several angiogenic factors are upregulated in the tau overexpressing mice and AD patients. However, it remains unknown if tau directly regulates cerebral angiogenesis attributing to the vascular changes in the tau overexpression mice. In our studies, we observed that cerebral vascular density was increased in the PS19 mice, another commonly used tauopathy mouse model which expresses 5-times higher neuronal tau in the brain, which indicates that tau directly induced the new blood vessels formation. Furthermore, we found that tau induced potent human brain endothelial cell migration, proliferation and cord formation in vitro and angiogenesis in vivo. By angiogenesis protein array screening, we uncovered that tau downregulated the expression of chemokine CXCL10, a potent anti-angiogenic factor. The supplement of CXCL10 inhibited tau-induced angiogenic activities in vitro and angiogenesis in vivo. In addition, we also observed that tau-induced angiogenic activities depends on endothelial cell surface receptor LRP1. The mRNA expression level of CXCL10 was increased in LRP1 knockout cell line with tau treatment. These observations led to a conclusion that tau binds to endothelial cell surface LRP1, leading to the downregulation of CXCL10 expression and further increasing the balance toward angiogenesis.
The uptake of extracellular, pathogenic tau by healthy neurons is a crucial step of tauopathy propagation in the brain. Heparan sulfate is essential to mediate this process through direct binding to tau, but the critical heparan sulfate structures that exert the functions remain elusive. 3-O-sulfation is a rare modification in heparan sulfate with limited known biological functions. By testing with chemoenzymatic synthesized 3-O-sulfated heparan sulfate oligosaccharide and 3-O-sulfation-deficient mouse lung endothelial cells, we determined that the 3-O-sulfation increased about 9-fold binding affinity of heparan sulfate to tau. The 3-O-sulfate oligo showed more potent inhibition of tau uptake than the control heparan sulfate oligo which lacks the 3-O-sulfation and otherwise is structurally identical. In agreement, the 3-O-sulfation-deficient mouse lung endothelial cells showed impaired tau uptake compared to its wild-type control cells. The observations highlighted the importance of 3-O-sulfation for heparan sulfate to mediate tau uptake and suggested that targeting 3-O-sulfation of heparan sulfate might effectively block pathogenic tau propagation, thereby slowing down and treating tauopathy.
The interaction between VEGF and VEGFRs, especially VEGFR2, is the master angiogenic signaling pathway. Many studies have established that heparan sulfate interacts with VEGFRs, possibly VEGFR2, to be obligated for the signaling-driving angiogenesis. Early studies also have reported that heparan sulfate binds VEGFR1. However, it is unknown the heparan sulfate structures involved in the binding and if the heparan sulfate binding regulates VEGFR1 signaling in angiogenesis. By testing with chemically modified and sized heparin, a commonly used heparan sulfate analog, and a serial heparan sulfate mutant mouse lung endothelial cell lines which are deficient in various types of sulfation modification(s), we uncovered that the binding of heparin and heparan sulfate to VEGFR is size- and sulfation-dependent, especially the N-sulfation. In endothelial cells, endogenous deletion of HS expression enhances the VEGFR1 signaling elicited by PIGF1, a non-heparin-binding PIGF isoform. These findings revealed that endothelial heparan sulfate binds VEGFR1 to suppress PIGF1-VEGFR1 signaling in endothelial cells.
In summary, these studies have significantly advanced our understanding of the biological functions of extracellular tau on brain vasculature as well as the biological function of heparan sulfate in both tau propagation process and VEGFR1 signaling.
Scholar Commons Citation
Zhu, Yanan, "The biological roles of tau protein in the vasculature and the regulation of VEGFR1 signaling by heparan sulfate" (2022). USF Tampa Graduate Theses and Dissertations.
https://digitalcommons.usf.edu/etd/10417
