Date Available

10-13-2022

Year of Publication

2022

Document Type

Doctoral Dissertation

Degree Name

Doctor of Philosophy (Medical Science)

College

Medicine

Department/School/Program

Neuroscience

Advisor

Dr. Donna M. Wilcock

Co-Director of Graduate Studies

Dr. Daniel Lee

Abstract

Vascular contributions to cognitive impairment and dementia (VCID) is one of the leading causes of dementia, along with Alzheimer’s disease (AD), and often manifests as a comorbidity of AD. Elevated plasma homocysteine, known as hyperhomocysteinemia (HHcy), is a risk factor for VCID; however, the mechanism underlying the connection between HHcy and development of VCID pathology remains elusive. Understanding this mechanism would reveal novel therapeutic targets with the potential to be disease modifying, which are a critical need for a disease that lacks any approved treatment. Previous studies from our lab have strongly associated neuroinflammation and blood brain barrier (BBB) dysregulation with VCID. Based on extensive data analysis in humans and animal models, we have identified proteinases that degrade essential components of the BBB, in particular matrix metalloproteinase 9 (MMP9), as being strongly associated with both HHcy and the progression of VCID pathology. We hypothesize that HHcy induces neuroinflammation, upregulating MMP9 which results in the dissociation of perivascular astrocytes from their vessels, endothelial cell tight-junction degradation, decreased BBB integrity, and the progression toward VCID pathology.

Treatment of WT and MMP9KO primary astrocytes and primary endothelial cells with HHcy and inflammation independently showed that while HHcy acting directly on astrocytes initiated a series of pathologic downstream effects, the MMP9 system was primarily activated by the inflammatory cascade initiated by microglia in response to HHcy. Additionally, we found that MMP2 may play a compensatory role in the absence of MMP9 which has many potential consequences when considering MMP9 inhibition as a therapeutic target.

When we placed C57Bl/6J WT and MMP9KO mice on a control diet or a HHcy inducing diet for 4, 8, 12, and 16 weeks we found that MMP9KO has the most impact on reducing the downstream pathologic effects of HHcy on the cerebrovasculature at more acute timepoints. Additionally, we found that impacting levels of MMP9 in our knockout mice may have produced an increase in NF-κB signaling to compensate for the lack of this important enzyme. These results have a significant impact on many fields currently attempting to inhibit MMP9 as a therapeutic target for a diverse array of pathologies.

Finally, following HHcy induction in both WT and MMP9KO mice we found that while hyperhomocysteinemia was induced in both the WT and MMP9KO mice, there were no robust significant differences between groups using novel object recognition or spontaneous alternation testing. Assessment of sensory-

motor coordination did reveal a significant genotype effect showing MMP9KO mice outperformed WT mice on the rotarod test regardless of which diet was administered.

From these studies it is clear that inflammation induced increases in MMP9 activity plays a complex and important role in the progression of VCID pathology and by better understanding this pathway can propose relevant and accessible targets for therapeutic intervention.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2022.370

Funding Information

This study was supported by the National Institute of Health's Fellowship Grant (Number: F31AG074530-01) in 2021 and 2022, the National Institute of Health's Training Grant (Number: T32NS077889) in 2020 and 2021 and the National Institute of Health's Training Grant (Number: T32GM118292) in 2020.

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