miR330-3p Dysregulation Plays a Critical Role in Disrupting the Endothelial Barrier During Staphylococcus aureus Infection

Sepsis is a life-threatening condition defined as an extreme inflammatory host response to infection. One of the most commonly isolated causative microorganisms from sepsis patients is the Gram-positive bacteria Staphylococcus aureus (S. aureus). Upon entry to the bloodstream, S. aureus locates and adheres to the inner lining of the blood vessels, the endothelium, and triggers the typical characteristics of sepsis such as a sustained and dysregulated immune response, hyper-coagulation and endothelial dysfunction.

These manifestations are thought to result from early dysregulation of the host endothelium in response to infection, of which one of the primary characteristics is loss of endothelial barrier integrity. Loss of barrier integrity is attributed to a loss of critical junction proteins such as VE-cadherin and ZO-1, located at cell-to-cell junction points. VE-cadherin is a central regulator of endothelial junctions and plays a pivotal role in mediating the permeability of the endothelial monolayer.

Currently, recommended management strategies for sepsis include an aggressive empiric administration of antibiotics and fluid resuscitation. Antibiotic treatment however has been inadequate and indeed, eradication of bacteria from the host is not sufficient to reverse adverse effects of the infection. This is in part due to the gap in understanding as to how infection of the endothelium triggers early endothelial dysfunction and as a result, many clinical trials and treatments have been ineffective and disappointing. Investigating these early pathogenic events in sepsis will be critical for preventing progression of the infection, as breakdown of the endothelial barrier provides a gateway for bacteria to access metastatic sites and major organs throughout the body. If left untreated, bacterial dissemination can lead to organ infection, organ failure and death.

miRNAs are signalling molecules responsible for regulating approximately 60% of all genes within the human genome. They are implemented in nearly all human biological processes including inflammation, immune responses

and endothelial function. Unsurprisingly, their dysregulation has been associated with a variety of clinically important human diseases such as cancers and immune diseases. They however, have not been widely studied in the pathogenesis of sepsis.

The overall goal of the work presented in this thesis was to investigate early signals triggered by S. aureus infection of the human endothelium that lead to the endothelial dysfunction associated with sepsis. We examine the impact of S. aureus infection upon the intrinsic expression of endothelial miRNAs, and focus on identifying the role of miRNAs in maintaining the integrity of the endothelial barrier through regulating the expression of important junction proteins.

In this study, we uncover the vast interference implemented by S. aureus into the expression of intrinsic endothelial miRNA. Specifically, we demonstrate that S. aureus infection causes dysregulated and enhanced expression of intrinsic miR330-3p. We identify a previously unknown role for miR330-3p as an important regulator of critical junction protein VE-cadherin within the endothelium. In addition, we demonstrate that the increase of miR330-3p implemented by S. aureus leads to the vascular leak observed during sepsis as the enhanced expression of miR330-3p leads to a harmful loss of junction protein VE-cadherin.

Our work suggests that S. aureus manipulates host signalling mechanisms by triggering abnormal expression of miRNA in the endothelium, in order to enhance the infection. By increasing expression of miR330-3p, and thereby hindering expression of VE-cadherin, S. aureus ensures that the endothelium provides a passageway to secondary sites and organs throughout the body.

Further work is required to investigate the expression of miR330-3p in response to S. aureus infection in vivo.