Ingrid H. Sarelius
My research focus is on vascular cell communication and signaling. Our lab is specifically interested in how the cells of the blood vessel wall communicate with each other, with the surrounding tissue, and with the blood itself. Basically, we’re interested in figuring out how microvessels work! Since cardiovascular disease is a major killer of adult populations, and since much of this is vascular disease or disorder, understanding normal blood vessel function seems like a good place to start.
The major tool that we use to ask our questions is confocal immunofluorescent intravital microscopy; we are one of a handful of labs worldwide that is able to image local events in cells of the vascular wall in real time in blood perfused microvessels in living animals. We also use single microparticle tracking to define the local fluid dynamic environment in vivo, and confocal imaging of signaling molecules and leukocyte populations in the in situ blood vessel.
Our research is supported by several NIH grants that are either funded or pending, and we are also part of an NIH Program Project, which is a large multi-investigator grant that is focused on understanding leukocyte biology and biophysics.
At present we have two major areas of investigation, with several projects underway in each.
Inflammation. During inflammation, different populations of white blood cells emigrate from the vasculature into the tissue: also during inflammation, fluid and proteins leave the vasculature, causing edema in the surrounding tissue. We recently made the discovery that the adhesion molecule ICAM-1, which is mediates neutrophil adhesion to the endothelium, also initiates signals that regulate the permeability of the wall to albumin. Albumin is a molecule that is critical in maintaining fluid balance and is also a critical carrier of hormones and other essential proteins. We discovered that different signaling pathways are turned on in normal versus inflamed endothelium; now we need to find out whether these signals regulate different pathways through the endothelial barrier, and their characteristics. Where leukocytes interact within the vasculature is determined by adhesion molecule expression on the vessel wall, by the local fluid dynamics, and by the morphology of the vasculature. Leukocytes roll, adhere, crawl along the lumen and then emigrate (mostly, although not always) through junctions between endothelial cells. We want to know why these events happen in some areas of the microcirculation and not others, and what the different mechanisms are. We combine quantitation of leukocyte behavior with calcium signaling measurements, measurements of adhesion- and junctional-protein densities, and analysis of the local fluid microenvironment.
Metabolic vasodilation. When skeletal muscle contracts, metabolites are released that cause vasodilation in nearby arterioles, thus matching blood flow to demand. Mechanisms underlying this very local response are unresolved, and, importantly, how it is spread along the vessel wall so as to enable a co-ordinated increase in blood flow is not known. Gap junctions are important in many communicated responses, but we showed that the metabolic response is propaged differently. We have published work showing that endothelial calcium changes are essential, and we have also shown that purines (such as adenosine) that are released when muscles contract, initiate propagated responses. We are trying to find out which receptors, and which endothelial signaling mechanisms, are critical for this response.
We recently showed, in an important new paper, that the extracellular matrix proteins that surround arterioles respond to the mechanical events associated with muscle contraction with signals that cause vasodilation. The ECM protein fibronectin is key in this newly identified mechanosignaling pathway. We are collaborating with the Hocking lab to explore the mechanisms underlying the response. Because ECM proteins change as people age, and because it is well known that the vasculature becomes less responsive in older adults, this finding is likely to be very important in planning therapeutic interventions as our population ages.
Portals for leukocyte emigration from blood vessels.
We found recently that leukocytes transmigrate across blood vessel walls at very specific sites, that we have termed “portals”. These portals are located at some, but not all, endothelial junctions, and are associated with high local surface expression of ICAM-1. This project will explore what makes these portals special, and how they are regulated.
Mechanisms of intralumenal crawling in different leukocyte sub-populations.
Recent work has shown that after blood-born monocytes and neutrophils adhere to the microvessel wall, they activate, flatten and crawl for a limited amount of time on the endothelial surface. We don’t yet know what signaling mechanisms underly this very specific amount of crawling. If they encounter a portal, they transmigrate: the majority do not, and deactivate, detach, and return to the flowing blood. We have also discovered that the primary integrins that mediate this crawling (LFA-1 and Mac-1) induce different characteristic crawling behaviors. The project will relate these differences to mechanisms of crawling and emigration in monocytes and neutrophils.
Microvascular permeability: regulation by ICAM-1.
We showed that ICAM-1 ligation activates mechanisms in endothelial cells that regulate permeability of the microvessel wall to essential molecules such as albumin. We found that regulation of permeability in non-inflamed tissues has different signaling characteristics than in inflamed tissues, despite all being dependent on ICAM-1 ligation. The project will define mechanisms for regulation of permeability by ICAM-1 under these different conditions – we are particularly interested in identifying the “switch” that shifts signaling pathways.
Endothelial calcium changes in exercise.
To model exercise we electrically induce contractions in small bundles of myocytes in muscles of anesthetized animals; responses of cells in the arteriolar wall are imaged and quantified using real time confocal intravital microscopy. In an earlier project, we made the surprising finding that endothelial calcium increases are required for this metabolically-coupled dilation. The new project will define mechanisms responsible for these calcium changes, and will explore how the resulting signals are communicated to adjacent smooth muscle cells.
A role for fibronectin in arteriolar responses (with the Hocking lab).
This project will extend earlier work in which we found that mechanical signals from the extracellular matrix protein fibronectin contribute to vascular responses in normal resting and exercising tissue (by apparently different mechanisms). We will use FN-mimetic peptides, in combination with fluorescence intravital microscopy, to probe responses in small arterioles at rest and during muscle contraction. The goal of this project is to identify the key mechanotransducing mechanisms that underly this response.
Click here for a list of PubMed publications.