Our Work

The Surveillance and Traffic of Fluid

All forms of endocytosis involve the uptake of extracellular fluid yet the volume of the endocytic pathway and that of the cell remains unchanged. The pathways used to resolve the ingested volumes require ion transport followed by membrane remodeling and recycling.

 

In cells that take up fluid in bulk as part of their ongoing surveillance programs, there is considerable pressure for these pathways to act efficiently. There is also the need to distill and breakdown the ingested macromolecules.

 

We are interested in the surveillance and resolution of fluids, especially as performed by cells of the innate immune system. We are currently investigating ion transport by Two Pore Channels and organic solute transport by members of the Solute carrier family (SLCs) resident to the endocytic pathway.

image depicting surveillance and traffic of fluid in cells

Barriers to Phagocytosis

Phagocytosis, the ingestion of particulate matter, plays an essential role in the maintenance of tissue homeostasis.  It serves as a first line of defense in the elimination of invading pathogens.  Phagocytosis also prevents secondary necrosis and unwanted inflammation by efficiently recognizing and disposing of apoptotic bodies and debris.  In addition, phagocytic clearance of malignant cells is fundamental in the innate immune surveillance for cancerous growth; indeed, suppression of phagocytosis facilitates tumor-mediated immune evasion.  Given these essential functions, phagocytes reside in virtually all tissues of the body, where they constantly survey their surroundings for prey.

 

In their surveillance, phagocytes must rapidly distinguish harmful from healthy components by detecting features exposed on the surface of their putative targets.  Features that trigger phagocytosis can be intrinsic to the target or facilitated by the deposition of soluble opsonins on the target. These ligands are called “eat me” signals as they engage phagocytic receptors that trigger extensive remodeling of the plasma membrane and of the actin cytoskeleton, culminating in the extension of pseudopods that surround and engulf the target.

 

In addition to scanning for “eat me” signals, phagocytes also recognize surface molecules that serve as “don’t eat me” signals.  These include CD47, PD-L1, and CD24 that engage their cognate receptors SIRPα, PD-1, and Siglec-10, respectively, to exert an inhibitory effect on phagocytosis.  When engaged, these inhibitory receptors arrest signaling pathways by recruiting otherwise cytosolic phosphatases that suppress phagocytic signaling.  Certain tumors have found ways to usurp these mechanisms to support their growth. IgG-based biologics that target the “don’t eat me” pathways of cancerous lesions have been deployed with some success. Augmenting these responses would be advantageous. Since phagocytosis is a tunable and translationally impactful process, dependent on the features of the target and the microenvironment of the phagocyte, understanding the mechanisms that modulate receptor activation is of great importance.

 

We are interested in how pathogens and malignant cells avoid close contacts with immune cells by a glycocalyx barrier. Targeted removal of the tumour glycocalyx may unleash immune responses against it.

Purposed role of the tumor glycocalyx and immune surveillance
A scientific illustration depicting the a membrane coating that surrounds the cell membrane
A glycocalyx coating comprised of glycoproteins/proteoglycans and glycolipids surrounds the cell membrane of all cells. Pathogens and malignant cells embellish this coating to protect them from immune attack.

Membrane Oxidation

The ongoing metabolic and microbicidal pathways that support and protect cellular life generate potentially damaging reactive oxygen species. To counteract damage, cells express peroxidases, antioxidant enzymes that catalyze the reduction of oxidized biomolecules. A single hydroperoxidase is responsible for reducing lipid peroxides called Glutathione peroxidase 4 (GPX4); its activity is essential and its inhibition causes a unique type of lytic cell death, ferroptosis.

Recently, we have been working to understand the mechanisms that lead to cell lysis in ferroptosis. Using sensors for membrane oxidation that we can image with high resolution microscopy, we found that the lipid peroxides formed during ferroptosis accumulate preferentially at the plasma membrane. The oxidation of the membrane increases in its tension, resulting in the constitutive gating mechanosensitive channels, ultimately making the membrane permeable to cations. Blocking or deleting the channels prevents ferroptosis.

We’re now interested in how oxidized membranes feature more generally in mechanosensitive channel gating, including for organelles.