Research

Dendritic spines

Communication between neurons in the brain requires exquisitely controlled, precise connections between neighbouring cells. The connections, or synapses, are composed of a presynaptic bouton at the end of axons and a postsynaptic dendritic spine that protrude from the surface of dendrites.

Within the dendritic spine are different condensates that can form through the process of biological phase separation, including postsynaptic densities. This condensate controls dendritic spine plasticity that is required for basic brain functions including memory, behavior, and emotions.

Mutations that have been identified in many proteins that localize to the postsynaptic density are linked to neurodevelopmental disorders such as autism spectrum disorder, schizophrenia, and intellectual disabilities. Moreover, the biochemical and biophysical mechanisms by which these mutations alter normal dendritic spine function are unknown.

We are particularly interested in understanding the role of phase separation in regulating postsynaptic density controlled local RNA translation and actin polymerization; two processes that regulate synaptic plasticity and communication between neurons.

T cell signaling: immune synapse organization

T cell response to infection relies on the recognition of peptide major histocompatibility complexes (pMHC) presented on the surface of antigen presenting cells by T cell receptors on the surface of T cells.

Upon T cell receptor binding to pMHC, kinases are activated within the T cell and signaling clusters composed of proteins form on the membrane. Binding of T cell receptor to pMHC also alters the organization of lipid domains on the T cell membrane.

It is unclear whether lipid and protein domain organization is coupled, and whether potential coupling is required for T cell activation. We are interested in understanding organization at the membrane of the immune synapse and the role that co-existing lipid and protein domains play in regulating T cell activation.

T cells can also experience exhaustion that prevents the cell from mounting a response to threats including cancer. When T cells receive signals telling them to activate without co-stimulation or when repressive receptors are activated, T cells become ineffective. We study how the organization of co-stimulatory or co-repressive receptors that are expressed on antigen presenting cells promote T cell activation versus exhaustion.

Cystic Fibrosis Transmembrane Conductance Regulator functional organization

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is a chloride channel that maintains ion homeostasis in cells. CFTR is organized on cell surfaces into clusters composed of it and its binding partners. These clusters as a discrete structure are thought to broadly regulate ion homeostasis. We discovered that CFTR, its binding partners, and membrane cholesterol undergo phase separation that is regulated by calcium and phosphorylation. CFTR protein phase separation is coupled to cholesterol phase separation into cholesterol-rich membrane domains, much like T cell signaling proteins.

Dysregulation of channel activity caused by Cystic Fibrosis-linked mutations results in the accumulation of ions within cells, dehydration of extracellular surfaces, and subsequent damage to airway and ductal organs. Corrector and potentiator therapeutics have been discovered that can at least partially rescue CFTR function for a majority of CF patients; however, a small but significant portion of patients do not respond well to existing therapies.

We are particularly interested in how cells use the principles of phase separation to regulate CFTR function. We also study how CF-linked mutations dysregulate CFTR functional organization on membranes and whether rescuing mutant CFTR organization can contribute to restoring its function in patients who are not responsive to existing treatments.

Heterogeneous condensation

Many biomolecular condensates on membranes, in the cytosol, and in the nucleus form through the process of biological phase separation.

Initial biophysical analysis of biological phase separation was often performed on single component systems where a single protein interacted with itself and underwent phase separation in specific buffer conditions. In these simple systems, the concentrations of protein inside and outside of the condensates remain constant while the volume of condensed material increases when additional protein is added to the solution.

Many condensates are complex structures whose existence relies on interactions between multiple binding partners. In these more complex systems, it is not clear how the concentrations of proteins inside and outside the condensate will change as the total concentration of proteins is increased in solution. Furthermore, the role that each component plays in promoting biological phase separation is unclear.

Using model systems, we are interested in understanding a potential buffering role for complex condensates and deciphering general principles that underlie the contribution of different components to multi-component condensates.