Endothelial cells that line the blood vessels of the mammalian central nervous system (CNS) form a unique, tight barrier that maintains the homeostasis necessary for neuronal and glial function, and protects the CNS from pathological agents and immune cell invasion. Barrier properties of CNS endothelial cells (ECs) are mediated by three distinct cell biological mechanisms: a) extremely tight junctions that prevent diffusion of small molecules between endothelial cells (paracellular pathway); b) very few endocytotic vesicles (caveolae) that transcytose slowly and thereby reduce transport of large molecules across the brain endothelium (transcellular pathway); and c) transporters that shuttle only selected molecules between the blood and the brain (Figure 1). This barrier breaks down during CNS insults such as stroke or in CNS autoimmune diseases such as multiple sclerosis (MS) or PANDAS/PANS.
What are the cellular and molecular mechanisms that govern development and maintenance of the BBB?
We are interested in identifying key signaling pathways that regulate formation of the BBB in order to repair BBB function in CNS diseases in which the barrier breaks down. We have found that the Wnt/beta-catenin pathway plays an important role in regulating both CNS angiogenesis and some properties of the barrier. We have identified a novel inhibitor of this pathway named Apcdd1 that is expressed in CNS endothelial cells when they acquire their barrier properties during development. We are currently investigating the roles of Apcdd1 in CNS angiogenesis and blood-brain barrier development. We are also interested in understanding how tight junctions between endothelial cells in the CNS develop and mature during BBB formation. We have developed proteomic discovery platforms for: a) identifying novel junctional proteins important for barrier development and function; and b) potential structural modifications of tight junction proteins that underlie CNS diseases exhibiting reduced barrier function.
How are structural components of the BBB affected in diseases of the brain and spinal cord, where barrier function is impaired?
Remarkably little is known about how the BBB breaks down in CNS diseases with impaired barrier function such as stroke, or autoimmune diseases such as multiple sclerosis. In particular, the roles of BBB structural components such as tight junctions and transcytosis vesicles (caveolae), that regulate the transport of small or large molecules, respectively, to cross the BBB, are poorly defined for the pathogenesis of brain disorders associated with leaky BBB. We have created a novel transgenic strain where tight junctions are labeled with GFP (Tie2p::eGFP::Claudin-5). This strain allows real-time analysis of tight junction dynamics in CNS disease models where the BBB is compromised. We are using this novel transgenic strain to image tight junctions in brain capillaries using two-photon microscopy in vivo, combined with quantitative analysis of fluorescent tracer leakage across the BBB. We are currently using these approaches to analyze tight junction dynamics in stroke and Experimental Autoimmune Encephalitis (EAE), a model for MS.
How does repeated Streptococcus pyogenes infection induce rapid onset of neuropsychiatric symptoms in children?
Despite emerging evidence that inflammatory molecules (e.g. cytokines and complement) alter synapse formation, neuronal connectivity and behavior, the molecular mechanisms underlying impairment of brain development and function by infections that induce an aberrant immune response remain poorly understood. Group A Streptococci (S. pyogenes; GAS), the primary agent for acute pharyngitis in children, is associated with several autoimmune diseases, including the central nervous system (CNS) autoimmune motor and behavioral disorders Sydenham’s chorea and Pediatric Acute-onset Neuropsychiatric Syndrome (PANS). Recurrent GAS mucosal infections induce a strong antigen-specific Th17 cellular response in mice and humans. Th17 cells have been implicated in many autoimmune diseases including multiple sclerosis, where they trigger inflammation and destruction of the blood-brain barrier (BBB). Our preliminary experiments show that these T cells specifically home to the brain, localizing primarily in the olfactory bulb (OB) and along the olfactory nerve as well as some other brain regions where OB neurons send projections and eventually make synaptic connections. We are investigating the mechanisms of how these T cells induce neuronal damage and BBB breakdown and the consequences for behavioral outcomes.