Harvard Medical School
Department of Neurobiology
1. What are the cellular and molecular mechanisms governing the formation, function, and regulation of the blood brain barrier (BBB)?
A major obstacle in treating neurological diseases and brain tumors is to deliver drugs or antibodies across the ‘blood brain barrier’ (BBB). The BBB, which is composed of blood vessels whose endothelial cells display unique features, acts as gatekeeper for the brain. In concert with pericytes and astrocytes, the BBB protects the brain from various toxins and pathogens and provides the proper chemical composition for synaptic transmission. This tightly controlled environment is essential for normal brain function. Indeed BBB breakdown has recently been shown to be involved in the initiation and perpetuation of some neurological diseases. A lack of understanding of the molecular mechanisms that control BBB formation has hampered our ability to manipulate the BBB in disease and therapy.
There are several unique feathers of CNS endothelium determine the BBB integrity. One is the specialized tight junctions between endothelial cells lining the CNS capillaries that form the physical seal between the blood and brain parenchyma, which is much “tighter” than the junctions between peripheral endothelial cells. In addition, CNS endothelial cells are characterized by unusually low rates of transcytosis, and express transporters to selectively traffic materia across the BBB. We aim to understand when and how these unique features are aquired, and identify the key regulators governing the formation and function of BBB.
Our progress towards this goal has been very fruitful. We have identifed genes that are specifically required for BBB but not for CNS vascular morphogenesis, demonstrating that BBB genesis is a uniqure biological process. We have identified genes that are specifically requied for one BBB property but not others, which revealed the importance of previously unappreciated features of BBB-contianing CNS endothelium in BBB function. So far sevel genes have been validated for CNS-endothelial cell-specific expression, and elucidating functions of these genes will allow us to begin to build our knowledge of what and how brain signals induce BBB formation. We will expand our studies in the future to understand how signals from pericytes and astrocytes continue to maintain the integrity of BBB. Ultimately, we hope that the identification of key molecular regulators and mechanisms that control the formation and maintenance of BBB functionality will provide important insights into BBB restoration or BBB manipulations.
Micrographs illustrating the functionality of the blood-brain barrier, in a normal situation (top images) where the fluids contained within blood vessels are confined in these vessels, and in a situation of leakiness (bottom images) where extravasation of blood contents can be detected in the brain.
(News & Views in Nature 509(7501):432-3).
(Preview in Neuron 82(4):728-30.)
2. What are the mechanisms underlying the cross-talk between neural activity and vascular structure dynamics?
Neurovascular coupling is a very well known phenomena in which the magnitude and spatial location of blood flow changes are tightly linked to changes in neural activity. Many vascular-based functional brain imaging techniques such as fMRI rely on this coupling to infer changes in neural activity. However, whether neural activity or/and neuronal structural organization have any impact on vascular structure is still elusive. We are developing a combination of genetic, imaging, and computational tools allowing simultaneous analysis of neuronal and vascular components to examine the effect of manipulations of sensory‑related neural activity on vascular structure and plasticity.
Micrograph of a brain section, from a mouse in which the axons are genetically labeled in red and the blood vessels in green, allowing for the simultaneous analysis of both neuronal and vascular systems.
Lacoste, B., Comin, C.H., Ben-Zvi, A., Kaeser, P.S., Xu, X., Costa, L.D., Gu, C., (2014) Sensory-Related Neural Activity Regulates the Structure of Vascular Networks in the Cerebral Cortex. Neuron, 83(5): 1117-1130.
3. Characterize how common guidance cues and their receptors function in wiring neural and vascular networks
(1)Role in synapse specificity in mouse basal ganglia system
Cre-mCherry AAV into plexin-D1f/f; Drd2-GFP
Ding, J.B., Oh, W., Sabatini, B.L., Gu, C., (2011) Semaphorin3E-Plexin-D1 signaling controls pathway-specific synapse formation in the striatum. Nature Neuroscience. 15(2):215-23. PMC3267860
(2)Role in vascular network formation in mouse retina system
Reciprocal interaction between VEGF and Semaphorin 3E-Plexin-D1 signaling regulates the formation of the retina vascular network topology. (See Kim et al 2011)
(Cover illustration + Preview in Developmental Cell. 2011 16;21(2):189-90.)
(3)Signaling cascade operating in neurons and endothelial cells, common or different
Tata, A., Stoppel, D., Hong, S., Ben-Zvi, A., Xie, T., Gu, C., (2014) An image-based RNAi screen identifies SH3BP1 as a key effector of Semaphoring 3E-PlexinD1 signaling. JCB, 205(4):573-590.
4. What are the molecular mechanisms underlying the establishment of neurovascular congruency?
Nervous and vascular systems are both highly-branched and complicated networks; yet both networks have remarkably stereotyped patterns. Moreover, nerves and vessels often run adjacent to each other. In principle, there are 3 possible mechanisms control the establishment of the congruency (interdependent: one follows the other; co-patterning: both systems are patterned by common environment cues; independent). We aim to uncover the precise mechanism underlying the neurovascular congruency.
Examples of neurovascular congruency. Left: E16.5 mouse whisker pad was immunostained with anti-neurofilament (green) for trigeminal axons and anti-PECAM (red) for blood vessels. Right:An E11.5 mouse embryo was immunostained with anti-neurofilament (red) for spinal nerves and anti-PECAM (green) for intersomitic vessels. We found in both cases, neurovascular congruency is established via a co-patterning mechanism by common environmental cues, and patterned independently.
Oh, W., Gu, C., (2013) Establishment of neurovascular congruency in the mouse whisker system by an independent patterning mechanism. Neuron. 80(2):458-469. PMC in process.
(Preview in Neuron 80(2): 262-265).
Copyright 2011 by the President and Fellows of Harvard College