Our laboratory is interested in understanding how central nervous system (CNS) synapses are formed. In the CNS, neurons make very specific connections with distant targets and establishment of these synaptic contacts are meticulously timed, demonstrating the presence of a complex control of synapse formation. However our knowledge on regulation of synaptogenesis is still limited.
Synapses are essentially asymmetric cell adhesions. Since synapses in the CNS are formed between neurons, traditional studies in synapse formation primarily focuses on neuronal cell surface molecules. However in the last decade astrocytes, the most abundant cell type in the CNS, emerged as an important player in the formation of synapses.
We are able to study the effect of astrocytes in synapse formation thanks to our retinal ganglion neuron culture system. Unlike many other primary neuronal culture systems, retinal ganglion cells (RGCs) can be isolated from rodent retina as a pure population and can be cultured in the absence of any other cell type, in a serum free well-defined culture media. Interestingly, RGCs cultured in the absence of astrocytes form very few synapses. In contrast, neurons cultured in the presence of astrocytes or astrocyte conditioned media form many synapses. This is due to the fact that astrocytes secrete factors that regulate synapse formation. We work to identify these factors and elucidate the molecular and cellular mechanisms by which astrocytes regulate synapse formation both in vitro and in vivo.
Understanding synaptogenesis is crucial for understanding how our brains are sculpted during development, and how we learn and remember as adults. In addition, knowledge on how synaptogenesis can go awry has important health implications for understanding the pathophysiology of diseases such as Alzheimer's disease, epilepsy, and drug addiction.
What are the secreted signals coming from astrocytes that regulate synapse formation?
Using our culture system we screen candidate secreted proteins for their role in synapse formation. Thrombospondin (TSP), a 450kDa extracellular matrix protein coming from astrocytes, has previously been shown to be sufficient and necessary for astrocyte-induced synapse formation in vitro. In addition, we have identified two other extracellular matrix proteins expressed by astrocytes that regulate synapse formation. Interestingly, one of these proteins is a negative regulator of synapse formation indicating that astrocytes not only provide positive but also negative cues for synaptogenesis. We are continuing to investigate the role of other astrocyte secreted factors in synapse formation and CNS development.
How do astrocyte-secreted factors lead to synapse formation?
To answer this question we are investigating the identity of the neuronal cell surface receptors for these astrocyte secreted signals and characterizing their mechanisms of action in synapse formation.
What is the role of astrocyte-induced synapse formation in development and maintenance of the CNS?
Since astrocytes secrete regulators of synapse formation, they might play important roles in the development and function of the CNS. The expression of Thrombospondins and several other astrocyte-secreted factors are developmentally regulated and their expression coincides with the synaptogenic period of the brain. In addition, astrocytes have been implicated to play an important role in the regulation of the developmental synaptic plasticity. Using knockout mice and inhibitors of astrocyte induced synapse formation we study different developmental plasticity and disease paradigms to get a molecular grasp on how astrocytes might participate in these processes.
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Wan Y., Ade K., Caffall Z., Ozlu M.I., Eroglu C., Feng G., and Calakos N. (2013). Circuit-selective striatal synaptic dysfunction in the Sapap3 knockout mouse model of obsessive-compulsive disorder. Biol Psych. pii: S0006-3223(13)00048-6.
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Eroglu, C., Barres, B.A. (2010). Regulation of synaptic connectivity by glia. Nature, Nov 11;468(7321):223-31.
Ippolito D.M., Eroglu C. (2010). Quantifying synapses: An immunocytochemistry based assay to quantify synapse number. JoVE., doi: 10.3791/2270.
Eroglu, C., Allen, N.J., Susman, M.W., O'Rourke, N.A., Park, C.Y., Ozkan, E., Chakraborty, C., Mulinyawe, S.B., Annis, D.S., Huberman, A.D., et al. (2009). Gabapentin receptor alpha2delta-1 is a neuronal thrombospondin receptor responsible for excitatory CNS synaptogenesis. Cell 139, 380-392.
Bolton, M.M., and Eroglu, C. (2009). Look who is weaving the neural web: glial control of synapse formation. Current opinion in neurobiology.
Eroglu, C. (2009). The role of astrocyte-secreted matricellular proteins in central nervous system development and function. Journal of cell communication and signaling.