The correct formation and maturation of neuronal synapses during development is essential for neuronal circuit function throughout life. The ability of neurons to form synapses is not entirely intrinsic, as synapse formation and function can be regulated by interactions with other cell types in the brain, including astrocytes. Astrocytes secrete factors that regulate the formation of glutamatergic synapses, including factors that increase the number of AMPA glutamate receptors (AMPARs) at synapses. Glypicans 4 and 6 (Gpc4 and 6) are astrocyte secreted factors that are necessary and sufficient to increase synaptic levels of AMPARs and synaptic strength. Mice globally lacking Gpc4 have weaker glutamatergic synapses in hippocampal neurons, demonstrating an important in vivo role for astrocytes and Gpc4 in synaptic development. The levels of AMPARs at a synapse determine the size of the synaptic response, and the regulated addition and removal of AMPARs at synaptic sites is the molecular mechanism underlying learning and memory. Hence, identifying the mechanisms that Gpc4 uses to regulate synaptic AMPARs has important implications for understanding how synaptic strength is normally regulated during development, how it is altered during learning and memory, and how it can be misregulated in neurological disorders such as autism and schizophrenia. This proposal addresses major unanswered questions regarding the mechanism of action of Gpc4 which will increase knowledge of astrocyte-neuron interactions in the developing brain. 1) The action of Gpc4 in recruiting AMPARs to synapses is relatively slow, and in Aim 1 the hypothesis that Gpc4 acts by regulating the transcription of pro-synaptogenic factors in neurons is investigated. 2) Astrocytes express high levels of Gpc4 in early postnatal development during periods of intense synapse formation, and Gpc4 expression in astrocytes decreases with maturation when synapse numbers have stabilized. Aim 2 investigates the hypothesis that active neurons signal to downregulate the expression of Gpc4 in astrocytes, thus limiting neuronal excitability. 3) Gpc4 deficient mice have defects in hippocampal synapse formation, but it is not known whether all synapses are affected equally, or there are defects in specific subsets of synapses. Aim 3 will determine if Gpc4 regulates the strength of all synapses on a neuron equally e.g. by transcriptional control (Aim 1), or can locally regulate the strength o specific subsets of synapses. This will differentiate between a globally permissive and locally instructive role for astrocytes and Gpc4 in synaptic development. 4) In the adult brain subsets of excitatory neurons upregulate expression of Gpc4, at a time when astrocyte expression of Gpc4 has decreased. The relative contribution of astrocytic and neuronal Gpc4 to neuronal synaptic function in development and plasticity are investigated in Aim 4. These experiments will give important insight into the molecular mechanisms astrocytes and neurons use to bidirectionally interact throughout life, leading to the correct formation and function of neural circuits.