NMDA Receptors Mediate Synaptic Competition in Culture

Background: Activity through NMDA type glutamate receptors sculpts connectivity in the developing nervous system. This topic is typically studied in the visual system in vivo, where activity of inputs can be differentially regulated, but in which individual synapses are difficult to visualize and mechanisms governing synaptic competition can be difficult to ascertain. Here, we develop a model of NMDA-receptor dependent synaptic competition in dissociated cultured hippocampal neurons. Methodology/Principal Findings: GluN1-/- (KO) mouse hippocampal neurons lacking the essential NMDA receptor subunit were cultured alone or cultured in defined ratios with wild type (WT) neurons. The absence of functional NMDA receptors did not alter neuron survival. Synapse development was assessed by immunofluorescence for postsynaptic PSD-95 family scaffold and apposed presynaptic vesicular glutamate transporter VGlut1. Synapse density was specifically enhanced onto minority wild type neurons co-cultured with a majority of GluN1-/- neighbour neurons, both relative to the GluN1-/neighbours and relative to sister pure wild type cultures. This form of synaptic competition was dependent on NMDA receptor activity and not conferred by the mere physical presence of GluN1. In contrast to these results in 10 % WT and 90

Key Mechanisms Regulating Synaptic and Cell Wide Forms of Homeostatic Plasticity.

Sustained alterations in neuron activity elicit compensatory changes in synaptic function, a form of adaptation known as homeostatic plasticity. Homeostatic forms of plasticity are thought to maintain neural circuit activity within a dynamic, yet stable, functional range in the face of potentially destabilizing environmental influences. In recent years, homeostatic plasticity has received considerable attention as its dysregulation may lead to instability of neuronal circuits which, in turn, may contribute to the development of neurological disorders such as epilepsy. Typically, sustained changes in network activity drive a slow form of homeostatic plasticity that emerges over an 18-24 hr period. However, neurons also exhibit homeostatic adaptations that emerge 1-3 hr following direct disruption of synaptic activity, suggesting that separate slow and rapid forms of homeostatic plasticity exist at synapses. Both slow and rapid forms of homeostatic plasticity can emerge as changes in presynaptic neurotransmitter release (presynaptic compensation) or in the abundance of postsynaptic neurotransmitter receptors (postsynaptic compensation). However, the molecular mechanisms underlying these unique slow and rapid forms of homeostatic plasticity remain largely unknown. Here, key molecular events underlying these homeostatic forms of regulation are elucidated. Slow homeostatic plasticity requires targeted protein degradation by the postsynaptic ubiquitin proteasome system (UPS). Postsynaptic blockade of the UPS can both mimic and occlude slow homeostatic plasticity expression mechanisms suggesting that network driven changes in activity engage proteasome function to drive slow homeostatic adaptations at synapses. In contrast, rapid homeostatic plasticity requires presynaptic UPS function. Rapid homeostatic plasticity mechanisms require coupling of presynaptic UPS function with postsynaptic protein translation, retrograde synaptic signaling by BDNF/TrkB and the presence of presynaptic action potential activity. Together, these results demonstrate that slow (disruption of network activity) and rapid (disruption of synaptic activity) forms of homeostatic plasticity require unique pre- and post- synaptic mechanisms that additionally work together to coordinate expression of pre- and post- synaptic functional compensation. Understanding key molecular components underlying homeostatic plasticity mechanisms may lead to an advanced understanding of destabilizing neurological disorders, such as epilepsy.Ph.D.NeuroscienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/84493/1/jakawich_1.pd

BDNF and its pro-peptide are stored in presynaptic dense core vesicles in brain neurons

Although brain-derived neurotrophic factor (BDNF) regulates numerous and complex biological processes including memory retention, its extremely low levels in the mature central nervous system have greatly complicated attempts to reliably localize it. Using rigorous specificity controls, we found that antibodies reacting either with BDNF or its pro-peptide both stained large dense core vesicles in excitatory presynaptic terminals of the adult mouse hippocampus. Both moieties were ?10-fold more abundant than pro-BDNF. The lack of postsynaptic localization was confirmed in Bassoon mutants, a seizure-prone mouse line exhibiting markedly elevated levels of BDNF. These findings challenge previous conclusions based on work with cultured neurons, which suggested activity-dependent dendritic synthesis and release of BDNF. They instead provide an ultrastructural basis for an anterograde mode of action of BDNF, contrasting with the long-established retrograde model derived from experiments with nerve growth factor in the peripheral nervous system