Tumor necrosis factor-α signaling maintains the ability of cortical synapses to express synaptic scaling.

Glial tumor necrosis factor-α (TNFα) is essential for scaling up of synapses during prolonged activity blockade, but whether TNFα is an instructive or permissive signal is not known. Here we show in rat cortical neurons that the effects of TNFα and activity blockade are not additive; whereas TNFα increased AMPA quantal amplitude at control synapses, TNFα reduced quantal amplitude at prescaled synapses, demonstrating state-dependent effects of TNFα signaling on the scaling process. Whereas synaptic scaling during prolonged activity blockade [24 h tetrodotoxin (TTX)] was prevented by blocking TNFα signaling, early scaling (6 h TTX) was not, unless TNFα signaling was first blocked for 24 h. Moreover, when synapses were prescaled, prolonged (24 h) but not brief (6 h) blockade of TNFα signaling reversed scaling. Finally, prolonged block of TNFα signaling modified the synaptic localization of several scaffold proteins, suggesting that maintenance of postsynaptic density composition is TNFα dependent. Together, these data suggest that TNFα is not an instructive signal for scaling but rather is critical for maintaining synapses in a plastic state in which synaptic scaling can be expressed.

Glial cells promote dendrite formation and the reception of synaptic input in Purkinje cells from postnatal mice.

Previous studies suggest that glial cells contribute to synaptogenesis in specific neurons from the postnatal CNS. Here, we studied whether this is true for Purkinje cells (PCs), which represent a unique neuronal cell type due to their large size, massive synaptic input, and high vulnerability. Using new glia-free cultures enriched in PCs from postnatal mice we show that these neurons survived and grew, but displayed only low levels of excitatory and inhibitory synaptic activity. Coculture with glial cells strongly enhanced the frequency and size of spontaneous and miniature excitatory synaptic currents as well as neurite growth and branching. Immunocytochemical staining for microtubule-associated protein 2- (MAP2-) positive neurites revealed impaired dendrite formation in PCs under glia-free conditions, which can explain the absence of synaptic activity. Glial signals strongly enhanced dendritogenesis in PCs and thus their ability to receive excitatory synaptic input from granule cells (GCs). The enhancement of dendrite formation was mimicked by glia-conditioned medium (GCM), whereas the increase in synaptic activity required physical presence of glia. This indicated that dendrite development is necessary but not sufficient for PCs to receive excitatory synaptic input and that synaptogenesis requires additional signals. The level of inhibitory synaptic activity was low even in cocultures due to a low incidence of inhibitory interneurons. Taken together, our results reinforce the idea that glial cells promote synaptogenesis in specific neuronal cell types.

Upregulation of µ3A Drives Homeostatic Plasticity by Rerouting AMPAR into the Recycling Endosomal Pathway.

Synaptic scaling is a form of homeostatic plasticity driven by transcription-dependent changes in AMPA-type glutamate receptor (AMPAR) trafficking. To uncover the pathways involved, we performed a cell-type-specific screen for transcripts persistently altered during scaling, which identified the µ subunit (µ3A) of the adaptor protein complex AP-3A. Synaptic scaling increased µ3A (but not other AP-3 subunits) in pyramidal neurons and redistributed dendritic µ3A and AMPAR to recycling endosomes (REs). Knockdown of µ3A prevented synaptic scaling and this redistribution, while overexpression (OE) of full-length µ3A or a truncated µ3A that cannot interact with the AP-3A complex was sufficient to drive AMPAR to REs. Finally, OE of µ3A acted synergistically with GRIP1 to recruit AMPAR to the dendritic membrane. These data suggest that excess µ3A acts independently of the AP-3A complex to reroute AMPAR to RE, generating a reservoir of receptors essential for the regulated recruitment to the synaptic membrane during scaling up.