If you were able to remember where you put your keys on your way out the door this morning, it’s because – somehow – synapses in your brain changed their properties to encode this information and store it until you needed it. This process, known as “synaptic plasticity”, is essential for the continuity of our memory and sense of self, and yet we are only beginning to grasp the molecular mechanisms that enable this amazing feat of constant information storage and retrieval. Now a collaborative paper from the Turrigiano and Nelson labs just published in Cell Reports sheds important new light into how experience interacts with the genome to allow synapses to change their strength to store information.
Synapses are the connections between neurons, and it has long been appreciated that information is stored in large part through changes in the strength of these connections. Changes in strength at many synapses are in turn determined by the number of neurotransmitter receptors that are clustered at synaptic sites – the more receptors synapses have, the easier it is for neurons to excite each other to transmit information. Synapses are highly complicated molecular machines that utilize at least 300 different proteins that interact to traffic these receptors to synapses and sequester them there, and exactly how a change in experience alters the function of this nano-machine to enhance the number of synaptic receptors is still a matter of puzzlement.
In this study the Brandeis team devised a way to screen for candidate proteins that are critical for a particular form of synaptic plasticity: “synaptic scaling”, thought to be especially important for maintaining brain stability during learning and development. They were able to induce synaptic scaling within specific labelled neurons in the intact mouse brain (layer 4 star pyramidal neurons), and then sort out those labelled neurons from the rest of the brain and probe for changes in gene expression that were correlated with (and potentially causally involved in) the induction of plasticity. This approach produced a small number of candidate genes that were up- or down-regulated during plasticity, to produce more or less of a given protein. The team then went on to show that – when upregulated – one of these candidates (known as µ3A) acts to prevent neurotransmitter receptors from going into the cellular garbage bin (the lysosomes, where proteins are degraded) and instead recycles them to the synapse. Thus increased µ3A flips a switch within cells to enhance receptor recycling, and this in turn increases synaptic strength.
A screen for genes with altered expression during synaptic plasiticity in specific neurons revealed that µ3A plays a critical role in the recycling of AMPA-type neurotransmitter receptors at the synapse. When this protein is upregulated, it prevents receptors from being trafficked into lysosomes, and instead allows them to be recycled back to synapses, increasing synapse number and enhancing synaptic strength.
It turns out that many other forms of synaptic plasticity use the same receptor recycling machinery as synaptic scaling, so it is likely that this mechanism represents an important and general way for neurons to alter synaptic strength. This study also raises the possibility that defects in this pathway might contribute to the genesis of neurological disorders in which the stability of brain circuits is disrupted, such as epilepsy and autism. So next time you complain about having to sort your garbage, consider that your neurons do it all the time – and what’s good for the planet turns out to be good for your brain as well.
Steinmetz CC, Tatavarty V, Sugino K, Shima Y, Joseph A, Lin H, Rutlin M, Lambo M, Hempel CM, Okaty BW, Paradis S, Nelson SB, Turrigiano G. Upregulation of μ3A Drives Homeostatic Plasticity by Rerouting AMPAR into the Recycling Endosomal Pathway. Cell reports. 2016.
