Remote-control in the cortex: How distant synapses are equipped for change

New study finds extensive mRNA localization and local protein synthesis in cortical layer 1, a key site for synaptic plasticity, perception and learning

The outermost layer of the brain’s cerebral cortex is a major meeting point for information coming from across the brain. Researchers at the Max Planck Institute for Brain Research have now discovered that synapses in this thin layer can produce proteins locally. Published in Cell Reports, the study provides the first comprehensive molecular map of cortical layer 1 and its synapses, revealing a specialized signature and unexpected similarities with the hippocampus.

A molecular supply problem at the brain’s surface

The cerebral cortex processes sensory information and supports functions ranging from movement to memory and cognition. Its outermost layer, layer 1, is unusual: unlike deeper cortical layers, it contains very few neuronal cell bodies and consists largely of the distant dendrites of neurons located deeper in the cortex. These dendrites receive many synaptic inputs from many brain areas, including signals carrying “top-down” information (representing environmental context, expectations and internal states of the animal).

Synapses in layer 1 are highly plastic, changing in response to experience and learning. But the molecular mechanisms enabling this plasticity are not well understood. The long distance of layer 1 synapses from the neuronal cell body raises a logistical challenge: how do they obtain the proteins they need to adapt? One solution is to transport messenger RNAs (mRNAs) from the cell body to distant synapses, where they can serve as instructions to produce proteins locally, at the right place and time. While this process has been studied extensively in the hippocampus, little was known about its role in cortical layer 1.

“We wanted to know whether mRNAs are transported all the way into layer 1 and its synapses, and whether they are actually used there to make new proteins,” says Teresa Spanò, graduate student in the lab of Prof. Erin Schuman and first author of the study. “Because these synapses are so distant from the neuronal cell body, local protein synthesis could be a powerful strategy to allow them to respond rapidly and precisely to changes in activity.”

More than 1,000 mRNAs at excitatory synapses

Using a combination of laser-capture microdissection of tissue, RNA sequencing, fluorescence in situ hybridization and biochemical approaches, the researchers showed that layer 1 contains localized mRNAs and actively synthesizes new proteins. To determine which transcripts were present directly at synaptic sites, they then isolated excitatory and inhibitory synapses from layer 1. At excitatory synapses alone, the team identified more than 1,000 different localized mRNAs.

The researchers also compared the synapses in layer 1 with those found in deeper layers of the cortex (where the neuronal cell bodies are found) and discovered substantial differences in their transcriptomes - the collections of RNA molecules present. The results indicate that mRNA localization is not uniform throughout the cortex: Instead, it is specialized according to the location of the synapses in the cortical network.

An unexpected connection to the hippocampus

A surprising finding emerged when the researchers compared cortical layer 1 with the hippocampus, where local mRNA translation has been extensively studied. They found a striking similarity with one hippocampal layer in particular: the stratum lacunosum moleculare, which, like cortical layer 1, contains distal portions of neuronal dendrites.

Unexpectedly, this similarity was not primarily driven by synaptic mRNAs, but also by transcripts associated with glial and immune cells, and the extracellular matrix. This suggests that these distal layers share a broader molecular architecture extending beyond dendrites and synapses.

“Our findings suggest that distal dendritic compartments in different parts of the brain may have a conserved molecular organization, which could be important for their function” says Spanò. “They also highlight components such as glial cells and the extracellular matrix, which may play a very important role for neuronal function in these distal layers.”

A new resource for understanding cortical plasticity

The findings establish local protein synthesis as a key feature of cortical layer 1 and provide a molecular framework for understanding how its synapses are maintained and modified. This local protein production could help synapses adapt during learning, while differences between layer 1 and deeper layers point to distinct molecular programs across the cortex.

The work may also help researchers study disorders involving altered cortical circuitry and synaptic function, including autism spectrum disorders. The new datasets, available at www.syndive.org, allow scientists to examine whether disease-associated molecules are localized to layer 1 and its synapses.

“Our study provides a molecular map of a part of the cortex that has been difficult to investigate,” says Erin Schuman. “It shows that layer 1 is not simply a place where distant synapses receive information - it has a rich local molecular mRNA environment that may help those synapses maintain themselves and adapt, processing information locally”.