Francisco José López-Murcia, from the Faculty of Medicine and Health Sciences, the Institute of Neurosciences of the University of Barcelona (UBneuro) and the Bellvitge Biomedical Research Institute (IDIBELL).
Photo Credit: Courtesy of University of Barcelona
Scientific Frontline: Extended "At a Glance" Summary: Brain Mechanisms of Working Memory
The Core Concept: Working memory is a critical cognitive function that enables the temporary retention and processing of information necessary for carrying out everyday activities, learning, and managing controlled behavioral responses.
Key Distinction/Mechanism: At the synaptic level, working memory relies on the temporary strengthening of neural connections during repeated activity. This process is governed by the synaptic protein Munc13-1, which must be precisely regulated by calcium through two complementary mechanisms: calcium-phospholipid signaling (via the C2B domain of Munc13-1) and the calcium-calmodulin pathway. If Munc13-1 fails to accurately detect calcium signals, synapses lose their capacity to temporarily strengthen, thereby degrading short-term information retention.
Major Frameworks/Components:
- Munc13-1 Protein: A crucial presynaptic protein responsible for regulating the release of neurotransmitters.
- Calcium-Phospholipid Signaling: One of the primary regulatory pathways operating through the C2B domain of the Munc13-1 protein.
- Calcium-Calmodulin Pathway: A secondary, complementary regulatory pathway operating via a specific calmodulin-binding region on the protein.
- Synaptic Plasticity/Strengthening: The physiological process where repeated neural activity temporarily enhances synaptic efficacy, forming the cellular basis of working memory.
Branch of Science: Neuroscience, Molecular Neurobiology, and Biochemistry.
Future Application: The identification of these molecular pathways provides highly specific targets for pharmacological interventions aimed at restoring or enhancing synaptic plasticity. This could lead to novel treatments for maintaining cognitive function and mitigating memory loss in clinical settings.
Why It Matters: Working memory is frequently one of the first cognitive abilities impaired in various neurodegenerative diseases and cognitive disorders. Mapping the precise molecular and calcium-dependent foundations of how synapses hold short-term information is vital for developing targeted therapies to combat these debilitating neurological conditions.
The study, conducted using animal models, is led by Francisco José López-Murcia, a professor at the Faculty of Medicine and Health Sciences and the Institute of Neurosciences of the University of Barcelona (UBneuro), and a member of the Bellvitge Biomedical Research Institute (IDIBELL). The team led by Professor Nils Brose at the Max Planck Institute for Multidisciplinary Sciences (MPI-NAT, Göttingen, Germany) is also participating in the project.
How synapses prepare for neural transmission
Neurons do not always communicate at a constant rate. In many neural circuits, brief bursts of activity occur that temporarily strengthen synapses, allowing for more efficient transmission of information. Two such transient strengthening processes are short-term facilitation and post-tetanic potentiation (PTP), both of which are particularly prominent at mossy fiber synapses, which are thought to contribute to working memory.
At the molecular level, the team focused on studying the Munc13-1 protein, a key factor that prepares synaptic vesicles for the release of neurotransmitters, a process known as vesicular priming. The study demonstrates that Munc13-1 must be regulated by calcium via two complementary pathways: calcium-phospholipid signaling (via the C2B domain of Munc13-1) and the calcium-calmodulin pathway (via a region that binds to this protein).
Analyzing the molecular sensors of the Munc13-1 protein
In animal models with these signaling pathways disrupted, the authors measured synaptic responses at mossy fiber synapses in the hippocampus during stimulation patterns that mimic physiological activity.
“The results show that when Munc13-1 was unable to detect calcium signals properly, the synapses lost much of their ability to temporarily strengthen during repeated activity,” says Francisco José López-Murcia, a professor at the Department of Pathology and Experimental Therapeutics at the UB.
By identifying a specific molecular mechanism that links short-term synaptic strengthening to working memory performance, this study expands our understanding of how the brain rapidly stores and updates information.
“Disruption of the calcium-phospholipid signaling pathway increased the threshold for inducing post-tetanic potentiation and reduced its magnitude, suggesting that this pathway is particularly important for triggering strong short-term increases in synaptic transmission,” explains the researcher.
A maze of errors: when memory fails at the synapse
To study whether these synaptic alterations influence behavior, the team assessed the animal models in a spatial working memory task (an eight-arm radial maze). Mice carrying the Munc13-1 mutation — which disrupts calcium-mediated binding to cell membrane phospholipids — showed pronounced deficits consistent with impaired working memory, such as repeatedly returning to reward locations after having obtained the reward.
“These results provide experimental evidence that working memory may depend not only on sustained neuronal activation, but also on transient, activity-dependent changes in synaptic transmission that temporarily retain information within neural circuits,” says López-Murcia.
The study also highlights the role of the Munc13-1 protein as a key component that enables synapses to sustain to adapt to transfer and reinforce information during peaks of activity, an essential feature of neuronal activity in the hippocampus.
Previous studies have identified mutations in the human UNC13A gene that alter the sequence of multiple protein domains — including those examined in this study — in people with a wide range of neurological symptoms, notably intellectual disability. The findings of the new study highlight the crucial role of the Munc13-1 protein in healthy brain function and its clinical relevance in neurodevelopmental disorders.
Published in journal: Cell Reports
Authors: Francisco José López-Murcia, Dilja Krueger-Burg, Sally Wenger, Tania López-Hernández, Noa Lipstein, Holger Taschenberger, and Nils Brose
Source/Credit: University of Barcelona
Reference Number: ns031626_03