Neuronal function of the endocytic adaptor CALM in the sorting of SNAREs and AMPARs

• Applicants: Professor Volker Haucke, Ph.D.; Professorin Dr. Tanja Maritzen
• Subject Area: Molecular Biology and Physiology of Neurons and Glial Cells ; Cell Biology
• Term: from 2013 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 241672096

• Final Report Year: 2023

Final Report Abstract

For our brain to function properly, its individual neurons need to be able to efficiently communicate with each other. While long-distance transmission of information in the brain relies on electrical signals, the gap between two neurons has to be bridged by chemical signals. This occurs at dedicated contact points, the synapses, and involves the release of neurotransmitters from synaptic vesicles. For this, the synaptic vesicles carry specific proteins such as VAMP2 that allow them to fuse with the presynaptic membrane. The released neurotransmitters bind to neurotransmitter receptors present on the postsynaptic neuron and thereby induce a signal cascade that triggers again an electrical signal. For this communication system to work, the presynaptic neuron needs to constantly replenish its supply of fusion-competent synaptic vesicles, and the postsynaptic neuron needs to always have the right type and amount of neurotransmitter receptors on its membrane, for example AMPA receptors to rely excitatory signals. This requires dynamic processes to place the required proteins where they are needed. For example, when synaptic vesicles fuse, they collapse into the membrane so that their VAMP2 protein is then stranded there. To generate a new fusion-competent synaptic vesicle, part of the presynaptic membrane has to bend inwards and pinch off, and in doing so, the VAMP2 protein has to be retrieved as well. This process is called endocytosis and requires specific endocytic sorting adaptors to link the proteins that need to be retrieved to the endocytic machinery. In our project we have studied the function and importance of two related endocytic adaptors called AP180 and CALM using mice in which we deleted either the one or the other adaptor, so-called knockout mice. With these, we discovered that AP180 is crucial for efficiently retrieving VAMP2 for the generation of new fusion-competent vesicles. In the absence of AP180, neurotransmission is impaired leading to behavioral alterations such as the inability to build a nest, to seizures and to decreased survival. While the adaptor CALM can support AP180 in this function, we discovered that it is more important at the postsynapse for the endocytic internalization of AMPA receptors. While neurotransmitter receptors need to be present on the postsynapse for the neuron to be able to respond to a presynaptic signal, their levels vary. In fact, by lowering the receptor number, the synapse can weaken its response. This is important when we have to forget old things to learn something new. Our work revealed that learning is in fact impaired when CALM is not there to mediate the internalization of a certain type of AMPA receptor. In summary, our studies demonstrate that endocytic sorting adaptors are crucial to safeguard and dynamically regulate pre- and postsynaptic proteins that are essential for neurotransmission and thus for brain function.

Publications

• Diffusional spread and confinement of newly exocytosed synaptic vesicle proteins. Nature Communications, 6(1).
  Gimber, Niclas; Tadeus, Georgi; Maritzen, Tanja; Schmoranzer, Jan & Haucke, Volker
  
• Vesicular Synaptobrevin/VAMP2 Levels Guarded by AP180 Control Efficient Neurotransmission. Neuron, 88(2), 330-344.
  Koo, Seong Joo; Kochlamazashvili, Gaga; Rost, Benjamin; Puchkov, Dmytro; Gimber, Niclas; Lehmann, Martin; Tadeus, Georgi; Schmoranzer, Jan; Rosenmund, Christian; Haucke, Volker & Maritzen, Tanja
  
• Endocytic proteins: An expanding repertoire of presynaptic functions. Current Opinion in Neurobiology, 73, 102519.
  Azarnia, Tehran Domenico & Maritzen, Tanja
  
• Selective endocytosis of Ca 2+ -permeable AMPARs by the Alzheimer’s disease risk factor CALM bidirectionally controls synaptic plasticity. Science Advances, 8(21).
  Azarnia, Tehran Domenico; Kochlamazashvili, Gaga; Pampaloni, Niccolò P.; Sposini, Silvia; Shergill, Jasmeet Kaur; Lehmann, Martin; Pashkova, Natalya; Schmidt, Claudia; Löwe, Delia; Napieczynska, Hanna; Heuser, Arnd; Plested, Andrew J. R.; Perrais, David; Piper, Robert C.; Haucke, Volker & Maritzen, Tanja