Turbocharger for nerve cells: operation of ADHS gene investigated
2014-06-05 / Individuals with attention-deficit / hyperactivity syndrome (ADHS) often have a higher frequency of mutations in the gene for GIT1 – the research group headed by Volker Haucke has now established the role played by this protein at neuronal synapses. In a second study, the group elucidated fundamentally different pathways through which nerve cells recycle vesicles that release neurotransmitters. Both publications (Cell Reports and Neuron) deal with the age-old, yet unresolved question of how nerve cells are able to fire rapidly and adaptably.
Electron tomograms and 3D reconstructions of presynaptic terminals from the somatosensory cortex of wild-type (A) and AP-2(μ) KO mice (B). AP-2(μ) KO synapses display a drastic reduction in SV number and an accumulation of endosomes-like vacuoles (ELVs), most of which were disconnected from the plasma membrane (PM). Adapted from Kononenko et al. Neuron (2014). FMP
A single nerve cell must often remain in a dormant state and then very suddenly engage in intense activity – up to 800 electrical impulses per second may reach certain synapses in some cases. This kind of bandwidth and speed are essential for many processes in the brain, such as for processing acoustical and visual stimulation for example. Neurotransmitters that are packaged in vesicles and kept ready at the cell membrane are released at synapses in response to such electrical impulses. “It is as if a sports car were poised at a red light with the driver revving the engine, ready to take off at any moment,” as Volker Haucke describes the situation. This poses a non-trivial problem for the nerve cell in managing the reverse process as well: it has to rapidly recycle vesicles as needed, which have fused with the cell membrane. An extensive protein machinery at the so-called active zones coordinates this endocytosis/exocytosis circuit. One of these proteins is GIT1, as the researchers at FMP were able to establish.
The precise way that GIT1 works remained a mystery up until now – only that there was a relationship with attention deficit / hyperactivity syndrome (ADHS). Mutations in the corresponding gene occurred with a higher frequency in affected people, and mice with mutated GIT1 displayed symptoms reminiscent of ADHS. The exact function of GIT1 was unknown until now, however. Jasmin Podufall, a member of Volker Haucke’s group working together with the laboratory headed by Stephan Sigrist at Freie Universität Berlin, has now created fruit flies with mutations in GIT. In addition, Jan Schmoranzer and Mathias Böhme have made high-resolution images of synapses using dSTORM and STED fluorescence microscopy. They were able to show that GIT1 is part of the protein matrix at active zones, where it associates with the endocytic protein stonin. The picture that emerges is as follows: calcium channels are localized in the central area of the active zone where they mediate the influx of calcium ions to trigger exocytosis. Other proteins are arranged around this area like a wreath, including GIT1 and stonin, a protein involved in vesicle recycling by endocytosis. As a result, GIT1 acts as a link between endocytosis and exocytosis.
“Nerve cells can also operate without GIT1, but the efficiency of neurotransmission is impeded,” says Haucke. Inhibitory nerve cells in particular often need to fire at a high rate – the brakes are more important in the human brain than the accelerator pedal in many respects. “We can now speculate that defects in the mechanism of endocytic vesicle recycling, such as in GIT1 for example, particularly impair the operation of inhibitory nerve cells and thus lead to an excessively excited brain,” Haucke explains.
At the same time, the group pursued the question of how nerve cells can adapt to and maintain high rates of firing. Quite varied theories about the precise sequence of steps in endocytosis at synapses have developed over the last forty years. Volker Haucke and his postdoc Natalia Kononenko stimulated nerve cells from mice at varying intensities and then followed vesicle recycling by fluorescence imaging in living nerve cells and by electron microscopy. As they present in a parallel publication in Neuron, endocytosis in nerve cells can proceed in two fundamentally different ways. At low synaptic firing rates, fused vesicle membranes are retrieved directly from the nerve cell membrane as small individual vesicles coated with the proteins clathrin and AP2. At high intensities though, the nerve cells pull a large part of their membranes into the cell at once. This rapid endocytosis is mediated by the proteins dynamin 1/3 and endophilin. Larger, initially irregular endosomes arise this way, from which individual vesicles are then reformed in the cells’ cytoplasm– again with the assistance of clathrin and AP2.
“Our understanding of how nerve cells operate is becoming increasingly more precise,” says Haucke. We see over and over again that a great many components must interact, many of which appear to be redundant or only subtly produce fine adjustments. “It is these subtleties that are interesting, for the subtle changes can lead to neurological diseases like ADHS, epilepsy, schizophrenia, or Alzheimer’s.” The individual components of the synapses can also now be described in kinetic or even molecular models. “And with these we are now pushing up against the limits of what computers can even calculate,” explains Haucke.
Jasmin Podufall, Rui Tian, Elena Knoche, Dmytro Puchkov, Alexander M. Walter, Stefanie Rosa, Christine Quentin, Anela Vukoja, Nadja Jung, Andre Lampe, Carolin Wichmann, Mathias Böhme, Harald Depner, Yong Q. Zhang, Jan Schmoranzer, Stephan J. Sigrist and Volker Haucke: “A presynaptic role for the cytomatrix protein GIT in synaptic vesicle recycling”, Cell Reports, 29 May 2014; DOI: http://dx.doi.org/10.1016/j.celrep.2014.04.051.
Natalia L. Kononenko, Dmytro Puchkov, Gala A. Classen, Alexander M. Walter, Arndt Pechstein, Linda Sawade, Natalie Kaempf, Thorsten Trimbuch, Dorothea Lorenz, Christian Rosenmund, Tanja Maritzen, Volker Haucke: “Clathrin/ AP-2 mediate synaptic vesicle reformation from endosome-like vacuoles but are not essential for membrane retrieval at central synapses”, Neuron, 4 June 2014