Nano-scale organization and dynamics of presynaptic Ca2+ channels

PD Dr. Akos Kulik (Institute of Physiology II, University of Freiburg)

By connecting the gyrus dentatus with the hippocampus proper, the axons of granule cells, the mossy fibers (MFs), represent an important element of the main excitatory, trisynaptic pathway of the hippocampal formation. These axons form anatomically specialized synapses with different physiological properties depending on the nature of their postsynaptic targets: the large MF boutons innervate thorny excrescences of CA3 pyramidal neurons, dendrites of basket cell, and hilar mossy cells, whereas their small filopodial extensions contact with dendrites of local circuit GABAergic interneurons. The highly specialized function of large and small MF synaptic specializations critically depends on the fine control of transmitter release that requires appropriate subcellular distribution, dynamic reorganization, and functional coupling of Ca²⁺ channels and neurotransmitter receptors as well as well-regulated local Ca²⁺ signaling in axonal compartment of MFs. In this project we will study the ultrastructural organization and dynamic changes of the nano-environment of the two groups of interacting synapse-related proteins: voltage-activated Ca²⁺ (Ca_v) channels and their partner G-protein-coupled receptors at MF synapses using a combination of quantitative high-resolution SDS-FRL immunoelectron microscopic, electrophysiological, and pharmacological approaches. First, we will investigate the nano-architecture of the Ca_v2.1 (P/Q-type) and Ca_v2.2 (N-type) channels in boutons and filopodial extensions of MFs by determining their number, density, and spatial arrangements in synaptic and extrasynaptic membranes of terminals at a resolution of 10 nm. Second, we will study the dynamics of the molecular reorganization of presynaptic MF terminals with focus on changes of Ca_v channels (including number and ultrastructural distribution pattern over the active zone) upon induction of long-term potentiation (by application of forskolin or tetanic stimulation of MFs). Finally, we will analyse the ultrastructural basis for interaction and functional coupling of Ca_v channels and metabotropic GABA_B receptors or group III glutamate receptors (mGluR7, mGluR8) at MF synapses.