Morphology Induced Receptor Trapping in Artificial Dendritic Spines

Abstract

Memory and learning are believed to be regulated by the strength of the connections in the synapses of the 100 billion neurons in the human brain. In the synapse the signal is transmitted between the presynaptic axon and the dendritic spine by neurotransmitters. The number of receptors in the membrane of the dendritic spine defines the strength of a synapse. The persistent increased local concentration of receptors however, is contradicted by the finding of high receptor mobility within the synapse, dependent on spine morphology. The observation that synaptic strength correlates with dendritic spine morphology leads to the hypothesis that the mushroom-like shape of dendritic spines functions as a receptor trap.We developed a mimetic system to investigate dendritic spine morphology and its effects on receptor confinement and diffusion. Giant unilamellar vesicles (GUV's) are made from lipids using electroswelling. To mimic the mushroom-shaped morphologies of dendritic spines, a micromanipulator is used to pull membrane tubes from the GUV lipid bilayer. Trapping capabilities for different spine morphologies are assessed by tracking quantum dots attached to membrane lipids, thus mimicking receptors.Results show a strong dependence of escape times on GUV morphology, as quantified by GUV radius and tube length. Instead of a trivial quadratic dependence of escape times on GUV radius we find a powerlaw dependence with an exponent of 2.85. This confirms the idea that receptors can be trapped by the morphology of a dendritic spine. Therefore the connection strength of a mushroom-shaped dendritic spine is much more stable than the strengths of stubby shaped dendritic spines.