Gephyrin: A prime example of the importance of scaffolding proteins
Reflections on the minisymposium “Emerging Mechanisms Underlying Dynamics of GABAergic Synapses”
It started as most neuroscience symposiums do, with an economics lesson.
Dr. Shiva Tyagarajan, the host of the symposium, likened the role of inhibitory interneurons to the Pareto principle, also known as the ‘law of the vital few.’ The philosophy states that 80% of effects come from 20% of the causes or 80% of results come from 20% of the effort or 80% of resources are distributed to 20% of the population… you get the idea; the important bit is 80:20. Oddly enough, 20% of the neuronal population in the cortex is made up of GABAergic interneurons which control the remaining 80% of cells.
Photo courtesy of madscientistdigital.com
The purpose of this symposium was to highlight current work that explores the segregation, sorting and expression of that influential 20% of neurons. What are those distinct mechanisms that control dynamic GABAergic activity and expression? There were many fascinating aspects of regulation and coordination mentioned in the lectures (cannabinoid receptors, chloride transporters, the relationship between iLTP and LTD etc.), but one protein that was mentioned in multiple talks was gephyrin.
For those unfamiliar with proteins involved in synaptic scaffolding, gephyrin is likely new to you. A review co-authored by the host of this symposium, Dr. Tyagarajan, can be found here.
Essentially, gephyrin is a scaffolding protein that interacts with many other proteins to aid in expression and anchoring of GABARs to the membrane, making it an integral aspect of the postsynaptic lattice.
Dr. Charrier from the Institute of Biology (IBENS Inserm in Paris, France), started the symposium by highlighting work that shows interactions between a human-specific gene, SRGAP2, and gephyrin. SRGAP2A and SRGAP2C regulate inhibitory and excitatory synapse development, with SRGAP2A being involved in the maturation of synapses and accumulation of both AMPA and GABA receptors (providing an interesting link between excitation and inhibition). You can probably guess the protein that SRGAP2A interacts with to promote GABA accumulation (yup, gephyrin). Her work showed co-immunoprecipitation of gephyrin with SRGAP2A and demonstrated that knocking out SRGAP2A reverts gephyrin cluster density and size to levels that mimic an immature state, highlighting the importance of gephyrin and the proteins with which it interacts in the growth and maturation of inhibitory synapses. You can read more about her work here. It includes details about the paralog SRGAP2C and its role in inhibiting SRGAP2A functionality as it relates to gephyrin expression.
Gephyrin was also shown to be a dynamic protein that helps control synaptic plasticity. Dr. Tyagarajan discussed some of his work exploring changes in gephyrin homeostasis following theta burst stimulation (TBS) in organotypic hippocampal culture. 24 hours following TBS, gephyrin puncta were upregulated but returned to baseline levels after an additional 24 hour period. He has demonstrated in the past that gephyrin can be modulated via phosphorylation by calcium/calmodulin-dependent protein kinase II (CaMKII) (found here) and here demonstrated a loss of the homeostasis if the phosphorylation site of gephyrin was mutated. This adds to the ways in which proteins interacting with gephyrin can affect the inhibitory synapse.
There were many wonderful takeaways from this series of talks, but the link gephyrin providies between synaptic proteins and the function and growth of inhibitory synapses stuck out the most. I used gephyrin here as an example of just how important scaffolding proteins are. These networks are not always taken into consideration when discussing synaptic work, but they are too important to be ignored.