Max Planck Institute for Brain Research
(March 8, 2016)
Local Control Mechanisms at Neuronal Synapses
Messages are sent from one neuron to the next at the synapse (the junction where two neurons meet). Proteins located at the synapse control how that synapse functions. Any alterations in the levels of these different proteins can affect the activity of the neuron. What processes are in place to help neurons maintain the correct protein levels, and how does the synapse adjust when those levels are out of normal range? Dr. Schuman discussed the work of her group examining the functioning of the synapse, and how the individual neuron can control the proteins found in the synapse. They have developed imaging technologies that allow them to visualize newly formed proteins, in order to better understand how the neuron maintains a balance of protein levels.
The brain generates representations of environmental inputs received from sensory systems and must maintain and update these representations to allow humans and other animals to effectively interact with the environment. Brain cells, or neurons, connect and communicate at structures called synapses. Each neuron possesses about 104 synapses. Synapses are packed with over 500 different proteins that exist in copy numbers of ~20 to 2000 molecules per synapse. The proteins present at a given synapse determine how that synapse functions. When the brain adapts, such as during learning and memory, the properties of neurons and synapses change to encode and maintain new information. The adaptive response of neurons and synapses relies on modifications to existing proteins as well as changes in gene transcription, protein synthesis, and protein degradation. To endow synapses with independent control over their protein composition, neurons have delivered the cell biological machines for protein synthesis and degradation to synapses.
The Schuman lab is interested in understanding how synapses work. In particular, we focus on how an individual neuron controls the type, amount, and location of the proteins that populate its synapses. Research over the past 20 years or so has led to the identification and a basic functional understanding of the proteins that populate synapses. In order to understand how all of the proteins function together and maintain their concentrations in the appropriate range one must obtain quantitative information at high resolution. What are the populations of RNA molecules that code for protein (mRNA) and regulate synaptic function? What is the density of molecules in neurons? What is the lifetime of the different mRNAs and proteins at synapses? How do these numbers relate to the strength of individual synapses? How are these numbers altered during synaptic plasticity, and what effect does this have on the network of synaptic proteins? How do these molecular processes contribute to learning and memory in animals? These are the types of questions that we seek to answer with a combination of imaging, electrophysiology, biochemistry, molecular biology, bioinformatics, modeling, and behavioral analyses.
The Schuman group develops and uses advanced technologies (deep sequencing of neuronal mRNA 3’UTRs; fluorescent labeling and quantification of individual mRNAs; high-resolution in situ hybridization; labeling, identification, and visualization of newly synthesized proteins in identified neurons) to uncover the local cell biological mechanisms that allow synapses to function autonomously and be modified by experience. In order to identify or visualize recently synthesized proteins, we (together with Dave Tirrell at Caltech) developed a new suite of methods (BioOrthogonal NonCanonical Amino acid Tagging, or BONCAT) that use a non-canonical amino acid (NCAA), modeled after a natural amino acid, such as methionine. The NCAA is close enough in structure that the cellular protein synthesis machinery is “fooled” and uses the NCAA, instead of a natural amino acid, to make new proteins. Moreover, the NCAA has a special chemical group that serves as a molecular handle — allowing us to access to the newly synthesized protein — to identify it with mass spectrometry or to tag it with a fluorescent dye and visualize it within a neuron. Taken together, these technologies allow us and other researchers to unravel the molecular mechanisms that underlie synaptic protein homeostasis. Interestingly, several developmental (e.g. Fragile X Mental Retardation Syndrome) and degenerative (e.g.Parkinson’s disease) neural disorders appear to target the machinery that maintains synaptic proteins in a dynamic range — highlighting the importance of these local control mechanisms.