Our laboratory is interested in how synaptic activity modulates the molecular make-up of synaptic connections in the mammalian central nervous system (CNS), which in many cases leads to long-lasting changes in synaptic efficacy. The concerted regulation of protein synthesis and degradation is fundamental for the control of diverse cellular events. Many studies have provided evidence that new protein synthesis likely takes place at synapses and is required for plasticity. Protein degradation, on the other hand, provides another way to regulate protein levels. In fact the ability to dynamically control protein levels allows for very tight control of rapid signaling cascades.
We study the ubiquitin-proteasome system (UPS), one of the major cellular pathways controlling protein turnover in mammalian cells. The UPS is a complex proteolytic pathway whereby proteins are targeted to the 26S proteasome for degradation. Ubiquitin is covalently attached to a target protein through a series of steps: first an E1 ubiquitin activating enzymes pass ubiquitin to E2 transferase and E3 ligases. At this point, many times in concert with the help of an E2 enzyme, the E3 ligase binds and modifies the target protein with the ubiquitin. Multiple ubiquitin molecules are added and the protein is recognized and degraded by the 26S proteasome. Many cellular roles have been defined for the UPS such as cell cycle control, cell fate and growth determination, antigen presentation, and many cell signaling pathways. In contrast the mechanisms of how the UPS regulates the growth and development, maintenance, and remodeling of synaptic connection in the mammalian central nervous system (CNS) is less understood.
An interesting problem is how activated synapses of a single neuron become selectively modified as a result of synaptic plasticity. It is known, for example, that synaptic modifications can occur selectively at one group of synapses, but not at another group of synapses on the same neuron. This property is known as "input-" or "synapse specificity". It is plausible that the selective degradation of proteins that restrict or limit plasticity may be required for these synaptic changes to occur. Alternatively, various proteolytic activities may provide specificity for long-term synaptic changes. This could be accomplished through the degradation of some proteins at specific locations or by targeting regulatory components of a proteolytic pathway to modified or unmodified sites.
We study the UPS’s role in synaptic plasticity. One approach we take is to use genetically encoded fluorescent-based proteasome (or “degradation”) reporters and time-lapse confocal microscopy to assay how neuronal activity modulates the rate of degradation within the spines and dendrites of neurons. Using these techniques, we hope to understand how and why the UPS is activated at or recruited to synapses in response to neuronal activity.
We are also interested in identifying the protein targets of the UPS at synapses. In particular, we study how the UPS regulates alpha-amino-3-hydroxy-5-methyl-4-isoxaolepropionic acid (AMPA) type ionotropic glutamate receptor trafficking in neurons. The removal and insertion of AMPA receptors at glutamatergic synapses is one mechanism to either decrease or increase synaptic strength. We and others have found that the activity of the UPS is required for agonist-mediated endocytosis of AMPA receptors. The simple model is that protein degradation via the UPS is required for normal AMPA receptor trafficking in neurons. Using molecular and biochemical approaches, we hope to identify the components of the UPS and protein targets involved AMPA receptor trafficking.
We are also interested in neurodegenerative disease. In humans, many neurodegenerative diseases are characterized post-mortem by anatomical hallmarks visible by standard light microscopy. Alzheimer’s and Lewy body disease are the most common causes of dementia in elderly populations. These diseases are characterized by neurofibrillary tangles (NFT), plaques, and lewy bodies (large dense inclusions). Familial Parkinson’s disease is also characterized by Lewy bodies. In addition, large dense inclusions are characteristic of many of the CAG repeat diseases such as Huntington and ALS. Biochemical and immunohistochemical characterization of these inclusions, tangles, and neuritic plaques has indicated that they contain high concentrations of ubiquitin and ubiquitinated proteins. Does the ubiquitin-proteasome pathway contribute to these abnormalities and to neuronal death? We hope that the understanding of UPS in normal neuronal function will help elucidate how it may be involved in neuronal dysfunction and disease.
Dr. Patrick received his Ph.D. from Harvard University in 1999 after working in the laboratory of Dr. Li-Huei Tsai. He was a Damon Runyon Cancer Research Foundation and a United Negro College Fund/Merck postdoctoral fellow with Dr. Erin Schuman at the California Institute of Technology. Dr. Patrick joined the faculty in 2004.