What we do and why we do it
All cells maintain a delicate balance between protein synthesis and turnover, accomplished by a vast and intricate network of molecular factors. This network of protein homeostasis, or proteostasis, is essential for maintaining protein integrity and thereby, cellular and physiological health. Typically, prolonged, repeated, or severe challenges to proteostasis lead to deleterious outcomes, such as protein aggregation or cell death. Perhaps there is no system where dealing with these challenges is of such paramount importance as in the brain, where neurons cannot be replaced or cannot divide to deal with stressors. Failures in this system lead to severe consequences as observed in most major neurodegenerative disorders, such as Alzheimer’s Disease, Parkinson’s Disease, Huntington’s Disease, and Amyotrophic Lateral Sclerosis (ALS). This common root suggests that failed neuronal proteostasis may be relevant, or even etiological, to the onset and progression of neurodegenerative disorders.
We discovered a neuronal-specific variant of the core 20S-like proteasome (a “neuroproteasome”) which is somehow situated at the plasma membrane and has access to both the intra- and extracellular space, making it functionally transmembrane. This unique localization enables this membrane neuroproteasome to degrade substrates across the membrane into small polypeptides that are directly used as neuromodulators. Surprisingly, unlike the regular intracellular 26S proteasome, degradation through this membrane neuroproteasome does not require a ubiqutinated protein for substrate recognition. Recognition seems to be a neuronal activity-dependent process, whereby nascent polypeptides off the ribosome are directly delivered to the neuroproteasome. The function of these neuronal plasma-membrane proteasome complexes is largely a huge mystery.
In many ways, these discoveries are a departure from the classic paradigm of ubiquitin-proteasome degradation. First, a membrane-bound proteasome is highly unusual as is a new form of localized degradation. Second, this is a ubiquitin-independent proteasomal system. Third, this represents a new mechanism of proteasomal modulation of a biological system through peptide fragments used as a novel class of neuromodulators.
Our approach to hard problems
A model of NMP-mediated degradation (left) and approaches we use to approach this phenomenon (right)
Our goal is to determine how this neuronal membrane proteasome complex (NMP) is assembled, how degradation is regulated, and its function in neurons. We have a series of questions we’re addressing.
We are approach-agnostic to answer the questions we find most pertinent to move this field forward. Currently, we utilize techniques across biochemistry, genetics, proteomics, protein engineering, cell biology, chemical biology, and neurobiology to tackle NMP biology. Here is a small snapshot of what we are solving:
How does the NMP associate with the plasma membrane?
What are the physiological functions that NMPs regulate in vivo? How do NMPs contribute to neuronal plasticity?
How do substrates get targeted to the NMP without ubiquitin?
How is NMP-derived peptide signaling coded and decoded?
How and why does NMP localization and function become dysregulated over aging and in neurodegenerative disease? Are these changes etiological to these processes?
How can we manipulate NMP biology at will?