Colin Johnson - US grants

Item 6 of R&R Other Project Information: Summary/Abstract The aim of this project is to determine the functional and structural properties of the hearing associated protein otoferlin. This protein is believed to directly regulate membrane fusion and the resultant neurotransmitter release from inner hair cells in the cochlea. Mutations in otoferlin result in profound hearing loss at birth. However, the exact role of otoferlin in hearing is not well understood. The following specific aims pursued under this award are intended to improve our understanding of otoferlin in the auditory pathway. If otoferlin is indeed directly involved in exocytosis, it should posses certain characteristic functional properties which allow it to partake in membrane fusion events. In the first aim, the hypothesized ability of otoferlin to bind SNARE proteins and accelerate membrane fusion will be directly tested for the first time using an in vitro fusion assay. This will establish a functional role for otoferlin in the exocytosis pathway. The calcium binding properties of otoferlin will also be probed in an attempt to determine whether the protein is a calcium sensor. The second aim will elucidate the structure of the protein using X-ray crystallographic methods so as to provide a structural basis for the observed functional properties. Several pathological mutant forms of the protein will also be examined so as to determine the molecular basis for the pathology. Third, the relationship between otoferlin and lipid membranes will be elucidated. Manipulation of lipid bilayers is a common characteristic of proteins involved in membrane fusion and exocytosis. Yet it is currently unknown whether otoferlin can indeed bind to and alter membrane morphology. This aim will test the hypothesis that otoferlin can penetrate and induce curvature in lipid bilayers in a calcium dependent manner as a mechanistic step in accelerating membrane fusion.

? DESCRIPTION (provided by applicant): Congenital hearing loss is a common disorder, with approximately 1 out of every 600 children suffering from profound deafness. A central player in hearing is the protein otoferlin. More than 60 pathogenic mutations in otoferlin are known, and up to 8% of all forms of prelingual autosomal recessive hearing loss are due to otoferlin mutations. Currently the only known bodily function for otoferlin is in mediating neurotransmitter release from hair cells, making it a hearing specific protein. While otoferlin is essential for hearing, current approaches to study otoferlin in vivo have not elucidated the exact function of the protein or provided a molecular level explanation for mutations associated with deafness. Further, the large size of otoferlin has prevented rescue experiments and prohibited viral-based gene transfer, representing a major hurdle toward the use of gene therapy as a treatment. To screen for the effects of pathogenic mutations on otoferlin function, develop truncated forms of otoferlin that can be packaged into existing gene delivery vehicles, and determine the function of otoferlin, we will use zebrafish as a model system. A major advantage of using zebrafish as a model is the ability to easily transfect hair cells with mutant and truncated forms of otoferlin, something that cannot be achieved easily in a mouse model. This work builds off studies conducted under a K99/R00 award devoted to characterizing otoferlin using recombinant protein. Our development of zebrafish as a model for testing results of recombinant protein studies uniquely positions us to pursue a powerful two-pronged approach to probe and engineer otoferlin on both the molecular and organismal level. In specific aim 1 of this proposal we will engineer and test truncated forms of otoferlin to determine the minimal regions of the protein required for hearing. Determination of the minimal sequence of otoferlin capable of restoring hearing will be critical for the design of therapeutics, including the design of truncated forms of otoferlin small enough to be packaged for viral mediated gene therapy. Specific aim 2 will use zebrafish and recombinant proteins to study pathological missense mutations associated with deafness in human patients. The goal of this aim will be to establish the molecular basis for why certain otoferlin missense mutations result in hearing loss in humans. Specific aim 3 will characterize the biophysical properties of otoferlin, with the goal of determining the unique functional properties of otoferlin that are needed for hearing. The results of these studies will directly impact the development of therapeutics for treating deafness, explain the basis for several human pathological missense mutations, and establish the mechanisms otoferlin utilizes for the encoding of sound. The PI is well suited for the proposed work, having already established zebrafish as a model for otoferlin studies, and having carried out fundamental biophysical studies under a K99/R00 award. The excellent zebrafish facilities at OSU, coupled with collaborations and support from specialists in the area of exocytosis and hair cell physiology make for an excellent overall environment for conducting the proposed studies.

The protein dysferlin initiates and controls the process of repairing damage to the cell membranes of muscles during physical movement. With this award, the Chemistry of Life Processes Program in the Chemistry Division is funding a team led by Professor Joe Baio from Oregon State University to investigate the key chemical interactions that dictate dysferlin repairs and manipulate biological membranes. Certain types of muscular dystrophy, including Limb-girdle muscular dystrophy type 2B and Miyoshi distal myopathy muscular dystrophy are connected to mutations within the dysferlin protein. This project specifically studies how the interactions of dysferlin with components of the cell membrane start the repair process. Results from this research help unlock answers to how chemical miscues lead to downstream musculoskeletal diseases. Professor Baio plans to integrate this project into an outreach program designed to increase the representation of Native Americans in STEM disciplines by providing training opportunities for both Native students and instructors from a local American Indian school.

The overall goal of this project is to establish the chemical principles that dictate how key proteins repair and manipulate biological membranes. When muscle sarcolemma is damaged, calcium leaks into the cell and exposure to calcium ions trigger a binding event between dysferlin's outer domain and an intracellular lipid vesicle. It has been postulated that following lipid binding, dysferlin then directs this liposome towards the damaged portion of the membrane, where it fuses and patches the damaged membrane. In this project, sum frequency vibrational spectroscopic approaches is applied to resolve the chemical interactions between dysferlin and phospholipids that trigger the initial lipid binding step and guide vesicle shuttling. This approach provides the geometry and positions of important atoms and protein structures at the dysferlin-plasma membrane interface. These approaches are then repeated for dysferlin variants associated with dysferlinopathies demonstrating how mutations within the protein contribute to a failure of this mechanism. Finally, this experimental work also provides crucial evidence that sum frequency vibrational spectroscopy-based approaches can potentially be applied to the dissection of any complex membrane protein-lipid interaction. In addition, Professor Baio works with teachers and students from a local Native American boarding school and provides them with opportunities to engage in research during the summers.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

Membrane trafficking is regarded as a crucial and distinctive characteristic of eukaryotic cells, with both intracellular transport and secretion events playing an essential role in homeostasis and signaling. While much of what is known about membrane trafficking comes from studies in yeast, there are important proteins involved in membrane trafficking in multicellular organisms not seen in yeast. One family of these proteins are the ferlins, an evolutionarily ancient family of trafficking proteins linked to an increasingly diverse list of physiological activities, including fertility, the encoding of sound, muscle development, and repair of damaged cell membranes. This diversity of physiological roles has made establishing a common underlying molecular function for this family challenging. This project will address this gap in our knowledge of the Ferlin family proteins, and in so doing add to our understanding of fertility, hearing, and muscle development. The proposed work will also develop undergraduate laboratory research training opportunities and lab classes focused on membrane biology and provide graduate students with state-of-the-art training in interdisciplinary research.

The project tests the idea that ferlins share a common function as calcium sensitive scaffolds for regulated exocytosis and endocytosis. Challenges due to the large size of the proteins, their transmembrane domains, and the high number of endogenous cysteines typical of ferlins have precluded many of the traditional recombinant protein assays typically used to test this hypothetical function. To overcome these challenges a novel single molecule protein-protein interaction assay that allows for probing the contacts of large multivalent membrane proteins has been developed. In addition, genetic code expansion techniques will be exploited to incorporate environmentally sensitive fluorescent unnatural amino acids into the ferlins. These technologies will be exploited to determine the functions that underlie ferlin activity. The proposed studies will define a set of underlying molecular-level functions that unite the ferlins, despite their disparate physiological roles. This information will allow integrating ferlins into the larger picture of membrane trafficking and cell signaling. The development of a novel single molecule fluorescence technique will provide a method for the study of large multi-domain membrane proteins and thus have applications beyond the study of ferlins.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.