ROLE OF THE SPECTRIN BASED MEMBRANE SKELETON IN PLASMA MEMBRANE PROTEIN PRESENTATION: A PROTEOMIC APPROACH
Open Access
- Author:
- Khanna, Mansi Rajendra
- Graduate Program:
- Integrative Biosciences
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- May 09, 2011
- Committee Members:
- Graham Hugh Thomas, Dissertation Advisor/Co-Advisor
Graham Hugh Thomas, Committee Chair/Co-Chair
Richard W Ordway, Committee Member
Hong Ma, Committee Member
Melissa Rolls, Committee Member
Bruce A Stanley, Committee Member - Keywords:
- spectrin
proteomics - Abstract:
- The plasma membrane (PM) and its associated proteins play an important role in determining how a cell interacts with its neighbours as well as how it responds to the components of, and conditions in the extracellular environment. As a reflection of this, more than 50% of the current drug targets lie at the cell surface. The amount of a protein at the cell surface is determined by its rate of delivery, internalization, recycling and degradation. All these parameters are subject to change under normal physiological adjustments, development, varying environmental influences and pathological conditions. The spectrin based membrane skeleton (SBMS) is a ubiquitous feature of all metazoan cells. This network of spectrin and associated proteins is attached directly or indirectly to plasma membrane proteins. Among the numerous functions of the SBMS, are roles in protein trafficking and turnover that determine levels of plasma membrane proteins. Regulated spectrin proteolysis, mediated by calpain, occurs under normal physiological conditions for various cellular processes, including establishment of synaptic contacts, long-term potentiation and platelet activation. Spectrin cleavage is also observed in age-related pathologies such as stroke and other ischemic events, Alzheimer’s and Parkinson’s diseases. However, little is known about the consequences of spectrin breakdown per se under these conditions. The goal of this work is to establish the fruit fly, Drosophila melanogaster as model system for some aspects of ischemic stroke, of which spectrin breakdown is a hallmark. The hypothesis being tested in this work is that disruption of the SBMS will affect the levels of many proteins at the cell surface under pathological conditions. The significance of this with respect to an ischemic stroke is that changes in levels of cell surface proteins in the event of SBMS breakdown need to be compensated for in post-stroke drug therapies and rehabilitation. In order to observe changes in the cell surface proteome under SBMS breakdown, one would first need to establish what it looks in normal conditions. Towards this, I developed a technique to isolate pure PM that employs a unique combination of density gradient centrifugation and aqueous two-phase affinity partitioning (2PAP). Density gradient centrifugation is an accepted method of fractionation on the basis of the buoyant densities of biomolecules and organelles but results in an overlap in cellular compartments due to similarities in densities. 2PAP is an established method for PM isolation in vertebrate model systems and makes use of the glycosylation pattern of PM proteins to isolate them from those in other compartments. My work demonstrates that a novel combination of both the techniques results in a robust PM preparation, which neither technique can achieve on its own. Using the new technique, 432 proteins were identified from Drosophila heads by MudPIT, of which 22% were found to be known PM residents and 34% are currently unassigned to any compartment and represent candidate PM proteins. The SBMS was disrupted in Drosophila with the help of a temperature sensitive α-spectrin allele (α–specR22S, R22S) that causes spectrin network disassembly at non-permissive temperature (29oC) and has been previously characterized. The R22S allele was introduced as a transgene into wild-type flies as well as an α-spectrin null background. My data suggests that although there is no precedent for an effect of α-spectrin R22S (α(R22S) being expressed in a wild-type background, the mutant protein in my hands, shows a difference in distribution in the ovaries in comparison to its wild-type counterpart. α(R22S) is excluded from the spectrin network at the apical membrane of the ovarian follicle cells and is instead exclusively localized in the basal membrane in the presence of the wild-type α-spectrin. It is also unstable in the presence of the wild-type α-spectrin, while in an α-spectrin null background, it is stable and its distribution in the ovaries resembles that of the wild-type protein. These data suggest a faster turnover of α(R22S) in the presence of wild-type α-spectrin due its selective exclusion from the membrane bound spectrin network. The work presented also shows similarity in phenotypes in the ovaries when α(R22S) is present as the sole source of α−spectrin and when it is present in a background heterozygous for α−spectrin (heterozygous flies): at a frequency of about 8%, abnormal late stage egg chambers are observed, with gross morphological defects in the follicle cell layer, absence of nurse cells and necrosis. This effect seems to be independent of temperature and genotype. Also, flies with α(R22S) as the sole source of α-spectrin (rescued flies) don’t produce any progeny. Since the heterozygous flies show similar phenotypes as the rescued flies, I decided to record significant changes in the Drosophila plasma membrane proteome by iTRAQ in these flies. iTRAQ analysis revealed a significant change in 18 proteins, of which 4 are PM proteins and have been established in literature to being affected by the SBMS. The structural role of the SBMS has been long established. The results presented in this thesis point to a more dynamic role for the SBMS in maintaining intercellular communication and maintaining surface protein levels.