- Title
- The function of molecular chaperones in human sperm-egg recognition
- Creator
- Bromfield, Elizabeth Grace
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Sperm-egg recognition, adhesion and fusion are amongst the most fundamental interactions to occur between two cells in the body. As such, these processes are tightly regulated by numerous proteins adorning the surface of both the male and female gametes. However, given the importance of these proteins for fertilization success, even a slight deregulation of sperm-egg recognition machinery can have devastating effects on reproduction. This is evidenced by the distressingly common occurrence of sperm-egg recognition defects in the spermatozoa of male infertility patients. Unfortunately, as a majority of these men do not possess easily detectable defects in their spermatozoa, their infertility often goes undiagnosed with a large proportion of patients classified as having idiopathic infertility. Recent work has shown that a deficiency in the molecular chaperone, Heat Shock Protein A2 (HSPA2) in the spermatozoa of these patients may be partly responsible for their inability to recognize the outer vestments of the egg, the zona pellucida (ZP). HSPA2 is known to be heavily involved in the remodeling of the sperm plasma membrane that occurs as these cells transcend the female reproductive tract. Moreover, our previous studies suggest that this protein regulates the assembly and presentation of important ZP-receptor protein complexes at the sperm surface that are required for interaction with the ZP. The studies within this thesis aimed to build upon this body of work by determining the molecular basis by which these cells lose the function and expression of HSPA2, and with it their ability to recognize the egg. Herein, we provide the first evidence for a critical link between the attenuation of HSPA2 function and the presence of oxidative stress in human spermatozoa. By exploring this, we have revealed both the sensitivity of HSPA2 to oxidative modification by the lipid peroxidation product, 4-hydroxynonenal (4HNE) and an ensuing pathway that leads to the severe loss of ZP binding ability in human spermatozoa. Importantly, we have shown that the regulation of protein complex dynamics by HSPA2 and the perturbation of these events under conditions of oxidative stress extend to the novel HSPA2 client proteins, angiotensin converting enzyme (ACE) and protein disulfide isomerase A6 (PDIA6). In accounting for the underrepresentation of HSPA2 in the patient population, we hypothesize that oxidative stress occurring in the developing germ cells of the testis, likely promotes similar modifications of HSPA2 by 4HNE but that these changes may result in more severe consequences for the stability of this protein. In the mouse testis, the proteolysis of HSPA2 has been demonstrated in the absence of its stabilizing chaperone, BCL-2 associated athanogene 6 (BAG6). In this thesis we have demonstrated a stable interaction between HSPA2 and BAG6 in the testicular germ cells and mature spermatozoa of our own species, suggesting that a similar dependency on BAG6 may exist. Excitingly, our studies evaluating protein deficiency in the patient population have revealed that spermatozoa that lack the ability to interact with homologous human zonae pellucidae, associated with dysregulation of HSPA2 protein expression, also have a severe deficiency in BAG6 protein expression compared with fertile controls. These data lead us to propose that infertile spermatozoa are predisposed to a loss of HSPA2 protein expression through an absence of a BAG6-dependent protective mechanism against enzymes such as ubiquitin ligase that result in its degradation in testicular germ cells. This also provides impetus for further study into BAG6 as a potential molecular target for male factor infertility. Taken together, the findings of this thesis contribute to our understanding of idiopathic male infertility by providing distinct links between oxidative stress and failed sperm-egg recognition. Importantly, this collection of studies offers a molecular understanding of the predictive value of HSPA2 in determining the fertilizing capacity of human spermatozoa. This in turn takes us closer to the development of HSPA2 as a positive molecular biomarker for sperm-egg binding competence, a crucial tool for the more accurate diagnosis of male factor infertility. Furthermore, our ability to recover a degree of sperm function through use of the nucleophile penicillamine, suggests that the deleterious effects of lipid aldehydes on HSPA2-mediated sperm-egg recognition may be ameliorated through antioxidant supplementation strategies. This gives credence to new practices that could improve the state of male reproductive health.
- Subject
- sperm-egg recognition; molecular chaperones; reproduction; spermatoza; fertility; infertility; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1310106
- Identifier
- uon:21984
- Rights
- Copyright 2015 Elizabeth Grace Bromfield
- Language
- eng
- Full Text
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| View Details Download | ATTACHMENT02 | Thesis | 4 MB | Adobe Acrobat PDF | View Details Download |