Functional Transformations in the Sperm Glycome
Spermatozoa, or sperm for short, are the male gametes which must successfully navigate the female reproductive tract and fertilize an oocyte in natural conception. Sperm face a number of challenges in the completion of this task. This dissertation addresses two of these challenges, one immunological and one functional. First, the immunological challenge involves the fact that sperm must survive in a non-self body. The female immune system closely regulates the environment of the female reproductive tract to protect against potentially pathogenic non-self organisms while tolerating symbiotic microbes. The endometrium initiates the leukocytic reaction in response to insemination, triggering a migration of various leukocytes to destroy threatening foreign cells. A successful sperm must evade destruction by these leukocytes. In chapter 2, I show that a subset of infertile human males exhibit a non-human cell-surface molecule, N-glycolylneuraminic acid (Neu5Gc), that is targeted by circulating antibodies within the female reproductive tract.
The second major challenge concerns the cellular functioning of the sperm. Sperm undergo a series of changes, collectively called capacitation, upon exposure to the female reproductive tract. The functional challenge is highlighted by the fact that a sperm cell must perform its various functions in the absence of gene expression and translation. Compared to somatic cells, the DNA in the mature sperm head is tightly packaged and inaccessible. Furthermore, sperm are virtually absent of ribosomes and thus are unable to mediate translation. Thus, sperm must survive and achieve fertilization using only post-translational modifications (PTMs). The PTMs examined in this dissertation include changes in the glycocalyx, phosphorylation of peptide residues, and a specific type of intracellular glycosylation of serine/threonine residues called O-GlcNAcylation. In chapter 3, I provide evidence that endogenous sialidases begin cleaving sialic acids from the sperm glycocalyx immediately upon exposure to capacitating conditions. Surprisingly, the lesser-sialylated capacitated sperm do not increase immune cell
activation in vitro. In chapter 4 I focus on O-GlcNAcylation, a lesser studied and often entirely overlooked PTM implicated in various cell processes that may compete with phosphorylating enzymes for certain peptide residues. Our data suggest that sperm exhibit capacitation-induced changes in O-GlcNAcylation.