The divergence of the two superfamilies belonging to the Infraorder Catarrhini – Cercopithecoidea (Old World monkeys) and Hominoidea (apes, including humans) – is generally assumed to have occurred during the Oligocene, between 38 and 20 million years ago. Genetic studies indicate that this time period was one of active genetic evolution under strong purifying selection for catarrhine primates. This includes selective pressures on the glycoprotein galactosyltransferase 1 (GGTA1) gene and subsequent inactivation "clocked" at approximately 28 ma, possibly prior to the Cercopithecoidea/Hominoidea split. The GGTA1 gene codes for an α1,3 galactosyltransferase (GT) enzyme that synthesizes a terminal disaccharide, αgalactosyl (αGal), found on glycoproteins and glycolipids on the surface of cells in the tissues of most mammals. Currently, catarrhines are the only mammals studied for the terminal αGal residue that do not express this sugar on their cell surfaces. The proposed selective advantage of this mutation for catarrhines is the ability to produce anti-Gal antibodies, which may be an effective immune component in neutralizing αGal-expressing pathogens, as certain helminthes, many bacteria, including those found in primate guts, and some viruses derived from GGTA1 positive species express αGal on their surfaces. However, many viruses are known to utilize host cell carbohydrates in various ways such as binding receptors or attachment proteins, making these moieties "hot spots" for selective evolution. Cell surface αGal may have predisposed ancestral catarrhines to pathogens and toxins that could utilize the terminal sugar moieties on host cells as binding sites or in other capacities during infection. I found that, in fact, the presence or absence of cell surface αGal affects the course of certain viral infections. Infections of paired cell lines with differential expression of GT showed that Sindbis viruses (SINV) preferentially replicate in αGal-positive cells, whereas herpes simplex viruses type 1 and type 2 (HSV-1 and HSV-2) preferentially grow in cells lacking αGal. In both cases, differences in infection levels resulted from the ability of the virus to successfully initiate infection. This points to a role for αGal in the early stages of viral infections. I also showed that GT knockout mice infected with HSV-2 had higher viral load and greater pathology compared to WT B6 mice that naturally express αGal. The increased susceptibility of KO mice to HSV-2 was not due to an immune component as differences in viral load and pathology were even more evident in immunocompromised mice. This clearly indicates that αGal expression in cells or animal hosts can affect the course of viral infections. I was not able to further confirm differences in susceptibility to HSV 1 and 2 using mouse backcrosses (KO x WT). Unknown genetic factors, that are independent of αGal expression, may be introduced during the crosses that need to be further investigated. Infections of KO and WT mice with other herpes viruses did not yield definitive data and require further studies with suitable reagents. The mechanism by which GT-dependent differential susceptibility to viruses operates still remains to be deciphered. However, it is clear that susceptibility to certain viral infections is tied to the presence or absence of αGal on the surface of host cells. Overall, these results have implications for the evolution of resistance to viral infections in catarrhines. Pathogens exert great selective pressure on their hosts, and it is possible that a pathogen, able to exploit αGal, could have helped shape primate lineage evolution during the Oligocene.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-7231 |
| Date | 01 January 2014 |
| Creators | Rodriguez Ayala, Idalia Aracely |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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