Absence of Neu5Gc and Presence of Anti-Neu5Gc Antibodies in Humans-An Evolutionary Perspective

Abstract

The glycocalyx of human cells differs from that of many other mammals by the lack of the sialic acid N-glycolylneuraminic acid (Neu5Gc) and increased abundance of its precursor N-acetylneuraminic acid (Neu5Ac). Most humans also have circulating antibodies specifically targeting the non-human sialic acid Neu5Gc. Recently, several additional mammalian species have been found to also lack Neu5Gc. In all cases, loss-of-function mutations in the gene encoding the sialic acid-modifying enzyme CMAH are responsible for the drastic change in these species. Unlike other glycan antigens, Neu5Gc apparently cannot be produced by microbes, raising the question about the origin of these antibodies in humans. Dietary exposure and presentation on bacteria coating themselves with Neu5Gc from the diet are distinct possibilities. However, the majority of the non-human species that lack Neu5Gc do not consume diets rich in Neu5Gc, making it unlikely that they will have been immunized against this sialic acid. A notable exception are mustelids (ferrets, martens and their relatives) known for preying on various small mammal species rich in Neu5Gc. No studies exist on levels of anti-Neu5Gc antibodies in non-human species. Evolutionary scenarios for the repeated, independent fixation of CMAH loss-of-function mutations at various time points in the past include strong selection by parasites, especially enveloped viruses, stochastic effects of genetic drift, and directional selection via female immunity to paternal Neu5Gc. Convergent evolution of losses of the vertebrate-specific self-glycan Neu5Gc are puzzling and may represent a prominent way in which glycans become agents of evolutionary change in their own right. Such change may include the reconfiguration of innate immune lectins that use self-sialic acids as recognition patterns.

Keywords: Neu5Ac; Neu5Gc; anti-glycan antibodies; sialic acid; xeno-antigen; xenoglycan.

Figure 1

Modification of CMP-Neu5Ac to CMP-Neu5Gc. (A) The enzyme CMAH, encoded by a single gene in all mammals, catalyzes the derivatization of Neu5Ac to Neu5Gc in the form of their sugar nucleotides, cytidine monophosphate (CMP). (B) Due to the large number of sialic acids terminating many of the glycan chains on the glycocalyces of most cells, the loss of function of the CMAH gene leads to a drastic change in the molecular identity or “flavor” of the glycocalyx, as indicated by a small fraction of a red blood cell membrane, redrawn and modified from Viitala and Järnefelt (7). (C) Micrographs showing green immunofluorescent staining of Neu5Gc on chimpanzee but not human sperm cells stained with affinity purified chicken anti-Neu5Gc IgY antibody and fluorescent secondary, controls include sialidase treated or anti-IgY secondary antibody alone, nuclei stained by DAPI (blue) reprinted from Ghaderi et al. (8) with permission.

Figure 2

Parallel evolution and loss of an innate self-signal. Humans cannot synthesize Neu5Gc, because human CMAH was inactivated over two million years ago (red). The inactivating mutation apparently fixed rapidly after originating, which suggests that the loss could have been adaptive—driven by pathogen avoidance, reproductive conflict, or a combination of the two. Independent losses of Cmah function have recently been found in New World Primates, Mustelids and several other groups. Figure modified from Springer and Gagneux (9). In some lineages, such as bats and toothed whales, only certain species lost the capacity to make Neu5Gc (indicated by lines that are both blue and red).

Figure 3

Schematic of the interplay of natural and sexual selection acting on cell-surface sialic acids. (A) Natural selection by pathogens recognizing and exploiting Neu5Gc (blue diamond) as a receptor on host target cells can select for mutant CMAH⁽⁻⁾ alleles that abolish expression of Neu5Gc in homozygote individuals and prevent infection. Such homozygous null individuals have only Neu5Ac on their cells (red diamonds) and at higher frequencies would be targeted by other pathogens adapted or adapting to the host glycan change (magenta). This equilibrium would result in maintenance of glycan polymorphism by balancing selection. (B) Anti-Neu5Gc antibody-expressing CMAH^(−/−) females, immunized by dietary consumption of Neu5Gc rich food (red meat) or by sperm antigens containing Neu5Gc, favor loss-of-function alleles on sperm due to reproductive incompatibility with CMAH^(−/+) or CMAH^(+/+) males expressing Neu5Gc on their sperm. Once the frequency of the CMAH⁽⁻⁾ allele reaches a certain level, this process can drive the fixation of the CMAH⁽⁻⁾ allele in a population via directional selection. Figure modified from Ghaderi et al. (8).

Figure 4

Exogenous (dietary) Neu5Gc and anti-Neu5Gc antibody as contributing factors to unexplained human infertility. The combination of incorporated dietary xenoglycan Neu5Gc (even in trace amounts) from red meat and milk products including cow milk-based infant formula and anti-Neu5Gc antibodies in males or females (A) could have deleterious consequences on human fertility via a number of potentially additive mechanisms which include: (B) coating of sperm by male and/or female anti-Neu5Gc antibodies, interference with sperm function, including passage through cervical mucins and/or other restrictive parts of the female reproductive tract such as the utero-tubal junction, (C) increased sperm death via female cellular and humoral immunity, where most sperm are killed by female immunity, (D) interference with sperm capacitation when sperm membranes get dynamically reconfigured, (E) interference with sperm penetration of vestment, interference with sperm-egg interactions, (F) interference with endometrial decidualization and receptivity resulting in reduced success of implantation. Figure modified from Ma et al. (75) with permission.