Four of my favorite things are development, evolution, and breasts, and now I have an article that ties them all together in one pretty package. It’s a speculative story at this point, but the weight of the evidence marshaled in support of the premise is impressive: the mammalian breast first evolved as an immunoprotective gland that produced bacteriocidal secretions to protect the skin and secondarily eggs and infants, and that lactation is a highly derived kind of inflammation response. That mammary glands may have had their origin as inflamed glands suppurating mucus may not be the most romantic image to arise in a scientific study, but really—they got better and better over the years.
Vorbach et al. have carried out a descriptive analysis of the elements of breast milk and lactation and come to this conclusion on the basis of three general lines of evidence.
- Immunoprotective proteins are a significant component of breast milk.
- The nutritional components of milk are synthesized by enzymes that are derived from immunoprotective proteins.
- Many of the molecular regulators of lactation are shared with inflammation pathways.
My previous article on mother’s milk gave a highly abbreviated list of the components found in milk, but there’s more detail in the list below. Milk is much, much more than baby food—it’s an incredible cocktail of interesting proteins, and 1) many of them are shared with mucus secretions produced by other secretory epithelia, and 2) many of them are elements of the innate immune system. Milk is actually a kind of anti-microbial snot mixed in with a lot of fat and sugar.
The innate immune system is a primitive defense system that uses peptides that recognize various common microbial surface molecules, and also uses enzymes that produce chemical agents lethal to bacteria. It’s a first line of defense that differs from the more specific immune system in that it doesn’t require specialized immune system cells and also doesn’t acquire the kind of highly refined specificity we see in antibodies (note, too, that antibodies are also secreted in milk—they are the IgG, IgE, and IgM molecules listed above.)
One key element in milk is XOR, xanthine oxidoreductase. This enzyme mediates purine catabolism and, for instance, the synthesis of uric acid, but also generates reactive oxygen species and reactive nitrogen species, compounds that can be voraciously reactive and interact destructively with bacteria. You may have heard of one reactive nitrogen species, the nitrites that hot dogs are loaded with as preservatives. Reactive oxygen species are chemicals like hydrogen peroxide, which is often used as a disinfectant, and superoxide anions and hydroxyl radicals.
Another is lysozyme, a protein that hydrolyzes the polysaccharides (sugars) that make up bacterial cell walls. This is a very common component of the innate immune system of many animals: you ooze this protein out of your skin, your mucus is loaded with it, and it’s also found in high concentrations in other places, such as egg whites. It’s simply generally useful protection to be able to chemically strip bacteria of their protective coats.
Other proteins also have bacteriocidal functions, and are found in other glandular secretions: lactoferrin and transferrin, lactoperoxidase (which as you might guess from the name, synthesizes reactive oxygen species), defensins (which disrupt bacterial membranes), and pattern recognition molecules (PRMs), which bind to certain common molecular motifs on bacterial surfaces. It’s so sweet and tasty, yet so deadly to microbes!
Now wait, you may be wondering…how is this evidence that milk’s original function was immunoprotective? If you’re secreting a sweet and fatty nutritious substance for your young, it would definitely be advantageous to load it up with antimicrobials to keep the nasty beasties from growing in it. XOR and lysozyme and all those others could have been added secondarily.
One reason to think otherwise is that those protective molecules are universal to vertebrates—lizards and birds have them, too. These enzymes and other proteins came first.
Those of you wise in the ways of evolution are saying to yourselves, “So what? It’s another example of cooption, taking a protein used for one function and adapting it to another.” And you’d be right: that’s only suggestive. Another good reason, though, is to look at the enzymes responsible for generating the nutritive components of milk, the lactose sugars and the fat droplets.
Breaking up fat into droplets and enveloping them with some kind of coat to promote suspension in water is an essential function in the production of milk. The protein that is responsible for this structural function, and which if knocked out prevents lactation and actually leads to collapse of the mammary gland, is…XOR! This is a wonderful example of a protein doing double duty, as an enzyme responsible for bacteriocidal action and as a structural protein involved in solubilizing fats. It almost certainly had the enzymatic function first, and if any cooption was going on, it was to recruit an immunoprotective protein to secondarily assist in a nutritive function.
The other essential nutritive component of milk is lactose, the milk sugar. Lactose is an odd and unusual sugar—that’s actually it’s advantage, that it is a sugar that many bacteria have difficulty digesting (it’s not just people that can be lactose intolerant!)—and it requires a specific synthetic complex consisting of β-1,4 galactosyltransferase and α-lactalbumin for its production. Where did α-lactalbumin come from? It’s sequence makes the homologies clear: the α-lactalbumin gene is a modified copy of…lysozyme!
I think that’s remarkable. The two primary nutritive components of milk, the sugars and fats, are the product of novel activity by proteins that are clearly primitively associated with innate immunity.
To further the similarities, the authors give a long list of components of signaling pathways that are typically associated with inflammatory and immune system responses and are also essential in lactation. For instance, the transcription factor NF-kB, which is also a hot candidate molecule in cancer research, is involved in regulating the expression of various cytokines and antimicrobial agents; transgenic mice that knock out this pathway also exhibit developmental failures in the differentiation of the mammary gland. They’ve modified other elements of this pathway (RANKL, C/EBPβ, TNF-α) which act in inflammation responses, and they all also induce developmental problems in mammary gland tissue and reduce or shut down lactation.
I’ve just finished teaching a human physiology course in which we learned the basics of endocrinology, and I gave my students the usual story on the pituitary hormone prolactin: it regulates milk production. It’s too bad I hadn’t read this paper earlier, because it makes the story much, much more complicated (my students are probably relieved, though—the class was complicated enough as it was).
Prolactin is known as a key lactogenic hormone but,
depending on the cellular context, prolactin can also act as an
anti-inflammatory or proinflammatory cytokine. Interestingly, it has been demonstrated that prolactin is involved in the
protective as well as the nutritional role of milk. Prolactin
participates in regulating the secretion of immunoglobin A
(IgA), the prominent Ig in mucus and milk that inhibits the
colonization of pathogenic bacteria on mucosal surfaces.
Changes in the secretion of IgA are associated with the anti-
inflammatory potential of epithelial tissues. In addition,
prolactin stimulates the uptake of some amino acids and
glucose, as well as the synthesis of casein, α-lactalbumin,
lactose and milk fat droplets in the lactating mammary
epithelium. Finally, prolactin and IFN-γ also stimulate the
expression of XOR in mammary epithelial cells via the Jak/Stat
signaling pathway. Thus, multiple small molecules and
ligand-receptor systems that have critical roles in inflammatory responses exert dual and, in many cases, essential functions in immunity and mammary gland biology.
Gee, I’m going to have to revise and add some stuff in that class next time I teach it. Very cool.
Their model for the evolution of the mammary gland is illustrated below. It’s reasonable, and the molecular evidence is persuasive. Basically, the process began as the secretion of antimicrobials agents from the skin of the early mammal (something we still do) as a protective function. This function was elaborated by infoldings of epithelia to increase surface area and generate reservoirs of mucus and the antimicrobials. Eggs and infants would have benefitted from more copious secretions from the mother, coating them as well with this immunoprotective substance. The young would have also lapped up the tasty rich goo, and infant survival would have been promoted by changes that caused the secretion of ever-richer substances.
One question not addressed by the paper is why only females lactate—you’d think the young would have benefitted if Papa Proto-mammal was also slathering them with immunoprotective slime. I’d guess that this supports the idea that those ancient males weren’t particularly involved in caring for their progeny, so it made little difference in infant survival if the father turned these secretions down to a level sufficient to selfishly protect just himself. These secretions are also expensive—I’ve seen figures that suggest that a third to half of the energy budget of nursing small mammals may be leaking out their lactating teats—so it would have been advantageous for those slacking males to shut down the production so necessary for female reproductive success.
Vorbach C, Capecchi MR, Penninger JM (2006) Evolution of the mammary gland from the innate immune system? BioEssays 28:606-616.