Written by: Ross Tellam, Ph.D. | Issue # 1 | 2012
Fresh out of the womb, a newborn baby is challenged with armies of disease-causing microbes. How does he survive this onslaught? In some parts of the world, he doesn’t. Millions of babies die each year in the first few months of life from common infections.
A recent publication by Barboza and colleagues1 reinforces the importance of mother’s milk as a major factor protecting a newborn from infection. Milk is much more than just a convenient source of high quality nutrition for the newborn.
The researchers have now expanded our knowledge about the biological defenses in mother’s milk by focusing on the antimicrobial properties of a highly abundant human milk protein, lactoferrin. They characterised the amazing versatility of this protein, both in terms of its structure and antimicrobial activities.
The newborn has an immature immune defence system. Consequently, it strongly depends on maternal factors to help protect it against bacterial and viral infections. This first line of defence against infection must rapidly neutralise threats and discriminate between foreign and self as well as pathogenic (disease-causing) and non-pathogenic bacteria.
While milk provides the newborn with the fundamental building blocks for growth and development, including amino acids derived from the digestion of milk proteins, and energy originating from the metabolism of fatty acids, it has a number of other biologically important functional roles, too. Components in milk directly promote maturation of the newborn gut mucosa, the growth of beneficial bacteria that aid digestion, and protection of digestive epithelial cells against attachment and invasion by pathogenic bacteria.
Barboza and colleagues investigated the finer structure and antimicrobial activities of one of the most abundant human milk proteins, lactoferrin. This protein is an iron-binding glycoprotein that is found in the milk of most mammals. A glycoprotein is simply a protein that has sugars, often complex sugars, attached to specific regions of the protein. Researchers have long known that a protein component of lactoferrin, a peptide called lactoferricin, has antimicrobial activity. This peptide binds bacteria and uses the iron bound by lactoferrin to produce toxic peroxides which then destroy the bacteria.
The researchers have now uncovered remarkable and previously unrecognised variation in the antimicrobial activities of lactoferrin that are separate from those of lactoferricin and mediated by an array of complex sugars attached to different regions of the protein. The big surprises from the results of this research are the extent and variability of these complex sugars as well as the range of their antimicrobial activities.
Complex sugars are often attached to proteins secreted from cells. The general view asserts these complex sugars have simple physical roles that help stabilise protein structure and increase protein solubility. However, emerging research indicates that complex sugars provide other services as well. In the case of lactoferrin, it plays key roles in generating a diversity of protective antimicrobial activities.
Barboza and colleagues first identified the spectrum of different types of complex sugars attached to human lactoferrin using a highly sensitive and selective technique called mass spectrometry – a technique that uses a targeted laser beam to energise small molecules which are then accelerated in a strong magnetic field thereby enabling their masses and structures to be determined. The initial revelation Barboza and colleagues made about lactoferrin using mass spectrometry was the considerable diversity of complex sugars attached to lactoferrin.
They then analysed these complex sugars, isolating them from the milk from five mothers over a time period from one day until 10 weeks after birth. The researchers showed the complex sugars attached to lactoferrin were very dynamic both in type and overall quantity during this period, which paralleled the lactational transition from colostrum to milk production in the mothers. There was also considerable variation of the complex sugars between milk samples from individual mothers at the same lactation time.
Another result from Barboza and colleagues pertains to the mode of bacterial infiltration into the body. Some pathogenic bacteria can adhere to digestive epithelial cells before gaining entry into the cell and thereby entry into an individual. Barboza and colleagues showed that the different types of complex sugars attached to lactoferrin help prevent enteropathogenic bacterial attachment and in some cases invasion of colonic epithelial cells.
However, the most remarkable discovery they made is that different complex sugars have selective effects on attachment to and invasion by different microbial species. Thus, the complex sugars attached to lactoferrin are exquisitely variable and specific in function, presumably enabling efficient neutralisation of the unpredictable onslaught on a newborn by a wide variety of bacterial challenges.
The new information about lactoferrin is very interesting as it emphasises the advantages of structural variation and provides us a glimpse into a fundamental evolutionary strategy designed to improve the survival chances of a newborn. Apart from demonstrating how little we know about even one of the most studied milk proteins, this research hints at future applications and investigations.
Although most proteins are strongly conserved in mammals, one of the greatest differences between species relates to the types of complex sugars attached to glycoproteins. Thus, lactoferrins from the milk of other species, such as cow, may have unique antimicrobial properties moulded by the specific microbial challenges in the environment of that species. Some of these specific antimicrobial activities could be very useful to humans.
If lactoferrins of other mammals possess different types of antimicrobial activities, it may be possible to isolate these other lactoferrins, such as those from a cow, and use them to specifically boost the antimicrobial properties of human infant formula, or to produce a food product that acts as a prophylactic to prevent specific gastrointestinal diseases prevalent in at-risk individuals, such as travellers. There may be many additional potential uses for lactoferrins from other mammals as well.
The source of maternal variation in the complex sugars is also of great interest. Is it genetically based, or can it be manipulated by diet, perhaps even before lactation and during pregnancy? Other questions raised by these results relate to mechanism. How do these complex sugars exert their antimicrobial activity? Are they just decoys that prevent pathogenic bacteria from binding to their target cells or do they have more direct protective roles?
Lactoferrin is clearly a multifaceted protein par excellence. The name represents an array of physical forms that confer a spectrum of antimicrobial activities. Lactoferrin probably has many more functional faces yet to be discovered.
Barboza et al. Glycosylation of human milk lactoferrin exhibits dynamic changes during early lactation enhancing its role in pathogenic bacteria-host interactions. Molecular & Cellular Proteomics (first published January 19, 2012).