Australasian Science: Australia's authority on science since 1938

Born Too Soon

herjua/iStockphoto

Credit: herjua/iStockphoto

By Sarah Robertson & Mark Hutchinson

Each year a million babies die after premature birth, but researchers have now identified a potential treatment.

Premature birth is defined as birth before 37 completed weeks of gestation. It is now identified by the World Health Organisation as the number one global killer of children under 5 years of age, with a high prevalence in Australia (8%) and even higher rates in many developing countries and the US, where up to 12% of babies are preterm. Every year around the world, 15 million babies are born prematurely and one million of those will die.

The proportion of pregnancies ending prematurely is escalating annually. Large cohort studies point to environmental causes such as infection, pollution, poor diet, stress and socio­economic status, but the underlying causes and the specific factors accounting for this rise in prevalence are unknown.

Modern post-natal health care now enables the survival of most preterm babies born after 32 weeks, but they are at higher risk of developmental problems and complications. Infants born early, especially those born less than 34 weeks gestation, are prone to cerebral palsy, mental retardation, visual impairment, hearing loss and less obvious central nervous system disorders, including language and learning disabilities, attention-deficit hyperactivity disorder and behavioural problems.

The average cost of neonatal care is estimated at more than A$50,000 for each preterm infant. The life-long economic health, psychological and social costs to the child, his family and society are considerably higher.

The major culprit is spontaneous preterm labour, which is often linked with bacterial or viral infection of the placental membranes. However non-infectious insults such as multiple pregnancy, blood supply issues to the placenta or immune imbalance can also cause preterm labour. All of these causes operate through a converging pathway leading to delivery.

Inflammatory mechanisms are clearly implicated in all of these insults, but how these channel both infectious and sterile triggers has remained elusive. Also, how the cascade of preterm birth overlaps with normal birth isn’t clear, mainly because the biological processes of normal birth are not fully understood.

In the past, research has focused on improving outcomes for the infant, such as the use of glucocorticoid drugs before birth to mature the fetal lungs.

However, the ultimate goal is to prevent premature birth, and to date there are no drugs or interventions that are very successful in slowing or arresting preterm labour. Although tocolytic agents that stall uterine contractions have shown promise, their use is contentious and they may even harm the fetus since prolonged exposure to high inflammatory mediators can damage the fetal brain.

The problem with many of these drugs is that they target what is effectively the last phase of the labour process. They do little to slow the inflammatory response that drives labour forward.

In theory, agents that target the initiating events of preterm labour should be more effective than the inhibitors of uterine activation that are currently used. An additional benefit of targeting events at the top of the inflammatory pathway would be to reduce the exposure of the fetal brain to inflammatory damage.

Pharmacological inhibitors of inflammatory cytokines are one prospect, but blocking signals upstream of these would be an even better strategy. Reaching this goal requires a complete and detailed understanding of the signals involved in initiating preterm labour, particularly the point at which different triggers converge, and how these relate to the events of normal term labour.

It’s commonly believed that term and preterm labour are fundamentally the same process that share a common pathway. In the case of term labour, a series of activation events gain momentum over several days and weeks to ultimately cause uterine muscles to develop the ability to contract. The same underlying events cause the cervix to soften and relax, allowing the fetal head to pass. Preterm labour is believed to result when pathological processes prematurely activate one or more components of the same common pathway.

The major knowledge gap lies in defining the critical activators in term labour and the point at which this pathway is usurped in preterm labour. Human clinical data is limited since cause and effect relationships are extremely difficult to establish without experimental interventions. Fortunately, many key features of human childbirth are evident in rodents and sheep models.

It’s now emerging from mouse studies that bacterial infection induces preterm delivery by increasing inflammatory cytokines that recruit immune cells to the uterus and stimulate contractions. The cytokine IL6 accelerates the inflammatory cascade in both term and preterm deliveries, while IL6 deficiency delays birth. Other cytokines, such as IL10, are negative regulators, with genetic IL10 deficiency predisposing to infection-induced preterm delivery. These animal studies provide an explanation for how subtle DNA changes in cytokine genes in women might influence their susceptibility to preterm delivery.

However, not all preterm births are caused by infection: toll-like receptors (TLRs) are another key mediator of the birth process. TLRs are abundant in the maternal uterine and placental membranes, and elicit inflammatory cytokines when stimulated.

One particular form, TLR4, appears to be a key mediator of childbirth that can be activated by bacteria as well as during normal cellular processes. Mice with a natural mutation in the TLR4 gene, or that are modified to be deficient in TLR4 signalling, do not experience infection-associated preterm delivery because the induction of inflammatory cytokines, leukocyte recruitment and uterine activation fail to occur.

Strikingly, these TLR4-deficient mice also have delayed timing of normal birth and a high rate of fetal death at the time of birth, indicating that this molecule is required for the normal timing of the birth cascade.

Now the search is on for the naturally occurring molecules that trigger TLR4 to control labour at the end of healthy pregnancy. Several candidate molecules, released from the placenta and maturing fetus in late gestation, are being explored.

These TLR ligands appear to interact with various hormones to trigger the downstream inflammatory cascade. The stress hormone corticotrophin-releasing hormone is induced by, and then amplifies, TLR-mediated responses in placental cells. The sex hormone progesterone, which sustains pregnancy and may protect from preterm birth, also attenuates the TLR-induced inflammatory cascade. Human studies show that the gestational tissues become more responsive to inflammatory triggers with advancing gestation as responsiveness to progesterone declines.

Together, these emerging observations are explaining how the same downstream pathways can be activated by a wide range of different events, and why several inflammatory mediators are commonly regulated in gestational tissues and the maternal blood prior to both term and preterm birth. In vitro experiments with uterine muscle cells show how these inflammatory cells and cytokines can elevate the synthesis of contraction-inducing prostaglandins and matrix-softening enzymes that in turn cause the changes in uterine and cervical tissues that allow the child to be born.

These new insights raise the prospect of new drugs that target the upstream events of labour, and the possibility of more efficacious therapies for preterm labour. We have recently discovered that a novel small molecule inhibitor of TLR4, (+)-naloxone, can prevent infection-induced preterm labour in mice. (+)-naloxone is the positive isomer of (–)-naloxone, an opioid antagonist with blood–brain permeability that is used to treat an opioid overdose. Both of these isomers block TLR4 signalling, but (+)-naloxone does not bind opioid receptors.

Our studies in mice show that the administration of (+)-naloxone can abrogate premature birth and increase the number of viable pups born. This result indicates the utility of (+)-naloxone as a powerful tool to define the causal pathways through which TLR4 controls inflammatory pathways that lead to preterm delivery. Importantly, (+)-naloxone and related drugs may also have value as new therapeutic drugs for delaying preterm delivery in humans.

Sarah Robertson is Director of The University of Adelaide’s Robinson Research Institute. Mark R. Hutchinson is the Director of the ARC Centre of Excellence for Nanoscale Biophotonics at The University of Adelaide.