Int. J. Devl Neuroscience 37 (2014) 8–14

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International Journal of Developmental Neuroscience journal homepage: www.elsevier.com/locate/ijdevneu

Review

Neurotrophins: Role in adverse pregnancy outcome Madhavi Dhobale Bharati Vidyapeeth University, Pune 411043, India

a r t i c l e

i n f o

Article history: Received 22 March 2014 Received in revised form 12 June 2014 Accepted 12 June 2014 Keywords: Brain derived neurotrophic factor Nerve growth factor Preterm delivery Angiogenesis Feto-placental unit

a b s t r a c t Proper placental development is essential during pregnancy since it forms the interface between the maternal–foetal circulations and is critical for foetal nutrition and oxygenation. Neurotrophins such as nerve growth factor (NGF), brain derived neurotrophin (BDNF), neurotrophin-3 (NT-3) and neurotrophin4/5 (NT-4/5) are naturally occurring molecules that regulate development of the placenta and brain. BDNF and NGF also involved in the regulation of angiogenesis. Recent studies suggest that the levels of BDNF and NGF are regulated by docosahexaenoic acid (DHA) which is an important omega-3 fatty acid and is a structural component of the plasma membrane. Oxidative stress during pregnancy may lower the levels of DHA and affecting the fluidity of the membranes leading to the changes in the levels and expression of BDNF and NGF. Therefore altered levels and expression of NGF and BDNF may lead to abnormal foetal growth and brain development that may increase the risk for cardiovascular disease, metabolic syndromes and neurodevelopmental disorders in children born preterm. This review discuss about the neurotrophins and their role in the feto-placental unit during critical period of pregnancy. © 2014 ISDN. Published by Elsevier Ltd. All rights reserved.

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Types of neurotrophins and their structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Nerve growth factor (NGF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Brain derived neurotrophic factor (BDNF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Neurotrophin-3 (NT-3) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Neurotrophin-4/5 (NT-4/5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Role of neurotrophins in the development of feto-placental unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neurotrophins in pregnancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Intrauterine growth restriction (IUGR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Preeclampsia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Small for gestational age (SGA), appropriate for gestational age (AGA), large for gestational age (LGA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Preterm delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interaction between micronutrients, oxidative stress, docosahexaenoic acid and neurotrophins (BDNF and NGF) in preterm pregnancy . . . . . . . 5.1. Micronutrients, docosahexaenoic acid (DHA) and neurotrophins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Oxidative stress and neurotrophins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Angiogenesis and neurotrophins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Altered levels of neurotrophins and their implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction The placenta is known to make adaptations to ensure optimal foetal growth during pregnancy. It has been suggested that the

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placenta hold clues for predicting the individuals who would be at risk of developing chronic diseases in childhood or in adult life (Faye-Petersen, 2008). Studies also reports that children born with preterm delivery, low birth weight, intra uterine growth restriction (IUGR) and preeclampsia have been associated with metabolic and neurodevelopmental disorders. However, the mechanisms are not clearly understood.

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Growth factors like neurotrophins and cytokines are known to act in paracrine and/or autocrine manner through their receptors in the cell for the development of feto-placental unit (GuzelogluKayisli et al., 2009). Neurotrophins are a family of polypeptide growth factors that influence proliferation, differentiation, survival and death of neuronal and non neuronal cells (Kim et al., 2004). The structure and function of these neurotrophins are described below

2. Types of neurotrophins and their structures There are four major types of neurotrophins i.e. nerve growth factor (NGF), brain derived neurotrophin (BDNF), neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5). Amongst these, BDNF and NGF are suggested to play an important role in placental and foetal growth and development (Zheng and Shao, 2012; Mayeur et al., 2010; Nico et al., 2008; Toti et al., 2006).

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2.3. Neurotrophin-3 (NT-3) NT-3 is also a member of neurotrophin family and plays an essential role in the development of both the neural-crestderived peripheral nervous system and the central nervous system (Chalazonitis, 2004). NT-3 has a high affinity for TrkC as a chemokine receptor (Chen et al., 2013) and also binds to TrkA and TrkB with low affinity (Skaper, 2008). It also has the same structure like other neurotrophins that contain a tertiary fold and cysteine knot. The NT3 promoter contributes to the dimer to form heterodimers (Robinson et al., 1995). The gene encoding human NT-3 (gene symbol designated NTF3) to chromosome 12 (Maisonpierre et al., 1991). The distribution of NT-3 messenger RNA and its biological activity on a variety of neuronal populations clearly distinguish NT-3 from NGF and BDNF. 2.4. Neurotrophin-4/5 (NT-4/5)

2.1. Nerve growth factor (NGF) The biologically active NGF consists of a dimer of 13-kDa polypeptide chains, each of which has three intrachain disulfide bridges. The crystal structure of NGF has been resolved (McDonald et al., 1991). The NGF gene is located on human chromosome one and is expressed as two major splice variants (Edwards et al., 1988). NGF has two known receptors, receptor tyrosine kinase A (TrkA) and p75NTR (Gigante et al., 2003). Upon binding of NGF to TrkA, the receptor is subjected to a series of events that characterize Trk signalling. These include receptor dimerization and transphosphorylation of tyrosines leading to activation of kinase activity, followed by autophosphorylation of tyrosines outside of the activation loop. Subsequent phosphorylation and activation of accessory proteins lead to the generation of a cascade of receptor-independent signalling pathways (Ras (Rat Sarcoma), Phosphoinositide 3-kinase (PKC), Phospholipase C (PI3) kinase pathway) (Sofroniew et al., 2001). 2.2. Brain derived neurotrophic factor (BDNF) The human BDNF gene has seven noncoding exons that are associated with distinct promoters and one coding exon that encode the mature BDNF proteins (Liu et al., 2005). BDNF protein shares about 50% amino acid identity with NGF, NT-3 and NT-4/5. It contains a signal peptide following the initiation codon and a pro-region containing an N-linked glycosylation site (Binder and Scharfman, 2004). It also shares a distinctive three-dimensional structure containing two pairs of antiparallel ␤-strands and cysteine residues in a cystine knot motif. Human BDNF transcripts are highest in the brain and several alternative BDNF mRNAs showed relatively high expression levels in nonneural tissues. For example, expression levels of transcripts containing exons VI and IXabcd were high in the heart, placenta, and prostate (Pruunsild et al., 2007). BDNF binds to its specific receptor i.e. Trk B, although some nonselective binding also occurs (Thoenen, 1995). Ligand induced receptor dimerization results in kinase activation; subsequent receptor autophosphorylation creates specific binding sites for intracellular target proteins (PLC-␥1 (phospholipase C), p85 (the noncatalytic subunit of PI-3 kinase) and Shc (SH2-containing sequence)), which bind to the activated receptor via SH2 domains (Patapoutian and Reichardt, 2001; Barbacid, 1995). This activation can then lead to a variety of intracellular signalling cascades such as the Ras-MAP (mitogen-activated protein) kinase cascade and phosphorylation of cyclic AMP-response element binding protein (CREB) (Segal, 2003; Patapoutian and Reichardt, 2001).

A fourth neurotrophin is NT-4/5 (also known as NT-4, NT-5) has been molecularly cloned from Xenopus (Ip et al., 1992). NT-4/5 contain six cysteine residues and also includes an insertion of seven amino acids between its second and third cysteines (Ip et al., 1992; Berkemeier et al., 1991) and its encoded gene is located on chromosome 19 in human. NT-4/5 shares a 95% amino-acid-sequence identity with BDNF and it is the only family member that has a truncated precursor region. NT-4/5 bind to the TrkB receptor corresponds with the onset of neurogenesis in the neural tube during brain development and is differentially regulated in later development (Bartkowska et al., 2010). There are very limited studies that report the role of these neurotrophins in the development of the placenta. 3. Role of neurotrophins in the development of feto-placental unit BDNF, NGF, NT-3 and NT-4/5 play a vital role during pregnancy in the mother, placenta and foetus (Fig. 1). BDNF regulate the cytotrophoblast differentiation, proliferation and survival of the placenta (Kawamura et al., 2009, 2011; Mayeur et al., 2010). BDNF plays a key role in the regulation of angiogenesis and is reported to protect the endothelial progenitor cells by increasing the expression of superoxide dismutase (He and Katusic, 2012; Jiang et al., 2011). Changes in levels of neurotrophins can produce long lasting effects on neurotrophic processes (neuron number, synapse), which alter neuronal maturation and plasticity in later life (Vicario-Abejón et al., 2002). BDNF/TrkB-stimulated intracellular signalling is critical for neuronal survival, morphogenesis and plasticity (Numakawa et al., 2010). It has been reported that both foetal and maternal tissues (trophoblast, amnion/chorion and maternal deciduas) express the NGF mRNA both in early gestation and at term (Toti et al., 2006). Role of NGF in mouse placentation during the post implantation period has been described (Kanai-Azuma et al., 1997). Furthermore, NGF and its receptors are suggested to play an important role during organogenesis (Miralles et al., 1998). NGF acts as an angiogenic factor by contributing to the maintenance, survival and function of endothelial cells by autocrine and/or paracrine mechanisms (Nico et al., 2008). In addition, NGF has been associated with functional activities of cells that include immune and endocrine systems and act as an inflammatory mediator (Berry et al., 2012). There are very few studies, which have discussed the role of NT-3 and NT-4 in the development of the feto-placental unit. It has been hypothesized that NT-3 function in the regulation of placental and foetal brain development and for the maternal inflammatory responses (Casciaro et al., 2009). Kawamura et al. (2009) have

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M. Dhobale / Int. J. Devl Neuroscience 37 (2014) 8–14

Fig. 1. Role of neurotrophins in pregnancy. DHA: docosahexaenoic acid; NGF: nerve growth factor; BDNF: brain derived neurotrophin; NT-3: neurotrophin-3; NT-4/5: neurotrophin 4/5 tyrosine kinase receptor

demonstrated the expression of neurotrophin 4/5 and their receptors TrkB in trophoblast cells and placentas during different stages of pregnancy in mice. The altered level and expression of these neurotrophins have been also indicated in complicated pregnancies such as intrauterine growth restriction (IUGR), preeclampsia and preterm delivery. 4. Neurotrophins in pregnancy Many studies have reported that alterations in the levels of neurotrophins (NGF, BDNF, NT-3, NT-4/5) in mother, cord, amniotic fluid and placenta in pregnancy complications and have been suggested their role in placental and foetal development (Malamitsi-Puchner et al., 2004, 2007; Toti et al., 2006; Marx et al., 1999). It has been suggested that BDNF and NGF could be a marker for the presence of central nervous system abnormalities, infectious insults in utero or both (Marx et al., 1999). Further the expression and localization of NGF has been observed in the trophoblast, decidua and foetal membranes and support the concept that human placenta is a potent neuroendocrine organ throughout gestation (Toti et al., 2006). Chouthai et al. (2003) demonstrated lower cord BDNF levels might have an implications for neural maturity in the premature infants. However, very few studies have examined the levels and expression of NT-3 and NT-4 in pregnancy. Study suggests that NT-3 may be playing a regulatory function on placenta and foetal brain development and maternal inflammatory response. It has been reported that the circulating NT-3 levels increased in early neonatal life, possibly due to exposure to various stimuli soon after birth (Malamitsi-Puchner et al., 2007). NT-3 and NT-4 have been documented to act at early stages of neuronal development and to decrease after hypoxia-ischaemia (Nikolaou et al., 2006). 4.1. Intrauterine growth restriction (IUGR) There are very few studies, which have reported the levels of these neurotrophins and their role in IUGR complicated pregnancies. Malamitsi-Puchner et al. (2007) showed no differences in the circulating levels of BDNF, NT-3 and NT-4 in IUGR pregnancies. NGF was the only neurotrophin that higher in the maternal and foetal plasma and further positively correlated with the infants’ centiles and birth weights. This no change in the neurotrophins could possibly be attributed to the activation of the brain-sparing effect (Malamitsi-Puchner et al., 2006). One of the rat IUGR animal

model showed less expression of BDNF and NT-3 in the cerebral cortex than controls. These alterations may be related in delay of neuronal migration (Fukami et al., 2000). 4.2. Preeclampsia Lower maternal and higher cord BDNF levels were indicated in women with preeclampsia when compared to normotensive women and suggested a possible role for BDNF in the pathophysiology of preeclampsia (D’Souza et al., 2014). BNDF levels were significantly higher in umbilical cord blood from preeclamptic pregnancies (Bienertova-Vasku et al., 2013). The expression of BDNF and its receptor TrkB, have been shown to be expressed in membranous chorion and villous tissue and was significantly higher in maternal plasma in preeclampsia than in controls (Fujita et al., 2011). Casciaro et al. (2009) have observed the expression of NT-3 in the human placenta during normal pregnancy and in preeclampsia. NGF levels are differently regulated in preeclamptic and normotensive mothers delivering low birth weight babies (Kilari et al., 2011). 4.3. Small for gestational age (SGA), appropriate for gestational age (AGA), large for gestational age (LGA) BDNF levels in the SGA groups was significantly higher than that in the AGA and LGA groups suggesting the BDNF is correlating with birth weight (Wang and Ye, 2008). Further, NGF levels have shown to be higher in the AGA compared to the IUGR group and were associated with the birth weight of the infants (Malamitsi-Puchner et al., 2006). Nikolaou et al. (2006) have examined the pattern of perinatal changes in NGF, BDNF, NT-3, and NT-4 in AGA full-term foetuses and neonates by determining their circulating levels. The data suggested that a gradual decrease of NT-3 and NT-4 from umbilical cord (UC) to neonates day 4 (N4), while levels of NGF and BDNF were decreasing suggesting NT3 and NT-4 have been documented to act at early stages of neuronal development and to decrease after hypoxia-ischaemia while NGF and BDNF to increase. 4.4. Preterm delivery There are very few studies, which have reported the levels of neurotrophins in preterm deliveries (Malamitsi-Puchner et al., 2004; Haddad et al., 1994). Higher levels of BDNF were observed in women delivering full term neonates as compared with preterm neonates suggesting that BDNF levels may reflect the mature

M. Dhobale / Int. J. Devl Neuroscience 37 (2014) 8–14

Fig. 2. Association of DHA and neurotrophins in cell membrane. DHA: docosahexaenoic acid; NGF: nerve growth factor; BDNF: brain derived neurotrophin; Trk: tyrosine kinase receptor; TrkA: tyrosine kinase receptor A; Trk B: tyrosine kinase receptor B

nervous and immune systems of mothers (Malamitsi-Puchner et al., 2004). Maternal NGF levels were lower in the mothers delivering preterm babies as compared with full term babies (Haddad et al., 1994). Cord NT-3 levels were significantly decreased in the presence of placental inflammation preterm infants (Kumar et al., 2011). NT-3 levels were decreased in preterm birth and suggested to play a role in neuronal growth and differentiation (Matoba et al., 2009). The alterations in the levels and expression of these neurotrophins during pregnancy could be due to their interaction with biomolecules such as micronutrients (folic acid, vitamin B12 ) and long chain polyunsaturated fatty acid (docosahexaenoic acid) which is explained in the following section. 5. Interaction between micronutrients, oxidative stress, docosahexaenoic acid and neurotrophins (BDNF and NGF) in preterm pregnancy Proper growth and development of the placenta is determined by maternal nutrition and plays a critical role in foetal growth and development. The peri or post conceptional period represents a particularly sensitive window for feto-placental development during which, suboptimal maternal micronutrients may alter the expression of the foetal genome leading to lifelong consequences. These neurotrophins may involve in the preterm delivery through different mechanisms which are discussed below. 5.1. Micronutrients, docosahexaenoic acid (DHA) and neurotrophins Micronutrients like folic acid and vitamin B12 have a major role in the one carbon metabolism since they are required for the transfer of methyl groups for methylation of DNA, RNA, proteins and membrane phospholipids. Studies carried out on humans and animals have extensively discussed the interaction of folic acid, vitamin B12 and DHA in the one carbon cycle (van Wijk et al., 2012; Kulkarni et al., 2011a,b; Kale et al., 2010). Deficiency of these micronutrients lead to increased homocysteine levels which are associated with reduced DNA methylation potential (Zijno et al., 2003). Further, it has been discussed that when DHA levels are low, influx of methyl groups may be diverted towards histone and DNA, resulting in altered methylation patterns (Kale et al., 2010). The levels of BDNF are known to be regulated by omega 3 fatty acids like DHA (Balogun and Cheema, 2014; Wu et al., 2004). DHA is a structural component of the plasma membrane and reductions in DHA can have a direct influence on membrane function (Fig. 2). Disruptions in membrane fluidity due to DHA deficiency have been

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suggested to lower the levels of BDNF in the rat brain (Bhatia et al., 2011). Animal studies have also demonstrated altered levels and expression of BDNF and NGF in the brain of the offspring at birth and in later life because of altered maternal micronutrients (Sable et al., 2011, 2012, 2014). These studies highlight the importance of altered early life nutrition serving as a predisposition to noncommunicable diseases like cardiovascular diseases, diabetes and neurodevelopmental disorders in adult life. Recent human studies have demonstrated the association between altered maternal micronutrients like folate and vitamin B12 ; reduced DHA levels; increased homocysteine and oxidative stress in preterm deliveries (Dhobale et al., 2011, 2012a,b). Based on this, we have recently hypothesized that increased oxidative stress and decreased DHA may alter the levels and expression of neurotrophins in preterm deliveries, since balance between neurotrophins and oxidative stress is critical for normal growth and development (Dhobale and Joshi, 2012). 5.2. Oxidative stress and neurotrophins Reactive oxygen species and antioxidant defence mechanism play a vital role in placental growth and development in the normal pregnancy (Al-Gubory et al., 2010). Increased oxidative stress may disrupt collagen and further lead to preterm delivery (Wall et al., 2002). Gardiner et al. (2009) suggested that increased oxidative stress can lead to down regulation of neurotrophins. We have observed higher oxidative stress with lower maternal and cord plasma NGF levels in preterm deliveries (Dhobale et al., 2012c; Joshi et al., 2008). Further, this increased oxidative stress was negatively associated with maternal and cord plasma MDA implying that oxidative stress may play a role in determining the levels of NGF. The maternal plasma NGF levels were also positively associated with baby weight suggesting the role of NGF in determining the foetal growth that support to an earlier study in IUGR infants (Malamitsi-Puchner et al., 2007). This altered levels of NGF in preterm deliveries could be due to increased oxidative stress that may disturb the membrane fluidity which will affect NGF signalling cascade (Fig. 3A and B). These changes in the levels of NGF mRNA and protein may lead to poor foetal/placental growth and development in preterm deliveries. 5.3. Angiogenesis and neurotrophins The process of new vessel growth from the pre-existing ones is commonly called angiogenesis (Potente et al., 2011). NGF, BDNF and NT-3 might have the potential to greatly increase angiogenesis in pathological situations (Blais et al., 2013; Jadhao et al., 2012). During the embryonic development, vessel formation and maturation is a very dynamic process, involving growth factors like vascular endothelial growth factor (VEGF), BDNF that widely modulates endothelial proliferation and migration (Koch and Claesson-Welsh, 2012; Kermani and Hempstead, 2007). NGF is reported to induce a potent angiogenic response in the developing chick embryo (Cantarella et al., 2002). It has been observed that BDNF and NGF process angiogenic activity through crucial signalling pathways such as mitogenactivated protein kinases/extracellular signal-regulated kinases (MAPK/ERK) and protein kinase B (PKB) pathways (Jadhao et al., 2012; Julio-Pieper et al., 2009; Wang et al., 2008; Lazarovici et al., 2006). One of an animal study has reported the role of TrkB in embryonic blood vessel development (Wagner et al., 2005). Further, Nakamura et al. (2006) has suggested TrkB increases HIF-1␣ through BDNF that stimulate VEGF expression in neuroblastoma cells.

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Fig. 3. (A and B) Possible association of oxidative stress and NGF in pregnancy (A) term and (B) preterm birth. DHA: docosahexaenoic acid; NGF: nerve growth factor; TrkA: tyrosine kinase A receptor; MDA: malondialdehyde, arrows indicate the direction of change

The lower maternal VEGF, BDNF and NGF levels were reported in women with preeclampsia (D’Souza et al., 2014; Kulkarni et al., 2010; Kilari et al., 2010). VEGF which is representative of angiogenesis can be a new and useful predictor of preterm delivery (Kim et al., 2013). Interestingly, we have observed lower maternal NGF (Dhobale et al., 2012c), higher placental TrkB levels which was the response to the higher levels of its ligand i.e. BDNF in preterm delivery (Dhobale et al., 2012b). The lower placental BDNF and NGF mRNA levels in the preterm deliveries suggesting that the lower mRNA levels of BDNF and NGF may possibly be a result of altered epigenetic mechanisms (Dhobale et al., 2013). From these studies one of the postulated mechanisms for angiogenesis through neurotrophins could be that BDNF/NGF-TrkB/TrkA signalling casacade increases hypoxia-inducible factor 1-alpha (HIF-1␣) and may regulate VEGF through mitogen-activated protein kinases/extracellular signal-regulated kinases (MAPK/ERK) and protein kinase B (PKB) pathways in the placenta in early pregnancy (Fig. 4).

6. Altered levels of neurotrophins and their implications The altered levels of folate, vitamin B12 and homocysteine may epigenetically regulate the levels and expression neurotrophins needed for the development of the feto-placental unit and may result in preterm delivery. Children born to preterm mothers may be at an increased risk for neurodevelopmental disorders in later life (Johnson and Marlow, 2011; Lindström et al., 2011). These altered levels of BDNF and NGF may be a consequence of altered gene expression and promoter methylation due to changes in the one carbon cycle. It is likely that these changes in the one carbon cycle influence epigenetic programming of the placenta and may have implications for micronutrient mediated foetal programming of diseases in adult life. The lower levels of cord BDNF and NGF in preterm pregnancies may provide clues to predict cognitive deficits in children. Identification of these genes can be of use in elucidating the molecular basis of the pathology and may further also lead to identify suitable disease early biomarkers in pregnancy. Preterm birth is a multifactorial aetiology and therefore role of these components discussed in this review will throw light in understanding the mechanisms that leading to preterm delivery.

Fig. 4. Cross talk between neurotrophins and angiogenic factor in the placenta. DHA: docosahexaenoic acid; NGF: nerve growth factor; BDNF: brain derived neurotrophic factor; Trk: tyrosine kinase receptor; TrkA: tyrosine kinase receptor A; Trk B: tyrosine kinase receptor B; VEGF: vascular endothelial growth factor; VEGFR1: vascular endothelial growth factor receptor-1; PI3K: phosphoinositide 3-kinase; AKT: protein kinase B; Raf: rapidly accelerated fibrosarcoma; ERK: extracellular signal-regulated kinases; RAS-MAPK: mitogen-activated protein kinases; HIF-1␣: hypoxia-inducible factor 1-alpha.

Further, it may be likely that these neurotrophins influences birth outcome and neurodevelopmental risk through two mechanisms i.e. angiogenesis and cellular growth, survival and maturation. Their levels may be regulated by prenatal epigenetic and foetal programming suggesting a need to examine these issues. Understanding the levels of these neurotrophins in the cord blood may be useful in predicting the consequences of central nervous system abnormalities while in utero. In future there is need to analyze the levels of neurotrophins in early pregnancy in order to understand their role in preterm deliveries. Children born preterm need to be followed up to assess the risk for cognitive and behavioural disorders to better understand

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the possible role of altered levels of BDNF and NGF at birth. Animal studies need to be carried out to understand the synergistic effect of maternal micronutrients and omega 3 fatty acid supplementation on brain neurotrophins and cognitive performance in the offspring.

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