The role of omega-3 fatty acids in adult hippocampal neurogenesis

hippo-campal-dependent

Neurogenesis is the process of generation of new neurons from neuronal precursor cells, and was first described in adult mammals in 1965, where it was identified in rodents (Altman and Das, 1965). Adult neurogenesis has subsequently been shown to occur in two specific regions of the adult brain, the subventricular zone of the olfactory bulb and the subgranular layer of the hippocampal dentate gyrus, where is has been identified in all mammals studied to date, including man (Ehninger and Kempermann, 2008). Adult neurogenesis involves several distinct stages, beginning with the proliferation of resident neural stem and progenitor cells, followed by differentiation, migration, selection and ultimately functional integration into the pre-existing circuitry (Ehninger and Kempermann, 2008).
The rate of neurogenesis and survival of new neurons in adults are enhanced by many factors, such as, growth factors and neurotransmitters, living in an enriched environment, voluntary exercise and antidepressant treatment (Emsley et al., 2005). Importantly, several lines of evidence indicate that neurogenesis plays an important role in learning, memory and neural plasticity (Yirmiya and Goshen, 2011). For example, neurogenesis increases following specific forms of learning and memory formation, there is a positive correlation between the rate of neurogenesis and hippocampal-dependent memory formation and ablation of neuro-genesis induces learning and memory impairments. Furthermore, increased neurogenesis in rodents has also been described after ischaemia (Takagi et al., 1999), stroke (Darsalia et al., 2005) and following seizures (Parent et al., 1997), and neurogenesis is also increased, in the hippocampus of patients with Alzheimer's disease (Jin et al., 2004); where these increases may be an attempt at brain self-repair.
The greatest negative regulator of neurogenesis is ageing, where the rate of neurogenesis in the dentate gyrus declines significantly with ageing (Kuhn et al., 1996), and there is a strong correlation between memory impairment in hippocampal-dependent tasks, such as those Abstract: Neurogenesis occurs in limited areas of the adult mammalian brain, and has been reported in the hippocampus of rodents and man. Neurogenesis is enhanced in conditions associated with enhanced synaptic plasticity and following neuronal injury, suggesting a role for neurogenesis in cognition and brain repair. Omega-3 polyunsaturated fatty acids (PUFAs) have been shown to promote hippocampal neurogenesis in a variety of models. Importantly, recent work has shown that the fat-1 transgenic mouse, an animal model of endogenous omega-3 PUFA enrichment, exhibits enhanced neurogenesis, with concomitant improvements in spatial memory compared to wild type mice. During ageing, the rate of neurogenesis declines significantly and there is a strong correlation between memory impairment in hippocampal-dependent tasks and this decline. Interestingly, there is a strong correlation between omega-3 PUFA and hippocampal-dependent memory tasks, and we have recently shown that supplementation of aged rats with omega-3 PUFAs partially reverses the age-related decline in neurogenesis. Thus omega-3 PUFAs positively influence neurogenesis, and these effects may contribute to improved cognitive performance. However, the mechanisms by which omega-3 PUFAs regulate neurogenesis remain unclear, although a number or putative targets have been suggested. The aims of this paper are to review the role of omega-3 PUFA in hippocampal neurogenesis, and explore some of the potential mechanisms of action which may underlie the observed effects. based on spatial memory, and this agerelated decline (Drapeau et al., 2003), suggesting a link between decreased neurogenesis and memory impairments.
Overall, adult hippocampal neurogenesis has been shown to be important in learning and memory in a variety of paradigms, linking neurogenesis to both normal brain function and disease. This raises the intriguing possibility that treatments that enhance the rate of neurogenesis and survival of new neurons in adults may have a potential therapeutic role in the treatment of neurodegenerative and psychiatric disorders. This review provides an overview of the effects of omega-3 polyunsaturated fatty acids (PUFAs) in adult neurogenesis in the dentate gyrus region of the hippocampus, and explores some of the potential mechanisms of action which may underlie the observed effects.

Omega-3 PUFAs and adult hippocampal neurogenesis
Omega-3 PUFAs have an essential role in brain development and function (Innis, 2007) and beneficial effects of omega-3 PUFA treatment have consistently been demonstrated in a variety of hippocampal-dependent tasks. For example, omega-3 PUFAs enhance spatial memory tasks in adult and old rats (Gamoh et al., 2001;Gamoh et al., 1999, possess antidepressant effects (Freeman, 2009), increase synaptogenesis (Cao et al., 2009) and enhance hippocampal neurite outgrowth (Calderon and Kim, 2004). However, the mechanisms underlying these effects are still unclear. Omega-3 PUFAs have been shown to influence developmental neurogenesis, where several studies have reported that omega-3 PUFA deficiency in embryonic and newborn rats leads to decreased neurogenesis and delay or inhibition of normal development (Coti Bertrand et al., 2006;Yavin et al., 2009;Kawakita et al., 2006). It may therefore be hypothesised that omega-3 PUFA may enhance hippocampal function via effects on adult neurogenesis.
The first published evidence of omega-3 PUFAs enhancing adult hippocampal neurogenesis was provided by Kawakita and co-workers (Kawakita et al., 2006). In this study adult rats were fed doco-sahexaenoic acid (DHA) at 300 mg/kg for seven weeks. DHA treatment significantly increased the number of BrdU positive and NeuN positive newborn neurons in dentate gyrus, indicating enhanced neuronal proliferation and maturation. In the second part of the study, neural stem cells were cultured under differential conditions with or without DHA for 4 and 7 days. DHA significantly increased the number of Tuj1-positive neurons compared with the control groups on both culture days, and the newborn neurons in the DHA group were morphologically more mature than in the control group. DHA also significantly decreased the incorporation ratio of BrdU during the first 24 h period; it also significantly decreased the number of pyknotic (degenerating) cells on day 7, indicating that DHA promotes the differentiation of neural stem cells into neurons by promoting cell cycle exit and suppressing cell death.
The fat-1 transgenic mouse expresses the fat-1 gene from Caenorhabditis elegans, which encodes an omega-3 desaturase, and is therefore able to convert omega-6 to omega-3 PUFAs, and is thought to model the effects of dietary enrichment with omega-3 PUFA (Kang et al., 2004). Fat-1 mice have increased brain DHA content compared with wild type litter mates. Importantly they also have significantly enhanced hippocampal neurogenesis, with an increased number of proliferating neurons and neuritogenesis, as evidenced by increased density of dendritic spines of CA1 pyramidal neurons in the hippocampus (He et al., 2009). They also exhibit a better spatial learning performance in the Morris water maze compared with wild type littermates, suggesting the positive effects on neurogenesis by omega-3 PUFAs may contribute to improved cognitive performance.
Positive effects of omega-3 PUFA treatment have also been reported in the lobster brain, a model of adult neurogenesis, where short-term omega-3 PUFA dietary enrichment significantly upregulates neurogenesis (Beltz et al., 2007). Taken together, these observations strongly support enhancing effects of omega-3 PUFAs on adult neurogenesis and neuritogenesis, and also suggest that this effect may be a potential mechanism underlying the beneficial effects observed on hippocampal-dependent functions. A number of putative targets have been suggested for the positive effects of omega-3 PUFA on neurogenesis.

Mechanisms of action
Adult neurogenesis occurs in a complex microenvironment and the progression from neural stem cells to mature neurons is subject to tightly coordinated control by a multitude of cell-extrinsic and intrinsic factors (Johnson et al., 2009;Mu et al., 2010). Extrinsic factors which have previously been shown to be modifiable by omega-PUFA treatment include glutamatergic signalling and neurotrophic factors, whereas intrinsic factors include a variety of transcription factors. The effects of omega-3 PUFAs on these regulatory factors in adult hippocampal neurogenesis have begun to be explored.
Brain-derived neurotrophic factor (BDNF), a neurotrophin involved in spatial learning and memory, plays an important role in dietary restriction-induced neurogenesis (Lee et al., 2002). Several studies have reported enhanced hippocampal neurogenesis in parallel with increase levels of BDNF levels following omega-3 PUFA treatment (Cysneiros et al., 2010;Venna et al., 2009;Blondeau et al., 2009). For example, three sequential injections of alinolenic acid significantly enhances adult hippocampal neurogenesis in mice, and increases in the expression of BDNF in hippocampal neurons and cortical and hippocampal tissue (Blondeau et al., 2009). In this study, the treatment also significantly increases the expression of proteins critical for synaptogenesis and synapse function, syntophysin-1, VAMP-2 and SNAP-25, and proteins supporting glutamatergic neurotransmission, V-GLUT-1 and V-GLUT-2. These studies therefore indicate that the positive effects of omega-3 PUFA on neurogenesis may at least in part be mediated via effects on BDNF expression.
There is strong evidence indicating the importance of the retinoic acid receptor family of transcription factors in regulating neural plasticity and neurogenesis in the hippocampus (Mccaffery et al., 2006;Jacobs et al., 2006). Furthermore, retinoic acid receptor agonists enhance BDNF expression (Katsuki et al., 2009), and are involved in the induction of neural progenitor cells to become doublecortin-expressing cells (Goncalves et al., 2009). Doublecortin is a microtubule associated protein associated with neuronal differentiation and migration. Dietary supplementation of aged rats with an omega-3 PUFA enriched diet for 12 weeks partially reverses the agerelated decline in neurogenesis in the dentate gyrus, as assessed by doublecortin expression, with a concomitant reversal of the age-related decreases in retinoic acid receptor-a (RARa) and retinoid X receptor-b (RXRb) expression in the dentate gyrus (Dyall et al., 2010). It remains to be established if the effects of omega-3 PUFA on neurogenesis are directly linked to the restoration of retinoid signalling in ageing; however, it is likely that omega-PUFAs possess cellintrinsic effects mediated by actions at transcription factors.
In neural stem cells, neurogenesis is regulated by activator-type and repressortype basic helix-loop-helix (bHLH) transcription factors (Johnson et al., 2009;Mu et al., 2010). Neurogenesis is promoted by activator-type bHLH transcription factors, such as neurogenin, NeuroD and Ascl1 (also known as Mash1), whereas, hairy and enhancer of split 1 and 5 (Hes1 and Hes5) prevent terminal differentiation and preserve a pool of stem cells. An interesting recent study has investigated the effects of DHA treatment on the expression of these bHLH transcription factors in neural stem cells (Katakura et al., 2009). DHA treatment significantly decreased the expression of Hes1 and increased neurogenin1 and NeuroD. MAP2 expression, a neuron specific protein, was also significantly increased. Since MAP2 is activated by NeuroD and repressed by Hes1, these results suggest that DHA stimulates neuronal differentiation by altering the balance of these bHLH transcription factors. However, the mechanisms by which DHA alter the expression of these transcription factors remain to be established.
It should also be noted that calorific or dietary restriction have been shown to closely relate to hippocampal neurogenesis (Lee et al., 2000). For this reason, in our study we monitored the food intake and weight of the animals throughout both the mixed omega-3 PUFA and DHA treatments (Dyall et al., 2010). In both studies, the omega-3 PUFA treated animals ate a similar amount of food to the control and untreated old animals, and indeed there was even a small non-significant increase in the average weight of the omega-3 PUFA treated animals compared to the other groups, indicating dietary restriction was not a factor in the observed effects on neurogenesis.
In summary, omega-3 PUFA treatment has consistently been shown to enhance adult hippocampal neurogenesis in a variety of animal models. Elucidating the mechanisms for this effect has been complicated by the convergent pathways involved in regulating neurogenesis and the pleiotropic effects of omega-3 PUFAs. However, a number of potential targets have been identified. This article has briefly reviewed some of the evidence of effects on cellextrinsic and intrinsic regulatory factors by omega-3 PUFAs. These include, iinfluencing neurotrophin levels, such as BDNF, modulation of transcription factors, such as retinoid receptors and the bHLH transcription factors. However, the effects of omega-3 PUFA are undoubtedly mediated by further mechanisms, which may include biophysical effects in neuronal membranes, modulation of pro-inflammatory cytokine levels, and interaction with cannabinoid signalling pathways. In order for omega-3 PUFA to achieve their therapeutic potential it is imperative to understand the molecular mechanisms that mediate their observed effects, and understanding their regulatory effects neurogenesis is an important avenue for exploration.