Changes in organ structure and function 23!

There are several key differences in the developmental timing of organ maturation between rodents and humans. Importantly, the third trimester in humans is roughly equivalent to the first postnatal weeks in rodents with respect to renal maturation. Kidney maturation does not occur until postnatal day 20 in the rodent whereas in humans, kidney development is completed by 36 weeks of gestation (89). Adipose tissue develops early in gestation in humans whereas in rodents adipose tissue is not laid down until late gestation and early postnatal life, and there is relatively more brown fat in the mouse compared to the human (90). In contrast to humans, whose hypothalamic pathways are fully developed at birth, development of hypothalamic neurocircuits in rodents is not complete until the third week of postnatal life making maternal diet during lactation highly influential on hypothalamic

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development (91). Given the significant differences in organ maturation between rodents and humans, care should be taken when applying knowledge from rodent models to the human situation.

Figure 3.1. A schematic representation depicting the key players in developmental programming

25 2.3.1.1 Adipose tissue

In humans, obese mothers have an increased risk of delivering offspring with LGA or macrosomia (14, 15). In contrast, rodent models of maternal obesity do not always report increased birth weight of offspring. Nonetheless, despite no difference in body weight per se, many studies have demonstrated changes in body fat distribution post birth, either by measuring fat deposits or by imaging techniques (92). Offspring of obese mothers are predisposed to adiposity, adipocyte hypertrophy and weight gain in adulthood as a result of upregulation of adipogenesis and lipogenesis (93). In particular, visceral adipose tissue is increased (including increased epididymal/periuterine, perirenal, omental and mesenteric fat deposits), which have been shown to have particularly adverse metabolic consequences to the offspring in relation to insulin resistance and metabolic risk (94).

Adipogenesis, or increased adipocyte number, results from differentiation of preadipocytes into adipocytes. Lipogenesis occurs as a result of triglyceride synthesis and storage within mature adipocyte and leads to adipocyte hypertrophy (enlarged size). Offspring of obese dams have larger adipocytes (> 150 µm) indicating increased lipogenesis and the number of very small adipocytes (<25 µm) is also increased likely due to an increase in adipogenesis (92, 94). Key transcription factors involved with adipogenesis and lipogenesis include peroxisome proliferator-activated receptor-γ (PPARγ), CCAAT/enhancer binding protein (C/EBPα, β, γ), the sterol regulatory element-binding protein 1c (SREBP1c) as well as fatty acid synthesis enzymes such as fatty acid synthase (FAS). All of them are upregulated in the adipose tissue of offspring of obese mothers (95). PPARγ has been shown to be upregulated in adipose tissue of offspring exposed to maternal obesity both prenatally and up to postnatal

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Day 130 (92, 96). FAS and multiple fatty acid transporters have been shown to be upregulated in retroperitoneal, omental, mesenteric and subcutaneous fat deposits (94).

Adipose tissue produces adipocytokines including adiponectin and leptin which have autocrine, paracrine and endocrine effects and influence whole-body insulin sensitivity and hence the development of metabolic diseases (97). Adiponectin promotes insulin sensitivity and has anti-inflammatory properties; decreased circulating levels are associated with obesity, insulin resistance, and T2D (97). Pregnant obese dams have lower adiponectin levels and similarly offspring of obese mothers also have lower adiponectin levels (98). In contrast, leptin plays important roles in modulating satiety and energy homeostasis. Although leptin is elevated in offspring of obese mothers, the offspring do not demonstrate reduced food intake suggesting that maternal obesity induces leptin resistance (83). The mechanism of leptin resistance due to maternal obesity may be permanently programmed by intrauterine overnutrition as a result of alterations in neural circuitry that is similar to that induced by HFD consumption (99).

As a result of increased adiposity in offspring, particularly visceral adiposity, pathological processes inherent to adipose tissue ensue. Adipocytes secrete inflammatory mediators including chemokines and cytokines which lead to both local and systemic inflammation (100). These include tumour necrosis factor- α (TNF-α), monocyte chemoattractant protein-1 (MCP-1), interleukin-8 (IL-8) and interleukin-6 (IL-6) predominantly secreted by adipose tissue macrophages. Excessive lipids that cannot be stored in adipocytes are released into the blood and ectopically deposited in the liver, muscle and pancreas. At these sites, inflammatory cytokines secreted by adipocytes, cause cellular functional injury. Together these metabolic abnormalities lead to insulin resistance and the predisposition towards

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metabolic disorders, which may lead to end-organ effects such as cardiovascular disease and CKD (101).

2.3.1.2 Neural circuitry is permanently altered in offspring of obese mothers

The hypothalamus is important for regulating appetite and energy metabolic homeostasis in response to peripheral key hormones including insulin and leptin (102). Leptin receptors are predominantly located in the hypothalamus, which are increased in response to food ingestion. As a result, the expression and release of neuropeptide Y (NPY) and agouti-related protein (AgRP) are inhibited, whereas pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcripts are stimulated to reduce appetite and thus inhibit food intake. Despite high leptin levels in obese subjects proportional to adipose tissue mass, weight loss does not ensue and this is mainly due to the concept of central leptin resistance. Furthermore, offspring of obese mothers also demonstrate leptin resistance in several animal models (99, 103).

There is debate regarding the effect of maternal obesity on offspring appetite regulation and hypothalamic neurocircuitry. Many studies have found up-regulation of appetite regulatory neuropeptides and hormones suggesting that appetite may be increased (75, 104-106). In a sheep model, lambs from obese mothers demonstrate increased appetite in the context of a blunted postnatal leptin surge (107). However, when a meta-analysis was conducted to determine the effect of maternal obesity on offspring food intake in rodents, there was no significant increase in food consumption in offspring of obese mothers when food consumption was allometrically scaled to body weight (66). The authors hypothesised that

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the juxtaposition between increased body weight and appetite in rodents could be explained by slowing down of energy metabolism, rather than energy overconsumption.

Offspring of obese mothers have a higher propensity towards HFD-induced obesity than offspring of lean mothers (75, 108, 109). When studies have interrogated differences in neural pathways, it appears that offspring of obese mothers have altered reward pathways for palatable food in the brain (110). They favour increased palatable food intake due to dysregulation of the reward pathway involving the mesocorticolimbic dopamine pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc). Ong et al found that offspring of obese dams (fed a cafeteria style diet) had increased expression of µ-opioid receptor (Mu) and reduced expression of dopamine active transporter (DAT) at 6 weeks of age, although downregulated Mu and upregulated DAT were found by 3 months. Their findings suggest that the normal response of the reward pathway may be desensitised in adult offspring of the obese mothers, which drives them towards increased food-seeking behaviour (111). In addition, maternal HFD was shown to alter endocannabinoid pathways within the hippocampus and increase anxiety-like behaviour in adult rat offspring (73). Furthermore, epigenetic changes mediated by maternal obesity in early life may have permanent effects in neural pathways within the brain (112, 113).

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