The hypothalamic nucleus that has been the most investigated in recent years with regard to energy homeostasis is undoubtedly the ARC (Gautron et al., 2015; Richard, 2015; Caron and Richard, 2017). The ARC is ventrally located on each side of the third ventricle just above the median eminence, a CVO allowing penetrance of peripheral hormones and nutrients (Schwartz et al., 2000; Richard, 2015). The ARC contains neurons producing proopiomelanocortin (POMC) and cocaine and amphetamine regulated transcript (CART) as well as neurons co-localizing agouti-related peptide (AgRP), neuropeptide Y (NPY), and gamma-aminobutyric acid (GABA), whose role in energy balance regulation have been acknowledged for years (Gropp et al., 2005; Luquet et al., 2005; Mayer and Belsham, 2009; Krashes et al., 2013). The NPY/AgRP/GABA- producing neurons exert anabolic effects while POMC/CART neurons are involved in catabolic processes. Those neurons have been referred to as “first order” neurons and project to “second order” neurons located in energy homeostasis-related nuclei, including the PVH and VMH, which individually form with the ARC prominent duets in the regulation of energy balance (Schwartz et al., 2000; Balthasar et al., 2005; Elmquist et al., 2005; Balthasar, 2006; Morton et al., 2006; Richard, 2015). Recently, single cell analysis performed in the hypothalamus revealed an important heterogeneity of ARC cells (Romanov et al., 2017), indicating that additional investigations will have to be make in order to fully characterized hypothalamic regulations of whole body homeostasis.

ARC POMC/CART and NPY/AgRP/GABA neurons are major constituents of the melanocortin system, which is recognized as playing a genuine role in energy balance regulation (Adan et al., 2006; Butler, 2006; Cone, 2006; Ellacott and Cone, 2006; De Jonghe et al., 2011; Xu et al., 2011). POMC/CART neurons exert their hypophagic and thermogenic effects mainly by releasing the melanocortins α- and β-melanocyte-stimulating hormone (MSHs). α- and β-MSHs activate the melanocortin receptors 3 and 4 (MC3R, MC4R) to increase food intake and reduce energy expenditure (Cone, 2006). Interestingly, recent report indicate that POMC neurons are also able to released β-endorphin instead of MSHs indicating that those neurons should also act as orexigenic neurons (Koch et al., 2015). However, investigations performed on POMC knockout revealed morbid obesity resulting from hyperphagia as well as hypometabolism (Yaswen et al., 1999), indicating that POMC neurons mainly act as anorexigenic neurons. While the Mc4r knockout mice exhibit marked obesity, resulting from hyperphagia and hypometabolism (Huszar et al., 1997; Butler and Cone, 2003; Butler, 2006), genetic disruption of the Mc3r lead to a modest obesity phenotype (Chen et al., 2000; Butler and Cone, 2002, 2003), suggesting that MC4R constitutes the major MCR receptor involved in energy homeostasis. The prominent role of Mc4r in the hypothalamic regulation of energy balance has been confirmed in humans, in whom the Mc4r mutation leads to one of the most common forms of monogenic obesity (Coll et al., 2004).

NPY/AgRP/GABA neurons are also part of the melanocortin system. Together with inhibiting the activity of the POMC/CART neurons through a GABAergic effect (Pu et al., 1999), they release AgRP, a characterized anabolic peptide able to competitively inhibit α–MSH binding to the MC4R (Ollmann et al., 1997). AgRP has also recently been described as a biased agonist of MCR coupling to the inwardly rectifying potassium channel Kir7.1 (Ghamari-Langroudi et al., 2015). AgRP production is increased by fasting (Liu et al., 2012), supporting a physiological role for this peptide in the ARC control of energy homeostasis. Interestingly, non-conditional single KO AgRP^−/− as well as double KO AgRP^−/−; NPY^−/− display normal energy homeostasis (Qian et al., 2002), suggesting that NPY/AgRP neurons are dispensable in the hypothalamic control of energy homeostasis. However, the post-natal genetic disruption of AgRP induces hypermetabolism and hypophagia (Luquet et al., 2005), demonstrating some physiological relevance of the neuron population. NPY has been considered as a robust orexigenic neuropeptide mainly by acting on Y1 and Y5 receptors (Richard, 2015). Interestingly, experiments performed in mice have indicated that the deletion of both Y1 and Y5 receptors induces anorexigenic effects (Nguyen et al., 2012), suggesting that Y1 and Y5 are receptors involved in energy balance. In contrast, the genetic disruption of Npy does not produce a lean phenotype or does not increase fasting-induced food intake, suggesting that Npy is not essential to the hypothalamic control of energy homeostasis. On the other hand, several investigations performed on Npy knockout mice revealed that those mice are less sensitive to high fat diet (Patel et al., 2006) (ref) and leptin (Erickson et al., 1996), which suggest that NPY could play a significant role in the hypothalamic integration of homeostatic signals. In that regard, future investigations appear require to further entirely decipher the role of NPY and its receptor in regulation of energy homeostasis.

As alluded to above, ARC cells, including POMC/CART and NPY/AgRP/GABA neurons, are strategically located to act as relays between peripheral homeostatic signals and other hypothalamic circuits involved in energy balance. The homeostatic signals can be anabolic and catabolic circulating hormones and nutrients.

Among all characterized anabolic hormones, ghrelin has emerged as one of the most significant ones. Ghrelin, which is largely produced by stomach cells under fasting (Sanchez et al., 2004a,b; Vallejo-Cremades et al., 2004), promotes food intake and decreases energy expenditure (Ueno et al., 2005) by acting on the growth hormone secretagogue receptor type 1A (GHS-R) (Kim et al., 2003; Egecioglu et al., 2006; Bresciani et al., 2008). The physiological role of ghrelin in the energy homeostasis is supported by investigations performed in mice revealing that genetic disruption of Ghrelin or Ghs-r can prevent high-fat induced obesity (Lee et al., 2016). GHS-R is widely produced in the central nervous system, where ghrelin exerts its effects in several brain regions (Zigman et al., 2006). Furthermore, experiments performed in GHS-R-deficient mice have shown that specific re-expression of the GHS-R in ARC AgRP neurons partially restores the orexigenic effect of the ghrelin (Wang et al., 2014), which confirms the involvement of AgRP-producing neurons as relays in the hypothalamic ghrelin signaling pathway.

Among all key homeostatic hormones, leptin, which is produced mainly by white adipose tissue (MacDougald et al., 1995; Cinti et al., 1997; Niijima, 1998) (WAT), is considered as one of the most prominent catabolic circulating hormones (Elmquist et al., 1997; Friedman and Halaas, 1998; Elias et al., 1999; Friedman, 1999; Gautron et al., 2010; Gautron and Elmquist, 2011). Leptin levels increase with fat mass (Lonnqvist et al., 1997). Its access to the brain is insured by an active transport system apparently involving tanycytes (Balland and Prevot, 2014; Balland et al., 2014). Leptin acts by activating the LepRb receptor, which can be found in several populations of ARC cells including POMC/CART and NPY/AgRP/GABA neurons. Genetic disruption of the gene encoding leptin (ob) or its receptor (Lepr) leads to marked obesity, hyperphagia and reduced BAT thermogenesis in mice (Thenen and Mayer, 1976; Leiter et al., 1983; Garris, 1987, 1989; Malik and Young, 1996; Mizuno et al., 1998; Garris and Garris, 2004; Goncalves et al., 2009).

Notably, the obesity induced by the disruption of leptin signaling resembles that observed following Pomc or Mc4r nullification (Trevaskis and Butler, 2005). In such context one may argue the presence of a functional link between leptin and the melanocortin system, all the more so that POMC/CART and NPY/AgRP/GABA neurons express LepR mRNA (Baskin et al., 1999; Elias et al., 1999; Wilson et al., 1999; Williams et al., 2010) and that leptin increases and decreases the mRNA levels of Agrp and Pomc respectively (Elias et al., 1999; Cowley et al., 2001; van den Top et al., 2004; Takahashi and Cone, 2005). However, experiments performed in mice revealed that mice lacking Lepr on POMC neurons (e.g., Pomc-Cre, Lepr^lox/lox mice) (Balthasar et al., 2004), on AgRP neurons (e.g., Agrp-Cre, Lepr^lox/lox mice) (Tong et al., 2008) and on both POMC and AgRP neurons (e.g., Pomc-Cre, Agrp-Cre, Lepr^lox/lox mice) (van de Wall et al., 2008) develop mild obesity, which suggests that the POMC/CART and NPY/AgRP/GABA could not be the only neurons interfacing the catabolic action of leptin. In that regard, the suggestion has been made that there could exist another population of neurons involved in the hypothalamic leptin signaling pathway. In that regard, mice lacking Lepr on GABA-producing neurons (Vong et al., 2011) (Vgat-Cre, Lepr^lox/lox mice) develop strong obesity. Apparently, there are LepR-expressing GABAergic neurons, distinct from NPY/AgRP neurons that exert an inhibitory tone onto POMC neurons, which could be blunted by leptin (Vong et al., 2011). This presumption has however been challenged by experiments performed in obese model mice homozygous for the Lepr^S1138 allele, in which the ability to acutely decrease the GABA inhibitory tone is unaltered, despite the loss of the catabolic effects of leptin. In such a context, the characterization of the unidentified neurons capable of modulating POMC/CART neurons in response to leptin can be seen as major challenge of the current research in the neurobiology of obesity.

ACBD7 is a member of the ACBD protein family, which includes proteins containing the acyl-coA-binding domain motif signature (Burton et al., 2005; Neess et al., 2015). This protein family contains the well-characterized ACBD1, also known as DBI/ACBP (see above), which is known to be involved in numerous intracellular processes including fatty acid, glycerolipid, and glycerophospholipid biosynthesis, cellular differentiation and proliferation, and β-oxydation. Several hypothetical related-proteins have been characterized in silico, including ACBD7, which represents the product of a well-conserved paralog gene of the DBI/ACBP. Interestingly, sequence analysis has revealed that ACBD7 contains all the residues relevant for DBI/ACBP stability and acyl-CoA binding efficiency (Burton et al., 2005). However, while the three-dimensional conformation of the ACBD7 has been characterized (Neess et al., 2015), its ability to bind acyl-CoA esters remains to be established.

Considering the highly conserved exon/intron structure (Lanfray et al., 2016) of Acbd7, it has been postulated that the duplication of the ancestor gene occurs prior to the divergence of fish and higher vertebrates (450 Mya) (Burton et al., 2005). Interestingly, as for its paralog gene product (i.e., Dbi/Acbp), ACBD7 contains strongly conserved lysine allowing for the production of potential bioactive peptides, including a potential bioactive central fragment, released from a tryptic maturation process. By using a mass spectrometry-multiple reaction monitoring MS-MRM approach, we demonstrated that a 19-amino acid peptide-derived from ACBD7 (called nonadecaneuropeptide—NDN) was present in the mouse hypothalamus, demonstrating that Acbd7 was produced and processed in vivo (Lanfray et al., 2016).

The expression of Acbd7 in the hypothalamus suggests that ACBD7 may exert specific autonomic functions (Neess et al., 2015). We recently confirmed that Acbd7 was expressed by ARC and PVN neurons (Lanfray et al., 2016), two structures described above as key in the hypothalamic regulation of energy balance. Additionally, immunohistochemistry experiments have shown that ACBD7 is produced by ARC neuronal cells apparently differing from NPY/AgRP/GABA and POMC/CART (Lanfray et al., 2016). Additionally, our investigation indicates that ACBD7 immunoreactivity is co-localized with VGAT immuno-labeling (Lanfray et al., 2016), demonstrating that ACBD7 is produced by GABAergic neurons in the hypothalamus.

The observation that ARC ACBD7 produced a fragment homologous to the anorexigenic DBI/ACBP-derived peptide ODN prompted us to hypothesize that ACBD7-containing neurons could be involved in energy balance, all the more so that there existed a Acbd7 polymorphism that had been associated to obesity in humans (Comuzzie et al., 2012).

To determine the role of ACBD7 on energy homeostasis, we assessed the effects of NDN on both food intake and energy expenditure. Our investigations performed in mice revealed that intracerebroventricular (icv) administration of NDN induced an early and marked inhibition of food intake in fasted mice (Lanfray et al., 2016). Our investigations also demonstrated that the icv injection of NDN increased both O₂ consumption and UCP-1 expression in interscapular BAT (Lanfray et al., 2016), suggesting that NDN could also enhance energy expenditure. We also observed that the subchronic treatment with NDN (5 days) reduced food efficiency. Notably, the anorexigenic effect of NDN was blunted by the antagonism of the MC4R (Lanfray et al., 2016), the main effector of the melanocortin signaling pathway, suggesting that NDN acted upstream to the melanocortin signaling pathway.

We also demonstrated that Acbd7 mRNA levels, the ACBD7 protein levels and hypothalamic NDN levels varied with energy availability (Lanfray et al., 2016). This supports the notion that ACBD7-producing neurons are stimulated by one or several catabolic hormones/factors. In that regard, we demonstrated that the leptin treatment could increase the production of both ACBD7 and NDN, suggesting that leptin is able to stimulate ACBD7-producing neurons (Lanfray et al., 2016). Moreover, we demonstrated that the acute pharmacological disruption of the endozepines metabotropic receptor signaling blunted the anorexigenic effect of leptin (Lanfray et al., 2016).

Altogether the data accumulated so far suggest that (i) ACBD7 and NDN are produced by ARC GABAergic neurons different from POMC/CART and NPY/AgRP/GABA neurons, (ii) NDN is a anorexigenic peptide acting probably via the activation of ARC POMC/CART neurons, and (iii) NDN signaling contributes to the leptin-melanocortin pathway. While the identity of the uncharacterized endozepine metabotropic receptor remains to be fully established, we can postulate that the ARC ACBD7 producing neuron represents a significant relay between leptin and the melanocortin signaling pathway.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.