Chronic subcutaneous infusion of neurosecretory protein GM increases body mass gain in chicks

Highlights

• The biological action of neurosecretory protein GM (NPGM) was surveyed in chicks.

• Chronic subcutaneous infusion of NPGM increased body mass gain.

• This effect seems to be due to increased mass of white adipose tissue.

Abstract

Recently we discovered a small hypothalamic protein in the chicken, named neurosecretory protein GL (NPGL), which is associated with body growth and energy metabolism in birds and rodents. Genome database analysis suggested that the NPGL gene has a paralogous gene in vertebrates, named neurosecretory protein GM (NPGM). However, the biological action of NPGM remains unclear. In this study, we investigated whether NPGM affects body growth in chicks. We found that subcutaneous infusion of NPGM for six days increased body mass gain in a dose-dependent manner. Despite the observed increase in body mass, infusion of NPGM did not alter food and water intake. Of note, we observed tendency of mass increase of several peripheral tissues, specifically. When we compared several tissue types, NPGM seemed to induce the largest growth increase in white adipose tissue mass. These results suggest that NPGM may accelerate fat accumulation and body growth. In addition, we analyzed whether NPGM increases body growth through the action of pituitary hormones. However, we observed no significant changes in mRNA expression of pituitary hormones or plasma levels of growth hormone in NPGM-treated chicks. This is the first report describing the biological action of NPGM in vertebrates.

Introduction

Body growth in animals is regulated by various hormones and genetic, nutritional, and environmental factors (Harvey, 2013). In particular, growth hormone (GH) is generally regarded as essential for body growth and development in vertebrates. GH excess results in gigantism, whereas GH deficiency results in dwarfism (Gahete et al., 2016, Hannah-Shmouni et al., 2016. Several neuropeptides, including growth hormone-releasing hormone (GHRH), ghrelin, and somatostatin (Burgus et al., 1973, Guillemin et al., 1982, Kojima et al., 1999), regulate production and secretion of GH. Growth in peripheral tissues, including liver, muscle, and adipose tissue, is mainly regulated by GH and insulin-like growth factors (IGFs) working with their respective receptors (Harvey, 2013, Hyun, 2013). However, as these tissues are associated with energy metabolism, increases in their masses are also influenced by energy state. Furthermore, various other bioactive substances can also influence body mass growth, such as neuropeptide Y (NPY), glucocorticoids, and leptin, which regulate feeding behavior and bone metabolism (Baldock et al., 2006, Engblom et al., 2007, Takeda et al., 2002). These neuropeptides, peripheral steroids, or peptide hormones are secreted from the hypothalamus, adrenal gland, and adipose tissue in response to physiological conditions. In this way, body growth is regulated by various hormones secreted from both central and peripheral tissues. Because of the complexity of growth regulation, the precise mechanism of control remains unknown and likely involves novel factors that remain to be elucidated.

We recently identified a novel cDNA that encodes the precursor of a small hypothalamic secretory protein in chickens, mice, and rats (Ukena et al., 2014, Matsuura et al., 2017, Iwakoshi-Ukena et al., 2017). The precursor protein is cleaved between a signal peptide sequence and a dibasic amino acids cleavage site to produce the mature protein, which is then modified with C-terminus amidation. Because the C-terminal amino acids of the small protein are Gly-Leu-NH₂, the small protein was named neurosecretory protein GL (NPGL) (Ukena et al., 2014). Morphological analysis indicated that NPGL was produced in the medial mammillary nucleus (MM) and the infundibular nucleus (IN) within the hypothalamic infundibulum of the chicken (Ukena et al., 2014, Shikano et al., 2018). NPGL mRNA levels increase during post-hatching development (Ukena et al., 2014). Chronic subcutaneous infusion of NPGL for four days promotes body growth gain without affecting food intake in chicks (Ukena et al., 2014). Additionally, intracerebroventricular infusion of NPGL increases food intake and body mass gain (Shikano et al., 2018). These findings suggest that NPGL participates in growth and feeding regulation in chicks. More recently, we also found that NPGL stimulates feeding behavior in rodents (Matsuura et al., 2017, Iwakoshi-Ukena et al., 2017). Thus, NPGL plays important roles in the regulation of growth and energy homeostasis in birds and rodents.

Furthermore, genome database search indicates that the NPGL gene has a paralogous gene named neurosecretory protein GM (NPGM) and that NPGM and NPGL are highly conserved in vertebrates, including chicken, rat, and human (Ukena et al., 2014). Homology between NPGM and NPGL is 54% in chicken (Table 1). Therefore, we expected that NPGM might also have some significant biological actions in vertebrates. In the present study, to elucidate biological actions of NPGM, we investigated the effect of chronic subcutaneous infusion of NPGM in chicks.