Characterization, phylogeny, and responses of leptin to different nutritional states in critically endangered Yangtze sturgeon (Acipenser dabryanus)

Highlights

• The full-length cDNAs of leptin A were identified in Yangtze sturgeon.

• Yangtze sturgeon Leptin A mRNA was primarily expressed in the liver.

• Postprandial and fasting feeding experiments showed that Yangtze sturgeon leptin A is an appetite suppressor.

• During weaning, leptin A expression is inhibited to increase the appetite of juveniles, and it is beneficial for juveniles to adapt to food changes.

Abstract

Leptin is an important energy homeostasis hormone in mammals. However, the role of leptin in energy balance of fish remains controversial. In this study, we characterized Yangtze sturgeon (Acipenser dabryanus) leptin (Adleptin) with a full-length 507 bp coding sequence (CDS) encoding 168 amino acids (aa) and we used phylogenetic analysis to identify it as leptin A. The tissue expression profiles suggested that Adleptin is widely expressed in various types of tissues, mainly in the liver. Moreover, the Adleptin level was detected during several important nutritional states. The Adleptin mRNA levels significantly increased in the liver after feeding while the levels were dramatically decreased during 10 days of fasting and increased upon refeeding. In comparison to the Tubificidae group, the Adleptin level in the formulated feed group was significantly lower on the 2nd and 5th days; after 2nd day, the level began to increase until it was significantly increased on the 8th day. These results indicate that leptin is an endocrine indicator of nutritional state in Yangtze sturgeon. This indicator can be used to monitor whether artificial nutrient conditions meet the nutritional requirements of Yangtze sturgeon and aid in developing and optimizing protection strategies for this species.

Introduction

Leptin, a 16-kDa hormone belonging to the type I helical α-cytokine family, is the protein product of the obese (OB) gene and was originally cloned in the adipose tissue of ob/ob mice (Zhang et al., 1994). Subsequently, leptin was discovered in humans and other mammals, a subject that was reviewed by Denver et al. (2011). Over the past 20 years, leptin, leptin has also been identified in other vertebrate groups, such as birds (Friedman-Einat et al., 2014; Seroussi et al., 2015), reptiles (Denver et al., 2011), amphibians (Boswell et al., 2006; Crespi and Denver, 2006), and teleosts, such as pufferfish Takifugu rubripes (Kurokawa et al., 2005), zebrafish Danio rerio (Gorissen et al., 2009) and medaka Oryzias latipes (Kurokawa and Murashita, 2009). Existing research has reported that mammals and amphibians express a single ortholog of leptin (Londraville et al., 2017; Londraville et al., 2014). However, multiple leptin genes have been identified in various teleosts, including Zebrafish (Gorissen et al., 2009), Medaka (Kurokawa and Murashita, 2009) and orange-spotted grouper Epinephelus coioides (Zhang et al., 2013).

In mammals, leptin plays a key role in several physiological processes, including appetite, energy homeostasis, immune response, bone formation and reproduction (Allison and Myers, 2014; Anubhuti, 2008; Park and Ahima, 2015; Roubos et al., 2012). In contrast to the body of research evidence on mammals, the role of leptin in fish is far from fully understood. Since the finding that leptin was present in fish (Kurokawa et al., 2005), research studies have mainly focused on energy balance by investigating feed intake and metabolic regulation. Leptin was reported to suppress feed intake in rainbow trout Oncorhynchus mykiss (Gong et al., 2016; Murashita et al., 2008), striped bass Morone saxatilis (Won et al., 2012), goldfish Carassius auratus (De Pedro et al., 2006; Yan et al., 2016) and grass carp Ctenopharyngodon idellus (Li et al., 2010). Moreover, evidence from medaka indicated that wild-type fish have a lower food intake than leptin-receptor-deficient fish (Chisada et al., 2014). The mRNA expression of leptin was influenced by many nutritional states (feeding and fasting) in Atlantic salmon Salmo salar (Rønnestad et al., 2010), striped bass (Won et al., 2012) and Ya-fish Schizothorax prenanti (Yuan et al., 2014). Leptin also plays a role in lipolysis and fatty acid β-oxidation (Lu et al., 2012), in inhibiting adipogenesis (Lu et al., 2012), in inducing glucosensing (Aguilar et al., 2010) and in decreasing hepatic glycogen levels (Baltzegar et al., 2014) in fish. Leptin seems to play an important role in the regulation of energy balance in fish.

In fish farming, aquatic scientists are most concerned about two areas of energy balance: feeding and weaning. The construction of dams (Fuller et al., 2018; Li et al., 2017) and environmental pollution (Dabney et al., 2018; Koski et al., 2017) have caused a sharp decline in live feed in aquatic environments. Lack of food may often occur in many fish species. To understand this ecological phenomenon, more attention should be paid to the adaptation mechanisms of fish. In the wild, larvae feed on different types of live feed, such as microalgae, ciliates, copepods, mollusk eggs, polychaetes, larval decapods, and other fish larvae (Fortier and Harris, 1989). In larval fish rearing, live feeds may cause slow growth during the late larval stages and introduce a high bacterial load (Grotkjær et al., 2016). Therefore, the weaning period, also known as the food conversion period, is a vital process that gradually replace live feeds with formulated diet. However, the main difficulty in weaning is that the larvae refuse to eat formulated feed. Appetite regulation plays an important role in feeding, fasting adaptation and weaning. Therefore, it is important to understand the molecular mechanisms involved in appetite regulation (such as leptin) for aquaculture activities and to support the conservation of fish.

The critically endangered Yangtze sturgeon (Acipenser dabryanus), also known as Dabry's sturgeon, is nationally protected in China (Sung, Peiqi, Yiyu, and Commission, 1998) and it was included in the International Union for Conservation of Nature and Natural Resources (IUCN) Red List in 2010. Many conservation efforts have been made, such as establishing a national nature reserve, captive rearing and re-stocking. However, the shortage of wildlife prey and high mortality rates during captivity (partly due to a lack of appropriate feed strategies) make it extremely difficult to restore the wild Yangtze sturgeon population.

This study aimed to understand the role of leptin during different nutrient states, especially those related to population recovery. Toward that end, Yangtze sturgeon leptin (Adleptin) cDNA and DNA were primarily identified and characterized, and phylogenetic analysis was used to demonstrate the evolutionary status of the Yangtze sturgeon leptin gene. The expression pattern was analyzed in various tissues; the tissue with the highest expression was adopted for the next stage of analysis in different nutritional states, including: the postprandial, fasting, and refeeding periods and the food conversion period. These data can provide basic information to develop a protection strategy for Yangtze sturgeon and can be used as a reference to protect other endangered fish.