Role of D-aspartate on biosynthesis, racemization, and potential functions: A mini-review.

D-aspartate, a natural and endogenous amino acid, widely exists in animal tissues and can be synthesized through aspartate racemase and transformed by D-aspartate oxidase (DDO). D-aspartate mainly serves as a neurotransmitter and has been demonstrated to exhibit various physiological functions, including nutritional potential, regulation on reproduction and hormone biology, and neuron protection. This article mainly reviews the synthesis, racemization, and physiological functions of D-aspartate with emphasis on the potential in diseases.

Introduction

Most amino acids include asymmetric-carbon atoms and exhibit 2 stereoisomers: left-handed (L-forms) and right-handed (D-forms) (Fig. 1) (Katane and Homma, 2011, Fujii et al., 2011), while only L-forms occupy the protein-building residues (Chung and Reed, 2015). Thus, some researchers speculated that only L-amino acids play functional roles in the body (Katane and Homma, 2011).

Fig. 1

D- and L-forms of amino acid (Chung and Reed, 2015).

Aspartate widely exists in animal tissues with D-enantiomers, especially in nervous and reproductive systems (Ota et al., 2012, Di Fiore et al., 2014). In 1977, high concentration of D-aspartate has been firstly found in the brain of Octopus vulgaris Lam (D'Aniello and Giuditta, 1977) and D-aspartate in other tissues has been also detected subsequently, including heart, lung, stomach, intestine (Motoie et al., 2009), salivary glands (Masuda et al., 2003), seminal plasma and spermatozoa (D'Aniello et al., 2005), neural system (D'Aniello and Garcia-Fernàndez, 2006), knee cartilage (Fisher et al., 2006), testis (Lamanna et al., 2006), pre-ovulatory ovarian follicle (D'Aniello et al., 2007), and retinae (Opere et al., 2009). Wide existence of D-aspartate in body tissues suggests a potential physiological function in animals. Indeed, various studies focused on the regulatory effects of D-aspartate on the nervous (D'Aniello, 2007), endocrine, and reproduction (Lamanna et al., 2007). For example, D-aspartate contributes to the synthesis and release of glucocorticoids, prolactin, oxytocin, and steroids (D'Aniello et al., 2017). Currently, nutritional potential of D-aspartate has been investigated and the results found that low-dosage of D-aspartate improved growth performance and stress-resistance in animal industries. Here, we reviewed the biosynthesis, racemization and functional roles of D-aspartate in animals with emphasis on D-aspartate related signaling pathways.

D-aspartate in biosynthesis and racemization

D-aspartate can be synthesized in body tissues, which was firstly found in rat pheochromocytoma PC-12 cells (Koyama et al., 2005). In the body, D-aspartate racemase mainly contributes to the biosynthesis of D-aspartate by converting L-aspartate to D-aspartate (D'Aniello et al., 2011, Kim et al., 2010). Mouse glutamic-oxaloacetic transaminase 1-like 1 has been reported to encode Asp racemase and synthesize substantially D-aspartate from L-aspartate in adult neurogenesis (Tanaka-Hayashi et al., 2015, Kim et al., 2010, Matsuda et al., 2015). Synthesized D-aspartate can be oxidized by D-aspartate oxidase (DDO) and tissue D-aspartate concentration is mainly controlled by DDO activity, as DDO is the only enzyme known to selectively degrade D-aspartate (Van Veldhoven and Mannaerts, 1991, D'Aniello and Petrucelli, 1993, Cristino et al., 2015). For example, D-aspartate is abundant during the animal embryo and parturient of the brain and markedly reduced in the maturity due to the change of DDO activity (Punzo et al., 2016, Han et al., 2015). Furthermore, inhibition of DDO activity shows a sustained overflow of extracellular D-aspartate (Punzo et al., 2016). In DDO−/− mice (lacking DDO gene), the amounts of D-aspartate drastically increase compared with the wild-type mice (DDO+/+) (Han et al., 2015).

Functions of D-aspartate

Potential nutrition

Lower-dosage D-aspartate improves growth performance, indicating a potential nutritional function in animal nutrition. For example, D-amino acid mixed with L-amino acid improve mouse growth performance, while excess D-amino acid inhibit growth (Berg, 1953). Similarly, oral administration of D-aspartate (0, 3.75, 7.5 and 15 mmol/kg body weight) linearly decreased feed intake (Erwan et al., 2013). Zhang et al. (2015) reported that excess D-aspartate acts as a potent damaging agent for DNA, vitamin C and E, and thus inhibits cell growth. The possible mechanism of D-aspartate in the body may be N-methyl-D-aspartate receptors (NMDAR) and L-Glu/L-aspartate transporter signals. D-aspartate can activate metabotropic glutamate receptors 5 (mGlu5) (Molinaro et al., 2010) and stimulate NMDAR-mediated neurotransmission by directly acting at the glutamate-binding site (GluN1 and GluN2B subunits) of NMDAR, thus functioning as an endogenous agonist for this subclass of glutamate receptors (Errico et al., 2013, Cristino et al., 2015). Moreover, when intracellular uptake the D-aspartate, D-aspartate can be released by L-Glu/L-aspartate transporter that makes use of the Ca2+-mediated manner after electrochemical stimulation (Savage et al., 2001, Muzzolini et al., 1997) and its uptake by Na+/K+ electrical or chemical manner to against their concentration gradient (Adachi et al., 2004, Koyama et al., 2005, Errico et al., 2015). Therefore, we speculate that the proper dose of D-aspartate may play a potential value in nutrition and excess D-aspartate blocks the synthesis and metabolism of other important substances in the body.

D-aspartate in reproduction

D-aspartate has a positive effect on reproduction in animals by improving sperm quality, testosterone synthesis, spermatogenesis, and steroidogenesis proliferation (Fiore et al., 2017). For example, dietary 200 mg/kg per day BW D-aspartate can improve fresh and post-thawed sperm quality and post-thawed sperm fertility in male broiler breed, especially for sperm cryopreservation (Ansari et al., 2017). Giacone et al. (2017) reported that the effect of D-aspartate on reproduction may be due to the antioxidant function and protecting DNA integrity, which may further improve sperm motility and embryo development (Barbato et al., 2017). Santillo et al. (2016) also reported that D-aspartate activates proliferative pathways in GC-1 cells (mitotic germ cell). Meanwhile, D-aspartate in rats improves testosterone synthesis and activates testicular NMDAR, extracellular signal-regulated kinase (ERK) pathway, and androgen receptor (Santillo et al., 2014). Similarly, D-aspartate stimulates spermatogonia proliferation through the NMDAR-mediated ERK and ser-ine-threonine protein kinase (AKT) pathways (Palazzo et al., 2016), which further activates the P450 aromatase/ERb pathway (Santillo et al., 2016) (Fig. 2). D-aspartate also either stimulates steroidogenesis and spermatogenesis proliferation by binding to the NMDAR and thereby increasing the expression of the steroidogenic cascade enzymes by cAMP or stimulates mitogen-activated protein kinase (MAPK) and AKT pathways and then increased the expression of androgen receptor (Fig. 3) (Di Fiore et al., 2016).

Fig. 2

Schematic representation of molecular pathways activated by D-aspartate in spermatogonia (Santillo et al., 2016). NMDAR = N-methyl-D-aspartate receptor; Ras = renin-angiotensin system; PI3-K = phosphatidylinositol-4,5-bisphosphate 3-kinase; ERK = extracellular signal-regulated kinase; AKT = serine-threonine kinase; ERβ = estrogen receptor β; PCNA = proliferation cell nuclear antigen.

Fig. 3

Schematic representation of molecular pathways activated by D-aspartete in Leydig cell (Di Fiore et al., 2016). cAMP = cyclic adenosine monophosphate; MAPK = mitogen-activated protein kinase; STAR = steroidogenic acute regulatory protein; CYP11AL = P450 cholesterol side-chain cleavage; HSD3B1 = 3β-hydroxysteroid dehydrogenase; HSD17B3 = 17β-hydroxysteroid dehydrogenase; CYP19A1 = cytochrome P450 aromatase; SRD5A1 = 5α-reductase; AR = androgen receptor; ESR = estrogen receptor.

Serving as endogenous neurotransmitter

D-aspartate is founded in the multitude brain regions (Palazzo et al., 2016), such as hippocampus and prefrontal. In mammals, D-aspartate is found in synaptic vesicles of nerve endings (D'Aniello et al., 2011) and maintains long-term potentiation and regulatory effect on the synaptic plasticity decay in the hippocampus of rodents (Errico et al., 2011a, Errico et al., 2011b). D-aspartate excites dopamine neurons and exerts a protective uptake mechanism by stimulating NMDA receptor and metabotropic glutamate receptors to prevent the neuronal degeneration (Krashia et al., 2016). Also, DDO with the change of D-aspartate concentration has been reported to regulate the homeostasis of glutamatergic system and, thus, suppresses neurodegenerative processes (Cristino et al., 2015). Moreover, D-aspartate plays important roles in plasticity, physiology, neuronal dendritic morphology, gray matter volume, and brain activity in mammals (Errico et al., 2014). For example, treatment with D-aspartate (20 mmol/L) in drinking water markedly alleviates mechanical allodynia through glutamate neurotransmission normalization after spared nerve injury in neuropathic mice (Palazzo et al., 2016).

Regulation of hormone release

D-aspartate plays an important role in the synthesis and release of hormones, such as glucocorticoids, prolactin, oxytocin, growth hormone (D’Aniello et al., 2000), and steroids in the prefrontal cortex and hippocampus (D'Aniello et al., 2017). D-aspartate was reported to promote the release of gonadotropin-releasing hormone by stimulating luteinizing hormone secretion in the pituitary gland (D'Aniello et al., 2000). Likewise, D-aspartate is positively associated with serum luteinizing hormone, androstenedione, and testosterone concentrations through increasing steroidogenic acute regulatory protein, P450 cholesterol side-chain cleavage enzyme, and 3b-hydroxysteroid dehydrogenase/D5-D4 isomerases gene expressions in rat testis and Leydig cells (Raucci et al., 2014).

D-aspartate in diseases

Due to antioxidant potential, D-aspartate plays an important role in the endocrine and nervous system to prevent or treat neurological diseases and reproductive diseases (Afraei et al., 2017, Boccella et al., 2015, D'Aniello et al., 2017). D-aspartate regulates nociceptive specific neuron electrophysiological activity and behavior to reflect the pain conditions (Boccella et al., 2015). Meanwhile, D-aspartate also serves as a pharmacological tool in chronic pain to relieve related psychiatric condition, including Alzheimer's disease (D'Aniello et al., 2017). Injection of D-aspartate has a sedative effect with and without a hypnotic effect (Erwan et al., 2014). The potential mechanism may be that D-aspartate can activate NMDAR-mediated neurotransmission in the core of the postsynaptic density and then enhances glutamate signaling in the postsynaptic site, which is responsible to antipsychotic and schizophrenia treatment (de Bartolomeis et al., 2015).

Conclusion

D-aspartate is widely detected in animals and human with D-enantiomers and can be metabolized by D-aspartate racemase and DDO. D-aspartate mainly serves as a neurotransmitter and plays important roles in growth performance, reproduction, nervous, and endocrine mediatory functions, whereas the effects of D-aspartate on intestinal absorption and mechanism of feed intake need further studies.

Conflicts of interest

No potential conflicts of interest were reported by the authors.