d-aspartate affects NMDA receptor-extracellular signal–regulated kinase pathway and upregulates androgen receptor expression in the rat testis

Barley sprouts and D-Aspartic acid supplementation improves fertility, hatchability, and semen quality in aging male broiler breeders by up-regulating StAR and P450SCC gene expressions

At 50 wk of age, broiler breeder roosters exhibit a significant decline of fertility. Therefore, the aim of this study was to assess the impact of incorporating barley sprout (BS) powder, D-aspartic acid (DA), or their combination into the diet on fertility, hatchability, semen quality, and the relative expression of StAR and P450SCC genes in aging broiler roosters. Aging (50 wk) male broiler breeders (n=32) were randomly assigned to one of four dietary treatments (2 × 2 factorial) with 2 levels of BS (0 or 2% basal diet) and DA (0 or 200 mg/kg/BW) for 12 wk. Roosters were individually housed under a 14-h light and 10-h dark cycle, with 150 g/d feed allocation and free access to fresh water, then euthanized. Throughout the study, the body weight of the broiler breeders was measured, along with various parameters related to semen quality, on a weekly basis. Additionally, artificial insemination was performed during the last 2 wk to evaluate reproductive endpoints. The results revealed that both BS and DA decreased (P < 0.01) body weight. Interestingly, the inclusion of BS, either alone or in combination with DA, resulted in a significant increase in total and forward sperm motility. Furthermore, it was demonstrated that the seminal concentration of malondialdehyde, a marker of oxidative stress, was significantly decreased by more than 20% in all groups compared to the control. The combination of both BS and DA led to the highest levels of circulating testosterone, as well as the functionality and membrane integrity of sperms. Additionally, it resulted in increased sperm concentrations, production, and penetration, ultimately leading to improved fertility rate and hatchability percentage. Moreover, a positive association between total motility and fertility was observed (P < 0.01). Furthermore, the combined supplementation of BS and DA up-regulated the relative mRNA expression of P450scc and StAR (P < 0.01). To summarize, dietary inclusion of BS, DA, or their combination have a potential to improve various aspects of reproductive performance in aging roosters.

Type 1 diabetes impairs the activity of rat testicular somatic and germ cells through NRF2/NLRP3 pathway-mediated oxidative stress

Background

It is well known that metabolic disorders, including type 1 diabetes (T1D), are often associated with reduced male fertility, mainly increasing oxidative stress and impairing the hypothalamus-pituitary-testis (HPT) axis, with consequently altered spermatogenesis and reduced sperm parameters. Herein, using a rat model of T1D obtained by treatment with streptozotocin (STZ), we analyzed several parameters of testicular activity.

Methods

A total of 10 adult male Wistar rats were divided into two groups of five: control and T1D, obtained with a single intraperitoneal injection of STZ. After 3 months, the rats were anesthetized and sacrificed; one testis was stored at -80°C for biochemical analysis, and the other was fixed for histological and immunofluorescence analysis.

Results

The data confirmed that T1D induced oxidative stress and, consequently, alterations in both testicular somatic and germ cells. This aspect was highlighted by enhanced apoptosis, altered steroidogenesis and Leydig cell maturity, and impaired spermatogenesis. In addition, the blood-testis barrier integrity was compromised, as shown by the reduced levels of structural proteins (N-cadherin, ZO-1, occludin, connexin 43, and VANGL2) and the phosphorylation status of regulative kinases (Src and FAK). Mechanistically, the dysregulation of the SIRT1/NRF2/MAPKs signaling pathways was proven, particularly the reduced nuclear translocation of NRF2, affecting its ability to induce the transcription of genes encoding for antioxidant enzymes. Finally, the stimulation of testicular inflammation and pyroptosis was also confirmed, as highlighted by the increased levels of some markers, such as NF-κB and NLRP3.

Conclusion

The combined data allowed us to confirm that T1D has detrimental effects on rat testicular activity. Moreover, a better comprehension of the molecular mechanisms underlying the association between metabolic disorders and male fertility could help to identify novel targets to prevent and treat fertility disorders related to T1D.

Steroidogenesis Upregulation through Mitochondria-Associated Endoplasmic Reticulum Membranes and Mitochondrial Dynamics in Rat Testes: The Role of D-Aspartate

Mitochondria-Associated Endoplasmic Reticulum Membranes (MAMs) mediate the communication between the Endoplasmic Reticulum (ER) and the mitochondria, playing a fundamental role in steroidogenesis. This study aimed to understand how D-aspartate (D-Asp), a well-known stimulator of testosterone biosynthesis and spermatogenesis, affects the mechanism of steroidogenesis in rat testes. Our results suggested that D-Asp exerts this function through MAMs, affecting lipid trafficking, calcium signaling, ER stress, and mitochondrial dynamics. After 15 days of oral administration of D-Asp to rats, there was an increase in both antioxidant enzymes (SOD and Catalase) and in the protein expression levels of ATAD3A, FACL4, and SOAT1, which are markers of lipid transfer, as well as VDAC and GRP75, which are markers of calcium signaling. Additionally, there was a decrease in protein expression levels of GRP78, a marker of aging that counteracts ER stress. The effects of D-Asp on mitochondrial dynamics strongly suggested its active role as well. It induced the expression levels of proteins involved in fusion (MFN1, MFN2, and OPA1) and in biogenesis (NRF1 and TFAM), as well as in mitochondrial mass (TOMM20), and decreased the expression level of DRP1, a crucial mitochondrial fission marker. These findings suggested D-Asp involvement in the functional improvement of mitochondria during steroidogenesis. Immunofluorescent signals of ATAD3A, MFN1/2, TFAM, and TOMM20 confirmed their localization in Leydig cells showing an intensity upgrade in D-Asp-treated rat testes. Taken together, our results demonstrate the involvement of D-Asp in the steroidogenesis of rat testes, acting at multiple stages of both MAMs and mitochondrial dynamics, opening new opportunities for future investigation in other steroidogenic tissues.

Evidence of the protective role of D-Aspartate in counteracting/preventing cadmium-induced oxidative stress in the rat testis

Cadmium (Cd), by producing oxidative stress and acting as an endocrine disruptor, is known to cause severe testicular injury, documented by histological and biomolecular alterations, such as decreased serum testosterone (T) level and impairment of spermatogenesis. This is the first report on the potential counteractive/preventive action of D-Aspartate (D-Asp), a well-known stimulator of T biosynthesis and spermatogenesis progression by affecting hypothalamic-pituitary-gonadal axis, in alleviating Cd effects in the rat testis. Our results confirmed that Cd affects testicular activity, as documented by the reduction of serum T concentration and of the protein levels of steroidogenesis (StAR, 3β-HSD, and 17β-HSD) and spermatogenesis (PCNA, p-H3, and SYCP3) markers. Moreover, higher protein levels of cytochrome C and caspase 3, together with the number of cells positive to TUNEL assay, indicated the intensification of the apoptotic process. D-Asp administered either simultaneously to Cd, or for 15 days before the Cd-treatment, reduced the oxidative stress induced by the metal, alleviating the consequent harmful effects. Interestingly, the preventive action of D-Asp was more effective than its counteractive effect. A possible explanation is that giving D-Asp for 15 days induces its significant uptake in the testes, reaching the concentrations necessary for optimum function. In summary, this report highlights, for the first time, the beneficial role played by D-Asp in both counteracting/preventing the adverse Cd effects in the rat testis, strongly encouraging further investigations to consider the potential value of D-Asp also in improving human testicular health and male fertility.

D-Aspartate Depletion Perturbs Steroidogenesis and Spermatogenesis in Mice

High levels of free D-aspartate (D-Asp) are present in vertebrate testis during post-natal development, coinciding with the onset of testosterone production, which suggests that this atypical amino acid might participate in the regulation of hormone biosynthesis. To elucidate the unknown role of D-Asp on testicular function, we investigated steroidogenesis and spermatogenesis in a one-month-old knockin mouse model with the constitutive depletion of D-Asp levels due to the targeted overexpression of D-aspartate oxidase (DDO), which catalyzes the deaminative oxidation of D-Asp to generate the corresponding α-keto acid, oxaloacetate, hydrogen peroxide, and ammonium ions. In the Ddo knockin mice, we found a dramatic reduction in testicular D-Asp levels, accompanied by a significant decrease in the serum testosterone levels and testicular 17β-HSD, the enzyme involved in testosterone biosynthesis. Additionally, in the testes of these Ddo knockin mice, the expression of PCNA and SYCP3 proteins decreased, suggesting alterations in spermatogenesis-related processes, as well as an increase in the cytosolic cytochrome c protein levels and TUNEL-positive cell number, which indicate an increase in apoptosis. To further investigate the histological and morphometric testicular alterations in Ddo knockin mice, we analyzed the expression and localization of prolyl endopeptidase (PREP) and disheveled-associated activator of morphogenesis 1 (DAAM1), two proteins involved in cytoskeletal organization. Our results showed that the testicular levels of DAAM1 and PREP in Ddo knockin mice were different from those in wild-type animals, suggesting that the deficiency of D-Asp is associated with overall cytoskeletal disorganization. Our findings confirmed that physiological D-Asp influences testosterone biosynthesis and plays a crucial role in germ cell proliferation and differentiation, which are required for successful reproduction.

Autophagy and mitochondrial damage in the testis of high-fat diet fed rats

High-fat diet (HFD) affects the physiology of reproduction in males, and many studies have investigated its detrimental effects. In this study, we investigated the cellular response induced by an HFD in the rat testis, focusing on the mitochondrial compartment. After five weeks of HFD, an increase in the levels of malondialdehyde and of reduced form of glutathione in the rat testis indicated an increase in lipid peroxidation. The results showed an increase in autophagy, apoptosis, and mitochondrial damage in the testis of HFD rats. We found a decrease in the protein expression of mitochondrial antioxidant enzymes, such as catalase and SOD2. Immunohistochemical analysis revealed a decrease in the immunofluorescent signal of SOD2, mainly in the spermatogonia and spermatocytes of HFD rats. HFD-induced mitochondrial damage caused a reduction in mitochondria, as evidenced by a decrease in the protein expression of TOM20, a mitochondrial outer membrane receptor. Consistently, HFD enhanced the levels of the PINK1 protein, a mitophagy marker, suggesting the removal of damaged mitochondria under these conditions. Induction of mtDNA damage and repair was stronger in the HFD rat testis. Finally, we found a decrease in the mtDNA copy number and expression of the POLG enzyme, which is involved in mtDNA replication. In conclusion, our results showed that autophagy and apoptosis are activated in the testis of HFD rats as a survival strategy to cope with oxidative stress. Furthermore, HFD-induced oxidative stress affects the mitochondria, inducing mtDNA damage and mtDNA copy number reduction. Mitophagy and mtDNA repair mechanisms might represent a mitochondrial adaptive response.

Differentiation of neonate mouse spermatogonia on two-dimensional and three-dimensional culture systems supplemented with d-Serine and Dizocilpine (MK-801)

N-methyl-d-aspartate (NMDA) modulates the spermatogenesis process through stimulating the steroid hormone biosynthesis. The aim of this study was to evaluate the effects of NMDA receptors agonists (d-Serine) and antagonists (MK801) on spermatogonia differentiation on decellularization testicular matrix (DTM) hydrogel scaffold. Four treatment groups were planned: 2D + D-Serine, 3D + D-Serine, 2D + MK801, and 3D + MK801. Results showed that cell viability was significantly decreased after 48 h in the 3D + D-Serine group and after 24 and 48 h in the 3D + MK801 group compared to the controls. The spermatogonia proliferation after two, four, and eight weeks was significantly increased in the 3D + D-Serine culture, while it was significantly reduced in the 2D + MK801 and 3D + MK801 groups after four and eight weeks. Real-time PCR results demonstrated that pre-meiotic gene (Plzf) expression was significantly increased only in the 3D + D-Serine culture compared to the control groups after four weeks of culture. The meiotic gene (Sycp3) expression was significantly increased in the 2D + D-Serine and 3D + D-Serine compared to the 2D controls after four and eight weeks. The post-meiotic gene (Tnp1) level in the 3D + D-Serine was significantly higher than the other groups. Flow-cytometry results indicated that the protein expression of Plzf (after four and eight weeks), Sycp3 (after eight weeks), and Tnp1 (after eight weeks) in the d-Serine-treated groups was significantly increased compared with the 2D control groups. There were not any significant changes in the gene expression of spermatogenic-related markers in MK801 culture media. However, a significant decrease in the protein levels of Plzf after eight weeks and Sycp3 after four and eight weeks was observed. In conclusion, the addition of NMDARs agonists (d-Serine) could be used to regulate the differentiation of spermatogonia in the 3D culture system.

Effect of NMDA receptor agonist and antagonist on spermatogonial stem cells proliferation in 2- and 3- dimensional culture systems

BACKGROUND

The main purpose of this study was to investigate the effect of D-serine (DS) and Dizocilpine (MK-801) on the proliferation of spermatogonial stem cells (SSCs) in two-dimensional (2D) and three-dimensional (3D) culture systems.

METHODS AND RESULTS

The SSCs of male NMRI mice were isolated by enzymatic digestion and cultured for two weeks. Then, the identity of SSCs was validated by anti-Plzf and anti-GFR-α1 antibodies via immunocytochemistry (ICC). The proliferation capacity of SSCs was evaluated by their culture on a layer of the decellularized testicular matrix (DTM) prepared from mouse testis, as well as two-dimensional (2D) with different mediums. After two weeks of the initiation of proliferation culture on 3D and 2D medium, the pre-meiotic at the mRNA and protein levels were evaluated via qRT-PCR and flow cytometry methods, respectively. The results showed that the proliferation rate of SSCs in 3D culture with 50 mM glutamic acid and 20 mM D-serine was significantly different from other groups after 14 days treatment. mRNA expression levels of promyelocytic leukemia zinc finger (Plzf) in 3D cultures supplemented by 20 mM D-serine and 50 mM glutamic acid were considerably higher than the 3D control group (p < 0.001). The flow cytometry analysis revealed that the amount of Plzf in the 2D-culture groups of SSCs with 20 mM MK-801 was considerably lower compared to the 2D-culture control group (p < 0.001).

CONCLUSIONS

This study indicated that decellularized testicular matrix supplemented with D-serine and glutamic acid could be considered a promising vehicle to support cells and provide an appropriate niche for the proliferation of SSCs.

d-aspartate and N-methyl-d-aspartate promote proliferative activity in mouse spermatocyte GC-2 cells

D-Aspartate (D-Asp) and its methylated form N-methyl-d-aspartate (NMDA) promote spermatogenesis by stimulating the biosynthesis of sex steroid hormones. d-Asp also induces spermatogonia proliferation directly by activating the ERK/Aurora B pathway. In the present study, a mouse spermatocyte-derived cell line (GC-2) which represents a stage between preleptotene spermatocyte and round spermatids was exposed to 200 μM d-Asp or 50 μM NMDA for 30 min, 2 h, and 4 h to explore the influence of these amino acids on cell proliferation and mitochondrial activities occurring during this process. By Western blotting analyses, the expressions of AMPAR (GluA1-GluA2/3 subunits), cell proliferation as well as mitochondria functionality markers were determined at different incubation times. The results revealed that d-Asp or NMDA stimulate proliferation and meiosis in the GC-2 cells via the AMPAR/ERK/Akt pathway, which led to increased levels of the PCNA, p-H3, and SYCP3 proteins. The effects of d-Asp and NMDA on the mitochondrial functionality of the GC-2 cells strongly suggested an active role of these amino acids in germ cell maturation. In both d-Asp- and NMDA-treated GC-2 cells mitochondrial biogenesis as well as mitochondrial fusion are increased while mitochondria fission is inhibited. Finally, the findings showed that NMDA significantly increased the expressions of the CII, CIII, CIV, and CV complexes of oxidative phosphorylation system (OXPHOS), whereas d-Asp induced a significant increase in the expressions only of the CIV and CV complexes. The present study provides novel insights into the mechanisms underlying the role of d-Asp and NMDA in promoting spermatogenesis.

Beneficial effects of memantine on ischemia/reperfusion injury following torsion/detorsion induced testicular damage in rats: Improvement in histological and biochemical parameters

INTRODUCTION

Testicular torsion is a common urologic emergency and one of the causes of infertility in males. Hence, prompt diagnosis and treatment are important to prevent testicular damages. It has been reported that memantine, a drug for Alzheimer's disease has anti-oxidative role against cerebral ischemic stroke and cardiac ischemia reperfusion.

OBJECTIVE

In this experimental study, the effects of memantine on a testicular torsion injury in adolescent rat testis after ischemia/reperfusion (I/R) were evaluated.

STUDY DESIGN

Thirty-six adolescent rats were divided into three groups with 12 rats per group including sham-operated, T/D (torsion/detorsion) + vehicle, and T/D + memantine (10 mg/kg). Testicular torsion was induced for 2 h by rotating right testis 720⁰ in the clockwise direction. In treated group 30 min before detorsion, a single intraperitoneal dose of memantine was administered. After 4 h of reperfusion, orchiectomy was conducted and Histopathological and biochemical evaluations of testicular tissue samples were performed.

RESULTS

The malondialdehyde level in the T/D group was significantly greater than in the sham operated group. Moreover, the testicular malondialdehyde values in the T/D + memantine group were significantly lower than in the T/D group. Also, significant decreases occurred in catalase and superoxide dismutase activities in the T/D group compared with sham operated group. These values were significantly greater in the memantine group than in the T/D group. Furthermore, after induction of T/D, histopathological evaluations also revealed severe testicular damages which were improved by memantine administration.

DISCUSSION

Memantine prevented increases in oxidative stress markers and reductions of antioxidants during I/R injury in the current study. Subsequently the histologic injury was reduced in rats treated with memantine. The antioxidant characteristics of memantine and its protective effects have been shown in our study.

CONCLUSION

These results suggest that administration of memantine before detorsion prevents I/R cellular damage in testicular torsion. This drug probably acts through reduction of reactive oxygen species and support antioxidant enzyme systems.

Proliferative and Apoptotic Pathways in the Testis of Quail Coturnix coturnix during the Seasonal Reproductive Cycle

Simple Summary

The quail Coturnix coturnix exhibits an annual cycle of testis size, sexual steroid production, and spermatogenesis. The testicular levels of both 17β-estradiol (E₂) and androgens are higher during the reproductive period compared to the non-reproductive period, suggesting that estrogens act in synergy with the androgens for the initiation of spermatogenesis. Therefore, the present study aimed to investigate the estrogen responsive system in quail testis in relation to the reproduction seasons, with a focus on the molecular pathways activated in both active and regressive quail testes. The results indicated that estrogens participated in the activation of mitotic and meiotic events during the reproductive period by activating the ERK1/2 and Akt-1 pathways. In the non-reproductive period, when the E₂/ERα levels are low, ERK1/2 and Akt-1 pathways remain inactive and apoptotic events occur. Our results suggest that the activation or inhibition of these molecular pathways plays a crucial role in the physiological switch “on/off” of the testicular activity in male quail during the seasonal reproductive cycle.

Abstract

The quail Coturnix coturnix is a seasonal breeding species, with the annual reproductive cycle of its testes comprising an activation phase and a regression phase. Our previous results have proven that the testicular levels of both 17β-estradiol (E₂) and androgens are higher during the reproductive period compared to the non-reproductive period, which led us to hypothesize that estrogens and androgens may act synergistically to initiate spermatogenesis. The present study was, therefore, aimed to investigate the estrogen responsive system in quail testis in relation to the reproduction seasonality, with a focus on the molecular pathways elicited in both active and regressive quail testes. Western blotting and immunohistochemistry analysis revealed that the expression of ERα, which is the predominant form of estrogen receptors in quail testis, was correlated with E₂ concentration, suggesting that increased levels of E₂-induced ERα could play a key role in the resumption of spermatogenesis during the reproductive period, when both PCNA and SYCP3, the mitotic and meiotic markers, respectively, were also increased. In the reproductive period we also found the activation of the ERK1/2 and Akt-1 kinase pathways and an increase in second messengers cAMP and cGMP levels. In the non-reproductive phase, when the E2/ERα levels were low, the inactivation of ERK1/2 and Akt-1 pathways favored apoptotic events due to an increase in the levels of Bax and cytochrome C, with a consequent regression of the gonad.

Simultaneous Measurement of Amino Acid Enantiomers in Aged Mouse Brain Samples by LC/MS/MS Combined with Derivatization Using N α-(5-Fluoro-2,4-dinitrophenyl)-l-leucinamide (l-FDLA)

d-amino acids have distinct roles from their l-enantiomer. In particular, some d-amino acids function as agonists or antagonists of neuronal receptors and are involved in higher brain functions. Thus, it is important to precisely measure the levels of these amino acid enantiomers in cells and tissues. Various quantification methods have been developed for measurements of chiral amino acids. However, each method has advantages and disadvantages. Additionally, measuring the amino acid enantiomers in crude biological samples requires a higher selectivity. In this study, we developed a quantification method for amino acid enantiomers using derivatization with N^α-(5-Fluoro-2,4-dinitrophenyl)-l-leucinamide (l-FDLA) followed by liquid chromatography–tandem mass spectrometry (LC/MS/MS) with a conventional reversed-phase column. We simultaneously identified 10 chiral amino acids. Furthermore, we applied this method to investigate murine tissue samples and examined the effect of aging on the amino acid levels in aged brain regions. We found that aging decreased the levels of both d-serine and d-aspartate in the hippocampus. In addition, d-Phenylalanine in the thalamus significantly increased with age. In conclusion, our method is suitable for the quantification of the d-amino acids in crude biological samples and may contribute to elucidating the biological roles of chiral amino acids.

A Proteomics Signature of Mild Hypospadias: A Pilot Study

Background and Objective: Mild hypospadias is a birth congenital condition characterized by the relocation of the male urethral meatus from its typical anatomical position near the tip of the glans penis, to a lower ventral position up to the brim of the glans corona, which can also be accompanied by foreskin ventral deficiency. For the most part, a limited number of cases have known etiology. We have followed a high-throughput proteomics approach to study the proteome in mild hypospadias patients. Methods: Foreskin samples from patients with mild hypospadias were collected during urethroplasty, while control samples were collected during elective circumcision (n = 5/group). A high-throughput, quantitative proteomics approach based on multiplexed peptide stable isotope labeling (SIL) and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis was used to ascertain protein abundance changes in hypospadias patients when compared to control samples. Results: A total of 4,815 proteins were quantitated (2,522 with at least two unique peptides). One hundred and thirty-three proteins from patients with mild hypospadias showed significant abundance changes with respect to control samples, where 38 proteins were increased, and 95 proteins were decreased. Unbiased functional biological analysis revealed that both mitochondrial energy production and apoptotic signaling pathways were enriched in mild hypospadias. Conclusions: This first comprehensive proteomics characterization of mild hypospadias shows molecular changes associated with essential cellular processes related to energy production and apoptosis. Further evaluation of the proteome may expand the search of novel candidates in the etiology of mild hypospadias and could also lead to the identification of biomarkers for this congenital urogenital condition.

New Evidence on the Role of D-Aspartate Metabolism in Regulating Brain and Endocrine System Physiology: From Preclinical Observations to Clinical Applications

The endogenous amino acids serine and aspartate occur at high concentrations in free D-form in mammalian organs, including the central nervous system and endocrine glands. D-serine (D-Ser) is largely localized in the forebrain structures throughout pre and postnatal life. Pharmacologically, D-Ser plays a functional role by acting as an endogenous coagonist at N-methyl-D-aspartate receptors (NMDARs). Less is known about the role of free D-aspartate (D-Asp) in mammals. Notably, D-Asp has a specific temporal pattern of occurrence. In fact, free D-Asp is abundant during prenatal life and decreases greatly after birth in concomitance with the postnatal onset of D-Asp oxidase expression, which is the only enzyme known to control endogenous levels of this molecule. Conversely, in the endocrine system, D-Asp concentrations enhance after birth during its functional development, thereby suggesting an involvement of the amino acid in the regulation of hormone biosynthesis. The substantial binding affinity for the NMDAR glutamate site has led us to investigate the in vivo implications of D-Asp on NMDAR-mediated responses. Herein we review the physiological function of free D-Asp and of its metabolizing enzyme in regulating the functions of the brain and of the neuroendocrine system based on recent genetic and pharmacological human and animal studies.

D-Amino acids in mammalian endocrine tissues

D-Aspartate, D-serine and D-alanine are a regular occurrence in mammalian endocrine tissues, though in amounts varying with the type of gland. The pituitary gland, pineal gland, thyroid, adrenal glands and testis contain relatively large amounts of D-aspartate in all species examined. D-alanine is relatively abundant in the pituitary gland and pancreas. High levels of D-serine characterize the hypothalamus. D-leucine, D-proline and D-glutamate are generally low. The current knowledge of physiological roles of D-amino acids in endocrine tissues is far from exhaustive, yet the topic is attracting increasing interest because of its potential in pharmacological application. D-aspartate is known to act at all levels of the hypothalamus-pituitary-testis axis, playing a key role in reproductive biology in several vertebrate classes. An involvement of D-amino acids in the endocrine function of the pancreas is emerging. D-Aspartate has been immunolocalized in insulin-containing secretory granules in INS-1 E clonal β cells and is co-secreted with insulin by exocytosis. Specific immunolocalization of D-alanine in pituitary ACTH-secreting cells and pancreatic β-cells suggests that this amino acid participates in blood glucose regulation in mammals. By modulating insulin secretion, D-serine probably participates in the control of systemic glucose metabolism by modulating insulin secretion. We anticipate that future investigation will significantly increase the functional repertoire of D-amino acids in homeostatic control.

Agonist and antagonist NMDA receptor effect on cell fate during germ cell differentiation and regulate apoptotic process in 3D organ culture

In this work, agonist and antagonist N-methyl-D-aspartate (NMDA) receptor activation effect on cell fate during germ cell differentiation and regulate apoptotic process in 3D organ culture were studied. Afterwards, the effect of D-serine, retinoic acid (RA) and MK801 on spermatogenesis development was investigated. The animals were injected a single dose (40 mg/kg, intraperitoneal) of busulfan. After confirming the model, ten 5-day-old NMRI mice were used as spermatogonial stem cells (SSCs) transplantation donors. The SSCs were confirmed by detecting the promyelocytic leukaemia zinc finger (PLZF) protein. Then, tissue culture of the azoospermia model which had received SSCs was performed in various conditions (seven groups). The apoptosis markers levels of cells were significantly decreased in differentiation media containing RA and serine. In contrast, the expression of apoptotic markers including caspase 3, caspase 9 and Bax was increased in the presence of MK801. In conclusion, a new in vitro system capable of producing mature spermatozoa was developed that would be useful for investigating the medicinal effects of agents on the male reproductive system. Also, a comparison of spermatogenesis development in different media revealed that the presence of D-serine and RA (retinoic acid) in the culture medium has a positive effect on spermatogenesis.

Roles of N-methyl-D-aspartate receptors and D-amino acids in cancer cell viability

N-methyl-D-aspartate (NMDA) receptors, which are widely present in the central nervous system, have also been found to be up-regulated in a variety of cancer cells and tumors and they can play active roles in cancer cell growth regulation. NMDA receptor antagonists have been found to affect cancer cell viability and interfere with tumor growth. Moreover, cancer cells also have been shown to have elevated levels of some D-amino acids. Two human skin cell lines: Hs 895.T skin cancer and Hs 895.Sk skin normal cells were investigated. They were derived from the same patient to provide tumor and normal counterparts for comparative studies. The expression of specific NMDA receptors was confirmed for the first time in both skin cell lines. Dizocilpine (MK-801) and memantine, NMDA receptor channel blockers, were found to inhibit the growth of human skin cells by reducing or stopping NMDA receptor activity. Addition of D-Ser, D-Ala, or D-Asp, however, significantly reversed the antiproliferative effect on the human skin cells triggered by MK-801 or memantine. Even more interesting was the finding that the specific intracellular composition of a few relatively uncommon amino acids was selectively elevated in skin cancer cells when exposed to MK-801. It appears that a few specific and upregulated D-amino acids can reverse the drug-induced antiproliferative effect in skin cancer cells via the reactivation of NMDA receptors. This study provides a possible innovative anticancer therapy by acting on the D-amino acid pathway in cancer cells either blocking or activating their regulatory enzymes.

Amino Acids in Health and Endocrine Function

Dietary amino acids play an important role in maintaining health. Branched chain amino acids can adversely increase blood pressure whereas arginine and citrulline can reduce it. D-amino acids play important roles in several cell types including testis, the nervous system and adrenal glands. Several amino acids also can have dramatic effects on diabetes; branched chain amino acids, phenylalanine and tyrosine have been implicated while others, namely arginine and citrulline can improve outcomes. Leucine has been shown to play important roles in muscle primarily through the mTOR pathway though this effect does not translate across every population. Glutamine, arginine and D-aspartate also exert their muscle effects through mTOR. Relationships between amino acids and endocrine function include that of glucocorticoids, thyroid function, glucagon-like peptide 1 (GLP-1), ghrelin, insulin-like growth factor-1 (IGF-1) and leptin. Leucine, for example, can alleviate the effect of dexamethasone on muscle protein accretion. Interestingly, amino acid transporters play an important role in thyroid function. Several amino acids have been shown to increase GLP-1 levels in non-diabetics when administered orally. Similarly, several amino acids increase ghrelin levels in different species while cysteine can decrease it in mice. There is evidence to suggest that the arginine/NO pathway may be involved in modulating some of the effects of ghrelin on cells. In regard to IGF-1, branched chain amino acids can increase levels in adults while tryptophan and phenylalanine have been shown to increase levels in infants. Finally, leptin levels can be elevated by branched chain amino acids while restricting leucine in high fat diets can increase leptin sensitivity.

D-Aspartate Upregulates DAAM1 Protein Levels in the Rat Testis and Induces Its Localization in Spermatogonia Nucleus

Cell differentiation during spermatogenesis requires a proper actin dynamic, regulated by several proteins, including formins. Disheveled-Associated-Activator of Morphogenesis1 (DAAM1) belongs to the formins and promotes actin polymerization. Our results showed that oral D-Aspartate (D-Asp) administration, an excitatory amino acid, increased DAAM1 protein levels in germ cells cytoplasm of rat testis. Interestingly, after the treatment, DAAM1 also localized in rat spermatogonia (SPG) and mouse GC-1 cells nuclei. We provided bioinformatic evidence that DAAM1 sequence has two predicted NLS, supporting its nuclear localization. The data also suggested a role of D-Asp in promoting DAAM1 shuttling to the nuclear compartment of those proliferative cells. In addition, the proliferative action induced by D-Asp is confirmed by the increased levels of PCNA, a protein expressed in the nucleus of cells in the S phase and p-H3, a histone crucial for chromatin condensation during mitosis and meiosis. In conclusion, we demonstrated, for the first time, an increased DAAM1 protein levels following D-Asp treatment in rat testis and also its localization in the nucleus of rat SPG and in mouse GC-1 cells. Our results suggest an assumed role for this formin as a regulator of actin dynamics in both cytoplasm and nuclei of the germ cells.

Severity-Dependent Profile of the Metabolome in Hypospadias

Background & Objective: Hypospadias, characterized by the displacement of the opening of the urethra at any point in the medial-ventral side of the penis, is classified upon severity as mild (Type I) and severe (Type II and Type III) hypospadias. Hypospadias' etiology is idiopathic in the majority of cases, and underlying causes seem of multifactorial origin. Studies regarding genetic variants support this notion. It is unknown whether downstream gene products fit this profile. This study evaluated the metabolome of hypospadias by using the emerging technology of metabolomics in the search for distinct cellular processes associated with hypospadias' etiology according to the severity of this congenital urogenital condition. Methods: Foreskin samples were collected during urethroplasty from boys with Type I, II, and III hypospadias or undergoing elective circumcision (N = 28) between 5 and 28 months of age. Samples were processed and submitted to gas chromatography-mass spectrometry (GC/MS). MetaboloAnalyst (http://www.metaboanalyst.ca/) online platform was used for bioinformatic analyses. Results: Thirty-five metabolites across experimental groups were identified by GC/MS. Principal component analysis (PCA) and partial least squares-discriminant analysis (PLS-DA) showed that the metabolome of Type II and Type III hypospadias patients differs from the metabolome of Type I hypospadias and control patients. Of those 35, 10 amino acids were found in significantly low concentrations in severe hypospadias: aspartate, glutamate, glycine, isoleucine, leucine, lysine, methionine, phenylalanine, proline, and tyrosine. A high concentration of the amino acid lysine was detected in mild hypospadias. Conclusions: The observed downregulation of specific amino acids in severe hypospadias provides alternative routes for future research aiming to identify disrupted networks and pathways while considering the severity of hypospadias.

D-Aspartic Acid in Vertebrate Reproduction: Animal Models and Experimental Designs‡

This article reviews the animal models and experimental designs that have been used during the past twenty years to demonstrate the prominent role played by d-aspartate (d-Asp) in the reproduction of vertebrates, from amphibians to humans. We have tabulated the findings of in vivo and in vitro experiments that demonstrate the effects of d-Asp uptake on hormone production and gametogenesis in vertebrate animal models. The contribution of each animal model to the existing knowledge on the role of d-Asp in reproductive processes has been discussed. A critical analysis of experimental designs has also been carried out. Experiments performed on wild animal species suggest a role of d-Asp in the mechanisms that regulate the reproductive cycle. Several in vivo and in vitro studies carried out on mouse and rat models have facilitated an understanding of the molecular pathways activated by D-Asp in both steroidogenesis and spermatogenesis, with particular emphasis on testosterone biosynthesis. Some attempts using d-Asp for the improvement of reproductive activity in animals of commercial interest have yielded mixed results. The increased transcriptome activity of enzymes and receptors involved in the reproductive activity in d-Asp-treated broiler roosters revealed further details on the mechanism of action of d-Asp on the reproductive processes. The close relationship between d-Asp and reproductive activity has emerged, particularly in relation to its effects exerted on semen quality, proposing therapeutic applications of this amino acid in andrology and in medically-assisted procreation techniques.

The Effects of D-aspartate on Neurosteroids, Neurosteroid Receptors, and Inflammatory Mediators in Experimental Autoimmune Encephalomyelitis

OBJECTIVE

Experimental autoimmune encephalomyelitis (EAE) is a widely used model for multiple sclerosis. The present study has been designed to compare the efficiencies of oral and intraperitoneal (IP) administration of D-aspartate (D-Asp) on the onset and severity of EAE, the production of neurosteroids, and the expression of neurosteroid receptors and inflammatory mediators in the brain of EAE mice.

METHODS

In this study, EAE was induced in C57BL/6 mice treated with D-Asp orally (D-Asp-Oral) or by IP injection (D-Asp-IP). On the 20th day, brains (cerebrums) and cerebellums of mice were evaluated by histological analyses. The brains of mice were analyzed for: 1) Neurosteroid (Progesterone, Testosterone, 17β-estradiol) concentrations; 2) gene expressions of cytokines and neurosteroid receptors by reverse transcription polymerase chain reaction, and 3) quantitative determination of D-Asp using liquid chromatography-tandem mass spectrometry. Further, some inflammatory cytokines and matrix metalloproteinase-2 (MMP-2) were identified in the mouse serum using enzyme-linked immunosorbent assay kits.

RESULTS

Our findings demonstrated that after D-Asp was administered, it was taken up and accumulated within the brain. Further, IP injection of D-Asp had more beneficial effects on EAE severity than oral gavage. The concentration of the testosterone and 17β-estradiol in D-Asp-IP group was significantly higher than that of the control group. There were no significant differences in the gene expression of cytokine and neurosteroid receptors between control, D-Asp-IP, and D-Asp-Oral groups. However, IP treatment with D-Asp significantly reduced C-C motif chemokine ligand 2 and MMP-2 serum levels compared to control mice.

CONCLUSION

IP injection of D-Asp had more beneficial effects on EAE severity, neurosteroid induction and reduction of inflammatory mediators than oral gavage.

Seasonal expression and cellular distribution of star and steroidogenic enzymes in quail testis

The quail Coturnix coturnix is a seasonal breeder with a physiological switch on/off of gonadic activity. Photoperiod and temperature are the major environmental factors regulating the spermatogenesis. To more thoroughly comprehend the steroidogenic pathways that govern the seasonal reproductive cycle, we have investigated the localization of StAR protein and steroidogenic enzymes (3β-HSD, 17β-HSD, P450 aromatase, and 5α-Red) as well as androgen and estrogen levels, in the testis of reproductive and nonreproductive quails. We demonstrated that StAR, 3β-HSD, 17β-HSD, P450 aromatase, and 5α-Red were always present in the somatic (Leydig and Sertoli cells) and germ cells (spermatogonia, spermatocytes I and II, spermatids, and spermatozoa). In addition, by western blot analysis, we demonstrated that 17β-HSD, P450 aromatase, and 5α-Red showed the highest expression levels during the reproductive testis compared with nonreproductive one. Accordingly, we also found that during the reproductive phase the highest titres of testosterone, 17β-estradiol, and 5α-dihydrotestosterone are recorded. In conclusion, our findings demonstrated that in C. coturnix: (a) both somatic and germ cells are involved in the local synthesis of sex hormones; (b) 17β-HSD, P450 aromatase, and 5α-Red expressions, as well as testicular androgens and estrogens, increased in reproductive quail testis. This study strongly indicates that the steroidogenic process in quail testis exhibits seasonal changes with the promotion of both androgenic and estrogenic pathways in the reproductive period, suggesting their synergic mechanism in the spermatogenesis regulation.

AMPA receptor expression in mouse testis and spermatogonial GC-1 cells: A study on its regulation by excitatory amino acids

Excitatory amino acids (EAAs) are found present in the nervous and reproductive systems of animals. Numerous studies have demonstrated a regulatory role for Glutamate (Glu), d-aspartate ( d-Asp) and N-methyl- d-aspartate (NMDA) in the control of spermatogenesis. EAAs are able to stimulate the Glutamate receptors, including the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR). Here in, we assess expression of the main AMPAR subunits, GluA1 and GluA2/3, in the mouse testis and in spermatogonial GC-1 cells. The results showed that both GluA1 and GluA2/3 were localized in mouse testis prevalently in spermatogonia. The subunit GluA2/3 was more highly expressed compared with GluA1 in both the testis and the GC-1 cells. Subsequently, GC-1 cells were incubated with medium containing l-Glu, d-Glu, d-Asp or NMDA to determine GluA1 and GluA2/3 expressions. At 30 minutes and 2 hours of incubation, EAA-treated GC-1 cells showed significantly higher expression levels of both GluA1 and GluA2/3. Furthermore, p-extracellular signal-regulated kinase (ERK), p-Akt, proliferating cell nuclear antigen (PCNA), and Aurora B expressions were assayed in l-Glu-, d-Glu-, and NMDA-treated GC-1 cells. At 30 minutes and 2 hours of incubation, treated GC-1 cells showed significantly higher expression levels of p-ERK and p-Akt. A consequent increase of PCNA and Aurora B expressions was induced by l-Glu and NMDA, but not by d-Glu. Our study demonstrates a direct effect of the EAAs on spermatogonial activity. In addition, the increased protein expression levels of GluA1 and GluA2/3 in EAA-treated GC-1 cells suggest that EAAs could activate ERK and Akt pathways through the AMPAR. Finally, the increased PCNA and Aurora B levels may imply an enhanced proliferative activity.

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.

StAR protein and steroidogenic enzyme expressions in the rat Harderian gland

The Harderian gland (HG) of the rat (Rattus norvegicus) secretes copious amounts of lipids, such as cholesterol. Here we report a study of the expressions of the StAR protein and key steroidogenic enzymes in the HG of male and female rats. The objective of the present investigation was to ascertain (a) whether the rat HG is involved in steroid production starting with cholesterol, and (b) whether the pattern of gene and protein expressions together with the enzymatic activities display sexual dimorphism. The results demonstrate, for the first time, the expression of StAR gene and protein, and Cyp11a1, Hsd3b1, Hsd17b3, Srd5a1, Srd5a2 and Cyp19a1 genes in the rat HG. StAR mRNA and protein expressions were much greater in males than in females. Immunohistochemical analysis demonstrated a non-homogeneous StAR distribution among glandular cells. Hsd17b3 and Cyp19a1 mRNA levels were higher in males than in females, whereas Srd5a1 mRNA levels were higher in females than in males. No significant differences were observed in mRNA levels of Cyp11a1, Hsd3b1 and Srd5a2 between sexes. Furthermore, the in vitro experiments demonstrated a higher 5α-reductase activity in the female as compared to the male HG vice versa a higher P450 aro activity in males as compared to females. These results suggest that the Harderian gland can be classified as a steroidogenic tissue because it synthesizes cholesterol, expresses StAR and steroidogenic enzymes involved in both androgen and estrogen synthesis. The dimorphic expression and activity of the steroidogenic enzymes may suggest sex-specific hormonal effects into the HG physiology.

D-Aspartate amends reproductive performance of aged roosters by changing gene expression and testicular histology

Male broiler breeders (n = 32) of 55 weeks of age were administered four different doses of capsulated d-aspartate (DA; 0, 100, 200 or 300 mg kg−1 day−1, p.o. (DA0, DA100, DA200 and DA300 respectively)) for 12 successive weeks to assess reproductive performance, blood testosterone, testicular histology and transcript levels of steroidogenic acute regulatory protein (StAR), cholesterol side-chain cleavage enzyme (P450scc), androgen receptor (AR), LH receptor (LHR), 3β-hydroxysteroid dehydrogenase (3BHSD), proliferating cell nuclear antigen (PCNA), glutamate ionotropic receptor NMDA type subunit 1 (GRIN1) and glutamate ionotropic receptor NMDA type subunit 2B (GRIN2B). Blood samples and ejaculates were collected, and bodyweight was recorded weekly for 10 weeks. AI was performed weekly for the last 2 weeks to determine the number of sperm penetration holes in the perivitelline layer, fertility and hatchability. Testes histology and transcript levels were evaluated in the 12th week. Bodyweight, numbers of Leydig cells and blood vessels, testis index and levels of sperm abnormalities were not affected (P > 0.05) by the treatment. However, sperm total and forward motility, plasma membrane integrity and functionality of sperm, ejaculate volume, testosterone concentration and fertility were higher (P < 0.05) in both the DA200 and DA300 groups compared with the other groups. In the DA100 and DA200 groups, sperm concentration, number of spermatogonia, thickness of the seminiferous epithelium and the diameter of tubules were significantly higher (P < 0.05) than the other DA-treated groups. The number of penetration holes, hatchability and malondialdehyde concentration were higher in the DA200, all DA-treated and DA300 groups respectively compared with the control and other treatment groups. Except for P450scc, AR, LHR and PCNA transcript levels in the DA300 groups, the relative expression of the genes evaluated improved significantly in the other DA-treated groups. Based on these experimental findings, it is concluded that DA improves reproductive performance of aged roosters.

Sex hormone levels in the brain of d-aspartate-treated rats

d-Aspartate (d-Asp) is an endogenous amino acid present in the central nervous system and endocrine glands of various animal taxa. d-Asp is implicated in neurotransmission, physiology of learning, and memory processes. In gonads, it plays a crucial role in sex hormone synthesis. We have investigated the effects of chronic (30 days d-Asp drinking solution) and acute (i.p. injection of 2μmol/g bw d-Asp) treatments on sex steroid synthesis in rat brain. Furthermore, to verify the direct effect of d-Asp on neurosteroidogenic enzyme activities, brain homogenates were incubated with different substrates (cholesterol, progesterone, or testosterone) with or without the addition of d-Asp. Enzyme activities were measured by evaluating the in vitro conversion rate of (i) cholesterol to progesterone, testosterone, and 17β-estradiol, (ii) progesterone to testosterone and 17β-estradiol, (iii) testosterone to 17β-estradiol. We found that d-Asp oral administration produced an increase of approximately 40% in progesterone, 110% in testosterone, and 35% in 17β-estradiol. Similarly, the results of the acute experiment showed that at 30min after d-Asp treatment, the progesterone, testosterone, and 17β-estradiol levels increased by 29-35%, and at 8h they further increased by a 100% increment. In vitro experiments demonstrate that the addition of d-Asp to brain homogenate+substrate induces a significant increase in progesterone, testosterone and 17β-estradiol suggesting that the amino acid upregulates the local activity of steroidogenic enzymes.

Seasonal changes and sexual dimorphism in gene expression of StAR protein, steroidogenic enzymes and sex hormone receptors in the frog brain

The brain of amphibians contains all the key enzymes of steroidogenesis and has a high steroidogenic activity. In seasonally-breeding amphibian species brain steroid levels fluctuate synchronously with the reproductive cycle. Here we report a study of gene expression of StAR protein, key steroidogenic enzymes and sex hormone receptors in the telencephalon (T) and diencephalon-mesencephalon (D-M) of male and female reproductive and post-reproductive Pelophylax esculentus, a seasonally breeding anuran amphibian. Significant differences in gene expression were observed between (a) the reproductive and post-reproductive phase, (b) the two brain regions and (c) male and female frogs. During the reproductive phase, star gene expression increased in the male (both T and D-M) but not in the female brain. Seasonal fluctuations in expression levels of hsd3b1, hsd17b1, srd5a1 and cyp19a1 genes for neurosteroidogenic enzymes occurred in D-M region of both sexes, with the higher levels in reproductive period. Moreover, the D-M region generally showed higher levels of gene expression than the T region in both sexes. Gene expression was higher in females than males for most genes, suggesting higher neurosteroid production in female brain. Seasonal and sex-linked changes were also observed in gene expression for androgen (ar) and estrogen (esr1, esr2) receptors, with the males showing the highest ar levels in reproductive phase and the highest esr1 and esr2 levels in post-reproductive phase; in contrast, females showed the maximum expression for all three genes in reproductive phase. The results are the first evidence for seasonal changes and sexual dimorphism of gene expression of the neurosteroidogenic pathway in amphibians.

Modulation of N-Methyl-D-Aspartate Receptors (NMDAR), Bcl-2 and C-Fos Gene Expressions on Exposure to Individual and Mixtures of Low Concentration Metals in Zebrafish (Danio rerio)

Currently, there is limited information on the toxicity of low concentration of metal mixtures in the environment. Of particular interest is the effect of low levels of metal mixtures on neurodevelopment of aquatic organisms. This study reports the neurological gene expressions after exposing zebrafish embryos to low concentration toxic heavy metals, 120 h post fertilization (hpf). Embryos were exposed to low concentration individual and mixtures of lead (Pb), mercury (Hg), arsenic (As), and cadmium (Cd). Quantitative real-time PCR was used to assess gene expressions. The findings of this study confirmed that exposure to low concentration heavy metals upregulated N-methyl-D-aspartate (NMDA) receptor subunits NMDAR2A (NR2A), NMDAR2B (NR2B), and NMDAR2D (NR2D) and B cell lymphoma (Bcl-2) genes. NR2A genes were significantly upregulated by 90 and 74%, respectively, on exposure to Pb + As and Pb + Cd. NR2B genes were upregulated by 85.3, 68.6, 62.7, and 62.7% on exposure to As, Pb + Hg, Pb + As, and Pb + Cd, respectively. Exposure to As, Pb + Cd, and Pb + Hg + As significantly upregulated Bcl-2 genes by 2.01-, 1.84-, and 1.80-fold, respectively. NR1A and C-fos gene expressions were not significantly different from control. Upregulation of NMDAR subunits and Bcl-2 genes in this study was largely a counter measure against insults from exposure to low concentration heavy metals. Principal component analysis confirmed the influence of low concentration individual and mixtures of Pb, Hg, As, and Cd on gene expression of NMDAR subunits and Bcl-2. These data suggest that altered expression of NMDA receptor subunits and Bcl-2 genes may explain toxicity of low concentration individual and mixtures of Pb, Hg, As, and Cd.

Molecular Mechanisms Elicited by d-Aspartate in Leydig Cells and Spermatogonia

A bulk of evidence suggests that d-aspartate (d-Asp) regulates steroidogenesis and spermatogenesis in vertebrate testes. This review article focuses on intracellular signaling mechanisms elicited by d-Asp possibly via binding to the N-methyl-d-aspartate receptor (NMDAR) in both Leydig cells, and spermatogonia. In Leydig cells, the amino acid upregulates androgen production by eliciting the adenylate cyclase-cAMP and/or mitogen-activated protein kinase (MAPK) pathways. d-Asp treatment enhances gene and protein expression of enzymes involved in the steroidogenic cascade. d-Asp also directly affects spermatogonial mitotic activity. In spermatogonial GC-1 cells, d-Asp induces phosphorylation of MAPK and AKT serine-threonine kinase proteins, and stimulates expression of proliferating cell nuclear antigen (PCNA) and aurora kinase B (AURKB). Further stimulation of spermatogonial GC-1 cell proliferation might come from estradiol/estrogen receptor β (ESR2) interaction. d-Asp modulates androgen and estrogen levels as well as the expression of their receptors in the rat epididymis by acting on mRNA levels of Srd5a1 and Cyp19a1 enzymes, hence suggesting involvement in spermatozoa maturation.

D-Aspartate Induces Proliferative Pathways in Spermatogonial GC-1 Cells

D-aspartate (D-Asp) is an endogenous amino acid present in vertebrate tissues, with particularly high levels in the testis. In vivo studies indicate that D-Asp indirectly stimulates spermatogenesis through the hypothalamic-pituitary-gonadal axis. Moreover, in vitro studies have demonstrated that D-Asp up-regulates testosterone production in Leydig cells by enhancing expression of the steroidogenic acute regulatory protein. In this study, a cell line derived from immortalized type-B mouse spermatogonia retaining markers of mitotic germ cells (GC-1) was employed to explore more direct involvement of D-Asp in spermatogenesis. Activity and protein expression of markers of cell proliferation were determined at intervals during incubation in D-Asp-containing medium. D-Asp induced phosphorylation of ERK and Akt proteins, stimulated expression of PCNA and Aurora B, and enhanced mRNA synthesis and protein expression of P450 aromatase and protein expression of Estrogen Receptor β (ERβ). These results are the first demonstration of a direct effect of D-Asp on spermatogonial mitotic activity. Considering that spermatogonia express the NR1 subunit of the N-Methyl-D-Aspartic Acid receptor (NMDAR), we suggest that their response to D-Asp depends on NMDAR-mediated activation of the ERK and Akt pathways and is further enhanced by activation of the P450 aromatase/ERβ pathway.

D-Aspartic acid stimulates steroidogenesis through the delay of LH receptor internalization in a mammalian Leydig cell line

PURPOSE

Recent experimental evidence on non-mammalian animal models showed that D-Aspartic acid (d-Asp) administration increases testosterone levels through upregulation of StAR in Leydig cells. In this study, we aimed to investigate in vitro the signaling pathway associated with d-Asp stimulation in MA-10 murine Leydig cells.

METHODS

MA-10 cells were stimulated with different concentrations of d-Asp, in presence or absence of hCG. Then total testosterone (T) levels in the culture medium were evaluated by electrochemiluminescence immunoassay, and StAR and LHR protein expressions were quantified by the means of Western blotting. LHR cellular localization after hormonal stimulation was assessed by immunofluorescence.

RESULTS

Stimulation with the sole d-Asp did not induce any relevant increase of T release from cultured cells. On the other hand, stimulation with hCG induced significant increase of T (P = 0.045). Concomitant stimulation with hCG and d-Asp, at the concentration of 0.1 and 1 nM, induced additional and significant increase of released T (P = 0.03 and P = 0.04, respectively). StAR protein levels increased after concomitant stimulation with hCG and d-Asp 0.1 nM, compared with stimulation with the sole hCG (P = 0.02), whereas no variation in LHR protein expression was observed. Finally, d-Asp attenuated displacement of LHR staining, from cell membrane to cytoplasm, subsequent to hCG stimulation.

CONCLUSIONS

In this study, we confirmed a steroidogenic role for d-Asp, in concert with hCG, on murine Leydig cells, which is mediated by an increase in StAR protein levels. In addition, we showed that the possible mechanism subtending the effect of d-Asp could rely on the modulation of LHR exposure on the cell membrane.

D-Amino Acids in the Nervous and Endocrine Systems

Amino acids are important components for peptides and proteins and act as signal transmitters. Only L-amino acids have been considered necessary in mammals, including humans. However, diverse D-amino acids, such as D-serine, D-aspartate, D-alanine, and D-cysteine, are found in mammals. Physiological roles of these D-amino acids not only in the nervous system but also in the endocrine system are being gradually revealed. N-Methyl-D-aspartate (NMDA) receptors are associated with learning and memory. D-Serine, D-aspartate, and D-alanine can all bind to NMDA receptors. H₂S generated from D-cysteine reduces disulfide bonds in receptors and potentiates their activity. Aberrant receptor activity is related to diseases of the central nervous system (CNS), such as Alzheimer's disease, amyotrophic lateral sclerosis, and schizophrenia. Furthermore, D-amino acids are detected in parts of the endocrine system, such as the pineal gland, hypothalamus, pituitary gland, pancreas, adrenal gland, and testis. D-Aspartate is being investigated for the regulation of hormone release from various endocrine organs. Here we focused on recent findings regarding the synthesis and physiological functions of D-amino acids in the nervous and endocrine systems.

Androgen and oestrogen modulation by D-aspartate in rat epididymis

Testosterone (T) synthesised in Leydig cells enters the epididymis and may there be converted into dihydrotestosterone (DHT) by 5α-reductase (5α-red) or into 17β-oestradiol (E2) by P450 aromatase (P450-aro). D-aspartate (D-Asp) is known to induce T synthesis in the testis. In this study, we investigated the effects of in vivo D-Asp administration in two major regions of the rat epididymis (Region I: initial segment, caput, corpus; Region II: cauda). The results suggest that exogenous D-Asp was taken up by both regions of rat epididymis. D-Asp administration induced a rapid increase in T, followed by a more gradual decrease in the T : DHT ratio in Region I. In Region II, T levels rapidly decreased and the T : DHT ratio was consistently lower relative to the control. Expression of 5α-red and androgen receptor genes showed a good correlation with DHT levels in both regions. D-Asp treatment also induced an increase of both E2 levels and oestradiol receptor-α (ERα) expression in Region I, whereas neither E2 levels nor ERα expression were affected in Region II. The early increase of P450-aro expression in Region I and late increase in Region II suggests a direct involvement of D-Asp modulation in P450-aro gene expression. Our results suggest that D-Asp modulates androgen and oestrogen levels and expression of androgen and oestrogen receptors in the rat epididymis by acting on the expression of 5α-red and P450-aro genes.

Identification and characterization of the NMDA receptor and its role in regulating reproduction in the cockroach Diploptera punctata

The NMDA receptor (NMDAR) plays important roles in excitatory neurotransmission and in the regulation of reproduction in mammals. NMDAR in insects comprises two subunits, NR1 and NR2. In this study, we identified two NR1 paralogs and eleven NR2 alternatively spliced variants in the cockroach Diploptera punctata. This is the first report of NR1 paralogs in insects. The tissue distributions and expression profiles of DpNR1A, DpNR1B and DpNR2 in different tissues were also investigated. Previous studies have demonstrated NMDA-stimulated biosynthesis of juvenile hormone (JH) in the corpora allata through the influx of extracellular Ca2+ in Diploptera punctata. However, our data show that the transcript levels of DpNR1A, DpNR1B and DpNR2 were low in the corpora allata. MK-801, a high-affinity antagonist of NMDAR, did not show any effect on JH biosynthesis in vitro. In addition, neither partial knockdown of DpNR2 nor in vivo treatment with a physiologically relevant dose of MK-801 resulted in any significant change in JH biosynthesis or basal oocyte growth. Injection of animals with a high dose of MK-801 (30 µg per animal per injection), which paralyzed the animals for 4–5 h, resulted in a significant decrease in JH biosynthesis on days 4 and 5. However, the reproductive events during the first gonadotrophic cycle in female D. punctata were unaffected. Thus, NMDAR does not appear to play important roles in the regulation of JH biosynthesis or mediate reproduction of female D. punctata.