D-AMINO ACIDS AND HEALTH
The presence of enzymes for synthesis and degradation of D-amino acids (D-AA) in humans has been demonstrated, and their endogenous origin has been confirmed. The most abundant D-AAs in mammals are D-serine (D-Ser) and D-aspartate (D-Asp): they are involved in many human pathologies such as schizophrenia, ALS (Amyotrophic Lateral Sclerosis), Alzheimer’s Disease (AD), Parkinson’s Disease (PD) and others. The investigation of the physiological role and possible implication of D-AAs in genesis and development of these pathologies is of utmost relevance.
D-Ser is present in the central nervous system of vertebrates, and it is the principal co-agonist of NMDA receptors (NMDAR) (Danysz et al., 1998; Billard, 2015). These receptors are involved in the transmission of excitatory signals and modulate some physiological processes like neuronal migration and synaptic plasticity. A number of studies have demonstrated that decreased levels of D-Ser lead to altered NMDAR neurotransmission, a process related to susceptibility to schizophrenia (Wang et al. 2001; Coyle and Tsai, 2003; Hashimoto et al., 2003; Ferraris and Tsukamoto, 2011; Labrie et al., 2012).
Based on scientific results of recent years, D-Ser has been proposed as a therapeutic agent for schizophrenia treatment. High doses of this D-AA (30-60 mg/kg/day) improved neurocognitive functions of patients, and the same dose administered to high-risk individuals resulted in a significant reduction in negative symptoms compared to a placebo group, reflecting a potential use of D-Ser for the treatment of prodromal symptoms (Kantrowitz et al., 2015; McKay et al., 2019). D-Ser is also involved in the modulation of some biomarkers, like MMN (auditory mismatch negativity), a measure known to be sensitive to NMDAR dysfunction in animal models. Clinical studies have demonstrated that frequency MMN is very sensitive to D-Ser effects, and an improvement of this biomarker correlates with an amelioration of clinical response (Kantrowitz et al., 2018). Unluckily, D-Ser has a short half-life (4 hours) and alternative therapeutic approaches were investigated. On this side, a number of molecules acting as inhibitors of the enzyme deputed to D-AA degradation, namely D-amino acid oxidase (DAAO), have been identified (Sacchi et al., 2013): their use is supposed to increase the D-Ser levels in schizophrenia-affected patients rescuing the hypo-functioning of NMDAR. E.g., DAAO inhibition by sodium benzoate improved quality of life and Clinical Global Impression of patients with schizophrenia (Lane et al., 2013).
There are many evidence that age-related decrease in D-Ser levels could take part to the progression of cognitive decline, lowering activation of NMDAR. Thus, it is possible that a restored NMDAR activity using D-Ser may counteract age-related cognitive decline (Billard, 2015; Guercio et al., 2018). Some studies have confirmed an improvement in many aspects of age-related cognitive decline after D-Ser administration: an acute oral administration of 30 mg/kg of D-Ser improved spatial learning and problem solving in older rodents.
D-Ser is also involved in Alzheimer’s disease: an abnormal metabolism of this D-AA may lead to excessive activation of NMDAR, which seems to correlate to AD development. Increased levels of D-Ser are recorded in CSF (cerebrospinal fluid) of AD patients, which contribute to neurogenesis, neuronal survival and apoptosis modulation. Cellular death is inhibited by increased D-Ser in the early-phase of the disease, whereas it is stimulated in the later phases. Accordingly, increasing D-Ser levels in the initial phases of AD may contribute to limit nervous cells loss and counteract disease progression (Brito-Moreira et al., 2011; Talantova et al., 2013; Guercio et al., 2018).
In patients with fALS (familial Amyotrophic Lateral Sclerosis) an abnormal activation of NMDAR induces an excessive excitability of neuronal cells, the loss of motor neurons of brain and spinal cord, and neurodegeneration. Various studies showed increased D-Ser levels (and serine racemase expression) in cells of a transgenic mouse model of ALS (G93A mice) (Sasabe et al., 2007). D-Ser administration leads to a slower disease’s progression. Using specific inhibitors of serine racemase to decrease cord D-Ser levels could represent a new therapeutic approach to ALS.
Recent findings suggest that D-Ser could be useful in treatment of Parkinson’s disease. PD is a debilitating nervous disorder which provokes both physical (tremors, rigidity, loss of balance) and emotional changes. NMDAR modulation seems to be involved in the disease, as suggested by the modification in receptors abundance, structure and function following treatment with PD dopaminergic drugs (Hallet et al., 2004). As reported by (Gelfin et al., 2012), administration of 30 mg/kg/day of D-Ser lead to significant improvements of both motor and behavioural PD manifestations.
Acute administration of D-Ser has shown antidepressant and anxiolytic effects in mice (Malkesman et al., 2012) while an acute single dose of this D-AA improved mood in adult humans (Otte et al., 2013).
Furthermore, the analysis of plasma D-Ser levels could be used for an early assessment of chronic kidney disorders. In such patients, an increase in the plasma levels of this D-AA has been reported, while the levels in the urine were low. This has also been confirmed by the decrease in the GFR (glomerular filtration ratio), currently the most reliable parameter for the evaluation of these pathologies. GFR is indirectly measured through the evaluation of inulin clearance. Measurement of plasma D-Ser levels would represent a more direct parameter for evaluation of kidney diseases (Hesaka et al., 2019)
During the years, based on the evidence gained for rats, some concerns emerged: D-Ser might induce nephrotoxicity in humans. Such a nephrotoxicity is mainly due to hydrogen peroxide generation by DAAO-induced d-Ser degradation. The co-administration of a DAAO inhibitor with d-Ser represents a way to increase oral bioavailability of D-Ser and to prevent nephrotoxicity (Williams and Lock, 2005).
D-Asp is an endogenous amino acid found in free form in the endocrine glands, in particular in those associated with the reproductive system (D’Aniello et al., 1998), and in the cells of nervous system (Hashimoto et al., 1993) of animals, including rat and humans. In the endocrine system, it is involved in the regulation of hormone synthesis and release. In rats, D-Asp modulates the release of gonadotropin-releasing hormone (GnRH), prolactin (PRL), luteinizing hormone (LH), growth hormone (GH), testosterone and progesterone (Genchi, 2017).
Recent studies have revealed that decrease in D-Asp levels may be involved in the pathophysiology of human infertility. Indeed, oligoasthenoteratospermic and azoospermic men present less D-Asp in seminal plasma and spermatozoa than normospermic ones (D’Aniello et al., 2005). Oral administration of D,L-Asp in rabbit leds to an increase in both sperm concentration and kinetic parameters, such as the overall percentage of motile spermatozoa, the average path velocity, the percentage of progressively motile spermatozoa (Macchia et al., 2010). Thus, a possible application of D-Asp for treatment of human infertility is desirable. In women, D-Asp is found in ovarian follicular fluids and its amount seems to correlate with oocyte quality and fertilization competence: older females subjected to in vitro fertilization presented lower D-Asp concentration than younger patients. D-Asp concentration may be considered as an additional biochemical marker for oocyte quality in patients undergoing in vitro fertilization (D’Aniello et al., 2007).
As D-Ser, also D-Asp can activate NMDAR and modulate a number of NMDAR-mediated processes like synaptic plasticity and memory (Errico et al., 2011; Errico et al., 2015). As mentioned above, hypo-functioning of NMDAR is correlated to schizophrenia. Actually, reduced levels of D-Asp and increased D-aspartate oxidase (the enzyme deputed to D-Asp degradation) mRNA expression is observed in prefrontal samples of schizophrenic patients (Errico et al., 2015). A study conducted on transgenic mice has revealed that constitutive high levels of D-Asp exert a putative protective effect against the schizophrenia-like symptoms induced by the NMDAR antagonist phencyclidine. A putative downregulation of the levels of D-Asp during the early phases of neuronal development and an abnormal metabolism of this molecule may be more dangerous during critical phases of brain development, when D-Asp levels are physiologically high. So, increasing D-Asp levels in schizophrenic patients might represent an interesting application of this D-AA in the treatment of this pathology.
D-Asp is involved also in Alzheimer’s disease: L-aspartic acid (L-Asp) residues of amyloid—β (Aβ) are partially converted to D-Asp, and this racemization may hinder the proteolytic attack on Aβ by aminopeptidases, resulting in the stabilization of Aβ deposits in stable plaques (Iwatsubo et al., 1996). Furthermore, Aβ containing D-Asp has caused apoptosis in PC12 cells (Kaneko et al., 2001) and produced a toxic peptide that damaged neurons. Thus, modulation of racemization of aspartic acid and regulation of D-Asp levels might be of interest in treating AD patients.
D-Asp brain’s levels are modified during aging. In humans and in rodents they are high in the embryonic brain and decrease to very low amounts during postnatal life (Wolosker et al., 2000; Errico et al., 2011). Catabolism of this D-AA is due to D-aspartate oxidase (DASPO or DDO), whose expression increases in the post-natal phase. Studies conducted on Ddo-/- transgenic mice, which do not degrade D-Asp and consequently present high levels of this D-AA, have revealed worst synaptic transmission and plasticity, ERK signaling, cognitive abilities, and increased sensitivity to phencyclidine-induced prepulse inhibition deficit (Errico et al., 2011; Cristino et al., 2015). On the other hand, an increase in free D-Asp content has beneficial NMDAR-mediated effects, counteracting bad symptoms listed above. It brings to hippocampal early- and late-phase LTP enhancement (Errico et al., 2008, 2011, 2014), ERK phosphorylation (Errico et al., 2011), and dendritic length and spine density (Errico et al., 2014), spatial memory improvement (Errico et al., 2008, 2011), and a reduction of prepulse inhibition deficits produced by psychotomimetic drugs (Errico et al., 2008, 2015). Thus, modulation of DDO and consequent regulation of D-Asp levels might represent a new treatment approach to contrast negative effects of aging (Punzo et al., 2016).
Recent studies have revealed that D-Asp may represent a new, important active compound in multiple sclerosis (MS) treatment. In an experimental autoimmune encephalomyelitis (EAE) model, usually used to mimic demyelization and axonal damage which occur in multiple sclerosis disease, a reduction in myelin loss is observed after D-Asp administration, preventing the onset of MS (Goudarzvand et al., 2019; Afraei et al., 2017 ).
OTHERS D-AMINO ACIDS
D-Asparagine, D-alanine and D-proline may be also of clinical interest. For example, their levels in plasma have been correlated with progression of renal pathologies. So, they could be used as biomarkers of renal damage, but also for the prognosis and presence of other pathologies (Kimura et al., 2016).
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Elena Crespi – FIIRV (DAAIR Center)
Loredano Pollegioni, University of Insubria