Interaction of the neutral amino acid transporter ASCT2 with basic amino acids

Cell cycle of microalga Isochrysis galbana arrested by neurotoxin β-N-methylamino-l-alanine and corresponding molecular mechanisms

The phycotoxin β-N-methylamino-l-alanine (BMAA) has attracted attention due to its risks to marine organisms and human health. In this study, approximately 85 % of synchronized cells of the marine microalga Isochrysis galbana were arrested at the cell cycle G1 phase by BMAA at 6.5 μM for a 24-h exposure. The concentration of chlorophyll a (Chl a) gradually decreased, while the maximum quantum yield of PSII (F_v/F_m), the maximum relative electron transport rate (rETR_max), light utilization efficiency (α) and half-saturated light irradiance (I_k) reduced early and recovered gradually in I. galbana exposed to BMAA in 96-h batch cultures. Transcriptional expression of I. galbana analyzed at 10, 12, and 16 h disclosed multiple mechanisms of BMAA to suppress the microalgal growth. Production of ammonia and glutamate was limited by the down-regulation of nitrate transporters, glutamate synthase, glutamine synthetase, cyanate hydrolase, and formamidase. Diverse extrinsic proteins related to PSII, PSI, cytochrome b6f complex, and ATPase were influenced by BMAA at transcriptional level. Suppression of the DNA replication and mismatch repair pathways increased the accumulation of misfolded proteins, which was reflected by the up-regulated expression of proteasome to accelerate proteolysis. This study improves our understanding of the chemical ecology impacts of BMAA in marine ecosystems.

Is Exposure to BMAA a Risk Factor for Neurodegenerative Diseases? A Response to a Critical Review of the BMAA Hypothesis

In a literature survey, Chernoff et al. (2017) dismissed the hypothesis that chronic exposure to β-N-methylamino-L-alanine (BMAA) may be a risk factor for progressive neurodegenerative disease. They question the growing scientific literature that suggests the following: (1) BMAA exposure causes ALS/PDC among the indigenous Chamorro people of Guam; (2) Guamanian ALS/PDC shares clinical and neuropathological features with Alzheimer's disease, Parkinson's disease, and ALS; (3) one possible mechanism for protein misfolds is misincorporation of BMAA into proteins as a substitute for L-serine; and (4) chronic exposure to BMAA through diet or environmental exposures to cyanobacterial blooms can cause neurodegenerative disease. We here identify multiple errors in their critique including the following: (1) their review selectively cites the published literature; (2) the authors reported favorably on HILIC methods of BMAA detection while the literature shows significant matrix effects and peak coelution in HILIC that may prevent detection and quantification of BMAA in cyanobacteria; (3) the authors build alternative arguments to the BMAA hypothesis, rather than explain the published literature which, to date, has been unable to refute the BMAA hypothesis; and (4) the authors erroneously attribute methods to incorrect studies, indicative of a failure to carefully consider all relevant publications. The lack of attention to BMAA research begins with the review's title which incorrectly refers to BMAA as a "non-essential" amino acid. Research regarding chronic exposure to BMAA as a cause of human neurodegenerative diseases is emerging and requires additional resources, validation, and research. Here, we propose strategies for improvement in the execution and reporting of analytical methods and the need for additional and well-executed inter-lab comparisons for BMAA quantitation. We emphasize the need for optimization and validation of analytical methods to ensure that they are fit-for-purpose. Although there remain gaps in the literature, an increasingly large body of data from multiple independent labs using orthogonal methods provides increasing evidence that chronic exposure to BMAA may be a risk factor for neurological illness.

The Human SLC1A5 Neutral Amino Acid Transporter Catalyzes a pH-Dependent Glutamate/Glutamine Antiport, as Well

ASCT2 is a neutral amino acid transporter, which catalyzes a sodium-dependent obligatory antiport among glutamine and other neutral amino acids. The human ASCT2 over-expressed in Pichia pastoris and reconstituted in proteoliposomes has been employed for identifying alternative substrates of the transporter. The experimental data highlighted that hASCT2 also catalyzes a sodium-dependent antiport of glutamate with glutamine. This unconventional antiport shows a preferred sidedness: glutamate is inwardly transported in exchange for glutamine transported in the counter direction. The orientation of the transport protein in proteoliposomes is the same as in the cell membrane; then, the observed sidedness corresponds to the transport of glutamate from the extracellular to the intracellular compartment. The competitive inhibition exerted by glutamate on the glutamine transport together with the docking analysis indicates that the glutamate binding site is the same as that of glutamine. The affinity for glutamate is lower than that for neutral amino acids, while the transport rate is comparable to that measured for the asparagine/glutamine antiport. Differently from the neutral amino acid antiport that is insensitive to pH, the glutamate/glutamine antiport is pH-dependent with optimal activity at acidic pH on the external (extracellular) side. The stimulation of glutamate transport by a pH gradient suggests the occurrence of a proton flux coupled to the glutamate transport. The proton transport has been detected by a spectrofluorometric method. The rate of proton transport correlates well with the rate of glutamate transport indicating a 1:1 stoichiometry H⁺: glutamate. The glutamate/glutamine antiport is also active in intact HeLa cells. On a physiological point of view, the described antiport could have relevance in some districts in which a glutamate/glutamine cycling is necessary, such as in placenta.