Genomic insights into the biosynthesis and physiology of the cyanobacterial neurotoxin 2,4-diaminobutanoic acid (2,4-DAB).

Cyanobacteria are an ancient clade of photosynthetic prokaryotes, whose worldwide occurrence, especially in water, presents health hazards to humans and animals due to the production of a range of toxins (cyanotoxins). These include the sometimes co-occurring, non-encoded diaminoacid neurotoxins 2,4-diaminobutanoic acid (2,4-DAB) and its structural analogue ß-N-methylaminoalanine (BMAA). Knowledge of the biosynthetic pathway for 2,4-DAB, and its role in cyanobacteria, is lacking. The aspartate 4-phosphate pathway is a known route of 2,4-DAB biosynthesis in other bacteria and in some plant species. Another pathway to 2,4-DAB has been described in Lathyrus species. Here, we use bioinformatics analyses to investigate hypotheses concerning 2,4-DAB biosynthesis in cyanobacteria. We assessed the presence or absence of each enzyme in candidate biosynthesis routes, the aspartate 4-phosphate pathway and a pathway to 2,4-DAB derived from S-adenosyl-L-methionine (SAM), in 130 cyanobacterial genomes using sequence alignment, profile hidden Markov models, substrate specificity/active site identification and the reconstruction of gene phylogenies. In the aspartate 4-phosphate pathway, for the 18 species encoding diaminobutanoate-2-oxo-glutarate transaminase, the co-localisation of genes encoding the transaminase with the downstream decarboxylase or ectoine synthase - often within hybrid non-ribosomal peptide synthetase (NRPS)-polyketide synthases (PKS) clusters, NRPS-independent siderophore (NIS) clusters and incomplete ectoine clusters - is compatible with the hypothesis that some cyanobacteria use the aspartate 4-phosphate pathway for 2,4-DAB production. Through this route, in cyanobacteria, 2,4-DAB may be functionally associated with environmental iron-scavenging, via the production of siderophores of the schizokinen/synechobactin type and of some polyamines. In the pathway to 2,4-DAB derived from SAM, eight cyanobacterial species encode homologs of SAM-dependent 3-amino-3-carboxypropyl transferases. Other enzymes in this pathway have not yet been purified or sequenced. Ultimately, the biosynthesis of 2,4-DAB appears to be either restricted to some cyanobacterial species, or there may be multiple and additional routes, and roles, for the synthesis of this neurotoxin.

The Regulation and Function of Lactate Dehydrogenase A: Therapeutic Potential in Brain Tumor.

There are over 120 types of brain tumor and approximately 45% of primary brain tumors are gliomas, of which glioblastoma multiforme (GBM) is the most common and aggressive with a median survival rate of 14 months. Despite progress in our knowledge, current therapies are unable to effectively combat primary brain tumors and patient survival remains poor. Tumor metabolism is important to consider in therapeutic approaches and is the focus of numerous research investigations. Lactate dehydrogenase A (LDHA) is a cytosolic enzyme, predominantly involved in anaerobic and aerobic glycolysis (the Warburg effect); however, it has multiple additional functions in non-neoplastic and neoplastic tissues, which are not commonly known or discussed. This review summarizes what is currently known about the function of LDHA and identifies areas that would benefit from further exploration. The current knowledge of the role of LDHA in the brain and its potential as a therapeutic target for brain tumors will also be highlighted. The Warburg effect appears to be universal in tumors, including primary brain tumors, and LDHA (because of its involvement with this process) has been identified as a potential therapeutic target. Currently, there are, however, no suitable LDHA inhibitors available for tumor therapies in the clinic.

Brain glutathione as a target for aetiological factors in neurolathyrism and konzo.

Both neurolathyrism and konzo are associated with the nutritional dependence of human populations on a single plant food. These diseases express themselves as chronic disorders of upper motor neurones, leading to signs and symptoms that characterise amyotrophic lateral sclerosis (motor neurone disease). The plant food associated with neurolathyrism is grass pea, which contains the neurotoxic ß-N-oxalyl-α,ß-diaminopropionic acid (ß-ODAP). The plant food associated with konzo is cassava, which may contain significant concentrations of cyanogenic glycosides and their degradation products. A monotonous diet of grass pea is likely to generate nutritional deficiencies; it is proposed that one of these, plasma methionine deficiency, may predispose neurones to the neurotoxic effects of ß-ODAP. Subjects suffering from konzo also have low concentrations of plasma methionine as a result of a dietary deficiency of this amino acid. However, the plasma cystine concentration is also compromised because cyanide released from cyanogenic glycosides in cassava probably reacts with plasma cystine non-enzymatically. The product of this reaction is 2-imino-4-thiazolidine carboxylic acid. Since both plasma methionine and cystine are used for glutathione synthesis it seems likely that one common feature that leads to motor neurone death in neurolathyrism and konzo is the depletion of glutathione in the central nervous system.

The Lathyrus excitotoxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid is a substrate of the L-cystine/L-glutamate exchanger system xc-.

Beta-N-oxalyl-L-alpha-beta-diaminopropionic acid (beta-L-ODAP) is an unusual amino acid present in seeds of plants from the Lathyrus genus that is generally accepted as the causative agent underlying the motor neuron degeneration and spastic paraparesis in human neurolathyrism. Much of the neuropathology produced by beta-L-ODAP appears to be a direct consequence of its structural similarities to the excitatory neurotransmitter L-glutamate and its ability to induce excitotoxicity as an agonist of non-NMDA receptors. Its actions within the CNS are, however, not limited to non-NMDA receptors, raising the likely possibility that the anatomical and cellular specificity of the neuronal damage observed in neurolathyrism may result from the cumulative activity of beta-L-ODAP at multiple sites. Accumulating evidence suggests that system xc-, a transporter that mediates the exchange of L-cystine and L-glutamate, is one such site. In the present work, two distinct approaches were used to define the interactions of beta-L-ODAP with system xc-: Traditional radiolabel-uptake assays were employed to quantify inhibitory activity, while fluorometrically coupled assays that follow the exchange-induced efflux of L-glutamate were used to assess substrate activity. In addition to confirming that beta-L-ODAP is an effective competitive inhibitor of system xc-, we report that the compound exhibits a substrate activity comparable to that of the endogenous substrate L-cystine. The ability of system xc- to transport and accumulate beta-L-ODAP identifies additional variables that could influence its toxicity within the CNS, including the ability to limit its access to EAA receptors by clearing the excitotoxin from the extracellular synaptic environment, as well as serving as a point of entry through which beta-L-ODAP could have increased access to intracellular targets.