Kainic Acid-Based Agonists of Glutamate Receptors: SAR Analysis and Guidelines for Analog Design

Algal Kainoid Synthases Exhibit Substrate-Dependent Hydroxylation and Cyclization Activities

FeII/α-ketoglutarate-dependent dioxygenases (Fe/αKG) make up a large enzyme family that functionalize C-H bonds on diverse organic substrates. Although Fe/αKG homologues catalyze an array of chemically useful reactions, hydroxylation typically predominates. Microalgal DabC uniquely forms a novel C-C bond to construct the bioactive pyrrolidine ring in domoic acid biosynthesis; however, we have identified that this kainoid synthase exclusively performs a stereospecific hydroxylation reaction on its cis substrate regioisomer. Mechanistic and kinetic analyses with native and alternative substrates identified a 20-fold rate increase in DabC radical cyclization over β-hydroxylation with no observable 1,5-hydrogen atom transfer. Moreover, this dual activity was conserved among macroalgal RadC1 and KabC homologues and provided insight into substrate recognition and reactivity trends. Investigation of this substrate-dependent chemistry improves our understanding of kainoid synthases and their biocatalytic application.

Organic Kainate Single Crystals for Second-Harmonic and Broadband THz Generation

Organic crystals with unique nonlinear optical properties have been attracting attention owing to their capability to outperform their conventional nonorganic counterparts. Since nonlinear material responses are linked to a crystal's internal microscopic structure, molecular engineering of maximally unharmonic quantum potentials can boost macromolecular susceptibilities. Here, large-scale kainic acid (kainate) single crystals were synthesized, and their linear and nonlinear optical properties were studied in a broad spectral range, spanning the visible to THz spectral regions. The non-centrosymmetric zwitterionic crystallization, molecular structure, and intermolecular arrangement were found to act as additive donor-acceptor domains, enhancing the efficiency of the intrinsic second-order optical nonlinearity of this pure enantiomeric crystal. Molecular simulations and experimental analysis were performed to retrieve the crystals' properties. The crystals were predicted and found to have good transparency in a broad spectral range from the UV to the infrared (0.2-20 μm). Second-harmonic generation was measured for ultrashort pumping wavelengths between 800 and 2400 nm, showing an enhanced response around 600 nm. Broadband THz generation was demonstrated with a detection limited bandwidth of >8 THz along with emission efficiencies comparable to and prevailing those of commercial ZnTe crystals. The broadband nonlinear response and high transparency make kainate crystals extremely attractive for realizing a range of nonlinear optical devices.

Acetyl-L-Carnitine Exerts Neuroprotective and Anticonvulsant Effect in Kainate Murine Model of Temporal Lobe Epilepsy

The most well-known type of focal epilepsy that is resistant to existing treatments is temporal lobe epilepsy (TLE), with seizure foci in various structures including temporal lobe, hippocampus, amygdala, entorhinal cortex, and subcortex. The most significant processes involved in the pathophysiology of temporal lobe epilepsy (TLE) are oxidative stress, inflammation, and pyroptosis. There are evidences indicating that acetyl-l-carnitine (ALC) has anti-oxidative, anti-inflammatory, and anti-pyroptotic effects. In the present study, rat model of TLE was induced by intrahippocampal kainate and animals received ALC (100 mg/kg, p.o.). ALC properly attenuated intensity of seizures and also incidence of kainate-induced status epilepticus (SE). As well, obtained findings showed that ALC can partially reverse hippocampal levels of MDA, ROS, SOD, TNFa, NF-kB, TLR4, GFAP, and caspase 1. Besides, treatment of kainate group with ALC exerted a protective effect against CA₁ neuronal loss and abnormal mossy fiber sprouting (MFS). Conclusively, these results suggest that ALC is capable to attenuate kainate-induced SE which is somewhat mediated through its lowering of oxidative stress, neuroinflammation, and pyroptosis that are related to its neuroprotective effect.

Current Trends and New Challenges in Marine Phycotoxins

Marine phycotoxins are a multiplicity of bioactive compounds which are produced by microalgae and bioaccumulate in the marine food web. Phycotoxins affect the ecosystem, pose a threat to human health, and have important economic effects on aquaculture and tourism worldwide. However, human health and food safety have been the primary concerns when considering the impacts of phycotoxins. Phycotoxins toxicity information, often used to set regulatory limits for these toxins in shellfish, lacks traceability of toxicity values highlighting the need for predefined toxicological criteria. Toxicity data together with adequate detection methods for monitoring procedures are crucial to protect human health. However, despite technological advances, there are still methodological uncertainties and high demand for universal phycotoxin detectors. This review focuses on these topics, including uncertainties of climate change, providing an overview of the current information as well as future perspectives.

TIGAR plays neuroprotective roles in KA-induced excitotoxicity through reducing neuroinflammation and improving mitochondrial function

Excitotoxicity refers to the ability of excessive extracellular excitatory amino acids to damage neurons via receptor activation. It is a crucial pathogenetic process in neurodegenerative diseases. TP53 is confirmed to be involved in excitotoxicity. It is demonstrated that TP53 induced glycolysis and apoptotic regulator (TIGAR)-regulated metabolic pathway can protect against neuronal injury. However, the role of TIGAR in excitotoxicity and specific mechanisms is still unknown. In this study, an in vivo excitotoxicity model was constructed via stereotypical kainic acid (KA) injection into the striatum of mice. KA reduced TIGAR expression levels, neuroinflammatory responses and mitochondrial dysfunction. TIGAR overexpression could reverse KA-induced neuronal injury by reducing neuroinflammation and improving mitochondrial function, thereby exerting neuroprotective effects. Therefore, this study could provide a potential therapeutic target for neurodegenerative diseases.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

A Diversity Oriented Synthesis Approach to New 2,3-trans-Substituted l-Proline Analogs as Potential Ligands for the Ionotropic Glutamate Receptors

Discovery of chemical tools for the ionotropic glutamate receptors continues to be a challenging task. Herein we report a diversity-oriented approach to new 2,3-trans-l-proline analogs whereby we study how the spatial orientation of the distal carboxylate group influences the binding affinity and receptor class and subtype selectivity. In total, 10 new analogs were synthesized and 14 stereoisomers characterized in binding assays at native rat ionotropic glutamate receptors, and at cloned human homomeric kainic acid (KA) receptor subtypes GluK1-3. The study identified isoxazole analogs 3d,e, which displayed selectivity in binding at native N-methyl-d-aspartate (NMDA) receptors over native α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and KA receptors, in the high nanomolar to low micromolar range. Furthermore, analogs 3i-A/B showed a preference in binding affinity for GluK3 over GluK1,2. Finally, analog 3j displayed high nanomolar affinity for native NMDA receptors as well as for homomeric GluK3 receptors.