Synaptic Zn2+ and febrile seizure susceptibility

Signaling pathway mechanisms of neurological diseases induced by G protein-coupled receptor 39

BACKGROUND

G protein-coupled receptor 39 (GPR39) is a transmembrane zinc receptor with two splice variants, which belongs to the G-protein-coupled receptor growth hormone-releasing peptide family. Its expression is induced by zinc, which activates GPR39, and its activation mediates cell proliferation, ion homeostasis, and anti-inflammatory, antioxidant, and other pathophysiological effects via different signaling pathways.

AIMS

The article reviews the latest literature in this field. In particular, the role of GPR39 in nervous system is discussed.

MATERIALS AND METHODS

GPR39 can be a promising target in neurological diseases for targeted therapy, which will help doctors overcome the associated problems.

DISCUSSION

GPR39 is expressed in vivo at several sites. Increasing evidence suggests that GPR39 plays an important role as a neuroprotective agent in vivo and regulates various neurological functions, including neurodegeneration, neuroelectrophysiology, and neurovascular homeostasis.

CONCLUSION

This review aims to provide an overview of the functions, signal transduction pathways, and pathophysiological role of GPR39 in neurological diseases and summarize the GPR39 agonists that have been identified in the recent years.

A Comparison Between Serum Selenium Level in Febrile Children with or Without Seizure

The role of trace elements in febrile seizure (FS) was considered recently. The present study was performed evaluating the serum level of selenium in febrile children aged 6-60 months with and without seizure. A cross-sectional study was performed in Mashhad University of Medical Sciences, Mashhad, Iran. Sixty patients aged 6-60 months including 30 children with FS and 30 febrile children without seizure were included. Blood sample was taken, and the serum level of selenium was measured. Data was analyzed using SPSS software. Sixteen patients in FS group (53.3%) and 10 patients in febrile group (33.3%) were males with an average age of 25.21 ± 15.91 and 26.47 ± 17.61 months, respectively. There was no significant difference between groups in age and sex (p = 0.77 and p = 0.19, respectively). The serum level of selenium was 87.34 ± 8.23 and 89.63 ± 9.83 µg/L in FS and febrile groups, respectively. Difference was not significant (p = 0.33). In children aged less than 1 year, the serum level of selenium in FS and febrile group was 83.32 ± 6.2 µg/L and 82.55 ± 8.32 µg/L, respectively. Difference was not significant (p = 0.87). In children aged more than 1 year, the serum level of selenium in FS significantly was lower compared to febrile group (87.96 ± 8.42 µg/L and 93.17 ± 8.66 µg/L, respectively, p = 0.04). The serum level of selenium was lower in children aged more than 1 year with febrile seizure compared to febrile ones.

Prenatal Zinc Deficient Mice as a Model for Autism Spectrum Disorders

Epidemiological studies have shown a clear association between early life zinc deficiency and Autism Spectrum Disorders (ASD). In line with this, mouse models have revealed prenatal zinc deficiency as a profound risk factor for neurobiological and behavioral abnormalities in the offspring reminiscent of ASD behavior. From these studies, a complex pathology emerges, with alterations in the gastrointestinal and immune system and synaptic signaling in the brain, as a major consequence of prenatal zinc deficiency. The features represent a critical link in a causal chain that leads to various neuronal dysfunctions and behavioral phenotypes observed in prenatal zinc deficient (PZD) mice and probably other mouse models for ASD. Given that the complete phenotype of PZD mice may be key to understanding how non-genetic factors can modify the clinical features and severity of autistic patients and explain the observed heterogeneity, here, we summarize published data on PZD mice. We critically review the emerging evidence that prenatal zinc deficiency is at the core of several environmental risk factors associated with ASD, being mechanistically linked to ASD-associated genetic factors. In addition, we highlight future directions and outstanding questions, including potential symptomatic, disease-modifying, and preventive treatment strategies.

GRIN2A Variant in A 3-Year-Old-An Expanding Spectrum?

Glutamate, the major excitatory neurotransmitter, plays a ubiquitous role in most aspects of normal brain functioning. Its indispensable position is paradoxically doubled by a high excitotoxic potential following disruption of its dynamic equilibrium. Several lines of evidence have suggested the involvement of the glutamatergic N-methyl-D-aspartate receptor (NMDAR) in learning, memory formation, and human cognition. Furthermore, NMDARs play a pivotal role in various neuropsychiatric disorders, recently being identified as an important locus for disease-associated genomic variation. The GRIN2A gene encodes the NMDAR’s GluN2A subunit. Genetic alterations of GRIN2A result in phenotypic pleiotropy, predisposing to a broad range of epilepsy syndromes, with an elusive and unpredictable evolution and response to treatment. The archetypal GRIN2A-related phenotype comprises the idiopathic focal epilepsies (IFEs), with a higher incidence of GRIN2A mutants among entities at the more severe end of the spectrum. We report the case of a patient heterozygous for GRIN2A, c.1081C>T, presenting with febrile convulsions and later superimposed atonic seizures, expressive language delay, and macrocephaly. As the number of reported GRIN2A variants is continuously increasing, the phenotypic boundaries gradually grow faint. Therefore, it is fundamental to maintain an acute critical awareness of the possible genetic etiology of different epilepsy syndromes. So far, therapeutic strategies rely on empirical observations relating genotypes to specific drugs, but the overall success of treatment remains unpredictable. Deciphering the functional consequences of individual GRIN2A variants could lead to the development of precision therapeutic approaches for patients carrying NMDAR mutations.

Zinc in the Brain: Friend or Foe?

Zinc is a trace metal ion in the central nervous system that plays important biological roles, such as in catalysis, structure, and regulation. It contributes to antioxidant function and the proper functioning of the immune system. In view of these characteristics of zinc, it plays an important role in neurophysiology, which leads to cell growth and cell proliferation. However, after brain disease, excessively released and accumulated zinc ions cause neurotoxic damage to postsynaptic neurons. On the other hand, zinc deficiency induces degeneration and cognitive decline disorders, such as increased neuronal death and decreased learning and memory. Given the importance of balance in this context, zinc is a biological component that plays an important physiological role in the central nervous system, but a pathophysiological role in major neurological disorders. In this review, we focus on the multiple roles of zinc in the brain.

The thalamic midline nucleus reuniens: potential relevance for schizophrenia and epilepsy

Anatomical, electrophysiological and behavioral studies in rodents have shown that the thalamic midline nucleus reuniens (RE) is a crucial link in the communication between hippocampal formation (HIP, i.e., CA1, subiculum) and medial prefrontal cortex (mPFC), important structures for cognitive and executive functions. A common feature in neurodevelopmental and neurodegenerative brain diseases is a dysfunctional connectivity/communication between HIP and mPFC, and disturbances in the cognitive domain. Therefore, it is assumed that aberrant functioning of RE may contribute to behavioral/cognitive impairments in brain diseases characterized by cortico-thalamo-hippocampal circuit dysfunctions. In the human brain the connections of RE are largely unknown. Yet, recent studies have found important similarities in the functional connectivity of HIP-mPFC-RE in humans and rodents, making cautious extrapolating experimental findings from animal models to humans justifiable. The focus of this review is on a potential involvement of RE in schizophrenia and epilepsy.

Carnosine as a Possible Drug for Zinc-Induced Neurotoxicity and Vascular Dementia

Increasing evidence suggests that the metal homeostasis is involved in the pathogenesis of various neurodegenerative diseases including senile type of dementia such as Alzheimer’s disease, dementia with Lewy bodies, and vascular dementia. In particular, synaptic Zn²⁺ is known to play critical roles in the pathogenesis of vascular dementia. In this article, we review the molecular pathways of Zn²⁺-induced neurotoxicity based on our and numerous other findings, and demonstrated the implications of the energy production pathway, the disruption of calcium homeostasis, the production of reactive oxygen species (ROS), the endoplasmic reticulum (ER)-stress pathway, and the stress-activated protein kinases/c-Jun amino-terminal kinases (SAPK/JNK) pathway. Furthermore, we have searched for substances that protect neurons from Zn²⁺-induced neurotoxicity among various agricultural products and determined carnosine (β-alanyl histidine) as a possible therapeutic agent for vascular dementia.

Zinc signaling and epilepsy

Evidence from both preclinical and clinical studies suggest the importance of zinc homeostasis in seizures/epilepsy. Undoubtedly, zinc, via modulation of a variety of targets, is necessary for maintaining the balance between neuronal excitation and inhibition, while an imbalance between excitation and inhibition underlies seizures. However, the relationship between zinc signaling and seizures/epilepsy is complex as both extracellular and intracellular zinc may produce either protective or detrimental effects. This review provides an overview of preclinical/behavioral, functional and molecular studies, as well as clinical data on the involvement of zinc in the pathophysiology and treatment of seizures/epilepsy. Furthermore, the potential of targeting elements associated with zinc signaling or homeostasis and zinc levels as a therapeutic strategy for epilepsy is discussed.

How cellular Zn2+ signaling drives physiological functions

Zinc is an essential micronutrient affecting many aspects of human health. Cellular Zn²⁺ homeostasis is critical for cell function and survival. Zn²⁺, acting as a first or second messenger, triggers signaling pathways that mediate the physiological roles of Zn²⁺. Transient changes in Zn²⁺ concentrations within the cell or in the extracellular region occur following its release from Zn²⁺ binding metallothioneins, its transport across membranes by the ZnT or ZIP transporters, or release of vesicular Zn²⁺. These transients activate a distinct Zn²⁺ sensing receptor, ZnR/GPR39, or modulate numerous proteins and signaling pathways. Importantly, Zn²⁺ signaling regulates cellular physiological functions such as: proliferation, differentiation, ion transport and secretion. Indeed, novel therapeutic approaches aimed to maintain Zn²⁺ homeostasis and signaling are evolving. This review focuses on recent findings describing roles of Zn²⁺ and its transporters in regulating physiological or pathological processes.

The Zinc Sensing Receptor, ZnR/GPR39, in Health and Disease

A distinct G-protein coupled receptor that senses changes in extracellular Zn²⁺, ZnR/GPR39, was found in cells from tissues in which Zn²⁺ plays a physiological role. Most prominently, ZnR/GPR39 activity was described in prostate cancer, skin keratinocytes, and colon epithelial cells, where zinc is essential for cell growth, wound closure, and barrier formation. ZnR/GPR39 activity was also described in neurons that are postsynaptic to vesicular Zn²⁺ release. Activation of ZnR/GPR39 triggers Gαq-dependent signaling and subsequent cellular pathways associated with cell growth and survival. Furthermore, ZnR/GPR39 was shown to regulate the activity of ion transport mechanisms that are essential for the physiological function of epithelial and neuronal cells. Thus, ZnR/GPR39 provides a unique target for therapeutically modifying the actions of zinc in a specific and selective manner.

Plasma Membrane Na⁺-Coupled Citrate Transporter (SLC13A5) and Neonatal Epileptic Encephalopathy

SLC13A5 is a Na⁺-coupled transporter for citrate that is expressed in the plasma membrane of specific cell types in the liver, testis, and brain. It is an electrogenic transporter with a Na⁺:citrate³⁻ stoichiometry of 4:1. In humans, the Michaelis constant for SLC13A5 to transport citrate is ~600 μM, which is physiologically relevant given that the normal concentration of citrate in plasma is in the range of 150–200 μM. Li⁺ stimulates the transport function of human SLC13A5 at concentrations that are in the therapeutic range in patients on lithium therapy. Human SLC13A5 differs from rodent Slc13a5 in two important aspects: the affinity of the human transporter for citrate is ~30-fold less than that of the rodent transporter, thus making human SLC13A5 a low-affinity/high-capacity transporter and the rodent Slc13a5 a high-affinity/low-capacity transporter. In the liver, SLC13A5 is expressed exclusively in the sinusoidal membrane of the hepatocytes, where it plays a role in the uptake of circulating citrate from the sinusoidal blood for metabolic use. In the testis, the transporter is expressed only in spermatozoa, which is also only in the mid piece where mitochondria are located; the likely function of the transporter in spermatozoa is to mediate the uptake of citrate present at high levels in the seminal fluid for subsequent metabolism in the sperm mitochondria to generate biological energy, thereby supporting sperm motility. In the brain, the transporter is expressed mostly in neurons. As astrocytes secrete citrate into extracellular medium, the potential function of SLC13A5 in neurons is to mediate the uptake of circulating citrate and astrocyte-released citrate for subsequent metabolism. Slc13a5-knockout mice have been generated; these mice do not have any overt phenotype but are resistant to experimentally induced metabolic syndrome. Recently however, loss-of-function mutations in human SLC13A5 have been found to cause severe epilepsy and encephalopathy early in life. Interestingly, there is no evidence of epilepsy or encephalopathy in Slc13a5-knockout mice, underlining the significant differences in clinical consequences of the loss of function of this transporter between humans and mice. The markedly different biochemical features of human SLC13A5 and mouse Slc13a5 likely contribute to these differences between humans and mice with regard to the metabolic consequences of the transporter deficiency. The exact molecular mechanisms by which the functional deficiency of the citrate transporter causes epilepsy and impairs neuronal development and function remain to be elucidated, but available literature implicate both dysfunction of GABA (γ-aminobutyrate) signaling and hyperfunction of NMDA (N-methyl-d-aspartate) receptor signaling. Plausible synaptic mechanisms linking loss-of-function mutations in SLC13A5 to epilepsy are discussed.

Synaptic Zn2+ and febrile seizure susceptibility

Zn²⁺ , the second most prevalent trace element in the body, is essential for supporting a wide range of biological functions. While the majority of Zn²⁺ in the brain is protein-bound, a significant proportion of free Zn²⁺ is found co-localized with glutamate in synaptic vesicles and is released in an activity-dependent manner. Clinical studies have shown Zn²⁺ levels are significantly lower in blood and cerebrospinal fluid of children that suffer febrile seizures. Likewise, investigations in multiple animal models demonstrate that low levels of brain Zn²⁺ increase seizure susceptibility. Recent work provides human genetic evidence that disruption of brain Zn²⁺ homeostasis at the level of the synapse is associated with increased seizure susceptibility. In this review, we have explored the clinical, functional and genetic data supporting the view that low synaptic Zn²⁺ increases cellular excitability and febrile seizure susceptibility. Finally, the review focuses on the potential of therapeutic Zn²⁺ supplementation for at risk patients.