Sueun Lee

Cranial irradiation regulates CREB-BDNF signaling and variant BDNF transcript levels in the mouse hippocampus

The brain can be exposed to ionizing radiation in various ways, and such irradiation can trigger adverse effects, particularly on learning and memory. However, the precise mechanisms of cognitive impairments induced by cranial irradiation remain unknown. In the hippocampus, brain-derived neurotrophic factor (BDNF) plays roles in neurogenesis, neuronal survival, neuronal differentiation, and synaptic plasticity. The significance of BDNF transcript variants in these contexts is becoming clearer. In the present study, both object recognition memory and contextual fear conditioning task performance in adult C57BL/6 mice were assessed 1 month after a single exposure to cranial irradiation (10 Gy) to evaluate hippocampus-related behavioral dysfunction following such irradiation. Furthermore, changes in the levels of BDNF, the cAMP-response element binding protein (CREB) phosphorylation, and BDNF transcript variants were measured in the hippocampus 1 month after cranial irradiation. On object recognition memory and contextual fear conditioning tasks, mice evaluated 1 month after irradiation exhibited significant memory deficits compared to sham-irradiated controls, but no apparent change was evident in locomotor activity. Both phosphorylated CREB and BDNF protein levels were significantly downregulated after irradiation of the hippocampus. Moreover, the levels of mRNAs encoding common BDNF transcripts, and exons IIC, III, IV, VII, VIII, and IXA, were significantly downregulated after irradiation. The reductions in CREB phosphorylation and BDNF expression induced by differential regulation of BDNF hippocampal exon transcripts may be associated with the memory deficits evident in mice after cranial irradiation.

Developmental and degenerative modulation of brain-derived neurotrophic factor transcript variants in the mouse hippocampus.

Brain‐derived neurotrophic factor (BDNF) is regarded as an important factor for neurogenesis, synaptic plasticity, and neuronal network organization in brain circuits. However, little is known about the regulation of BDNF transcript variants in the hippocampus during postnatal development and following chemically induced neurotoxicity. In the present study, we examined the expression of individual BDNF transcript variants in the mouse hippocampus on postnatal day (PD) 3, 7, 14, 21, and 56, as well as in the adult hippocampus 1, 2, 4, and 8 days after trimethyltin (TMT) treatment. During postnatal development, the expression levels of common BDNF‐coding transcripts and BDNF transcript variants increased gradually in the hippocampus, but the temporal patterns of each exon transcript showed significant differences. In the TMT‐treated hippocampus, the levels of common BDNF‐coding transcripts and exon I, IIC, III, VII, VIII, and IXA transcripts were significantly increased 1 day post‐treatment. These observations suggest that the differential regulation of BDNF exon transcripts may be associated with neuronal and synaptic maturation during postnatal development, and neuronal survival and synaptic plasticity in chemically induced neurodegeneration.

Developmental and degenerative modulation of GABAergic transmission in the mouse hippocampus

γ‐Aminobutyric acid (GABA) is the main inhibitory neurotransmitter involved in synaptic plasticity. GABAergic transmission is also implicated in developmental and degenerative processes in the brain. The goal of the present study was to understand the developmental and degenerative regulation of GABAergic transmission in the mouse hippocampus by examining changes in GABA receptor subunit mRNA levels and GABA‐related protein expression during postnatal development of the hippocampus and trimethyltin (TMT)‐induced neurodegeneration in the juvenile (postnatal day [PD] 24) and adult hippocampus (PD 56). During postnatal development, the mRNA levels of GABA A receptor (GABAAR) subunits, including α1, α4, β1, β2, and δ; GABA B receptor (GABABR) subunit 2; and the expression of GABA‐related proteins, including glutamic acid decarboxylase, vesicular GABA transporter (VGAT), and potassium chloride cotransporter 2 increased gradually in the mouse hippocampus. The results of seizure scoring and histopathological findings in the hippocampus revealed a more pronounced response to the same administered TMT dose in juvenile mice, compared with that in adult mice. The mRNA levels of most GABA receptor subunits in the juvenile hippocampus, excluding GABAAR subunit β3, were dynamically altered after TMT treatment. The mRNA levels of GABAAR subunits γ2 and δ decreased significantly in the adult hippocampus following TMT treatment, whereas the level of GABABR subunit 1 mRNA increased significantly. Among the GABA‐related proteins, only VGAT decreased significantly in the juvenile and adult mouse hippocampus after TMT treatment. In conclusion, regulation of GABAergic signaling in the mouse hippocampus may be related to maturation of the central nervous system and the degree of neurodegeneration during postnatal development and TMT‐induced neurodegeneration in the experimental animals.

Nestin expression and glial response in the hippocampus of mice after trimethyltin treatment.

Nestin is a protein of embryonic intermediate filaments expressed by multipotent neural stem cells. In the present study, the nestin expression pattern in the mouse hippocampus 1, 2, 3, 4, and 8 days after treatment with trimethyltin (TMT) was examined to explore the possible role played by nestin in chemically induced hippocampal injury. TMT treatment (2.5mg/kg, intraperitoneally) selectively injured the dentate gyrus (DG) of the mouse hippocampus. The level of hippocampal mRNA encoding nestin increased significantly 2 and 3 days post-treatment and thereafter decreased (at 4 and 8 days post-treatment). The level of nestin protein significantly increased 2 - 4 days post-treatment, particularly in the injured region of the DG, and predominantly in glial fibrillary acidic protein-positive astrocytes in the hippocampal DG. Ki67-positive proliferating cells were increased following TMT treatment and co-localized with nestin-positive reactive astrocytes. Thus, we suggest that nestin contributes to remodeling of the chemically injured DG via glial scar formation and the alteration of neurogenesis.

Trimethyltin-induced hippocampal neurodegeneration: A mechanism-based review

Trimethyltin (TMT), a toxic organotin compound, induces neurodegeneration selectively involving the limbic system and especially prominent in the hippocampus. Neurodegeneration-associated behavioral abnormalities, such as hyperactivity, aggression, cognitive deficits, and epileptic seizures, occur in both exposed humans and experimental animal models. Previously, TMT had been used generally in industry and agriculture, but the use of TMT has been limited because of its dangers to people. TMT has also been used to make a promising in vivo rodent model of neurodegeneration because of its region-specific characteristics. Several studies have demonstrated that TMT-treated animal models of epileptic seizures can be used as tools for researching hippocampus-specific neurotoxicity as well as the molecular mechanisms leading to hippocampal neurodegeneration. This review summarizes the in vivo and in vitro underlying mechanisms of TMT-induced hippocampal neurodegeneration (oxidative stress, inflammatory responses, and neuronal death/survival). Thus, the present review may be helpful to provide general insights into TMT-induced neurodegeneration and approaches to therapeutic interventions for neurodegenerative diseases, including temporal lobe epilepsy.

Possible involvement of hippocampal immediate–early genes in contextual fear memory deficit induced by cranial irradiation

Cranial irradiation can trigger adverse effects on brain functions, including cognitive ability. However, the cellular and molecular mechanisms underlying radiation-induced cognitive impairments remain still unknown. Immediate-early genes (IEGs) are implicated in neuronal plasticity and the related functions (i.e., memory formation) in the hippocampus. The present study quantitatively assessed changes in the mRNA and protein levels of the learning-induced IEGs, including Arc, c-fos, and zif268, in the mouse hippocampus after cranial irradiation using quantitative real-time reverse transcription polymerase chain reaction (qRT-PCR) and immunohistochemistry, respectively. Mice (male, 8-week-old C57BL/6) received whole-brain irradiation with 0 or 10Gy of gamma-ray and, 2weeks later, contextual fear conditioning (CFC) was used to induce IEGs. In the CFC task, mice evaluated 2weeks after irradiation exhibited significant memory deficits compared with sham (0Gy)-irradiated controls. The levels of mRNA encoding IEGs were significantly upregulated in the hippocampus 10 and 30min after CFC training. The mRNA levels in the irradiated hippocampi were significantly lower than those in the sham-irradiated controls. The IEG protein levels were significantly increased in all hippocampal regions, including the hippocampal dentate gyrus, cornu ammonis (CA)1, and CA3, after CFC training. The CFC-induced upregulation of Arc and c-fos in 10Gy-irradiated hippocampi was significantly lower than that in sham-irradiated controls, although there were no significant differences in the protein levels of the learning-induced zif268 between sham-irradiated and 10Gy-irradiated hippocampi. Thus, cranial irradiation with 10Gy of gamma-ray impairs the induction of hippocampal IEGs (particularly Arc and c-fos) via behavioral contextual fear memory, and this disturbance may be associated with the memory deficits evident in mice after cranial irradiation, possibly through the dysregulation of neuronal plasticity during memory formation.

Involvement of BDNF/ERK signaling in spontaneous recovery from trimethyltin-induced hippocampal neurotoxicity in mice

Trimethyltin (TMT) toxicity causes histopathological damage in the hippocampus and induces seizure behaviors in mice. The lesions and symptoms recover spontaneously over time; however, little is known about the precise mechanisms underlying this recovery from TMT toxicity. We investigated changes in the brain-derived neurotrophic factor/extracellular signal-regulated kinases (BDNF/ERK) signaling pathways in the mouse hippocampus following TMT toxicity. Mice (7 weeks old, C57BL/6) administered TMT (2.6 mg/kg intraperitoneally) showed acute and severe neurodegeneration with increased TUNEL-positive cells in the dentate gyrus (DG) of the hippocampus. The mRNA and protein levels of BDNF in the hippocampus were elevated by TMT treatment. Immunohistochemical analysis showed that TMT treatment markedly increased phosphorylated ERK1/2 expression in the mouse hippocampus 1-4 days after TMT treatment, although the intensity of ERK immunoreactivity in mossy fiber decreased at 1-8 days post-treatment. In addition, ERK-immunopositive cells were localized predominantly in doublecortin-positive immature progenitor neurons in the DG. In primary cultured immature hippocampal neurons (4 days in vitro), BDNF treatment alleviated TMT-induced neurotoxicity, via activation of the ERK signaling pathway. Thus, we suggest that BDNF/ERK signaling pathways may be associated with cell differentiation and survival of immature progenitor neurons, and will eventually lead to spontaneous recovery in TMT-induced hippocampal neurodegeneration.

Enhanced expression of immediate–early genes in mouse hippocampus after trimethyltin treatment

Immediate-early genes (IEGs) are transiently and rapidly activated in response to various cellular stimuli. IEGs mediate diverse functions during pathophysiologic events by regulating cellular signal transduction. We investigated the temporal expression of several IEGs, including c-fos, early growth response protein-1 (Egr-1), and activity-regulated cytoskeleton-associated protein (Arc), in trimethyltin (TMT)-induced hippocampal neurodegeneration. Mice (7 weeks old, C57BL/6) administered TMT (2.6mg/kg intraperitoneally) presented severe neurodegenerative lesions in the dentate gyrus (DG) and showed behavioral seizure activity on days 1-4 post-treatment, after which the lesions and behavior recovered spontaneously over time. c-fos, Egr-1, and Arc mRNA and protein levels significantly increased in the mouse hippocampus after TMT treatment. Immunohistochemical analysis showed that nuclear c-fos expression increased mainly in the DG, whereas nuclear Egr-1 expression was increased extensively in cornu ammonis (CA) 1, CA3, and the DG after TMT treatment. Increased Arc levels were detected in the cellular somata/dendrites of the hippocampal subregions after TMT treatment. Therefore, we suggest that increased IEGs are associated with TMT-induced pathological events in mouse hippocampus.

Changes in epigenetic markers, DNMT1 and HDAC1/2, in the adult mouse hippocampus after cranial irradiation

Brain exposure to ionizing radiation can cause functional deficits in the hippocampus, including memory impairment. However, the specific molecular mechanisms underlying irradiation-induced cognitive impairments are largely unknown. Changes in DNA methyltransferases (DNMTs) and histone deacetylases (HDACs), which are involved in DNA methylation and histone remodeling, may be associated with behavioral changes in learning and memory. We assessed changes in the levels of enzymes associated with the epigenetic modification of gene expression, including DNMT1, HDAC1, HDAC2, Sirtuin 1 (SIRT1), and acetylated histone H3 (Ace-H3) in the mouse hippocampus 1 and 30days after a single exposure to cranial irradiation (0 or 10Gy). mRNA levels of HDAC1 were significantly downregulated 1day after irradiation with 10Gy, and those of DNMT1, HDAC1, and HDAC2 were significantly downregulated 30days post-irradiation. Western blot analysis revealed significant decreases in DNMT1, HDAC1, and HDAC2 protein levels 1 and 30days after irradiation with 10Gy. Furthermore, protein levels of SIRT1 and Ace-H3 were significantly downregulated in the mouse hippocampus 1 and 30days after cranial irradiation. Our findings suggest that the reduction in epigenetic gene expression is associated with hippocampal dysfunction in mice exposed to cranial irradiation.

Brain-derived neurotropic factor and GABAergic transmission in neurodegeneration and neuroregeneration

Neurotoxicity induced by stress, radiation, chemicals, or metabolic diseases, is commonly associated with excitotoxicity, oxidative stress, and neuroinflammation. The pathological process of neurotoxicity induces neuronal death, interrupts synaptic plasticity in the brain, and is similar to that of diverse neurodegenerative diseases. Animal models of neurotoxicity have revealed that clinical symptoms and brain lesions can recover over time via neuroregenerative processes. Specifically, brain-derived neurotropic factor (BDNF) and gamma-aminobutyric acid (GABA)-ergic transmission are related to both neurodegeneration and neuroregeneration. This review summarizes the accumulating evidences that suggest a pathogenic role of BDNF and GABAergic transmission, their underlying mechanisms, and the relationship between BDNF and GABA in neurodegeneration and neuroregeneration. This review will provide a comprehensive overview of the underlying mechanisms of neuroregeneration that may help in developing potential strategies for pharmacotherapeutic approaches to treat neurotoxicity and neurodegenerative disease.