Loss of signal transducer and activator of transcription 3 (STAT3) signaling during elevated activity causes vulnerability in hippocampal neurons

Microglial repopulation induced by PLX3397 protects against ischemic brain injury by suppressing neuroinflammation in aged mice

As the resident immune cells in the central nervous system, microglia exhibit a 'sensitized' or 'primed' phenotype with dystrophic morphology and dysregulated functions in aged brains. Although studies have demonstrated the inflammatory profile of aged microglia in several neurological diseases, this issue is largely uncertain in stroke. Consequently, this study investigated the effects of primed and repopulated microglia on post-ischemic brain injury in aged mice. We replaced primed microglia with newly repopulated microglia through pharmacological administration and withdrawal of the colony-stimulating factor 1 receptor (CSF1R) inhibitor, PLX3397. Further, we performed a series of behavioral tests and flow cytometry in mouse models of middle cerebral artery occlusion (MCAO) to study the effects of microglial replacement on ischemic injury in the aged brain. With depletion and subsequent repopulation of microglia in MCAO mice, microglial replacement in aged mice improved neurological function and decreased brain infarction. This protective effect was accompanied by the reduction of peripheral immune cells infiltrating into brains. We showed that the repopulated microglia expressed elevated neuroprotective factors (including Cluster of Differentiation 206, transforming growth factor-β, and interleukin-10) and diminished expression of inflammatory markers (including Cluster of Differentiation 86, interleukin-6, and tumor necrosis factor α). Moreover, microglial replacement protected the blood-brain barrier and relieved neuronal death in aged mice subjected to 60 min of MCAO. These results imply that the replacement of microglia in the aged brain may alleviate brain damage and neuroinflammation, and therefore, ischemic brain damage. Thus, targeting microglia could be a promising therapeutic strategy for ischemic stroke.

From immunomodulation to therapeutic prospects: Unveiling the biology of butyrophilins in cancer

Butyrophilin (BTN) proteins are a type of membrane protein that belongs to the Ig superfamily. They exhibit a high degree of structural similarity to molecules in the B7 family. They fulfill a complex function in regulating immune responses, including immunomodulatory roles, as they influence γδ T cells. The biology of BTN molecules indicates that they are capable of inhibiting the immune system's ability to detect antigens within tumors. A dynamic association between BTN molecules and cellular surfaces is also recognized in specific contexts, influencing their biology. Notably, the dynamism of BTN3A1 is associated with the immunosuppression of T cells or the activation of Vγ9Vδ2 T cells. Cancer immunotherapy relies heavily on T cells to modulate immune function within the intricate interaction of the tumor microenvironment (TME). A significant interaction between the TME and antitumor immunity involves the presence of BTN, which should be taken into account when developing immunotherapy. This review explores potential therapeutic applications of BTN molecules, based on the current understanding of their biology.

Phytic acid improves osteogenesis and inhibits the senescence of human bone marrow mesenchymal stem cells under high-glucose conditions via the ERK pathway

Hyperglycaemia causes impairment of osteogenic differentiation and accelerates stem cell senescence, resulting in weakened osteogenesis and disordered bone metabolism. Phytic acid (PA) is an antioxidant that is reportedly beneficial to bone homeostasis. The present study aims to clarify how PA affects the osteogenic capacity and cellular senescence of bone marrow mesenchymal stem cells (BMSCs) exposed to high-glucose environments, as well as the potential molecular mechanisms. Our results indicate that osteogenic differentiation in BMSCs cultivated in high-glucose conditions is enhanced by PA, as evidenced by increased alkaline phosphatase activity and staining, Alizarin Red S staining, osteogenic marker in in vitro studies, and increased osteogenesis in animal experiments. PA also prevented high-glucose-induced senescence of BMSCs, as evidenced by the repression of reactive oxygen species production, senescence-associated β-galactosidase staining, and P21 and P53 expression. Furthermore, it was found that PA rescued the high-glucose-inhibited expression of phosphorylated extracellular regulated protein kinases (p-ERK). The inhibition of ERK pathway by the specific inhibitor PD98059 blocked the PA-enhanced osteogenesis of BMSCs and promoted cell senescence. Our results revealed that PA enhances osteogenic differentiation and inhibits BMSC senescence in a high-glucose environment. In addition, the activation of the ERK pathway seems to mediate the beneficial effects of PA. The findings provide novel insights that could facilitate bone regeneration in patients with diabetes.

STAT3 in medulloblastoma: a key transcriptional regulator and potential therapeutic target

Medulloblastoma is the most common malignant brain tumor of childhood accounting for about 60% of all pediatric embryonal tumors. Despite improvements in the overall survival rate, this tumor still lacks an efficient, reliable, and less toxic therapeutic approach. Characterization of the molecular mechanisms involved in medulloblastoma initiation and progression is a crucial step for the development of effective therapies. Signal transducer and activator of transcription 3 is a convergence point for several signaling cascades that are implicated in medulloblastoma tumorigenesis. Accumulated evidence has revealed the pivotal role of signal transducer and activator of transcription 3 in medulloblastoma pathogenesis such as proliferation, survival, angiogenesis, and immunosuppression as well as maintenance, drug resistance, and recurrence. In this review, we focus on the role of signal transducer and activator of transcription 3 in medulloblastoma tumorigenesis and discuss the recent advances of signal transducer and activator of transcription 3 inhibition as a promising developed strategy for medulloblastoma therapy.

Prenatal interleukin 6 elevation increases glutamatergic synapse density and disrupts hippocampal connectivity in offspring

Early prenatal inflammatory conditions are thought to be a risk factor for different neurodevelopmental disorders. Maternal interleukin-6 (IL-6) elevation during pregnancy causes abnormal behavior in offspring, but whether these defects result from altered synaptic developmental trajectories remains unclear. Here we showed that transient IL-6 elevation via injection into pregnant mice or developing embryos enhanced glutamatergic synapses and led to overall brain hyperconnectivity in offspring into adulthood. IL-6 activated synaptogenesis gene programs in glutamatergic neurons and required the transcription factor STAT3 and expression of the RGS4 gene. The STAT3-RGS4 pathway was also activated in neonatal brains during poly(I:C)-induced maternal immune activation, which mimics viral infection during pregnancy. These findings indicate that IL-6 elevation at early developmental stages is sufficient to exert a long-lasting effect on glutamatergic synaptogenesis and brain connectivity, providing a mechanistic framework for the association between prenatal inflammatory events and brain neurodevelopmental disorders.

Voltage-dependent potassium channel Kv4.2 alleviates the ischemic stroke impairments through activating neurogenesis

As well as their ion transportation function, the voltage-dependent potassium channels could act as the cell signal inducer in a variety of pathogenic processes. However, their roles in neurogenesis after stroke insults have not been clearly illustrated. In our preliminary study, the expressions of voltage-dependent potassium channels Kv4.2 was significantly decreased after stroke in cortex, striatum and hippocampus by real-time quantitative PCR assay. To underlie the neuroprotection of Kv4.2 in stroke rehabilitation, recombinant plasmids encoding the cDNAs of mouse Kv4.2 was constructed. Behavioral tests showed that the increased Kv4.2 could be beneficial to the recovery of the sensory, the motor functions and the cognitive deficits after stroke. Temozolomide (TMZ), an inhibitor of neurogenesis, could partially abolish the mentioned protections of Kv4.2. The immunocytochemical staining showed that Kv4.2 could promote the proliferations of neural stem cells and induce the neural stem cells to differentiate into neurons in vitro and in vivo. And Kv4.2 could up-regulate the expressions of ERK1/2, p-ERK1/2, p-STAT3, NGF, p-TrkA, and BDNF, CAMKII and the concentration of intracellular Ca²⁺. Namely, we concluded that Kv4.2 promoted neurogenesis through ERK1/2/STAT3, NGF/TrkA, Ca²⁺/CAMKII signal pathways and rescued the ischemic impairments. Kv4.2 might be a potential drug target for ischemic stroke intervention.

Carnosic acid increases sorafenib-induced inhibition of ERK1/2 and STAT3 signaling which contributes to reduced cell proliferation and survival of hepatocellular carcinoma cells

Hepatocellular carcinoma (HCC) has increasing worldwide incidence but when unresectable lacks curative options. Treatment with a kinase inhibitor Sorafenib (Sf), while initially effective, results in only short increases in patient survival, thus there is a need for improved treatment regimens. Numerous treatment regimens have been explored wherein Sf is combined with other agents, such as non-toxic botanicals like Curcumin or Silibinin. Recently, we have shown that carnosic acid (CA), a component of the food preservative Rosemary Extract, can markedly enhance the cytotoxic actions of Sf in several cell lines derived from HCC, but not in the cell line Hu1545 derived from normal hepatocytes. CA has been shown to enhance Sf-induced cell death in the neoplastic cell lines, principally due to the composite of increased apoptosis and cytotoxic autophagy. In the present study we focused on the mechanisms that underlie the reduced proliferation and survival of HCC cells when CA is added to Sf and how this relates to the increase in Sf-induced DNA damage as well as to the elevation of cytoplasmic levels of reactive oxygen species (ROS). Importantly, the elevation of ROS levels induced by Sf was increased by adding CA. We found that CA enhanced Sf-induced prolongation of cell cycle, and the overall decrease in cell growth was associated with reduced activation of both STAT3 transcription factor (TF) and extracellular signal-regulated protein kinase (Erk)1/2. Our data suggest that a regimen incorporating CA, an inexpensive and non-toxic food additive, in the treatment of advanced HCC merits clinical evaluation.

Protective Effect of Stachydrine Against Cerebral Ischemia-Reperfusion Injury by Reducing Inflammation and Apoptosis Through P65 and JAK2/STAT3 Signaling Pathway

Stachydrine, a constituent of Leonurus japonicus Houtt which also called Japanese motherwort has been shown to improve vascular microcirculation and ameliorate endothelial dysfunction. This study investigated the neuroprotective effect of stachydrine. Male Sprague-Dawley (SD) rats were randomly divided into sham, control, and stachydrine groups. The neurological deficit score was evaluated and the infarct size of the brain was measured using 2,3,5-triphenyltetra-zolium (TTC) chloride staining assay, and the pathological changes in the brain tissues were examined by HE staining. Nissl and terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick end labeling (TUNEL) staining were performed to assess the numbers of Nissl bodies and the levels of apoptosis in the neurons. The activities of superoxide dismutase (SOD) and the levels of malondialdehyde (MDA) were also measured. The release of inflammatory factors IL-1β and TNF-α were detected by Enzyme-linked immunosorbent assay (ELISA). Compared with the control group, the stachydrine group showed a significant prevention of neurological deficit, as indicated by the reduced infarct volume in the brain. Moreover, the stachydrine treatment reduced the activities of SOD, the levels of MDA and decreased the amount of IL-1β, and TNF-α, indicating that it could function to decrease the level of inflammation, thus reducing brain damage. The ischemic stroke model of PC12 cells was prepared via oxygen-glucose deprivation (OGD) protocol for 6 h. The expression of P65 and JAK2/STAT3 signaling pathway related proteins was measured by western blot. The treatment group was found to have the survival rate of PC12 cells improved and the release of inflammatory factors reduced when compared with the OGD group. This study demonstrated that stachydrine could improve nerve function by inhibiting the phosphorylation of P65/JAK2 and STAT3.

Mechanisms shaping the role of ERK1/2 in cellular senescence (Review)

Senescence is a result of cellular stress and is a potential mechanism for regulating cancer. As a member of the mitogen-activated protein kinase family, ERK1/2 (extracellular signal-regulated protein kinase) has an important role in delivering extracellular signals to the nucleus, and these signals regulate the cell cycle, cell proliferation and cell development. Previous studies demonstrated that ERK1/2 is closely associated with cell aging; however other previous studies suggested that ERK1/2 exerts an opposite effect on aging models and target proteins, even within the same cell model. Recent studies demonstrated that the effect of ERK1/2 on aging is likely associated with its target proteins and regulators, negative feedback loops, phosphorylated ERK1/2 factors and ERK1/2 translocation from the cytoplasm to the nucleus. The present review aims to examine the mechanism of ERK1/2 and discuss its role in cellular outcomes and novel drug development.

STAT3-Inducible Mouse ESCs: A Model to Study the Role of STAT3 in ESC Maintenance and Lineage Differentiation

Studies have demonstrated that STAT3 is essential in maintaining self-renewal of embryonic stem cells (ESCs) and modulates ESC differentiation. However, there is still lack of direct evidence on STAT3 functions in ESCs and embryogenesis because constitutive STAT3 knockout (KO) mouse is embryonic lethal at E6.5-E7.5, prior to potential functional role in early development can be assessed. Therefore, in this study, two inducible STAT3 ESC lines were established, including the STAT3 knockout (InSTAT3 KO) and pSTAT3 overexpressed (InSTAT3 CA) using Tet-on inducible system in which STAT3 expression can be strictly controlled by doxycycline (Dox) stimulation. Through genotyping, deletion of STAT3 alleles was detected in InSTAT3 KO ESCs following 24 hours Dox stimulation. Western blot also showed that pSTAT3 and STAT3 protein levels were significantly reduced in InSTAT3 KO ESCs while dominantly elevated in InSTAT3 CA ECSs upon Dox stimulation. Likewise, it was found that STAT3-null ESCs would affect the differentiation of ESCs into mesoderm and cardiac lineage. Taken together, the findings of this study indicated that InSTAT3 KO and InSTAT3 CA ESCs could provide a new tool to clarify the direct targets of STAT3 and its role in ESC maintenance, which will facilitate the elaboration of the mechanisms whereby STAT3 maintains ESC pluripotency and regulates ESC differentiation during mammalian embryogenesis.

Reactive oxygen species released from astrocytes treated with amyloid beta oligomers elicit neuronal calcium signals that decrease phospho-Ser727-STAT3 nuclear content

The transcription factor STAT3 has a crucial role in the development and maintenance of the nervous system. In this work, we treated astrocytes with oligomers of the amyloid beta peptide (AβOs), which display potent synaptotoxic activity, and studied the effects of mediators released by AβOs-treated astrocytes on the nuclear location of neuronal serine-727-phosphorylated STAT3 (pSerSTAT3). Treatment of mixed neuron-astrocyte cultures with 0.5µMAβOs induced in neurons a significant decrease of nuclear pSerSTAT3, but not of phosphotyrosine-705 STAT3, the other form of STAT3 phosphorylation. This decrease did not occur in astrocyte-poor neuronal cultures revealing a pivotal role for astrocytes in this response. To test if mediators released by astrocytes in response to AβOs induce pSerSTAT3 nuclear depletion, we used conditioned medium derived from AβOs-treated astrocyte cultures. Treatment of astrocyte-poor neuronal cultures with this medium caused pSerSTAT3 nuclear depletion but did not modify overall STAT3 levels. Extracellular catalase prevented the pSerSTAT3 nuclear depletion caused by astrocyte-conditioned medium, indicating that reactive oxygen species (ROS) mediate this response. This conditioned medium also increased neuronal oxidative tone, leading to a ryanodine-sensitive intracellular calcium signal that proved to be essential for pSerSTAT3 nuclear depletion. In addition, this depletion decreased BCL2 and Survivin transcription and significantly increased BAX/BCL2 ratio. This is the first description that ROS generated by AβOs-treated astrocytes and neuronal calcium signals jointly regulate pSerSTAT3 nuclear distribution in neurons. We propose that astrocytes release ROS in response to AβOs, which by increasing neuronal oxidative tone, generate calcium signals that cause pSerSTAT3 nuclear depletion and loss of STAT3 protective transcriptional activity.

Intracellular Ca2+ homeostasis and JAK1/STAT3 pathway are involved in the protective effect of propofol on BV2 microglia against hypoxia-induced inflammation and apoptosis

BACKGROUND

Perioperative hypoxia may induce microglial inflammation and apoptosis, resulting in brain injury. The neuroprotective effect of propofol against hypoxia has been reported, but the underlying mechanisms are far from clear. In this study, we explored whether and how propofol could attenuate microglia BV2 cells from CoCl2-induced hypoxic injury.

METHODS

Mouse microglia BV2 cells were pretreated with propofol, and then stimulated with CoCl2. TNF-α level in the culture medium was measured by ELISA kit. Cell apoptosis and intracellular calcium concentration were measured by flow cytometry analysis. The effect of propofol on CoCl2-modulated expression of Ca2+/Calmodulin (CaM)-dependent protein kinase II (CAMKIIα), phosphorylated CAMKIIα (pCAMKIIα), STAT3, pSTAT3Y705, pSTAT3S727, ERK1/2, pERK1/2, pNFκB(p65), pro-caspase3, cleaved caspase 3, JAK1, pJAK1, JAK2, pJAK2 were detected by Western blot.

RESULTS

In BV2 cell, CoCl2 treatment time-dependently increased TNF-α release and induced apoptosis, which were alleviated by propofol. CoCl2 (500μmol/L, 8h) treatment increased intracellular Ca2+ level, and caused the phosphorylation of CAMKIIα, ERK1/2 and NFκB (p65), as well as the activation of caspase 3. More importantly, these effects could be modulated by 25μmol/L propofol via maintaining intracellular Ca2+ homeostasis and via up-regulating the phosphorylation of JAK1 and STAT3 at Tyr705.

CONCLUSION

Propofol could protect BV2 microglia from hypoxia-induced inflammation and apoptosis. The potential mechanisms may involve the maintaining of intracellular Ca2+ homeostasis and the activation of JAK1/STAT3 pathway.

STAT3 degradation mediated by calcineurin involved in the neurotoxicity of isoflurane

Isoflurane, a commonly used volatile anesthetic, causes widespread neuronal apoptosis in the developing brain of rodents. Signal transducer and activator of transcription-3 (STAT3) signaling is crucial for cell survival during the neural network establishment period. The aim of this study was to determine whether isoflurane would target STAT3 to deliver its neurotoxicity. Mice at postnatal day 7 and primary cortical neurons cultured for 5 days were treated with isoflurane. Our data showed that isoflurane exposure downregulated the STAT3 survival pathway in the brain of mice and in primary neurons, whereas the mRNA levels of STAT3 remained unchanged after isoflurane exposure. We found that inhibiting the activity of calcineurin, which specifically promotes STAT3 degradation, alleviated isoflurane-induced neural apoptosis. Further studies showed that isoflurane increased calcineurin activity and that the inositol 1,4,5-trisphosphate-sensitive Ca(2+) channel was involved in these isoflurane-induced molecular cascades. These findings suggest that isoflurane-induced neurotoxicity may stem from STAT3 degradation, partially through the activation of calcineurin.

Impaired Focal Adhesion Kinase-Grb2 Interaction during Elevated Activity in Hippocampal Neurons

Excitatory/inhibitory imbalances are implicated in many neurological disorders. Previously, we showed that chronically elevated network activity induces vulnerability in neurons due to loss of signal transducer and activator of transcription 3 (STAT3) signaling in response to the impairment of the serine/threonine kinase, extracellular-signal-regulated kinases 1/2 (Erk1/2) activation. However, how phosphorylation of Erk1/2 decreases during elevated neuronal activity was unknown. Here I show the pErk1/2 decrease induced by 4-aminopyridine (4-AP), an A-type potassium channel inhibitor can be blocked by a broad-spectrum matrix-metalloproteinase (MMP) inhibitor, FN-439. Surface expression levels of integrin β1 dramatically decrease when neurons are challenged by chronically elevated activity, which is reversed by FN-439. Treatment with 4-AP induces degradation of focal adhesion kinase (FAK), the mediator of integrin signaling. As a result, interactions between FAK and growth factor receptor-bound protein 2 (Grb2), the adaptor protein that mediates Erk1/2 activation by integrin, are severely impaired. Together, these data suggest the loss of integrin signaling during elevated activity causes vulnerability in neurons.

Hippocampal volume and depressive symptoms are linked to serum IL-18 in schizophrenia

Objective:

Since schizophrenia (SCZ) is often accompanied by hippocampal abnormalities and dysregulation of cytokine production, this study aimed to investigate the impact of the cytokine interleukin (IL)-18, whose biological system appears to be perturbed in SCZ, on brain structure and clinical severity in patients with chronic SCZ.

Methods:

The serum levels of IL-18, including its free bioactive form (i.e., the cytokine fraction not bound to its specific endogenous inhibitor IL-18 binding protein), were evaluated in a case-control study involving 71 individuals with SCZ diagnosis and 29 healthy controls. All participants underwent brain MRI automatic evaluation for hippocampal volume estimation. The Positive and Negative Syndrome Scale (PANSS) was administered to measure severity of symptoms in patients with SCZ.

Results:

Lower amounts of free IL-18 were related to smaller hippocampal volume measures in patients with SCZ. Furthermore, in line with a possible neuroprotective effect of the cytokine, higher levels of free IL-18 corresponded to lower subscores of PANSS depression in patients with SCZ.

Conclusions:

These findings demonstrate that the levels of circulating bioactive IL-18 are related to both hippocampal volume and severity of psychopathologic symptoms in patients with SCZ, confirming the involvement of the cytokine in SCZ pathophysiology and suggesting hippocampal-dependent and neuroprotective functions of IL-18 in this clinical context.

Interleukin 18 activates MAPKs and STAT3 but not NF-κB in hippocampal HT-22 cells

Interleukin (IL)-18 is a cytokine previously demonstrated to participate in neuroinflammatory processes. Since the components of the IL-18 receptor complex are expressed in neurons throughout the brain, IL-18 is also believed to directly influence neuronal function. Here we tested this hypothesis on mouse hippocampal neurons by measuring the effects of IL-18 on three pathways previously shown to be regulated by this cytokine in non-neuronal cells: the MAPK pathways, p38 and ERK1/2 MAPKs, STAT3 and NF-κB. Experiments were carried out in vitro using the immortalized hippocampal neuronal line HT-22 or in vivo following i.c.v. injection with recombinant mouse IL-18. We showed that IL-18 did not activate NF-κB in HT-22 cells whereas it induced a rapid (within 15min) activation of the MAPK pathways. Moreover, we demonstrated that IL-18 treatment enhanced P-STAT3 (Tyr705)/STAT3 ratio in the nucleus of HT-22 cells after 30-60min of exposure. A similar increase in P-STAT3 (Tyr705)/STAT3 ratio was observed in the whole hippocampus one hour after i.c.v. injection. These data demonstrate that IL-18 can act directly on neuronal cells affecting the STAT3 pathway; therefore, possibly regulating the expression of specific genes within the hippocampus. This effect may help to explain some of the IL-18-induced effects on synaptic plasticity and functionality within the hippocampal system.

Neuronal activity-dependent STAT3 localization to nucleus is dependent on Tyr-705 and Ser-727 phosphorylation in rat hippocampal neurons

Signal transducer and activator of transcription 3 (STAT3) dramatically increases during the first post-natal week, and supports the survival of mature hippocampal neurons. Recently, we reported that chronic elevation of excitability leads to a loss of STAT3 signal, inducing vulnerability in neurons. The loss of STAT3 signal was due to impaired Erk1/2 activation. While overnight elevation of activity attenuated STAT3 signal, brief low-frequency stimuli, which induce long-term depression, have been shown to activate STAT3. Here we investigated how STAT3 responds to depolarization in mature neurons. A brief depolarization results in the transient activation of STAT3: it induces calcium influx through L-type voltage-gated calcium channels, which triggers activation of Src family kinases. Src family kinases are required for phosphorylation of STAT3 at Tyr-705 and Ser-727. PTyr-705 is Janus kinase (JAK)-dependent, while PSer-727 is dependent on Akt, the Ser/Thr kinase. Both PTyr-705 and PSer-727 are necessary for nuclear translocation of STAT3 in these neurons. Chronic elevation of spontaneous activity by an A-type potassium blocker, 4-aminopyridine (4-AP), also induced the transient phosphorylation of STAT3, which after 4 h fell to basal levels despite the presence of 4-AP. These results suggest that phasic and chronic neuronal activation induce distinct molecular pathways, resulting in opposing regulation of STAT3 signal.

A new model for developmental neuronal death and excitatory/inhibitory balance in hippocampus

The nervous system develops through a program that produces neurons in excess and then eliminates approximately half during a period of naturally occurring death. Neuronal activity has been shown to promote the survival of neurons during this period by stimulating the production and release of neurotrophins. In the peripheral nervous system (PNS), neurons depends on neurotrophins that activate survival pathways, which explains how the size of target cells influences number of neurons that innervate them (neurotrophin hypothesis). However, in the central nervous system (CNS), the role of neurotrophins has not been clear. Contrary to the neurotrophin hypothesis, a recent study shows that, in neonatal hippocampus, neurotrophins cannot promote survival without spontaneous network activity: Neurotrophins recruit neurons into spontaneously active networks, and this activity determines which neurons survive. By placing neurotrophin upstream of activity in the survival signaling pathway, these new results change our understanding of how neurotrophins promote survival. Spontaneous, synchronized network activity begins to spread through both principle neurons and interneurons in the hippocampus as they enter the death period. At this stage, neurotransmission mediated by γ-aminobutyric acid (GABA) is excitatory and drives the spontaneous activity. An important recent observation is that neurotrophins preferentially recruit GABAergic neurons into spontaneously active networks; thus, neurotrophins select for survival only those neurons joined to active networks with strong GABAergic inputs, which would later become inhibitory. A proper excitatory/inhibitory (E/I) balance is critical for normal adult brain function. This balance may be especially important in the hippocampus where impairments in E/I balance are associated with pathologies including epilepsy. Here, I discuss the molecular mechanisms for survival in neonatal neurons, how these mechanisms change during development, and how they may be linked to degenerative diseases.

Early methyl donor deficiency may induce persistent brain defects by reducing Stat3 signaling targeted by miR-124

The methyl donors folate (vitamin B9) and vitamin B12 are centrepieces of the one-carbon metabolism that has a key role in transmethylation reactions, and thus in epigenetic and epigenomic regulations. Low dietary intakes of folate and vitamin B12 are frequent, especially in pregnant women and in the elderly, and deficiency constitutes a risk factor for various diseases, including neurological and developmental disorders. In this respect, both vitamins are essential for normal brain development, and have a role in neuroplasticity and in the maintenance of neuronal integrity. The consequences of a methyl donor deficiency (MDD) were studied both in vivo in rats exposed in utero, and in vitro in hippocampal progenitors (H19-7 cell line). Deficiency was associated with growth retardation at embryonic day 20 (E20) and postnatally with long-term brain defects in selective areas. mRNA and protein levels of the transcription factor Stat3 were found to be decreased in the brains of deprived fetuses and in differentiating progenitors (62 and 48% for total Stat3 protein, respectively), along with a strong reduction in its phosphorylation at both Tyr⁷⁰⁵ and Ser⁷²⁷ residues. Vitamin shortage also affected upstream kinases of Stat3 signaling pathway (phospho-Erk1/2, phospho-Src, phospho-JNK, and phospho-p38) as well as downstream target gene products (Bcl-2 and Bcl-xL), thus promoting apoptosis. Conversely, the expression of the Stat3 regulator miR-124 was upregulated in deficiency conditions (≥65%), and its silencing by using siRNA partly restored Stat3 signaling in hippocampal neurons by increasing specifically the phosphorylation of Erk1/2 and Src kinases. Furthermore, miR-124 siRNA improved the phenotype of deprived cells, with enhanced neurite outgrowth. Taken together, our data suggest that downregulation of Stat3 signaling by miR-124 would be a key factor in the deleterious effects of MDD on brain development.

Signal transducer and activator of transcription 3 (STAT3) degradation by proteasome controls a developmental switch in neurotrophin dependence

Neonatal brains develop through a program that eliminates about half of the neurons. During this period, neurons depend on neurotrophins for their survival. Recently, we reported that, at the conclusion of the naturally occurring death period, neurons become neurotrophin-independent and, further, that this developmental switch is achieved by the emergence of a second survival pathway mediated by signal transducer and activator of transcription 3 (STAT3). Here I show that calcineurin plays a key role in controlling the developmental switch in mouse hippocampal neurons. Calcineurin promotes the degradation of STAT3 via the ubiquitin-proteasome pathway. Inhibition of calcineurin acutely increases total levels of STAT3 as well as its activated forms, resulting in decreased levels of the tumor suppressor p53 and its proapoptotic target, Bax. In vivo and in vitro, calcineurin regulates levels of STAT3 and neurotrophin dependence. TMF/ARA 160 (TATA element modulatory factor/androgen receptor co-activator 160), the key mediator of STAT3 ubiquitination, is required for calcineurin-dependent STAT3 degradation. Thus, these results show that the ubiquitin-proteasome pathway controls the critical developmental switch of neurotrophin dependence in the newborn hippocampus.