Phosphatidylinositol 3-kinase signaling is involved in neurogenesis during Xenopus embryonic development

Locational and functional characterization of PI4KB in the mouse embryo

Phosphatidylinositol 4-kinase beta (PI4KB) is a member of the PI4K family, which is mainly enriched and functions in the Golgi apparatus. The kinase domain of PI4KB catalyzes the phosphorylation of phosphatidylinositol to form phosphatidylinositol 4-phosphate, a process that regulates various sub-cellular events, such as non-vesicular cholesterol and ceramide transport, protein glycosylation, and vesicle transport, as well as cytoplasmic division. In this study, a strain of PI4KB knockout mouse, immunofluorescence, reverse transcription polymerase chain reaction and microinjection were used to characterize the cytological location and biological function of PI4KB in the mouse embryos. we found that knocking down Pi4kb in mouse embryos resulted in embryonic lethality at around embryonic day (E) 7.5. Additionally, we observed dramatic fluctuations in PI4KB expression during the development of preimplantation embryos, with high expression in the 4-cell and morula stages. PI4KB colocalized with the Golgi marker protein TGN46 in the perinuclear and cytoplasmic regions in early blastomeres. Postimplantation, PI4KB was highly expressed in the epiblast of E7.5 embryos. Treatment of embryos with PI4KB inhibitors was found to inhibit the development of the morula into a blastocyst and the normal progression of cytoplasmic division during the formation of a 4-cell embryo. These findings suggest that PI4KB plays an important role in mouse embryogenesis by regulating various intracellular vital functions of embryonic cells.

OTX Genes in Adult Tissues

OTX homeobox genes have been extensively studied for their role in development, especially in neuroectoderm formation. Recently, their expression has also been reported in adult physiological and pathological tissues, including retina, mammary and pituitary glands, sinonasal mucosa, in several types of cancer, and in response to inflammatory, ischemic, and hypoxic stimuli. Reactivation of OTX genes in adult tissues supports the notion of the evolutionary amplification of functions of genes by varying their temporal expression, with the selection of homeobox genes from the "toolbox" to drive or contribute to different processes at different stages of life. OTX involvement in pathologies points toward these genes as potential diagnostic and/or prognostic markers as well as possible therapeutic targets.

AKT signaling displays multifaceted functions in neural crest development

AKT signaling is an essential intracellular pathway controlling cell homeostasis, cell proliferation and survival, as well as cell migration and differentiation in adults. Alterations impacting the AKT pathway are involved in many pathological conditions in human disease. Similarly, during development, multiple transmembrane molecules, such as FGF receptors, PDGF receptors or integrins, activate AKT to control embryonic cell proliferation, migration, differentiation, and also cell fate decisions. While many studies in mouse embryos have clearly implicated AKT signaling in the differentiation of several neural crest derivatives, information on AKT functions during the earliest steps of neural crest development had remained relatively scarce until recently. However, recent studies on known and novel regulators of AKT signaling demonstrate that this pathway plays critical roles throughout the development of neural crest progenitors. Non-mammalian models such as fish and frog embryos have been instrumental to our understanding of AKT functions in neural crest development, both in neural crest progenitors and in the neighboring tissues. This review combines current knowledge acquired from all these different vertebrate animal models to describe the various roles of AKT signaling related to neural crest development in vivo. We first describe the importance of AKT signaling in patterning the tissues involved in neural crest induction, namely the dorsal mesoderm and the ectoderm. We then focus on AKT signaling functions in neural crest migration and differentiation.

Oligo-Porphyran Ameliorates Neurobehavioral Deficits in Parkinsonian Mice by Regulating the PI3K/Akt/Bcl-2 Pathway

Parkinson’s disease (PD) is a neurodegenerative movement disorder that is caused by a selective loss of dopaminergic neurons. Current PD treatments provide symptomatic relief but do not prevent or decelerate disease progression. Previous studies have suggested that acetylated and phosphorylated porphyran, derived from Porphyra, produces a neuroprotective effect against 6-OHDA-induced damage. Due to its antioxidant and neuroprotective potential, this study evaluates whether oligo-porphyran (OP) could be beneficial in an experimental model of PD in mice. The drug 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was intraperitoneally injected (20 mg/kg body weight) for seven days to simulate PD, followed by OP administration. We found that the behavioral deficits in spontaneous motor activity, latency to descend in a pole test, and suspension in a traction test were ameliorated, and excessive dopamine (DA) metabolism was suppressed after OP treatment. Additionally, we found that OP protected dopaminergic neurons by preventing MPTP-induced decreases in dopaminergic transporter and tyrosine hydroxylase protein levels. We speculated whether OP regulates a signaling pathway that affects the behavioral changes seen in PD mice. In this study, the PI3K/Akt/Bcl-2 pathway was detected. Our results demonstrate that OP increased the phosphorylation of PI3K/Akt/GSK-3β and inhibited the activation of caspase-3 and poly (ADP-ribose) polymerase, with changes in the Bax/Bcl-2 ratio. These results showed that OP might promote DA neuron survival in vivo by regulating the PI3K/Akt/Bcl-2 pathway, thereby ameliorating the neurobehavioral deficits in a PD mouse model and suggesting OP as a neuroprotective treatment for PD.

FGF mediated MAPK and PI3K/Akt Signals make distinct contributions to pluripotency and the establishment of Neural Crest

Early vertebrate embryos possess cells with the potential to generate all embryonic cell types. While this pluripotency is progressively lost as cells become lineage restricted, Neural Crest cells retain broad developmental potential. Here, we provide novel insights into signals essential for both pluripotency and neural crest formation in Xenopus. We show that FGF signaling controls a subset of genes expressed by pluripotent blastula cells, and find a striking switch in the signaling cascades activated by FGF signaling as cells lose pluripotency and commence lineage restriction. Pluripotent cells display and require Map Kinase signaling, whereas PI3 Kinase/Akt signals increase as developmental potential is restricted, and are required for transit to certain lineage restricted states. Importantly, retaining a high Map Kinase/low Akt signaling profile is essential for establishing Neural Crest stem cells. These findings shed important light on the signal-mediated control of pluripotency and the molecular mechanisms governing genesis of Neural Crest.

PIWI-like protein, HIWI2 is aberrantly expressed in retinoblastoma cells and affects cell-cycle potentially through OTX2

Retinoblastoma (RB), a childhood cancer, is caused by biallelic mutation of the RB1 gene, but its development is not clearly understood. Furthermore, the presence of a cancer stem cell subpopulation in RB might impact its treatment. PIWI protein, known for its role in stem cell self-renewal, is aberrantly expressed in cancers. We examined the role of the PIWI-like protein HIWI2 in RB and its effect on the stem cell markers in cells of the RB line, Y79. The expression of HIWI2 is significantly increased in Y79 compared with its level in HeLa and ARPE19 cells. The stem cell markers Oct-3/4, Nanog and Sox-2 were not altered upon HIWI2 knockdown in Y79 cells. Interestingly, OTX2 was significantly downregulated in the absence of HIWI2. Otx2 transcripts also decreased in HIWI2-silenced Y79 and ARPE19 cells. Moreover, silencing HIWI2 in Y79 accumulated the cells at G2–M phase and reduced the levels of proliferating cell nuclear antigen (PCNA) and the tumor suppressor, p16. Our results demonstrate that HIWI2 is aberrantly expressed in Y79 cells and silencing of HIWI2 downregulates OTX2, suggesting that HIWI2 might play a role in the progression of RB.

Developmental Dynamics of Rett Syndrome

Rett Syndrome was long considered to be simply a disorder of postnatal development, with phenotypes that manifest only late in development and into adulthood. A variety of recent evidence demonstrates that the phenotypes of Rett Syndrome are present at the earliest stages of brain development, including developmental stages that define neurogenesis, migration, and patterning in addition to stages of synaptic and circuit development and plasticity. These phenotypes arise from the pleotropic effects of MeCP2, which is expressed very early in neuronal progenitors and continues to be expressed into adulthood. The effects of MeCP2 are mediated by diverse signaling, transcriptional, and epigenetic mechanisms. Attempts to reverse the effects of Rett Syndrome need to take into account the developmental dynamics and temporal impact of MeCP2 loss.

Regeneration in the nervous system with erythropoietin

Globally, greater than 30 million individuals are afflicted with disorders of the nervous system accompanied by tens of thousands of new cases annually with limited, if any, treatment options. Erythropoietin (EPO) offers an exciting and novel therapeutic strategy to address both acute and chronic neurodegenerative disorders. EPO governs a number of critical protective and regenerative mechanisms that can impact apoptotic and autophagic programmed cell death pathways through protein kinase B (Akt), sirtuins, mammalian forkhead transcription factors, and wingless signaling. Translation of the cytoprotective pathways of EPO into clinically effective treatments for some neurodegenerative disorders has been promising, but additional work is necessary. In particular, development of new treatments with erythropoiesis-stimulating agents such as EPO brings several important challenges that involve detrimental vascular outcomes and tumorigenesis. Future work that can effectively and safely harness the complexity of the signaling pathways of EPO will be vital for the fruitful treatment of disorders of the nervous system.

Ethosuximide Induces Hippocampal Neurogenesis and Reverses Cognitive Deficits in an Amyloid-β Toxin-induced Alzheimer Rat Model via the Phosphatidylinositol 3-Kinase (PI3K)/Akt/Wnt/β-Catenin Pathway

Neurogenesis involves generation of new neurons through finely tuned multistep processes, such as neural stem cell (NSC) proliferation, migration, differentiation, and integration into existing neuronal circuitry in the dentate gyrus of the hippocampus and subventricular zone. Adult hippocampal neurogenesis is involved in cognitive functions and altered in various neurodegenerative disorders, including Alzheimer disease (AD). Ethosuximide (ETH), an anticonvulsant drug is used for the treatment of epileptic seizures. However, the effects of ETH on adult hippocampal neurogenesis and the underlying cellular and molecular mechanism(s) are yet unexplored. Herein, we studied the effects of ETH on rat multipotent NSC proliferation and neuronal differentiation and adult hippocampal neurogenesis in an amyloid β (Aβ) toxin-induced rat model of AD-like phenotypes. ETH potently induced NSC proliferation and neuronal differentiation in the hippocampus-derived NSC in vitro. ETH enhanced NSC proliferation and neuronal differentiation and reduced Aβ toxin-mediated toxicity and neurodegeneration, leading to behavioral recovery in the rat AD model. ETH inhibited Aβ-mediated suppression of neurogenic and Akt/Wnt/β-catenin pathway gene expression in the hippocampus. ETH activated the PI3K·Akt and Wnt·β-catenin transduction pathways that are known to be involved in the regulation of neurogenesis. Inhibition of the PI3K·Akt and Wnt·β-catenin pathways effectively blocked the mitogenic and neurogenic effects of ETH. In silico molecular target prediction docking studies suggest that ETH interacts with Akt, Dkk-1, and GSK-3β. Our findings suggest that ETH stimulates NSC proliferation and differentiation in vitro and adult hippocampal neurogenesis via the PI3K·Akt and Wnt·β-catenin signaling.

Insulin-like factor regulates neural induction through an IGF1 receptor-independent mechanism

Insulin receptor (IR) and insulin-like growth factor-1 receptor (IGF1R) signalling is required for normal embryonic growth and development. Previous reports indicated that the IGF/IGF1R/MAPK pathway contributes to neural induction and the IGF/IGF1R/PI3K/Akt pathway to eye development. Here, we report the isolation of insulin3 encoding a novel insulin-like ligand involved in neural induction. Insulin3 has a similar structure to pro-insulin and mature IGF ligands, but cannot activate the IGF1 receptor. However, similar to IGFs, Insulin3 induced the gene expression of an anterior neural marker, otx2, and enlarged anterior head structures by inhibiting Wnt signalling. Insulin3 are predominantly localised to the endoplasmic reticulum when otx2 is induced by insulin3. Insulin3 reduced extracellular Wnts and cell surface localised Lrp6. These results suggest that Insulin3 is a novel cell-autonomous inhibitor of Wnt signalling. This study provides the first evidence that an insulin-like factor regulates neural induction through an IGF1R-independent mechanism.

PFKFB4 controls embryonic patterning via Akt signalling independently of glycolysis

How metabolism regulators play roles during early development remains elusive. Here we show that PFKFB4 (6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 4), a glycolysis regulator, is critical for controlling dorsal ectoderm global patterning in gastrulating frog embryos via a non-glycolytic function. PFKFB4 is required for dorsal ectoderm progenitors to proceed towards more specified fates including neural and non-neural ectoderm, neural crest or placodes. This function is mediated by Akt signalling, a major pathway that integrates cell homeostasis and survival parameters. Restoring Akt signalling rescues the loss of PFKFB4 in vivo. In contrast, glycolysis is not essential for frog development at this stage. Our study reveals the existence of a PFKFB4-Akt checkpoint that links cell homeostasis to the ability of progenitor cells to undergo differentiation, and uncovers glycolysis-independent functions of PFKFB4.

IGF-1 promotes Brn-4 expression and neuronal differentiation of neural stem cells via the PI3K/Akt pathway

Our previous studies indicated that transcription factor Brn-4 is upregulated in the surgically denervated hippocampus in vivo, promoting neuronal differentiation of hippocampal neural stem cells (NSCs) in vitro. The molecules mediating Brn-4 upregulation in the denervated hippocampus remain unknown. In this study we examined the levels of insulin-like growth factor-1 (IGF-1) in hippocampus following denervation. Surgical denervation led to a significant increase in IGF-1 expression in vivo. We also report that IGF-1 treatment on NSCs in vitro led to a marked acceleration of Brn-4 expression and cell differentiation down neuronal pathways. The promotion effects were blocked by PI3K-specific inhibitor (LY294002), but not MAPK inhibitor (PD98059); levels of phospho-Akt were increased by IGF-1 treatment. In addition, inhibition of IGF-1 receptor (AG1024) and mTOR (rapamycin) both attenuated the increased expression of Brn-4 induced by IGF-1. Together, the results demonstrated that upregulation of IGF-1 induced by hippocampal denervation injury leads to activation of the PI3K/Akt signaling pathway, which in turn gives rise to upregulation of the Brn-4 and subsequent stem cell differentiation down neuronal pathways.

Akt1 mediates neuronal differentiation in zebrafish via a reciprocal interaction with notch signaling

Akt1 is well known for its role in regulating cell proliferation, differentiation, and apoptosis and is implicated in tumors and several neurological disorders. However, the role of Akt1 in neural development has not been well defined. We have isolated zebrafish akt1 and shown that this gene is primarily transcribed in the developing nervous system, and its spatiotemporal expression pattern suggests a role in neural differentiation. Injection of akt1 morpholinos resulted in loss of neuronal precursors with a concomitant increase in post-mitotic neurons, indicating that knockdown of Akt1 is sufficient to cause premature differentiation of neurons. A similar phenotype was observed in embryos deficient for Notch signaling. Both the ligand (deltaA) and the downstream target of Notch (her8a) were downregulated in akt1 morphants, indicating that Akt1 is required for Delta-Notch signaling. Furthermore, akt1 expression was downregulated in Delta-Notch signaling-deficient embryos and could be induced by constitutive activation of Notch signaling. In addition, knockdown of Akt1 was able to nullify the inhibition of neuronal differentiation caused by constitutive activation of Notch signaling. Taken together, these results provide in vivo evidence that Akt1 interacts with Notch signaling reciprocally and provide an explanation of why Akt1 is essential for the inhibition of neuronal differentiation.

Neuroprotective and Anti-Inflammatory Activities of Atorvastatin in a Rat Chronic Constriction Injury Model

Atorvastatin is an HMG-CoA reductase inhibitor used to treat hypercholesterolemic conditions associated with hypertension. This study aims to investigate the anti-inflammatory and neuroprotective effects of atorvastatin on peripheral neuropathic pain. Peripheral neuropathic pain was induced by chronic constriction injury (CCI) in Sprague-Dawley rats. Rats were divided into 3 groups including sham-operated, CCI, and atorvastatin-treated. Atorvastatin (10 mg/kg) or phosphate-buffered saline was orally administered for 2 weeks. All animals were assessed by neurobehavioral tests before surgery and at days 3, 7, 14 after surgery. Inflammatory and neuroprotective factors were evaluated by Western blot analysis. eNOS, COX2 and iNOS in the sciatic nerve were also studied using immunohistochemistry. Atorvastatin attenuated CCI-induced nociceptive sensitization and thermal hyperalgesia in a time-dependent manner. Atorvastatin improved CCI-induced neurobehavioral/inflammatory activity by inhibition of TGF-β, PIκB/IκB, NFκB, COX2, iNOS, EP1 and EP4 in the sciatic nerve. Atorvastatin was also found to increase neuroprotection factors pAkt/Akt, eNOS and VEGF. Taken together, these data indicate that atorvastatin could protect the sciatic nerve against CCI-induced neuroinflammation and nociception.

Pten coordinates retinal neurogenesis by regulating Notch signalling

Development of nervous tissue is a coordinated process of neural progenitor cell (NPC) proliferation and neuronal differentiation. Intracellular signalling events that regulate the balance between NPC proliferation and neuronal differentiation, therefore, determine the size and composition of nervous tissues. Here, we demonstrate that negative regulation of phosphoinosite 3-kinase (PI3K)-Akt signalling by phosphatase tensin homologue (Pten) is essential for maintaining NPC population in mouse retina. We found that mouse retinal progenitor cells (RPCs) lacking the Pten gene complete neurogenesis earlier than their normal developmental schedule, resulting in their premature depletion in the mature retina. We further discover that Notch intracellular domain (NICD) fails to form transcription activator complex in Pten-deficient RPCs, and thereby unable to support RPC maintenance. Taken together, our results suggest that Pten plays a pivotal role in retinal neurogenesis by supporting Notch-driven RPC maintenance against neurogenic PI3K-Akt signalling.

Role of the PI3K-TOR-S6K pathway in the onset of cell cycle elongation during Xenopus early embryogenesis

In the early embryogenesis of the frog, Xenopus laevis, cells proliferate by rapid and synchronous divisions, followed by cell cycle elongation and prolongation of the S phases, and then the appearance of the G2 and G1 phases after the midblastula transition (MBT). The beginning of cell cycle elongation was thought to depend on an increase in the nucleo-cytoplasmic (N/C) ratio in blastomeres and a decrease in cortical cytoplasmic factors necessary for cell cycle progression, although these factors are unknown. In the present study, we demonstrated that a regulatory subunit of PI3K (p85α) was localized in the cortical cytoplasm of the blastomere during the MBT. When the embryos were treated with a PI3K inhibitor, LY294002, or a TOR inhibitor, rapamycin, cell cycle elongation was initiated before the MBT. In addition, the inhibition of S6K expression by antisense morpholino oligo enhanced the initiation of cell cycle elongation. In contrast, the activation of PI3K-TOR by Rheb-S16H expression delayed the initiation of cell cycle elongation. These results indicate that a decrease in translational activity dependent on the PI3K-TOR-S6K pathway causes the initiation of cell cycle elongation at the onset of the MBT.

Phosphoinositide 3-kinase enhancer regulates neuronal dendritogenesis and survival in neocortex

Phosphoinositide 3-kinase enhancer (PIKE) binds and enhances phosphatidylinositol 3-kinase (PI3K)/Akt activities. However, its physiological functions in brain have never been explored. Here we show that PIKE is important in regulating the neuronal survival and development of neocortex. During development, enhanced apoptosis is observed in the ventricular zone of PIKE knock-out (PIKE(-/-)) cortex. Moreover, PIKE(-/-) neurons show reduced dendritic complexity, dendritic branch length, and soma size. These defects are due to the reduced PI3K/Akt activities in PIKE(-/-) neurons, as the impaired dendritic arborization can be rescued when PI3K/Akt cascade is augmented in vitro or in PIKE(-/-)PTEN(-/-) double-knock-out mice. Interestingly, PIKE(-/-) mice display behavioral abnormality in locomotion and spatial navigation. Because of the diminished PI3K/Akt activities, PIKE(-/-) neurons are more vulnerable to glutamate- or stroke-induced neuronal cell death. Together, our data established the critical role of PIKE in regulating neuronal survival and development by substantiating the PI3K/Akt pathway.

Asymmetric PI(3,4,5)P3 and Akt signaling mediates chemotaxis of axonal growth cones

The action of many extracellular guidance cues on axon pathfinding requires Ca2+ influx at the growth cone (Hong et al., 2000; Nishiyama et al., 2003; Henley and Poo, 2004), but how activation of guidance cue receptors leads to opening of plasmalemmal ion channels remains largely unknown. Analogous to the chemotaxis of amoeboid cells (Parent et al., 1998; Servant et al., 2000), we found that a gradient of chemoattractant triggered rapid asymmetric PI(3,4,5)P3 accumulation at the growth cone's leading edge, as detected by the translocation of a GFP-tagged binding domain of Akt in Xenopus laevis spinal neurons. Growth cone chemoattraction required PI(3,4,5)P3 production and Akt activation, and genetic perturbation of polarized Akt activity disrupted axon pathfinding in vitro and in vivo. Furthermore, patch-clamp recording from growth cones revealed that exogenous PI(3,4,5)P3 rapidly activated TRP (transient receptor potential) channels, and asymmetrically applied PI(3,4,5)P3 was sufficient to induce chemoattractive growth cone turning in a manner that required downstream Ca2+ signaling. Thus, asymmetric PI(3,4,5)P3 elevation and Akt activation are early events in growth cone chemotaxis that link receptor activation to TRP channel opening and Ca2+ signaling. Altogether, our findings reveal that PI(3,4,5)P3 elevation polarizes to the growth cone's leading edge and can serve as an early regulator during chemotactic guidance.

Identification of protein domains required for makorin-2-mediated neurogenesis inhibition in Xenopus embryos

Makorin-2, consisting of four highly conserved C(3)H zinc fingers, a Cys-His motif and a C(3)HC(4) RING zinc finger domain, is a putative ribonucleoprotein. We have previously reported that Xenopus makorin-2 (mkrn2) is a neurogenesis inhibitor acting upstream of glycogen synthase kinase-3beta (GSK-3beta) in the phosphatidylinositol 3-kinase/Akt pathway. In an effort to identify the functional domains required for its anti-neurogenic activity, we designed and constructed a series of N- and C-terminal truncation mutants of mkrn2. Concurred with the full-length mkrn2, we showed that overexpression of one of the truncation mutants mkrn2(s)-7, which consists of only the third C(3)H zinc finger, Cys-His motif and C(3)HC(4) RING zinc finger, is essential and sufficient to produce the phenotypical dorso-posterior deficiencies and small-head/short-tail phenotype in tadpoles. In animal cap explant assay, we further demonstrated that mkrn2(s)-7 not only inhibits activin and retinoic acid-induced animal cap neuralization and the expression of a pan-neural marker neural cell adhesion molecule, but also induces GSK-3beta expression. These results collectively suggest that the third C(3)H zinc finger, Cys-His motif and C(3)HC(4) RING zinc finger are indispensable for the anti-neurogenic activity of mkrn2.

Evidence that phosphatidylinositol 3-kinase is involved in sperm-induced tyrosine kinase signaling in Xenopus egg fertilization

BACKGROUND

Studies have examined the function of PI 3-kinase in the early developmental processes that operate in oocytes or early embryos of various species. However, the roles of egg-associated PI 3-kinase and Akt, especially in signal transduction at fertilization, are not well understood.

RESULTS

Here we show that in Xenopus eggs, a potent inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase), LY294002 inhibits sperm-induced activation of the tyrosine kinase Src and a transient increase in the intracellular concentration of Ca2+ at fertilization. LY294002 also inhibits sperm-induced dephosphorylation of mitogen-activated protein kinase, breakdown of cyclin B2 and Mos, and first embryonic cleavage, all of which are events of Ca2+-dependent egg activation. In fertilized eggs, an 85-kDa subunit of PI 3-kinase (p85) undergoes a transient translocation to the low-density, detergent-insoluble membranes (membrane microdomains) where Src tyrosine kinase signaling is operating. However, the tyrosine phosphorylation of p85 in fertilized eggs is not as evident as that in H2O2-activated eggs, arguing against the possibility that PI 3-kinase is activated by Src phosphorylation. Nevertheless, sperm-induced activation of PI 3-kinase has been demonstrated by the finding that Akt, a serine/threonine-specific protein kinase, is phosphorylated at threonine-308. The threonine-phosphorylated Akt also localizes to the membrane microdomains of fertilized eggs. Application of bp(V), an inhibitor of PTEN that dephosphorylates PIP3, the enzymatic product of PI 3-kinase, promotes parthenogenetic activation of Xenopus eggs. In vitro kinase assays demonstrate that PIP3 activates Src in a dose-dependent manner.

CONCLUSIONS

These results suggest that PI 3-kinase is involved in sperm-induced egg activation via production of PIP3 that would act as a positive regulator of the Src signaling pathway in Xenopus fertilization.

Crucial role of phosphatidylinositol 4-kinase IIIalpha in development of zebrafish pectoral fin is linked to phosphoinositide 3-kinase and FGF signaling

Phosphatidylinositol 4-kinases (PI4Ks) catalyze the first committed step in the synthesis of phosphoinositides, important lipid regulators of signaling and trafficking pathways. Here we cloned Pik4a, one of the zebrafish PI4K enzymes, and studied its role(s) in vertebrate development using morpholino oligonucleotide-based gene silencing in zebrafish. Downregulation of Pik4a led to multiple developmental abnormalities, affecting the brain, heart, trunk and most prominently causing loss of pectoral fins. Strikingly similar defects were caused by treatment of the developing embryos with the phosphoinositide 3-kinase (PI3K) inhibitor, LY294002. To investigate the cause of the pectoral fin developmental defect, we focused on fibroblast growth factor (FGF) signaling pathways because vertebrate limb development requires the concerted action of a series of FGF ligands. Using in situ hybridization, the pectoral fin defect was traced to disruption of the early FGF signaling loops that are crucial for the establishment of the sharp signaling center formed by the apical ectodermal ridge and the underlying mesenchyme. This, in turn caused a prominent loss of the induction of one of the mitogen-activated protein kinase (MAPK) phosphatases, Mkp3, an essential intermediate in vertebrate limb development. These changes were associated with impaired proliferation in the developing fin bud due to a loss of balance between the MAPK and PI3K branch of FGF-initiated signals. Our results identify Pik4a as an upstream partner of PI3Ks in the signaling cascade orchestrated by FGF receptors with a prominent role in forelimb development.

Intranasal administration of IGF-1 attenuates hypoxic-ischemic brain injury in neonatal rats

To determine whether intranasal administration (iN) of recombinant human insulin-like growth factor-1 (rhIGF-1) provides neuroprotection to the neonatal rat brain following cerebral hypoxia-ischemia (HI), two doses of rhIGF-1 (50 microg at a 1 h interval) were infused into the right naris of postnatal day 7 (P7) rat pups with or without a prior HI insult (right common carotid artery ligation, followed by an exposure to 8% oxygen for 2 h). Our result showed that rhIGF-1 administered via iN was successfully delivered into the brain 30 min after the second dose. In the following studies rhIGF-1 was administered to P7 rat pups at 0, 1 or 2 h after HI at the dose described above. Pups in the control group received cerebral HI and vehicle treatment. Pups that underwent sham operation and vehicle treatment served as the sham group. Brain pathological changes were evaluated 2 and 15 days after HI. Our results showed that rhIGF-1 treatment up to 1 h after cerebral HI effectively reduced brain injury as compared to that in the vehicle-treated rats. Moreover, rhIGF-1 treatment improved neurobehavioral performance (tested on P5-P21) in juvenile rats subjected to HI. Our results further showed that rhIGF-1 inhibited apoptotic cell death, possibly through activating the Akt signal transduction pathway. rhIGF-1 enhanced proliferation of neuronal and oligodendroglial progenitors after cerebral HI as well. These data suggest that iN administration of IGF-1 has the potential to be used for clinical treatment.

Cell biology and clinical promise of G-CSF: immunomodulation and neuroprotection

In the light of the enthusiasm to use of recombinant human granulocyte colony-stimulating factor (G-CSF) for immunomodulation and neuroprotection, it should be remembered that the current knowledge is based on a century of laborious research. G-CSF is a pleiotropic cytokine playing a major role as regulator of haematopoiesis. Although the precise mechanisms of G-CSF are not known, there is growing evidence supporting the notion that G-CSF also exerts profound immunoregulatory effect in adaptive immunity and has a neuroprotective role in both cerebral ischemia and neurodegeneration. Here, we describe the immunomodulation and the neuroprotection that can be achieved with G-CSF, and summarize possible mechanisms of G-CSF as a potential therapeutic agent in autoimmune diseases and neurological disorders. Our understanding of these novel sites of action of G-CSF has opened therapeutic avenues for the treatment of autoimmune diseases and neurological disorders, and has translated the beneficial effects of G-CSF from basic experiments to clinical patients.

Simvastatin-mediated upregulation of VEGF and BDNF, activation of the PI3K/Akt pathway, and increase of neurogenesis are associated with therapeutic improvement after traumatic brain injury

This study was undertaken to evaluate the effect of simvastatin, a cholesterol-lowering agent, on the Akt-mediated signaling pathway and neurogenesis in the dentate gyrus (DG) of the hippocampus in rats after traumatic brain injury (TBI). Adult male Wistar rats were divided into three groups: (1) sham group (n = 8); (2) saline control group (n = 40); and (3) simvastatin-treated group (n = 40). Controlled cortical impact (CCI) injury was performed over the left parietal lobe. Simvastatin was administered orally at a dose of 1 mg/kg starting at day 1 after TBI and then daily for 14 days. Bromodeoxyuridine (BrdU) was injected intraperitoneally into rats. A modified Morris Water Maze (WM) task was performed between 31 and 35 days after treatment to test spatial memory (n = 8/group). Animals were sacrificed at 1, 3, 7, 14, and 35 days after treatment (n = 8/group/time point). Western blot was utilized to investigate the changes in the Akt-mediated signaling pathway. Enzyme-linked immunosorbent assay (ELISA) analyses were employed to measure vascular endothelial growth factor (VEGF) and brain-derived neurotrophin factor (BDNF) expression. Immunohistochemical and fluorescent staining were performed to detect the BrdU- and neuronal nuclei (NeuN)/BrdU-positive cells. Our data show that simvastatin treatment increases phosphorylation of v-akt murine thymoma viral oncogene homolog (Akt), glycogen synthase kinase-3beta (GSK-3beta), and cAMP response element-binding proteins (CREB); elevates the expression of BDNF and VEGF in the DG; increases cell proliferation and differentiation in the DG; and enhances the recovery of spatial learning. These data suggest that the neurorestorative effect of simvastatin may be mediated through activation of the Akt-mediated signaling pathway, subsequently upregulating expression of growth factors and inducing neurogenesis in the DG of the hippocampus, thereby leading to restoration of cognitive function after TBI in rats.

Makorin-2 is a neurogenesis inhibitor downstream of phosphatidylinositol 3-kinase/Akt (PI3K/Akt) signal

Makorin-2 belongs to the makorin RING zinc finger gene family, which encodes putative ribonucleoproteins. Here we cloned the Xenopus makorin-2 (mkrn2) and characterized its function in Xenopus neurogenesis. Forced overexpression of mkrn2 produced tadpoles with dorso-posterior deficiencies and small-head/short-tail phenotype, whereas knockdown of mkrn2 by morpholino antisense oligonucleotides induced double axis in tadpoles. In Xenopus animal cap explant assay, mkrn2 inhibited activin, and retinoic acid induced animal cap neuralization, as evident from the suppression of a pan neural marker, neural cell adhesion molecule. Surprisingly, the anti-neurogenic activity of mkrn2 is independent of the two major neurogenesis signaling cascades, BMP-4 and Wnt8 pathways. Instead, mkrn2 works specifically through the phosphatidylinositol 3-kinase (PI3K) and Akt-mediated neurogenesis pathway. Overexpression of mkrn2 completely abrogated constitutively active PI3K- and Akt-induced, but not dominant negative glycogen synthase kinase-3beta (GSK-3beta)-induced, neural cell adhesion molecule expression, indicating that mkrn2 acts downstream of PI3K and Akt and upstream of GSK-3beta. Moreover, mkrn2 up-regulated the mRNA and protein levels of GSK-3beta. These results revealed for the first time the important role of mkrn2 as a new player in PI3K/Akt-mediated neurogenesis during Xenopus embryonic development.

Identification of peripherin as a Akt substrate in neurons

Activation of Akt-mediated signaling pathways is crucial for survival and regeneration of injured neurons. In this study, we attempted to identify novel Akt substrates by using an antibody that recognized a consensus motif phosphorylated by Akt. PC12 cells that overexpressed constitutively active Akt were used. Using two-dimensional PAGE, we identified protein spots that exhibited increased immunostaining of the antibody. Mass spectrometry revealed several major spots as the neuronal intermediate filament protein, peripherin. Using several peripherin fragments, the phosphorylation site was determined as Ser(66) in its head domain in vitro. Furthermore, a co-immunoprecipitation experiment revealed that Akt interacted with the head domain of peripherin in HEK 293T cells. An antibody against phosphorylated peripherin was raised, and induction of phosphorylated peripherin was observed not only in Akt-activated cultured cells but also in nerve-injured hypoglossal motor neurons. These results suggest that peripherin is a novel substrate for Akt in vivo and that its phosphorylation may play a role in motor nerve regeneration.

Transgenic Xenopus with prx1 limb enhancer reveals crucial contribution of MEK/ERK and PI3K/AKT pathways in blastema formation during limb regeneration

Understanding the mechanisms that control amphibian limb regeneration should allow us to decipher the critical differences between amphibians and humans, which have the limited ability of organ regeneration. However, many issues at the cellular and molecular levels still remain unresolved. We have generated a transgenic Xenopus laevis line that expresses green fluorescent protein (GFP) under the control of mouse prx1 limb enhancer, which directs reporter gene expression in limb mesenchyme in mice, and found that GFP accumulated in blastemal mesenchymal cells of the transgenic froglets after limb amputation. Thus, this transgenic line should provide a new approach to gain insights into the cellular dynamics and signaling pathways involved in limb blastema formation. We have also developed a culture system for forelimb explants of froglets and found that treatment with inhibitors of mitogen-activated protein kinase (MAPK)/extracellular signal-regulated protein kinase (ERK) kinase 1/2 (MEK1/2) and phosphatidylinositol 3-kinase (PI3K) repressed GFP expression. These effects were partially reversible, and down-regulation of GFP was associated with inhibition of cell-cycle progression and induction of ectopic apoptosis. In addition, we found that ERK1/2 and AKT, downstream mediators of MEK1/2 and PI3K pathways, were activated in amputated forelimb stumps. These results demonstrate that MEK/ERK and PI3K/AKT pathways regulate limb blastema formation in the X. laevis froglet.

PTEN is required for the normal progression of gastrulation by repressing cell proliferation after MBT in Xenopus embryos

PTEN phosphatase mediates several developmental cues involving cell proliferation, growth, death, and migration. We investigated the function of the PTEN gene at the transition from the cell proliferation state to morphogenesis around the midblastula transition (MBT) and gastrulation in Xenopus embryos. An immunoblotting analysis indicated that PTEN expresses constantly through embryogenesis. By up- or down-regulating PTEN activity using overexpression of the active form or C terminus of PTEN before MBT, we induced elongation of the cell cycle time just before MBT or maintained its speed even after MBT, respectively. The disruption of the cell cycle time by changing the activity of PTEN delayed gastrulation after MBT. In addition, PTEN began to localize to the plasma membranes and nuclei at MBT. Overexpression of a membrane-localizing mutant of PTEN caused dephosphorylation of Akt, whereas overexpression of the C terminus of PTEN caused phosphorylation of Akt and inhibited the localization of EGFP-PTEN to the plasma membranes and nuclei. These results indicate that an appropriate PTEN activity, probably regulated by its differential localization, is necessary for coordinating cell proliferation and early morphogenesis.

Neurogenin 1 mediates erythropoietin enhanced differentiation of adult neural progenitor cells

Proneuronal basic helix-loop-helix (bHLH) transcription factor, neurogenin 1 (Ngn1), regulates neuronal differentiation during development of the cerebral cortex. Akt mediates proneuronal bHLH protein function to promote neuronal differentiation. Here, we show that recombinant human erythropoietin (rhEPO) significantly increased Akt activity and Ngn1 mRNA levels in neural progenitor cells derived from the subventricular zone (SVZ) of adult rat, which was coincident with increases of neural progenitor cell proliferation, differentiation, and neurite outgrowth. Inhibition of Akt activity by the phosphatidylinositol 3-kinase/Akt (PI3K/Akt) inhibitor, LY294002, abolished rhEPO-increased Ngn1 mRNA levels and the effects of rhEPO on neural progenitor cells. In addition, reducing expression of endogenous Ngn1 by means of short-interfering RNA (siRNA) blocked rhEPO-enhanced neuronal differentiation and neurite outgrowth but not rhEPO-increased proliferation. Furthermore, treatment of stroke rat with rhEPO significantly increased Ngn1 mRNA levels in SVZ cells. These data suggest that rhEPO acts as an extracellular molecule that activates the PI3K/Akt pathway, which enhances adult neural progenitor cell proliferation, differentiation, and neurite outgrowth, and Ngn1 is required for Akt-mediated neuronal differentiation and neurite outgrowth.

An obligatory caravanserai stop on the silk road to neural induction: Inhibition of BMP/GDF signaling

Work in Xenopus laevis produced the first molecular explanation for neural specification, the default model, where inactivation of the BMP pathway in ectodermal cells changes fates from epidermal to neural. This review covers the present status of our understanding of neural specification, with emphasis on Xenopus, but including relevant facts in other model systems. While recent experiments have increased the complexity of the molecular picture, they have also provided additional support for the default model and the central position of the BMP pathway. We conclude that synergy between accumulated knowledge and technical progress will maintain Xenopus at the forefront of research in neural development.

Homeotic factor ATBF1 induces the cell cycle arrest associated with neuronal differentiation

The present study aimed to elucidate the function of AT motif-binding factor 1 (ATBF1) during neurogenesis in the developing brain and in primary cultures of neuroepithelial cells and cell lines (Neuro 2A and P19 cells). Here, we show that ATBF1 is expressed in the differentiating field in association with the neuronal differentiation markers beta-tubulin and MAP2 in the day E14.5 embryo rat brain, suggesting that it promotes neuronal differentiation. In support of this, we show that ATBF1 suppresses nestin expression, a neural stem cell marker, and activates the promoter of Neurod1 gene, a marker for neuronal differentiation. Furthermore, we show that in Neuro 2A cells, overexpressed ATBF1 localizes predominantly in the nucleus and causes cell cycle arrest. In P19 cells, which formed embryonic bodies in the floating condition, ATBF1 is mainly cytoplasmic and has no effect on the cell cycle. However, the cell cycle was arrested when ATBF1 became nuclear after transfer of P19 cells onto adhesive surfaces or in isolated single cells. The nuclear localization of ATBF1 was suppressed by treatment with caffeine, an inhibitor of PI(3)K-related kinase activity of ataxa-telangiectasia mutated (ATM) gene product. The cytoplasmic localization of ATBF1 in floating/nonadherent cells is due to CRM1-dependent nuclear export of ATBF1. Moreover, in the embryonic brain ATBF1 was expressed in the cytoplasm of proliferating stem cells on the ventricular zone, where cells are present at high density and interact through cell-to-cell contact. Conversely, in the differentiating field, where cell density is low and extracellular matrix is dense, the cell-to-matrix interaction triggered nuclear localization of ATBF1, resulting in the cell cycle arrest. We propose that ATBF1 plays an important role in the nucleus by organizing the neuronal differentiation associated with the cell cycle arrest.

Multiple signaling pathways regulate FGF-2-induced retinal ganglion cell neurite extension and growth cone guidance

Growth cones use cues in their environment in order to grow in a directed fashion to their targets. In Xenopus laevis, fibroblast growth factors (FGFs) participate in retinal ganglion cell (RGC) axon guidance in vivo and in vitro. The main intracellular signaling cascades known to act downstream of the FGF receptor include the mitogen-activated protein kinase (MAPK), phospholipase Cgamma (PLCgamma) and phosphotidylinositol 3-kinase (PI3K) pathways. We used pharmacological inhibitors to identify the signaling cascade(s) responsible for FGF-2-stimulated RGC axon extension and chemorepulsion. The MAPK, PI3K and PLCgamma pathways were blocked by U0126, LY249002 and U73122, respectively. D609 was used to test a role for the phosphotidylcholine-PLC (PC-PLC) pathway. We determined that the MAPK and two PLC pathways are required for FGF-2 to stimulate RGC neurite extension in vitro, but the response of axons to FGF-2 applied asymmetrically to the growth cone depended only on the PLC pathways.

Activation of the PI3-K/Akt pathway mediates cGMP enhanced-neurogenesis in the adult progenitor cells derived from the subventricular zone

The intracellular mechanisms that regulate neurogenesis remain unclear. Using neurospheres isolated from the subventricular zone (SVZ) of the adult rat, we investigated the effect of cyclic guanosine monophosphate (cGMP) and its signaling pathway on the induction of neurogenesis. Neurospheres expressed phosphodiesterase 5 (PDE5) and treatment of neurospheres with Sildenafil, a specific inhibitor of PDE5, significantly increased cGMP levels and neurogenesis. In addition, incubation of neurospheres with Sildenafil significantly phosphorylated Akt, which was associated with an increase of phosphorylation of glycogen synthase kinase 3 (GSK-3), a downstream target of Akt. Coincubation of neurospheres with Sildenafil and LY 294002, a pharmacological inhibitor of PI3-K/Akt, abolished Sildenafil-induced phosphorylated Akt and GSK-3. Furthermore, LY 294002 blocked Sildenafil-increased SVZ cell proliferation. These data suggest that Sildenafil-enhanced neurogenesis likely occurs through activation of the PI3-K/Akt/GSK-3 pathway.