Cell death in the nervous system: lessons from insulin and insulin-like growth factors

Placental MFSD2A expression in fetal growth restriction and maternal and fetal DHA status

INTRODUCTION

Fetal growth restriction (FGR) may affect placental transfer of key nutrients to the fetus, such as the fatty acid docosahexaenoic acid (DHA). Major facilitator superfamily domain containing 2A (MFSD2A) has been described as a specific DHA carrier in placenta, but its expression has not been studied in FGR. The aim of this study was to evaluate for the first time the placental MFSD2A levels in late-FGR pregnancies and the maternal and cord plasma DHA.

METHODS

87 pregnant women from a tertial reference center were classified into late-FGR (N = 18) or control (N = 69). Fatty acid profile was determined in maternal and cord venous plasma, as well as placental levels of MFSD2A and of insulin mediators like phospho-protein kinase B (phospho-AKT) and phospho-extracellular regulated kinase (phospho-ERK).

RESULTS

Maternal fatty acid profile did not differ between groups. Nevertheless, late-FGR cord vein presented higher content of saturated fatty acids than control, producing a concomitant decrease in the percentage of some unsaturated fatty acids. In the late-FGR group, a lower DHA fetal/maternal ratio was observed when using percentages, but not with concentrations. No alterations were found in the expression of MFSD2A in late-FGR placentas, nor in phospho-AKT or phospho-ERK.

DISCUSSION

MFSD2A protein expression was not altered in late-FGR placentas, in line with no differences in cord DHA concentration between groups. The increase in the saturated fatty acid content of late-FGR cord might be a compensatory mechanism to ensure fetal energy supply, decreasing other fatty acids percentage. Future studies are warranted to elucidate if altered saturated fatty acid profile in late-FGR fetuses might predispose them to postnatal catch-up and to long-term health consequences.

Normal development of the brain: a survey of joint structural-functional brain studies

Joint structural-functional (S-F) developmental studies present a novel approach to address the complex neuroscience questions on how the human brain works and how it matures. Joint S-F biomarkers have the inherent potential to model effectively the brain's maturation, fill the information gap in temporal brain atlases, and demonstrate how the brain's performance matures during the lifespan. This review presents the current state of knowledge on heterochronous and heterogeneous development of S-F links during the maturation period. The S-F relationship has been investigated in early-matured unimodal and prolonged-matured transmodal regions of the brain using a variety of structural and functional biomarkers and data acquisition modalities. Joint S-F unimodal studies have employed auditory and visual stimuli, while the main focus of joint S-F transmodal studies has been resting-state and cognitive experiments. However, nonsignificant associations between some structural and functional biomarkers and their maturation show that designing and developing effective S-F biomarkers is still a challenge in the field. Maturational characteristics of brain asymmetries have been poorly investigated by the joint S-F studies, and the results were partially inconsistent with previous nonjoint ones. The inherent complexity of the brain performance can be modeled using multifactorial and nonlinear techniques as promising methods to simulate the impact of age on S-F relations considering their analysis challenges.

Vestibular function in severe GH deficiency due to an inactivating mutation in the GH-releasing hormone receptor gene

PURPOSE

Body balance involves the vestibular, visual, and proprioceptive systems. IGF-I is a GH-dependent key factor in the development and postnatal differentiation of the inner ear in mice and men, but its role in the vestibular function in adult humans is unknown. We have previously described a cohort of individuals with severe isolated GH deficiency (IGHD) caused by a mutation in the GHRH receptor (GHRHR) gene. These individuals complain of dizziness, exhibit mild sensorineural loss, but have normal postural balance, without increase in falls risk. The aim of this study was to evaluate their vestibular function.

METHODS

We performed physical examination (clinical head impulse and Fukuda dynamic stepping test), oculomotor (saccadic eye movements, spontaneous, semi-spontaneous and opotokinetic nystagmus, and pendular tracking) and caloric stimulation (postcaloric reflex and ocular fixation index) tests, in 15 GH-naïve IGHD (seven males) and 15 controls (five males).

RESULTS

IGHD subjects showed lower height and weight, with similar BMI to controls, and higher number of individuals with abnormal clinical head impulse test and abnormal oculomotor tests, namely the saccadic movements and the spontaneous nystagmus. There was a nonsignificant trend in abnormalities in the Fukuda stepping test and postcaloric reflex test.

CONCLUSIONS

Adult untreated IGHD individuals have higher prevalence of moderate peripheral vestibular impairment, and of abnormal vestibular-ocular reflex.

Insulin-like growth factor 1 promotes cochlear synapse regeneration after excitotoxic trauma in vitro

In the context of acquired sensorineural hearing loss (SNHL), cochlear hair cells have long been thought to be among the most vulnerable elements in mammalian cochleae. However, recent studies have indicated that the synaptic connection between inner hair cells (IHC) and spiral ganglion neurons (SGN) can be an important target for the treatment of SNHL. Our previous studies in patients with sudden SNHL demonstrated delayed and gradual hearing recovery following topical application of insulin-like growth factor 1 (IGF-1), suggesting that not only protective but also regenerative mechanisms may account for hearing recovery after treatment with IGF-1. We then hypothesized that IGF-1 has the potential to drive the regeneration of IHC-SGN synapses. To test this hypothesis, we investigated the effects of IGF-1 on IHC-SGN synapses using cochlear explant cultures from postnatal day 2 mice that had been damaged by exposure to the excitatory amino acids N-methyl-d-aspartate and kainate. Cochlear explants that lost IHC-SGN synapses upon exposure to excitatory amino acids were cultured with exogenous IGF-1 for an additional 48 h. We observed increased numbers of IHC-SGN synapses after exogenous IGF-1 application. Pharmacological inhibition of the IGF-1 receptor attenuated the restoration of IHC-SGN synapses by exogenous IGF-1. These findings indicated that IGF-1 induces regeneration of IHC-SGN synapses in cochlear explant cultures from postnatal day 2 mice. Therefore, in a future study we will perform in vivo experiments using adult mice to ascertain the effects of IGF-1 on the regeneration of IHC-SGN synapses.

Trophic Factors in Inflammation and Regeneration: The Role of MANF and CDNF

Regeneration is an important process in multicellular organisms, responsible for homeostatic renewal and repair of different organs after injury. Immune cell activation is observed at early stages of the regenerative response and its regulation is essential for regenerative success. Thus, immune regulators play central roles in optimizing regenerative responses. Neurotrophic factors (NTFs) are secreted molecules, defined by their ability to support neuronal cell types. However, emerging evidence suggests that they can also play important functions in the regulation of immune cell activation and tissue repair. Here we discuss the literature supporting a role of NTFs in the regulation of inflammation and regeneration. We will focus, in particular, in the emerging roles of mesencephalic astrocyte-derived neurotrophic factor (MANF) and cerebral dopamine neurotrophic factor (CDNF) in the regulation of immune cell function and in the central role that immune modulation plays in their biological activity in vivo. Finally, we will discuss the potential use of these factors to optimize regenerative success in vivo, both within and beyond the nervous system.

Increased Sensitivity to Noise-Induced Hearing Loss by Blockade of Endogenous PI3K/Akt Signaling

The PI3K/Akt signaling pathway is involved in mediating survival of sensory hair cells. Here, we investigated the involvement of PI3K/Akt in noise-induced hearing loss in both temporary and permanent threshold shift noise models. The PI3K regulatory subunit p85α and phosphorylation of Akt on serine 473 (p-Akt S473) are downregulated in sensory hair cells, including both outer and inner hair cells, and supporting cells of the mouse organ of Corti 1 h after exposure to permanent-threshold-shift-inducing noise (PTS noise), but not with temporary-threshold-shift-inducing noise (TTS noise). In contrast, the PI3K catalytic subunit p110α and phosphorylation of Akt on threonine 308 (p-Akt T308) do not change with PTS or TTS noise. Additionally, mice pretreated with p85α small interfering RNA (siRNA) have decreased expression of p-Akt1 (S473) in their sensory hair cells and increased sensitivity to TTS noise-induced hearing loss. Finally, Akt1-knockout mice also have enhanced sensitivity to TTS noise-induced hearing loss. In conclusion, this study suggests that endogenous PI3K/Akt signaling is an intrinsic protective mechanism of the inner ear. Blockade of PI3K/Akt signaling pathways increases sensitivity to TTS noise-induced hearing loss.

Insulin-like growth factor 1 induces the transcription of Gap43 and Ntn1 during hair cell protection in the neonatal murine cochlea

We previously reported that insulin-like growth factor 1 (IGF-1) protects cochlear hair cells against aminoglycosides through activation of the PI3K/Akt and MEK/ERK pathways in supporting cells. In this study, we found that IGF-1 up-regulated the expression levels of Gap43 and Ntn1 as measured using cDNA microarray analysis and qRT-PCR. Using inhibitors of the PI3K/Akt and MEK/ERK pathways, we reveal that both pathways are involved in the up-regulation of Gap43 and Ntn1 expression. Moreover the time window of Gap43 and Ntn1 transcription was limited to within 12h after IGF-1 treatment, indicating that downstream gene expression was tightly controlled by IGF-1.

Early development of the vertebrate inner ear

This is a review of the biological processes and the main signaling pathways required to generate the different otic cell types, with particular emphasis on the actions of insulin-like growth factor I. The sensory organs responsible of hearing and balance have a common embryonic origin in the otic placode. Lineages of neural, sensory, and support cells are generated from common otic neuroepithelial progenitors. The sequential generation of the cell types that will form the adult inner ear requires the coordination of cell proliferation with cell differentiation programs, the strict regulation of cell survival, and the metabolic homeostasis of otic precursors. A network of intracellular signals operates to coordinate the transcriptional response to the extracellular input. Understanding the molecular clues that direct otic development is fundamental for the design of novel treatments for the protection and repair of hearing loss and balance disorders.

IRS2 signalling is required for the development of a subset of sensory spinal neurons

Insulin and insulin-like growth factor-I play important roles in the development and maintenance of neurons and glial cells of the nervous system. Both factors activate tyrosine kinase receptors, which signal through adapter proteins of the insulin receptor substrate (IRS) family. Although insulin and insulin-like growth factor-I receptors are expressed in dorsal root ganglia (DRG), the function of IRS-mediated signalling in these structures has not been studied. Here we address the role of IRS2-mediated signalling in murine DRG. Studies in cultured DRG neurons from different embryonic stages indicated that a subset of nerve growth factor-responsive neurons is also dependent on insulin for survival at very early time points. Consistent with this, increased apoptosis during gangliogenesis resulted in a partial loss of trkA-positive neurons in DRG of Irs2 mutant embryos. Analyses in adult Irs2(-/-) mice revealed that unmyelinated fibre afferents, which express calcitonin gene-related peptide/substance P and isolectin B4, as well as some myelinated afferents to the skin were affected by the mutation. The diminished innervation of glabrous skin in adult Irs2(-/-) mice correlated with longer paw withdrawal latencies in the hot-plate assay. Collectively, these findings indicate that IRS2 signalling is required for the proper development of spinal sensory neurons involved in the perception of pain.

AKT signaling mediates IGF-I survival actions on otic neural progenitors

BACKGROUND

Otic neurons and sensory cells derive from common progenitors whose transition into mature cells requires the coordination of cell survival, proliferation and differentiation programmes. Neurotrophic support and survival of post-mitotic otic neurons have been intensively studied, but the bases underlying the regulation of programmed cell death in immature proliferative otic neuroblasts remains poorly understood. The protein kinase AKT acts as a node, playing a critical role in controlling cell survival and cell cycle progression. AKT is activated by trophic factors, including insulin-like growth factor I (IGF-I), through the generation of the lipidic second messenger phosphatidylinositol 3-phosphate by phosphatidylinositol 3-kinase (PI3K). Here we have investigated the role of IGF-dependent activation of the PI3K-AKT pathway in maintenance of otic neuroblasts.

METHODOLOGY/PRINCIPAL FINDINGS

By using a combination of organotypic cultures of chicken (Gallus gallus) otic vesicles and acoustic-vestibular ganglia, Western blotting, immunohistochemistry and in situ hybridization, we show that IGF-I-activation of AKT protects neural progenitors from programmed cell death. IGF-I maintains otic neuroblasts in an undifferentiated and proliferative state, which is characterised by the upregulation of the forkhead box M1 (FoxM1) transcription factor. By contrast, our results indicate that post-mitotic p27(Kip)-positive neurons become IGF-I independent as they extend their neuronal processes. Neurons gradually reduce their expression of the Igf1r, while they increase that of the neurotrophin receptor, TrkC.

CONCLUSIONS/SIGNIFICANCE

Proliferative otic neuroblasts are dependent on the activation of the PI3K-AKT pathway by IGF-I for survival during the otic neuronal progenitor phase of early inner ear development.

The role of insulin-like growth factor-I in the physiopathology of hearing

Insulin-like growth factor-I (IGF-I) belongs to the family of polypeptides of insulin, which play a central role in embryonic development and adult nervous system homeostasis by endocrine, autocrine, and paracrine mechanisms. IGF-I is fundamental for the regulation of cochlear development, growth, and differentiation, and its mutations are associated with hearing loss in mice and men. Low levels of IGF-I have been shown to correlate with different human syndromes showing hearing loss and with presbyacusis. Animal models are fundamental to understand the genetic, epigenetic, and environmental factors that contribute to human hearing loss. In the mouse, IGF-I serum levels decrease with aging and there is a concomitant hearing loss and retinal degeneration. In the Igf1(-/-) null mouse, hearing loss is due to neuronal loss, poor innervation of the sensory hair cells, and age-related stria vascularis alterations. In the inner ear, IGF-I actions are mediated by intracellular signaling networks, RAF, AKT, and p38 MAPK protein kinases modulate the expression and activity of transcription factors, as AP1, MEF2, FoxM1, and FoxP3, leading to the regulation of cell cycle and metabolism. Therapy with rhIGF-I has been approved in humans for the treatment of poor linear growth and certain neurodegenerative diseases. This review will discuss these findings and their implications in new IGF-I-based treatments for the protection or repair of hearing loss.

The anemia of the newborn induces erythropoietin expression in the developing mouse retina

The hematopoietic hormone erythropoietin (Epo), regularly produced by the kidneys and the liver, is also expressed in neuronal tissue, where it has been found to mediate paracrine neuroprotective effects. In most studies exploring the rescue effects of Epo, apoptosis was exogenously induced by different cell death stimuli. Herein, we set out to study the expression and function of Epo in physiologically occurring apoptosis in a model of retinal development. We made use of an organotypic retinal wholemount culture system that resembles the physiological in vivo situation with cell connections still retained. Epo mRNA expression in the retina, liver, and kidney showed a significant increase during early development, coinciding with the anemia of the newborn. In the retina of Epo-green fluorescent protein transgenic mice, Epo-expressing cells were identified and found to be distributed in the retinal ganglion cell layer. Treatment of retinal wholemount cultures with recombinant Epo resulted in a significant decrease of apoptotic ganglion cells as well as photoreceptor cells throughout retinal development. Moreover, transforming growth factor-β-induced apoptosis was completely antagonized by Epo when both factors were simultaneously applied. Investigations on the signaling pathway revealed a decrease in Bax mRNA levels in Epo-treated retinal cells. We conclude that Epo exerts wide and prolonged neuroprotective activity in physiologically occurring apoptosis and thus contributes to proper retinal development.

A novel type of glial cell in the retina is stimulated by insulin-like growth factor 1 and may exacerbate damage to neurons and Müller glia

Recent studies have demonstrated that insulin can have profound affects on the survival of neurons within the retina. The purpose of this study was to determine how insulin-like growth factor 1 (IGF1) influences retinal cells; in particular, the glial cells. We identify a novel type of glial cell in the avian retina and provide evidence that these cells can respond to acute damage and IGF1. In normal retinas, we found a distinct cell-type, scattered across the ganglion cell and inner plexiform layers that express Sox2, Sox9, Nkx2.2, vimentin, and transitin, the avian homologue of mammalian nestin. These glial cells have a unique immunohistochemical profile, morphology, and distribution that are distinct among other known types of retinal glia, including microglia, oligodendrocytes, astrocytes, and Muller glia. We termed these cells nonastrocytic inner retinal glia-like (NIRG) cells. We found that the NIRG cells may express the IGF1 receptor and respond to IGF1 by proliferating, migrating distally into the retina, and upregulating transitin. In addition, IGF1 stimulated microglia to become reactive and upregulate lysosomal membrane glycoprotein and CD45. With microglia and NIRG cells stimulated by IGF1 there were elevated levels of cell death and numerous focal detachments across the retina in response to excitotoxic damage. Cell death was prominent within the areas of detachment coinciding with a stark loss of Müller glia and accumulation of NIRG cells. We conclude that NIRG cells are a novel type of retinal glia that is sensitive to IGF1 and whose activity may impact the survival of neurons and Müller glia.

IGF-I promotes neuronal migration and positioning in the olfactory bulb and the exit of neuroblasts from the subventricular zone

While insulin-like growth factor-I (IGF-I) supports neuronal and glial differentiation in the CNS, it is largely unknown whether IGF-I also influences neuronal migration and positioning. We show here that the pattern of olfactory bulb (OB) layering is altered in Igf-I (-/-) mice. In these animals, Tbr1(+)-glutamatergic neurons are misplaced in the mitral cell layer (ML) and the external plexiform layer (EPL). In addition, there are fewer interneurons in the glomerular layer and the EPL of the Igf-I (-/-) mice, and fewer newborn neurons are incorporated into the OB from the forebrain subventricular zone (SVZ). Indeed, neuroblasts accumulate in the postnatal/adult SVZ of Igf-I (-/-) mice. Significantly, the positioning of Tbr1(+)-cells in a primitive ML is stimulated by IGF-I in cultured embryonic OB slices, an effect that is partially repressed by the phosphoinositide 3-kinase (PI3K) inhibitor. In OB cell cultures, IGF-I increases the phosphorylation of disabled1 (P-Dab1), an adaptor protein that is a target of Src family kinases (SFK) in the reelin signalling pathway, whereas reduced P-Dab1 levels were found in Igf-I (-/-) mice. Neuroblast migration from the rostral migratory stream (RMS) explants of postnatal Igf-I (-/-) was similar to that from Igf-I (+/+) explants. However, cell migration was significantly enhanced by IGF-I added to the explants, an effect that was repressed by PI3K and SFK inhibitors. These findings suggest that IGF-I promotes neuronal positioning in the OB and support a role for IGF-I in stimulating neuroblast exit from the SVZ into the RMS, thereby promoting the incorporation of newly formed neurons into the OB.

Differential, age-dependent MEK-ERK and PI3K-Akt activation by insulin acting as a survival factor during embryonic retinal development

Programmed cell death is a genuine developmental process of the nervous system, affecting not only projecting neurons but also proliferative neuroepithelial cells and young neuroblasts. The embryonic chick retina has been employed to correlate in vivo and in vitro studies on cell death regulation. We characterize here the role of two major signaling pathways, PI3K-Akt and MEK-ERK, in controlled retinal organotypic cultures from embryonic day 5 (E5) and E9, when cell death preferentially affects proliferating neuroepithelial cells and ganglion cell neurons, respectively. The relative density of programmed cell death in vivo was much higher in the proliferative and early neurogenic stages of retinal development (E3-E5) than during neuronal maturation and synaptogenesis (E8-E19). In organotypic cultures from E5 and E9 retinas, insulin, as the only growth factor added, was able to completely prevent cell death induced by growth factor deprivation. Insulin activated both the PI3K-Akt and the MEK-ERK pathways. Insulin survival effect, however, was differentially blocked at the two stages. At E5, the effect was blocked by MEK inhibitors, whereas at E9 it was blocked by PI3K inhibitors. The cells which were found to be dependent on insulin activation of the MEK-ERK pathway at E5 were mostly proliferative neuroepithelial cells. These observations support a remarkable specificity in the regulation of early neural cell death.

Proinsulin/insulin is synthesized locally and prevents caspase- and cathepsin-mediated cell death in the embryonic mouse retina

Programmed cell death is an essential, highly regulated process in neural development. Although the role of insulin-like growth factor I in supporting the survival of neural cells has been well characterized, studies on proinsulin/insulin are scarce. Here, we characterize proinsulin/insulin effects on cell death in embryonic day 15.5 mouse retina. Both proinsulin mRNA and proinsulin/insulin immunoreactivity were found in the developing retina. Organotypic embryonic day 15.5 retinas cultured under growth factor deprivation showed an increase in cell death that was reversed by proinsulin, insulin and insulin-like growth factor I, with similar median effective concentration values via phosphatidylinositol-3-kinase activation. Although insulin and insulin-like growth factor I provoked a sustained Akt phosphorylation, proinsulin-induced phosphorylation of Akt was not found. Analysis of the growth factor deprivation-induced cell death mechanisms, using caspase and cathepsin inhibitors, demonstrated that both protease families were required for the effective execution of cell death. The insulin survival effect, which decreased the extent and distribution of cell death to levels similar to those found in vivo, was not enhanced by simultaneous treatment with caspase and cathepsin inhibitors, suggesting that insulin interferes with these protease pathways in the embryonic mouse retina. The mechanisms characterized in this study provide new details on early neural cell death and its genuine regulation by insulin/proinsulin.

Modulation of the PI 3-kinase-Akt signalling pathway by IGF-I and PTEN regulates the differentiation of neural stem/precursor cells

Neural stem cells depend on insulin-like growth factor I (IGF-I) for differentiation. We analysed how activation and inhibition of the PI 3-kinase-Akt signalling affects the number and differentiation of mouse olfactory bulb stem cells (OBSCs). Stimulation of the pathway with insulin and/or IGF-I, led to an increase in Akt phosphorylated on residues Ser473 and Thr308 (P-Akt(Ser473) and P-Akt(Thr308), respectively) in proliferating OBSCs, and in differentiating cells. Conversely, P-Akt(Ser473) levels decreased by 50% in the OB of embryonic day 16.5-18.5 IGF-I knockout mouse embryos. Overexpression of PTEN, a negative regulator of the PI 3-kinase pathway, caused a reduction in the basal levels of P-Akt(Ser473) and P-Akt(Thr308) and a minor reduction in IGF-I-stimulated P-Akt(Ser473). Although PTEN overexpression decreased the proportion of neurons and astrocytes in the absence of insulin/IGF-I, it did not alter the proliferation or survival of OBSCs. Accordingly, overexpression of a catalytically inactive PTEN mutant promoted OBSCs differentiation. Inhibition of PI 3-kinase by LY294002 produced strong and moderate reductions in IGF-I-stimulated P-Akt(Ser473) and P-Akt(Thr308), respectively. Consequently, LY294002 reduced the proliferation of OBSCs and the number of neurons and astrocytes, and also augmented cell death. These findings indicate that OBSC differentiation is more sensitive to lower basal levels of P-Akt than proliferation or death. By regulating P-Akt levels in opposite ways, IGF-I and PTEN contribute to the fine control of neurogenesis in the olfactory bulb.

Proinsulin in development: New roles for an ancient prohormone

In postnatal organisms, insulin is well known as an essential anabolic hormone responsible for maintaining glucose homeostasis. Its biosynthesis by the pancreatic beta cell has been considered a model of tissue-specific gene expression. However, proinsulin mRNA and protein have been found in embryonic stages before the formation of the pancreatic primordium, and later, in extrapancreatic tissues including the nervous system. Phylogenetic studies have also confirmed that production of insulin-like peptides antecedes the morphogenesis of a pancreas, and that these peptides contribute to normal development. In recent years, other roles for insulin distinct from its metabolic function have emerged also in vertebrates. During embryonic development, insulin acts as a survival factor and is involved in early morphogenesis. These findings are consistent with the observation that, at these stages, the proinsulin gene product remains as the precursor form, proinsulin. Independent of its low metabolic activity, proinsulin stimulates proliferation in developing neuroretina, as well as cell survival and cardiogenesis in early embryos. Insulin/proinsulin levels are finely regulated during development, since an excess of the protein interferes with correct morphogenesis and is deleterious for the embryo. This fine-tuned regulation is achieved by the expression of alternative embryonic proinsulin transcripts that have diminished translational activity.

Balance of pro-apoptotic transforming growth factor-β and anti-apoptotic insulin effects in the control of cell death in the postnatal mouse retina

Transforming growth factor (TGF)-beta and insulin display opposite effects in regulating programmed cell death during vertebrate retina development; the former induces apoptosis while the latter prevents it. In the present study we investigated coordinated actions of TGF-beta and insulin in an organotypic culture system of early postnatal mouse retina. Addition of exogenous TGF-beta resulted in a significant increase in cell death whereas exogenous insulin attenuated apoptosis and was capable of blocking TGF-beta-induced apoptosis. This effect appeared to be modulated via insulin-induced transcriptional down-regulation of TGF-beta receptor II levels. The analysis of downstream signalling molecules also revealed opposite effects of both factors; insulin provided survival signalling by increasing the level of anti-apoptotic Bcl-2 protein expression and phosphorylation and down-regulating caspase 3 activity whereas pro-apoptotic TGF-beta signalling reduced Bcl-2 mRNA levels and Bcl-2 phosphorylation and induced the expression of TGF-induced immediate-early gene (TIEG), a Krüppel-like zinc-finger transcription factor, mimicking TGF-beta activity.

Trophic effects of insulin-like growth factor-I (IGF-I) in the inner ear

Insulin-like growth factors (IGFs) have a pivotal role during nervous system development and in its functional maintenance. IGF-I and its high affinity receptor (IGF1R) are expressed in the developing inner ear and in the postnatal cochlear and vestibular ganglia. We recently showed that trophic support by IGF-I is essential for the early neurogenesis of the chick cochleovestibular ganglion (CVG). In the chicken embryo otic vesicle, IGF-I regulates developmental death dynamics by regulating the activity and/or levels of key intracellular molecules, including lipid and protein kinases such as ceramide kinase, Akt and Jun N-terminal kinase (JNK). Mice lacking IGF-I lose many auditory neurons and present increased auditory thresholds at early postnatal ages. Neuronal loss associated to IGF-I deficiency is caused by apoptosis of the auditory neurons, which presented abnormally increased levels of activated caspase-3. It is worth noting that in man, homozygous deletion of the IGF-1 gene causes sensory-neural deafness. IGF-I is thus necessary for normal development and maintenance of the inner ear. The trophic actions of IGF-I in the inner ear suggest that this factor may have therapeutic potential for the treatment of hearing loss.

Early neural cell death: dying to become neurons

The importance of programmed cell death (PCD) during vertebrate development has been well established. During the development of the nervous system in particular, neurotrophic cell death in innervating neurons matches the number of neurons to the size of their target field. However, PCD also occurs during earlier stages of neural development, within populations of proliferating neural precursors and newly postmitotic neuroblasts, all of which are not yet fully differentiated. This review addresses early neural PCD, which is distinct from neurotrophic death in differentiated neurons. Although early neural PCD is observed in a range of organisms, from Caenorhabditis elegans to mouse, the role and the regulation of early neural PCD are not well understood. The regulation of early neural PCD can be inferred from the function of factors such as bone morphogenetic proteins (BMPs), Wnts, fibroblast growth factors (FGFs), and Sonic Hedgehog (Shh), which regulate both early neural development and PCD occurring in other developmental processes. Cell number control, removal of damaged or misspecified cells (spatially or temporally), and selection are the proposed roles early neural PCDs play during neural development. Data from developmental PCD in C. elegans and Drosophila provide insights into the possible signaling pathways integrating PCD with other processes during early neural development and the roles they might play.

Upstream AUGs in embryonic proinsulin mRNA control its low translation level

Proinsulin is expressed prior to development of the pancreas and promotes cell survival. Here we study the mechanism affecting the translation efficiency of a specific embryonic proinsulin mRNA. This transcript shares the coding region with the pancreatic form, but presents a 32 nt extended leader region. Translation of proinsulin is markedly reduced by the presence of two upstream AUGs within the 5' extension of the embryonic mRNA. This attenuation is lost when the two upstream AUGs are mutated to AAG, leading to translational efficiency similar to that of the pancreatic mRNA. The upstream AUGs are recognized as initiator codons, because expression of upstream ORF is detectable from the embryonic transcript, but not from the mutated or the pancreatic mRNAs. Strict regulation of proinsulin biosynthesis appears to be necessary, since exogenous proinsulin added to embryos in ovo decreased apoptosis and generated abnormal developmental traits. A novel mechanism for low level proinsulin expression thus relies on upstream AUGs within a specific form of embryonic proinsulin mRNA, emphasizing its importance as a tightly regulated developmental signal.

Insulin-like growth factor 1 is required for survival of transit-amplifying neuroblasts and differentiation of otic neurons

Neurons that connect mechanosensory hair cell receptors to the central nervous system derive from the otic vesicle from where otic neuroblasts delaminate and form the cochleovestibular ganglion (CVG). Local signals interact to promote this process, which is autonomous and intrinsic to the otic vesicle. We have studied the expression and activity of insulin-like growth factor-1 (IGF-1) during the formation of the chick CVG, focusing attention on its role in neurogenesis. IGF-1 and its receptor (IGFR) were detected at the mRNA and protein levels in the otic epithelium and the CVG. The function of IGF-1 was explored in explants of otic vesicle by assessing the formation of the CVG in the presence of anti-IGF-1 antibodies or the receptor competitive antagonist JB1. Interference with IGF-1 activity inhibited CVG formation in growth factor-free media, revealing that endogenous IGF-1 activity is essential for ganglion generation. Analysis of cell proliferation cell death, and expression of the early neuronal antigens Tuj-1, Islet-1/2, and G4 indicated that IGF-1 was required for survival, proliferation, and differentiation of an actively expanding population of otic neuroblasts. IGF-1 blockade, however, did not affect NeuroD within the otic epithelium. Experiments carried out on isolated CVG showed that exogenous IGF-1 induced cell proliferation, neurite outgrowth, and G4 expression. These effects of IGF-1 were blocked by JB1. These findings suggest that IGF-1 is essential for neurogenesis by allowing the expansion of a transit-amplifying neuroblast population and its differentiation into postmitotic neurons. IGF-1 is one of the signals underlying autonomous development of the otic vesicle.