ADAPTIVE ROLES OF PROGRAMMED CELL DEATH DURING NERVOUS SYSTEM DEVELOPMENT

Enhanced neuronal loss under perinatal hypothyroidism involves impaired neurotrophic signaling and increased proteolysis of p75(NTR)

Recognition of the molecular events that lead to enhanced cell death is vital to understand the developmental cerebellar defects under hypothyroidism. Though neurotrophins promote the survival and development of neurons in the cerebellum, but the mechanism of their insufficiency mediated cell loss under hypothyroidism is unknown. Here in developmental hypothyroid rat model we report that hypothyroidism induced neuronal loss involve down regulation of neurotrophic survival signaling and increased truncation of the receptor p75(NTR). Results showed that perinatal hypothyroidism besides repressing the expression of BDNF also impairs the maturation of NGF which results in decreased activation of ERK, CREB, NF-kappaB and AKT. Furthermore hypothyroidism caused an enhanced expression and proteolysis of p75(NTR). The increased proteolysis of p75(NTR)in vivo and its association with death of granule neurons brings forward hitherto a p75(NTR) dependence signaling which along with compromised survival signaling could provide a neurotrophic basis of understanding the cause of enhanced cell death in developing cerebellum under hypothyroidism.

Requirement of 3-phosphoinositide-dependent protein kinase-1 for BDNF-mediated neuronal survival

Although PDK1 regulates several signaling pathways that respond to neurotrophins, direct evidence for its involvement in neurotrophin-mediated survival has not yet been reported. Here we show high neuronal expression of active PDK1 in the rat cortex and hippocampus at the developmental stages with pronounced dependence on extracellular survival signals. Also, in cultured cortical neurons from newborn rats, BDNF resulted in PDK1- and extracellular signal-regulated kinase-1/2 (ERK1/2)-mediated activation of their direct target, the p90 ribosomal S6 kinase 1/2 (RSK1/2). In trophic-deprived cortical neurons, knockdown of endogenous PDK1 attenuated the antiapoptotic survival response to 10 ng/ml BDNF, whereas an overexpressed active mutant form of PDK1 reduced apoptosis. The neuroprotection by BDNF or active PDK1 required RSK1/2. Conversely, PDK1 knockdown reversed the survival effects of combining the overexpressed RSK1 with a low, subprotective BDNF concentration of 2 ng/ml. Likewise, the protection by the overexpressed, active PDK1 was enhanced by coexpression of an active RSK1 mutant. Consistent with the observations that in BDNF-stimulated neurons RSK1/2 activation required both PDK1 and ERK1/2, ERK1/2 knockdown removed BDNF-mediated survival. Selective activation of ERK1/2 with an overexpressed active mutant form of MKK1 resulted in RSK1/2- and PDK1-dependent neuroprotection. Finally, at subprotective plasmid DNA dosage, overexpression of the active MKK1 and PDK1 mutants produced synergistic effect on survival. Our findings indicate a critical role for PDK1-RSK1/2 signaling in BDNF-mediated neuronal survival. Thus, the PDK1 is indispensable for the antiapoptotic effects of the ERK1/2 pathway offering previously unrecognized layer of survival signal processing and integration.

Enhanced sensitivity of midline glial cells to apoptosis is achieved by HOW(L)-dependent repression of Diap1

The selective sensitivity of cells to programmed cell death (PCD) depends on the positive and negative death-inducing signals that converge into the apoptotic pathway. In Drosophila, the midline glial (MG) cells undergo selective death during development. Here, we show that the long isoform of the RNA-binding protein Held Out Wing (HOW(L)) is essential for enhancing the sensitivity of the MG cells to PCD. In how mutant embryos, the number of MG cells was elevated. This phenotype could be rescued by midline expression of the HOW(L) repressor isoform. In how mutant embryos, the levels of the caspase inhibitor of apoptosis, Diap1 were elevated, in parallel to reduction in the levels of activated caspase. Similarly, reducing the levels of HOW in S2 cells led to elevation of Diap1, whereas over expression of HOW(L) promoted reduction of Diap1 protein as well as mRNA levels. Importantly, deletion of the two HOW binding sites from diap1 3'UTR abrogated HOW-dependent repression of Diap1, suggesting that HOW represses diap1 by binding to its 3'UTR. These results suggest that HOW(L) enhances the sensitivity of MG cells to apoptotic signals by reducing the levels of diap1 in these cells in, demonstrating a novel mode of regulation of PCD at the mRNA level.

Spatiotemporal recapitulation of central nervous system development by murine embryonic stem cell-derived neural stem/progenitor cells

Neural stem/progenitor cells (NS/PCs) can generate a wide variety of neural cells. However, their fates are generally restricted, depending on the time and location of NS/PC origin. Here we demonstrate that we can recapitulate the spatiotemporal regulation of central nervous system (CNS) development in vitro by using a neurosphere-based culture system of embryonic stem (ES) cell-derived NS/PCs. This ES cell-derived neurosphere system enables the efficient derivation of highly neurogenic fibroblast growth factor-responsive NS/PCs with early temporal identities and high cell-fate plasticity. Over repeated passages, these NS/PCs exhibit temporal progression, becoming epidermal growth factor-responsive gliogenic NS/PCs with late temporal identities; this change is accompanied by an alteration in the epigenetic status of the glial fibrillary acidic protein promoter, similar to that observed in the developing brain. Moreover, the rostrocaudal and dorsoventral spatial identities of the NS/PCs can be successfully regulated by sequential administration of several morphogens. These NS/PCs can differentiate into early-born projection neurons, including cholinergic, catecholaminergic, serotonergic, and motor neurons, that exhibit action potentials in vitro. Finally, these NS/PCs differentiate into neurons that form synaptic contacts with host neurons after their transplantation into wild-type and disease model animals. Thus, this culture system can be used to obtain specific neurons from ES cells, is a simple and powerful tool for investigating the underlying mechanisms of CNS development, and is applicable to regenerative treatment for neurological disorders.

Misplacement of Purkinje cells during postnatal development in Bax knock-out mice: a novel role for programmed cell death in the nervous system?

During early postnatal development, the orchestrated regulation of proliferation, migration and the survival versus elimination of neurons is essential for histogenesis of the cerebellum. For instance, Purkinje cells (PCs) promote the proliferation and migration of external granule cells (EGCs), whereas EGCs in turn play a role in the migration of PCs. Considering that a substantial number of neurons undergo programmed cell death (PCD) during cerebellar development, it seems likely that neuronal loss could have a significant role in the histogenesis of the cerebellum. To address this question, we examined postnatal development of the cerebellum in Bax-knock-out (KO) mice in which the PCD of PC has been reported to be selectively reduced or eliminated, whereas EGCs are unaffected. We confirmed the absence of PC PCD as well as the normal PCD of EGCs in Bax-KO mice. We also observed a subpopulation of PCs that were misplaced in the inner granule cell layer of Bax-KO mice on postnatal day 5 (P5) to P10 and that, by the end of the major period of cerebellar histogenesis (P14), lose expression of the PC marker calbindin. These results suggest that the removal of ectopically located neurons may be a previously unrecognized function of developmental PCD.

Developing postmitotic mammalian neurons in vivo lacking Apaf-1 undergo programmed cell death by a caspase-independent, nonapoptotic pathway involving autophagy

Previous studies have shown that caspases and Apaf-1 are required for the normal programmed cell death (PCD) in vivo of immature postmitotic neurons and mitotically active neuronal precursor cells. In contrast, caspase activity is not necessary for the normal PCD of more mature postmitotic neurons that are establishing synaptic connections. Although normally these cells use caspases for PCD, in the absence of caspase activity these neurons undergo a distinct nonapoptotic type of degeneration. We examined the survival of these more mature postmitotic neuronal populations in mice in which Apaf-1 has been genetically deleted and find that they exhibit quantitatively normal PCD of developing postmitotic neurons. We next characterized the morphological mode of PCD in these mice and show that the neurons degenerate by a caspase-independent, nonapoptotic pathway that involves autophagy. However, autophagy does not appear to be involved in the normal PCD of postmitotic neurons in which caspases and Apaf-1 are present and functional because quantitatively normal neuronal PCD occurred in the absence of a key gene required for autophagy (ATG7). Finally, we examined the possible role of another caspase-independent type of neuronal PCD involving the apoptosis-inducing factor (AIF). Mice deficient in AIF also exhibit quantitatively normal PCD of postmitotic neurons after caspase inhibition. Together, these data indicate that, when key components of the type 1 apoptotic pathway (i.e., caspases and Apaf-1) are perturbed in vivo, developing postmitotic neurons nonetheless undergo quantitatively normal PCD by a caspase-independent pathway involving autophagy and not requiring AIF.

Stability of the fittest: organizing learning through retroaxonal signals

Classically, neurons communicate by anterograde conduction of action potentials. However, information can also pass backward along axons, a process that is essential during the development of the nervous system. Here we propose a role for such 'retroaxonal' signals in adult learning. We hypothesize that strengthening of a neuron's output synapses stabilizes recent changes in the same neuron's inputs. During learning, the input synapses of many neurons undergo transient changes, resulting in altered spiking activity. If this in turn promotes strengthening of output synapses, the recent synaptic changes will be stabilized; otherwise they will decay. A representation of sensory stimuli therefore evolves that is tailored to the demands of behavioral tasks. We describe a candidate molecular mechanism for this process involving the activation of CREB by retrograde neurotrophin signals.

Impaired migration in the rostral migratory stream but spared olfactory function after the elimination of programmed cell death in Bax knock-out mice

Rats and mice exhibit neurogenesis of olfactory bulb (OB) interneurons throughout adulthood. To homeostatically maintain stable neuron numbers, it is necessary to continuously remove a subset of OB neurons by programmed cell death (PCD). Here we demonstrate that Bax is critical for the elimination of OB neurons by showing that Bax-KO mice exhibit greatly reduced PCD in the OB. Despite the reduction of PCD, however, proliferation of progenitors and the size of the OB were virtually unaffected in Bax-knock-out (KO) mice. However, reducing PCD by Bax deletion affected the migration of a subset of adult-produced neurons by the disruption of glial tube formation as well as by premature detachment of neuroblasts from the migratory chain. Rescued cells aberrantly remained in the subventricular zone (SVZ)-rostral migratory stream (RMS), in which they differentiated into calretinin+ or GABA-expressing interneurons. Because of the migratory deficit, OB cell homeostasis involving new cell entry and PCD (neuronal turnover) was virtually absent in adult Bax-KO mice. Despite this, Bax-KO mice exhibited normal olfactory behaviors such as odor discrimination and olfactory memory which are thought to be influenced by adult neurogenesis. These results demonstrate that PCD is involved in the regulation of RMS migration and differentiation after OB neurogenesis, but that animals maintain normal olfactory function in the absence of PCD.

Tissue inhibitor of metalloproteinases-3 (TIMP-3) expression is increased during serum deprivation-induced neuronal apoptosis in vitro and in the G93A mouse model of amyotrophic lateral sclerosis: a potential modulator of Fas-mediated apoptosis

Cortical neurons deprived of serum undergo apoptosis that is sensitive to inhibitors of macromolecule synthesis. Proteomic analysis revealed differential expression of 49 proteins in cortical neurons 8 h after serum deprivation. Tissue inhibitor of metalloproteinases-3 (TIMP-3), a pro-apoptotic protein in various cancer cells, was increased during serum deprivation-induced apoptosis (SDIA), but not during necrosis induced by excitotoxicity or oxidative stress. Levels of TIMP-3 were markedly increased in degenerating motor neurons in a transgenic model of familial amyotrophic lateral sclerosis. The TIMP-3 expression was accompanied by increase in Fas-FADD interaction, activated caspase-8, and caspase-3 during SDIA and in vulnerable spinal cord of the ALS mouse. SDIA and activation of the Fas pathway were prevented by addition of an active MMP-3. Timp-3 deletion by RNA interference attenuated SDIA in N2a cells. These findings provide evidence that TIMP-3 is an upstream mediator of neuronal apoptosis and likely contributes to neuronal loss in neurodegenerative diseases such as amyotrophic lateral sclerosis.

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.

Deletion of the Bax gene disrupts sexual behavior and modestly impairs motor function in mice

Cell death is a nearly ubiquitous feature of the developing nervous system, and differential death in males and females contributes to several well studied sex differences in neuron number. Nonetheless, the functional importance of neuronal cell death has been subjected to few direct tests. Bax, a pro-apoptotic protein, is required for cell death in many neural regions. Deletion of the Bax gene in mice increases neuron number in several areas and eliminates sex differences in cell number in the brain and spinal cord. Here, sexual and motor behaviors were examined in Bax-/- mice and their wild-type siblings to test the functional consequences of preventing Bax-dependent cell death. Animals were gonadectomized in adulthood and provided with ovarian hormones or with testosterone for tests of feminine and masculine sexual behaviors, respectively. Wild-type mice exhibited a sex difference in feminine sexual behavior, with high lordosis scores in females and low scores in males. This sex difference was eliminated by Bax deletion, with very low receptivity exhibited by both male and female Bax-/- mice. Masculine sexual behavior was not sexually dimorphic among wild-type mice, but mounts and pelvic thrusts were nearly eliminated in Bax-/- mice of both sexes. Motor strength and performance at low speeds on a RotaRod apparatus did not differ by sex or Bax gene status. However, Bax-/- animals exhibited impairments on the RotaRod at higher speeds. Thus, developmental cell death may be required for masculine and feminine sexual behaviors and the fine tuning of motor coordination.

The skin as a neurotrophic organ

The ability of the skin to serve as a protective shield against environmental challenges and as a sensitive detector and responder to thermal, chemical, and mechanical stimuli speaks to its exquisite design. A central feature of this design is the diverse array of neuronal afferents that convey and respond to sensory stimuli that the skin encounters. Cutaneous neuron development, form, and function are highly dependent on communication with the skin through its production of multiple growth factor proteins that modulate afferent development, maturation, and function. Production by the skin of neurotrophin growth factors and members of the glial cell line-derived neurotrophic factor family are particularly important for support of specific subsets of sensory neurons with unique phenotypic and functional properties. Although these proteins have central roles in afferent development and function, challenges remain in identifying specific molecular mechanisms of growth factor communication and understanding how activation of signaling pathways direct neuron differentiation and function under normal and pathological conditions.

Neuronal damage accompanies perinatal white-matter damage

Extremely low-gestational-age newborns have a prominently increased risk of brain dysfunctions attributed to white-matter damage, which is thought to result from the vulnerability of the oligodendrocyte. This white-matter damage now appears to be accompanied by cerebral-cortex and deep-gray-matter abnormalities, including excess apoptosis without replacement and the impairment of surviving neurons and resulting interference with synaptogenesis and connectivity. Recent advances in corticogenesis suggest that neurons migrate from the germinative zones through the white matter to the cortex when the white matter is most vulnerable and perhaps is being injured. Advances in developmental neuroscience also suggest that the excitotoxic and inflammatory processes that probably contribute to white-matter damage are also able to damage developing neurons. Together, these advances support the untested hypothesis that white-matter damage in the preterm newborn is accompanied by the death of neurons as they migrate through the dangerous minefield of white matter undergoing injury.

A new technique for real-time analysis of caspase-3 dependent neuronal cell death

Several markers are available to identify cells undergoing programmed cell death, but so far they are only applicable on fixed material. Therefore, no information on the kinetics of apoptosis can be obtained, although apoptosis is a dynamic cell process. Here, we describe a new technique that allows the real-time observation of the onset of apoptosis in primary neurons. Neurons are transfected with a plasmid that codes for a fluorescent protein localized in the soma. Upon activation of caspase-3, which represents the point-of-no-return in the apoptosis process, the fusion protein is cleaved and as a consequence translocates into the nucleus. The onset of apoptosis is thus visualized by translocation of the fluorescent signal from the soma to the nucleus. The translocation process was found to be specific for the apoptosis process as it correlates with the activation of caspase-3 and TUNEL staining. This tool does not require complex detection systems and allows for the first time the analysis of the kinetics of apoptosis in a simple and efficient manner.

Neuromuscular development in the absence of programmed cell death: phenotypic alteration of motoneurons and muscle

The widespread, massive loss of developing neurons in the central and peripheral nervous system of birds and mammals is generally considered to be an evolutionary adaptation. However, until recently, models for testing both the immediate and long-term consequences of preventing this normal cell loss have not been available. We have taken advantage of several methods for preventing neuronal death in vivo to ask whether rescued neurons [e.g., motoneurons (MNs)] differentiate normally and become functionally incorporated into the nervous system. Although many aspects of MN differentiation occurred normally after the prevention of cell death (including the expression of several motoneuron-specific markers, axon projections into the ventral root and peripheral nerves, ultrastructure, dendritic arborization, and afferent axosomatic synapses), other features of the neuromuscular system (MNs and muscle) were abnormal. The cell bodies and axons of MNs were smaller than normal, many MN axons failed to become myelinated or to form functional synaptic contacts with target muscles, and a subpopulation of rescued cells were transformed from alpha- to gamma-like MNs. Additionally, after the rescue of MNs in myogenin glial cell line-derived neurotrophic factor (MyoGDNF) transgenic mice, myofiber differentiation of extrafusal skeletal muscle was transformed and muscle physiology and motor behaviors were abnormal. In contrast, extrafusal myofiber phenotype, muscle physiology, and (except for muscle strength tests) motor behaviors were all normal after the rescue of MNs by genetic deletion of the proapoptotic gene Bax. However, there was an increase in intrafusal muscle fibers (spindles) in Bax knock-out versus both wild-type and MyoGDNF mice. Together, these data indicate that after the prevention of MN death, the neuromuscular system becomes transformed in novel ways to compensate for the presence of the thousands of excess cells.

The scoop on the fly brain: glial engulfment functions in Drosophila

Glial cells provide support and protection for neurons in the embryonic and adult brain, mediated in part through the phagocytic activity of glia. Glial cells engulf apoptotic cells and pruned neurites from the developing nervous system, and also clear degenerating neuronal debris from the adult brain after neural trauma. Studies indicate that Drosophila melanogaster is an ideal model system to elucidate the mechanisms of engulfment by glia. The recent studies reviewed here show that many features of glial engulfment are conserved across species and argue that work in Drosophila will provide valuable cellular and molecular insight into glial engulfment activity in mammals.

Cathepsin D gene expression outlines the areas of physiological cell death during embryonic development

The implication of lysosomes in the activation of physiological cell death (PCD) was proposed some decades ago. In this work, we show that the expression of the lysosomal enzyme cathepsin D is up-regulated in developing tissues undergoing apoptosis. By comparing vital and terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL) labeling patterns with in situ hybridization for this gene in a variety of tissues and organs, we show that this procedure constitutes a reliable technique to map the regions of PCD in the embryo. Using this methodological approach, we report the occurrence of two new areas of PCD in the developing limb.

Apoptosome dependent caspase-3 activation pathway is non-redundant and necessary for apoptosis in sympathetic neurons

Although sympathetic neurons are a well-studied model for neuronal apoptosis, the role of the apoptosome in activating caspases in these neurons remains debated. We find that the ability of sympathetic neurons to undergo apoptosis in response to nerve growth factor (NGF) deprivation is completely dependent on having an intact apoptosome pathway. Genetic deletion of Apaf-1, caspase-9, or caspase-3 prevents apoptosis after NGF deprivation, and importantly, allows these neurons to recover and survive long-term following readdition of NGF. The inability of caspase-3 deficient sympathetic neurons to undergo apoptosis is particularly striking, as apoptosis in dermal fibroblasts and cortical neurons proceeds even in the absence of caspase-3. Our results show that in contrast to dermal fibroblasts and cortical neurons, sympathetic neurons express no detectable levels of caspase-7. The strict requirement for an intact apoptosome, coupled with a lack of effector caspase redundancy, provides sympathetic neurons with a markedly increased control over their apoptotic pathway.