Alcohol and adult neurogenesis: roles in neurodegeneration and recovery in chronic alcoholism

The concept of "structural plasticity" has emerged as a potential mechanism in neurodegenerative and psychiatric diseases such as drug abuse, depression, and dementia. Chronic alcoholism is a progressive neurodegenerative disease while the person continues to abuse alcohol, though clinical and imaging studies show that some recovery may occur with abstinence. The neural plasticity observed in chronic alcoholism coupled with conflicting reports on alcohol-induced hippocampal neuropathology make this disease ripe for reconsideration in terms of the phenomenon of adult neurogenesis. This review describes opposing neurogenic processes that occur with alcohol intoxication and abstinence following alcohol dependence and how these opposing events relate to neurodegeneration and recovery from chronic alcoholism.     

Differential roles of TNFR1 and TNFR2 signaling in adult hippocampal neurogenesis

Tumor necrosis factor alpha (TNFα) is a potent inhibitor of neurogenesis in vitro but here we show that TNFα signaling has both positive and negative effects on neurogenesis in vivo and is required to moderate the negative impact of cranial irradiation on hippocampal neurogenesis. In vitro, basal levels of TNFα signaling through TNFR2 are required for normal neural progenitor cell proliferation while basal signaling through TNFR1 impairs neural progenitor proliferation. TNFR1 also mediates further reductions in proliferation and elevated cell death following exposure to recombinant TNFα. In vivo, TNFR1^−/− and TNFα^−/− animals have elevated baseline neurogenesis in the hippocampus, whereas absence of TNFR2 decreases baseline neurogenesis. TNFα is also implicated in defects in neurogenesis that follow radiation injury but we find that loss of TNFR1 has no protective effects on neurogenesis and loss of TNFα or TNFR2 worsened the effects of radiation injury on neurogenesis. We conclude that the immunomodulatory signaling of TNFα mediated by TNFR2 is more significant to radiation injury outcome than the proinflammatory signaling mediated through TNFR1.     

Deletion of the Pemt gene increases progenitor cell mitosis, DNA and protein methylation and decreases calretinin expression in embryonic day 17 mouse hippocampus

Choline is a required nutrient and is derived from the diet as well as from de novo synthesis catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). We previously reported that choline availability during pregnancy alters mitosis and neuronal protein expression during fetal brain development in wild-type mice and rats, and that Pemt-/- mice become choline deficient. In this study, we examined brain development in these knockout mice. Pregnant Pemt-/- and wild-type mice were fed AIN-76A diet until gestation day 17 (E17) when the fetal brains were harvested. Phosphorylation of histone H3 (a measure of mitosis) and calretinin (a GABAergic neuronal marker) were assessed in hippocampal regions. We observed increased numbers of phosphorylated histone H3 positive cells in the Pemt-/- mice (up 54% compared to wild-type mice; p<0.01). We also found decreased calretinin labeling in Pemt-/- (down to 43% compared to wild-type mice; p<0.01). Thus, there was increased stem cell proliferation in the neuroepithelium and decreased GABAergic neuronal differentiation of these animals on E17. These results are opposite to what would have been expected in choline-deficient mice. The concentrations of S-adenosylmethionine (up 21%; p<0.05) and methylation of DNA (up 46%) and proteins (up 12%; p<0.01) in hippocampus were significantly increased in Pemt-/- mice, suggesting that increased S-adenosylmethionine availability may mediate the observed developmental changes. This is the first report of altered brain development in Pemt-/- mice.     

DAGL-dependent endocannabinoid signalling: roles in axonal pathfinding, synaptic plasticity and adult neurogenesis

Until recently, endocannabinoid (eCB) signalling was largely studied in the context of synaptic plasticity in the postnatal brain in the absence of detailed knowledge of the nature of the enzyme(s) responsible for the synthesis of the eCBs. However, the identification of two diacylglycerol lipases (DAGLα and DAGLβ) responsible for the synthesis of 2-arachidonoylglycerol (2-AG) has increased the understanding of where this eCB is synthesised in relationship to the expression of cannabinoid receptor (CB)1 and CB2. Furthermore, the generation of knockout animals for each enzyme has allowed for the direct testing of their importance for established and emerging eCB functions. Based on this, we now know that DAGLα is enriched in dendritic spines that appose CB1-positive synaptic terminals, and that 2-AG functions as a retrograde signal controlling synaptic strength throughout the nervous system. Consequently, we have built on the principle that expression of eCB components dictates function to identify additional physiological functions of this signalling cassette. A number of studies have now provided support for DAGL-dependent eCB signalling playing important roles in brain development and in cellular plasticity in the adult nervous system. In this article, we will review evidence based on the localisation of the enzymes, as well as from genetic and pharmacological studies, that show DAGL-dependent eCB signalling to play an important role in axonal growth and guidance during development, in retrograde synaptic signalling at mature synapses, and in the control of adult neurogenesis in the hippocampus and subventricular zone.     

Shifting equilibriums in Alzheimer's disease: the complex roles of microglia in neuroinflammation,neuronal survival and neurogenesis

Alzheimer's disease is the leading cause of dementia.Its increased prevalence in developed countries,due to the sharp rise in ageing populations,presents one of the costliest challenges to modern medicine.In order to find disease-modifying therapies to confront this challenge,a more complete understanding of the pathogenesis of Alzheimer's disease is necessary.Recent studies have revealed increasing evidence for the roles played by microglia,the resident innate immune system cells of the brain.Reflecting the well-established roles of microglia in reacting to pathogens and inflammatory stimuli,there is now a growing literature describing both protective and detrimental effects for individual cytokines and chemokines produced by microglia in Alzheimer's disease.A smaller but increasing number of studies have also addressed the divergent roles played by microglial neurotrophic and neurogenic factors,and how their perturbation may play a key role in the pathogenesis of Alzheimer's disease.Here we review recent findings on the roles played by microglia in neuroinflammation,neuronal survival and neurogenesis in Alzheimer's disease.In each case,landmark studies have provided evidence for the divergent ways in which microglia can either promote neuronal function and survival,or perturb neuronal function,leading to cell death.In many cases,the secreted molecules of microglia can lead to divergent effects depending on the magnitude and context of microglial activation.This suggests that microglial functions must be maintained in a fine equilibrium,in order to support healthy neuronal function,and that the cellular microenvironment in the Alzheimer's disease brain disrupts this fine balance,leading to neurodegeneration.Thus,an understanding of microglial homeostasis,both in health and across the trajectory of the disease state,will improve our understanding of the pathogenic mechanisms underlying Alzheimer's disease,and will hopefully lead to the development of microglial-based therapeutic strategies to restore equilibrium in the Alzheimer's disease brain.     

The roles of reactive gliosis and mitosis on tropic factor production in traumatized nervous system tissue

Proximal stumps of rat sciatic nerves were attached to inlet ends of Y-shaped silastic implants and offered a 'choice' of growing toward an Elvax pellet containing homogenate from previously crushed optic nerve which had been exposed to saline or cytosine arabinofuranoside (AraC). Previous studies indicate that AraC administration inhibits reactive gliosis in crushed optic nerve. Preferential or exclusive growth of axons occurred in implant forks attached to pellets containing saline- (vs drug-) exposed optic nerve homogenates. In contrast, inhibition of Schwann cell mitosis had no discernible effect on tropic factor production in distal stumps of transected sciatic nerves. Tropic activity of homogenates from cultures containing reactive-like astrocytes was nearly 4 times higher than homogenates not containing these cells. Results suggest a possible link between formation of hypertrophic reactive astrocytes and production of neurotropic factor.     

Different Fgfs have distinct roles in regulating neurogenesis after spinal cord injury in zebrafish

Background

Despite conserved developmental processes and organization of the vertebrate central nervous system, only some vertebrates including zebrafish can efficiently regenerate neural damage including after spinal cord injury. The mammalian spinal cord shows very limited regeneration and neurogenesis, resulting in permanent life-long functional impairment. Therefore, there is an urgent need to identify the cellular and molecular mechanisms that can drive efficient vertebrate neurogenesis following injury. A key pathway implicated in zebrafish neurogenesis is fibroblast growth factor signaling.

Methods

In the present study we investigated the roles of distinct fibroblast growth factor members and their receptors in facilitating different aspects of neural development and regeneration at different timepoints following spinal cord injury. After spinal cord injury in adults and during larval development, loss and/or gain of Fgf signaling was combined with immunohistochemistry, in situ hybridization and transgenes marking motor neuron populations in in vivo zebrafish and in vitro mammalian PC12 cell culture models.

Results

Fgf3 drives neurogenesis of Islet1 expressing motor neuron subtypes and mediate axonogenesis in cMet expressing motor neuron subtypes. We also demonstrate that the role of Fgf members are not necessarily simple recapitulating development. During development Fgf2, Fgf3 and Fgf8 mediate neurogenesis of Islet1 expressing neurons and neuronal sprouting of both, Islet1 and cMet expressing motor neurons. Strikingly in mammalian PC12 cells, all three Fgfs increased cell proliferation, however, only Fgf2 and to some extent Fgf8, but not Fgf3 facilitated neurite outgrowth.

Conclusions

This study demonstrates differential Fgf member roles during neural development and adult regeneration, including in driving neural proliferation and neurite outgrowth of distinct spinal cord neuron populations, suggesting that factors including Fgf type, age of the organism, timing of expression, requirements for different neuronal populations could be tailored to best drive all of the required regenerative processes.

     

Huntingtin interacting protein 1 can regulate neurogenesis in Drosophila

Huntington's disease (HD) is associated with a range of cellular consequences including selective neuronal death and decreased levels of neurogenesis. Ultimately, these altered processes are dependent upon proteins that interact with Huntingtin (Htt) such as the Huntingtin-interacting protein 1 (Hip1) which has a reduced binding preference to expanded Htt. These effects are similar to those observed with modified Notch signal transduction. As Hip1 plays a key role in endocytosis and intracellular transport, and activation of the Notch signal requires both, we investigated putative links between Hip1 and Notch signaling in flies. We have identified two forms of Hip1 that may be produced through the use of alternative first exons: a version of Hip1 with a lipid-binding ANTH domain and Hip1ΔANTH lacking this domain. The directed expression of Hip1 decreases, while expression of Hip1ΔANTH increases, the density of sensory microchaetae on the dorsal notum, a classical model of neurogenesis. A reduction in microchaetae density associated with Notch ^{Microchaetae Deficient (MCD)} ( N ^MCD ) alleles is sensitive to both Hip1 and Hip1ΔANTH levels, as are the bristle phenotypes generated by misexpression of deltex , a key mediator of Notch signaling. Genetic studies further demonstrate that the observed effects of Hip1 and of Hip1ΔANTH are sensitive to achaete gene dosage while insensitive to the levels of E(Spl) , suggesting a non-canonical Notch neurogenic signal through a deltex- dependent pathway. The novel role we describe for Hip1 in Notch-mediated neurogenesis provides a functional link between Notch signaling and proteins related to HD.     

Expression of embryonic tau protein isoforms persist during adult neurogenesis in the hippocampus

Tau is a microtubule-associated protein with a developmentally regulated expression of multiple isoforms. The neonatal isoform is devoid of two amino terminal inserts and contains only three instead of four microtubule-binding repeats (0N/3R-tau). We investigated the temporal expression pattern of 0N-tau and 3R-tau in the rat hippocampus. After the decline of 0N- and 3R-tau immunoreactivity during the postnatal development both isoforms remain highly expressed in a few cells residing beneath the granule cell layer. Coexpression of the polysialylated neuronal cell adhesion molecule, doublecortin, and incorporated bromodeoxyuridine showed that these cells are proliferating progenitor cells. In contrast mature granule cells express the adult tau protein isoform containing one aminoterminal insert domain (1N-tau). Therefore a shift in tau isoform expression takes place during adult neurogenesis, which might be related to migration, differentiation, and integration in the granule cell layer. A model for studying shifts in tau isoform expression in a defined subset of neurons might help to understand the etiology of tauopathies, when isoform composition is crucial for neurodegeneration, as in Pick's disease or FTDP-17.     

TGF-alpha and olfactory marker protein enhance mitosis in rat olfactory epithelium in vivo

Previous studies demonstrated that both transforming growth factor-alpha (TGF-alpha) and olfactory marker protein (OMP) are potent enhancers of mitosis in fetal rat olfactory epithelium grown in organotypic culture. Here we show that when either of these two peptides is administered to adult rats they elicit a significant increase in uptake of tritiated thymidine ([3H]TdR) by olfactory epithelium. In addition OMP promotes an increase in uptake of [3H]TdR in liver, but TGF-alpha has no effect. The data argue that both peptides regulate the rate of cell division in rat olfactory epithelium in vivo and in vitro, and suggest there may be redundancy in the regulatory apparatus modulating cell division in this tissue.     

The immunolocalisation of the neuroendocrine specific protein PGP9.5 during neurogenesis in the rat.

We have examined the immunolocalisation of the protein gene product (PGP) 9.5 during neurogenesis in the rat embryo. PGP9.5 was first present at 11.5 days gestation (E11.5): all morphologically recognisable nerve cell bodies and fibres were immunoreactive. In routinely processed, wax-embedded tissues, using a standard immunocytochemical technique, PGP9.5 polyclonal antibody specifically demonstrated the developing nervous system and primitive adrenal chromaffin cells.     

Expression and phosphorylation of the mid-sized neurofilament protein NF-M during chick spinal cord neurogenesis

The middle molecular weight polypeptide of neurofilaments (NF-M) is modified posttranslationally by extensive phosphorylation. This modification is slow in mature neurons, requiring approximately 24-48 hr for completion and probably occurs outside of the cell soma (Bennett and DiLullo: J Cell Biol 100:1799, 1985c). Thus, NF-M synthesis and phosphorylation are separate events both temporally and spatially. Although it is known that NF-M is among the earliest neuron-specific gene products to be expressed during nervous system development, it is not known what the temporal relationship is between the initiation of NF-M translation and its phosphorylation. To address this question, we have produced an antiserum against the dephosphorylated form of NF-M (NF-M130) and have used this antiserum, together with a previously characterized antiserum against completely phosphorylated NF-M (NF-M160), in an immunohistochemical examination of neurogenesis and the initial period of neuronal differentiation in chick spinal cord. We found that 1) nonphosphorylated and partially phosphorylated NF-M cannot be detected prior to the completion of the terminal mitosis; 2) most postmitotic neuroblasts begin expressing NF-M as they commence migration, but do not contain the completely phosphorylated polypeptide until some time after completion of migration; and 3) those precursor cells of a subpopulation of neuroblasts that begin expressing completely phosphorylated NF-M during their terminal cell cycle (Bennett and DiLullo: Dev Biol 107:94, 1985a) contain no detectable nonphosphorylated or partially phosphorylated NF-M. These cells probably complete the phosphorylation step more rapidly than do mature neurons.     

Mitogen-activated protein kinase signaling, oxygen sensors and hypoxic induction of neurogenesis

In the adult nervous system, neuronal subpopulations sustain a hierarchical pattern of selective vulnerability to hypoxia. Hypoxia also activates quiescent neural progenitor cells (NPCs) resulting in their amplification and subsequent differentiation into neurons and glia. Use of rat organotypic hippocampal cultures facilitates examination of early signaling events in response to hypoxia and reoxygenation that result in neurogenesis. Cultures were exposed to hypoxia for up to 6 h followed by reoxygenation. CA1 neurons showed focal nuclear condensation by 2 h of hypoxia, but CA2 and CA3 neurons were spared. JNKs and c-Jun reached peak activation by 4 h, returning to basal levels by 6 h. Expression of oxygen sensors, hemoxygenase 2 and HIF1, were elevated by 30 min and 2 h, respectively. By 24 h of reoxygenation, there was proliferation of nestin-positive NPCs. With U0126, an upstream inhibitor of ERK activation, BrdU labeling was markedly reduced immunohistochemically as well as PCNA protein expression, suggesting a role for ERKs in the proliferation response. Immunohistochemically, antinestin detected NPCs and on Western blots reached peak levels by 24-48 h of reoxygenation. Proliferation and differentiation of endogenous NPCs in the area of neuronal loss further suggests that mechanisms potentially exist in vitro for replacement with functional neurons.     

Antagonism between Notch and bone morphogenetic protein receptor signaling regulates neurogenesis in the cerebellar rhombic lip

Background

During the embryonic development of the cerebellum, neurons are produced from progenitor cells located along a ventricular zone within dorsal rhombomere 1 that extends caudally to the roof plate of the fourth ventricle. The apposition of the caudal neuroepithelium and roof plate results in a unique inductive region termed the cerebellar rhombic lip, which gives rise to granule cell precursors and other glutamatergic neuronal lineages. Recently, we and others have shown that, at early embryonic stages prior to the emergence of granule cell precursors (E12), waves of neurogenesis in the cerebellar rhombic lip produce specific hindbrain nuclei followed by deep cerebellar neurons. How the induction of rhombic lip-derived neurons from cerebellar progenitors is regulated during this phase of cerebellar development to produce these temporally discrete neuronal populations while maintaining a progenitor pool for subsequent neurogenesis is not known.

Results

Employing both gain- and loss-of-function methods, we find that Notch1 signaling in the cerebellar primordium regulates the responsiveness of progenitor cells to bone morphogenetic proteins (BMPs) secreted from the roof plate that stimulate the production of rhombic lip-derived neurons. In the absence of Notch1, cerebellar progenitors are depleted during the early production of hindbrain neurons, resulting in a severe decrease in the deep cerebellar nuclei that are normally born subsequently. Mechanistically, we demonstrate that Notch1 activity prevents the induction of Math1 by antagonizing the BMP receptor-signaling pathway at the level of Msx2 expression.

Conclusion

Our results provide a mechanism by which a balance between neural induction and maintenance of neural progenitors is achieved in the rhombic lip throughout embryonic development.