Neuron Pruning Research Articles

Genomic Analysis of Drosophila Neuronal Remodeling: A Role for the RNA-Binding Protein Boule as a Negative Regulator of Axon Pruning

Drosophila mushroom body (MB) gamma neurons undergo axon pruning during metamorphosis through a process of localized degeneration of specific axon branches. Developmental axon degeneration is initiated by the steroid hormone ecdysone, acting through a nuclear receptor complex composed of USP (ultraspiracle) and EcRB1 (ecdysone receptor B1) to regulate gene expression in MB gamma neurons. To identify ecdysone-dependent gene expression changes in MB gamma neurons at the onset of axon pruning, we use laser capture microdissection to isolate wild-type and mutant MB neurons in which EcR (ecdysone receptor) activity is genetically blocked, and analyze expression changes by microarray. We identify several molecular pathways that are regulated in MB neurons by ecdysone. The most striking observation is the upregulation of genes involved in the UPS (ubiquitin-proteasome system), which is cell autonomously required for gamma neuron pruning. In addition, we characterize the function of Boule, an evolutionarily conserved RNA-binding protein previously implicated in spermatogenesis in flies and vertebrates. boule expression is downregulated by ecdysone in MB neurons at the onset of pruning, and forced expression of Boule in MB gamma neurons is sufficient to inhibit axon pruning. This activity is dependent on the RNA-binding domain of Boule and a conserved DAZ (deleted in azoospermia) domain implicated in interactions with other RNA-binding proteins. However, loss of Boule does not result in obvious defects in axon pruning or morphogenesis of MB neurons, suggesting that it acts redundantly with other ecdyonse-regulated genes. We propose a novel function for Boule in the CNS as a negative regulator of developmental axon pruning.

PiggyBac-Based Mosaic Screen Identifies a Postmitotic Function for Cohesin in Regulating Developmental Axon Pruning

Developmental axon pruning is widely used to refine neural circuits. We performed a mosaic screen to identify mutations affecting axon pruning of Drosophila mushroom body gamma neurons. We constructed a modified piggyBac vector with improved mutagenicity and generated insertions in >2000 genes. We identified two cohesin subunits (SMC1 and SA) as being essential for axon pruning. The cohesin complex maintains sister-chromatid cohesion during cell division in eukaryotes. However, we show that the pruning phenotype in SMC1(-/-) clones is rescued by expressing SMC1 in neurons, revealing a postmitotic function. SMC1(-/-) clones exhibit reduced levels of the ecdysone receptor EcR-B1, a key regulator of axon pruning. The pruning phenotype is significantly suppressed by overexpressing EcR-B1 and is enhanced by a reduced dose of EcR, supporting a causal relationship. We also demonstrate a postmitotic role for SMC1 in dendrite targeting of olfactory projection neurons. We suggest that cohesin regulates diverse aspects of neuronal morphogenesis.

Optimization and stabilization of sequential learning in RBF network for nonlinear function approximation

This paper proposes a solution for inconsistency pruning of neurons within a sequential learning Radial Basis Function (RBF) Network. This paper adopts the concept that a specific RBF neuron which continuously exhibits low output in a sequence of training patterns does not justify the proposition that the neuron is insignificant to the whole function to be approximated. We establish additional criterions to provide protection from error in pruning RBF neurons within the hidden layer, which we prove is able to improve consistency and stability of neuron evolution. With such stability within the sequential learning process, we also show how the convergence speed of the network can be improved by reducing the number of consecutive observations required to prune a neuron in the hidden layer.

Cell Signaling and Neuronal Death

The past few decades have revealed that cell death can be precisely programmed with two principal forms, apoptosis and necrosis. Besides pathophysiological alterations, physiologic processes, such as the pruning of neurons during normal development and the involution of the thymus, involve apoptosis. This review focuses on the role of inter- and intracellular signaling systems in cell death, especially in the nervous system. Among neurotransmitters, glutamate and nitric oxide have been most extensively characterized and contribute to cell death in excitotoxic damage, especially in stroke and possibly in neurodegenerative diseases. Within cells, calcium, the most prominent of all intracellular messengers, mediates diverse forms of cell death with actions modulated by many proteins, including IP3 receptors, calcineurin, calpain, and cytochrome c.

Identification of E2/E3 Ubiquitinating Enzymes and Caspase Activity Regulating Drosophila Sensory Neuron Dendrite Pruning

Ubiquitin-proteasome system (UPS) is a multistep protein degradation machinery implicated in many diseases. In the nervous system, UPS regulates remodeling and degradation of neuronal processes and is linked to Wallerian axonal degeneration, though the ubiquitin ligases that confer substrate specificity remain unknown. Having shown previously that class IV dendritic arborization (C4da) sensory neurons in Drosophila undergo UPS-mediated dendritic pruning during metamorphosis, we conducted an E2/E3 ubiquitinating enzyme mutant screen, revealing that mutation in ubcD1, an E2 ubiquitin-conjugating enzyme, resulted in retention of C4da neuron dendrites during metamorphosis. Further, we found that UPS activation likely leads to UbcD1-mediated degradation of DIAP1, a caspase-antagonizing E3 ligase. This allows for local activation of the Dronc caspase, thereby preserving C4da neurons while severing their dendrites. Thus, in addition to uncovering E2/E3 ubiquitinating enzymes for dendrite pruning, this study provides a mechanistic link between UPS and the apoptotic machinery in regulating neuronal process remodeling.

Clearing Damaged or Unneeded Axons

Three reports this week investigate the mechanisms by which excess axons are pruned during development to fine-tune synaptic connections and injured axons are cleared. Awasaki et al ., MacDonald et al ., and Hoopfer et al . investigated Drosophila axonal clearance. Awasaki et al . showed that axons of γ neurons in the mushroom body are removed during metamorphosis by recruitment of glia with processes that contain the draper and Ced-6 proteins. Ced-6 and draper, which is the fly homolog of the nematode Ced-1, are both involved in engulfment and removal of apoptotic cells. The γ neurons of the mushroom body do not undergo apoptosis; thus, a similar mechanism appears to mediate recognition of dying cells and neuronal processes destined for pruning during development. Draper mutant ( drpr Δ5 ) flies or flies expressing an RNAi in glia to suppress draper synthesis exhibited a lack of mushroom body γ neuron pruning and loss of infiltrating glia and glial engulfment activity. MacDonald et al . used the Drosophila olfactory system as the model to show that draper in glia was required for clearance of injured (transected) axons. Furthermore, transected axons underwent a process of Wallerian degeneration similar to that observed in mammals, because introduction of the mouse Wld s (a chimeric protein composed of nicotinamide mononucleotide adenylyltransferase and an E4 ubiquitin ligase) rescued transected axons from degeneration for weeks. Hoopfer et al . show that although Wld s rescues injured axons from degeneration, it does not prevent pruning of axons during development. Hoopfer et al . examined retinal ganglion cell (RGC) and cortical layer 5 pruning in wild-type and Wld s mice and found no difference in the remodeling that occurs during development. However, the Wld s mice showed delayed degeneration of RGC in response to transection. Hoopfer et al . found no effect of Wld s in Drosophila on the pruning of γ axons of the mushroom body and also reported that Wld s delayed degeneration of Drosophila olfactory neurons following transection. Awasaki et al . reported that ecdysone signaling was required for mushroom body pruning. A dominant-negative ecdysone receptor inhibited the increase in draper in the glia and the cytoskeletal disruption exhibited by the axons that were targeted for pruning. Hoopfer et al . showed that ecdysone signaling in the transected axons did not appear to be involved in their degeneration. The results from these three studies suggest that although the initiating events in axon pruning and degeneration are different, the mechanism by which glia recognize and remove these neuronal processes involve the same components, which are also common to the process by which apoptotic cells are recognized and eliminated. See Fainzilber and Twiss for a discussion of these articles and more about the Wld s mutation. T. Awasaki, R. Tatsumi, K. Takahashi, K. Arai, Y. Nakanishi, R. Ueda, K. Ito, Essential role of the apoptotic cell engulfment genes draper and ced-6 in programmed axon pruning during Drosophila metamorphosis. Neuron 50 , 855-867 (2006). [Online Journal] J. M. MacDonald, M. G. Beach, E. Porpiglia, A. E. Sheehan, R. J. Watts, M. R. Freeman, The Drosophila cell corpse engulfment receptor Draper mediates glial clearance of severed axons. Neuron 50 , 869-881 (2006). [Online Journal] E. D. Hoopfer, T. McLaughlin, R. J. Watts, O. Schuldiner, D. D. M. O'Leary, L. Luo, Wld s protection distinguishes axon degeneration following injury from naturally occurring developmental pruning. Neuron 50 , 883-895 (2006). [Online Journal] M. Fainzilber, J. L. Twiss, Tracking in the Wld s : The hunting of the SIRT and the luring of the Draper. Neuron 50 , 819-821 (2006). [Online Journal]

Essential Role of the Apoptotic Cell Engulfment Genes draper and ced-6 in Programmed Axon Pruning during Drosophila Metamorphosis

Axon pruning is a common phenomenon in neural circuit development. Previous studies demonstrate that the engulfing action of glial cells is essential in this process. The underlying molecular mechanisms, however, remain unknown. We show that draper (drpr) and ced-6, which are essential for the clearance of apoptotic cells in C. elegans, function in the glial engulfment of larval axons during Drosophila metamorphosis. The drpr mutation and glia-specific knockdown of drpr and ced-6 by RNA interference suppress glial engulfment, resulting in the inhibition of axon pruning. drpr and ced-6 interact genetically in the glial action. Disruption of the microtubule cytoskeleton in the axons to be pruned occurs via ecdysone signaling, independent of glial engulfment. These findings suggest that glial cells engulf degenerating axons through drpr and ced-6. We propose that apoptotic cells and degenerating axons of living neurons are removed by a similar molecular mechanism.

Morphology and metamorphosis of the peptidergic Va neurons and the median nerve system of the fruit fly, Drosophila melanogaster

Metamorphosis is a fundamental developmental process and has been intensively studied for various neuron types of Drosophila melanogaster. However, detailed accounts of the fate of identified peptidergic neurons are rare. We have performed a detailed study of the larval morphology and pupal remodelling of identified peptidergic neurons, the CAPA-expressing Va neurons of D. melanogaster. In the larva, Va neurons innervate abdominal median and transverse nerves that are typically associated with perisympathetic organs (PSOs), major neurohaemal release sites in insects. Since median and transverse nerves are lacking in the adult, Va neurites have to undergo substantial remodelling during metamorphosis. We have examined the hitherto uncharacterised gross morphology of the thoracic PSOs and the abdominal median and transverse nerves by scanning electron microscopy and found that the complete reduction of these structures during metamorphosis starts around pupal stage P7 and is completed at P9. Concomitantly, neurite pruning of the Va neurons begins at P6 and is preceded by the high expression of the ecdysone receptor (EcR) subtype B1 in late L3 larvae and the first pupal stages. New neuritic outgrowth mainly occurs from P7-P9 and coincides with the expression of EcR-A, indicating that the remodelling of the Va neurons is under ecdysteroid control. Immunogold-labelling has located the CAPA peptides to large translucent vesicles, which are released from the transverse nerves, as suggested by fusion profiles. Hence, the transverse nerves may serve a neurohaemal function in D. melanogaster.

Pruning the Brain

Thrombopoietin (TPO) and erythropoietin (EPO) both act as hematopoietic growth factors: TPO stimulates platelet formation, whereas EPO stimulates erythrocyte formation. Noting that EPO acts as a survival factor in the developing and stressed adult brain, Ehrenreich et al. investigated the possible role of brain TPO. Whereas the amount of mouse brain EPO mRNA decreased between embryonic day 11 (E11) and E15, that of TPO continued to rise into adulthood. Similarly, brain EPO protein expression was decreased in the adult compared with that at E11 or E13, whereas TPO protein expression was greater in the adult. In primary cultures of hippocampal neurons, hypoxia stimulated an increase in EPO mRNA and protein, whereas it caused a decrease in TPO mRNA and protein. Under normoxic conditions, concentrations of TPO that stimulated hematopoietic stem cell proliferation unexpectedly promoted the death of cultured hippocampal neurons, an effect that was blocked by EPO as well as by granulocyte colony-stimulating factor, neurotrophin-3, or brain-derived neurotrophic factor (BDNF). Pharmacological inhibition of the protein kinase JAK2, extracellular signal-regulated kinase (ERK1/2), or caspase-3 blocked TPO-induced neuronal death, whereas inhibition of phosphatidylinositol 3-kinase (PI3K) blocked the ability of EPO to rescue neurons from TPO-mediated death. TPO enhanced apoptosis of young neurons (neurons that expressed β-tubulin III) in juvenile rats in a model of moderate hypoxic-ischemic brain injury. Thus, whereas other growth factors enhance neuronal survival, the authors propose that TPO may contribute to the pruning of extraneous neurons that takes place during late stages of brain development. H. Ehrenreich, M. Hasselblatt, F. Knerlich, N. von Ahsen, S. Jacob, S. Sperling, H. Wolge, K. Vehmeyer, K.-A. Nave, A.-L. Sirén, A hematopoietic growth factor, thrombopoietin, has a proapoptotic role in the brain. Proc. Natl. Acad. Sci. U.S.A. 102 , 862-867 (2005). [Abstract] [Full Text]

Abuse Said to Interfere With Child Brain Development

Abuse Said to Interfere With Child Brain Development

Artificial neural networks estimate the contribution of taste neurons to coding

Taste qualities are believed to be coded in the activity of populations of taste neurons. However, it is not clear whether all neurons are equally responsible for coding. To clarify the point the relative contribution of each taste neuron to coding was assessed by constructing simple three-layer neural networks with input neurons that represent cortical taste neurons of the rat. The networks were trained by the back-propagation learning algorithm to classify the neural response patterns to the basic taste stimuli (sucrose, HCl, quinine–hydrochloride, and NaCl). The networks had four output neurons representing the basic taste qualities, the values of which provide a measure for similarity of test stimuli to the basic taste stimuli. We estimated relative contributions of input neurons to the taste discrimination of the network by examining their significance S j , which is defined as the sum of the absolute values of the connection weights from the jth input neuron to the hidden layer. When the input neurons with a smaller S j (e.g., 15 out of 39 input units) were “pruned” from the trained network, the ability of the network to discriminate the basic taste qualities was not greatly affected. On the other hand, the taste discrimination of the network progressively deteriorated much more rapidly with pruning of input neurons with a larger S j . These results suggest that cortical taste neurons differentially contribute to the coding of taste qualities. Input neurons with a larger S j tended to be with a larger variation of neural discharge rates to the basic taste stimuli. The variation of neural discharges may be important in the coding of taste qualities.

The Application of Hybrid Evolving Connectionist Systems to Image Classification

This paper presents a methodology for image classification of both spatial and spectral data with the use of hybrid evolving fuzzy neural networks (EFuNNS). EFuNNs are five layer sparsely connected networks. EFuNNs contain dynamic structures that evolve by growing and pruning of neurons and connections. EFuNNS merge three supervised classification methods: connectionism, fuzzy logic, and case-based reasoning. By merging these strategies, this new structure is capable of learning and generalising from a small sample set of large attribute vectors as well as from large sample sets and small feature vectors. Two case studies data are used to demonstrate the effectiveness of the methodology. First, an environmental remote sensing application, and second, large scale images of fruit for automated grading. The proposed methodology provides fast and accurate adaptive learning for image classification. It is also applicable for on-line, real-time learning and classification.

Neurodevelopmental abnormalities in schizophrenia: insights from neuropathology.

Growing epidemiological, genetic, and clinical neurobiological evidence indicates that abnormalities in brain development play determining roles in the pathobiology of schizophrenia. Neuropathological research has made significant progress in delineating cellular and molecular abnormalities in schizophrenia that have relevance to neurodevelopment. This paper reviews the neurodevelopmental processes of neurogenesis, neuronal migration, differentiation, synaptogenesis, neuron and synaptic pruning, and myelination and the reported neuropathological findings in schizophrenia that may be a consequence of disturbances in these processes. While many neuropathological findings in schizophrenia are controversial or await confirmation, reported abnormalities in neuron density, number and morphology, cytoarchitecture, dendritic arbors and spines, synapse-related proteins, and the well-established absence of gliosis or any other evidence of neurodegeneration or neural injury all provide support for the neurodevelopmental model of schizophrenia.

Is schizophrenia a lifetime disorder of brain plasticity, growth and aging?

Chronic schizophrenia is characterized by change in the normal brain cortical structure, asymmetric reduction, and ventricular enlargement. The debate continues as to whether these anomalies occur early in development or represent an active progressive process continuing after the onset of psychosis. The case is made in the present manuscript for a continuing aberrant lifetime brain process. It is proposed that the underlying basis for the neuropathology of schizophrenia resides in the periodic activation of a defective gene or genes that determine the rate of cerebral growth. This process causes subtle cortical maldevelopment prenatally and through early childhood, is activated again during adolescent pruning of neurons, and again during the gradual aging process in the brain throughout adulthood. The case for a progressive active brain process in schizophrenia is thus presented.

An Adaptive Structure Neural Networks with Application to EEG Automatic Seizure Detection

This paper introduces a new algorithm for adaptively adjusting the structure of a multi-layer back-propagation network. The proposed algorithm belongs to the class of neuron generating strategies as opposed to the class of neuron pruning strategies. Initially a “small” multi-layer perceptron network is selected. The stabilized error is used as an index to determine whether the network needs to generate a new neuron or not. If after a period of learning the error is stabilized, but the error is larger than a desired value, then new neuron(s) is (are) generated. The new neurons are placed at locations that contribute most to the network error behavior through the fluctuation in their input weight vectors. Among the features of the new architecture are its improved performance and generalization capabilities compared to a standard fixed-structure back-propagation network. Application to an electroencephalogram (EEG) automatic epileptic seizure detection is presented to illustrate advantages and capabilities of the proposed algorithm. Using an actual data from five patients it is shown that the proposed approach correctly identifies all true seizures that are also identified by an expert physician. The new algorithm provides a reduction of 60–70% in the training epochs as compared to a back-propagation algorithm. Furthermore, it is shown that by utilizing a new training algorithm it is possible to reduce the false seizure detections to zero while resulting in a 5.1% error in identifying the true seizures. Copyright © 1996 Elsevier Science Ltd

Pruning of rat cortical taste neurons by an artificial neural network model.

1. Taste qualities are believed to be coded in the activity of ensembles of taste neurons. However, it is not clear whether all neurons are equally responsible for coding. To clarify the point, the relative contribution of each taste neuron to coding needs to be assessed. 2. We constructed simple three-layer neural networks with input units representing cortical taste neurons of the rat. The networks were trained by the back-propagation learning algorithm to classify the neural response patterns to the basic taste stimuli (sucrose, HCl, quinine hydrochloride, and NaCl). The networks had four output units representing the basic taste qualities, the values of which provide a measure for similarity of test stimuli (salts, tartaric acid, and umami substances) to the basic taste stimuli. 3. Trained networks discriminated the response patterns to the test stimuli in a plausible manner in light of previous physiological and psychological experiments. Profiles of output values of the networks paralleled those of across-neuron correlations with respect to the highest or second-highest values in the profiles. 4. We evaluated relative contributions of input units to the taste discrimination of the network by examining their significance Sj, which is defined as the sum of the absolute values of the connection weights from the jth input unit to the hidden layer. When the input units with weaker connection weights (e.g., 15 of 39 input units) were "pruned" from the trained network, the ability of the network to discriminate the basic taste qualities as well as other test stimuli was not greatly affected. On the other hand, the taste discrimination of the network progressively deteriorated much more rapidly with pruning of input units with stronger connection weights. 5. These results suggest that cortical taste neurons differentially contribute to the coding of taste qualities. The pruning technique may enable the evaluation of a given taste neuron in terms of its relative contribution to the coding, with Sj providing a quantitative measure for such evaluation.

Deviations in brain development of F2 generation on caloric undernutrition and scope of their prevention by rehabilitation: alterations in dendritic spine production and pruning of pyramidal neurons of lower laminae of motor cortex and visual cortex

This is a report of comparison of developmental changes of spine densities on the different categories of dendrites of neocortical pyramidal neurons (V and VI layers of motor and visual areas) of Wistar rat, during 11–150 days of age, under conditions of normal nutrition and under chronic caloric but not protein deprivation. The studied animals were of F2 generation born to parents undernourished to a degree that their weights were only 40–50% of normal control. At such a level they would be active, reproduce, and not morbid. Similar level of undernutrition also continued in the F2 group studies. A group of undernourished animals was also, for rehabilitation, put on normal diet from 21 days of age. Visual and motor cortical area pieces were impregnated by Stensaas' rapid Golgi method. Spines were counted on successive 20-μm segments (I-IV) of both apical and basal dendritic main shafts as well as primary and secondary branches. The spine count per segment (density) in the normal population reached exuberant values by 26–50 days of age and later underwent a progressive decline or pruning by 30–50% or more by 150 days of age. The degrees of exuberance and pruning varied in different categories of dendrites, generally being more conspicuous in motor than visual cortex, and more in basal than apical dendrites. Under the conditions of chronic caloric restriction, the phenomenon of exuberance was retarded and pruning was not observed. On the contrary, there was a progressive increase in the spine densities on both basal and apical dendrites, in motor and visual cortex. By 150 days of age, the spine densities were not only greater than the final counts for respective segments in the normal animal, but even greater than the exuberant counts. Postweaning caloric rehabilitation had only a modest impact against this deviation. Preliminary data (intersections) of dendritic branching also indicated a similar pattern of changes (lag followed by increase), but of a lesser degree. These alterations in neuronal development are interpreted as biological adaptations evoked in shaping the homeostasis of the organism's brain and behaviour by factors of nurture.

Inhibitory of glycine on spinal neurons in the cat.

Inhibitory of glycine on spinal neurons in the cat.

Excitability and inhibitability of motoneurons of different sizes.

Excitability and inhibitability of motoneurons of different sizes.