Period Of Cell Death Research Articles

CD24−/low stem-like breast cancer marker defines the radiation-resistant cells involved in memorization and transmission of radiation-induced genomic instability

A growing body of evidence attributes properties of chemo- and/or radiation-resistance to cancer stem cells (CSCs). Moreover, non-targeted delayed effects such as genomic instability, transmitted through many generations, can be observed in the progeny of surviving irradiated cells. As a consequence, we propose that radiation-resistance properties associated to CSCs could confer a key role to this subpopulation in the transmission of genomic instability. To test this hypothesis, we searched the CSC markers associated to radiation-resistance in breast cancer cell lines and studied the role of the resistant cells in the transmission of genomic instability. First, we show that irradiation induces a 2-4 weeks period of intense cell death leading to the emergence of chromosomal unstable cells during more than 35 population doublings. Then, among seven breast CSC markers, we identify CD24(-/low) labelling as a marker of radiation-resistance. We demonstrate that CD24(+) progeny of irradiated cells exclusively descends from CD24(-/low) cells. Finally, we show that delayed chromosomal instability is only expressed by CD24(+) cells, but is transmitted by stable surviving CD24(-/low) cells. So, for the first time a CSC marker, CD24, is associated with the transmission of genomic instability. This work may assign a new deleterious role to breast CSCs in aggressive recurrence after radiotherapy, as the transmitted genomic instability potentially leads tumour cells to acquire more aggressive characteristics.

Necdin Protects Embryonic Motoneurons from Programmed Cell Death

NECDIN belongs to the type II Melanoma Associated Antigen Gene Expression gene family and is located in the Prader-Willi Syndrome (PWS) critical region. Necdin-deficient mice develop symptoms of PWS, including a sensory and motor deficit. However, the mechanisms underlying the motor deficit remain elusive. Here, we show that the genetic ablation of Necdin, whose expression is restricted to post-mitotic neurons in the spinal cord during development, leads to a loss of 31% of specified motoneurons. The increased neuronal loss occurs during the period of naturally-occurring cell death and is not confined to specific pools of motoneurons. To better understand the role of Necdin during the period of programmed cell death of motoneurons we used embryonic spinal cord explants and primary motoneuron cultures from Necdin-deficient mice. Interestingly, while Necdin-deficient motoneurons present the same survival response to neurotrophic factors, we demonstrate that deletion of Necdin leads to an increased susceptibility of motoneurons to neurotrophic factor deprivation. We show that by neutralizing TNFα this increased susceptibility of Necdin-deficient motoneurons to trophic factor deprivation can be reduced to the normal level. We propose that Necdin is implicated through the TNF-receptor 1 pathway in the developmental death of motoneurons.

Coordinated expression of cell death genes regulates neuroblast apoptosis

Properly regulated apoptosis in the developing central nervous system is crucial for normal morphogenesis and homeostasis. In Drosophila, a subset of neural stem cells, or neuroblasts, undergo apoptosis during embryogenesis. Of the 30 neuroblasts initially present in each abdominal hemisegment of the embryonic ventral nerve cord, only three survive into larval life, and these undergo apoptosis in the larvae. Here, we use loss-of-function analysis to demonstrate that neuroblast apoptosis during embryogenesis requires the coordinated expression of the cell death genes grim and reaper, and possibly sickle. These genes are clustered in a 140 kb region of the third chromosome and show overlapping patterns of expression. We show that expression of grim, reaper and sickle in embryonic neuroblasts is controlled by a common regulatory region located between reaper and grim. In the absence of grim and reaper, many neuroblasts survive the embryonic period of cell death and the ventral nerve cord becomes massively hypertrophic. Deletion of grim alone blocks the death of neuroblasts in the larvae. The overlapping activity of these multiple cell death genes suggests that the coordinated regulation of their expression provides flexibility in this crucial developmental process.

Activated Kras Alters Epidermal Homeostasis of Mouse Skin, Resulting in Redundant Skin and Defective Hair Cycling

Germline mutations in the RAS-mitogen-activated protein kinase (RAS/MAPK) pathway are associated with genodermatoses, characterized by cutaneous, cardiac, and craniofacial defects, and cancer predisposition. Whereas activating mutations in HRAS are associated with the vast majority of patients with Costello syndrome, mutations in its paralog, KRAS, are rare. To better understand the disparity among RAS paralogs in human syndromes, we generated mice that activate a gain-of-function Kras allele (Lox-Stop-Lox (LSL)-Kras(G12D)) in ectodermal tissue using two different Cre transgenic lines. Using Msx2-Cre or ligand-inducible keratin 15 (K15)-CrePR, the embryonic effects of activated Kras were bypassed and the effects of Kras(G12D) expression from its endogenous promoter were determined. We found that Kras(G12D) induced redundant skin, papillomas, shortened nails, and hair loss. Redundant skin was associated with basal keratinocyte hyperplasia and an increase in body surface area. Paradoxically, Kras(G12D) also prevented hair cycle activation. We find that Kras(G12D) blocks proliferation in the bulge region of the hair follicle, when activated through Msx2-Cre but not through K15-CrePR. These studies reveal that KRAS, although infrequently involved in RAS/MAPK syndromes, is capable of inducing multiple cutaneous features that grossly resemble human RAS/MAPK syndromes.

Long-Distance Control of Synapse Assembly by Target-Derived NGF

We report a role for long-distance retrograde neurotrophin signaling in the establishment of synapses in the sympathetic nervous system. Target-derived NGF is both necessary and sufficient for formation of postsynaptic specializations on dendrites of sympathetic neurons. This, in turn, is a prerequisite for formation of presynaptic specializations, but not preganglionic axonal ingrowth from the spinal cord into sympathetic ganglia. We also find that NGF-TrkA signaling endosomes travel from distal axons to cell bodies and dendrites where they promote PSD clustering. Furthermore, the p75 neurotrophin receptor restricts PSD formation, suggesting an important role for antagonistic NGF-TrkA and p75 signaling pathways during retrograde control of synapse establishment. Thus, in addition to defining the appropriate number of sympathetic neurons that survive the period of developmental cell death, target-derived NGF also exerts control over the degree of connectivity between the spinal cord and sympathetic ganglia through retrograde control of synapse assembly.

Targeted Overexpression of a Golli-Myelin Basic Protein Isoform to Oligodendrocytes Results in Aberrant Oligodendrocyte Maturation and Myelination

Recently, several in vitro studies have shown that the golli–myelin basic proteins regulate Ca2+ homoeostasis in OPCs (oligodendrocyte precursor cells) and immature OLs (oligodendrocytes), and that a number of the functions of these cells are affected by cellular levels of the golli proteins. To determine the influence of golli in vivo on OL development and myelination, a transgenic mouse was generated in which the golli isoform J37 was overexpressed specifically within OLs and OPCs. The mouse, called JOE (J37-overexpressing), is severely hypomyelinated between birth and postnatal day 50. During this time, it exhibits severe intention tremors that gradually abate at later ages. After postnatal day 50, ultrastructural studies and Northern and Western blot analyses indicate that myelin accumulates in the brain, but never reaches normal levels. Several factors appear to underlie the extensive hypomyelination. In vitro and in vivo experiments indicate that golli overexpression causes a significant delay in OL maturation, with accumulation of significantly greater numbers of pre-myelinating OLs that fail to myelinate axons during the normal myelinating period. Immunohistochemical studies with cell death and myelin markers indicate that JOE OLs undergo a heightened and extended period of cell death and are unable to effectively myelinate until 2 months after birth. The results indicate that increased levels of golli in OPC/OLs delays myelination, causing significant cell death of OLs particularly in white matter tracts. The results provide in vivo evidence for a significant role of the golli proteins in the regulation of maturation of OLs and normal myelination.

Cell-generated forces influence the viability, metabolism and mechanical properties of fibroblast-seeded collagen gel constructs

The aim of this study was to investigate the influence of the endogenous forces generated by fibroblast-mediated contraction, using four individual collagen gel models that differed with respect to the ability of the cells to contract the gel. Human neonatal dermal fibroblasts were seeded in type I collagen and the gels were cast in a racetrack-shaped mould containing a removable central island. Two of the models were mechanically stressed (20 mm and 10 mm), as complete contraction was prevented by the presence of a central island. The central island was removed in the third model (released) and the final model was cast in a Petri dish and detached, allowing full multi-axial contraction (SR). Cell viability was maintained in the 10 mm, released and SR models over a 6 day culture period but localized regions of cell death were evident in the 20 mm model. Cell and collagen alignment was developed in the 20 mm and 10 mm models and to a lesser extent in the released model, but was absent in the SR model. Cell proliferation and collagen synthesis was lower in the 20 mm model compared to the other systems and there was evidence of enhanced matrix metalloproteinase production. The mechanical properties of the 20 mm model system were inferior to the 10 mm and released systems. The 10 mm model system induced a high level of cell and matrix orientation and may, therefore, represent the best option for tissue-engineered ligament repair involving an orientated fibroblast-seeded collagen gel.

O.619 How does nicotine affect bone regeneration?

Maternal cigarette use during pregnancy is associated with increased incidence of neural impairments in offspring, but nicotine's unique contribution to any neuropathology remains unclear, and nicotine's neurodevelopmental effects assessed in animal models vary with concentration. During ontogenesis, the chick oculomotor complex (OMN) is regulated by central nervous system (CNS) afferent-derived and target-derived trophic factors, allowing assessment of nicotine's potential interference in receptor-mediated CNS trophic phenomena, unconfounded by myriad other compounds in cigarette smoke. In the current study, 100 ng nicotine applied daily in ovo to yolk during embryonic days (E) 1–7 mimicked maternal plasma nicotine concentrations during fetal cranial nerve development. Nicotine-treated embryos exhibited a 15% decrease in whole body weight and 7% decrease in brain weight at E16. However, at E16, nicotine-treated embryos had 37% and 15% increases in the combined ventromedial+lateral (v) OMN motoneuron density and soma area, respectively, effects not observed in the optic tectum, in which nicotine cholinergic receptor expression is delayed until E8–12. Incorporation of tritiated thymidine into whole brain DNA demonstrated that the nicotine treatment did not cause increased rates of whole brain mitosis, suggesting that the dosage regimen did not elicit a cytotoxic, wound-healing, response of differentiating cells. As determined by DNA fragment-labeling assay during the normal period of cell death, vOMN apoptosis occurs maximally on E11 during a normal period of declining cell density, and a dose–response study demonstrated 78% E11 vOMN apoptotic suppression at approximately 0.30 μM cumulative yolk nicotine with an inhibition threshold between 0.10 and 0.20 μM. These results suggest that plasma nicotine concentrations resulting from tobacco use or nicotine replacement therapy (NRT) are sufficient to inhibit motoneuron apoptosis and enhance neuronal growth.

Prosurvival and proapoptotic intracellular signaling in rat spiral ganglion neurons in vivo after the loss of hair cells

Neurons depend on afferent input for survival. Rats were given daily kanamycin injections from P8 to P16 to destroy hair cells, the sole afferent input to spiral ganglion neurons (SGNs). Most SGNs die over an approximately 14-week period after deafferentation. During this period, the SGN population is heterogeneous. At any given time, some SGNs exhibit apoptotic markers--TUNEL and cytochrome c loss--whereas others appear nonapoptotic. We asked whether differences among SGNs in intracellular signaling relevant to apoptotic regulation could account for this heterogeneity. cAMP response element binding protein (CREB) phosphorylation, which reflects neurotrophic signaling, is reduced in many SGNs at P16, P23, and P32, when SGNs begin to die. In particular, nearly all apoptotic SGNs exhibit reduced phospho-CREB, implying that apoptosis is due to insufficient neurotrophic support. However, >32% of SGNs maintain high phospho-CREB levels, implying access to neurotrophic support. By P60, when approximately 50% of the SGNs have died, phospho-CREB levels in surviving neurons are not reduced, and SGN death is no longer correlated with reduced phospho-CREB. Activity in the proapoptotic Jun N-terminal kinase (JNK)-Jun signaling pathway is elevated in SGNs during the cell death period. This too is heterogeneous: <42% of the SGNs exhibited high phospho-Jun levels, but nearly all SGNs undergoing apoptosis exhibited elevated phospho-Jun. Thus, heterogeneity among SGNs in prosurvival and proapoptotic signaling is correlated with apoptosis. SGN death following deafferentation has an early phase in which apoptosis is correlated with reduced phospho-CREB and a later phase in which it is not. Proapoptotic JNK-Jun signaling is tightly correlated with SGN apoptosis.

In Vivo Restoration of Physiological Levels of Truncated TrkB.T1 Receptor Rescues Neuronal Cell Death in a Trisomic Mouse Model

Imbalances in neurotrophins or their high-affinity Trk receptors have long been reported in neurodegenerative diseases. However, a molecular link between these gene products and neuronal cell death has not been established. In the trisomy 16 (Ts16) mouse there is increased apoptosis in the cortex, and hippocampal neurons undergo accelerated cell death that cannot be rescued by administration of brain-derived neurotrophic factor (BDNF). Ts16 neurons have normal levels of the TrkB tyrosine kinase receptor but an upregulation of the TrkB.T1 truncated receptor isoform. Here we show that restoration of the physiological level of the TrkB.T1 receptor by gene targeting rescues Ts16 cortical cell and hippocampal neuronal death. Moreover, it corrects resting Ca2+ levels and restores BDNF-induced intracellular signaling mediated by full-length TrkB in Ts16 hippocampal neurons. These data provide a direct link between neuronal cell death and abnormalities in Trk neurotrophin receptor levels.

Synaptically-silent immature neurons show gaba and glutamate receptor-mediated currents in adult rat dentate gyrus.

The fate of adult-generated neurons in dentate gyrus is mainly determined early, before they receive synapses. In developing brain, classical neurotransmitters such as GABA and glutamate exert trophic effects before synaptogenesis. In order for this to occur in adult brain as well, immature non-contacted cells must express functional receptors to GABA and glutamate. In this investigation, patch-clamp recordings were used in adult rat dentate gyrus slices to assess the presence and analyze the characteristics of GABA- and glutamate-evoked currents in highly immature, synaptically-silent granule cells. Whole-cell patch-clamp recordings showed that all the analyzed cells responded to puff application of GABA and most of them responded to glutamate. Currents evoked by GABA were mediated exclusively by GABAA receptors and those elicited by glutamate were mediated by NMDA and AMPA/Kainate receptors. GABAA receptor-mediated currents were reduced by furosemide, which suggests that synaptically-silent immature neurons express high-affinity, alpha4-subunit-containing GABAA receptors. Gramicidin-perforated-patch recordings showed that GABAA receptor-mediated currents exerted a depolarizing effect due to high intracellular chloride concentration. Synaptically-silent immature cells shared morphological and electrophysiological properties with GFP-expressing, 7-day-old adult-generated granule layer cells, indicating that they could be in the first week of life, the period of maximal newborn cell death. Moreover, the presence of functional GABA and glutamate receptors was confirmed in these GFP-expressing cells. Present findings are mostly consistent with previous data obtained in female mice undergoing spontaneous activity and in transgenic mice, except for some inconsistencies about the presence of functional glutamatergic receptors. We speculate that adult-generated, non-contacted granule cells may be able to sense activity-related variations of GABA and glutamate extracellular levels. This condition is necessary, even if not sufficient, for these neurotransmitters to have a direct role in addressing cell survival.

Evolution of broad host range in retroviruses leads to cell death mediated by highly cytopathic variants.

The ability of many retroviruses to cause disease can be correlated to their cytopathic effect (CPE) in tissue culture characterized by an acute period of cell death and viral DNA accumulation. Here, we show that mutants of a subgroup B avian retrovirus (Alpharetrovirus) cause a very dramatic CPE in certain susceptible avian cells that is coincident with elevated levels of apoptosis, as measured by nuclear morphology, and persistent viral DNA accumulation. These mutants also have a broadly extended host range that includes rodent, cat, dog, monkey, and human cells (31). Previously, we have shown that the mutants exhibit diminished resistance to superinfection. The results presented here have important implications for the process of evolution of retroviruses to use distinct cellular receptors.

P63 Is an Essential Proapoptotic Protein during Neural Development

The p53 family member p63 is required for nonneural development, but has no known role in the nervous system. Here, we define an essential proapoptotic role for p63 during naturally occurring neuronal death. Sympathetic neurons express full-length TAp63 during the developmental death period, and TAp63 levels increase following NGF withdrawal. Overexpression of TAp63 causes neuronal apoptosis in the presence of NGF, while cultured p63-/- neurons are resistant to apoptosis following NGF withdrawal. TAp63 is also essential in vivo, since embryonic p63-/- mice display a deficit in naturally occurring sympathetic neuron death. While both TAp63 and p53 induce similar apoptotic signaling proteins and require BAX expression and function for their effects, TAp63 induces neuronal death in the absence of p53, but p53 requires coincident p63 expression for its proapoptotic actions. Thus, p63 is essential for developmental neuronal death, likely functioning both on its own, and as an obligate proapoptotic partner for p53.

Activity Regulates Positive and Negative Neurotrophin-Derived Signals to Determine Axon Competition

Developmental axon competition plays a key role in sculpting neural circuitry. Here, we have asked how activity and neurotrophins could interact to select one axon over another. Using compartmented cultures of sympathetic neurons, we show that, in the presence of NGF, local depolarization confers a competitive growth advantage on the depolarized axon collaterals and at the same time disadvantages the growth of unstimulated axons from the same and competing neurons. Depolarization mediates the competitive advantage by activating a CaMKII-MEK pathway, which converges to enhance local NGF-mediated downstream growth signals. Patterned electrical stimulation also acts via this pathway to enhance NGF-promoted axonal growth. In contrast, the competitive disadvantage is due to BDNF secreted from and acting on the unstimulated, competing axons through p75NTR. Thus, activity regulates both positive and negative neurotrophin-derived signaling cascades to confer a competitive growth advantage on one axon versus another, thereby providing a cellular mechanism for developmental axon selection.

Prenatal auditory enrichment with species-specific calls and sitar music modulates expression of Bcl-2 and Bax to alter programmed cell death in developing chick auditory nuclei

Postnatal auditory stimulation influences early perceptual learning. Previously we reported morphological effects of prenatal auditory stimulation by species-specific and sitar musical sounds on the chick brainstem auditory nuclei—nucleus magnocellularis and nucleus laminaris. At hatching, these two nuclei of auditory enriched embryos showed higher neuronal numbers, amongst other morphological changes. There were also increases in synaptophysin and syntaxin1 expressions in the sound enriched groups and modulation of the developmental expression of transcription factors c-Fos and c-Jun. We hypothesized that prenatal auditory enrichment may have reduced embryonic apoptosis in these nuclei with possible alteration of molecular mechanisms enhancing the postsynaptic neuron's ability to survive. In the present study, therefore, we examined apoptotic cell death by TUNEL technique and Bcl-2 expression using immunohistochemistry and immunoblotting. In the controls, a peak percentage in the TUNEL-positive cells was noted in the auditory nuclei at embryonic day 12, which was reduced at embryonic day 16. Bcl-2 immunoreactivity decreased from embryonic day 8 to embryonic day 12 overlapping the period of embryonic cell death in these nuclei. The stimulated groups, however, showed fewer apoptotic neurons and higher Bcl-2 level than that in the controls. On the other hand, Bax immunohistochemistry showed correlated reverse changes compared to Bcl-2 expression. Thus prenatal extra-acoustic stimulation appears to alter Bcl-2 and Bax expression to support cell survival and differentiation, thereby augmenting the development of auditory nuclei.

TRAIL-related death receptors in normal, Lurcher and weaver mutant mouse brain

In this study, we searched for murine analogues of the four death-receptor types (TRAIL-R1 to R4), targeted by the tumour necrosis factor related apoptosis inducing ligand (TRAIL), which were recently identified in the human brain. The expression of TRAIL-receptors in the normal murine brain was investigated using antibodies directed against different epitopes of the human TRAIL-receptors. Mouse mutants, in particular weaver and Lurcher with their well defined spatio-temporal patterns of neurodegeneration in the cerebellum, the inferior olive and the substantia nigra, were used as a model for investigating a potential contribution of TRAIL-receptors to the genetically determined cell death observed in these mutants. Although all antibodies used, recognized the respective human antigens, only the murine analogue of the human TRAIL-R2 epitope was also identified in the mouse brain. Antisera against human TRAIL-R1, TRAIL-R3 and TRAIL-R4 failed to reveal any other murine TRAIL-receptor analogue. In normal mice, TRAIL-R2 is not universally expressed throughout the brain but rather restricted to specific neuronal populations predominantly consisting of large neurons. In weaver, the spatial patterns and relative densities of TRAIL-R2 labelling were virtually identical to those seen in wild-types during the period of cell death in the cerebellum and the substantia nigra. In Lurcher, TRAIL-R2 expression in cerebellar granule cells and inferior olivary neurons was identical to that in wildtypes but significantly reduced in Purkinje cells undergoing degeneration. Thus, although TRAIL-R2 is found to be expressed in various cell types of the murine brain, cell death in weaver and Lurcher mutants is apparently not accompanied by an upregulation of TRAIL-receptors.

Factors promoting survival of mesencephalic dopaminergic neurons.

Growth factors promoting survival of mesencephalic dopaminergic neurons are discussed in the context of their requirement during development and adulthood. The expression of growth factors should be detectable in the nigrostriatal system during critical periods of development, i.e., during the period of ontogenetic cell death and synaptogenesis and during neurite extension and neurotransmitter synthesis. Growth factors discussed include members of the family of glial-cell-line-derived neurotrophic factors (GDNF), neurotrophins, transforming growth factors beta, and low molecular compounds mimicking growth factor activities. To date, the available data support the notion that GDNF is a highly promising candidate, although GDNF-null mice lack a dopaminergic phenotype. There remains a possibility that endogenous dopaminotrophic factors remain to be discovered.

6-WEEK AD LIBITUM ADMINISTRATION OF NICOTINE SIGNIFICANTLY ALTERS THE GENE EXPRESSION PROFILE IN THE BONE MARROW AND SPLEEN OF C57BL6 MICE

Maternal cigarette use during pregnancy is associated with increased incidence of neural impairments in offspring, but nicotine's unique contribution to any neuropathology remains unclear, and nicotine's neurodevelopmental effects assessed in animal models vary with concentration. During ontogenesis, the chick oculomotor complex (OMN) is regulated by central nervous system (CNS) afferent-derived and target-derived trophic factors, allowing assessment of nicotine's potential interference in receptor-mediated CNS trophic phenomena, unconfounded by myriad other compounds in cigarette smoke. In the current study, 100 ng nicotine applied daily in ovo to yolk during embryonic days (E) 1–7 mimicked maternal plasma nicotine concentrations during fetal cranial nerve development. Nicotine-treated embryos exhibited a 15% decrease in whole body weight and 7% decrease in brain weight at E16. However, at E16, nicotine-treated embryos had 37% and 15% increases in the combined ventromedial+lateral (v) OMN motoneuron density and soma area, respectively, effects not observed in the optic tectum, in which nicotine cholinergic receptor expression is delayed until E8–12. Incorporation of tritiated thymidine into whole brain DNA demonstrated that the nicotine treatment did not cause increased rates of whole brain mitosis, suggesting that the dosage regimen did not elicit a cytotoxic, wound-healing, response of differentiating cells. As determined by DNA fragment-labeling assay during the normal period of cell death, vOMN apoptosis occurs maximally on E11 during a normal period of declining cell density, and a dose–response study demonstrated 78% E11 vOMN apoptotic suppression at approximately 0.30 μM cumulative yolk nicotine with an inhibition threshold between 0.10 and 0.20 μM. These results suggest that plasma nicotine concentrations resulting from tobacco use or nicotine replacement therapy (NRT) are sufficient to inhibit motoneuron apoptosis and enhance neuronal growth.

Learning may reduce neurogenesis in adult rat dentate gyrus

Neuron production carries on throughout adult life in granule cell layer of mammalian dentate gyrus. The acquisition of a hippocampus-dependent task enhances newborn cell survival in granule cell layer (GCL) during the period in which several newborn cells die. In this paper the effect of learning, occurring after the maximal period of newborn cell death, on newborn cells was investigated. Rats were trained for hippocampus-dependent learning in Morris water maze. 5-bromo-2′-deoxyuridine was administered 8–10 days before training beginning and labeled cells were counted after training. Learning decreased BrdU-labeled cell density in GCL, and increased TUNEL-positive cells. Moreover, learning diminished immature neuron density prevalently in the internal blade of GCL. Therefore, a different effect of learning on immature neuron survival, depending on the time elapsing from mitosis to learning, is suggested.

Cochlear ablation in mice lacking SHP-1 results in an extended period of cell death of anteroventral cochlear nucleus neurons

Cochlear ablation results in the death of anteroventral cochlear nucleus (AVCN) neurons from birth to approximately postnatal day 14 (P14) in the murine brainstem. It is not known whether microglial activation contributes to AVCN neuronal death following deafferentation. In order to determine whether microglial activation helps to define the period of neuronal susceptibility within AVCN, we performed unilateral cochlear ablation on mice lacking the protein tyrosine phosphatase SHP-1 ( me/ me). These mice have been shown to have an exaggerated microglial response following ischemic injury. In the present study, the glial and neuronal response to deafferentation within AVCN was examined in wild-type and me/ me mice at P5, P14, and P21. Lack of SHP-1 results in robust microglial but not astrocyte activation within the ablated P14 me/ me AVCN. These mice also exhibit approximately 28% neuronal death at P14, a time when normal wild-type littermate controls show little cell death. Glial activation and neuronal loss at P5 and P21 were similar between the two phenotypes, suggesting a role of activated microglia in inducing neuronal death beyond P14 but not P21. These results indicate that activated microglia may participate in determining whether neurons in AVCN live or die following deafferentation.