Polymer-Encapsulated PC12 Cells: Long-Term Survival and Associated Reduction in Lesion-Induced Rotational Behavior

Intrastriatal implantation of a dopaminergic cell line surrounded by a permeable, thermoplastic membrane was investigated as a method of long-term dopamine (DA) delivery within the central nervous system (CNS). An increase in DA release from PC12 cell-loaded capsules maintained in vitro was associated with an increase in mitotic activity of the encapsulated cell line. A significant reduction in apomorphine-induced rotational behavior was observed after PC12 cell-containing capsules were implanted into unilaterally 6-hydroxydopamine (6-OHDA) lesioned rats, which was sustained for 24 wk. Four wk after implantation, micro-dialysis studies revealed the presence of DA near PC12 cell-containing capsules, which was comparable to extracellular striatal levels of unlesioned controls. Extracellular striatal DA was undetectable by microdialysis in lesioned animals near empty polymer capsules. Histological analysis after 24 wk in vivo demonstrated that encapsulated PC12 cells survived, continued to express tyrosine hydroxylase, and that encapsulation prevented tumorigenesis. The data suggested that the release of a diffusible substance, most likely DA, from an implant is sufficient to exert a long-term functional influence upon 6-OHDA unilaterally lesioned rats and that capsules containing DA-secreting cells may be an effective method of long-term DA delivery in the CNS.

Lack of anti-factor Xa assay standardization results in significant low molecular weight heparin (enoxaparin) dose variation in neonates and children

INTRODUCTION

Enoxaparin is a frequently used anticoagulant in children. Unlike in adults, consensus guidelines recommend therapeutic monitoring to a target anti-factor Xa level of 0.5-1 U mL(-1) . Therapeutic ranges are not well correlated with clinical outcomes (e.g. thrombosis or hemorrhage), and assays are not standardized. Owing to limited reagent supplies, our clinical laboratory conducted a validation process and switched anti-FXa assays. Although the assays correlated well with each other, anti-FXa values were, on average, 33% higher with the new assay. The target anti-FXa range was not altered. We evaluated how this change in anti-FXa assays influenced enoxaparin dosing (mg kg(-1) ).

METHODS

Enoxaparin dosing and anti-FXa values for all patients started on enoxaparin for the 6 months before and after assay change were retrospectively compiled and analyzed with a Student's t-test.

RESULTS

One hundred and nine children were started on enoxaparin before assay change, and 104 after assay change. The mean therapeutic enoxaparin dose (mg kg(-1) ) was significantly lower in subjects aged < 3 months (P = 0.01) and 3 months to 2 years (P < 0.0001), but not in subjects aged > 2 years (P = 0.18), after assay change. The median number of enoxaparin dose changes required to achieve the target range was significantly reduced after assay change, from 1 to 0 (P = 0.004).

CONCLUSIONS

The current pediatric practice of dose adjustment to achieve and maintain a target anti-FXa range is vulnerable to assay determination, which may provide false reassurance of efficacy and safety and represent misappropriation of time and resources. These data support a pediatric randomized controlled clinical trial comparing the safety and efficacy of enoxaparin weight-based dosing with or without dose titration based on anti-FXa.

Use of global assays to understand clinical phenotype in congenital factor VII deficiency

Congenital factor VII (FVII) deficiency is characterized by genotypic variability and phenotypic heterogeneity. Traditional screening and factor assays are unable to reliably predict clinical bleeding phenotype and guide haemorrhage prevention strategy. Global assays of coagulation and fibrinolysis may better characterize overall haemostatic balance and aid in haemorrhagic risk assessment. We evaluated the ability of novel global assays to better understand clinical bleeding severity in congenital FVII deficiency. Subjects underwent central determination of factor VII activity (FVII:C) as well as clot formation and lysis (CloFAL) and simultaneous thrombin and plasmin generation (STP) global assay analysis. A bleeding score was assigned to each subject through medical chart review. Global assay parameters were analysed with respect to bleeding score and FVII:C. Subgroup analyses were performed on paediatric subjects and subjects with FVII ≥ 1 IU dL(-1). CloFAL fibrinolytic index (FI2 ) inversely correlated with FVII:C while CloFAL maximum amplitude (MA) and STP maximum velocity of thrombin generation (VT max) varied directly with FVII:C. CloFAL FI2 directly correlated with bleeding score among subjects in both the total cohort and paediatric subcohort, but not among subjects with FVII ≥ 1 IU dL(-1) . Among subjects with FVII ≥ 1 IU dL(-1), STP time to maximum velocity of thrombin generation and time to maximum velocity of plasmin generation inversely correlated with bleeding score. These preliminary findings suggest a novel potential link between a hyperfibrinolytic state in bleeding severity and congenital FVII deficiency, an observation that should be further explored.

Regulated ATF5 loss-of-function in adult mice blocks formation and causes regression/eradication of gliomas

Glioblastomas are among the most incurable cancers. Our past findings indicated that glioblastoma cells, but not neurons or glia, require the transcription factor ATF5 (activating transcription factor 5) for survival. However, it was unknown whether interference with ATF5 function can prevent or promote regression/eradication of malignant gliomas in vivo. To address this issue, we created a mouse model by crossing a human glial fibrillary acidic protein (GFAP) promoter-tetracycline transactivator mouse line with tetracycline operon-dominant negative-ATF5 (d/n-ATF5) mice to establish bi-transgenic mice. In this model, d/n-ATF5 expression is controlled by doxycycline and the promoter for GFAP, a marker for stem/progenitor cells as well as gliomas. Endogenous gliomas were produced with high efficiency by retroviral delivery of platelet-derived growth factor (PDGF)-B and p53-short hairpin RNA (shRNA) in adult bi-transgenic mice in which expression of d/n-ATF5 was spatially and temporally regulated. Induction of d/n-ATF5 before delivery of PDGF-B/p53-shRNA virus greatly reduced the proportion of mice that formed tumors. Moreover, d/n-ATF5 induction after tumor formation led to regression/eradication of detectable gliomas without evident damage to normal brain cells in all 24 mice assessed.

Selective destruction of glioblastoma cells by interference with the activity or expression of ATF5

Glioblastoma multifome is the most common and most aggressive primary brain tumor with no current curative therapy. We found expression of the bZip transcription factor ATF5 in all 29 human glioblastomas and eight human and rat glioma cell lines assessed. ATF5 is not detectably expressed by mature brain neurons and astrocytes, but is expressed by reactive astrocytes. Interference with ATF5 function or expression in all glioma cell lines tested causes marked apoptotic cell death. In contrast, such manipulations do not affect survival of ATF5-expressing cultured astrocytes or of several other cell types that express this protein. In a proof-of-principle experiment, retroviral delivery of a function-blocking mutant form of ATF5 into a rat glioma model evokes death of the infected tumor cells, but not of infected brain cells outside the tumors. The widespread expression of ATF5 in glioblastomas and the selective effect of interference with ATF5 function/expression on their survival suggest that ATF5 may be an attractive target for therapeutic intervention in such tumors.

RAIDD is required for apoptosis of PC12 cells and sympathetic neurons induced by trophic factor withdrawal

Caspase 2 has been implicated in trophic deprivation-induced neuronal death. We have shown that overexpression of the caspase 2-binding protein RAIDD induces neuronal apoptosis, acting synergistically with trophic deprivation. Currently, we examine the role of endogenous RAIDD in apoptosis of PC12 cells and sympathetic neurons. Expression of a truncated caspase recruitment domain-only form of caspase 2, which presumably disrupts the RAIDD interaction with endogenous caspase 2, attenuated trophic deprivation-induced apoptosis. Furthermore, downregulation of RAIDD by small interfering RNA led to inhibition of trophic deprivation-induced death, whereas death induced by DNA damage, which is not caspase 2-mediated, was not inhibited. Therefore, RAIDD, likely through interaction with caspase 2, is involved in trophic deprivation-induced neuronal apoptosis. This is the first demonstration of the involvement of RAIDD in apoptosis, and provides further support for the idea that apoptotic pathways in the same system may differ depending on the initiating stimulus.

Cell cycle molecules and vertebrate neuron death: E2F at the hub

Vertebrate neuron cell death is both a normal developmental process and the catastrophic outcome of nervous system trauma or degenerative disorders. Although the mechanisms of such death include an evolutionarily conserved core apoptotic pathway that is highly homologous to that first described by Horvitz and co-workers in Caenorhabditis elegans, it appears that many instances of neuron death additionally require the transcription-dependent induction of proapoptotic molecules. One such proapoptotic transcriptional pathway revealed by studies over the past decade revolves about the transcription factor E2F and those molecules that either regulate E2F activity or that are direct or indirect transcriptional targets of E2F. Many of the molecules associated with the E2F apoptotic pathway in postmitotic neurons also participate in the cell cycle in proliferating cells. Observations in human material and in animal and cell culture models show widespread correlation between changes in expression, activity and subcellular localization of E2F-related cell cycle molecules and developmental and catastrophic neuron death. A variety of experimental approaches support a causal role for such changes in the death process and are beginning to indicate how the neuronal E2F pathway activates the core apoptotic machinery. The discovery and elaboration of the neuronal apoptotic E2F pathway provides abundant targets as well as small molecule candidates for potential therapeutic intervention in nervous system trauma and degenerative disease.

Expression of A53T mutant but not wild-type alpha-synuclein in PC12 cells induces alterations of the ubiquitin-dependent degradation system, loss of dopamine release, and autophagic cell death

Alpha-synuclein mutations have been identified in certain families with Parkinson's disease (PD), and alpha-synuclein is a major component of Lewy bodies. Other genetic data indicate that the ubiquitin-dependent proteolytic system is involved in PD pathogenesis. We have generated stable PC12 cell lines expressing wild-type or A53T mutant human alpha-synuclein. Lines expressing mutant but not wild-type alpha-synuclein show: (1) disruption of the ubiquitin-dependent proteolytic system, manifested by small cytoplasmic ubiquitinated aggregates and by an increase in polyubiquitinated proteins; (2) enhanced baseline nonapoptotic death; (3) marked accumulation of autophagic-vesicular structures; (4) impairment of lysosomal hydrolysis and proteasomal function; and (5) loss of catecholamine-secreting dense core granules and an absence of depolarization-induced dopamine release. Such findings raise the possibility that the primary abnormality in these cells may involve one or more deficits in the lysosomal and/or proteasomal degradation pathways, which in turn lead to loss of dopaminergic capacity and, ultimately, to death. These cells may serve as a model to study the effects of aberrant alpha-synuclein on dopaminergic cell function and survival.

Regulation of neuronal survival and death by E2F-dependent gene repression and derepression

Neuronal death induced by a variety of means requires participation of the E2F family of transcription factors. Here, we show that E2F acts as a gene silencer in neurons and that repression of E2F-responsive genes is required for neuronal survival. Moreover, neuronal death evoked by DNA damaging agents or trophic factor withdrawal is characterized by derepression of E2F-responsive genes. Such derepression, rather than direct E2F-promoted gene activation, is required for death. Among the genes that are derepressed in neurons subjected to DNA damage or trophic factor withdrawal are the transcription factors B- and C-myb. Overexpression of B- and C-myb is sufficient to evoke neuronal death. These findings support a model in which E2F-dependent gene repression and derepression play pivotal roles in neuronal survival and death, respectively.

Neuronal apoptosis at the G1/S cell cycle checkpoint

Apoptosis is a fundamental and essential process in development and tissue homeostasis of multicellular organisms. Roughly half of all the neurons produced during neurogenesis die apoptotically before the nervous system matures. Apoptosis is also involved in various neurodegenerative disorders such as Alzheimer's disease and neuronal trauma. Investigation of the mechanisms underlying neuronal apoptosis led to an unexpected discovery that in many cases revival of the quiescent and dormant cell cycle machinery is a common theme. Recent data suggest that uncoordinated expression of cell cycle molecules and the consequent breach of cell cycle checkpoints could be one of the primary mechanisms by which postmitotic neurons undergo apoptotic death. Evidence indicates that upregulation of cyclin-D-CDK4/6 activity and deregulation of E2F transcription factors mark key events in early stages of neuronal apoptosis. Active E2F repression by Rb family members is required for the survival of neurons. Apoptotic signals promote successive phosphorylation and dysfunction of Rb family members, resulting in sequential E2F derepression and expression of selective E2F-responsive genes. Thus, expression of derepressed E2F-responsive genes may be instrumental in propagating and amplifying the apoptotic signals instructing neuronal cells to carry out the apoptotic program.

Death in the balance: alternative participation of the caspase-2 and -9 pathways in neuronal death induced by nerve growth factor deprivation

The data presented here demonstrate that sympathetic neurons have the potential to activate two alternative caspase-dependent pathways either of which is capable of mediating death induced by NGF deprivation and that these neurons have the potential to switch from one pathway to the other. The presence of these two alternative pathways to trophic factor deprivation-induced death may have implications for ensuring the correct development of the nervous system. In wild-type neurons, a caspase-2-dependent pathway is required for death, and a caspase-9-dependent pathway appears to be suppressed by endogenous inhibitors of apoptosis proteins (IAPs). In contrast, for caspase-2-null neurons, death is dependent on the caspase-9 pathway. The mechanism underlying the shift is the result of a threefold compensatory elevation of caspase-9 expression and a doubling of levels of direct IAP binding protein with low pI/(DIABLO)/second mitochondria-derived activator of caspase (Smac), an IAP inhibitor, both at the mRNA and protein levels [corrected]. These findings resolve seemingly discrepant findings regarding the roles of various caspases after NGF deprivation and raise a cautionary note regarding the interpretation of findings with caspase-null animals. The choice of the death-mediating caspase pathway in the sympathetic neurons is thus dependent on the regulated relative expression of components of the pathways including those of caspases, IAPs, and IAP inhibitors.

Cep-1347 (KT7515), a semisynthetic inhibitor of the mixed lineage kinase family

CEP-1347 (KT7515) promotes neuronal survival at dosages that inhibit activation of the c-Jun amino-terminal kinases (JNKs) in primary embryonic cultures and differentiated PC12 cells after trophic withdrawal and in mice treated with 1-methyl-4-phenyl tetrahydropyridine. In an effort to identify molecular target(s) of CEP-1347 in the JNK cascade, JNK1 and known upstream regulators of JNK1 were co-expressed in Cos-7 cells to determine whether CEP-1347 could modulate JNK1 activation. CEP-1347 blocked JNK1 activation induced by members of the mixed lineage kinase (MLK) family (MLK3, MLK2, MLK1, dual leucine zipper kinase, and leucine zipper kinase). The response was selective because CEP-1347 did not inhibit JNK1 activation in cells induced by kinases independent of the MLK cascade. CEP-1347 inhibition of recombinant MLK members in vitro was competitive with ATP, resulting in IC(50) values ranging from 23 to 51 nm, comparable to inhibitory potencies observed in intact cells. In addition, overexpression of MLK3 led to death in Chinese hamster ovary cells, and CEP-1347 blocked this death at doses comparable to those that inhibited MLK3 kinase activity. These results identify MLKs as targets of CEP-1347 in the JNK signaling cascade and demonstrate that CEP-1347 can block MLK-induced cell death.

The MLK family mediates c-Jun N-terminal kinase activation in neuronal apoptosis

Neuronal apoptotic death induced by nerve growth factor (NGF) deprivation is reported to be in part mediated through a pathway that includes Rac1 and Cdc42, mitogen-activated protein kinase kinases 4 and 7 (MKK4 and -7), c-Jun N-terminal kinases (JNKs), and c-Jun. However, additional components of the pathway remain to be defined. We show here that members of the mixed-lineage kinase (MLK) family (including MLK1, MLK2, MLK3, and dual leucine zipper kinase [DLK]) are expressed in neuronal cells and are likely to act between Rac1/Cdc42 and MKK4 and -7 in death signaling. Overexpression of MLKs effectively induces apoptotic death of cultured neuronal PC12 cells and sympathetic neurons, while expression of dominant-negative forms of MLKs suppresses death evoked by NGF deprivation or expression of activated forms of Rac1 and Cdc42. CEP-1347 (KT7515), which blocks neuronal death caused by NGF deprivation and a variety of additional apoptotic stimuli and which selectively inhibits the activities of MLKs, effectively protects neuronal PC12 cells from death induced by overexpression of MLK family members. In addition, NGF deprivation or UV irradiation leads to an increase in both level and phosphorylation of endogenous DLK. These observations support a role for MLKs in the neuronal death mechanism. With respect to ordering the death pathway, dominant-negative forms of MKK4 and -7 and c-Jun are protective against death induced by MLK overexpression, placing MLKs upstream of these kinases. Additional findings place the MLKs upstream of mitochondrial cytochrome c release and caspase activation.

Characterization of a novel isoform of caspase-9 that inhibits apoptosis

We have identified a novel isoform of rat caspase-9 in which the C terminus of full-length caspase-9 is replaced with an alternative peptide sequence. Casp-9-CTD (where CTD is carboxyl-terminal divergent) is expressed in multiple tissues, with the relative highest expression observed in ovary and heart. Casp-9-CTD was found primarily in the cytoplasm and was not detected in the nucleus. Structural predictions suggest that in contrast to full-length caspase-9, casp-9-CTD will not be processed. Our model is supported by reduced protease activity of casp-9-CTD preparations in vitro and by the lack of detectable processing of casp-9-CTD proenzyme or the induction of cell death following transfection into cells. Both neuronal and non-neuronal cell types transfected with casp-9-CTD were resistant to death evoked by trophic factor deprivation or DNA damage. In addition, cytosolic lysates prepared from cells permanently expressing exogenous casp-9-CTD were resistant to caspase induction by cytochrome c in reconstitution assays. Taken together, our observations indicate that casp-9-CTD acts as a dominant-negative variant. Its expression in various tissues indicates a physiological role in regulating cell death.

beta-Amyloid-induced neuronal apoptosis requires c-Jun N-terminal kinase activation

beta-Amyloid (A beta) has been strongly implicated in the pathophysiology of Alzheimer's disease (AD), but the means by which the aggregated form of this molecule induces neuronal death have not been fully defined. Here, we examine the role of the c-Jun N-terminal kinases (JNKs) and of their substrate, c-Jun, in the death of cultured neuronal PC12 cells and sympathetic neurons evoked by exposure to aggregated A beta. The activities of JNK family members increased in neuronal PC12 cells within 2 h of A beta treatment and reached 3--4-fold elevation by 6 h. To test the role of these changes in death caused by A beta, we examined the effects of CEP-1347 (KT7515), an indolocarbazole that selectively blocks JNK activation. Inclusion of CEP-1347 (100--300 nM) in the culture medium effectively blocked the increases in cellular JNK activity caused by A beta and, at similar concentrations, protected both PC12 cells and sympathetic neurons from A beta-evoked-death. Effective protection required addition of CEP-1347 within 2 h of A beta treatment, indicating that the JNK pathway acts relatively proximally and as a trigger in the death mechanism. A dominant-negative c-Jun construct also conferred protection from A beta-evoked death, supporting a model in which JNK activation contributes to death via activation of c-Jun. Finally, CEP-1347 blocked A beta-stimulated activation of caspase-2 and -3, placing these downstream of JNK activation. These observations implicate the JNK pathway as a required element in death evoked by A beta and hence identify it as a potential therapeutic target in AD.

Synuclein-1 is selectively up-regulated in response to nerve growth factor treatment in PC12 cells

Mutations in the alpha-synuclein gene have recently been identified in families with inherited Parkinson's disease and the protein product of this gene is a component of Lewy bodies, indicating that alpha-synuclein is involved in Parkinson's disease pathogenesis. A role for normal alpha-synuclein in synaptic function, apoptosis or plasticity responses has been suggested. We show here that in rat pheochromocytoma PC12 cells synuclein-1, the rat homolog of human alpha-synuclein, is highly and selectively up-regulated at the mRNA and protein levels after 7 days of nerve growth factor treatment. Synuclein-1 expression appears neither sufficient nor necessary for the neuritic sprouting that occurs within 1-2 days of nerve growth factor treatment. Rather, it likely represents a component of a late neuronal maturational response. Synuclein-1 redistributes diffusely within the cell soma and the neuritic processes in nerve growth factor-treated PC12 cells. Cultured neonatal rat sympathetic neurones express high levels of synuclein-1, with a diffuse intracellular distribution, similar to neuronal PC12 cells. These results suggest that levels of synuclein-1 may be regulated by neurotrophic factors in the nervous system and reinforce a role for alpha-synuclein in plasticity-maturational responses. In contrast, there is no correlation between synuclein expression and apoptotic death following trophic deprivation.

Cell cycle regulators in neuronal death evoked by excitotoxic stress: implications for neurodegeneration and its treatment

Excitotoxic stress is potentially an important component of disorders such as stroke and neurodegenerative diseases. Its toxic effects appear to be transduced through mechanisms that result in both acute and delayed forms of death. We examined here whether cyclin dependent kinases (CDKs), molecules normally associated with cell cycle control, may be involved in delayed excitotoxic death in two different excitotoxin models. We show that nuclear localized cyclin D1, an activator of Cdk4/6, is upregulated during kainic acid evoked death of CA3/CA1 neurons and that this upregulation is associated with increased phosphorylation of a critical CDK substrate, pRb. In addition, we find that the CDK inhibitor, flavopiridol blocks the delayed death of cultured cortical neurons evoked by 3-nitroproprionic acid, an inhibitor of the mitochondrial electron transport chain, treatment and that the NMDA antagonist, MK801 provides short term protection in this model. Full, long-term protection occurs when both flavopiridol and MK-801 are present. Taken together, these data support a role for cell cycle regulators in neuronal death evoked by excitotoxic stress and indicate a potential therapeutic target for treatment of excitotoxicity-related disorders.

Neuromelanin biosynthesis is driven by excess cytosolic catecholamines not accumulated by synaptic vesicles

Melanin, the pigment in hair, skin, eyes, and feathers, protects external tissue from damage by UV light. In contrast, neuromelanin (NM) is found in deep brain regions, specifically in loci that degenerate in Parkinson's disease. Although this distribution suggests a role for NM in Parkinson's disease neurodegeneration, the biosynthesis and function of NM have eluded characterization because of lack of an experimental system. We induced NM in rat substantia nigra and PC12 cell cultures by exposure to l-dihydroxyphenylalanine, which is rapidly converted to dopamine (DA) in the cytosol. This pigment was identical to human NM as assessed by paramagnetic resonance and was localized in double membrane autophagic vacuoles identical to NM granules of human substantia nigra. NM synthesis was abolished by adenoviral-mediated overexpression of the synaptic vesicle catecholamine transporter VMAT2, which decreases cytosolic DA by increasing vesicular accumulation of neurotransmitter. The NM is in a stable complex with ferric iron, and NM synthesis was inhibited by the iron chelator desferrioxamine, indicating that cytosolic DA and dihydroxyphenylalanine are oxidized by iron-mediated catalysis to membrane-impermeant quinones and semiquinones. NM synthesis thus results from excess cytosolic catecholamines not accumulated into synaptic vesicles. The permanent accumulation of excess catechols, quinones, and catechol adducts into a membrane-impermeant substance trapped in organelles may provide an antioxidant mechanism for catecholamine neurons. However, NM in organelles associated with secretory pathways may interfere with signaling, as it delays stimulated neurite outgrowth in PC12 cells.

Identification of diverse nerve growth factor-regulated genes by serial analysis of gene expression (SAGE) profiling

Neurotrophic factors such as nerve growth factor (NGF) promote a wide variety of responses in neurons, including differentiation, survival, plasticity, and repair. Such actions often require changes in gene expression. To identify the regulated genes and thereby to more fully understand the NGF mechanism, we carried out serial analysis of gene expression (SAGE) profiling of transcripts derived from rat PC12 cells before and after NGF-promoted neuronal differentiation. Multiple criteria supported the reliability of the profile. Approximately 157,000 SAGE tags were analyzed, representing at least 21,000 unique transcripts. Of these, nearly 800 were regulated by 6-fold or more in response to NGF. Approximately 150 of the regulated transcripts have been matched to named genes, the majority of which were not previously known to be NGF-responsive. Functional categorization of the regulated genes provides insight into the complex, integrated mechanism by which NGF promotes its multiple actions. It is anticipated that as genomic sequence information accrues the data derived here will continue to provide information about neurotrophic factor mechanisms.

NADE, a p75NTR-associated cell death executor, is involved in signal transduction mediated by the common neurotrophin receptor p75NTR

The low affinity neurotrophin receptor p75NTR can mediate cell survival as well as cell death of neural cells by NGF and other neurotrophins. To elucidate p75NTR-mediated signal transduction, we screened p75NTR-associated proteins by a yeast two-hybrid system. We identified one positive clone and named NADE (p75NTR-associated cell death executor). Mouse NADE has marked homology to the human HGR74 protein. NADE specifically binds to the cell-death domain of p75NTR. Co-expression of NADE and p75NTR induced caspase-2 and caspase-3 activities and the fragmentation of nuclear DNA in 293T cells. However, in the absence of p75NTR, NADE failed to induce apoptosis, suggesting that NADE expression is necessary but insufficient for p75NTR-mediated apoptosis. Furthermore, p75NTR/NADE-induced cell death was dependent on NGF but not BDNF, NT-3, or NT-4/5, and the recruitment of NADE to p75NTR (intracellular domain) was dose-dependent. We obtained similar results from PC12 cells, nnr5 cells, and oligodendrocytes. Taken together, NADE is the first signaling adaptor molecule identified in the involvement of p75NTR-mediated apoptosis induced by NGF, and it may play an important role in the pathogenesis of neurogenetic diseases.

Involvement of retinoblastoma family members and E2F/DP complexes in the death of neurons evoked by DNA damage

Neuronal death evoked by DNA damage requires cyclin-dependent kinase 4 (Cdk4) and 6 activity and is accompanied by elevation of cyclin D1-associated kinase activity. Because Cdk4/6 phosphorylates retinoblastoma protein (pRb) family members that then modulate the transcriptional activity of E2F/DP1 complexes, we examined the involvement of these components in DNA damage-evoked neuronal death. Camptothecin induced rapid pRb and p107 phosphorylation at a Cdk4/6 phosphorylation site followed by selective loss of Rb and p107. The CDK inhibitor flavopiridol suppressed pRb and p107 phosphorylation and loss, implicating CDK activity in these events. Moreover, the loss of pRb and p107 appeared to be mediated by caspases because it was blocked by general caspase inhibitors. The role of phosphorylation and pRb and p107 loss in the death pathway was indicated by observations that virally mediated expression of pRb mutated at sites of phosphorylation, including the Cdk4/6 site, inhibited death. Finally, expression of dominant-negative versions of DP1, known to compromise E2F transcriptional activity, protects cortical neurons from death induced by camptothecin and sympathetic neurons from death evoked by UV treatment. Taken together, these results implicate the CDK-pRb/E2F/DP pathway as a required element in the neuronal death evoked by DNA damage.

Neurotrophin signaling via Trks and p75

This review focuses on recent advances in our understanding of receptor-mediated signaling by the neurotrophins NGF, BDNF, NT3, and NT4/5. Two distinct receptor types have been distinguished, Trks and p75. The Trks are receptor tyrosine kinases that utilize a complex set of substrates and adapter proteins to activate defined secondary signaling cascades required for neurotrophin-promoted neuronal differentiation, plasticity, and survival. A specialized aspect of Trk/neurotrophin action in neurons is the requirement for retrograde signaling from the distal periphery to the cell body. p75 is a universal receptor for neurotrophins that is a member of the TNF receptor/Fas/CD40 superfamily. p75 appears to modify Trk signaling when the two receptor types are coexpressed. When expressed in the absence of Trks, p75 mediates responses to neurotrophins including promotion of apoptotic death. The mechanisms of p75 receptor signaling remain to be fully understood.

Stimulus-coupled interaction of tyrosine hydroxylase with 14-3-3 proteins

Tyrosine hydroxylase (TH) is phosphorylated by CaM kinase II and is activated in situ in response to a variety of stimuli that increase intracellular Ca(2+). We report here, using baculovirus-expressed TH, that the 14-3-3 protein binds and activates the expressed TH when the enzyme is phosphorylated at Ser-19, a site of CaM kinase II-dependent phosphorylation located in the regulatory domain of TH. Site-directed mutagenesis showed that a TH mutant in which Ser-19 was substituted by Ala retained enzymatic activity at the same level as the non-mutated enzyme, but was a poor substrate for CaM kinase II and did not bind the 14-3-3 protein. Likewise, a synthetic phosphopeptide (FRRAVpSELDA) corresponding to the part of the TH sequence, including phosphoSer-19, inhibited the interaction between the expressed TH and 14-3-3, while the phosphopeptide (GRRQpSLIED) corresponding to the site of cAMP-dependent phosphorylation (Ser-40) had little effect on complex formation. The complex was very stable with a dissociation constant of 3 nM. Furthermore, analysis of PC12nnr5 cells transfected with myc-tagged 14-3-3 showed that 14-3-3 formed a complex with endogenous TH when the cultured cells were exposed to a high K(+) concentration that increases intracellular Ca(2+) and phosphorylation of Ser-19 in TH. These findings suggest that the 14-3-3 protein participates in the stimulus-coupled regulation of catecholamine synthesis that occurs in response to depolarization-evoked, Ca(2+)-dependent phosphorylation of TH.

CEP-1347 (KT7515), an inhibitor of JNK activation, rescues sympathetic neurons and neuronally differentiated PC12 cells from death evoked by three distinct insults

The c-Jun N-terminal kinase signaling cascade appears to play a role in some cases of cell death, including neuronal apoptosis. CEP-1347 (KT7515), an indolocarbazole of the K252a family, blocks this stress signaling cascade and promotes survival. Here, we used CEP-1347 to probe whether neuronal death pathways activated by distinct insults also possess elements in common. Cultured rat sympathetic neurons and neuronally differentiated PC12 cells were induced to die by withdrawal of nerve growth factor, exposure to ultraviolet irradiation, or subjection to oxidative stress. In each case, death was prevented by 100-200 nM CEP-1347. Moreover, in each of these death paradigms, c-Jun N-terminal kinase 1 activity in neuronally differentiated PC12 cells was elevated by two- or threefold, and this increase was totally blocked by CEP-1347 at concentrations that promoted survival. In contrast, 200 nM CEP-1347 did not block death due to serum withdrawal from undifferentiated PC12 cells or to activation of Fas in Jurkat T cell cultures, even though in each case c-Jun N-terminal kinase 1 activation occurred and was inhibited by CEP-1347. These observations suggest that some but not all death pathways triggered by different insults can include a common mechanistic component, a likely candidate for which is activation of the c-Jun N-terminal kinase signaling cascade.

Role of cell cycle regulatory proteins in cerebellar granule neuron apoptosis

Cerebellar granule neurons (CGNs) undergo apoptosis when deprived of depolarizing concentrations of KCl, but the underlying molecular mechanisms are not yet clear. Although caspases have been postulated to be involved in CGN cell death, inhibitors of caspases failed to prevent apoptosis under our culture conditions, suggesting an involvement of other molecules and pathways. We find that inhibitors of cyclin-dependent kinases--flavopiridol, olomoucine, and roscovitine--protect CGNs from KCl withdrawal-induced apoptosis, suggesting that cell cycle components play a significant role in the death of these neurons. Analysis of the different cell cycle regulatory elements in this model revealed that apoptosis is preceded by an increase in the level of cyclin E protein, with elevated nuclear levels of cyclin D1 and with enhanced activity of the cyclin D1- and E- associated kinases. In addition, there was a significant decrease in the level of the cyclin-dependent kinase (cdk) inhibitor p27. In agreement with these changes, analysis of a major substrate of cyclin-activated cdks, retinoblastoma protein (Rb), showed an increase in the level of phosphorylated forms within 1 hr of KCl withdrawal. Moreover, the overall levels of Rb protein were significantly reduced within 6-12 hr of KCl withdrawal and did so by a caspase-independent mechanism. All of these responses were blocked by cdk inhibitors. These findings indicate that cdks act at an early step in the pathway by which KCl withdrawal induces apoptotic death of cerebellar granule cells and suggest that additional elements of the cell cycle machinery participate in this mechanism.

Caspase-dependent and -independent death of camptothecin-treated embryonic cortical neurons

This study investigates the mechanisms underlying death of cultured embryonic cortical neurons exposed to the DNA-damaging agent camptothecin and in particular the interdependence of the roles of cyclin-dependent kinases (Cdks), caspases, and mitochondrial function. Camptothecin evokes rapid neuronal death that exhibits nuclear features of apoptosis. This death is accompanied by loss of cytochrome c and mitochondrial transmembrane potential as well as by induction of caspase-3-like activity and caspase-2 processing. The Cdk inhibitor flavopiridol provides long-term rescue from death and prevents loss of cytochrome c and mitochondrial transmembrane potential as well as caspase activation and processing. General caspase inhibitors rescue neurons from this rapid apoptotic death but do not prevent them from undergoing delayed death in which nuclear features of apoptosis are absent. Moreover, the caspase inhibitors do not affect early cytochrome c release and delay but do not prevent the loss of transmembrane potential. Agents that directly disrupt mitochondrial function without inducing cytochrome c release lead to a caspase-independent death. These observations favor a model in which (1) DNA damage leads to Cdk activation, which lies upstream of release of cytochrome c and caspase activation; (2) cytochrome c release is caspase-independent and may occur upstream of caspase activation; (3) early apoptotic death requires caspases; and (4) delayed nonapoptotic death that occurs in the presence of caspase inhibitors is a consequence of prolonged loss of mitochondrial function. These findings shed light on the mechanisms by which DNA damage kills neurons and raise questions regarding the general utility of caspase inhibitors as neurotherapeutic agents.

A function-structure model for NGF-activated TRK

Mechanisms regulating transit of receptor tyrosine kinases (RTKs) from inactive to active states are incompletely described, but require autophosphorylation of tyrosine(s) within a kinase domain 'activation loop'. Here, we employ functional biological assays with mutated TRK receptors to assess a 'switch' model for RTK activation. In this model: (i) ligand binding stimulates activation loop tyrosine phosphorylation; (ii) these phosphotyrosines form specific charge pairs with nearby basic residues; and (iii) the charge pairs stabilize a functionally active conformation in which the activation loop is restrained from blocking access to the kinase catalytic core. Our findings both support this model and identify residues that form specific charge pairs with each of the three TRK activation loop phosphotyrosines.

Caspase-2 (Nedd-2) processing and death of trophic factor-deprived PC12 cells and sympathetic neurons occur independently of caspase-3 (CPP32)-like activity

We have previously shown that caspase-2 (Nedd-2) is required for apoptosis induced by withdrawal of trophic support from PC12 cells and sympathetic neurons. Here, we examine the relationship of caspase-2 processing and cell death to induction of caspase-3 (CPP32)-like activity in PC12 cells. Caspase-2 processing, at a site tentatively identified as D333, led to the formation of an N-terminal 37 kDa product. This processing correlated temporally with induction of caspase-3-like activity. Agents previously shown to inhibit caspase-3-like activation, such as bcl-2 and the Cdk inhibitor flavopiridol, also acted upstream of caspase-2 processing. The general caspase inhibitors BAF and zVAD-FMK inhibited N-terminal caspase-2 processing. In contrast, the more selective caspase inhibitor DEVD-FMK inhibited the induction of caspase-3-like activity but did not affect caspase-2 processing or significantly suppress death in PC12 cells or sympathetic neurons. This indicates that caspase-3-like activity is not required for either caspase-2 processing or apoptosis in this paradigm. An antisense oligonucleotide to caspase-2 inhibited cell death but did not affect caspase-3-like activity, indicating that caspase-2 is not upstream of this activity and that activation of caspase-3-like caspases is not sufficient for death. Thus, in our paradigm, caspase-2 processing and caspase-3-like activity are induced independently of each other. Moreover, although death requires caspase-2, caspase-3-like activity is neither necessary nor sufficient for death.

Cyclin-dependent kinases participate in death of neurons evoked by DNA-damaging agents

Previous reports have indicated that DNA-damaging treatments including certain anticancer therapeutics cause death of postmitotic nerve cells both in vitro and in vivo. Accordingly, it has become important to understand the signaling events that control this process. We recently hypothesized that certain cell cycle molecules may play an important role in neuronal death signaling evoked by DNA damage. Consequently, we examined whether cyclin-dependent kinase inhibitors (CKIs) and dominant-negative (DN) cyclin-dependent kinases (CDK) protect sympathetic and cortical neurons against DNA-damaging conditions. We show that Sindbis virus-induced expression of CKIs p16(ink4), p21(waf/cip1), and p27(kip1), as well as DN-Cdk4 and 6, but not DN-Cdk2 or 3, protect sympathetic neurons against UV irradiation- and AraC-induced death. We also demonstrate that the CKIs p16 and p27 as well as DN-Cdk4 and 6 but not DN-Cdk2 or 3 protect cortical neurons from the DNA damaging agent camptothecin. Finally, in consonance with our hypothesis and these results, cyclin D1-associated kinase activity is rapidly and highly elevated in cortical neurons upon camptothecin treatment. These results suggest that postmitotic neurons may utilize Cdk4 and 6, signals that normally control proliferation, to mediate death signaling resulting from DNA-damaging conditions.

Neuroprotective actions of dipyridamole on cultured CNS neurons

We report that dipyridamole is neuroprotective for a variety of rat embryonic CNS neurons cultured in serum-free basal medium lacking trophic factors or other additives. We also describe the mechanism underlying this action. Neurons died rapidly in basal medium but were rescued in large measure by 10 microM dipyridamole. The protective action of dipyridamole seems to be attributable to its antioxidant property. Vitamin E and N-acetylcysteine provided comparable neuroprotection in basal medium, whereas an array of compounds that mimic other actions of dipyridamole (inhibition of phosphodiesterases, blockade of nucleoside and chloride transport, interference with the multidrug resistance protein, and enhancement of prostacyclin synthesis) failed to promote survival. Thus, a major cause of neuronal death in this system seems to be oxidative stress that is relieved by dipyridamole. Iron plays a significant role in generation of such stress, as indicated by the observations that addition of apotransferrin or iron chelators to basal medium or use of iron-free medium also afforded protection. Although oxidative stress was a major determinant of neuronal death, it was not the only factor. Dipyridamole or other antioxidant measures did not provide sustained neuroprotection. However, provision of insulin, which was not protective alone in basal medium, along with dipyridamole significantly enhanced long-term neuronal survival. Hence, optimal protection requires both trophic support and relief from oxidative stress. These findings lend credence to the potential use of dipyridamole or its derivatives in prevention and/or treatment of CNS injuries and degenerative disorders in which oxidative stress is a significant component.

Promotion of neuronal survival by GM1 ganglioside. Phenomenology and mechanism of action

The purpose of this article is to review recent findings regarding the mechanisms by which GM1 may mimic or potentiate certain actions of neurotrophic factors, including promotion of neuronal survival. It is proposed that the neuroprotective activity of GM1 is due, at least in part, to its ability to favor the dimerization of neurotrophic factor tyrosine kinases and thereby mimicking the action of their corresponding ligands. This may manifest both in the absence of ligand (thereby triggering a subset of neurotrophic-factor responses such as prevention of apoptosis) and in the presence of ligand (thereby potentiating responses to neurotrophic factors).

The Src homology domain 3 (SH3) of a yeast type I myosin, Myo5p, binds to verprolin and is required for targeting to sites of actin polarization

The budding yeast contains two type I myosins, Myo3p and Myo5p, with redundant functions. Deletion of both myosins results in growth defects, loss of actin polarity and polarized cell surface growth, and accumulation of intracellular membranes. Expression of myc-tagged Myo5p in myo3Delta myo5Delta cells fully restores wild-type characteristics. Myo5p is localized as punctate, cortical structures enriched at sites of polarized cell growth. We find that latrunculin-A-induced depolymerization of F-actin results in loss of Myo5p patches. Moreover, incubation of yeast cells at 37 degrees C results in transient depolarization of both Myo5p patches and the actin cytoskeleton. Mutant Myo5 proteins with deletions in nonmotor domains were expressed in myo3Delta myo5Delta cells and the resulting strains were analyzed for Myo5p function. Deletion of the tail homology 2 (TH2) domain, previously implicated in ATP-insensitive actin binding, has no detectable effect on Myo5p function. In contrast, myo3Delta myo5Delta cells expressing mutant Myo5 proteins with deletions of the src homology domain 3 (SH3) or both TH2 and SH3 domains display defects including Myo5p patch depolarization, actin disorganization, and phenotypes associated with actin dysfunction. These findings support a role for the SH3 domain in Myo5p localization and function in budding yeast. The proline-rich protein verprolin (Vrp1p) binds to the SH3 domain of Myo3p or Myo5p in two-hybrid tests, coimmunoprecipitates with Myo5p, and colocalizes with Myo5p. Immunolocalization of the myc-tagged SH3 domain of Myo5p reveals diffuse cytoplasmic staining. Thus, the SH3 domain of Myo5p contributes to but is not sufficient for localization of Myo5p either to patches or to sites of polarized cell growth. Consistent with this, Myo5p patches assemble but do not localize to sites of polarized cell surface growth in a VRP1 deletion mutant. Our studies support a multistep model for Myo5p targeting in yeast. The first step, assembly of Myo5p patches, is dependent upon F-actin, and the second step, polarization of actin patches, requiresVrp1p and the SH3 domain of Myo5p.