Expression pattern of a novel death-promoting gene, DP5, in the developing murine nervous system

Apoptotic cell death regulation in neurons

Apoptosis plays a major role in shaping the developing nervous system during embryogenesis as neuronal precursors differentiate to become post-mitotic neurons. However, once neurons are incorporated into functional circuits and become mature, they greatly restrict their capacity to die via apoptosis, thus allowing the mature nervous system to persist in a healthy and functional state throughout life. This robust restriction of the apoptotic pathway during neuronal differentiation and maturation is defined by multiple unique mechanisms that function to more precisely control and restrict the intrinsic apoptotic pathway. However, while these mechanisms are necessary for neuronal survival, mature neurons are still capable of activating the apoptotic pathway in certain pathological contexts. In this review, we highlight key mechanisms governing the survival of post-mitotic neurons, while also detailing the physiological and pathological contexts in which neurons are capable of overcoming this high apoptotic threshold.

STAT1 is a master regulator of pancreatic {beta}-cell apoptosis and islet inflammation

Cytokines produced by islet-infiltrating immune cells induce β-cell apoptosis in type 1 diabetes. The IFN-γ-regulated transcription factors STAT1/IRF-1 have apparently divergent effects on β-cells. Thus, STAT1 promotes apoptosis and inflammation, whereas IRF-1 down-regulates inflammatory mediators. To understand the molecular basis for these differential outcomes within a single signal transduction pathway, we presently characterized the gene networks regulated by STAT1 and IRF-1 in β-cells. This was done by using siRNA approaches coupled to microarray analysis of insulin-producing cells exposed or not to IL-1β and IFN-γ. Relevant microarray findings were further studied in INS-1E cells and primary rat β-cells. STAT1, but not IRF-1, mediates the cytokine-induced loss of the differentiated β-cell phenotype, as indicated by decreased insulin, Pdx1, MafA, and Glut2. Furthermore, STAT1 regulates cytokine-induced apoptosis via up-regulation of the proapoptotic protein DP5. STAT1 and IRF-1 have opposite effects on cytokine-induced chemokine production, with IRF-1 exerting negative feedback inhibition on STAT1 and downstream chemokine expression. The present study elucidates the transcriptional networks through which the IFN-γ/STAT1/IRF-1 axis controls β-cell function/differentiation, demise, and islet inflammation.

Hrk/DP5 contributes to the apoptosis of select neuronal populations but is dispensable for haematopoietic cell apoptosis

The pro-apoptotic BH3-only members of the Bcl2 family, crucial initiators of cell death, are activated by a diverse array of developmental cues or experimentally applied stress stimuli. We have investigated, through gene targeting in mice, the biological roles for the BH3-only family member HRK (also known as DP5) in apoptosis regulation. Hrk gene expression was found to be restricted to cells and tissues of the central and peripheral nervous systems. Sensory neurons from mice lacking Hrk were less sensitive to apoptosis induced by nerve growth factor (NGF) withdrawal, consistent with the induction of Hrk following NGF deprivation. By contrast, cerebellar granule neurons that upregulate Hrk upon transfer to low-K+ medium underwent apoptosis normally under these conditions in the absence of Hrk. Furthermore, loss of Hrk was not sufficient to rescue the neuronal degeneration in lurcher mutant mice. Despite previous reports, no evidence was found for Hrk expression or induction in growth-factor-dependent haematopoietic cell lines following withdrawal of their requisite cytokine, and haematopoietic progenitors lacking HRK died normally in response to cytokine deprivation. These results demonstrate that HRK contributes to apoptosis signalling elicited by trophic factor withdrawal in certain neuronal populations but is dispensable for apoptosis of haematopoietic cells.

dp5/HRK is a c-Jun target gene and required for apoptosis induced by potassium deprivation in cerebellar granule neurons

In cerebellar granule neurons, a BH3-only Bcl-2 family member, death protein 5/harakiri, is up-regulated in a JNK-dependent manner during apoptosis induced by potassium deprivation. However, it is not clear whether c-Jun is directly involved in the induction of dp5. Here, we showed that the up-regulation of dp5, but not fas ligand and bim, after potassium deprivation was suppressed by the expression of a dominant negative form of c-Jun. Deletion analysis of the 5'-flanking sequence of the dp5 gene revealed that a major responsive element responsible for the induction by potassium deprivation is an ATF binding site located at -116 to -109 relative to the transcriptional start site. Mutation of this site completely abolished promoter activation. Furthermore, a gel shift assay showed that a specific complex containing c-Jun and ATF2 recognized this site and increased in potassium-deprived cerebellar granule neurons. Chromatin immunoprecipitation demonstrated that c-Jun was able to bind to this site in vivo. Finally, we demonstrated that knockdown of Dp5 by small interfering RNA rescued neurons from potassium deprivation-induced apoptosis. Taken together, these results suggest that dp5 is a target gene of c-Jun and plays a critical role in potassium deprivation-induced apoptosis in cerebellar granule neurons.

Erythropoietin Downregulates Bax and DP5 ProApoptotic Gene Expression in Neonatal Hypoxic-Ischemic Brain Injury

BACKGROUND

Perinatal asphyxia is an important cause of neonatal mortality and subsequent serious sequelae such as motor and cognitive deficits and seizures. The ameliorative effect of erythropoietin (Epo) on experimental hypoxic-ischemic brain injury in neonatal rats has been recently reported. Recent studies also confirm the antiapoptotic effect of Epo in a variety of in vitro and in vivo neuronal injury models including hypoxic-ischemic brain injury. However, molecular mechanisms of Epo protection and antiapoptotic effect in this model are unclear. Epo may exert its antiapoptotic effect via the differential regulation of the expression of genes involved in the apoptotic process.

OBJECTIVES

Thus, in the present study, we studied the effects of systemically administered Epo on antiapoptotic (bcl-2, bcl-XL), proapoptotic (bax and DP5) gene expression following hypoxic-ischemic brain injury in neonatal rats.

METHODS

Seven- day-old Wistar rat pups were divided into three groups: control group (n=15), saline-treated group (n=17), and Epo-treated group (n=18). Rat pups were subjected to left carotid artery occlusion followed by 2.5 h of hypoxic exposure. Epo-treated group received an intraperitoneal injection of recombinant human Epo at a dose of 1,000 units/kg, saline-treated group received an intraperitoneal injection of saline at the same volume of Epo. Forty-eight hours after hypoxia, 3 animals in each group were killed for histopathological evaluation. To detect DNA fragmentation in cell nuclei, terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling reaction was applied. Bcl-2 and bax protein expression were also analyzed with immunohistochemistry. For reverse transcriptase-polymerase chain reaction (RT-PCR) analysis, rats were sacrificed 4, 12, and 24 h after hypoxia. Bcl-2, bcl-XL, bax, and DP5 mRNA expression were analyzed by RT-PCR.

RESULTS

Epo significantly prevented hypoxia-ischemia-induced bax and DP5 mRNA upregulation in brain tissue. Epo did not show any effect on bcl-XL transcription altered by injury. However, Epo reversed injury-induced downregulation in bcl-2 transcription. Modulating effects of Epo on bcl-2 and bax protein expression were also revealed by immunohistochemistry.

CONCLUSIONS

These results suggest that Epo exerts a neuroprotective effect against hypoxic-ischemic brain injury, at least partially, via the differential regulation of the expression of genes involved in apoptotic process.

The pro-apoptotic human BH3-only peptide harakiri is expressed in cryptococcus-infected perivascular macrophages in HIV-1 encephalitis patients

In the central nervous system (CNS), HIV-1 targets mainly microglia/macrophages. Like the CD4+ T cell depletion and neuronal loss in AIDS, apoptosis is thought to be involved in eliminating infected macrophages. In this study, we examined the expression of the pro-apoptotic BH3-peptide harakiri (Hrk) in brain tissues of AIDS patients. Immunoreactivity against Hrk was positive in perivascular macrophages infiltrated into some restricted lesions. Most of these immunopositive cells contained small inclusions positive for Grocott's methenamine silver staining. Confocal laser microscopy demonstrated that Hrk expression coincided with immunoreactivities against HIV-1 and Cryptococcus neoformans. Expression of Hrk mRNA was demonstrated in these cells by in situ hybridization, which indicated that Hrk is not phagocytosed material. Some pro-apoptotic bcl-family members, including Hrk, may contribute to the delayed hypersensitive reaction in AIDS, in macrophages eliminating opportunistic infection.

Brain ischemia/reperfusion-induced expression of DP5 and its interaction with Bcl-2, thus freeing Bax from Bcl-2/Bax dimmers are mediated by c-Jun N-terminal kinase (JNK) pathway

Our previous studies and those of others have strongly suggested that c-Jun N-terminal kinase (JNK) signaling pathway plays a critical role in ischemic brain injury. But the downstream mechanism that accounts for the proapoptotic actions of JNK during cerebral ischemia/reperfusion still remains to be investigated in detail. DP5, one of the mammalian BH3-only proteins, was cloned as a neuronal apoptosis-inducing gene. In this study, we examined the changes of protein level of DP5 and its interaction with Bcl-2 family members in a rat model of global ischemia and reperfusion by immunoprecipitation and immunoblotting; furthermore, we investigated the effect of activated JNK on DP5-signaling pathway. We show here that DP5 was induced and interacted with Bcl-2 but not Bax in hippocampal CA1 6 h to 3 days after ischemia, while the interaction of Bcl-2 with Bax decreased. Systemic administration of SP600125, a small molecule JNK-specific inhibitor, diminished the induction of DP5 and its interaction with Bcl-2 after 2 days of ischemia. At the same time, SP600125 increased the interaction of Bax with Bcl-2 after 2 days of reperfusion. Thus, these results indicate that brain ischemia/reperfusion-induced activation of DP5 signaling pathway is mediated by JNK in postischemic rat hippocampal CA1.

Bcl-2 family regulation of neuronal development and neurodegeneration

Neuronal cell death is a key feature of both normal nervous system development and neuropathological conditions. The Bcl-2 family, via its regulation of both caspase-dependent and caspase-independent cell death pathways, is uniquely positioned to critically control neuronal cell survival. Targeted gene disruptions of specific bcl-2 family members and the generation of transgenic mice overexpressing anti- or pro-apoptotic Bcl-2 family members have confirmed the importance of the Bcl-2 family in the nervous system. Data from studies of human brain tissue and experimental animal models of neuropathological conditions support the hypothesis that the Bcl-2 family regulates cell death in the mature nervous system and suggest that pharmacological manipulation of Bcl-2 family action could prove beneficial in the treatment of human neurological conditions such as stroke and neurodegenerative diseases.

Methamphetamine induces oligodendroglial cell death in vitro

We investigated whether the psychostimulant methamphetamine (METH) has a cytotoxic effect on oligodendrocytes and which cell-death pathways are involved in the cytotoxic process. METH caused concentration- and time-dependent cytotoxicity in rat oligodendrocyte cultures. METH induced apoptotic cell death and mRNA expression of pro-apoptotic proteins (bax and DP5), but not anti-apoptotic proteins (bcl-2 and bcl-XL). These results suggest that METH induces cytotoxicity in rat oligodendrocytes via the differential regulation of the expression of genes involved in the apoptotic process.

Up-regulation of Hrk, a regulator of cell death, in retinal ganglion cells of axotomized rat retina

Hrk, a regulator of cell death, belongs to the family of pro-apoptotic BH3-only proteins and is known to induce apoptosis in nerve tissue. We examined Hrk gene expression to clarify the role of BH3-only proteins in apoptosis of retinal ganglion cells (RGCs) after optic nerve transection in the adult rat. Reverse transcription-polymerase chain reaction showed that Hrk was up-regulated in retina from 12 h after axotomy, and continued to be elevated for 1 week. In situ hybridization histochemistry revealed that Hrk was expressed in a sub-population of axotomized RGCs. These results suggest that Hrk is involved in the induction of apoptosis in RGCs after optic nerve transection.

Upregulation of the pro-apoptotic BH3-only peptide harakiri in spinal neurons of amyotrophic lateral sclerosis patients

DNA fragmentation and activation of caspase-1, implicating involvement of apoptosis, have been reported in the spinal cord of amyotrophic lateral sclerosis (ALS) patients and transgenic mouse models of ALS. Because BH3-only members of the Bcl-2 family have pro-apoptotic activity, we examined the expression of the BH3-only peptide harakiri (Hrk) in the spinal cord of ALS patients. In situ expression of Hrk mRNA and immunoreactivity against the Hrk peptide were verified in the spinal neurons. In the immunoblot analysis, upregulated Hrk protein migrated at 16 kDa. Heterodimerization of Hrk with Bcl-2 was detected by immunoprecipitation, which suggests the competition of Hrk and anti-apoptotic Bcl-2. These findings suggest that Hrk plays a role in apoptotic events in ALS pathogenesis.

Expression of sonic hedgehog signal transducers, patched and smoothened, in human basal cell carcinoma

In basal cell nevus syndrome (BCNS) patients, mutations of a gene, patched (ptc), which encodes a putative signal transducer of sonic hedgehog protein (SHH), were found and are thought to be one of the major causes of BCNS. The SHH signaling pathway is an important developmental pathway, and ptc protein (PTC) is a suppressive component serving as a receptor for the secreted SHH. Another transmembrane protein, smoothened (SMO), forms a complex with PTC and regulates this signaling pathway. Recent transgenic studies have strengthened the importance of the SHH signaling system in the etiology of basal cell carcinoma (BCC). In this study, we examined the expression patterns of mRNA for ptc and smo in two different BCC subtypes and normal skin. We found that the expressions of ptc and smo mRNA were enhanced in the tumor nests of the nodular BCC, especially at the advancing portions, but were under the detectable level in the superficial BCC cases examined, indicating that ptc and smo mRNA expressions might be associated with BCC tumor progression and divide the BCC histologic types into two subtypes, superficial and nodular types. In addition, no obvious signals for ptc and smo mRNA were detected in the normal human epidermis, appendages, or seborrheic keratosis, indicating that the abnormal proliferation of follicular epithelial cells caused by ptc, smo and/or other genetic changes, which also cause ptc and smo overexpressions, might result in BCC tumor formation.

Eos: a novel member of the Ikaros gene family expressed predominantly in the developing nervous system

We identified a novel member of the Ikaros gene family, which has critical roles in the development of lymphoid lineages. This gene, which we named Eos, was expressed predominantly in the developing central and peripheral nervous system. Eos protein could interact with itself and Ikaros protein through its C-terminal portion in the yeast two hybrid assay. These findings suggested that Eos may have important roles in neural development similarly to the Ikaros family in the development of hemolymphoid tissue.

The cell death-promoting gene DP5, which interacts with the BCL2 family, is induced during neuronal apoptosis following exposure to amyloid beta protein

DP5, which contains a BH3 domain, was cloned as a neuronal apoptosis-inducing gene. To confirm that DP5 interacts with members of the Bcl-2 family, 293T cells were transiently co-transfected with DP5 and Bcl-xl cDNA constructs, and immunoprecipitation was carried out. The 30-kDa Bcl-xl was co-immunoprecipitated with Myc-tagged DP5, suggesting that DP5 physically interacts with Bcl-xl in mammalian cells. Previously, we reported that DP5 is induced during neuronal apoptosis in cultured sympathetic neurons. Here, we analyzed DP5 gene expression and the specific interaction of DP5 with Bcl-xl during neuronal death induced by amyloid-beta protein (A beta). DP5 mRNA was induced 6 h after treatment with A beta in cultured rat cortical neurons. The protein encoded by DP5 mRNA showed a specific interaction with Bcl-xl. Induction of DP5 gene expression was blocked by nifedipine, an inhibitor of L-type voltage-dependent calcium channels, and dantrolene, an inhibitor of calcium release from the endoplasmic reticulum. These results suggested that the induction of DP5 mRNA occurs downstream of the increase in cytosolic calcium concentration caused by A beta. Moreover, DP5 specifically interacts with Bcl-xl during neuronal apoptosis following exposure to A beta, and its binding could impair the survival-promoting activities of Bcl-xl. Thus, the induction of DP5 mRNA and the interaction of DP5 and Bcl-xl could play significant roles in neuronal degeneration following exposure to A beta.