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Molecular and Cellular Biology, October 2002, p. 6866-6870, Vol. 22, No. 19
0270-7306/02/$04.00+0 DOI: 10.1128/MCB.22.19.6866-6870.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Department of Genetics, St. Jude Children's Research Hospital, Memphis, Tennessee 38105
Received 5 February 2002/ Returned for modification 19 June 2002/ Accepted 2 July 2002
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The Bcl-2 family of proteins is functionally important in apoptosis and often acts in a tissue-specific manner (1, 15, 21). The canonical member of this family, Bcl-2, was first identified as a component of a translocation in B-cell malignancies, and when overexpressed was found to inhibit apoptosis in a variety of biological systems (15). Other Bcl-2 family members act as general effectors of cell death by either promoting or protecting against cell death (1, 3). Pro-apoptotic members include Bax, Bad, Bid, Bak, and Bik, while antiapoptotic members include Bcl-2, Bcl-X, Mcl-1, Bcl-w, and A-1. Bcl-2 family proteins can contain four conserved domains, designated Bcl-2 homology regions (BH1 to BH4) (1, 3, 15). The BH1 and BH2 motifs of the death antagonists (such as Bcl-2 and Bcl-X) and the BH3 domain of the death agonists (such as Bax and Bak) are important for homo- or heterodimerization between family members and facilitate control of apoptosis (1, 3, 15, 30). The BH4 domain, found in several antiapoptotic homologues, is essential for the death-repressing activity (10). Some Bcl-2 family members share sequence homology only with the BH3 domain (11, 26). These BH3-domain-only proteins are thought to activate multidomain Bcl-2 members to initiate apoptosis (4).
As demonstrated in mice with null mutations for Bcl-2 family members, this group of proteins plays important roles during development and homeostasis (21). For example, Bcl-2 inactivation leads to polycystic kidney disease, while inactivation of Bax in the mouse resulted in hyperplasia of thymocytes and male infertility due to spermatocyte hypoplasia (14). Bax-/- animals also show a decrease in normal programmed cell deaths in a number of nervous system tissues, including peripheral ganglia and the trigeminal brainstem nuclear complex, and neuronal cultures derived from Bax-/- animals are resistant to a number of death-inducing agents (5, 6, 28, 29). More dramatic effects are found in Bcl-X-null mice, where embryonic survival requires the presence of this protein (17). Tissue-specific effects are observed in other cases; for example, Bcl-W inactivation results in infertility due to arrested sperm development associated with a gradual loss of germ cells and Sertoli cells from the testis (23, 24). In this report we have investigated the consequences of inactivating the Bcl-2-related protein Diva (12, 25). This gene contains several BH domains (BH1, BH2, and BH4), with some contention existing regarding the presence of a BH3 domain (12, 13, 25), and it can modulate apoptosis in vitro (2, 12, 13, 16, 19, 25). Diva is also relatively restricted in expression, with high levels of expression confined to the ovary (12, 25). Here we report that Diva-null mice are fertile, respond normally to apoptotic stimuli, and do not have any obvious developmental defects.
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8 kb) immediately 5' to Diva exon 1 was cloned into the BglII site of pNTK1901 (Stratagene, San Diego, Calif.) to generate pDiva-1, and a 2.5-kb BamHI/BglII fragment immediately 3' of Diva exon 2 was cloned into the BamHI site of pDiva-1 to generate pDiva-KO. This construct was linearized with SalI and electroporated into W9.5 embryonic stem (ES) cells. Targeted ES cells were identified by Southern blot analysis of EcoRV-digested ES genomic DNA using a genomic BglII/EcoRV fragment 3' of Diva. The probe,
200 bp in length, was generated by PCR using mouse genomic DNA as a template with the following primers: forward, 5' AGA TCT ACT GAA CTC AGC, and reverse, 5' ATA TCT GAG AAG CCA AGG. EcoRV digestion of G418-resistant ES cells identified a 4-kb fragment in targeted clones due to loss of an EcoRV site in the mutant allele that was readily distinguishable from the endogenous 2.9-kb Diva genomic EcoRV fragment. Resulting clones were injected into C57BL/6 blastocysts and then implanted into the uteri of pseudopregnant F1 B/CBA foster mothers and allowed to develop to term. Male chimeras were selected for high percentage of agouti coat color and were mated to C57BL/6 females to obtain germ line transmission. The presence of the mutated allele was confirmed by Southern blot analysis or PCR, and heterozygous F1 males and females were interbred to generate F2 animals for subsequent study. Genotyping of Diva mutant mice was done from tail DNA using PCR with the following primers to identify the Diva wild-type (WT) allele: GDP 1, 5' CAG ACG ATT GCCC CGG C, and GDP 4, 5' GGT AAC ATC AGC ATC ACA GAA TGC. The Neor marker gene was identified using the following primers: Neo 4, 5' CGG GAG CGG CGA TAC CGT AAA GC, and Neo 7, 5' GAA GCG GGA AGG GAC TGG CTG CTA. Histology. Ovaries were obtained from 2-month-old mice 6 h after 18 Gy of whole-body ionizing radiation from a cesium irradiator (delivered at a rate of 1.2 Gy/min) and were placed in 10% formalin. Histology of unirradiated tissues was done using age-matched Diva-null mice and littermate controls. Ovaries were paraffin embedded, sectioned into 8-µm sections with an HM325 microtome (Microm), and hematoxylin and eosin stained according to standard procedures. For studies using nervous system tissues, mice were used 5 days after birth (P5; day of birth is P0) and irradiated with 18 Gy. Nervous system tissues were collected after fixation by transcardial perfusion with 4% paraformaldehyde, cryoprotected in 20% sucrose-phosphate-buffered saline, and cryosectioned (12-µm coronal sections) using an HM500 M cryostat (Microm). Neutral red staining was performed with 1% neutral red (Aldrich Chemical) in 0.1 M acetic acid (pH 4.8) for 1 min followed by dehydration in ethanol and mounting with Permount (Fisher). In all cases, experiments were done in triplicate and comparative studies of Diva-null mice used WT littermates as controls.
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To determine the biological role of Diva, we used gene targeting to inactivate mouse Diva. This gene (GenBank no. NM013479, NM013479, and AF102501) is located on chromosome 9 and contains two coding exons. Inactivation of Diva was achieved by replacing an
3-kb genomic region containing both exons with a Neor selection cassette to delete the entire Diva open reading frame (ORF) (Fig. 1A). Targeting of ES cells occurred at a frequency of approximately 1/25 (Fig. 1A), and two of these targeted ES lines were used to generate chimeras and, subsequently, Diva heterozygous mice. Interbreeding of Diva heterozygotes generated Diva-null mice, which were born at the expected frequency of 1/4. Southern blot analysis using a Diva cDNA probe also showed an absence of Diva coding sequence in Diva-/- mice, while both WT and Diva+/- mice contained Diva ORF sequence (Fig. 1B). We further confirmed that Diva expression was disrupted in the Diva-/- mice using Northern blot analysis; Diva mRNA of 1.2 kb was identified in RNA obtained from WT and heterozygous, but not homozygous Diva-/-, ovaries (Fig. 1C).
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FIG. 1. Inactivation of mouse Diva. (A) Diva was inactivated by replacing both exons 1 and 2 with a neomycin selection cassette driven by the PGK promoter derived from pNTK901. Selected restriction sites relevant to the generation and analysis of Diva inactivation are indicated. Homologous recombination removed an EcoRV site from the Diva locus, resulting in a 4-kb mutant Diva allele fragment after EcoRV digestion of genomic DNA; Southern blot analysis was done using a probe encompassing the genomic region contained in a 3' BglII/EcoRV fragment (probe). (B) The Southern blot shown was probed with Diva cDNA, is a representative analysis of mice derived from mating Diva heterozygotes, and shows that exons 1 and 2 containing the Diva ORF are absent from Diva-/- mice. (C) Northern blot analysis shows that the Diva message (1.2 kb) is present in the WT but not Diva-/- ovaries, while no Diva signal is detected in the testis. The control probe (reticulon, 1.6 kb; GenBank no. AF133669) was used to ensure RNA integrity in samples used for Northern analysis.
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FIG. 2. Radiation-induced apoptosis in Diva-null ovaries. Ovaries from WT (a) or Diva-/- (e) mice are histologically indistinguishable, as was apoptosis after ionizing radiation treatment (b, c, f, and g). However, p53-/- ovaries (d, h) were completely resistant to radiation-induced apoptosis. Panels b to d and f to h represent two different comparative views through the ovaries. Panels a and e are unirradiated WT and Diva-/- mice, respectively. Arrows identify pyknotic cells indicative of apoptosis. Magnification, x200.
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To determine if Diva-/- mice were differentially sensitive to genotoxic stress compared to WT littermates, we examined ionizing radiation (IR)-induced apoptosis in these mice. Pronounced apoptosis as determined histologically was observed at 6 h following IR in granulosa cells in both Diva-null and WT controls (Fig. 2b, c, f, and g). However, while no differences were found between Diva-/- and WT mice, there is a clear genetic basis for ovarian radiation-induced apoptosis as p53-/- null mice were completely resistant to IR-induced apoptosis in the ovary (Fig. 2d and h). Because Diva was detected in the developing brain we also examined IR-induced apoptosis in various developing nervous system tissues of Diva-/- and WT controls. Widespread IR-induced apoptosis was found throughout susceptible regions of the developing nervous system (5), including the cerebellar external granule layer (Fig. 3b and c), the hippocampal dentate gyrus (Fig. 3e and f), and the retina (Fig. 3h and i), and was identical in both Diva-/- and WT controls. Therefore, while other Bcl-2-related members can modulate the response to radiation (5), Diva is not required for IR-induced apoptosis in the developing nervous system.
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FIG. 3. Radiation-induced apoptosis in the Diva-null developing nervous system. The nervous system of Diva-/- mice was histologically indistinguishable from that of WT controls. Apoptosis 6 h after IR in the P5 Diva-/- cerebellar external granule layer (EGL; b and c), dentate gyrus (DG; e and f), and retina (h and i) was similar to that in WT tissue. Apoptosis was assessed by the presence of pyknotic nuclei (arrows) by use of neutral red staining (5). Magnification, x400.
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