Genetic Analysis of Susceptibility to Radiation-induced Apoptosis of Thymocytes in Mice

Delayed Reaction of Radiation on the Central Nervous System and Bone System in C57BL/6J Mice

The aim of this study was to provide a suitable mouse model of radiation-induced delayed reaction and identify potential targets for drug development related to the prevention and treatment of radiation injury. C57BL/6J mice were subjected to singular (109 cGy/min, 5 Gy*1) and fractional (109 cGy/min, 5 Gy*2) total body irradiation. The behavior and activity of mice were assessed 60 days after ionizing radiation (IR) exposure. After that, the pathological changes and mechanism of the mouse brain and femoral tissues were observed by HE, Nissl, Trap staining micro-CT scanning and RNA sequencing (RNA-Seq), and Western blot. The results show that singular or fractional IR exposure led to a decrease in spatial memory ability and activity in mice, and the cognitive and motor functions gradually recovered after singular 5 Gy IR in a time-dependent manner, while the fractional 10 Gy IR group could not recover. The decrease in bone density due to the increase in osteoclast number may be relative to the down-regulation of RUNX2, sclerostin, and beta-catenin. Meanwhile, the brain injury caused by IR exposure is mainly linked to the down-regulation of BNDF and Tau. IR exposure leads to memory impairment, reduced activity, and self-recovery, which are associated with time and dose. The mechanism of cognitive and activity damage was mainly related to oxidative stress and apoptosis induced by DNA damage. The damage caused by fractional 10 Gy TBI is relatively stable and can be used as a stable multi-organ injury model for radiation mechanism research and anti-radiation medicine screening.

Influence of Microgravity on Apoptosis in Cells, Tissues, and Other Systems In Vivo and In Vitro

All life forms have evolved under the constant force of gravity on Earth and developed ways to counterbalance acceleration load. In space, shear forces, buoyance-driven convection, and hydrostatic pressure are nullified or strongly reduced. When subjected to microgravity in space, the equilibrium between cell architecture and the external force is disturbed, resulting in changes at the cellular and sub-cellular levels (e.g., cytoskeleton, signal transduction, membrane permeability, etc.). Cosmic radiation also poses great health risks to astronauts because it has high linear energy transfer values that evoke complex DNA and other cellular damage. Space environmental conditions have been shown to influence apoptosis in various cell types. Apoptosis has important functions in morphogenesis, organ development, and wound healing. This review provides an overview of microgravity research platforms and apoptosis. The sections summarize the current knowledge of the impact of microgravity and cosmic radiation on cells with respect to apoptosis. Apoptosis-related microgravity experiments conducted with different mammalian model systems are presented. Recent findings in cells of the immune system, cardiovascular system, brain, eyes, cartilage, bone, gastrointestinal tract, liver, and pancreas, as well as cancer cells investigated under real and simulated microgravity conditions, are discussed. This comprehensive review indicates the potential of the space environment in biomedical research.

Differential sensitivity of inbred mouse strains to ovarian damage in response to low-dose total body irradiation†

Radiation induces ovarian damage and accelerates reproductive aging. Inbred mouse strains exhibit differential sensitivity to lethality induced by total body irradiation (TBI), with the BALB/cAnNCrl (BALB/c) strain being more sensitive than the 129S2/SvPasCrl (129) strain. However, whether TBI-induced ovarian damage follows a similar pattern of strain sensitivity is unknown. To examine this possibility, female BALB/c and 129 mice were exposed to a single dose of 1 Gy (cesium-137 γ) TBI at 5 weeks of age, and ovarian tissue was harvested for histological and gene expression analyses 2 weeks post exposure. Sham-treated mice served as controls. 1 Gy radiation nearly eradicated the primordial follicles and dramatically decreased the primary follicles in both strains. In contrast, larger growing follicles were less affected in the 129 relative to BALB/c strain. Although this TBI paradigm did not induce detectable ovarian fibrosis in either of the strains, we did observe strain-dependent changes in osteopontin (Spp1) expression, a gene involved in wound healing, inflammation, and fibrosis. Ovaries from BALB/c mice exhibited higher baseline Spp1 expression that underwent a significant decrease in response to radiation relative to ovaries from the 129 strain. A correspondingly greater change in the ovarian matrix, as evidenced by reduced ovarian hyaluronan content, was also observed following TBI in BALB/c mice relative to 129 mice. These early changes in the ovary may predispose BALB/c mice to more pronounced late effects of TBI. Taken together, our results demonstrate that aspects of ovarian damage mirror other organ systems with respect to overall strain-dependent radiation sensitivity.

Variations in Prkdc encoding the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) and susceptibility to radiation-induced apoptosis and lymphomagenesis

DNA double-strand breaks (DSBs) induced by ionizing radiation enforce cells to die, if unrepaired; while if misrepaired, DSBs may cause malignant transformation. The DSB repair system predominant in mammals requires DNA-dependent protein kinase (DNA-PK). Previously, we identified the apoptosis susceptibility gene Radiation-induced apoptosis 1 (Rapop1) on mouse chromosome 16. The STS/A (STS) allele at Rapop1 leads to decreased sensitivity to apoptosis in the BALB/cHeA (BALB/c) background. In the present study, we established Rapop1 congenic strains C.S-R1 and C.S-R1L, which contain the STS genome in a 0.45 cM interval critical for Rapop1 in common in the BALB/c background. Within the segment critical for Rapop1, Prkdc encoding the catalytic subunit of DNA-PK (DNA-PKcs) was assigned. Two variations T6,418C and G11,530A, which induce amino acid substitutions C2,140R downstream from the putative leucine zipper motif and V3,844M near the kinase domain, respectively, were found between BALB/c and STS for Prkdc. The majority of inbred strains such as C57BL/6J carried the STS allele at Prkdc; a few strains including 129/SvJ and C.B17 carried the BALB/c allele. DNA-PK activity as well as DNA-PKcs expression was profoundly diminished in BALB/c and 129/SvJ mice as compared with C57BL/6 and C.S-R1 mice. In the crosses (C.S-R1 x BALB/c)F(1) x 129/SvJ and (C.S-R1 x BALB/c)F(1) x C.B17, enhanced apoptosis occurred in the absence of the wild-type allele at Prkdc. C.S-R1 and C.S-R1L were both less sensitive to radiation lymphomagenesis than BALB/c. Our study provides strong evidence for Prkdc as a candidate for Rapop1 and a susceptibility gene for radiation lymphomagenesis as well.

Modulations of glucocorticoid-induced apoptosis linked to the p53 deletion and to the apoptosis susceptibility gene Rapop1 (Radiation-induced apoptosis 1)

We have analysed the effects of p53 and of the apoptosis susceptibility gene Rapop1 (Radiation-induced apoptosis 1) located on chromosome 16 on glucocorticoid- and radiation-induced in vivo apoptosis of thymocytes. For those analyses, we used Rapop1 semicongenic mice heterozygous for the STS and BALB/cHeA alleles in the chromosomal segment containing Rapop1 in the BALB/cHeA background, mice bearing a p53 deficient allele in the BALB/cHeA background and the genetic crosses between these mice. The p53 wild type mice with a STS/A allele at the Rapop1 locus were less susceptible to both radiation- and glucocorticoid-induced apoptosis than those with homozygous BALB/cHeA alleles at this locus. Surprisingly, glucocorticoid-induced apoptosis was enhanced in the p53 hemizygous mice and considerably increased in the p53 nullizygous mice. In contrast, a sizable reduction of radiation-induced apoptosis was seen in the p53 hemizygous mice. The low susceptiblity to glucocortocoid-induced apoptosis linked to the STS allele of Rapop1 was less pronounced in the p53 hemizygous mice and a diminished effect of Rapop1 on radiation-induced apoptosis was seen in these mice. Although it remains to be established whether the genes modulating glucocortocoid-induced apoptosis are identical to p53 and Rapop1, our data suggest that p53 and Rapop1 may participate in glucocorticoid-induced apoptosis of thymocytes.

Modification of immune response by low dose ionizing radiation: role of apoptosis

Acute as well as fractionated whole body exposures to low doses (< 50 cGy) of ionizing radiation (LDR) have been reported to alter several immunological parameters in experimental animals. It is, however, not clear whether the augmentation of immune response by LDR will be observed for all responses and across genetic barriers. Since several proteins including p53 are synthesized following radiation exposure, the role of p53 and consequently that of activation induced apoptosis in the immunomodulation by LDR also remained to be evaluated. Experiments were, therefore, carried out in two different strains of inbred mice viz. C57BL/6 and BALB/c, exposed to fractionated LDR (4 cGy/day, 5 days/week, total dose 20 cGy) and subsequently stimulated with the polyclonal mitogen Con A or immunized with Mycobacterium vaccae or dinitrofluorobenzene (DNFB) for delayed type hypersensitivity (DTH) response. The proliferation of spleen cells in response to con A as measured by [3H]thymidine incorporation was significantly higher in 20 cGy-irradiated C57BL/6 mice as compared with that in the Con A-stimulated cells from sham-irradiated controls. The same response was suppressed by LDR in BALB/c mice. On the other hand, DTH to M. vaccae as well as DNFB was suppressed in C57BL/6 mice while DTH to M. vaccae was augmented in BALB/c mice and that to DNFB was not significantly affected following same dose. The augmentation of response to con A in C57BL/6 mice was prominent in CD4- (CD8+) T cells and was marked by the decrease in the proportion of cells expressing p53 as estimated by flow cytometry. Reduction in expression of p53 was accompanied by reduced apoptosis, as measured by TUNEL assay, in the Con A-stimulated spleen cells of irradiated C57BL/6 mice when compared with that in the sham-treated controls. The spleen cells of BALB/c mice showed exactly opposite profiles in this respect. Thus alteration in the immune response following LDR depends on antigen, type of response as well as the strain of mice used. Furthermore, the alterations in the expression of pro-apoptosis gene p53 and activation induced apoptosis in the effector or regulatory cells seem to contribute to the end result.

Genetics of susceptibility to radiation-induced apoptosis in colon: two loci on chromosomes 9 and 16

Apoptosis, a mechanism for removal of genetically damaged cells and for maintenance of desired size of cell populations, has been implicated in tumor development. Previously, we defined polymorphic loci for susceptibility to apoptosis of thymocytes Rapop1, Rapop2, and Rapop3 on mouse Chromosomes 16, 9, and 3, respectively, using recombinant congenic CcS/Dem strains, each of which contains a random set of 12.5% STS/A genome in the genetic background of BALB/cHeA. The STS/A alleles at these loci confer lower susceptibility to radiation-induced apoptosis of thymocytes than the BALB/cHeA. In the present study, we tested susceptibility of colon crypt cells to radiation-induced apoptosis. In contrast to apoptosis in thymus, the STS/A mice were more susceptible to apoptosis in colon than the BALB/cHeA. Among the CcS/Dem strains, CcS-4, CcS-7, and CcS-16 were more susceptible to apoptosis in colon than the BALB/cHeA; in thymus, the CcS-7 mice are less susceptible, and the CcS-4 and CcS-16 are not different from the BALB/cHeA. Thus, individual CcS/Dem strains showed different apoptosis susceptibility in the two organs. Analysis of (CcS-7 x BALB/cHeA)F2 hybrids revealed linkage of susceptibility to radiation-induced apoptosis of colon crypt cells to two loci on Chrs 9 and 16, to which Rapop2 and Rapop1 are mapped. The STS/A allele at the locus on chromosome 9 results in high susceptibility to apoptosis of colon crypt cells in mice homozygous for the BALB/cHeA allele at the locus on Chr 16. Although these two loci may be identical to Rapop1 and Rapop2, they affect apoptosis in colon in a way different from that in thymus.

[Significance of apoptotic processes in radiotherapy. II]

Apoptosis is known as an active process of cell death forced by radio- and chemotherapy. Therefore, established concepts (terms, therapy schemes) will reflect a picture different from that usually seen, when examined under the apoptotic point of view. Furthermore, the development of new concepts for innovative diagnosis, prognosis and therapy could be accomplished. This is an attempt to reveal actual features of both aspects.

Identification of quantitative trait loci controlling levels of radiation-induced thymocyte apoptosis in mice

Thymocyte apoptosis levels are higher in C57BL/6J mice than in C3Hf/Kam mice. Low-dose irradiation increases the numbers of thymocytes undergoing apoptosis, but the strain difference persists. We mapped three loci controlling radiation-induced thymocyte apoptosis levels in F2 intercross progeny of these strains. The strongest association of a genomic region with an apoptosis level occurred in a region of chromosome 11 known to harbor a locus (or loci) important in the pathogenesis of several rodent models of autoimmune disease. Additional loci influencing radiation-induced thymocyte apoptosis were identified on chromosomes 9 and 16. The genetic polymorphisms underlying these loci may have an evolutionary role in fine-tuning the apoptotic response in T cells and may be important in the etiology of lymphoproliferative disorders and autoimmunity.

Genetic dissection of susceptibility to radiation-induced apoptosis of thymocytes and mapping of Rapop1, a novel susceptibility gene

Genetic dissection of susceptibility to radiation-induced apoptosis of thymocytes was performed by counting dead cells in histologically processed thymuses after 0.5 Gy of whole-body X-irradiation, using recombinant congenic (CcS/Dem) strains derived from inbred mouse strains BALB/cHeA (susceptible) and STS/A (resistant). A high (8/20) number of strains with lower dead cell scores than BALB/cHeA among CcS/Dem recombinant congenic strains (RCS), which contain 12.5% of STS/A genome in the genetic background of BALB/cHeA strain, indicates that the difference between BALB/cHeA and STS/A is caused by several genes and that susceptibility probably requires BALB/cHeA alleles at more than one locus. Similar results were obtained with CXS/Hg recombinant inbred (CXS/Hg) strains. Analysis of F2 hybrids between BALB/cHeA and CcS-7, one of the CcS/Dem strains that showed lower dead cell scores than BALB/cHeA, demonstrated that a novel gene (Rapop1, radiation-induced apoptosis 1) controlling susceptibility to radiation-induced apoptosis in the thymus is located in the proximal region of mouse chromosome 16.

Effects of X-irradiation on mouse testicular cells and sperm chromatin structure

The testicular regions of male mice were exposed to x-ray doses ranging from 0 to 400 rads. Forty days after exposure the mice were killed and the testes and cauda epididymal sperm removed surgically. Flow cytometric measurements of acridine orange stained testicular samples indicated a repopulation of testicular cell types following x-ray killing of stem cells. Cauda epididymal sperm were analyzed by the sperm chromatin structure assay (SCSA), a flow cytometric measurement of the susceptibility of the sperm nuclear DNA to in situ acid denaturation. The SCSA detected increased susceptibility to DNA denaturation in situ after 12.5 rads of x-ray exposure, with significant increases following 25 rads. Abnormal sperm head morphology was not significantly increased until the testes were exposed to 60 rads of x-rays. These data suggest that the SCSA is currently the most sensitive, non-invasive method of detecting x-ray damage to testicular stem spermatogonia.

p53 is required for radiation-induced apoptosis in mouse thymocytes

The p53 tumour suppressor gene is the most widely mutated gene in human tumorigenesis. p53 encodes a transcriptional activator whose targets may include genes that regulate genomic stability, the cellular response to DNA damage, and cell-cycle progression. Introduction of wild-type p53 into cell lines that have lost endogenous p53 function can cause growth arrest or induce a process of cell death known as apoptosis. During normal development, self-reactive thymocytes undergo negative selection by apoptosis, which can also be induced in immature thymocytes by other stimuli, including exposure to glucocorticoids and ionizing radiation. Although normal negative selection involves signalling through the T-cell receptor, the induction of apoptosis by other stimuli is poorly understood. We have investigated the requirement for p53 during apoptosis in mouse thymocytes. We report here that immature thymocytes lacking p53 die normally when exposed to compounds that may mimic T-cell receptor engagement and to glucocorticoids but are resistant to the lethal effects of ionizing radiation. These results demonstrate that p53 is required for radiation-induced cell death in the thymus but is not necessary for all forms of apoptosis.