Autophagy and Apoptosis Act as Partners to Induce Germ Cell Death after Heat Stress in Mice
et al. (2012) Autophagy and Apoptosis Act as Partners to Induce Germ Cell Death after Heat Stress in Mice. PLoS
ONE 7(7): e41412. doi:10.1371/journal.pone.0041412
Autophagy and Apoptosis Act as Partners to Induce Germ Cell Death after Heat Stress in Mice
Mianqiu Zhang 0
Min Jiang 0
Ye Bi 0
Hui Zhu 0
Zuomin Zhou 0
Jiahao Sha 0
Janine Santos, University of Medicine and Dentistry of New Jersey, United States of America
0 State Key Laboratory of Reproductive Medicine, Department of Histology and Embryology, Nanjing Medical University , Nanjing, Jiangsu Province , China
Testicular heating suppresses spermatogenesis which is marked by germ cell loss via apoptotic pathways. Recently, it is reported that autophagy also can be induced by heat treatment in somatic cells. In this study, the status of autophagy in germ cells after heat treatment, as well as the partnership between autophagy and apoptosis in these cells was investigated. The results demonstrated that besides initiating apoptotic pathways, heat also induced autophagic pathways in germ cells. Exposure of germ cells to hyperthermia resulted in several specific features of the autophagic process, including autophagosome formation and the conversion of LC3-I to LC3-II. Furthermore, the ubiquitin-like protein conjugation system was implicated as being likely responsible for heat-induced autophagy in germ cells since all genes involving this system were found to be expressed in the testes. In addition, the upstream protein in this system, Atg7 (Autophagy-related gene 7), was found to be expressed in all types of spermatogenic cells, and its expression level was positively correlated with the level of autophagy in germ cells. As a result, Atg7 was selected as the investigative target to further analyze the role of autophagy in heat-induced germ cell death. It was shown that down expression of Atg7 protein resulted in the notable decrease in the level of autophagy in heat-treated germ cells, and this down-regulation of autophagy caused by Atg7 knockdown further reduced the apoptotic rate of germ cells. These results suggest that autophagy plays a positive role in the process of germ cell apoptosis after heat treatment. In conclusion, this study demonstrates that heat triggers autophagy and apoptosis in germ cells. These two mechanisms might act as partners, not antagonist, to induce cell death and lead to eventual destruction of spermatogenesis.
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Spermatogenesis is a highly ordered process that is dependent
on well-balanced germ cell proliferation, differentiation and death
in the testes [1,2]. This process can be disturbed by several factors,
including chemical insults, withdrawl of gonadotropin or
testosterone, heat, radiation, etc [37]. Testicular heating suppresses
spermatogenesis in several mammalian species, including human.
Exposure of the testis to body temperature (physiologic heating)
[4,8] or above (supraphysiologic heating) [912] results in
increased germ cell degeneration, death and loss. Previous studies
show that apoptosis, a process of programmed cell death, is
a potential mechanism responsible for the germ cell death and loss
induced by both physiologic heating [4] and supraphysiologic
heating [1013].
Recently, it was reported that autophagy could be activated in
cells under varied stress conditions, such as ischemia injury [14],
oxidative stress [15], endoplasmic reticulum stress [16], growth
factor deprivation [17], heat stress [18], etc. Heat can induce an
increase of autophagy in rat hepatocytes [18]. Autophagy is
a process of self-degradation whereby cellular organelles and
proteins are phagocytosed during metabolic stress [19]. It is an
evolutionarily conserved physiological process that is thought to
promote cell survival because nucleotides, amino acids, and free
fatty acids can be generated during the degradation of cellular
contents and then recycled and reused for macromolecular
synthesis and ATP generation [20]. On the other hand, autophagy
has also been shown to promote cell death under certain
conditions. Apoptosis is not the sole means by which the cell can
undergo a genetically programmed death. Autophagy has also
been linked to the actual death process [21]. In fact, autophagy is
often referred to as type II programmed cell death (distinct from
type I programmed cell death, apoptosis) because it does not
require caspase activation or DNA fragmentation, which are
classical characteristics of apoptosis [20]. During the process of cell
death, cross-talk between autophagy and apoptosis is very complex
and still controversial. Different types of interplay (partnership or
antagonist) between the autophagy and apoptosis have been
indicated [2123]. At present, the molecular mechanisms of
autophagic process are still not fully understood. However, genetic
studies in yeast have identified a set of autophagy-related genes
(Atg genes) that are required for initiation and performance of
autophagy.
In the protein profile of mouse spe (...truncated)