c-Flip expression and function in fetal mouse gonocytes

c-FLIP regulates autophagy by interacting with Beclin-1 and influencing its stability

c-FLIP (cellular FLICE-like inhibitory protein) protein is mostly known as an apoptosis modulator. However, increasing data underline that c-FLIP plays multiple roles in cellular homoeostasis, influencing differently the same pathways depending on its expression level and isoform predominance. Few and controversial data are available regarding c-FLIP function in autophagy. Here we show that autophagic flux is less effective in c-FLIP−/− than in WT MEFs (mouse embryonic fibroblasts). Indeed, we show that the absence of c-FLIP compromises the expression levels of pivotal factors in the generation of autophagosomes. In line with the role of c-FLIP as a scaffold protein, we found that c-FLIP_L interacts with Beclin-1 (BECN1: coiled-coil, moesin-like BCL2-interacting protein), which is required for autophagosome nucleation. By a combination of bioinformatics tools and biochemistry assays, we demonstrate that c-FLIP_L interaction with Beclin-1 is important to prevent Beclin-1 ubiquitination and degradation through the proteasomal pathway. Taken together, our data describe a novel molecular mechanism through which c-FLIP_L positively regulates autophagy, by enhancing Beclin-1 protein stability.

Function and transcriptomic dynamics of Sertoli cells during prospermatogonia development in mouse testis

In mammals, spermatogonial stem cells (SSCs) arise from a subpopulation of prospermatogonia during neonatal testis development. Currently, molecular mechanisms directing the prospermatogonia to spermatogonial transition are not well understood. In the study, we found that reducing Sertoli cells number by Amh-cre mediated expression of diphtheria toxin (AC;DTA) in murine fetal testis caused defects in prospermatogonia fate decisions. Histological and immunohistochemical analyses confirmed that Sertoli cells loss occurred at embryonic day (E) 14.5. Prospermatogonia maintained mitotic arrest at E16.5 in control animals, in contrast, 13.4% of germ cells in AC;DTA testis reentered cell cycle and expressed gH2A.X and Sycp3, indicating the commitment to meiosis. After birth, the number of prospermatogonia resuming mitosis was significantly affected by Sertoli cell loss in AC;DTA animals. Lastly, we isolated primary Sertoli cells using a Sertoli cell specific GFP reporter line and showed dynamics of Sertoli cell transcriptomes at E12.5, E13.5, E16.5 and P1. By further analysis, we revealed unique gene expression patterns and potential candidate genes regulating Sertoli cell development and likely mediating interactions between Sertoli cells, prospermatogonia and other testicular cells.

c-Flip KO fibroblasts display lipid accumulation associated with endoplasmic reticulum stress

c-Flip proteins are well-known apoptosis modulators. They generally contribute to tissue homeostasis maintenance by inhibiting death-receptor-mediated cell death. In the present manuscript, we show that c-Flip knock-out (KO) mouse embryonic fibroblasts (MEFs) kept in culture under starvation conditions gradually modify their phenotype and accumulate vacuoles, becoming progressively larger according to the duration of starvation. Large vacuoles are present in KO MEFs though not in WT MEFs, and are Oil Red-O positive, which indicates that they represent lipid droplets. Western blot experiments reveal that, unlike WT MEFs, KO MEFs express high levels of the lipogenic transcription factor PPAR-γ. Lipid droplet accumulation was found to be associated with endoplasmic reticulum (ER) stress activation and autophagic modulation valuated by means of BIP increase, LC3 lipidation and AMP-activated protein kinase (AMPK) phosphorylation, and p62 accumulation. Interestingly, XBP-1, an ER stress-induced lipogenic transcription factor, was found to preferentially localize in the nucleus rather than in the cytoplasm of KO MEFs. These data demonstrate that, upon starvation, c-Flip affects lipid accumulation, ER stress and autophagy, thereby pointing to an important role of c-Flip in the adaptive response and ER stress response programs under both normal and pathological conditions.

c-Flip overexpression affects satellite cell proliferation and promotes skeletal muscle aging

This study shows that forcing c-Flip overexpression in undifferentiated skeletal myogenic cells in vivo results in early aging muscle phenotype. In the transgenic mice, adult muscle histology, histochemistry and biochemistry show strong alterations: reduction of fibers size and muscle mass, mitochondrial abnormalities, increase in protein oxidation and apoptosis markers and reduced AKT/GSK3β phosphorylation. In the infant, higher levels of Pax-7, PCNA, P-ERK and active-caspase-3 were observed, indicating enhanced proliferation and concomitant apoptosis of myogenic precursors. Increased proliferation correlated with NF-κB activation, detected as p65 phosphorylation, and with high levels of embryonic myosin heavy chain. Reduced regenerative potential after muscle damage in the adult and impaired fiber growth associated with reduced NFATc2 activation in the infant were also observed, indicating that the satellite cell pool is prematurely compromised. Altogether, these data show a role for c-Flip in modulating skeletal muscle phenotype by affecting the proliferative potential of undifferentiated cells. This finding indicates a novel additional mechanism through which c-Flip might possibly control tissue remodeling.

A critical role of PUMA in maintenance of genomic integrity of murine spermatogonial stem cell precursors after genotoxic stress

Neonatal gonocytes are precursors of spermatogonial stem cells. Preserving their integrity by elimination of damaged germ cells may be crucial to avoid the transmission of genetic alterations to progeny. Using gamma-irradiation, we investigated by immunohistochemistry, flow cytometry and real-time PCR components of the death machinery in neonatal gonocytes. Their death was correlated with caspase 3 activation but not with AIF translocation into the nucleus. The in vivo contribution of both the extrinsic and the intrinsic pathways was then investigated. We focused on the roles of TRAIL/Death Receptor 5 (DR5) and PUMA. Our results were validated using knockout mice. Whereas DR5 expression was upregulated at the cell surface after radiation, caspase 8 was not activated. However, we detected caspase 9 cleavage associated with cytochrome c release. In mice deficient for PUMA, radiation-induced gonocyte apoptosis was reduced, whereas invalidation of TRAIL had no effect. Overall, our results show that genotoxic stress-induced apoptosis of gonocytes is caspase-dependent and involves almost exclusively the intrinsic pathway. Furthermore, PUMA plays a critical role in the maintenance of genomic integrity of spermatogonial stem cell precursors.

Testis atrophy and reduced sperm motility in transgenic mice overexpressing c-FLIP(L)

OBJECTIVE

To study the effect of c-FLIP overexpression in testicular germ cells.

DESIGN

A novel transgenic mouse model overexpressing the apoptotic modulator c-FLIP in the testis was generated.

SETTING

Animal facility and university research laboratory.

ANIMAL(S)

Transgenic mice overexpressing the long isoform of c-FLIP (c-FLIP(L)) under the transcriptional control of a 400 bp long regulatory region of the Stra8 promoter.

INTERVENTION(S)

Spermatozoa motility and testis histological, immunohistochemical, and Western blot analyses were carried out in transgenic and control derived specimens.

MAIN OUTCOME MEASURE(S)

Testis morphology, sperm motility, and germ cell apoptosis were assayed.

RESULTS

Stra8 promoter was found to activate the ectopic overexpression of c-FLIP(L) in round and elongated spermatids. As a consequence of such overexpression, a dramatic loss of germ cells was observed, resulting in testicular atrophy associated with reduced sperm motility.

CONCLUSION(S)

The data show that c-FLIP(L) forced expression in haploid male germ cells has detrimental effects on spermatogenesis and sperm quality and reveal a possible mechanism underlying the onset of testicular atrophy.

c-Flip overexpression reduces cardiac hypertrophy in response to pressure overload

OBJECTIVE

Activation of Fas signaling has been associated with the development of cardiomyocyte hypertrophy. In the present study, we investigated the effects of increased expression of c-Flip, a natural modulator of Fas receptor signaling, in a mouse model of cardiac growth response to pressure overload.

METHODS

A transgenic mouse overexpressing c-Flip in the heart was generated in FVB/N strain. Echocardiographic, hemodynamic, histological and molecular analyses were carried out under basal conditions and after transverse aortic constriction (TAC)-induced pressure overload.

RESULTS

Overexpression of c-Flip in ventricular heart tissue was functionally silent under basal conditions affecting neither cardiac morphology nor basal cardiac function. Transgenic mice were then subjected to pressure overload by TAC procedure. Under such conditions, c-Flip transgenic mice showed normal left ventricular function with a significantly reduced left ventricular hypertrophy compared with wild-type mice and reduced induction of the cardiac "fetal" gene programme. Further, analysis of intracellular signaling pathways indicated that c-Flip overexpression reduced phosphorylation of both the glycogen synthase kinase 3beta (GSK3 beta) and Akt as compared with controls. Finally, the reduction of the TAC-induced hypertrophy was not accompanied by significant apoptosis increase.

CONCLUSION

Altogether, these findings indicate c-Flip as a key regulator of the cardiac response to ventricular pressure overload.

Apaf-1 and caspase-9 deficiency prevents apoptosis in a Bax-controlled pathway and promotes clonogenic survival during paclitaxel treatment

Taxane derivatives such as paclitaxel elicit their antitumor effects at least in part by induction of apoptosis, but the underlying mechanisms are incompletely understood. Here, we used different cellular models with deficiencies in key regulators of apoptosis to elucidate the mechanism of paclitaxel-induced cell death. Apoptosis by paclitaxel was reported to depend on the activation of the initiator caspase-10; however, we clearly demonstrate that paclitaxel kills murine embryonic fibroblasts (MEFs) devoid of caspase-10 as well as human tumor cell lines deficient in caspase-10, caspase-8, or Fas-associating protein with death domain. In contrast, the lack of Apaf-1 or caspase-9, key regulators of the mitochondrial pathway, not only entirely protected against paclitaxel-induced apoptosis but could even confer clonogenic survival, depending on the cell type and drug concentration. Thus, paclitaxel triggers apoptosis not through caspase-10, but via caspase-9 activation at the apoptosome. This conclusion is supported by the fact that Bcl-2-overexpressing cells and Bax/Bak doubly-deficient MEFs were entirely resistant to paclitaxel-induced apoptosis. Interestingly, also the single knockout of Bim or Bax, but not that of Bak or Bid, conferred partial resistance, suggesting a particular role of these mediators in the cell-death pathway activated by paclitaxel.

c-Flip(L) is expressed in undifferentiated mouse male germ cells

Apoptosis represents a fundamental process during fetal/post-natal testis development. Therefore pro- and anti-apoptotic proteins are essential to regulate testis physiology. c-Flip(L) is a known inhibitor of caspase 8/10 activity; in this study its perinatal expression in mouse male germ cells was investigated. In testis sections and seminiferous tubule whole mount c-Flip(L) was found to be expressed in undifferentiated spermatogonia and to co-localize with germ stem cells markers. In vivo investigations in the vitamin-A deficient mouse, lacking differentiated germ cells, confirmed c-Flip(L) expression in undifferentiated spermatogonia. Further analyses showed Fas expression but no significant caspase 8/10 activity when c-Flip(L) was highly expressed. Altogether these data suggest that c-Flip may control the survival rate of undifferentiated spermatogonia.