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Putting things in place for fertilization: discovering roles for importin proteins in cell fate and spermatogenesis.

Loveland KL, Major AT, Butler R, Young JC, Jans DA, Miyamoto Y - Asian J. Androl. (2015 Jul-Aug)

Bottom Line: Knowledge of importin function has expanded substantially in regard to three key developmental systems: embryonic stem cells, muscle cells and the germ line.In the decade since the potential for regulated nucleocytoplasmic transport to contribute to spermatogenesis was proposed, we and others have shown that the importins that ferry transcription factors into the nucleus perform additional roles, which control cell fate.These studies of germline genesis illuminate new ways in which importin proteins govern cellular differentiation, including via directing proteins to distinct intracellular compartments and by determining cellular stress responses.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology;Department of Anatomy and Developmental Biology, Monash University; Hudson Institute of Medical Research, Monash Medical Centre; School of Clinical Sciences, Monash University, Clayton, VIC, Australia, .

ABSTRACT
Importin proteins were originally characterized for their central role in protein transport through the nuclear pores, the only intracellular entry to the nucleus. This vital function must be tightly regulated to control access by transcription factors and other nuclear proteins to genomic DNA, to achieve appropriate modulation of cellular behaviors affecting cell fate. Importin-mediated nucleocytoplasmic transport relies on their specific recognition of cargoes, with each importin binding to distinct and overlapping protein subsets. Knowledge of importin function has expanded substantially in regard to three key developmental systems: embryonic stem cells, muscle cells and the germ line. In the decade since the potential for regulated nucleocytoplasmic transport to contribute to spermatogenesis was proposed, we and others have shown that the importins that ferry transcription factors into the nucleus perform additional roles, which control cell fate. This review presents key findings from studies of mammalian spermatogenesis that reveal potential new pathways by which male fertility and infertility arise. These studies of germline genesis illuminate new ways in which importin proteins govern cellular differentiation, including via directing proteins to distinct intracellular compartments and by determining cellular stress responses.

No MeSH data available.


Related in: MedlinePlus

Importin function in cellular stress. (a) There is a well-defined association between changes in importin protein localization and function and the responses of cells to stress. In cultured cells, exposure to stresses that include oxidative stress (H2O2), ultraviolet light (UV) and heat shock, leads to rapid IMPα nuclear accumulation. This altered IMPα subcellular localization is considered to result from the collapsing RanGTP–GDP gradient, which results from the stress-induced decrease in cellular ATP levels. In the absence of high nuclear RanGTP, IMPα-mediated nuclear export and import halt. Significantly, these nuclear-located IMPa proteins associate strongly with chromatin, and are thought to directly induce specific alterations in gene expression.25,26 (b) Examination of mouse models with altered importin expression revealed a role for IMPα4 protein in haploid germ cell stress responses. Three mouse strains were compared to assess this: wild type (IMPα4 WT; presence of IMPα4 protein indicated as a blue hatched band), transgenic mice overexpressing IMPα4 with enhanced green fluorescent protein (EGFP) under the protamine promoter for exclusive expression in post-meiotic germ cells (IMPα4-EGFP; solid green band), and an IMPα4 complete knockout line (IMPα4-KO; white band). When testicular germ cells were exposed to high levels of oxidative stress through H2O2, the haploid germ cells–which express IMPα4-EGFP– exhibited increased resistance to stress-induced cell death. Conversely, the absence of IMPα4 led to increased sensitivity in this population. This indicates that IMPα4 aids in protecting the male germline against stress.27
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Figure 3: Importin function in cellular stress. (a) There is a well-defined association between changes in importin protein localization and function and the responses of cells to stress. In cultured cells, exposure to stresses that include oxidative stress (H2O2), ultraviolet light (UV) and heat shock, leads to rapid IMPα nuclear accumulation. This altered IMPα subcellular localization is considered to result from the collapsing RanGTP–GDP gradient, which results from the stress-induced decrease in cellular ATP levels. In the absence of high nuclear RanGTP, IMPα-mediated nuclear export and import halt. Significantly, these nuclear-located IMPa proteins associate strongly with chromatin, and are thought to directly induce specific alterations in gene expression.25,26 (b) Examination of mouse models with altered importin expression revealed a role for IMPα4 protein in haploid germ cell stress responses. Three mouse strains were compared to assess this: wild type (IMPα4 WT; presence of IMPα4 protein indicated as a blue hatched band), transgenic mice overexpressing IMPα4 with enhanced green fluorescent protein (EGFP) under the protamine promoter for exclusive expression in post-meiotic germ cells (IMPα4-EGFP; solid green band), and an IMPα4 complete knockout line (IMPα4-KO; white band). When testicular germ cells were exposed to high levels of oxidative stress through H2O2, the haploid germ cells–which express IMPα4-EGFP– exhibited increased resistance to stress-induced cell death. Conversely, the absence of IMPα4 led to increased sensitivity in this population. This indicates that IMPα4 aids in protecting the male germline against stress.27

Mentions: A crucial feature enabling importins to function in nucleocytoplasmic transport is their relocation to the cytoplasm after binding a cargo molecule and releasing it into the nucleus (Figure 1a). This normally occurs because a high Ran-GTP: RanGDP gradient between the nucleus and cytoplasm is maintained by cytoplasmic RanGAP1/RanBP1 and nuclear-localized RCC1.16 This gradient is lost following exposure of cells to stressors such as H2O2, ultraviolet irradiation or heat shock, and importin α proteins accumulate in the nucleus (Figure 3a).26 We recently identified that one functional outcome of this unusual localization of importin α is altered transcription of a discrete subset of genes, including the downregulation those encoding several replication-dependent histones.25 Given that both importin α2 and α4 are nuclear-localized in postmitotic germ cells, with importin α4 appearing to be exclusively nuclear, we considered that this unusual location might relate to the need for germ cells to manage oxidative and other stressors during their development within the seminiferous epithelium. Through analysis of two unique mouse models (Figure 3b), we obtained evidence that importin α4 levels directly and selectively determine the capacity for spermatids to resist oxidative stress.27 Haploid germ cells from transgenic male mice overexpressing importin α4 had a greater resistance (>30%) to transient H2O2 exposure, while a higher proportion (~30%) of spermatids exhibited decreased viability from importin α4 gene knockout mice compared with their littermate controls. The mechanisms underpinning this outcome remain to be determined, but we propose that genes that are downstream targets of nuclear importin α proteins in spermatogenesis are crucial for enabling male germline cells to recover from exposure to stress.


Putting things in place for fertilization: discovering roles for importin proteins in cell fate and spermatogenesis.

Loveland KL, Major AT, Butler R, Young JC, Jans DA, Miyamoto Y - Asian J. Androl. (2015 Jul-Aug)

Importin function in cellular stress. (a) There is a well-defined association between changes in importin protein localization and function and the responses of cells to stress. In cultured cells, exposure to stresses that include oxidative stress (H2O2), ultraviolet light (UV) and heat shock, leads to rapid IMPα nuclear accumulation. This altered IMPα subcellular localization is considered to result from the collapsing RanGTP–GDP gradient, which results from the stress-induced decrease in cellular ATP levels. In the absence of high nuclear RanGTP, IMPα-mediated nuclear export and import halt. Significantly, these nuclear-located IMPa proteins associate strongly with chromatin, and are thought to directly induce specific alterations in gene expression.25,26 (b) Examination of mouse models with altered importin expression revealed a role for IMPα4 protein in haploid germ cell stress responses. Three mouse strains were compared to assess this: wild type (IMPα4 WT; presence of IMPα4 protein indicated as a blue hatched band), transgenic mice overexpressing IMPα4 with enhanced green fluorescent protein (EGFP) under the protamine promoter for exclusive expression in post-meiotic germ cells (IMPα4-EGFP; solid green band), and an IMPα4 complete knockout line (IMPα4-KO; white band). When testicular germ cells were exposed to high levels of oxidative stress through H2O2, the haploid germ cells–which express IMPα4-EGFP– exhibited increased resistance to stress-induced cell death. Conversely, the absence of IMPα4 led to increased sensitivity in this population. This indicates that IMPα4 aids in protecting the male germline against stress.27
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4492042&req=5

Figure 3: Importin function in cellular stress. (a) There is a well-defined association between changes in importin protein localization and function and the responses of cells to stress. In cultured cells, exposure to stresses that include oxidative stress (H2O2), ultraviolet light (UV) and heat shock, leads to rapid IMPα nuclear accumulation. This altered IMPα subcellular localization is considered to result from the collapsing RanGTP–GDP gradient, which results from the stress-induced decrease in cellular ATP levels. In the absence of high nuclear RanGTP, IMPα-mediated nuclear export and import halt. Significantly, these nuclear-located IMPa proteins associate strongly with chromatin, and are thought to directly induce specific alterations in gene expression.25,26 (b) Examination of mouse models with altered importin expression revealed a role for IMPα4 protein in haploid germ cell stress responses. Three mouse strains were compared to assess this: wild type (IMPα4 WT; presence of IMPα4 protein indicated as a blue hatched band), transgenic mice overexpressing IMPα4 with enhanced green fluorescent protein (EGFP) under the protamine promoter for exclusive expression in post-meiotic germ cells (IMPα4-EGFP; solid green band), and an IMPα4 complete knockout line (IMPα4-KO; white band). When testicular germ cells were exposed to high levels of oxidative stress through H2O2, the haploid germ cells–which express IMPα4-EGFP– exhibited increased resistance to stress-induced cell death. Conversely, the absence of IMPα4 led to increased sensitivity in this population. This indicates that IMPα4 aids in protecting the male germline against stress.27
Mentions: A crucial feature enabling importins to function in nucleocytoplasmic transport is their relocation to the cytoplasm after binding a cargo molecule and releasing it into the nucleus (Figure 1a). This normally occurs because a high Ran-GTP: RanGDP gradient between the nucleus and cytoplasm is maintained by cytoplasmic RanGAP1/RanBP1 and nuclear-localized RCC1.16 This gradient is lost following exposure of cells to stressors such as H2O2, ultraviolet irradiation or heat shock, and importin α proteins accumulate in the nucleus (Figure 3a).26 We recently identified that one functional outcome of this unusual localization of importin α is altered transcription of a discrete subset of genes, including the downregulation those encoding several replication-dependent histones.25 Given that both importin α2 and α4 are nuclear-localized in postmitotic germ cells, with importin α4 appearing to be exclusively nuclear, we considered that this unusual location might relate to the need for germ cells to manage oxidative and other stressors during their development within the seminiferous epithelium. Through analysis of two unique mouse models (Figure 3b), we obtained evidence that importin α4 levels directly and selectively determine the capacity for spermatids to resist oxidative stress.27 Haploid germ cells from transgenic male mice overexpressing importin α4 had a greater resistance (>30%) to transient H2O2 exposure, while a higher proportion (~30%) of spermatids exhibited decreased viability from importin α4 gene knockout mice compared with their littermate controls. The mechanisms underpinning this outcome remain to be determined, but we propose that genes that are downstream targets of nuclear importin α proteins in spermatogenesis are crucial for enabling male germline cells to recover from exposure to stress.

Bottom Line: Knowledge of importin function has expanded substantially in regard to three key developmental systems: embryonic stem cells, muscle cells and the germ line.In the decade since the potential for regulated nucleocytoplasmic transport to contribute to spermatogenesis was proposed, we and others have shown that the importins that ferry transcription factors into the nucleus perform additional roles, which control cell fate.These studies of germline genesis illuminate new ways in which importin proteins govern cellular differentiation, including via directing proteins to distinct intracellular compartments and by determining cellular stress responses.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology;Department of Anatomy and Developmental Biology, Monash University; Hudson Institute of Medical Research, Monash Medical Centre; School of Clinical Sciences, Monash University, Clayton, VIC, Australia, .

ABSTRACT
Importin proteins were originally characterized for their central role in protein transport through the nuclear pores, the only intracellular entry to the nucleus. This vital function must be tightly regulated to control access by transcription factors and other nuclear proteins to genomic DNA, to achieve appropriate modulation of cellular behaviors affecting cell fate. Importin-mediated nucleocytoplasmic transport relies on their specific recognition of cargoes, with each importin binding to distinct and overlapping protein subsets. Knowledge of importin function has expanded substantially in regard to three key developmental systems: embryonic stem cells, muscle cells and the germ line. In the decade since the potential for regulated nucleocytoplasmic transport to contribute to spermatogenesis was proposed, we and others have shown that the importins that ferry transcription factors into the nucleus perform additional roles, which control cell fate. This review presents key findings from studies of mammalian spermatogenesis that reveal potential new pathways by which male fertility and infertility arise. These studies of germline genesis illuminate new ways in which importin proteins govern cellular differentiation, including via directing proteins to distinct intracellular compartments and by determining cellular stress responses.

No MeSH data available.


Related in: MedlinePlus