*3.4.7. Fertilization and development*

cancer and represses *MTNR1A* gene by directly interacting with its genomic site [145]. PiRNAs also modulate stability of their PIWI partners by promoting direct molecular interactions with specific proteins. During late mouse spermatogenesis, piRNAs regulate ubiquitination of Miwi through its binding to APC/C complex [146]. Furthermore, piRNAs can modify activity and expression of many distant genes. PiRNA-36026 interacts with suppressor proteins Serpin peptidase inhibitor, clade A, member 1 (SERPINA1), and lecithin retinol acyltransferase (LRAT). However, the PIWI-piRNA pathway is also present at lower levels in somatic pluripotent stem cells to differentiated cells [147, 148]. In adult somatic cells of *Drosophila melanogaster*, this pathway is active in ovarian follicle cells, in salivary glands, and in the brain [149]. The pathway is principally observed in stem cells with pluripotent capacities, including mesenchymal and hematopoietic stem cells, but rarely in adult stem cells with limited differentiation capacity [150]. Furthermore, the PIWI-piRNA pathway seems also to regulate protein-coding genes. The first piRNAs identified in *Drosophila melanogaster* were transcribed from the Suppressor of Stellate locus located on the Y chromosome and targeted the protein-coding gene Stellate on the X chromosome [151]. More recently, genome-wide mapping techniques have demonstrated that genic piRNAs derive from TEs and 3′ UTRs of coding genes [56]. Mounting evidence suggests that germline genes could have ancestral implication in regulating stemness. The "nuage" is located in lower metazoan stem cells but restricted to germline cells in upper metazoans [152]. The PIWI-piRNA pathway is expressed

in stem cells of metazoans with partial or whole-body regeneration capabilities [153].

resulting in c-Fos mRNA instability and repression of its translation [151–154].

The PIWI-piRNA axis promotes activation of numerous prosurvival molecules. PIWIL1 induced cell survival by upregulating expression of antiapoptotic molecule FGF8 and downregulating expression of proapoptotic Bax and p21. In blastema cells of Mexican *axolotl*,

PIWI proteins play pivotal roles in cell differentiation during early embryogenesis. In *Drosophila* ovary, self-renewal of differentiated germline stem cells is located in niches composed of different types of cells, including escort cells (ECs). Experimental deregulation of PIWI proteins expression in EC cells was associated with reduction of EC cell population and predominance of undifferentiated germline stem cells. PIWI proteins induced germline cell differentiation by promoting direct interaction between germline stem cells and escort cells through repression of the TGFβ signaling and bone morphogenetic protein (BMP) pathway by preventing edification of Smad complexes. PIWIL2 is the major PIWI protein implicated in cell differentiation through inhibition of the TGFβ signaling pathway. PIWIL2 directly interacts with Smad4 and HSP90 and prevents HSP90-TβR complex formation, resulting in inhibition of the TGFβ signaling pathway. Furthermore, PIWIL2 promotes degradation of TGFβ receptor (TβR) and Smad by upregulating ubiquitination and degradation of TβR by the ubiquitin E3 ligase Smurf2. PIWI proteins contribute to germline stem cells differentiation by repressing c-Fos at posttranscriptional. These proteins promote piRNAs synthesis from 3′ UTR region of c-Fos mRNA,

*3.4.5. Differentiation*

246 Chromatin and Epigenetics

*3.4.6. Cell survival*

Although most attention has been given to the pivotal role of the PIWI-piRNA pathway in germline TEs silencing, mounting evidence has revealed their implication in germline and somatic epigenetic and posttranscriptional regulation of gene expression [151]. This pathway is mainly implicated in the germline biology, including maintenance, differentiation, and function of *Drosophila* and murine GSCs. Furthermore, piRNAs epigenetically activate gene expression with transgenerational epigenetic effects by inducing euchromatin through activation of H3K4me3 and inhibition of H3K27me3 in subtelomeric heterochromatin [56, 73, 152–155]. The PIWI-piRNA axis is implicated in embryonic development, including cell cycle progression, nuclear division, chromatin organization, chromosome integrity during mitosis, control of mRNA translation, and embryonic sex determination [144, 156–158]. Spatialtemporal activation and regulation of PIWI proteins and piRNAs are of pivotal importance during mammalian oogenesis and spermatogenesis, early embryogenesis, organogenesis, and postbirth [159]. PIWIL2 is upregulated in germline cells and appears instrumental in maintaining genome stability, an open state of chromatin and DNA repair via silencing TEs and histones modifications, thus preventing TEs propagation, chromosome rearrangements, oncogenic mutations, and gene dysregulation [160].

#### *3.4.7.1. Oogenesis*

In *Drosophila*, PIWI-piRNA complexes promote TEs silencing at embryonic germ cell stage, mediate cellular memory of TEs repression, and thus maintain this mechanism in ovaries at the adult stage. In mouse ovary, Miwi upregulation is observed during neonatal stage and its expression is lower in adult ovaries [161]. Human PIWI proteins expression profiling is also variable, depending on the stage of development. PIWIL1 and PIWIL2 are highly upregulated in oocytes of human adult ovary that present a strong activity of TEs, whereas fetal oocytes, whose TEs propagation is lower, overexpress PIWIL2 but not other PIWI proteins [162].

#### *3.4.7.2. Spermatogenesis*

In mouse, Miwi inactivation occurs during late spermatogenesis and is induced by the anaphase promoting complex (APC)/C-26S proteasomal pathway [159]. Functional destruction box (D-box) is required for Miwi ubiquitination and degradation by (APC)/C system. A genetic analysis in mouse azoospermia showed that mutations in D-box favor Hiwi stabilization in late spermatogenesis. Stabilized mutant Hiwi interacts with RNF8 implicated in histone ubiquitination and prevents its nuclear translocation and ubiquitin ligase activity [163]. Human PIWIL4 function is crucial in accurate spermatogenesis, and genetic polymorphisms of PIWIL4 gene are significantly correlated with defective spermatogenesis associated with spermatogenesis defect and male infertility [164].

**4. Deregulation of the PIWI/piRNA pathway in pathological** 

Mounting evidence has revealed that many transcription factors and signaling molecules interact with the PIWI-piRNA pathway and represent downstream targets of these complexes under pathological conditions. PIWI proteins and piRNAs are deregulated, and their levels of expression are highly altered in various pathological processes. The PIWI-piRNA pathway is pivotal for regeneration after amputation in *Botrylloides leachi* [172]. In Mexican axolotl, PIWIL1 and PIWIL2 transient upregulation in limb blastemal cells induces regeneration of wounded limb [173]. In rat, PIWIL2 expression increases after 24 h of partial hepatectomy, and a set of 72 piR-NAs is deregulated during 48 h of posthepatectomy [58]. In rodents, expression of more than 100 piRNAs is deregulated in brain during ischemic condition [174]. In rat, PIWIL2 enhances activity of the autophagic process in diabetic nephropathy by regulating expression of beclin 1 and LC3A study in diabetic rat kidney [175]. Pro-inflammatory cytokines IL1β and TNFα promote PIWIL2 and PIWIL4 upregulation in synovial fibroblasts of rheumatoid arthritis [176].

Part 1: The PIWI-piRNA Pathway Is an Immune-Like Surveillance Process That Controls...

http://dx.doi.org/10.5772/intechopen.79974

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**nonneoplastic disorders**

**Acknowledgements**

**Author details**

**References**

**Disclosure: conflict of interest**

Didier Meseure1,2\* and Kinan Drak Alsibai<sup>3</sup>

This work was supported by grant INCa-DGOS-4654.

The authors declare that they have no competing interests.

\*Address all correspondence to: didier.meseure@curie.fr

goes awry. Developmental Cell. 2010;**19**:698-711

3 Institut de Pathologie de Paris, Malakoff, France

1 Platform of Experimental Pathology, Curie Institute, Paris, France

2 Department of Diagnostic and Theranostic Medicine, Curie Institute, Paris, France

[1] Berdasco M, Esteller M. Aberrant epigenetic landscape in cancer: How cellular identity

[2] Kanwal R, Gupta S. Epigenetic modifications in cancer. Clinical Genetics. 2012;**81**:303-311

#### *3.4.7.3. Organogenesis*

In *Drosophila*, PIWI proteins induce ovary tissue morphogenesis through c-Fos inactivation at posttranscriptional level [165]. They are implicated in development of eye color [144]. In silkworm, fempiRNA, a piRNA located on female W-chromosome, is pivotal in sex determination by repressing masculinization mRNA at posttranscriptional level [167]. At early stages of human embryonic lungs development, PIWIL1, PIWIL2, and PIWIL4 levels of expression are strongly upregulated from 6th week to 9th week and then decline [168]. In human, PIWIL1 and PIWIL2 have crucial role in neural polarization and radial migration during maturation of the cerebral cortex region of the brain [59].

#### *3.4.8. Physiological processes*

The PIWI-piRNA pathway has pivotal role in numerous physiological processes.

#### *3.4.8.1. Brain plasticity*

PIWI proteins and piRNAs are instrumental in synaptic plasticity and stabilization of longterm memory through serotonin-dependent suppression of CREB2 at transcription level that is induced by methylation of CpG islands in the promoter region of the *CREB2* gene [59]. In rodents, several piRNAs are upregulated in hippocampal neurons and Miwi associated with piRNAs control dentritic spine development and morphogenesis [149]. Mili expression is associated with anxiety and locomotory drive [54]. In humans, PIWIL1 controls cortical neuron activity through modulation of microtubule-associated proteins (MAPs) expression [169]. Furthermore, mutations of PIWIL2 and PIWIL4 are significantly correlated with autism [170].

#### *3.4.8.2. Regeneration*

PIWI proteins and piRNAs have crucial role in self-renewal, regeneration, and homeostasis. In planarian *Schmidtea mediterranea*, SMEDWI-2 and SMEDWI-3 increase division of adult stem cells to induce regeneration in injured tissues [113]. In jellyfish, Cniwi is upregulated during transdifferentiation of striated muscle into smooth muscle [171]. In humans, PIWI proteins promote hepatocyte regeneration [61] and maintain integrity of retinal cells [130].

#### *3.4.8.3. Metabolism*

The PIWI-piRNA pathway controls fat metabolism through repression of TEs, and fat metabolism inactivation is associated with depletion of lipid synthesis and storage [60]. PIWIL2 and PIWIL4 modulate pancreatic β-cells function and insulin secretion. Alterations of their levels of expression were observed in diabetic conditions [55].
