DHATUROOPI SHUKRA AND BEEJAROOPI SHUKRA: A CRITICAL REVIEW

Shukra is studied in Ayurveda both as a dhatu and beeja. As a mammalian human body comprises both somatic and gonadal cells. Somatic cells help for growth and regeneration through mitosis. Meiotic cell division causes equal contribution for the inheritance from maternal and paternal sides. Beejartham (reproduction) is the supreme function attributed to Shukra. Reproduction refers to the formation of new cells for tissue growth, repair/replacement (sukshmavayavantarotpatti), or the production of a new individual (shareerantarotpatti). Regenerative capacity is distributed unequally among species, individuals, and tissues. The affliction of shukrastana by kusthadosha (skin disease) causes a failure in regeneration. The affliction of parents' shukra and artava (gametes) by kusthadosha (skin disease) inherits to the next generation. Vrushan (testis) and medru (penis) are the moola of the shukravahavaha srotus, which is meant to fertilise the ovum (beejarupishukra). Majja (bone marrow) and stana (breasts) are the moola of the shukravaha srotus of the one pervading the entire body (dhaturupishukra).

A sensitized genetic screen to identify regulators of C. elegans germline stem cells

GLP-1/Notch signaling and a downstream RNA regulatory network maintain germline stem cells (GSCs) in Caenorhabditis elegans. In mutants lacking the GLP-1 receptor, all GSCs enter the meiotic cell cycle precociously and differentiate into sperm. This dramatic GSC defect is called the “Glp” phenotype. The lst-1 and sygl-1 genes are direct targets of Notch transcriptional activation and functionally redundant. Whereas single lst-1 and sygl-1 mutants are fertile, lst-1 sygl-1 double mutants are sterile with a Glp phenotype. We set out to identify genes that function redundantly with either lst-1 or sygl-1 to maintain GSCs. To this end, we conducted forward genetic screens for mutants with a Glp phenotype in genetic backgrounds lacking functional copies of either lst-1 or sygl-1. The screens generated nine glp-1 alleles, two lst-1 alleles, and one allele of pole-1, which encodes the catalytic subunit of DNA polymerase ε. Three glp-1 alleles reside in Ankyrin (ANK) repeats not previously mutated. pole-1 single mutants have a low penetrance Glp phenotype that is enhanced by loss of sygl-1. Thus, the screen uncovered one locus that interacts genetically with sygl-1 and generated useful mutations for further studies of GSC regulation.

Mammalian SWI/SNF chromatin remodeler is essential for reductional meiosis in males

The mammalian SWI/SNF nucleosome remodeler is essential for spermatogenesis. Here, we identify a role for ARID2, a PBAF (Polybromo - Brg1 Associated Factor)-specific subunit, in meiotic division. Arid2cKO spermatocytes arrest at metaphase-I and are deficient in spindle assembly, kinetochore-associated Polo-like kinase1 (PLK1), and centromeric targeting of Histone H3 threonine3 phosphorylation (H3T3P) and Histone H2A threonine120 phosphorylation (H2AT120P). By determining ARID2 and BRG1 genomic associations, we show that PBAF localizes to centromeres and promoters of genes known to govern spindle assembly and nuclear division in spermatocytes. Consistent with gene ontology of target genes, we also identify a role for ARID2 in centrosome stability. Additionally, misexpression of genes such as Aurkc and Ppp1cc (Pp1γ), known to govern chromosome segregation, potentially compromises the function of the chromosome passenger complex (CPC) and deposition of H3T3P, respectively. Our data support a model where-in PBAF activates genes essential for meiotic cell division.

Meiotic cell cycle progression requires adaptation to a constitutive DNA damage signal

Meiotic chromosome segregation relies on synapsis and crossover recombination between homologous chromosomes. These processes require multiple steps that are coordinated by the meiotic cell cycle and monitored by surveillance mechanisms. In the nematode Caenorhabditis elegans, CHK-2 kinase is activated at meiotic entry; its activity is essential for homologous synapsis and DSB formation. CHK-2 is normally inactivated at mid-prophase, but how this occurs has not been established. Defects in synapsis or establishment of crossover intermediates delay meiotic progression by prolonging the activity of CHK-2. We report that CHK-2 is necessary and sufficient to inhibit crossover designation. We further find that CHK-2 is inactivated at mid-prophase by a pathway that mediates DNA damage checkpoint adaptation in proliferating human cells: Polo-like kinases, particularly PLK-2, phosphorylate and inhibit CHK-2 in response to formation of crossover intermediates. These findings help to illuminate the mechanisms of crossover assurance and meiotic cell cycle control.

CRISPR-based targeting of DNA methylation in Arabidopsis thaliana by a bacterial CG-specific DNA methyltransferase

CRISPR-based targeted modification of epigenetic marks such as DNA cytosine methylation is an important strategy to regulate the expression of genes and their associated phenotypes. Although plants have DNA methylation in all sequence contexts (CG, CHG, CHH, where H = A, T, C), methylation in the symmetric CG context is particularly important for gene silencing and is very efficiently maintained through mitotic and meiotic cell divisions. Tools that can directly add CG methylation to specific loci are therefore highly desirable but are currently lacking in plants. Here we have developed two CRISPR-based CG-specific targeted DNA methylation systems for plants using a variant of the bacterial CG-specific DNA methyltransferase MQ1 with reduced activity but high specificity. We demonstrate that the methylation added by MQ1 is highly target specific and can be heritably maintained in the absence of the effector. These tools should be valuable both in crop engineering and in plant genetic research.

Phospho-Regulation of Meiotic Prophase

Germ cells undergoing meiosis rely on an intricate network of surveillance mechanisms that govern the production of euploid gametes for successful sexual reproduction. These surveillance mechanisms are particularly crucial during meiotic prophase, when cells execute a highly orchestrated program of chromosome morphogenesis and recombination, which must be integrated with the meiotic cell division machinery to ensure the safe execution of meiosis. Dynamic protein phosphorylation, controlled by kinases and phosphatases, has emerged as one of the main signaling routes for providing readout and regulation of chromosomal and cellular behavior throughout meiotic prophase. In this review, we discuss common principles and provide detailed examples of how these phosphorylation events are employed to ensure faithful passage of chromosomes from one generation to the next.

YTHDC2 Is Essential for Pachytene Progression and Prevents Aberrant Microtubule-Driven Telomere Clustering in Male Meiosis

SUMMARYMechanisms driving the prolonged meiotic prophase I are poorly understood. The RNA helicase YTHDC2 is critical for mitosis to meiosis transition, as YTHDC2-deficient mouse germ cells initiate meiosis but arrest with mixed characteristics of mitotic and meiotic cell types. However, YTHDC2 is also highly expressed in normal pachytene cells. Here we identify an essential role for YTHDC2 in meiotic progression. Specifically, we find that YTHDC2 deficiency causes microtubule-dependent telomere clustering and apoptosis at the pachytene stage of prophase I, and thus a failure to advance to the diplotene stage. Depletion of YTHDC2 results in a massively dysregulated transcriptome in pachytene cells, with a tendency toward upregulation of genes normally expressed in mitotic germ cells and downregulation of meiotic transcripts. Dysregulation does not correlate with the m6A status of RNAs and YTHDC2-bound mRNAs are enriched in genes upregulated in mutant germ cells, revealing that YTHDC2 primarily targets its substrate mRNAs for degradation. Finally, altered transcripts in YTHDC2-deficient pachytene cells encode microtubule network proteins and inhibition of microtubule polymerization disperses clustered telomeres. Together, our results demonstrate that YTHDC2 regulates the prolonged pachytene stage of prophase I by perpetuating a meiotic transcriptome and preventing changes in the microtubule network that could lead to aberrant telomere clustering.

Critical Functions of PP2A-Like Protein Phosphotases in Regulating Meiotic Progression

Meiosis is essential to the continuity of life in sexually-reproducing organisms through the formation of haploid gametes. Unlike somatic cells, the germ cells undergo two successive rounds of meiotic divisions after a single cycle of DNA replication, resulting in the decrease in ploidy. In humans, errors in meiotic progression can cause infertility and birth defects. Post-translational modifications, such as phosphorylation, ubiquitylation and sumoylation have emerged as important regulatory events in meiosis. There are dynamic equilibrium of protein phosphorylation and protein dephosphorylation in meiotic cell cycle process, regulated by a conservative series of protein kinases and protein phosphatases. Among these protein phosphatases, PP2A, PP4, and PP6 constitute the PP2A-like subfamily within the serine/threonine protein phosphatase family. Herein, we review recent discoveries and explore the role of PP2A-like protein phosphatases during meiotic progression.

Osmolarity-regulated swelling initiates egg activation in Drosophila

ABSTRACTEgg activation is a series of highly coordinated processes that prepare the mature oocyte for embryogenesis. Typically associated with fertilisation, egg activation results in many downstream outcomes, including the resumption of the meiotic cell cycle, translation of maternal mRNAs and cross-linking of the vitelline membrane. While some aspects of egg activation, such as initiation factors in mammals and environmental cues in sea animals, have been well-documented, the mechanics of egg activation in insects are less well understood. For many insects, egg activation can be triggered independently of fertilisation. In Drosophila melanogaster, egg activation occurs in the oviduct resulting in a single calcium wave propagating from the posterior pole of the oocyte.Here we use physical manipulations, genetics and live imaging to demonstrate the requirement of a volume increase for calcium entry at egg activation in mature Drosophila oocytes. The addition of water, modified with sucrose to a specific osmolarity, is sufficient to trigger the calcium wave in the mature oocyte and the downstream events associated with egg activation. We show that the swelling process is regulated by the conserved osmoregulatory channels, aquaporins (AQPs) and DEGenerin/Epithelial Na+ (DEG/ENaC) channels. Furthermore, through pharmacological and genetic disruption, we reveal a concentration-dependent requirement of Trpm channels to transport calcium, most likely from the perivitelline space, across the plasma membrane into the mature oocyte.Our data establishes osmotic pressure as the mechanism that initiates egg activation in Drosophila and is consistent with previous work from evolutionarily distant insects, including dragonflies and mosquitos, and shows remarkable similarities to the mechanism of egg activation in some plants.