Biology subjects

Haseeb, M. A.

Publications and source records attributed to Haseeb, M. A..

5 recordsLinked to original sources

bioRxiv2025

A persistent intracellular bridge and cell cycle misregulation enable polar body cell divisions and tumor formation in Mos-deficient eggs

Mammalian female meiosis is uniquely regulated to produce a developmentally competent egg capable of supporting embryogenesis. During meiosis I, homologous chromosomes segregate, with half extruded into the first polar body. The egg then arrests at metaphase II and only resumes meiosis and extrudes the second polar body following fertilization. The MOS/MAPK signaling pathway is important for maintaining the metaphase II arrest; in mos-/- mutants, a subset of eggs undergo spontaneous parthenogenetic activation and exhibit additional abnormal cell divisions. To further understand the cell cycle mis-regulation in activated mos-/- eggs, we used time-lapse microscopy to monitor the abnormal divisions. We discovered that, following parthenogenetic activation, the first polar body can assemble a spindle, segregate chromosomes, and divide with timings similar to anaphase II onset in the egg. This behavior contrasts with wildtype polar bodies, which do not divide and are typically degenerated. We demonstrate that mos-/- eggs and polar bodies can exchange cytoplasm at the time of meiosis II spindle assembly, likely allowing the transfer of cell cycle regulators between the two compartments. Further inspection revealed that mos-/- eggs have defective meiotic midbody assembly with most eggs lacking a cap structure, which is needed to separate the two compartments. We report that polar bodies of mos-/- eggs can re-enter the cell cycle and undergo additional aberrant divisions. These findings identify MOS as a critical regulator of meiotic midbody formation and uncover a novel consequence of disrupted MOS/MAPK signaling: the potential for polar bodies to become mitotically active and contribute to tumor formation.

cell biology↗

bioRxiv2025

Basal autophagy during meiotic prophase I is required for accurate chromosome segregation in Drosophila oocytes and declines during oocyte aging

Meiotic segregation errors in human oocytes are the leading cause of miscarriages and trisomic pregnancies and their frequency increases exponentially for women in their thirties. One factor that contributes to increased segregation errors in aging oocytes is premature loss of sister chromatid cohesion. However, the mechanisms underlying age-dependent deterioration of cohesion are not well-defined. Autophagy, a cellular degradation process critical for cellular homeostasis, is known to decline with age in various organisms and cell types. Here we quantify basal autophagy in Drosophila oocytes and use GAL4/UAS inducible knockdown to ask whether disruption of autophagy in prophase oocytes impacts the fidelity of chromosome segregation. We find that individual knockdown of autophagy proteins in Drosophila oocytes during meiotic prophase causes a significant increase in segregation errors. In addition, Atg8a knockdown in prophase oocytes leads to premature loss of arm cohesion and missegregation of recombinant homologs during meiosis I. Using an oocyte aging paradigm that we have previously described, we show that basal autophagy decreases significantly when Drosophila oocytes undergo aging. Our data support the model that a decline in autophagy during oocyte aging contributes to premature loss of meiotic cohesion and segregation errors.

cell biology↗

bioRxiv2023

An RNAi screen to identify proteins required for cohesion rejuvenation during meiotic prophase in Drosophila oocytes

Accurate chromosome segregation during meiosis requires maintenance of sister chromatid cohesion, initially established during premeiotic S phase. In human oocytes, DNA replication and cohesion establishment occur decades before chromosome segregation and deterioration of meiotic cohesion is one factor that leads to increased segregation errors as women age. Our previous work led us to propose that a cohesion rejuvenation program operates to establish new cohesive linkages during meiotic prophase in Drosophila oocytes and depends on the cohesin loader Nipped-B and the cohesion establishment factor Eco. In support of this model, we recently demonstrated that chromosome-associated cohesin turns over extensively during meiotic prophase and failure to load cohesin onto chromosomes after premeiotic S phase results in arm cohesion defects in Drosophila oocytes. To identify proteins required for prophase cohesion rejuvenation but not S phase establishment, we conducted a Gal4-UAS inducible RNAi screen that utilized two distinct germline drivers. Using this strategy, we identified 29 gene products for which hairpin expression during meiotic prophase, but not premeiotic S phase, significantly increased segregation errors. Prophase knockdown of Brahma or Pumilio, two positives with functional links to the cohesin loader, caused a significant elevation in the missegregation of recombinant homologs, a phenotype consistent with premature loss of arm cohesion. Moreover, fluorescence in situ hybridization confirmed that Brahma, Pumilio and Nipped-B are required during meiotic prophase for maintenance of arm cohesion. Our data support the model that Brahma and Pumilio regulate Nipped-B dependent cohesin loading during rejuvenation. Future analyses will better define the mechanism(s) that govern meiotic cohesion rejuvenation and whether additional prophase-specific positives function in this process.

cell biology↗

bioRxiv2023

Chromatin-associated cohesin turns over extensively and forms new cohesive linkages during meiotic prophase

In dividing cells, accurate chromosome segregation depends on sister chromatid cohesion, protein linkages that are established during DNA replication. Faithful chromosome segregation in oocytes requires that cohesion, first established in S phase, remain intact for days to decades, depending on the organism. Premature loss of meiotic cohesion in oocytes leads to the production of aneuploid gametes and contributes to the increased incidence of meiotic segregation errors as women age (maternal age effect). The prevailing model is that cohesive linkages do not turn over in mammalian oocytes. However, we have previously reported that cohesion-related defects arise in Drosophila oocytes when individual cohesin subunits or cohesin regulators are knocked down after meiotic S phase. Here we use two strategies to express a tagged cohesin subunit exclusively during mid-prophase in Drosophila oocytes and demonstrate that newly expressed cohesin is used to form de novo linkages after meiotic S phase. Moreover, nearly complete turnover of chromosome-associated cohesin occurs during meiotic prophase, with faster replacement on the arms than at the centromeres. Unlike S-phase cohesion establishment, the formation of new cohesive linkages during meiotic prophase does not require acetylation of conserved lysines within the Smc3 head. Our findings indicate that maintenance of cohesion between S phase and chromosome segregation in Drosophila oocytes requires an active cohesion rejuvenation program that generates new cohesive linkages during meiotic prophase.

cell biology↗

bioRxiv2022

Genome-wide binding of Drosophila Mask reveals its role in maintenance of gene activation by Trithorax group

The Trithorax group (trxG) proteins counteract repressive effect of Polycomb group (PcG) complexes and maintain transcriptional memory of active states of key developmental genes. Although, chromatin structure and modifications appear to play a fundamental role in this process, it is not clear how trxG prevents PcG-silencing and heritably maintain an active gene expression state. Here, we report a hitherto unknown role of Drosophila Multiple ankyrin repeats single KH domain (Mask), which emerged as one of the candidate trxG genes in our reverse genetic screen. The genome-wide binding profile of Mask correlates with known trxG binding sites across Drosophila genome. In particular, association of Mask at chromatin overlaps with CBP and H3K27ac, which are known hallmarks of actively transcribed genes by trxG. Importantly, Mask predominantly associates with actively transcribed genes in Drosophila. Depletion of Mask not only results in downregulation of trxG targets but also correlates with drastic reduction in H3K27ac levels and an increased H3K27me3 levels. The fact that MASK positively regulates H3K27ac levels in flies was also found to be conserved in human cells. Finally, strong suppression of Pc mutant phenotype by mutation in mask provides physiological relevance that Mask contributes to the anti-silencing effect of trxG, maintaining expression of key developmental genes. Since Mask is a downstream effector of multiple cell signaling pathways, we propose that Mask may connect cell signaling with chromatin mediated epigenetic cell memory governed by trxG.

genomics↗