Drosophila casein kinase I alpha regulates homolog pairing and genome organization by modulating condensin II subunit Cap-H2 levels

Condensin I folds the Caenorhabditis elegans genome

The structural maintenance of chromosome (SMC) complexes-cohesin and condensins-are crucial for chromosome separation and compaction during cell division. During the interphase, mammalian cohesins additionally fold the genome into loops and domains. Here we show that, in Caenorhabditis elegans, a species with holocentric chromosomes, condensin I is the primary, long-range loop extruder. The loss of condensin I and its X-specific variant, condensin IDC, leads to genome-wide decompaction, chromosome mixing and disappearance of X-specific topologically associating domains, while reinforcing fine-scale epigenomic compartments. In addition, condensin I/IDC inactivation led to the upregulation of X-linked genes and unveiled nuclear bodies grouping together binding sites for the X-targeting loading complex of condensin IDC. C. elegans condensin I/IDC thus uniquely organizes holocentric interphase chromosomes, akin to cohesin in mammals, as well as regulates X-chromosome gene expression.

The zinc-finger protein Z4 cooperates with condensin II to regulate somatic chromosome pairing and 3D chromatin organization

Chromosome pairing constitutes an important level of genome organization, yet the mechanisms that regulate pairing in somatic cells and the impact on 3D chromatin organization are still poorly understood. Here, we address these questions in Drosophila, an organism with robust somatic pairing. In Drosophila, pairing preferentially occurs at loci consisting of numerous architectural protein binding sites (APBSs), suggesting a role of architectural proteins (APs) in pairing regulation. Amongst these, the anti-pairing function of the condensin II subunit CAP-H2 is well established. However, the factors that regulate CAP-H2 localization and action at APBSs remain largely unknown. Here, we identify two factors that control CAP-H2 occupancy at APBSs and, therefore, regulate pairing. We show that Z4, interacts with CAP-H2 and is required for its localization at APBSs. We also show that hyperosmotic cellular stress induces fast and reversible unpairing in a Z4/CAP-H2 dependent manner. Moreover, by combining the opposite effects of Z4 depletion and osmostress, we show that pairing correlates with the strength of intrachromosomal 3D interactions, such as active (A) compartment interactions, intragenic gene-loops, and polycomb (Pc)-mediated chromatin loops. Altogether, our results reveal new players in CAP-H2-mediated pairing regulation and the intimate interplay between inter-chromosomal and intra-chromosomal 3D interactions.

Genome control by SMC complexes

Many cellular processes require large-scale rearrangements of chromatin structure. Structural maintenance of chromosomes (SMC) protein complexes are molecular machines that can provide structure to chromatin. These complexes can connect DNA elements in cis, walk along DNA, build and processively enlarge DNA loops and connect DNA molecules in trans to hold together the sister chromatids. These DNA-shaping abilities place SMC complexes at the heart of many DNA-based processes, including chromosome segregation in mitosis, transcription control and DNA replication, repair and recombination. In this Review, we discuss the latest insights into how SMC complexes such as cohesin, condensin and the SMC5-SMC6 complex shape DNA to direct these fundamental chromosomal processes. We also consider how SMC complexes, by building chromatin loops, can counteract the natural tendency of alike chromatin regions to cluster. SMC complexes thus control nuclear organization by participating in a molecular tug of war that determines the architecture of our genome.

Somatic chromosome pairing has a determinant impact on 3D chromatin organization

In the nucleus, chromatin is intricately structured into multiple layers of 3D organization important for genome activity. How distinct layers influence each other is not well understood. In particular, the contribution of chromosome pairing to 3D chromatin organization has been largely neglected. Here, we address this question in Drosophila, an organism that shows robust chromosome pairing in interphasic somatic cells. The extent of chromosome pairing depends on the balance between pairing and anti-pairing factors, with the anti-pairing activity of the CAP-H2 condensin II subunit being the best documented. Here, we identify the zinc-finger protein Z4 as a strong anti-pairer that interacts with and mediates the chromatin binding of CAP-H2. We also report that hyperosmotic cellular stress induces fast and reversible chromosome unpairing that depends on Z4/CAP-H2. And, most important, by combining Z4 depletion and osmostress, we show that chromosome pairing reinforces intrachromosomal 3D interactions. On the one hand, pairing facilitates RNAPII occupancy that correlates with enhanced intragenic gene-loop interactions. In addition, acting at a distance, pairing reinforces chromatin-loop interactions mediated by Polycomb (Pc). In contrast, chromosome pairing does not affect which genomic intervals segregate to active (A) and inactive (B) compartments, with only minimal effects on the strength of A-A compartmental interactions. Altogether, our results unveil the intimate interplay between inter-chromosomal and intra-chromosomal 3D interactions, unraveling the interwoven relationship between different layers of chromatin organization and the essential contribution of chromosome pairing.

Regulation of the mitotic chromosome folding machines

Over the last several years enormous progress has been made in identifying the molecular machines, including condensins and topoisomerases that fold mitotic chromosomes. The discovery that condensins generate chromatin loops through loop extrusion has revolutionized, and energized, the field of chromosome folding. To understand how these machines fold chromosomes with the appropriate dimensions, while disentangling sister chromatids, it needs to be determined how they are regulated and deployed. Here, we outline the current understanding of how these machines and factors are regulated through cell cycle dependent expression, chromatin localization, activation and inactivation through post-translational modifications, and through associations with each other, with other factors and with the chromatin template itself. There are still many open questions about how condensins and topoisomerases are regulated but given the pace of progress in the chromosome folding field, it seems likely that many of these will be answered in the years ahead.

Loss of IRF2BPL impairs neuronal maintenance through excess Wnt signaling

De novo truncations in Interferon Regulatory Factor 2 Binding Protein Like (IRF2BPL) lead to severe childhood-onset neurodegenerative disorders. To determine how loss of IRF2BPL causes neural dysfunction, we examined its function in Drosophila and zebrafish. Overexpression of either IRF2BPL or Pits, the Drosophila ortholog, represses Wnt transcription in flies. In contrast, neuronal depletion of Pits leads to increased wingless (wg) levels in the brain and is associated with axonal loss, whereas inhibition of Wg signaling is neuroprotective. Moreover, increased neuronal expression of wg in flies is sufficient to cause age-dependent axonal loss, similar to reduction of Pits. Loss of irf2bpl in zebrafish also causes neurological defects with an associated increase in wnt1 transcription and downstream signaling. WNT1 is also increased in patient-derived astrocytes, and pharmacological inhibition of Wnt suppresses the neurological phenotypes. Last, IRF2BPL and the Wnt antagonist, CKIα, physically and genetically interact, showing that IRF2BPL and CkIα antagonize Wnt transcription and signaling.

MCPH1 inhibits Condensin II during interphase by regulating its SMC2-Kleisin interface

Dramatic change in chromosomal DNA morphology between interphase and mitosis is a defining features of the eukaryotic cell cycle. Two types of enzymes, namely cohesin and condensin confer the topology of chromosomal DNA by extruding DNA loops. While condensin normally configures chromosomes exclusively during mitosis, cohesin does so during interphase. The processivity of cohesin's loop extrusion during interphase is limited by a regulatory factor called WAPL, which induces cohesin to dissociate from chromosomes via a mechanism that requires dissociation of its kleisin from the neck of SMC3. We show here that a related mechanism may be responsible for blocking condensin II from acting during interphase. Cells derived from patients affected by microcephaly caused by mutations in the MCPH1 gene undergo premature chromosome condensation. We show that deletion of Mcph1 in mouse embryonic stem cells unleashes an activity of condensin II that triggers formation of compact chromosomes in G1 and G2 phases, accompanied by enhanced mixing of A and B chromatin compartments, and this occurs even in the absence of CDK1 activity. Crucially, inhibition of condensin II by MCPH1 depends on the binding of a short linear motif within MCPH1 to condensin II's NCAPG2 subunit. MCPH1's ability to block condensin II's association with chromatin is abrogated by the fusion of SMC2 with NCAPH2, hence may work by a mechanism similar to cohesin. Remarkably, in the absence of both WAPL and MCPH1, cohesin and condensin II transform chromosomal DNAs of G2 cells into chromosomes with a solenoidal axis.

Cell-cycle regulation of non-enzymatic functions of the Drosophila methyltransferase PR-Set7

Tight cell-cycle regulation of the histone H4-K20 methyltransferase PR-Set7 is essential for the maintenance of genome integrity. In mammals, this mainly involves the interaction of PR-Set7 with the replication factor PCNA, which triggers the degradation of the enzyme by the CRL4CDT2 E3 ubiquitin ligase. PR-Set7 is also targeted by the SCFβ-TRCP ligase, but the role of this additional regulatory pathway remains unclear. Here, we show that Drosophila PR-Set7 undergoes a cell-cycle proteolytic regulation, independently of its interaction with PCNA. Instead, Slimb, the ortholog of β-TRCP, is specifically required for the degradation of the nuclear pool of PR-Set7 prior to S phase. Consequently, inactivation of Slimb leads to nuclear accumulation of PR-Set7, which triggers aberrant chromatin compaction and G1/S arrest. Strikingly, these phenotypes result from non-enzymatic PR-Set7 functions that prevent proper histone H4 acetylation independently of H4K20 methylation. Altogether, these results identify the Slimb-mediated PR-Set7 proteolysis as a new critical regulatory mechanism required for proper interphase chromatin organization at G1/S transition.

Condensin II Counteracts Cohesin and RNA Polymerase II in the Establishment of 3D Chromatin Organization

Interaction domains in Drosophila chromosomes form by segregation of active and inactive chromatin in the absence of CTCF loops, but the role of transcription versus other architectural proteins in chromatin organization is unclear. Here, we find that positioning of RNAPII via transcription elongation is essential in the formation of gene loops, which in turn interact to form compartmental domains. Inhibition of transcription elongation or depletion of cohesin decreases gene looping and formation of active compartmental domains. In contrast, depletion of condensin II, which also localizes to active chromatin, causes increased gene looping, formation of compartmental domains, and stronger intra-chromosomal compartmental interactions. Condensin II has a similar role in maintaining inter-chromosomal interactions responsible for pairing between homologous chromosomes, whereas inhibition of transcription elongation or cohesin depletion has little effect on homolog pairing. The results suggest distinct roles for cohesin and condensin II in the establishment of 3D nuclear organization in Drosophila.

Casein kinase 1α: biological mechanisms and theranostic potential

Casein kinase 1α (CK1α) is a multifunctional protein belonging to the CK1 protein family that is conserved in eukaryotes from yeast to humans. It regulates signaling pathways related to membrane trafficking, cell cycle progression, chromosome segregation, apoptosis, autophagy, cell metabolism, and differentiation in development, circadian rhythm, and the immune response as well as neurodegeneration and cancer. Given its involvement in diverse cellular, physiological, and pathological processes, CK1α is a promising therapeutic target. In this review, we summarize what is known of the biological functions of CK1α, and provide an overview of existing challenges and potential opportunities for advancing theranostics.

Guiding Mitotic Progression by Crosstalk between Post-translational Modifications

Cell division is tightly regulated to disentangle copied chromosomes in an orderly manner and prevent loss of genome integrity. During mitosis, transcriptional activity is limited and post-translational modifications (PTMs) are responsible for functional protein regulation. Essential mitotic regulators, including polo-like kinase 1 (PLK1) and cyclin-dependent kinases (CDK), as well as the anaphase-promoting complex/cyclosome (APC/C), are members of the enzymatic machinery responsible for protein modification. Interestingly, communication between PTMs ensures the essential tight and timely control during all consecutive phases of mitosis. Here, we present an overview of current concepts and understanding of crosstalk between PTMs regulating mitotic progression.

Axin phosphorylation in both Wnt-off and Wnt-on states requires the tumor suppressor APC

The aberrant activation of Wnt signal transduction initiates the development of 90% of colorectal cancers, the majority of which arise from inactivation of the tumor suppressor Adenomatous polyposis coli (APC). In the classical model for Wnt signaling, the primary role of APC is to act, together with the concentration-limiting scaffold protein Axin, in a “destruction complex” that directs the phosphorylation and consequent proteasomal degradation of the transcriptional activator β-catenin, thereby preventing signaling in the Wnt-off state. Following Wnt stimulation, Axin is recruited to a multiprotein “signalosome” required for pathway activation. Whereas it is well-documented that APC is essential in the destruction complex, APC’s role in this complex remains elusive. Here, we demonstrate in Drosophila that Axin exists in two distinct phosphorylation states in Wnt-off and Wnt-on conditions, respectively, that underlie its roles in the destruction complex and signalosome. These two Axin phosphorylation states are catalyzed by glycogen synthase kinase 3 (GSK3), and unexpectedly, completely dependent on APC in both unstimulated and Wnt-stimulated conditions. In a major revision of the classical model, we show that APC is essential not only in the destruction complex, but also for the rapid transition in Axin that occurs after Wnt stimulation and Axin’s subsequent association with the Wnt co-receptor LRP6/Arrow, one of the earliest steps in pathway activation. We propose that this novel requirement for APC in Axin regulation through phosphorylation both prevents signaling in the Wnt-off state and promotes signaling immediately following Wnt stimulation.

The Wnt signal transduction pathway directs fundamental cellular processes during development and in homeostasis. Wnt signaling is deregulated in 90% of colorectal cancers, most of which are triggered by inactivation of the tumor suppressor Adenomatous polyposis coli (APC). In the classical model, APC’s sole role in Wnt signaling is to target the transcriptional coactivator β-catenin for phosphorylation and subsequent degradation, and thereby to inhibit signaling in the unstimulated state. However, the mechanisms by which APC functions remain unknown. Herein, we provide evidence in Drosophila that supports a major role for APC in the direct regulation of the scaffold protein Axin in both Wnt-on and Wnt-off conditions. Our results indicate that APC promotes Axin phosphorylation, which is required not only to inhibit signaling in the unstimulated state, but also to activate signaling following Wnt stimulation. These unanticipated findings support a more active and multifaceted role for APC in Wnt signaling than previously known, and force revision of the current model for APC function.

Condensins promote chromosome individualization and segregation during mitosis, meiosis, and amitosis in Tetrahymena thermophila

Condensin is a protein complex with diverse functions in chromatin packaging and chromosome condensation and segregation. We studied condensin in the evolutionarily distant protist model Tetrahymena, which features noncanonical nuclear organization and divisions. In Tetrahymena, the germline and soma are partitioned into two different nuclei within a single cell. Consistent with their functional specializations in sexual reproduction and gene expression, condensins of the germline nucleus and the polyploid somatic nucleus are composed of different subunits. Mitosis and meiosis of the germline nucleus and amitotic division of the somatic nucleus are all dependent on condensins. In condensin-depleted cells, a chromosome condensation defect was most striking at meiotic metaphase, when Tetrahymena chromosomes are normally most densely packaged. Live imaging of meiotic divisions in condensin-depleted cells showed repeated nuclear stretching and contraction as the chromosomes failed to separate. Condensin depletion also fundamentally altered chromosome arrangement in the polyploid somatic nucleus: multiple copies of homologous chromosomes tended to cluster, consistent with a previous model of condensin suppressing default somatic pairing. We propose that failure to form discrete chromosome territories is the common cause of the defects observed in the absence of condensins.

An SMC-like protein binds and regulates Caenorhabditis elegans condensins

Structural Maintenance of Chromosomes (SMC) family proteins participate in multisubunit complexes that govern chromosome structure and dynamics. SMC-containing condensin complexes create chromosome topologies essential for mitosis/meiosis, gene expression, recombination, and repair. Many eukaryotes have two condensin complexes (I and II); C. elegans has three (I, II, and the X-chromosome specialized condensin IDC) and their regulation is poorly understood. Here we identify a novel SMC-like protein, SMCL-1, that binds to C. elegans condensin SMC subunits, and modulates condensin functions. Consistent with a possible role as a negative regulator, loss of SMCL-1 partially rescued the lethal and sterile phenotypes of a hypomorphic condensin mutant, while over-expression of SMCL-1 caused lethality, chromosome mis-segregation, and disruption of condensin IDC localization on X chromosomes. Unlike canonical SMC proteins, SMCL-1 lacks hinge and coil domains, and its ATPase domain lacks conserved amino acids required for ATP hydrolysis, leading to the speculation that it may inhibit condensin ATPase activity. SMCL-1 homologs are apparent only in the subset of Caenorhabditis species in which the condensin I and II subunit SMC-4 duplicated to create the condensin IDC- specific subunit DPY-27, suggesting that SMCL-1 helps this lineage cope with the regulatory challenges imposed by evolution of a third condensin complex. Our findings uncover a new regulator of condensins and highlight how the duplication and divergence of SMC complex components in various lineages has created new proteins with diverse functions in chromosome dynamics.

Condensin Regulation of Genome Architecture

Condensin complexes exist across all domains of life and are central to the structure and organization of chromatin. As architectural proteins, condensins control chromatin compaction during interphase and mitosis. Condensin activity has been well studied in mitosis but have recently emerged as important regulators of genome organization and gene expression during interphase. Here, we focus our discussion on recent findings on the molecular mechanism and how condensins are used to shape chromosomes during interphase. These findings suggest condensin activity during interphase is required for proper chromosome organization. J. Cell. Physiol. 232: 1617-1625, 2017. © 2016 Wiley Periodicals, Inc.

The functional role for condensin in the regulation of chromosomal organization during the cell cycle

In all organisms, the control of cell cycle progression is a fundamental process that is essential for cell growth, development, and survival. Through each cell cycle phase, the regulation of chromatin organization is essential for natural cell proliferation and maintaining cellular homeostasis. During mitosis, the chromatin morphology is dramatically changed to have a "thread-like" shape and the condensed chromosomes are segregated equally into two daughter cells. Disruption of the mitotic chromosome architecture physically impedes chromosomal behaviors, such as chromosome alignment and chromosome segregation; therefore, the proper mitotic chromosome structure is required to maintain chromosomal stability. Accumulating evidence has demonstrated that mitotic chromosome condensation is induced by condensin complexes. Moreover, recent studies have shown that condensin also modulates interphase chromatin and regulates gene expression. This review mainly focuses on the molecular mechanisms that condensin uses to exert its functions during the cell cycle progression. Moreover, we discuss the condensin-mediated chromosomal organization in cancer cells.

Oxidative stress in oocytes during midprophase induces premature loss of cohesion and chromosome segregation errors

In humans, errors in meiotic chromosome segregation that produce aneuploid gametes increase dramatically as women age, a phenomenon termed the "maternal age effect." During meiosis, cohesion between sister chromatids keeps recombinant homologs physically attached and premature loss of cohesion can lead to missegregation of homologs during meiosis I. A growing body of evidence suggests that meiotic cohesion deteriorates as oocytes age and contributes to the maternal age effect. One hallmark of aging cells is an increase in oxidative damage caused by reactive oxygen species (ROS). Therefore, increased oxidative damage in older oocytes may be one of the factors that leads to premature loss of cohesion and segregation errors. To test this hypothesis, we used an RNAi strategy to induce oxidative stress in Drosophila oocytes and measured the fidelity of chromosome segregation during meiosis. Knockdown of either the cytoplasmic or mitochondrial ROS scavenger superoxide dismutase (SOD) caused a significant increase in segregation errors, and heterozygosity for an smc1 deletion enhanced this phenotype. FISH analysis indicated that SOD knockdown moderately increased the percentage of oocytes with arm cohesion defects. Consistent with premature loss of arm cohesion and destabilization of chiasmata, the frequency at which recombinant homologs missegregate during meiosis I is significantly greater in SOD knockdown oocytes than in controls. Together these results provide an in vivo demonstration that oxidative stress during meiotic prophase induces chromosome segregation errors and support the model that accelerated loss of cohesion in aging human oocytes is caused, at least in part, by oxidative damage.

Interallelic Transcriptional Enhancement as an in Vivo Measure of Transvection in Drosophila melanogaster

Transvection—pairing-dependent interallelic regulation resulting from enhancer action in trans—occurs throughout the Drosophila melanogaster genome, likely as a result of the extensive somatic homolog pairing seen in Dipteran species. Recent studies of transvection in Drosophila have demonstrated important qualitative differences between enhancer action in cisvs.in trans, as well as a modest synergistic effect of cis- and trans-acting enhancers on total tissue transcript levels at a given locus. In the present study, we identify a system in which cis- and trans-acting GAL4-UAS enhancer synergism has an unexpectedly large quantitative influence on gene expression, boosting total tissue transcript levels at least fourfold relative to those seen in the absence of transvection. We exploit this strong quantitative effect by using publicly available UAS-shRNA constructs from the TRiP library to assay candidate genes for transvection activity in vivo. The results of the present study, which demonstrate that in trans activation by simple UAS enhancers can have large quantitative effects on gene expression in Drosophila, have important new implications for experimental design utilizing the GAL4-UAS system.

Investigating the Interplay between Sister Chromatid Cohesion and Homolog Pairing in Drosophila Nuclei

Following DNA replication, sister chromatids must stay connected for the remainder of the cell cycle in order to ensure accurate segregation in the subsequent cell division. This important function involves an evolutionarily conserved protein complex known as cohesin; any loss of cohesin causes premature sister chromatid separation in mitosis. Here, we examined the role of cohesin in sister chromatid cohesion prior to mitosis, using fluorescence in situ hybridization (FISH) to assay the alignment of sister chromatids in interphase Drosophila cells. Surprisingly, we found that sister chromatid cohesion can be maintained in G2 with little to no cohesin. This capacity to maintain cohesion is widespread in Drosophila, unlike in other systems where a reduced dependence on cohesin for sister chromatid segregation has been observed only at specific chromosomal regions, such as the rDNA locus in budding yeast. Additionally, we show that condensin II antagonizes the alignment of sister chromatids in interphase, supporting a model wherein cohesin and condensin II oppose each other’s functions in the alignment of sister chromatids. Finally, because the maternal and paternal homologs are paired in the somatic cells of Drosophila, and because condensin II has been shown to antagonize this pairing, we consider the possibility that condensin II-regulated mechanisms for aligning homologous chromosomes may also contribute to sister chromatid cohesion.

As cells grow, they replicate their DNA to give rise to two copies of each chromosome, known as sister chromatids, which separate from each other once the cell divides. To ensure that sister chromatids end up in different daughter cells, they are kept together from DNA replication until mitosis via a connection known as cohesion. A protein complex known as cohesin is essential for this process. Our work in Drosophila cells suggests that factors other than cohesin also contribute to sister chromatid cohesion in interphase. Additionally, we observed that the alignment of sister chromatids is regulated by condensin II, a protein complex involved in the compaction of chromosomes prior to division as well as the regulation of inter-chromosomal associations. These findings highlight that, in addition to their important individual functions, cohesin and condensin II proteins may interact to organize chromosomes over the course of the cell cycle. Finally, building on prior observations that condensin II is involved in the regulation of somatic homolog pairing in Drosophila, our work suggests that the mechanisms underlying homolog pairing may also contribute to sister chromatid cohesion.

Levels of Ycg1 Limit Condensin Function during the Cell Cycle

During mitosis chromosomes are condensed to facilitate their segregation, through a process mediated by the condensin complex. Although several factors that promote maximal condensin activity during mitosis have been identified, the mechanisms that downregulate condensin activity during interphase are largely unknown. Here, we demonstrate that Ycg1, the Cap-G subunit of budding yeast condensin, is cell cycle-regulated with levels peaking in mitosis and decreasing as cells enter G1 phase. This cyclical expression pattern is established by a combination of cell cycle-regulated transcription and constitutive degradation. Interestingly, overexpression of YCG1 and mutations that stabilize Ycg1 each result in delayed cell-cycle entry and an overall proliferation defect. Overexpression of no other condensin subunit impacts the cell cycle, suggesting that Ycg1 is limiting for condensin complex formation. Consistent with this possibility, we find that levels of intact condensin complex are reduced in G1 phase compared to mitosis, and that increased Ycg1 expression leads to increases in both levels of condensin complex and binding to chromatin in G1. Together, these results demonstrate that Ycg1 levels limit condensin function in interphase cells, and suggest that the association of condensin with chromosomes must be reduced following mitosis to enable efficient progression through the cell cycle.

Jade-1S phosphorylation induced by CK1α contributes to cell cycle progression

The PHD zinc finger protein Jade-1S is a component of the HBO1 histone acetyltransferase complex and binds chromatin in a cell cycle-dependent manner. Jade-1S also acts as an E3 ubiquitin ligase for the canonical Wnt effector protein β-catenin and is influenced by CK1α-mediated phosphorylation. To further elucidate the functional impact of this phosphorylation, we used a stable, low-level expression system to express either wild-type or mutant Jade-1S lacking the N-terminal CK1α phosphorylation motif. Interactome analyses revealed that the Jade-1S mutant unable to be phosphorylated by CK1α has an increased binding affinity to proteins involved in chromatin remodelling, histone deacetylation, transcriptional repression, and ribosome biogenesis. Interestingly, cells expressing the mutant displayed an elongated cell shape and a delay in cell cycle progression. Finally, phosphoproteomic analyses allowed identification of a Jade-1S site phosphorylated in the presence of CK1α but closely resembling a PLK1 phosphorylation motif. Our data suggest that Jade-1S phosphorylation at an N-terminal CK1α motif creates a PLK1 phospho-binding domain. We propose CK1α phosphorylation of Jade 1S to serve as a molecular switch, turning off chromatin remodelling functions of Jade-1S and allowing timely cell cycle progression. As Jade-1S protein expression in the kidney is altered upon renal injury, this could contribute to understanding mechanisms underlying epithelial injury repair.

Niche signaling promotes stem cell survival in the Drosophila testis via the JAK-STAT target DIAP1

Tissue-specific stem cells are thought to resist environmental insults better than their differentiating progeny, but this resistance varies from one tissue to another, and the underlying mechanisms are not well-understood. Here, we use the Drosophila testis as a model system to study the regulation of cell death within an intact niche. This niche contains sperm-producing germline stem cells (GSCs) and accompanying somatic cyst stem cells (or CySCs). Although many signals are known to promote stem cell self-renewal in this tissue, including the highly conserved JAK-STAT pathway, the response of these stem cells to potential death-inducing signals, and factors promoting stem cell survival, have not been characterized. Here we find that both GSCs and CySCs resist cell death better than their differentiating progeny, under normal laboratory conditions and in response to potential death-inducing stimuli such as irradiation or starvation. To ask what might be promoting stem cell survival, we characterized the role of the anti-apoptotic gene Drosophila inhibitor of apoptosis 1 (diap1) in testis stem cells. DIAP1 protein is enriched in the GSCs and CySCs and is a JAK-STAT target. diap1 is necessary for survival of both GSCs and CySCs, and ectopic up-regulation of DIAP1 in somatic cyst cells is sufficient to non-autonomously rescue stress-induced cell death in adjacent differentiating germ cells (spermatogonia). Altogether, our results show that niche signals can promote stem cell survival by up-regulation of highly conserved anti-apoptotic proteins, and suggest that this strategy may underlie the ability of stem cells to resist death more generally.

Condensin II Regulates Interphase Chromatin Organization Through the Mrg-Binding Motif of Cap-H2

The spatial organization of the genome within the eukaryotic nucleus is a dynamic process that plays a central role in cellular processes such as gene expression, DNA replication, and chromosome segregation. Condensins are conserved multi-subunit protein complexes that contribute to chromosome organization by regulating chromosome compaction and homolog pairing. Previous work in our laboratory has shown that the Cap-H2 subunit of condensin II physically and genetically interacts with the Drosophila homolog of human MORF4-related gene on chromosome 15 (MRG15). Like Cap-H2, Mrg15 is required for interphase chromosome compaction and homolog pairing. However, the mechanism by which Mrg15 and Cap-H2 cooperate to maintain interphase chromatin organization remains unclear. Here, we show that Cap-H2 localizes to interband regions on polytene chromosomes and co-localizes with Mrg15 at regions of active transcription across the genome. We show that co-localization of Cap-H2 on polytene chromosomes is partially dependent on Mrg15. We have identified a binding motif within Cap-H2 that is essential for its interaction with Mrg15, and have found that mutation of this motif results in loss of localization of Cap-H2 on polytene chromosomes and results in partial suppression of Cap-H2-mediated compaction and homolog unpairing. Our data are consistent with a model in which Mrg15 acts as a loading factor to facilitate Cap-H2 binding to chromatin and mediate changes in chromatin organization.