Temporal control of epigenetic centromere specification

Extraordinary centromeres: differences in the meiotic chromosomes of two rock lizards species Darevskia portschinskii and Darevskia raddei

According to the synthesis of 30 years of multidisciplinary studies, parthenogenetic species of rock lizards of genus Darevskia were formed as a result of different combination patterns of interspecific hybridization of the four bisexual parental species: Darevskia raddei, D. mixta, D. valentini, and D. portschinskii. In particular, D. portschinskii and D. raddei are considered as the parental species for the parthenogenetic species D. rostombekowi. Here for the first time, we present the result of comparative immunocytochemical study of primary spermatocyte nuclei spreads from the leptotene to diplotene stages of meiotic prophase I in two species: D. portschinskii and D. raddei. We observed similar chromosome lengths for both synaptonemal complex (SC) karyotypes as well as a similar number of crossing over sites. However, unexpected differences in the number and distribution of anti-centromere antibody (ACA) foci were detected in the SC structure of bivalents of the two species. In all examined D. portschinskii spermatocyte nuclei, one immunostained centromere focus was detected per SC bivalent. In contrast, in almost every studied D. raddei nuclei we identified three to nine SCs with additional immunostained ACA foci per SC bivalent. Thus, the obtained results allow us to identify species-specific karyotype features, previously not been detected using conventional mitotic chromosome analysis. Presumably the additional centromere foci are result of epigenetic chromatin modifications. We assume that this characteristic of the D. raddei karyotype could represent useful marker for the future studies of parthenogenetic species hybrid karyotypes related to D. raddei.

Imaging and Quantitation of Assembly Dynamics of the Centromeric Histone H3 Variant CENP-A in Drosophila melanogaster Spermatocytes by Immunofluorescence and Fluorescence In-Situ Hybridization (Immuno-FISH)

This chapter describes a method used to assay the cell cycle dynamics of the centromeric histone H3 variant CENP-A in meiosis using Drosophila males as the experimental system. Specifically, we describe a method that combines Immunofluorescence (IF) and Fluorescence in-situ Hybridization (FISH) protocols, performed on fixed Drosophila testes. An advantage of this protocol is the ability to localize individual centromeres on the four Drosophila homologous chromosomes that form distinct nuclear territories in spermatocytes. We also describe a method to quantify CENP-A focal intensities using Image J software.

Chromatin assembly: Journey to the CENter of the chromosome

All eukaryotic genomes are packaged into basic units of DNA wrapped around histone proteins called nucleosomes. The ability of histones to specify a variety of epigenetic states at defined chromatin domains is essential for cell survival. The most distinctive type of chromatin is found at centromeres, which are marked by the centromere-specific histone H3 variant CENP-A. Many of the factors that regulate CENP-A chromatin have been identified; however, our understanding of the mechanisms of centromeric nucleosome assembly, maintenance, and reorganization remains limited. This review discusses recent insights into these processes and draws parallels between centromeric and noncentromeric chromatin assembly mechanisms.

Nucleolar activity and CENP-C regulate CENP-A and CAL1 availability for centromere assembly in meiosis

The centromere-specific histone CENP-A is the key epigenetic determinant of centromere identity. Whereas most histones are removed from mature sperm, CENP-A is retained to mark paternal centromeres. In Drosophila males we show that the centromere assembly factors CAL1 and CENP-C are required for meiotic chromosome segregation, CENP-A assembly and maintenance on sperm, as well as fertility. In meiosis, CENP-A accumulates with CAL1 in nucleoli. Furthermore, we show that CENP-C normally limits the release of CAL1 and CENP-A from nucleoli for proper centromere assembly in meiotic prophase I. Finally, we show that RNA polymerase I transcription is required for efficient CENP-A assembly in meiosis, as well as centromere tethering to nucleoli.

Summary: Novel roles are uncovered for centromere assembly factors CENP-C and CAL1 in meiotic chromosome segregation, CENP-A assembly and maintenance of sperm, as well as fertility in Drosophila males.

The past, present, and future of human centromere genomics

The centromere is the chromosomal locus essential for chromosome inheritance and genome stability. Human centromeres are located at repetitive alpha satellite DNA arrays that compose approximately 5% of the genome. Contiguous alpha satellite DNA sequence is absent from the assembled reference genome, limiting current understanding of centromere organization and function. Here, we review the progress in centromere genomics spanning the discovery of the sequence to its molecular characterization and the work done during the Human Genome Project era to elucidate alpha satellite structure and sequence variation. We discuss exciting recent advances in alpha satellite sequence assembly that have provided important insight into the abundance and complex organization of this sequence on human chromosomes. In light of these new findings, we offer perspectives for future studies of human centromere assembly and function.

CENP-A is essential for cardiac progenitor cell proliferation

Centromere protein A (CENP-A) is a homolog of histone H3 that epigenetically marks the heterochromatin of chromosomes. CENP-A is a critical component of the cell cycle machinery that is necessary for proper assembly of the mitotic spindle. However, the role of CENP-A in the heart and cardiac progenitor cells (CPCs) has not been previously studied. This study shows that CENP-A is expressed in CPCs and declines with age. Silencing CENP-A results in a decreased CPC growth rate, reduced cell number in phase G₂/M of the cell cycle, and increased senescence associated β-galactosidase activity. Lineage commitment is not affected by CENP-A silencing, suggesting that cell cycle arrest induced by loss of CENP-A is a consequence of senescence and not differentiation. CENP-A knockdown does not exacerbate cell death in undifferentiated CPCs, but increases apoptosis upon lineage commitment. Taken together, these results indicate that CPCs maintain relatively high levels of CENP-A early in life, which is necessary for sustaining proliferation, inhibiting senescence, and promoting survival following differentiation of CPCs.

Kinetochore function and chromosome segregation rely on critical residues in histones H3 and H4 in budding yeast

Accurate chromosome segregation requires that sister kinetochores biorient and attach to microtubules from opposite poles. Kinetochore biorientation relies on the underlying centromeric chromatin, which provides a platform to assemble the kinetochore and to recruit the regulatory factors that ensure the high fidelity of this process. To identify the centromeric chromatin determinants that contribute to chromosome segregation, we performed two complementary unbiased genetic screens using a library of budding yeast mutants in every residue of histone H3 and H4. In one screen, we identified mutants that lead to increased loss of a nonessential chromosome. In the second screen, we isolated mutants whose viability depends on a key regulator of biorientation, the Aurora B protein kinase. Nine mutants were common to both screens and exhibited kinetochore biorientation defects. Four of the mutants map near the unstructured nucleosome entry site, and their genetic interaction with reduced IPL1 can be suppressed by increasing the dosage of SGO1, a key regulator of biorientation. In addition, the composition of purified kinetochores was altered in six of the mutants. Together, this work identifies previously unknown histone residues involved in chromosome segregation and lays the foundation for future studies on the role of the underlying chromatin structure in chromosome segregation.

Functions of the centromere and kinetochore in chromosome segregation

Centromeres play essential roles in equal chromosome segregation by directing the assembly of the microtubule binding kinetochore and serving as the cohesion site between sister chromatids. Here, we review the significant recent progress in our understanding of centromere protein assembly and how centromere proteins form the foundation of the kinetochore.

Basic properties of epigenetic systems: lessons from the centromere

Chromatin-based epigenetic inheritance cooperates with cis-acting DNA sequence information to propagate gene expression states and chromosome architecture across cell division cycles. Histone proteins and their modifications are central components of epigenetic systems but how, and to what extent, they are propagated is a matter of continued debate. Centromeric nucleosomes, marked by the histone H3 variant CENP-A, are stable across mitotic divisions and are assembled in a locus specific and cell cycle controlled manner. The mechanism of inheritance of this unique chromatin domain has important implications for how general nucleosome transmission is controlled in space and time.