An unexpected localization of basonuclin in the centrosome, mitochondria, and acrosome of developing spermatids

Toward clinical exomes in diagnostics and management of male infertility

Infertility, affecting ∼10% of men, is predominantly caused by primary spermatogenic failure (SPGF). We screened likely pathogenic and pathogenic (LP/P) variants in 638 candidate genes for male infertility in 521 individuals presenting idiopathic SPGF and 323 normozoospermic men in the ESTAND cohort. Molecular diagnosis was reached for 64 men with SPGF (12%), with findings in 39 genes (6%). The yield did not differ significantly between the subgroups with azoospermia (20/185, 11%), oligozoospermia (18/181, 10%), and primary cryptorchidism with SPGF (26/155, 17%). Notably, 19 of 64 LP/P variants (30%) identified in 28 subjects represented recurrent findings in this study and/or with other male infertility cohorts. NR5A1 was the most frequently affected gene, with seven LP/P variants in six SPGF-affected men and two normozoospermic men. The link to SPGF was validated for recently proposed candidate genes ACTRT1, ASZ1, GLUD2, GREB1L, LEO1, RBM5, ROS1, and TGIF2LY. Heterozygous truncating variants in BNC1, reported in female infertility, emerged as plausible causes of severe oligozoospermia. Data suggested that several infertile men may present congenital conditions with less pronounced or pleiotropic phenotypes affecting the development and function of the reproductive system. Genes regulating the hypothalamic-pituitary-gonadal axis were affected in >30% of subjects with LP/P variants. Six individuals had more than one LP/P variant, including five with two findings from the gene panel. A 4-fold increased prevalence of cancer was observed in men with genetic infertility compared to the general male population (8% vs. 2%; p = 4.4 × 10-3). Expanding genetic testing in andrology will contribute to the multidisciplinary management of SPGF.

BNC1 deficiency induces mitochondrial dysfunction-triggered spermatogonia apoptosis through the CREB/SIRT1/FOXO3 pathway: the therapeutic potential of nicotinamide riboside and metformin†

Male infertility is a global health problem that disturbs numerous couples worldwide. Basonuclin 1 (BNC1) is a transcription factor mainly expressed in proliferative keratinocytes and germ cells. A frameshift mutation of BNC1 was identified in a large Chinese primary ovarian insufficiency pedigree. The expression of BNC1 was significantly decreased in the testis biopsies of infertile patients with nonobstructive azoospermia. Previous studies have revealed that mice with BNC1 deficiency are generally subfertile and undergo gradual spermatogenic failure. We observed that apoptosis of spermatogonia is tightly related to spermatogenic failure in mice with a Bnc1 truncation mutation. Such impairment is related to mitochondrial dysfunction causing lower mitochondrial membrane potential and higher reactive oxygen species. We showed that downregulation of CREB/SIRT1/FOXO3 signaling participates in the above impairment. Administration of nicotinamide riboside or metformin reversed mitochondrial dysfunction and inhibited apoptosis in Bnc1-knockdown spermatogonia by stimulating CREB/SIRT1/FOXO3 signaling. Dietary supplementation with nicotinamide riboside or metformin in mutated mice increased SIRT1 signaling, improved the architecture of spermatogenic tubules, inhibited apoptosis of the testis, and improved the fertility of mice with a Bnc1 truncation mutation. Our data establish that oral nicotinamide riboside or metformin can be useful for the treatment of spermatogenic failure induced by Bnc1 mutation.

Truncating PICK1 Variant Identified in Azoospermia Affected Mitochondrial Dysfunction in Knockout Mice

OBJECTIVE

The protein interacting with C kinase 1 (PICK1) plays a critical role in vesicle trafficking, and its deficiency in sperm cells results in abnormal vesicle trafficking from Golgi to acrosome, which eventually disrupts acrosome formation and leads to male infertility.

METHODS

An azoospermia sample was filtered, and the laboratory detection and clinical phenotype indicated typical azoospermia in the patient. We sequenced all of the exons in the PICK1 gene and found that there was a novel homozygous variant in the PICK1 gene, c.364delA (p.Lys122SerfsX8), and this protein structure truncating variant seriously affected the biological function. Then we constructed a PICK1 knockout mouse model using clustered regularly interspaced short palindromic repeat cutting technology (CRISPRc).

RESULTS

The sperm from PICK1 knockout mice showed acrosome and nucleus abnormalities, as well as dysfunctional mitochondrial sheath formation. Both the total sperm and motility sperm counts were decreased in the PICK1 knockout mice compared to wild-type mice. Moreover, the mitochondrial dysfunction was verified in the mice. These defects in the male PICK1 knockout mice may have eventually led to complete infertility.

CONCLUSION

The c.364delA novel variant in the PICK1 gene associated with clinical infertility, and pathogenic variants in the PICK1 may cause azoospermia or asthenospermia by impairing mitochondrial function in both mice and humans.

Basonuclin 1 deficiency is a cause of primary ovarian insufficiency

Primary ovarian insufficiency (POI) leads to infertility and premature menopause in young women. The genetic etiology of this disorder remains unknown in most patients. Using whole exome sequencing of a large Chinese POI pedigree, we identified a heterozygous 5 bp deletion inducing a frameshift in BNC1, which is predicted to result in a non-sense-mediated decay or a truncated BNC1 protein. Sanger sequencing identified another BNC1 missense mutation in 4 of 82 idiopathic patients with POI, and the mutation was absent in 332 healthy controls. Transfection of recombinant plasmids with the frameshift mutant and separately with the missense mutant in HEK293T cells led to abnormal nuclear localization. Knockdown of BNC1 was found to reduce BMP15 and p-AKT levels and to inhibit meiosis in oocytes. A female mouse model of the human Bnc1 frameshift mutation exhibited infertility, significantly increased serum follicle-stimulating hormone, decreased ovary size and reduced follicle numbers, consistent with POI. We report haploinsufficiency of BNC1 as an etiology of human autosomal dominant POI.

Is transcription in sperm stationary or dynamic?

Transcriptional activity is repressed due to the packaging of sperm chromatins during spermiogenesis. The detection of numerous transcripts in sperm, however, raises the question whether transcriptional events exist in sperm, which has been the central focus of the recent studies. To summarize the transcriptional activity during spermiogenesis and in sperm, we reviewed the documents on transcript differences during spermiogenesis, in sperm with differential motility, before and after capacitation and cryopreservation. This will lay a theoretical foundation for studying the mechanism(s) of gene expression in sperm, and would be invaluable in making better use of animal sires and developing reproductive control technologies.

The importance of basonuclin 2 in adult mice and its relation to basonuclin 1

BNC2 is an extremely conserved zinc finger protein with important functions in the development of craniofacial bones and male germ cells. Because disruption of the Bnc2 gene in mice causes neonatal lethality, the function of the protein in adult animals has not been studied. Until now BNC2 was considered to have a wider tissue distribution than its paralog, BNC1, but the precise cell types expressing Bnc2 are largely unknown. We identify here the cell types containing BNC2 in the mouse and we show the unexpected presence of BNC1 in many BNC2-containing cells. BNC1 and BNC2 are colocalized in male and female germ cells, ovarian epithelial cells, sensory neurons, hair follicle keratinocytes and connective cells of organ capsules. In many cell lineages, the two basonuclins appear and disappear synchronously. Within the male germ cell lineage, BNC1 and BNC2 are found in prospermatogonia and undifferentiated spermatogonia, and disappear abruptly from differentiating spermatogonia. During oogenesis, the two basonuclins accumulate specifically in maturing oocytes. During the development of hair follicles, BNC1 and BNC2 concentrate in the primary hair germs. As follicle morphogenesis proceeds, cells possessing BNC1 and BNC2 invade the dermis and surround the papilla. During anagen, BNC1 and BNC2 are largely restricted to the basal layer of the outer root sheath and the matrix. During catagen, the compartment of cells possessing BNC1 and BNC2 regresses, and in telogen, the two basonuclins are confined to the secondary hair germ. During the next anagen, the BNC1/BNC2-containing cell population regenerates the hair follicle. By examining Bnc2(-/-) mice that have escaped the neonatal lethality usually associated with lack of BNC2, we demonstrate that BNC2 possesses important functions in many of the cell types where it resides. Hair follicles of postnatal Bnc2(-/-) mice do not fully develop during the first cycle and thereafter remain blocked in telogen. It is concluded that the presence of BNC2 in the secondary hair germ is required to regenerate the transient segment of the follicle. Postnatal Bnc2(-/-) mice also show severe dwarfism, defects in oogenesis and alterations of palatal rugae. Although the two basonuclins possess very similar zinc fingers and are largely coexpressed, BNC1 cannot substitute for BNC2. This is shown incontrovertibly in knockin mice expressing Bnc1 instead of Bnc2 as these mice invariably die at birth with craniofacial abnormalities undistinguishable from those of Bnc2(-/-) mice. The function of the basonuclins in the secondary hair germ is of particular interest.

The zinc-finger protein basonuclin 2 is required for proper mitotic arrest, prevention of premature meiotic initiation and meiotic progression in mouse male germ cells

Absence of mitosis and meiosis are distinguishing properties of male germ cells during late fetal and early neonatal periods. Repressors of male germ cell meiosis have been identified, but mitotic repressors are largely unknown, and no protein repressing both meiosis and mitosis is known. We demonstrate here that the zinc-finger protein BNC2 is present in male but not in female germ cells. In testis, BNC2 exists as several spliced isoforms and presumably binds to DNA. Within the male germ cell lineage, BNC2 is restricted to prospermatogonia and undifferentiated spermatogonia. Fetal prospermatogonia that lack BNC2 multiply excessively on embryonic day (E)14.5 and reenter the cell cycle prematurely. Mutant prospermatogonia also engage in abnormal meiosis; on E17.5, Bnc2(-/-) prospermatogonia start synthesizing the synaptonemal protein SYCP3, and by the time of birth, many Bnc2(-/-) prospermatogonia have accumulated large amounts of nonfilamentous SYCP3, thus appearing to be blocked at leptonema. Bnc2(-/-) prospermatogonia do not undergo proper male differentiation, as they lack almost all the mRNA for the male-specific methylation protein DNMT3L and have increased levels of mRNAs that encode meiotic proteins, including STRA8. Bnc2(-/-) prospermatogonia can produce spermatogonia, but these enter meiosis prematurely and undergo massive apoptotic death during meiotic prophase. This study identifies BNC2 as a major regulator of male germ stem cells, which is required for repression of meiosis and mitosis in prospermatogonia, and for meiosis progression during spermatogenesis. In view of the extreme evolutionary conservation of BNC2, the findings described here are likely to apply to many species.

BNC1 is required for maintaining mouse spermatogenesis

Basonuclin (BNC1) is a zinc finger protein expressed primarily in gametogenic cells and proliferative keratinocytes. Our previous work suggested that BNC1 is present in spermatogonia, spermatocytes, and spermatids, but absent in the Sertoli cells. BNC1's role in spermatogenesis is unknown. Here, we show that BNC1 is required for the maintenance of spermatogenesis. Bnc1-null male mice were sub-fertile, losing germ cells progressively with age. The Bnc1-null seminiferous epithelia began to degenerate before 8 weeks of age and eventually became Sertoli cell-only. Sperm count and motility also declined with age. Furthermore, Bnc1 heterozygotes, although fertile, showed a significant drop in sperm count and in testis weight by 24 weeks of age, suggesting a dosage effect of Bnc1 on testis development. In conclusion, our data demonstrate for the first time BNC1's essential role in maintaining mouse spermatogenesis.

Human balanced translocation and mouse gene inactivation implicate Basonuclin 2 in distal urethral development

We studied a man with distal hypospadias, partial anomalous pulmonary venous return, mild limb-length inequality and a balanced translocation involving chromosomes 9 and 13. To gain insight into the etiology of his birth defects, we mapped the translocation breakpoints by high-resolution comparative genomic hybridization (CGH), using chromosome 9- and 13-specific tiling arrays to analyze genetic material from a spontaneously aborted fetus with unbalanced segregation of the translocation. The chromosome 13 breakpoint was ∼400  kb away from the nearest gene, but the chromosome 9 breakpoint fell within an intron of Basonuclin 2 (BNC2), a gene that encodes an evolutionarily conserved nuclear zinc-finger protein. The BNC2/Bnc2 gene is abundantly expressed in developing mouse and human periurethral tissues. In all, 6 of 48 unrelated subjects with distal hypospadias had nine novel nonsynonymous substitutions in BNC2, five of which were computationally predicted to be deleterious. In comparison, two of 23 controls with normal penile urethra morphology, each had a novel nonsynonymous substitution in BNC2, one of which was predicted to be deleterious. Bnc2(-/-) mice of both sexes displayed a high frequency of distal urethral defects; heterozygotes showed similar defects with reduced penetrance. The association of BNC2 disruption with distal urethral defects and the gene's expression pattern indicate that it functions in urethral development.

The transcriptional repressor Kaiso localizes at the mitotic spindle and is a constituent of the pericentriolar material

Kaiso is a BTB/POZ zinc finger protein known as a transcriptional repressor. It was originally identified through its in vitro association with the Armadillo protein p120ctn. Subcellular localization of Kaiso in cell lines and in normal and cancerous human tissues revealed that its expression is not restricted to the nucleus. In the present study we monitored Kaiso's subcellular localization during the cell cycle and found the following: (1) during interphase, Kaiso is located not only in the nucleus, but also on microtubular structures, including the centrosome; (2) at metaphase, it is present at the centrosomes and on the spindle microtubules; (3) during telophase, it accumulates at the midbody. We found that Kaiso is a genuine PCM component that belongs to a pericentrin molecular complex. We analyzed the functions of different domains of Kaiso by visualizing the subcellular distribution of GFP-tagged Kaiso fragments throughout the cell cycle. Our results indicate that two domains are responsible for targeting Kaiso to the centrosomes and microtubules. The first domain, designated SA1 for spindle-associated domain 1, is located in the center of the Kaiso protein and localizes at the spindle microtubules and centrosomes; the second domain, SA2, is an evolutionarily conserved domain situated just before the zinc finger domain and might be responsible for localizing Kaiso towards the centrosomal region. Constructs containing both SA domains and Kaiso's aminoterminal BTB/POZ domain triggered the formation of abnormal centrosomes. We also observed that overexpression of longer or full-length Kaiso constructs led to mitotic cell arrest and frequent cell death. Knockdown of Kaiso accelerated cell proliferation. Our data reveal a new target for Kaiso at the centrosomes and spindle microtubules during mitosis. They also strongly imply that Kaiso's function as a transcriptional regulator might be linked to the control of the cell cycle and to cell proliferation in cancer.

Basonuclin-2 requirements for zebrafish adult pigment pattern development and female fertility

Relatively little is known about the generation of adult form. One complex adult trait that is particularly amenable to genetic and experimental analysis is the zebrafish pigment pattern, which undergoes extensive remodeling during post-embryonic development to form adult stripes. These stripes result from the arrangement of three classes of neural crest-derived pigment cells, or chromatophores: melanophores, xanthophores, and iridophores. Here, we analyze the zebrafish bonaparte mutant, which has a normal early pigment pattern but exhibits a severe disruption to the adult stripe pattern. We show that the bonaparte mutant phenotype arises from mutations in basonuclin-2 (bnc2), encoding a highly conserved, nuclear-localized zinc finger protein of unknown function. We show that bnc2 acts non-autonomously to the melanophore lineage and is expressed by hypodermal cells adjacent to chromatophores during adult pigment pattern formation. In bonaparte (bnc2) mutants, all three types of chromatophores differentiate but then are lost by extrusion through the skin. We further show that while bnc2 promotes the development of two genetically distinct populations of melanophores in the body stripes, chromatophores of the fins and scales remain unaffected in bonaparte mutants, though a requirement of fin chromatophores for bnc2 is revealed in the absence of kit and colony stimulating factor-1 receptor activity. Finally, we find that bonaparte (bnc2) mutants exhibit dysmorphic ovaries correlating with infertility and bnc2 is expressed in somatic ovarian cells, whereas the related gene, bnc1, is expressed within oocytes; and we find that both bnc2 and bnc1 are expressed abundantly within the central nervous system. These findings identify bnc2 as an important mediator of adult pigment pattern formation and identify bonaparte mutants as an animal model for dissecting bnc2 functions.

Basonuclin 2 has a function in the multiplication of embryonic craniofacial mesenchymal cells and is orthologous to disco proteins

Basonuclin 2 is a recently discovered zinc finger protein of unknown function. Its paralog, basonuclin 1, is associated with the ability of keratinocytes to multiply. The basonuclin zinc fingers are closely related to those of the Drosophila proteins disco and discorelated, but the relation between disco proteins and basonuclins has remained elusive because the function of the disco proteins in larval head development seems to have no relation to that of basonuclin 1 and because the amino acid sequence of disco, apart from the zinc fingers, also has no similarity to that of the basonuclins. We have generated mice lacking basonuclin 2. These mice die within 24 h of birth with a cleft palate and abnormalities of craniofacial bones and tongue. In the embryonic head, expression of the basonuclin 2 gene is restricted to mesenchymal cells in the palate, at the periphery of the tongue, and in the mesenchymal sheaths that surround the brain and the osteocartilagineous structures. In late embryos, the rate of multiplication of these mesenchymal cells is greatly diminished. Therefore, basonuclin 2 is essential for the multiplication of craniofacial mesenchymal cells during embryogenesis. Non-Drosophila insect databases available since 2008 reveal that the basonuclins and the disco proteins share much more extensive sequence and gene structure similarity than noted when only Drosophila sequences were examined. We conclude that basonuclin 2 is both structurally and functionally the vertebrate ortholog of the disco proteins. We also note the possibility that some human craniofacial abnormalities are due to a lack of basonuclin 2.

The human basonuclin 2 gene has the potential to generate nearly 90,000 mRNA isoforms encoding over 2000 different proteins

The number of mRNAs and proteins that can be produced from a single gene is known to be increased by the number of start sites and by multiple splicing of products. A few genes have been found to generate extraordinarily large numbers of splicing isoforms. In the human, the largest number, nearly 2000 mRNA isoforms, has been reported for the neurexin 3alpha gene. However, the biological significance of alternative splicing often remains unclear because many alternative transcripts contain early translational stops and are thought to be rapidly degraded. We demonstrate here that human basonuclin 2 (bn2; approved gene symbol BNC2) transcripts are initiated from six promoters, are alternatively spliced at multiple positions, and are polyadenylated at four sites. Characterization of nearly 100 bn2 mRNA isoforms suggests that each promoter, splice site, and poly(A) addition site is used independently. The bn2 gene has therefore the potential to generate up to 90,000 mRNA isoforms encoding more than 2000 different proteins. Because alternative exons affect the position of the first methionine codon, the length of the coding region, and the position of the translational stop, the encoded proteins range in size from 43 to 1211 amino acids and some bear no sequence similarity to others. PCR analysis and transient expression in HeLa cells show that the major bn2 mRNA isoforms are stable and are translated into equally stable proteins, even when the mRNA bears an early translational stop.

Cell-type-specific regulation of RNA polymerase I transcription: a new frontier

Ribosomal RNA transcription was one of the first model systems for molecular characterization of a transcription regulatory mechanism and certainly one of the best studied in the widest range of organisms. In multicellular organisms, however, the issue of cell-type-specific regulation of rRNA transcription has not been well addressed. Here I propose that a systematic study of cell-type-specific regulation of rRNA transcription may reveal new regulatory mechanisms that have not been previously realized. Specifically, issues concerning the cell-type-specific requirement for rRNA production, the universality of Pol I transcription complex and the division of rDNA into regulatory subdomains are discussed.

Basonuclins 1 and 2, whose genes share a common origin, are proteins with widely different properties and functions

Basonuclin (bn) 1 possesses three separated pairs of zinc fingers and a nuclear localization signal. It is largely confined to the basal cells of stratified squamous epithelia and to reproductive germ cells. bn1 can shuttle between the nucleus and the cytoplasm, and its location is correlated with the proliferative potential of the cell. The recently discovered bn2 also possesses three separated pairs of zinc fingers and a nuclear localization signal. Conservation of the zinc fingers and the nuclear localization signal by bn1 and bn2 indicates a common origin. However, in contrast to bn1, bn2 is found in virtually every cell type and is confined to the nucleus. Bn2 but not bn1 colocalizes with SC35 in nuclear speckles and, therefore, is likely to have a function in nuclear processing of mRNA.

Mouse SYCP2 is required for synaptonemal complex assembly and chromosomal synapsis during male meiosis

During meiosis, the arrangement of homologous chromosomes is tightly regulated by the synaptonemal complex (SC). Each SC consists of two axial/lateral elements (AEs/LEs), and numerous transverse filaments. SC protein 2 (SYCP2) and SYCP3 are integral components of AEs/LEs in mammals. We find that SYCP2 forms heterodimers with SYCP3 both in vitro and in vivo. An evolutionarily conserved coiled coil domain in SYCP2 is required for binding to SYCP3. We generated a mutant Sycp2 allele in mice that lacks the coiled coil domain. The fertility of homozygous Sycp2 mutant mice is sexually dimorphic; males are sterile because of a block in meiosis, whereas females are subfertile with sharply reduced litter size. Sycp2 mutant spermatocytes exhibit failure in the formation of AEs and chromosomal synapsis. Strikingly, the mutant SYCP2 protein localizes to axial chromosomal cores in both spermatocytes and fetal oocytes, but SYCP3 does not, demonstrating that SYCP2 is a primary determinant of AEs/LEs and, thus, is required for the incorporation of SYCP3 into SCs.

Re-epithelialisation and the possible involvement of the transcription factor, basonuclin

This article briefly summarises the basic mechanism of re-epithelialisation and discusses the possible role of the cell-type-specific transcription factor, basonuclin. Re-epithelialisation is initiated by a signal resulting from the absence of neighbouring cells at the wound edge. Basal cells at the wound edge become flattened and lose their intercellular desmosomes and substratum attachment. The amount of cytoplasmic actinomyosin filaments that insert into the new adhesion complexes is increased, and contraction of those filaments produces cell movement. The epithelial cells at the wound edge migrate on a provisional matrix using the newly expressed integrin receptors. Once re-epithelialisation is complete, the epithelial cells revert to the normal phenotype of basal epidermal cells, firmly attach to the newly developed basement membrane zone through hemidesmosomes and resume standard differentiation. Protein synthesis increases in the epidermal cells at the wound edge during re-epithelialisation. Active protein synthesis requires accelerated transcription of ribosomal RNA genes. The transcription factor basonuclin binds to the ribosomal RNA gene promoter and increases the transcription of the genes. Therefore, it is speculated that basonuclin in epithelial cells is required in the process of re-epithelialisation.

Basonuclin: a novel mammalian maternal-effect gene

Basonuclin is a zinc-finger protein found in abundance in oocytes. It qualifies as a maternal-effect gene because the source of pre-implantation embryonic basonuclin is maternal. Using a transgenic-RNAi approach, we knocked down basonuclin specifically in mouse oocytes, which led to female sub-fertility. Basonuclin deficiency in oocytes perturbed both RNA polymerase I- and II-mediated transcription, and oocyte morphology was affected (as evidenced by cytoplasmic and cell surface abnormalities). Some of the affected oocytes, however, could still mature to and arrest at metaphase II, and be ovulated. Nevertheless, fertilized basonuclin-deficient eggs failed to develop beyond the two-cell stage, and this pre-implantation failure accounted for the sub-fertility phenotype. These results suggest that basonuclin is a new member of the mammalian maternal-effect genes and, interestingly, differs from the previously reported mammalian maternal-effect genes in that it also apparently perturbs oogenesis.

Dynamics and unexpected localization of the plakin binding protein, kazrin, in mouse eggs and early embryos

The cell uses the cytoskeleton in virtually every aspect of cell survival and function. One primary function of the cytoskeleton is to connect to and stabilize intercellular junctions. To accomplish this task, microtubules, actin filaments, and intermediate filaments utilize cytolinker proteins, which physically bind the cytoskeletal filament to the core proteins of the adhesion junction. The plakin family of linker proteins have been in the spotlight recently as critical components for embryo survival and, when mutated, the cause of diseases such as muscular dystrophy and cardiomyopathies. Here, we reveal the dynamics of a recently discovered plakin binding protein, kazrin (kaz), during early mouse development. Kaz was originally found in adult tissues, primarily epidermis, linking periplakin to the plasma membrane and colocalizing with desmoplakin in desmosomes. Using reverse transcriptase-polymerase chain reaction, Western blots, and confocal microscopy, we found kaz in unfertilized eggs associated with the spindle apparatus and cytoskeletal sheets. As quickly as 5 min after egg activation, kaz relocates to a diffuse peri-spindle position, followed 20-30 min later by clear localization to the presumptive cytokinetic ring. Before the blastocyst stage of development, kaz associates with the nuclear matrix in a cell cycle-dependent manner, and also associates with the cytoplasmic actin cytoskeleton. After blastocyst formation, kaz localization and potential function(s) become highly complex as it is found associating with cell-cell junctions, the cytoskeleton, and nucleus. Postimplantation stages of development reveal that kaz retains a multifunctional, tissue-specific role as it is detected at diverse locations in various embryonic tissue types.

Identification of Basonuclin2, a DNA-binding zinc-finger protein expressed in germ tissues and skin keratinocytes

We used a bioinformatics approach to identify Basonuclin2, the second member of the Basonuclin zinc-finger family of transcription factors. The mouse Basonuclin2 protein consists of 1049 amino acids and contains three pairs of zinc fingers in the C-terminus that show a high level of amino acid sequence similarity with Basonuclin1. In addition, other characteristic domains of Basonuclin1, such as the serine strip and a nuclear localization signal, are also present in Basonuclin2. We used genomic and in silico database analysis to identify the human and rat homologs of basonuclin2. A search of the mouse genome showed that the basonuclin2 gene maps to chromosome 4 and consists of six exons spanning approximately 300 kb. Northern blot analysis revealed multiple transcripts of basonuclin2 in tissues of the reproductive system (ovary and testis) and also in kidney and skin. We demonstrate that, as expected from sequence conservation, recombinant Basonuclin2 can bind to a sequence in the promoter of a rRNA gene previously characterized as a Basonuclin-binding site. Full-length Basonuclin2 exclusively localizes to the nucleus, indicating that it likely plays an important role in nuclear function, probably in gene regulation. Our study establishes Basonuclin2 as a novel member of the Basonuclin family. Moreover, the structural and functional similarities with Basonuclin1 suggest that Basonuclin2 may play an analogous function in germ cells and skin keratinocytes.

Gli proteins up-regulate the expression of basonuclin in Basal cell carcinoma

Tumorigenesis is frequently accompanied by enhanced rRNA transcription, but the signaling mechanisms responsible for such enhancement remain unclear. Here, we report evidence suggesting a novel link between deregulated Hedgehog signaling and the augmented rRNA transcription in cancer. Aberrant activation of the Hedgehog pathway in keratinocytes is a hallmark of basal cell carcinoma (BCC), the most common cancer in light-skinned individuals. We show that Gli proteins, downstream effectors of the Hedgehog pathway, increase expression of a novel rRNA gene (rDNA) transcription factor, basonuclin, whose expression is markedly elevated in BCCs. The promoter of the human basonuclin gene contains a Gli-binding site, which is required for Gli protein binding and transcriptional activation. We show also that the level of 47S pre-rRNA is much higher in BCCs than in normal epidermis, suggesting an accelerated rRNA transcription in the neoplastic cells. Within BCC, those cells expressing the highest level of basonuclin also exhibit the greatest increase in 47S pre-rRNA, consistent with a role for basonuclin in increasing rRNA transcription in these cells. Our data suggest that Hedgehog-Gli pathway enhances rRNA transcription in BCC by increasing basonuclin gene expression.

Basonuclin 2: an extremely conserved homolog of the zinc finger protein basonuclin

Basonuclin is a zinc finger protein specific to basal keratinocytes and germ cells. In keratinocytes, basonuclin behaves as a stem cell marker and is thought to be a transcription factor that maintains proliferative capacity and prevents terminal differentiation. The human gene is located on chromosome 15. We have discovered in the chicken the existence of basonuclin 2, a basonuclin homolog. We also report the entire sequence of mouse and human basonuclin 2; the corresponding genes are located on mouse chromosome 4 and human chromosome 9. Although the amino acid sequence of basonuclin 2 differs extensively from that of basonuclin 1, the two proteins share essential features. Both contain three paired zinc fingers, a nuclear localization signal, and a serine stripe. The basonuclin 2 mRNA has a wider tissue distribution than the basonuclin 1 mRNA: it is particularly abundant in testis, kidney, uterus, and intestine. The extreme conservation of the basonuclin 2 amino acid sequence across vertebrates suggests that basonuclin 2 serves an important function, presumably as a regulatory protein of DNA transcription.

Marker succession during the development of keratinocytes from cultured human embryonic stem cells

Human embryonic stem cells injected into scid mice produce nodules containing differentiated somatic tissues. From the trypsinized cells of such a nodule, we have recovered keratinocytes that can be grown in cell culture. The method of recovery is sensitive enough to detect small numbers of keratinocytes formed in the nodule, but for purposes of analysis, it is preferable to study the development of the entire keratinocyte lineage in culture. The principle of our analysis is the successive appearance of markers, including transcription factors with considerable specificity for the keratinocyte (p63 and basonuclin) and differentiation markers characteristic of its final state (keratin 14 and involucrin). We have determined the order of marker succession during the time- and migration-dependent development of keratinocytes from single embryoid bodies in cell culture. Of the markers we have examined, p63 was the earliest to appear in the keratinocyte lineage. The successive accumulation of later markers provides increasing certainty of emergence of the definitive keratinocyte.

Telomere-Binding TRF2/MTBP Localization during Mouse Spermatogenesis and Cell Cycle of the Mouse Cells L929

Observations of the organization and distribution of telomeres (Tel) in somatic tissues still remain controversial. The Tel topography revealed by modern microscopy shows them to be associated with the nuclear envelope (NE) in a wide variety of eukaryotic cells, although not at the Rabl orientation (peripheral position at one pole of the nucleus at prophase). We used two cell types that have different nuclear architectures. The cell line L929 shows lack of any rigid Tel architecture in the nucleus. In contrast, spermatozoa have a precise architecture established during spermiogenesis. We observed Tel and membrane Tel binding protein (MTBP/TRF2) position by immunoFISH in L929 cells and by immunofluorescence and immunogold electron microscopy, using antibodies against Membrane Tel Binding Protein (MTBP/TRF2), during different stages of spermiogenesis. At all stages of the L929 cell cycle, MTBP/TRF2 is co-localized with Tel. The only Tel order found in this cell type is similar to the Rabl-orientation, probably due to fast divisions. In the mouse pachytene spermatocytes, the membrane structures abut on the synaptonemal complex (SC) attachment sites contain MTBP/TRF2. In fully formed spermatozoa and during spermiogenesis, apart from the expected MTBP/TRF2 position at the nuclear periphery, MTBP/TRF2 unexpectedly localized at the acrosomal membrane that is adjacent to the nucleus. The difference in the MTBP/TRF2 distribution in the oocyte and spermatozoa leads to the suggestion that the MTBP/TRF2 location might reflect preparation for fertilization events. The Tel distribution is not static in cultured cells throughout the cell cycle or during spermatogenesis. When the Tel are attached to the NE, as during SC formation, MTBP/TRF2 is the member of the protein complex, which appears to be responsible for this attachment.

Non-cyclic and developmental stage-specific expression of circadian clock proteins during murine spermatogenesis

The central circadian clock in mammals is housed in the brain and is based on cyclic transcription and translation of clock proteins. How the central clock regulates peripheral organ function is unclear. However, cyclic expression of circadian genes in peripheral tissues is well established, suggesting that these tissues have their own endogenous oscillators. Reproduction is a process influenced by circadian rhythms in many organisms, thus making the testis an attractive model for studying clock function in peripheral organs. However, results addressing cyclic expression of clock genes in the mammalian testis are inconsistent. To resolve this issue, RNA was extracted from testes of mice at various times of day. Expression of the circadian clock genes mPer1, mPer2, Bmal1, Clock, mCry1, and Npas2 was constant at all times. Immunohistochemical localization of mPER1 and CLOCK proteins revealed restricted expression only in cells at specific developmental stages of spermatogenesis. For mPER1, these stages are the spermatogonia and the condensing spermatids. In contrast, CLOCK expression was restricted to round spermatids, specifically within the developing acrosome. Expression of mPER1 and CLOCK was constant at all times of day. These results suggest that clock proteins have noncircadian functions in spermatogenesis. Noncircadian expression of clock genes was also found in the thymus, which, like the testis, is composed primarily of differentiating cells. We propose that cyclic expression of clock genes is suspended during cellular differentiation.

MSJ-1, a mouse testis-specific DnaJ protein, is highly expressed in haploid male germ cells and interacts with the testis-specific heat shock protein Hsp70-2

The MSJ-1 gene encodes a murine DnaJ homologue that is expressed specifically in adult testis. DnaJ proteins act as cochaperones of Hsp70 proteins in promoting diverse cellular functions. In this study we used recombinant MSJ-1 proteins to produce MSJ-1 antiserum and to carry out in vitro binding assays. In a wide immunoscreening of mouse tissues, affinity-purified MSJ-1 antibodies recognize a unique protein of 30 kDa in male germ cells only. MSJ-1 is able to interact with the testis-specific Hsp70-2 protein and can be coimmunoprecipitated with Hsp70-2 from spermatogenic cells; binding of these two chaperones is consistent with the presence of a third component, which is so far unknown. MSJ-1 is weakly detected in early round spermatids, and its protein content increases in cytodifferentiating spermatids where it colocalizes with the developing acrosome and their postnuclear region. Hsp70-2, which is known to be highly expressed in meiotic cells, shows a subcellular localization in late differentiating spermatids that overlaps that of MSJ-1. MSJ-1 is also maintained in testicular and epididymal spermatozoa, where it sharply demarcates into two distinct cell areas; the outer surface of the acrosomal vesicle, and the centrosomal area. On the whole, our findings are consistent with a role for MSJ-1 in acrosome formation and centrosome adjustment during spermatid development, whereas its presence in mature spermatozoa suggests a special function during fertilization, shortly afterward, or both.

Function of basonuclin in increasing transcription of the ribosomal RNA genes during mouse oogenesis

Active protein synthesis during early oogenesis requires accelerated transcription of ribosomal RNA genes (rDNAs). In response to this demand, rDNAs are amplified more than 1000-fold early in Xenopus oogenesis. Here, we report evidence that rDNA is not amplified in mouse oocytes, but these cells may instead employ the zinc-finger protein basonuclin, a putative rDNA transcription factor, to enhance rRNA synthesis. This conclusion is based on observations that basonuclin is localized in the nucleolus in the mouse oocyte early in its growth phase, when rRNA transcription is highly active; and that the binding sites of basonuclin zinc fingers on the human and mouse rDNA promoters are homologous. In a co-transfection assay, basonuclin can elevate transcription from an rDNA promoter, and its zinc-finger domain can inhibit RNA polymerase I transcription, as detected by a run-on assay, in growing mouse oocytes.

The Drosophila genes disconnected and disco-related are redundant with respect to larval head development and accumulation of mRNAs from deformed target genes

HOM-C/hox genes specify body pattern by encoding regionally expressed transcription factors that activate the appropriate target genes necessary for differentiation of each body region. The current model of target gene activation suggests that interactions with cofactors influence DNA-binding ability and target gene activation by the HOM-C/hox proteins. Currently, little is known about the specifics of this process because few target genes and fewer cofactors have been identified. We undertook a deficiency screen in Drosophila melanogaster in an attempt to identify loci potentially encoding cofactors for the protein encoded by the HOM-C gene Deformed (Dfd). We identified a region of the X chromosome that, when absent, leads to loss of specific larval mouthpart structures producing a phenotype similar to that observed in Dfd mutants. The phenotype is correlated with reduced accumulation of mRNAs from Dfd target genes, though there appears to be no effect on Dfd protein accumulation. We show that these defects are due to the loss of two functionally redundant, neighboring genes encoding zinc finger transcription factors, disconnected and a gene we call disco-related. We discuss the role of these genes during differentiation of the gnathal segments and, in light of other recent findings, propose that regionally expressed zinc finger proteins may play a central role with the HOM-C proteins in establishing body pattern.

The spatial and temporal expression of Tekt1, a mouse tektin C homologue, during spermatogenesis suggest that it is involved in the development of the sperm tail basal body and axoneme

Tektins comprise a family of filament-forming proteins that are known to be coassembled with tubulins to form ciliary and flagellar microtubules. Recently we described the sequence of the first mammalian tektin protein, Tekt1 (from mouse testis), which is most homologous with sea urchin tektin C. We have now investigated the temporal and spatial expression of Tekt1 during mouse male germ cell development. By in situ hybridization analysis TEKT1 RNA expression is detected in spermatocytes and in round spermatids in the mouse testis. Immunofluorescence microscopy analysis with anti-Tekt1 antibodies showed no distinct labeling of any subcellular structure in spermatocytes, whereas in round spermatids anti-Tekt1 antibodies co-localize with anti-ANA antibodies to the centrosome. At a later stage, elongating spermatids display a larger area of anti-Tektl staining at their caudal ends; as spermiogenesis proceeds, the anti-Tekt1 staining disappears. Together with other evidence, these results provide the first intraspecies evidence that Tekt1 is transiently associated with the centrosome, and indicates that Tekt1 is one of several tektins to participate in the nucleation of the flagellar axoneme of mature spermatozoa, perhaps being required to assemble the basal body.

Centrosome reduction during Rhesus spermiogenesis: gamma-tubulin, centrin, and centriole degeneration

Centrosome reduction during spermiogenesis has been studied using anti-gamma-tubulin and anti-centrin antibodies and electron microscopy in nonhuman primates. Rhesus spermatids possess apparently normal centrosomes comprising a pair of centrioles associated with gamma-tubulin and centrin. However, they do not nucleate detectable microtubules. The spermatids discard gamma-tubulin in the residual bodies during the spermiation stage. Mature sperm do not have any detectable gamma-tubulin. About half of the centrin associated with the distal centriole degenerates during spermiogenesis and the remainder is intimately bound to the centriolar microtubules. The mature sperm possess highly degenerated distal centrioles. The centriolar microtubules degenerate in the rostral region and the ventral side of the sperm. The study indicates that the centrosome is reduced during rhesus spermiogenesis, but not completely as in mice.

XYbp, a novel RING-finger protein, is a component of the XY body of spermatocytes and centrosomes

RING-finger proteins participate in developmental processes, including gametogenesis. A fetal oocyte cDNA library was used to select genes expressed during male germ-cell differentiation. A novel RING-finger protein, XYbp (XY body protein), participating in mouse spermatogenesis has been identified. This novel gene generates a ubiquitously expressed transcript of 4.2 kb and a testis-specific one of 2.8 kb, processed by an alternative polyadenylation mechanism from a non-canonical polyadenylation signal. Transcription of XYbp is regulated during spermatocyte differentiation. The antiserum raised against the XYbp peptide demonstrated that XYbp is localised mainly in the XY bivalent of spermatocytes (XY body) and in the centrosomes of somatic and germ cells in all phases of the cell cycle. These studies indicate that we have identified a new member of the RING-finger family of proteins associated with the XY meiotic bivalent during spermatogenesis development and with the centrosomes of all cells.

Basonuclin in murine corneal and lens epithelia correlates with cellular maturation and proliferative ability

Basonuclin is a zinc finger protein with highly restricted tissue distribution. It has been found in abundance only in keratinocytes of stratified epithelia and the germ cells of the testis and ovary. We studied the expression pattern of basonuclin in relation to cellular proliferation and differentiation in murine corneal and lens epithelia, two self-renewing tissues in the eye which contain cells that proliferate throughout life. Mouse corneal and lens epithelial cells at various stages of development were labeled with BrdU for 90 min to detect cells in S phase and to establish proliferative rates. Whole eyes of mouse or rat were processed for frozen sections and cellular basonuclin was detected by either a rabbit antimouse- or a rabbit anti-human-basonuclin antibody. Basonuclin was expressed in virtually all cells in the basal layer of corneal epithelium and in the pre-equatorial lens epithelium, the respective proliferative compartments of adult corneal and lens epithelia. Basonuclin expression in corneal epithelium began at post-natal life day 4, first in a few cells and then spread to virtually all basal cells at day 20. Basonuclin was consistently absent in limbal epithelium. Lens basonuclin, which was detected earlier than that of the cornea, was confined to the pre-equatorial epithelium and was absent in equatorial cells that expressed p57KIP2, an early differentiation marker for these cells. An important distinction between corneal and lens basonuclin is that the former is predominantly nuclear whereas the latter cytoplasmic.

Association of p63 with proliferative potential in normal and neoplastic human keratinocytes

p63, a recently identified member of the p53 gene family, encodes multiple products with transactivating, death-inducing, and dominant-negative activities. We show that in normal human epidermis, in hair follicles, and in stratified epidermal cultures, p63 protein is principally restricted to cells with high proliferative potential and is absent from the cells that are undergoing terminal differentiation. In normal human epidermis and in hair follicles, basal cells with abundant p63 are interspersed with cells with little or no p63. Whenever p63 mRNA is present, it encodes mainly truncated, potentially dominant-negative isotypes. In squamous cell carcinomas, the number of cells containing p63 and their distribution depends on the degree of anaplasia. In highly differentiated tumors, p63 is confined to a ring of basal-like cells surrounding, but at a distance from, centers of terminal differentiation. In less differentiated tumors, most cells contain p63 and their distribution is chaotic with respect to centers of terminal differentiation. p63 appears to be a valuable diagnostic marker for anaplastic keratinocytes.

Basonuclin is associated with the ribosomal RNA genes on human keratinocyte mitotic chromosomes

Basonuclin is a zinc finger protein mainly expressed in keratinocytes of the basal layer of epidermis and the outer root sheath of hair follicles. It is also found in abundance in the germ cells of testis and ovary. In cultured keratinocytes, basonuclin is associated with chromatin in all phases of the cell cycle, including mitosis. By immunocytochemical methods, we demonstrate here that in mitosis basonuclin is associated with the short arms of the acrocentric chromosomes and with other loci on many metaphase chromosomes of human keratinocytes. Using the evolutionarily highly conserved N-terminal pair of zinc fingers in an electrophoresis mobility shift assay, we demonstrate that the DNA target sequences of basonuclin on the acrocentric chromosomes are likely to be within the promoter region of the 45S rRNA gene transcription unit. DNase I footprinting shows that basonuclin zinc fingers interact with the upstream control element of this promoter, which is necessary for the high level of transcription of the rRNA genes. This result suggests that basonuclin may be a tissue-specific transcription factor for the ribosomal RNA genes.

Basonuclin, a zinc finger protein of keratinocytes and reproductive germ cells, binds to the rRNA gene promoter

Basonuclin is a protein containing three pairs of C(2)H(2) zinc fingers. The protein has been found in the basal (germinal) cell layer of stratified squamous epithelia, such as the epidermis, and in germ cells of the testis and ovary. We show here that the human protein has specific affinity for a segment of the promoter of the gene for rRNA. Basonuclin interacts with two separate parts of the promoter, each possessing dyad symmetry. The upstream part, but not the downstream part, is known to bind UBF1, a transcription factor for rDNA. Basonuclin is likely to be a cell-type-specific regulatory protein for rDNA transcription.

Molecular characteristics of the centrosome

As an organizer of the microtubule cytoskeleton in animals, the centrosome has an important function. From the early light microscopic observation of the centrosome to examination by electron microscopy, the centrosome field is now in an era of molecular identification and precise functional analyses. Tables compiling centrosomal proteins and reviews on the centrosome are presented here and demonstrate how active the field is. However, despite this intense research activity, many classical questions are still unanswered. These include those regarding the precise function of centrioles, the mechanism of centrosome duplication and assembly, the origin of the centrosome, and the regulation and mechanism of the centrosomal microtubule nucleation activity. Fortunately, these questions are becoming elucidated based on experimental data discussed here. Given the fact that the centrosome is primarily a site of microtubule nucleation, special focus is placed on the process of microtubule nucleation and on the regulation of centrosomal microtubule nucleation capacity during the cell cycle and in some tissues.

Flagellar mitochondrial association of the male-specific Don Juan protein in Drosophila spermatozoa

The Drosophila don juan gene encodes a basic protein (Don Juan protein), which is solely expressed postmeiotically during spermiogenesis in elongated spermatids and in mature sperm. Transgenic expression of a GFP-tagged Don Juan protein (DJ-GFP) in the male germ line showed an association of the fusion protein with the sperm tail. Detailed examination of DJ-GFP localization revealed novel insights into its distinct temporal and spatial distribution along the sperm tail during the last phase of spermatid maturation. Co-localization of DJ-GFP with actin-labeled cysts demonstrated its emergence in elongated spermatids during individualization. Additionally, the endogenous Don Juan protein was detected with epitope-specific antibodies in finally elongated nuclei of spermatids. After completion of nuclear shaping Don Juan is no longer detectable in the sperm heads with the onset of individualization. Mislocalization of the DJ-GFP protein in flagella of a mutant with defective mitochondrial differentiation provides evidence of mitochondrial association of the fusion protein with flagellar mitochondrial arrays. Ectopically expressed DJ-GFP in premeiotic germ cells as well as salivary gland cells confirmed the capability of the fusion protein to associate with mitochondria. Therefore we suppose that Don Juan is a nuclear-encoded, germ-cell specifically expressed mitochondrial protein, which might be involved in the final steps of mitochondrial differentiation within the flagellum.

Basonuclin as a cell marker in the formation and cycling of the murine hair follicle

Basonuclin, a zinc-finger protein, is found in stratified squamous epithelia and hair follicles. In the basal keratinocytes of mouse epidermis, basonuclin is detected mainly in the cytoplasm. During the development of murine hair follicles, this protein concentrates in the nuclei of the basal cells that form the primary hair germs. As follicle morphogenesis proceeds, the epithelial cells possessing nuclear basonuclin invade the dermis and surround the follicular papilla. In mature anagen follicles, nuclear basonuclin is principally restricted to the basal layers of the outer root sheath and bulbar matrix; these regions are known to contain cells capable of proliferation, and to lack the features of terminal differentiation. During catagen, the compartment of cells containing nuclear basonuclin regresses, and in telogen, only a small number of these cells remain to form the secondary hair germ at the follicle base. During the next anagen, this basonuclin-containing population expands and regenerates the hair-producing portion of the follicle. It is concluded that in all hair cycles, the transient segment of the follicle originates from germinative cells possessing nuclear basonuclin.

Translocation of the zinc finger protein basonuclin from the mouse germ cell nucleus to the midpiece of the spermatozoon during spermiogenesis

Basonuclin was first described as a human keratinocyte zinc finger protein present in the nuclei of proliferative basal keratinocytes in the epidermis. It disappears from keratinocytes that have lost their proliferative ability and have entered terminal differentiation. We now report that basonuclin is present also in the germ cells of the mouse testis and ovary. Immunocytochemical staining detected basonuclin in the nuclei of spermatogonia and spermatocytes at various developmental stages. During spermiogenesis, it relocated from the nucleus to the midpiece of the flagellum of the spermatozoa. In the ovary, basonuclin was found mainly in the nuclei of developing oocytes. The dual presence of basonuclin in differentiated spermatozoa and oocytes suggests that it may play a role in their differentiation and the early development of an embryo.

A novel 95-kD protein is located in a linker between cytoplasmic microtubules and basal bodies in a green flagellate and forms striated filaments in vitro

The flagellar basal apparatus comprises the basal bodies and the attached fibrous structures, which together form the organizing center for the cytoskeleton in many flagellated cells. Basal apparatus were isolated from the naked green flagellate Spermatozopsis similis and shown to be composed of several dozens of different polypeptides including a protein band of 95 kD. Screening of a cDNA library of S. similis with a polyclonal antibody raised against the 95-kD band resulted in a full-length clone coding for a novel protein of 834 amino acids (90.3 kD). Sequence analysis identified nonhelical NH2- and COOH-terminal domains flanking a central domain of approximately 650 residues, which was predicted to form a series of coiled-coils interrupted by short spacer segments. Immunogold labeling using a polyclonal antibody raised against the bacterially expressed 95-kD protein exclusively decorated the striated, wedge-shaped fibers, termed sinister fibers (sf-fibers), attached to the basal bodies of S. similis. Striated fibers with a periodicity of 98 nm were assembled in vitro from the purified protein expressed from the cloned cDNA indicating that the 95-kD protein could be a major component of the sf-fibers. This structure interconnects specific triplets of the basal bodies with the microtubular bundles that emerge from the basal apparatus. The sf-fibers and similar structures, e.g., basal feet or satellites, described in various eukaryotes including vertebrates, may be representative for cytoskeletal elements involved in positioning of basal bodies/centrioles with respect to cytoskeletal microtubules and vice versa.