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From CellBiology
Diagram showing major elements of the SC: SYCP2/SYCP3/STAG3 Axial Element; SYCP1 Transverse Filament; SYCE2/TEX12 Cental Element. Ref: Costa & Cooke Chromosome Research (2007) 15:579-589.


SYNAPTONEMAL COMPLEX PROTEIN 3 - SYCP3

Synaptonemal Complex

The synaptonemal complex is a ladder-like protein complex consisting of three distinct components – axial elements (lateral elements), central elements and transverse filaments – that forms between and then holds together homologous chromosomes during the first prophase of meiosis [1].


The synaptonemal complex is a structure unique to maturing germ cells undergoing meiosis and is critically involved in the synapsis, recombination and segregation of homologous chromosomes [2]. The synaptonemal complex was discovered 50 years ago and has been used since as a chronometer for the progression of the long and intricate prophase I with the sub-stages - Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis corresponding to the stages of assembly and disassembly of the synaptonemal complex [3].


Ref: Fuente et al PLoS Genetics (2007) 3(11):2122-2134.Image: Immunolabelling of sprematocytes with Anti-SYCP3 (Green) and Anti-Centromere (Red) Antibodies:(A.) Leptotene: Axial elements appear as thin discrete threads(B.) Zygotene: Axial elements are thicker where synapse is proceeding(C.) Pachytene: Autosomes have completed synapse ans synaptonemal complexes at full length(D.) Diplotene: Synaptonemal complexes begin to disassemble

However, it is only in the last 15 years that major technical developments have allowed understanding as to how it is assembled and disassembled during meiotic prophase and identification of the key molecular players in the process. Mouse genetic approaches, initially as a spin off from cancer genetic research, have yielded the role of mismatch repair genes Mlh1 and Pms2 in meiotic recombination and led to the generation of a range of meiotic knock-out mice including SYCP1, SYCP2, SYCP3 and meiotic cohesins SM1beta and REC8 [4]. This, along with antibody-based immuno-localisation techniques have added greatly to the elucidation of SYCP3 and its relevance to human infertility and aneupliody[5].

References

  1. Costa and Cooke Chromosome Research (2007) 15:579-589
  2. Baier et al Biochem Biophys Acta. 2007 May: 1774(5)
  3. Lammars et al Molecular and Cellular Biology, Feb 1994:1137-1146
  4. Vallente et al Chromosoma (2006) 115: 214-249 lente et al 2006
  5. Vallente et al Chromosoma (2006) 115: 214-249 2006

Function of SYCP3

SYCP3 is a major structural protein of the synaptonemal complex in mammals, which along with SYCP2 forms the axial element. It is required in the assembly of synaptonemal complex and is therefore necessary for: structural integrity of meiotic chromosomes; homologous chromosome synapse; and chiasma formation [1]. The axial element also participates in the organization of the chromatin both directly and via the function of other intimately connected proteins: recombination proteins Rad51, SYCP1; and Replication Protein A (RPA). In the absence of SYPC3, Rad 51 becomes less organized, SYCP1 fibers are fragmented and RPA fails to localize as it should on SYCP1 [2].


Without this structural support, studies with Sycp3 knockout female mice indicate that correct disjunction of chromosomes is disturbed [3]. As well, its expression is required for testis development in adult males and for fertility. In male SYCP3 knockout mice the testes are about 40% smaller than wild type and no post meiotic cells survive. Disruption of the spermatogenesis occurs at the Zygotene stage. It does not affect ovarian morphology or follicular development and it has no role in embryo development or in somatic cell division [4].
Ref: Vallente et al Chromosoma (2006) 115: 214-249. Image: Visualization of the prophase sub stages in mouse oocyte using immuno-flourescence staining for SYCP3: a. Leptotene, b. Zygotene, c. Pachytene


As is apparent in the image to the right, at the Leptotene phase, SYCP3 is well distributed along short linear segments which form along each chromosome with chromatin loops extending from the protein backbone. In Zygotene, the two AEs are at full length and are brought together by the transverse filaments which are composed of SYCP1. At Pachytene, the chromosomes are completely synapsed and the 23 centromeres are visible in humans. In the Diplotene phase, the homologous chromosomes move apart except at the points of recombination, the chiasma, and the SC disassembles.


The key events of meiosis – homologous recognition, synapsis, recombination and segregation - and the proteins associated with these are intimately connected and almost always occur in an ordered fashion starting with the repair of programmed DNA double strand breaks (DSBs) made by the SPO11 protein at the beginning of prophase. Other double stranded break proteins, RAD51, MLH1 and gamma-H2AX along with the cohesin proteins, REC8 and STAG3, involved in the cohesion of sister chromatids are also implicated in the functioning of SYCP3 though not necessarily directly. It appears that REC8 and STAG3 are not required for axial element formation but are required for the completion of synapse and may therefore have a greater implication for SYCP1 [5]

References

  1. Yuan et al Molecular Cell, 5:73-83, 2000
  2. Yuan et al Molecular Cell, 5:73-83, 2000
  3. Yuan et al Science, May 2002:1115-1123
  4. Yuan et al Science, May 2002:1115-1123
  5. Costa and Cooke Chromosome Research (2007) 15:579-589

Expression of SYCP3

Lammars et al cloned and sequenced SYCP3 from the testes of the male rat in 1994. SYCP3 is a small protein with a molecular mass of around 30kDa and is expressed only in meiotic germ cells. It has a C-terminal coiled-coil domain, nucleotide binding motifs and has two c-AMP-cGMP – dependent protein kinase target sites which function in the regulation of assembly and disassembly of the axial elements of the synaptonemal complex. SYCP3 appears to be only expressed in the prophase of meiosis in the testes of male mice within the synaptonemal complex and in small cytoplasmic aggregations before Diplotene and after Diakinesis but, it does not appear to be responsible for the maintenance of cohesiveness in Diakinesis and metaphase [1] [2] although there is some variation between species. In some species synaptonemal complexes are not formed at all – Drosophila males, grasshopper females and fission yeast, and, in some mammals, the X and Y chromosomes do not form functional synaptonemal complexes. Marsupials and in some species of gerbils the X and Y do not synapse or recombine but the axial elements are formed and SYCP3 is expressed throughout meiosis 1 maintaining association between the sex chromosomes until the first round of cell division [3].


When complete synapsis between non-sister homologues occurs at the end of Leptotene, the axial elements are termed lateral elements and the transverse filament protein SCP1 is expressed [4]. Both sexes express SYCP3 throughout the five sub stages of prophase and it is present in aggregates in the nucleoplasm of all meiotic cells. Gender specific temporal patterns of protein expression were not apparent in SYCP3, however, the implications of a deletion in the Sycp3 gene are gender specific [5].

Diagram of the male/female protein expression Ref: Vallente et al Chromosoma (2006) 115:241-249

References

  1. Lammars et al Molecular & Cellulear Biology, Feb 1994:1137-1146
  2. Dietrich et al Genome 35:492-497 1992
  3. Fuente et al, PLoS 3:11: 2122-2134, 2007
  4. Costa & Cook Chromosome Research (2007) 15:579-589
  5. Vallente et al Chromosoma (2006) 115: 214-249 2006


Regulation of SYCP3

Regulation of the assembly and disassembly of the synaptonemal complex is still poorly understood [1] but it does appear that DAZ related gene proteins are required for the efficient translation of the Sycp3 mRNA in vivo. Dazl knockout mice have markedly reduced levels of SYCP3 protein [2]. But it is not sufficient for SYCP3(Sycp3)initiation. The protein SPO11 that occurs at the very beginning of the first meiotic prophase and initiates of recombination events in the repair of programmed double stranded breaks (DSBs) is also required and appears to be implicated in the cyclin dependent kinase activity required for the phosphorylation of SYCP3 and hence its activation [3]; [4]. Other double stranded break associated proteins, RAD51 and histone gamma-H2AX occur in advance of SYCP3, with Gamma – H2AX being maximally expressed at late Zygotene, when the axial elements of the synaptonemal complex are at full length and the synapse of homologous chromosomes complete [5]. The nature of the relationship between SYCP3 and the double stranded break proteins is unclear but their co-expression has been demonstrated.


Image: Co-expression of sycp3-gammaH2AX

SYCP3 polymerization and de-polymerization may also hold answers as to the process of assembly and disassembly of the synaptonemal complex. It appears that the alpha –helix structure together with the two flanking motifs CM1 and CM2 are necessary and sufficient for the polymerization of the SYCP3 into the axial element of the synaptonemal complex. Deletion leads to interruption in the synaptonemal complex formation and infertility in humans [6]. Truncation of the Sycp2 gene in which the required DNA binding motif and the coiled coil domain are deleted produces a similar effect, and the SYCP3 proteins do not associate and polymerise, the synaptonemal complex fails to form resulting in male infertility and reduced litter sizes for female mice [7].

Disassembly of the synaptonemal complex and an end to the expression of SYCP3 appears to be controlled by a global regulating factor such as Okadaic Acid, inhibiting all protein phosphatase activity and, enzymatically via small molecule inhibitors of CDK’s and aurora kinases: Butyrolactone I (BLI) and ZM447439 (ZM) respectively [8].

References

  1. Sun & Handel Chromosoma 117(5):471-85 2008
  2. Reynolds et al RNA 13(7): 974-981
  3. Fuente et al, PLoS 3:11: 2122-2134, 2007
  4. Sun & Handel Chromosoma 117(5):471-85 2008
  5. Vallente et al Chromosoma 115:241-249 (2006)
  6. Baier et al Sex Dev. 2007; 1(3):161-8
  7. Costa & Cook Chromosome Research (2007) 15:579-589
  8. Sun & Handel Chromosoma 117(5):471-85 2008


Allelic variations in Humans

Mutations in the Sycp3 gene have been linked to miscarriage in humans and male infertility. Aneuploidy, either trisomy or monosomy, is the leading cause of human miscarriage accounting for the loss of around 25% of conceptions and is usually due to aberrant female chromosome segregation, with increasing age playing a role in the process. Female SYCP3 knockout mice, are fertile and give birth to healthy young, however, the litter size is reduced due to a dramatic increase in embryonic death in uterus. The reason for the embryonic death is the irregular karyotype caused by the reduced chiasma formation during the Pachytene stage of prophase. One role of SYCP3 is to establish chiasmata, and where the protein is absent, less chiasma form, putting short chromosomes at particular risk of forming a univalent and should this occur, an embryo may develop that is chromosomally deficient. In humans, mutations in the Sycp3 gene, mapped to chromosome 12, occur and have been linked to pregnancy loss in a few cases. Hetrozygosity for a defective copy of the gene may be sufficient to cause miscarriage, perhaps due to chromosomal abnormality, with recombinant mutant SYCP3 showing reduced interaction with wild type thus reducing the polymerization and lateral element formation [1].


References

  1. Bolor et al Am. Journal of Human Genetics 84:1 14-22 2009