SKAP, an outer kinetochore protein, is required for mouse germ cell development

Abstract

In sexually reproducing organisms, accurate gametogenesis is crucial for the transmission of genetic material from one generation to the next. This requires the faithful segregation of chromosomes during mitotic and meiotic divisions. One of the main players in this process is the kinetochore, a large multi-protein complex that forms at the interface of centromeres and microtubules. Here, we analyzed the expression profile and function of small kinetochore-associated protein (SKAP) in the mouse. We found that two distinct SKAP isoforms are specifically expressed in the germline: a smaller isoform, which is detected in spermatogonia and spermatocytes and localized in the outer mitotic and meiotic kinetochores from metaphase to telophase, and a larger isoform, which is expressed in the cytoplasm of elongating spermatids. We generated SKAP-deficient mice and found that testis size and sperm production were severely reduced in mutant males. This phenotype was partially caused by defects during spermatogonia proliferation before entry into meiosis. We conclude that mouse SKAP, while being dispensable for somatic cell divisions, has an important role in the successful outcome of male gametogenesis. In germ cells, analogous to what has been suggested in studies using immortalized cells, SKAP most likely stabilizes the interaction between kinetochores and microtubules, where it might be needed as an extra safeguard to ensure the correct segregation of mitotic and meiotic chromosomes.

A mitotic SKAP isoform regulates spindle positioning at astral microtubule plus ends

“…We did not detect spindle localization of the testis-specific SKAP isoform in mitotic cells of the testis, attesting to both the timing of expression and the specificity of the antibody. We also note that recent work found that both the short and long SKAP isoforms detected by Western blotting and by immunostaining using the antibody we generated were abolished in SKAP-knockout mice, and SKAP-knockout mice displayed spermatogenesis defects ( Grey et al, 2016 ). Thus, SKAP is expressed as two isoforms with distinct transcriptional start sites, resulting in a shorter mitotic form (which we will refer to as “short SKAP”) and a longer testis-specific form (which we will refer to as “long SKAP”).…”

Aurora-B kinase pathway controls the lateral to end-on conversion of kinetochore-microtubule attachments in human cells

“…Future structure studies on Astrin’s hinge domain and the kinetochore receptor(s) of Astrin-SKAP complex are likely to reveal crucial insights into how human cells overcome physical barriers in converting the plane of kinetochore-microtubule attachment. Although Astrin-SKAP complex is essential for normal chromosome segregation in human cells 41 , 55 – 58 , a truncation mutation in exon-3 of SKAP did not cause somatic defects in mice 59 , suggesting that redundant players may support the loss of SKAP in mice.…”

Astrin-SKAP complex reconstitution reveals its kinetochore interaction with microtubule-bound Ndc80

“…The Ndc80 complex is the core microtubule binding unit of the kinetochore and is widely conserved across eukaryotes. Intriguingly, although the Astrin-SKAP complex is conserved throughout vertebrates, SKAP and Astrin knockouts in mice and rats have shown that this complex is dispensable for viability in rodents ( Grey et al, 2016 ; Xue et al, 2002 ). Given the Astrin-SKAP complex’s established role in human cells, it is possible that the requirements for the Astrin-SKAP complex may be an important mitotic distinction between rodent and human kinetochores ( Akhmanova and van den Heuvel, 2016 ).…”