CENP-C functions in centromere assembly, the maintenance of CENP-A asymmetry and epigenetic age in Drosophila germline stem cells

Abstract

Germline stem cells divide asymmetrically to produce one new daughter stem cell and one daughter cell that will subsequently undergo meiosis and differentiate to generate the mature gamete. The silent sister hypothesis proposes that in asymmetric divisions, the selective inheritance of sister chromatids carrying specific epigenetic marks between stem and daughter cells impacts cell fate. To facilitate this selective inheritance, the hypothesis specifically proposes that the centromeric region of each sister chromatid is distinct. In Drosophila germ line stem cells (GSCs), it has recently been shown that the centromeric histone CENP-A (called CID in flies)—the epigenetic determinant of centromere identity—is asymmetrically distributed between sister chromatids. In these cells, CID deposition occurs in G2 phase such that sister chromatids destined to end up in the stem cell harbour more CENP-A, assemble more kinetochore proteins and capture more spindle microtubules. These results suggest a potential mechanism of ‘mitotic drive’ that might bias chromosome segregation. Here we report that the inner kinetochore protein CENP-C, is required for the assembly of CID in G2 phase in GSCs. Moreover, CENP-C is required to maintain a normal asymmetric distribution of CID between stem and daughter cells. In addition, we find that CID is lost from centromeres in aged GSCs and that a reduction in CENP-C accelerates this loss. Finally, we show that CENP-C depletion in GSCs disrupts the balance of stem and daughter cells in the ovary, shifting GSCs toward a self-renewal tendency. Ultimately, we provide evidence that centromere assembly and maintenance via CENP-C is required to sustain asymmetric divisions in female Drosophila GSCs.

Centromere function in asymmetric cell division in Drosophila female and male germline stem cells

“…Consistent with findings in females, an asymmetric distribution of CID was detected in Drosophila male GSCs at pro-metaphase with an approximate 50% enrichment of CID on sister chromatids that end up in the stem cell versus the daughter cell [ 16 ]. Furthermore, an asymmetric distribution for CENP-C was also detected in female GSCs [ 17 , 23 ], as well as the outer kinetochore protein Ndc80 in males [ 16 ] ( figure 3 ). Taken together, these studies propose that enhanced centromere, kinetochore and microtubule strength mediate non-random sister chromatid segregation in GSCs [ 16 , 17 ].…”

“…But if and how this impacts on cell fate remains enigmatic. Functional studies, again in Drosophila female and male GSCs, have recently attempted to shed light on this [ 16 , 17 , 23 ]. In female GSCs, CID and CAL1 knockdown resulted in germaria with very few or no germ cells [ 17 ].…”

“…This result is perhaps not surprising, given the essential function of both genes in cell division. Partial depletion of CENP-C (an approximate 60% reduction of CENP-C in GSCs) was however possible in females, which allowed phenotypic analysis [ 23 ]. In this case, a reduction in CENP-C level enhanced CID asymmetry such that even more CID is retained by the GSC.…”

“…This result suggested a shift towards GSC self-renewal upon disrupted centromere asymmetry. Interestingly, CENP-C knockdown after 10 days resulted in germaria that had lost the GSC pool, implicating CENP-C in GSC long-term maintenance [ 23 ]. Therefore, it appears that a reduction in CENP-C initially leads to an increase in GSC self-renewal, yet this GSC pool is finite and becomes exhausted over time.…”

“…Taken together, CAL1 depletion in males resulted in failed GSC self-renewal, while CAL1 depletion in females blocked GSCs differentiation or division. By contrast, depletion of CENP-C in females resulted in more GSC self-renewal, eventually leading to a depletion of the GSC pool over time [ 23 ]. Ultimately, knockdown of CAL1 or CENP-C in GSCs reduces the stem cell pool in either testes or ovaries [ 16 , 23 ].…”