Centrosome and microtubule dynamics during early stages of meiosis in mouse oocytes

Abstract

Centrosomes, major regulatory sites for the microtubule (MT) nucleation, are regulated in a dynamic manner throughout the process of meiotic maturation. Recently, centrosome orientation in mouse oocytes has been demonstrated in metaphase I through metaphase II. However, centrosomal protein expression in concordance with MT polymerization in earlier stages of oocyte maturation from germinal vesicle stage (GV) to prometaphase I still remains unclear. The present study aims to assess the centrosome-microtubule remodelling during the onset of meiosis based on strict criteria of nuclear maturation. Six consecutive stages were determined for scoring the oocytes as unrimmed nucleolus (UR), partially rimmed nucleolus (PR), fully rimmed nucleolus (FR), nuclear lamina dissolution (NLD), disappearance of nucleolus (DON), and chromatin condensation (CC). A centrosomal protein, pericentrin, was found tightly localized adjacent to nuclear lamina in UR, lacking any MT nucleation activity. In concordance with the competency to resume meiosis, an increase in the amount and nucleation capacity of pericentrin is noted. In FR, cytoplasmic MT almost disappeared while de-novo microtubule polymerization was found in small aggregates of pericentrin localized around the nucleus. Towards the end of DON and CC, a sudden burst of pericentrin was noted with an extreme MT nucleation activity in an organized fashion that is essential for the rapid formation of first meiotic spindle. The results show that centrosomes display precisely controlled spatio-temporal changes during the onset of meiotic maturation. Accumulation of centrosomal proteins to a single locus followed by a sequestration to several spots might be evidence of a mechanism by which the proper distribution of centrosomal material during nuclear breakdown and subsequently formation of spindle are regulated in concordance with the nuclear maturation.

The kinase VRK1 is required for normal meiotic progression in mammalian oogenesis

“…6A). We initially detected similar numbers of rimmed and condensed oocytes, as described in (Can et al, 2003) from both Vrk1 +/+ and Vrk1 Gt/Gt mice. Subsequently, significantly fewer oocytes from Vrk1 Gt/Gt females reached the ''parachute'' stage, a point at the completion of GVDB during which bivalents clump together prior to entrance into prometaphase I as described in (Debey et al, 1993), within 2.5 h, compared to Vrk1 +/+ females (15.6% versus 49.1% respectively, p < 0.0005).…”

Rebuilding MTOCs upon centriole loss during mouse oogenesis

“…The origin of these acentriolar MTOCs remains controversial. Initially thought to be present and active in prophase arrested oocytes (Mattson and Albertini, 1990;Can et al, 2003), they have been more recently proposed to form at the onset of the first meiotic division only, either through microtubule selforganization mechanisms (Schuh and Ellenberg, 2007), or via the activation of large and quiescent structures assembled in prophase-arrested oocytes (Calarco, 2000).…”

The position of the germinal vesicle and the chromatin organization together provide a marker of the developmental competence of mouse antral oocytes

“…For example, the heterochromatin surrounding the nucleolus in mouse SN oocytes is mainly AT-rich satellite-DNA of centromeric origin and involves most of the centromeres, whereas in NSN oocytes only the centromeres carrying the nucleolar organizing regions are associated with the nucleolus (Longo et al 2003). Microtubule organizing centers form around the nucleus of SN oocytes, but not around that of NSN oocytes (Wickramasinghe & Albertini 1992, Can et al 2003. In addition, the nucleoli of NSN oocytes are vacuolated and less compact than those of SN oocytes (Debey et al 1993).…”