.
S O U R C E : eLIFE
Abstract
Production of healthy gametes in meiosis relies on the quality control and proper distribution of both nuclear and cytoplasmic contents. Meiotic differentiation naturally eliminates age-induced cellular damage by an unknown mechanism. Using time-lapse fluorescence microscopy in budding yeast, we found that nuclear senescence factors – including protein aggregates, extrachromosomal ribosomal DNA circles, and abnormal nucleolar material – are sequestered away from chromosomes during meiosis II and subsequently eliminated. A similar sequestration and elimination process occurs for the core subunits of the nuclear pore complex in both young and aged cells. Nuclear envelope remodeling drives the formation of a membranous compartment containing the sequestered material. Importantly, de novo generation of plasma membrane is required for the sequestration event, preventing the inheritance of long-lived nucleoporins and senescence factors into the newly formed gametes. Our study uncovers a new mechanism of nuclear quality control and provides insight into its function in meiotic cellular rejuvenation.
Aging occurs as an organism loses its ability to maintain homeostasis over time. The cellular changes that accompany aging have been most extensively characterized in the budding yeast, Saccharomyces cerevisiae (Figure 1A; Denoth Lippuner et al., 2014; Kaeberlein, 2010; Longo et al., 2012). Disrupted protein homeostasis results in the accumulation of protein aggregates that contain oxidatively damaged proteins (Aguilaniu et al., 2003; Erjavec et al., 2007). Many organelles exhibit signs of dysfunction: mitochondria fragment and aggregate, mitochondrial membrane potential decreases, and the vacuole becomes less acidic (Henderson et al., 2014; Hughes and Gottschling, 2012; Veatch et al., 2009). Notably, the nucleus also undergoes a number of changes including enlargement of the nucleolus (Lewinska et al., 2014; Morlot et al., 2019; Sinclair et al., 1997), misorganization of nuclear pore complexes (Lord et al., 2015; Rempel et al., 2019), and accumulation of extrachromosomal ribosomal DNA (rDNA) circles (Denoth-Lippuner et al., 2014; Sinclair and Guarente, 1997). Many of the cellular changes that accrue with age are conserved across eukaryotes (Colacurcio and Nixon, 2016; David et al., 2010; Sun et al., 2016; Tiku et al., 2017).
Figure 1 with 2 supplements
(A) Schematic depiction of a young and aged budding yeast cell. (B) (left panel) Montage of a young cell (one generation old) with diffuse Hsp104-eGFP progressing through meiosis (UB9724). (right … see more
Numerical values corresponding to the graph in Figure 1D.
Figure 1—source data 2Numerical values corresponding to the graph in Figure 1E.
https://doi.org/10.7...eLife.47156.007 https://doi.org/10.7...eLife.47156.008
In budding yeast mitosis, age-induced damage is asymmetrically retained by the mother cell resulting in the formation of an aged mother cell and a young daughter cell (Mortimer and Johnston, 1959). In contrast, meiotic cells reset aging symmetrically such that all of the meiotic products are born young, independent of their progenitor’s age (Unal et al., 2011). Importantly, senescence factors originally present in the aged precursor cells, including protein aggregates, nucleolar damage, and rDNA circles, are no longer present in the newly formed gametes (Ünal and Amon, 2011; Unal et al., 2011). How gametes avoid inheriting age-associated damage and how this event is coupled to the meiotic differentiation program remains unknown.
Meiotic differentiation, also known as gametogenesis, is a tightly regulated developmental program whereby a progenitor cell undergoes two consecutive nuclear divisions, meiosis I and meiosis II, to form haploid gametes. Meiotic differentiation requires extensive cellular remodeling to ensure that gametes inherit the necessary nuclear and cytoplasmic contents. In yeast gametogenesis, the nucleus undergoes a closed division, with the nuclear envelope remaining continuous until karyokinesis forms four new nuclei (Moens, 1971; Moens and Rapport, 1971; Neiman, 2011). Mitochondria and cortical endoplasmic reticulum also undergo regulated morphological changes, separating from the cellular cortex and localizing near the nuclear envelope at the transition between meiosis I and II (Gorsich and Shaw, 2004; Miyakawa et al., 1984; Sawyer et al., 2019; Stevens, 1981; Suda et al., 2007). Around the same time, new plasma membranes, also known as prospore membranes, grow from the centrosome-like spindle pole bodies embedded in the nuclear envelope. This directed growth of plasma membrane ensures that nascent nuclei and a fraction of the cytoplasmic contents are encapsulated to form gametes (Brewer et al., 1980; Byers, 1981; Knop and Strasser, 2000; Moens, 1971; Neiman, 1998). Subsequently, the uninherited cellular contents are destroyed by proteases released upon permeabilization of the progenitor cell’s vacuole, the yeast equivalent of the mammalian lysosome (Eastwood et al., 2012; Eastwood and Meneghini, 2015). Whether these cellular remodeling events are integral to the removal of age-induced damage has not been characterized.
In this study, we aimed to determine the mechanism by which nuclear senescence factors are eliminated during budding yeast meiosis. Using time-lapse fluorescence microscopy, we found that protein aggregates, rDNA circles, and a subset of nucleolar proteins are sequestered away from chromosomes during meiosis II. Importantly, we show that the core subunits of the nuclear pore complex (NPC) also undergo a similar sequestration process in both young and aged cells. The damaged material localizes to a nuclear envelope-bound compartment containing the excluded NPCs that is eliminated upon vacuolar lysis. Finally, we found that the proper development of plasma membranes is required for the sequestration of core NPCs and senescence factors away from the newly forming gametes. Our study defines a key nuclear remodeling event and demonstrates its involvement in the elimination of age-induced cellular damage during meiotic differentiation.
.../...
F O R T H E R E S T O F T H E S T U D Y , P L E A S E V I S I T T H E S O U R C E
.
