SummaryFluorescence loss in photobleaching experiments and analysis of mitochondrial function using superoxide and redox potential biosensors revealed that mitochondria within individual yeast cells are physically and functionally distinct. Mitochondria that are retained in mother cells during yeast cell division have a significantly more oxidizing redox potential and higher superoxide levels compared to mitochondria in buds. Retention of mitochondria with more oxidizing redox potential in mother cells occurs to the same extent in young and older cells and can account for the age-associated decline in total cellular mitochondrial redox potential in yeast as they age from 0 to 5 generations. Deletion of Mmr1p, a member of the DSL1 family of tethering proteins that localizes to mitochondria at the bud tip and is required for normal mitochondrial inheritance, produces defects in mitochondrial quality control and heterogeneity in replicative lifespan (RLS). Long-lived mmr1D cells exhibit prolonged RLS, reduced mean generation times, more reducing mitochondrial redox potential and lower mitochondrial superoxide levels compared to wild-type cells. Short-lived mmr1D cells exhibit the opposite phenotypes. Moreover, short-lived cells give rise exclusively to short-lived cells, while the majority of daughters of long-lived cells are long lived. These findings support the model that the mitochondrial inheritance machinery promotes retention of lower-functioning mitochondria in mother cells and that this process contributes to both mother-daughter age asymmetry and age-associated declines in cellular fitness.

Summary Actin cables of budding yeast are bundles of F-actin that extend from the bud tip or neck to the mother cell tip, serve as tracks for bidirectional cargo transport, and undergo continuous movement from buds towards mother cells [1]. This movement, retrograde actin cable flow (RACF), is similar to retrograde actin flow in lamellipodia, growth cones, immunological synapses, dendritic spines and filopodia [2–5]. In all cases, actin flow is driven by the push of actin polymerization and assembly at the cell cortex, and myosin-driven pulling forces deeper within the cell [6–10]. Therefore, for movement and inheritance from mothers to buds, mitochondria must “swim upstream” against the opposing force of RACF [11]. We find that increasing RACF rates results in increased fitness of mitochondria inherited by buds, and that the increase in mitochondrial fitness leads to extended replicative lifespan and increased cellular healthspan. The sirtuin SIR2 is required for normal RACF and mitochondrial fitness, and increasing RACF rates in sir2Δ cells increases mitochondrial fitness and cellular healthspan but does not affect replicative lifespan. These studies support the model that RACF serves as a filter for segregation of fit from less fit mitochondria during inheritance, which controls celllular lifespan and healthspan. They also support a role for Sir2p in these processes.

Abstract. Using fluorescent membrane potential sensing dyes to stain budding yeast, mitochondria are resolved as tubular organeUes aligned in radial arrays that converge at the bud neck. Time-lapse fluorescence microscopy reveals region-specific, directed mitochondrial movement during polarized yeast cell growth and mitotic cell division. Mitochondria in the central region of the mother cell move linearly towards the bud, traverse the bud neck, and progress towards the bud tip at an average velocity of 49 _ 21 nm/sec. In contrast, mitochondria in the peripheral region of the mother cell and at the bud tip display significantly less movement. Yeast strains containing temperature sensitive lethal mutations in the actin gene show abnormal mitochondrial distribution. No mitochondrial movement is evident in these mutants after short-term shift to semipermissive temperatures. Thus, the actin cytoskeleton is important for normal mitochondrial movement during inheritance. To determine the possible role of known myosin genes in yeast mitochondrial motility, we investigated mitochondrial inheritance in myol, rnyo2, myo3 and myo4 single mutants and in a myo2, myo4 double mutant. Mitochondrial spatial arrangement and motility are not significantly affected by these mutations. We used a microfilament sliding assay to examine motor activity on isolated yeast mitochondria. Rhodamine-phalloidin labeled yeast actin filaments bind to immobilized yeast mitochondria, as well as unilamellar, right-side-out, sealed mitochondrial outer membrane vesicles. In the presence of low levels of ATP (0.1-100 ~M), we observe F-actin sliding on immobilized yeast mitochondria. In the presence of high levels of ATP (500 ~M-2 mM), bound filaments are released from mitochondria and mitochondrial outer membranes. The maximum velocity of mitochondriadriven microfilament sliding (23 + 11 nm/sec) is similar to that of mitochondrial movement in living cells. This motor activity requires hydrolysis of ATP, does not require cytosolic extracts, is sensitive to protease treatment, and displays an ATP concentration dependence similar to that of members of the myosin family of actin-based motors. This is the first demonstration of an actin-based motor activity in a defined organelle population. MITOCHONDRIAL inheritance is the process whereby mitochondria are transferred from mother cells to developing daughter cells during cell division.In the budding yeast, Saccharomyces cerevisiae, this process begins at the G1/S boundary of the cell division cycle (Stevens, 1981). Since mitochondria are produced only from preexisting mitochondria (Criddle and Schatz, 1969;Schatz and Saltzgaber, 1969), this inheritance process ensures that each daughter cell contains this indispensable organelle. We used budding yeast to study the dynamics of mitochondrial movements leading to inheritance, and the organelle-cytoskeletal interactions that control these movements.Cell division in S. cerevisiae proceeds by asymmetric growth of the developing daughter cell. In yeast and otherPl...

Mitochondria accumulate at neuronal and immunological synapses [1, 2] and yeast bud tips [3], and associate with the ER during phospholipid biosynthesis, calcium homeostasis and mitochondrial fission [4, 5]. We show that mitochondria are associated with cortical ER (cER) sheets [6, 7] underlying the plasma membrane in the bud tip, and confirmed that a deletion in YPT11, which inhibits cER accumulation in the bud tip [8], also inhibits bud tip anchorage of mitochondria [9]. Time-lapse imaging reveals that mitochondria are anchored at specific sites in the bud tip. Mmr1p, a member of the DSL1 family of tethering proteins, localizes to punctate structures on opposing surfaces of mitochondria and cER sheets underlying the bud tip, and is recovered with isolated mitochondria and ER. Deletion of MMR1 impairs bud tip anchorage of mitochondria, without affecting mitochondrial velocity or cER distribution. Deletion of the phosphatase PTC1 results in increased Mmr1p phosphorylation, mislocalization of Mmr1p, defects in association of Mmr1p with mitochondria and ER, and defects in bud tip anchorage of mitochondria. These findings indicate that Mmr1p contributes to mitochondrial inheritance as a mediator of anchorage of mitochondria to cER sheets in the yeast bud tip, and that Ptc1p regulates Mmr1p phosphorylation, localization and function.

SUMMARY Myeloid-biased hematopoietic stem cells (MB-HSCs) play critical roles in recovery from injury, but little is known about how they are regulated within the bone marrow niche. Here, we describe an auto/paracrine physiologic circuit that controls quiescence of MB-HSCs and hematopoietic progenitors marked by histidine decarboxylase (Hdc). Committed Hdc+ myeloid cells lie in close anatomical proximity to MB-HSCs and produce histamine, which activates the H2 receptor on MB-HSCs to promote their quiescence and self-renewal. Depleting histamine-producing cells enforces cell cycle entry, induces loss of serial transplant capacity, and sensitizes animals to chemotherapeutic injury. Increasing demand for myeloid cells via LPS treatment specifically recruits MB-HSCs and progenitors into the cell cycle; cycling MB-HSCs fail to revert into quiescence in the absence of histamine feedback, leading to their depletion, while an H2 agonist protects MB-HSCs from depletion after sepsis. Thus, histamine couples lineage-specific physiological demands to intrinsically-primed MB-HSCs to enforce homeostasis.

We propose a new approach to lung regeneration by replacement of damaged epithelium with full preservation of lung vasculature.

SUMMARY The urothelium is an epithelia barrier lined by a luminal layer of binucleated, octoploid, superficial cells. Superficial cells are critical for production and transport of uroplakins, a family of proteins that assemble into a waterproof crystalline plaque that helps protect against infection and toxic substances. Adult urothelium is nearly quiescent, but rapidly regenerates in response to injury. Yet the mechanism by which binucleated, polyploid, superficial cells are produced remains unclear. Here, we show that superficial cells are likely to be derived from a population of binucleated intermediate cells, which are produced from mononucleated intermediate cells via incomplete cytokinesis. We show that binucleated intermediate and superficial cells increase DNA content via endoreplication, passing through S phase without entering mitosis. The urothelium can be permanently damaged by repetitive or chronic injury or disease. Identification of the mechanism by which superficial cells are produced may be important for developing strategies for urothelial repair.

Mitochondria are indispensable for normal eukaryotic cell function. As they cannot be synthesized de novo and are self-replicating, mitochondria must be transferred from mother to daughter cells. Studies in the budding yeast Saccharomyces cerevisiae indicate that mitochondria enter the bud immediately after bud emergence, interact with the actin cytoskeleton for linear, polarized movement of mitochondria from mother to bud, but are equally distributed among mother and daughter cells [1] [2] [3]. It is not clear how the mother cell maintains its own supply of mitochondria. Here, we found that mother cells retain mitochondria by immobilization of some mitochondria in the 'retention zone', the base of the mother cell distal to the bud. Retention requires the actin cytoskeleton as mitochondria colocalized with actin cables in the retention zone, and mutations that perturb actin dynamics or actin-mitochondrial interactions produced retention defects. Our results support the model that equal distribution of mitochondria during cell division is a consequence of two actin-dependent processes: movement of some mitochondria into the daughter bud and immobilization of others in the mother cell.