Mechanism for Anaphase B: Evaluation of “Slide-and-Cluster” versus “Slide-and-Flux-or-Elongate” Models

Elongation of the mitotic spindle during anaphase B contributes to chromosome segregation in many cells. Here, we quantitatively test the ability of two models for spindle length control to describe the dynamics of anaphase B spindle elongation using experimental data from Drosophila embryos. In the slide-and-flux-or-elongate (SAFE) model, kinesin-5 motors persistently slide apart antiparallel interpolar microtubules (ipMTs). During pre-anaphase B, this outward sliding of ipMTs is balanced by depolymerization of their minus ends at the poles, producing poleward flux, while the spindle maintains a constant length. Following cyclin B degradation, ipMT depolymerization ceases so the sliding ipMTs can push the poles apart. The competing slide-and-cluster (SAC) model proposes that MTs nucleated at the equator are slid outward by the cooperative actions of the bipolar kinesin-5 and a minus-end-directed motor, which then pulls the sliding MTs inward and clusters them at the poles. In assessing both models, we assume that kinesin-5 preferentially cross-links and slides apart antiparallel MTs while the MT plus ends exhibit dynamic instability. However, in the SAC model, minus-end-directed motors bind the minus ends of MTs as cargo and transport them poleward along adjacent, parallel MT tracks, whereas in the SAFE model, all MT minus ends that reach the pole are depolymerized by kinesin-13. Remarkably, the results show that within a narrow range of MT dynamic instability parameters, both models can reproduce the steady-state length and dynamics of pre-anaphase B spindles and the rate of anaphase B spindle elongation. However, only the SAFE model reproduces the change in MT dynamics observed experimentally at anaphase B onset. Thus, although both models explain many features of anaphase B in this system, our quantitative evaluation of experimental data regarding several different aspects of spindle dynamics suggests that the SAFE model provides a better fit.     

Moralizing regulation: the implications of policing “good” versus “bad” immigrants

Recently, the US has dramatically expanded immigration enforcement. At the same time, some advocates have sought to support “good” immigrants. This paper considers how the resulting good/bad binaries affect undocumented immigrants. I examine a case study in Los Angeles, where policing intertwined with protection. Based on participant observation and interviews, I show that respondents believed state agents classified them either as “bad” criminals or “good”, immigrants. To the extent immigrants identified as “good”, they credited the US with offering them “freedom” and hoped for political inclusion. At the same time, in what I call moralizing regulation, they also performed “good” behaviour and distinguished themselves from those seen as “bad”. Some also tied “good” behaviour to femininity and “acting white”. At the extreme, they blamed other migrants for inviting state mistreatment. The effects were ambivalent: while immigrants appreciated US support, they also adopted and adapted to the state’s moral norms.     

“Self” versus “Non-Self” Connectivity Dictates Properties of Synaptic Transmission and Plasticity

Autapses are connections between a neuron and itself. These connections are morphologically similar to “normal” synapses between two different neurons, and thus were long thought to have similar properties of synaptic transmission. However, this has not been directly tested. Here, using a micro-island culture assay in which we can define the number of interconnected cells, we directly compared synaptic transmission in excitatory autapses and in two-neuron micronetworks consisting of two excitatory neurons, in which a neuron is connected to one other neuron and to itself. We discovered that autaptic synapses are optimized for maximal transmission, and exhibited enhanced EPSC amplitude, charge, and RRP size compared to interneuronal synapses. However, autapses are deficient in several aspects of synaptic plasticity. Short-term potentiation only became apparent when a neuron was connected to another neuron. This acquisition of plasticity only required reciprocal innervation with one other neuron; micronetworks consisting of just two interconnected neurons exhibited enhanced short-term plasticity in terms of paired pulse ratio (PPR) and release probability (Pr), compared to autapses. Interestingly, when a neuron was connected to another neuron, not only interneuronal synapses, but also the autaptic synapses on itself exhibited a trend toward enhanced short-term plasticity in terms of PPR and Pr. Thus neurons can distinguish whether they are connected via “self” or “non-self” synapses and have the ability to adjust their plasticity parameters when connected to other neurons.     

Comparison of “Live High-Train Low” in Normobaric versus Hypobaric Hypoxia

We investigated the changes in both performance and selected physiological parameters following a Live High-Train Low (LHTL) altitude camp in either normobaric hypoxia (NH) or hypobaric hypoxia (HH) replicating current “real” practices of endurance athletes. Well-trained triathletes were split into two groups (NH, n = 14 and HH, n = 13) and completed an 18-d LHTL camp during which they trained at 1100–1200 m and resided at an altitude of 2250 m (P_iO₂  = 121.7±1.2 vs. 121.4±0.9 mmHg) under either NH (hypoxic chamber; F_iO₂ 15.8±0.8%) or HH (real altitude; barometric pressure 580±23 mmHg) conditions. Oxygen saturations (S_pO₂) were recorded continuously daily overnight. P_iO₂ and training loads were matched daily. Before (Pre-) and 1 day after (Post-) LHTL, blood samples, VO_2max, and total haemoglobin mass (Hb_mass) were measured. A 3-km running test was performed near sea level twice before, and 1, 7, and 21 days following LHTL. During LHTL, hypoxic exposure was lower for the NH group than for the HH group (220 vs. 300 h; P<0.001). Night S_pO₂ was higher (92.1±0.3 vs. 90.9±0.3%, P<0.001), and breathing frequency was lower in the NH group compared with the HH group (13.9±2.1 vs. 15.5±1.5 breath.min⁻¹, P<0.05). Immediately following LHTL, similar increases in VO_2max (6.1±6.8 vs. 5.2±4.8%) and Hb_mass (2.6±1.9 vs. 3.4±2.1%) were observed in NH and HH groups, respectively, while 3-km performance was not improved. However, 21 days following the LHTL intervention, 3-km run time was significantly faster in the HH (3.3±3.6%; P<0.05) versus the NH (1.2±2.9%; ns) group. In conclusion, the greater degree of race performance enhancement by day 21 after an 18-d LHTL camp in the HH group was likely induced by a larger hypoxic dose. However, one cannot rule out other factors including differences in sleeping desaturations and breathing patterns, thus suggesting higher hypoxic stimuli in the HH group.     

Pools versus Queues: The Variable Dynamics of Stochastic “Steady States”

Mathematical models in ecology and epidemiology often consider populations “at equilibrium”, where in-flows, such as births, equal out-flows, such as death. For stochastic models, what is meant by equilibrium is less clear – should the population size be fixed or growing and shrinking with equal probability? Two different mechanisms to implement a stochastic steady state are considered. Under these mechanisms, both a predator-prey model and an epidemic model have vastly different outcomes, including the median population values for both predators and prey and the median levels of infection within a hospital (P < 0.001 for all comparisons). These results suggest that the question of how a stochastic steady state is modeled, and what it implies for the dynamics of the system, should be carefully considered.     

Sprouting of adult Purkinje cell axons in lesioned mouse cerebellum: “Non-permissive” versus “permissive” environment

Adult rat Purkinje cells are extremely resistant to axotomy and, although they lack spontaneous regeneration, are able to sprout. Axon sprouting is a late process that occurs mainly 6 to 18 months after the lesion and results from an interplay between Purkinje cell intrinsic properties and chemical remodeling of the glial scar. To better appraise the role of the local environment in the late sprouting, we performed new axotomy experiments in mice. In this species, unlike the rat, there is no cavitation because the post-lesional necrotic tissue is invaded by astrocytes and incorporated into the glial scar. In this scarring tissue, chondroitin sulfate proteoglycans (CS-PGs) and PSA-NCAM are present one week after the lesion, but the time courses of their expression differ: the former are transiently expressed and rapidly disappears (by one month), thus preventing early sprouting and providing a negative spatiotemporal correlation with the late sprouting. PSA-NCAM expression, which is maintained up till 12 months, is by itself not sufficient to attract the sprouts, since the core of the glial scar—which exhibits high level of PSA-NCAM—is always devoid of them. Finally, by using a double experimental approach (lesion and graft) aimed at providing a permissive environment to the terminal bulbs of axotomized Purkinje cells, we show that the presence of grafted cerebellum at the lesion site neither changes the time course of the sprouting nor enhances the Purkinje cell axonal regeneration. Nevertheless, these experiments have revealed a new type of altered Purkinje cells, the “irritated” Purkinje cells with a high potentiality for axon sprouting.     

Pluripotent versus totipotent plant stem cells: dependence versus autonomy?

Little is known of the mechanisms that induce the dedifferentiation of a single somatic cell into a totipotent embryogenic cell that can either be regenerated or develop into an embryo and subsequently an entire plant. In this Opinion article, we examine the cellular, physiological and molecular similarities and differences between different plant stem cell types. We propose to extend the plant stem cell concept to include single embryogenic cells as a totipotent stem cell based on their capacity to regenerate or develop into an embryo under certain conditions. Our survey suggests that differences in chromatin structure might ensure that meristem-localized stem cells have supervised freedom and are pluripotent, and that embryogenic stem cells are unsupervised, autonomous and, hence, freely totipotent.     

Killing pigs and talking to nonna: “wog” versus “cosmopolitan” Italianitá among second-generation Italian-Australians and the role of family

This paper extends the literature on second-generation migrants by examining the construction of ethnicity (Italianitá) over time. We compare two cohorts of second-generation Italian-Australians: the post-World War II cohort and the post-1980s cohort. Ethnographic data for this research were collected with second-generation Italian-Australians in Perth over a thirty-year period. Our findings highlight important differences between these two groups based on socio-historical context and transnational experiences. Informants draw on these differences to distinguish between “wog” vs. “cosmopolitan” forms of Italianitá. While these contrasting identities highlight cultural discontinuities between cohorts, both groups construct their ethnicity through the trope of the Italian migrant family. Employing the theoretical notions of “intimate culture” and “familial habitus” we theorize family as integral to conceptualizations of ethnic field and show how it has been overlooked and devalued in analyses of diaspora politics and identity.     

Immunogenicity of allograft components: II. Relative immunogenicity of rat kidney parenchymal versus “passenger” cells

Well-perfused adult DA kidneys were enzymatically dispersed under conditions which do not affect the expression of cell surface major histocompatibility antigens. The kidney cell suspensions were separated via sedimentation at unit gravity into three fractions: I, rapidly sedimenting (>6.5 mm/hr) enriched for kidney tubular and glomerular cells and depleted of passenger leukocytes (76 and 8%, respectively); II, intermediate (5.1–6.0 mm/hr) mixed population equivalent to the unseparated kidney cell suspension (52% tubular and glomerular cells, 20% endothelial cells, and 28% passenger leukocytes); and III, slow sedimenting (<5.0 mm/hr) enriched for passenger leukocytes (63%). The three isolated fractions were analyzed for their ability to accelerate allograft rejection in the “primed heart rejection assay.” The cells in fraction I were unable to reduce heart allograft survival, while the cells from fraction III reduced it significantly. Cells from fraction II were intermediary effective. The results are in agreement with the hypothesis that the urine-producing apparatus of rat kidney is relatively nonimmunogenic, while the main stimulus for graft rejection is provided by the “passenger” cell component.