Voluntary exercise attenuates LPS-induced reductions in neurogenesis and increases microglia expression of a proneurogenic phenotype in aged mice.

BACKGROUND: Microglia can acquire various phenotypes of activation that mediate their inflammatory and neuroprotective effects. Aging causes microglia to become partially activated towards an inflammatory phenotype. As a result, aged animals display a prolonged neuroinflammatory response following an immune challenge. Currently unknown is whether this persistent neuroinflammation leads to greater reductions in hippocampal neurogenesis. Exercise has been shown to alter microglia activation in aged animals, but the nature of these changes has yet to be fully elucidated. The present study assessed whether aged mice show enhanced reductions in hippocampal neurogenesis following an acute immune challenge with lipopolysaccharide (LPS). Further, we assessed whether voluntary wheel running protects against the effects of LPS. METHODS: Adult (4 months) and aged (22 months) male C57BL6/J mice were individually housed with or without a running wheel for a total of 9 weeks. After 5 weeks, mice received a single intraperitoneal LPS or saline injection in combination with four daily injections of bromodeoxyuridine (BrdU) to label dividing cells. Tissue was collected 4 weeks later and immunohistochemistry was conducted to measure new cell survival, new neuron numbers, and microglia activation. RESULTS: Data show that LPS reduced the number of new neurons in aged, but not adult, mice. These LPS-induced reductions in neurogenesis in the aged mice were prevented by wheel running. Further, exercise increased the proportion of microglia co-labeled with brain-derived neurotrophic factor (BDNF) in the aged. CONCLUSIONS: Collectively, findings indicate that voluntary wheel running may promote a neuroprotective microglia phenotype and protect against inflammation-induced reductions in hippocampal neurogenesis in the aged brain.

Ring bands in fish skeletal muscle: reorienting the myofibrils and microtubule cytoskeleton within a single cell.

Skeletal muscle cells (fibers) contract by shortening their parallel subunits, the myofibrils. Here we show a novel pattern of myofibril orientation in white muscle fibers of large black sea bass, Centropristis striata. Up to 48% of the white fibers in fish >1168 g had peripheral myofibrils undergoing an ∼90(o) shift in orientation. The resultant ring band wrapped the middle of the muscle fibers and was easily detected with polarized light microscopy. Transmission electron microscopy showed that the reoriented myofibrils shared the cytoplasm with the central longitudinal myofibrils. A microtubule network seen throughout the fibers surrounded nuclei but was mostly parallel to the long-axis of the myofibrils. In the ring band portion of the fibers the microtubule cytoskeleton also shifted orientation. Sarcolemmal staining with anti-synapsin was the same in fibers with or without ring bands, suggesting that fibers with ring bands have normal innervation and contractile function. The ring bands appear to be related to body-mass or age, not fiber size, and also vary along the body, being more frequent at the midpoint of the anteroposterior axis. Similar structures have been reported in different taxa and appear to be associated with hypercontraction of fibers not attached to a rigid structure (bone) or with fibers with unusually weak links between the sarcolemma and cytoskeleton, as in muscular dystrophy. Fish muscle fibers are attached to myosepta, which are flexible and may allow for fibers to hypercontract and thus form ring bands. The consequences of such a ring band pattern might be to restrict the further expansion of the sarcolemma and protect it from further mechanical stress.

Growth patterns and nuclear distribution in white muscle fibers from black sea bass, Centropristis striata: evidence for the influence of diffusion.

This study investigated the influence of fiber size on the distribution of nuclei and fiber growth patterns in white muscle of black sea bass, Centropristis striata, ranging in body mass from 0.45 to 4840 g. Nuclei were counted in 1 µm optical sections using confocal microscopy of DAPIand Acridine-Orange-stained muscle fibers. Mean fiber diameter increased from 36±0.87 µm in the 0.45 g fish to 280±5.47 µm in the 1885 g fish. Growth beyond 2000 g triggered the recruitment of smaller fibers, thus significantly reducing mean fiber diameter. Nuclei in the smaller fibers were exclusively subsarcolemmal (SS), whereas in larger fibers nuclei were more numerous and included intermyofibrillar (IM) nuclei. There was a significant effect of body mass on nuclear domain size (F=118.71, d.f.=3, P<0.0001), which increased to a maximum in fish of medium size (282-1885 g) and then decreased in large fish (>2000 g). Although an increase in the number of nuclei during fiber growth can help preserve the myonuclear domain, the appearance of IM nuclei during hypertrophic growth seems to be aimed at maintaining short effective diffusion distances for nuclear substrates and products. If only SS nuclei were present throughout growth, the diffusion distance would increase in proportion to the radius of the fibers. These observations are consistent with the hypothesis that changes in nuclear distribution and fiber growth patterns are mechanisms for avoiding diffusion limitation during animal growth.

Does 5-bromo-2'-deoxyuridine (BrdU) disrupt cell proliferation and neuronal maturation in the adult rat hippocampus in vivo?

5-Bromo-2'-deoxyuridine (BrdU) is frequently used as a mitotic marker in studies of cell proliferation. Recent studies have reported cytotoxic effects of BrdU on neural progenitor cells in embryonic and neonatal brains in vivo and in adult tissue studied in vitro. The present study was conducted to assess whether BrdU interferes with cell proliferation and neuronal maturation in the rat adult hippocampus in vivo. BrdU effects across a wide range of doses (40-480 mg/kg) on cell proliferation and the population of immature neurons in the adult hippocampus were investigated using immunohistochemical labeling methods for the cell cycle marker Ki67 and a marker for immature neurons, doublecortin. BrdU did not influence cell proliferation in the dentate gyrus or the population of immature neurons observed in the adult hippocampus relative to those observed in saline treated controls. Thus, in contrast with reports of deleterious effects of BrdU in embryonic and neonatal tissue and adult tissue studied in vitro, BrdU does not appear to have cytotoxic effects on proliferating hippocampal cells or immature neurons in vivo in rats.

Persistent increases in the pool of doublecortin-expressing neurons in the hippocampus following spatial navigation training.

In addition to its role in neuronal migration during embryonic development, doublecortin (DCX) plays a role in hippocampal neurogenesis across the lifespan. Hippocampal neurons exhibit a high degree of synaptic plasticity while they are in the DCX phase. While previous studies have reported that behavioral training on hippocampus-dependent tasks can enhance neuron survival, little was known about the stage of development of those neurons and, particularly, whether a large pool of the surviving new neurons remains in the DCX phase for a prolonged period after training. Here we report that spatial navigation training increases the pool of neurons that are in the DCX phase 4 weeks after training ended. Thus, the stock of DCX-expressing neurons in the hippocampus is affected by whether a hippocampus-dependent task has been encountered during the preceding few weeks.

Ultrastructure, histochemistry, and mineralization patterns in the ecdysial suture of the blue crab, Callinectes sapidus.

The ecdysial suture is the region of the arthropod exoskeleton that splits to allow the animal to emerge during ecdysis. We examined the morphology and composition of the intermolt and premolt suture of the blue crab using light microscopy and scanning electron microscopy. The suture could not be identified by routine histological techniques; however 3 of 22 fluorescein isothiocyanate-labeled lectins tested (Lens culinaris agglutinin, Vicia faba agglutinin, and Pisum sativum agglutinin) differentiated the suture, binding more intensely to the suture exocuticle and less intensely to the suture endocuticle. Back-scattered electron (BSE) and secondary electron observations of fracture surfaces of intermolt cuticle showed less mineralized regions in the wedge-shaped suture as did BSE analysis of premolt and intermolt resin-embedded cuticle. The prism regions of the suture exocuticle were not calcified. X-ray microanalysis of both the endocuticle and exocuticle demonstrated that the suture was less calcified than the surrounding cuticle with significantly lower magnesium and phosphorus concentrations, potentially making its mineral more soluble. The presence or absence of a glycoprotein in the organic matrix, the extent and composition of the mineral deposited, and the thickness of the cuticle all likely contribute to the suture being removed by molting fluid, thereby ensuring successful ecdysis.