Lack of effect of microfilament or microtubule cytoskeleton-disrupting agents on restriction of externalized phosphatidylserine to rod photoreceptor outer segment tips.

In the mammalian retina, life-long renewal of rod photoreceptor outer segments involves circadian shedding of distal outer segment tips and their prompt phagocytosis by the adjacent retinal pigment epithelium (RPE) every morning after light onset. Failure of this process causes retinal dystrophy in animal models and its decline likely contributes to retinal aging and some forms of degeneration of the human retina. We previously found that surface exposure of the membrane phospholipid phosphatidylserine (PS) is restricted to outer segment tips with discrete boundaries in mouse retina and that both frequency and length of tips exposing PS peak after light onset.

Diurnal, localized exposure of phosphatidylserine by rod outer segment tips in wild-type but not Itgb5-/- or Mfge8-/- mouse retina.

In the mammalian retina, life-long renewal of light-sensitive photoreceptor outer segments (POS) involves circadian shedding of distal rod POS tips and their subsequent phagocytosis by the adjacent retinal pigment epithelium (RPE) every morning after light onset. Molecular mechanisms that promote or synchronize POS tip shedding have thus far remained unknown. Here we examined plasma membrane asymmetry of living POS by quantifying surface exposure of the membrane phospholipid phosphatidylserine (PS) using antibodies, annexin V, and pSIVA (polarity-sensitive indicator of viability and apoptosis), an annexin-based biosensor with switchable states of fluorescence.

Mice with early retinal degeneration show differences in neuropeptide expression in the suprachiasmatic nucleus.

Background: In mammals, the brain clock responsible for generating circadian rhythms is located in the suprachiasmatic nucleus (SCN) of the hypothalamus. Light entrainment of the clock occurs through intrinsically photosensitive retinal ganglion cells (ipRGCs) whose axons project to the SCN via the retinohypothalamic tract. Although ipRGCs are sufficient for photoentrainment, rod and cone photoreceptors also contribute.

The development of melanopsin-containing retinal ganglion cells in mice with early retinal degeneration.

In mammals, the neuronal pathways by which rod and cone photoreceptors mediate vision have been well documented. The roles that classical photoreceptors play in photoentrainment, however, have been less clear. In mammals, intrinsically photosensitive retinal ganglion cells (ipRGCs) that express the photopigment melanopsin project directly to the suprachiasmatic nucleus of the hypothalamus, the site of the circadian clock, and thereby contribute to non-image-forming responses to light.

Actin filament bundles in Drosophila wing hairs: hairs and bristles use different strategies for assembly.

Actin filament bundles can shape cellular extensions into dramatically different forms. We examined cytoskeleton formation during wing hair morphogenesis using both confocal and electron microscopy. Hairs elongate with linear kinetics (approximately 1 microm/h) over the course of approximately 18 h.

Actin filament turnover regulated by cross-linking accounts for the size, shape, location, and number of actin bundles in Drosophila bristles.

Drosophila bristle cells are shaped during growth by longitudinal bundles of cross-linked actin filaments attached to the plasma membrane. We used confocal and electron microscopy to examine actin bundle structure and found that during bristle elongation, snarls of uncross-linked actin filaments and small internal bundles also form in the shaft cytoplasm only to disappear within 4 min. Thus, formation and later removal of actin filaments are prominent features of growing bristles.

Long continuous actin bundles in Drosophila bristles are constructed by overlapping short filaments.

The actin bundles essential for Drosophila bristle elongation are hundreds of microns long and composed of cross-linked unipolar filaments. These long bundles are built from much shorter modules that graft together. Using both confocal and electron microscopy, we demonstrate that newly synthesized modules are short (1-2 microm in length); modules elongate to approximately 3 microm by growing over the surface of longitudinally adjacent modules to form a graft; the grafted regions are initially secured by the forked protein cross-bridge and later by the fascin cross-bridge; actin bundles are smoothed by filament addition and appear continuous and without swellings; and in the absence of grafting, dramatic alterations in cell shape occur that substitutes cell width expansion for elongation.