A steady state and differential polarised phase fluorimetric study of the liver microsomal and mitochondrial membranes of thermally acclimated green sunfish (Lepomis cyanellus)

The liver mitochondrial and microsomal membranes of green sunfish and rat were examined by steady state polarisation and differential polarised phase fluorimetry to determine the effects of seasonal adaptation of membrane dynamic structure to temperature. Steady state polarisation studies indicated that the liver mitochondria of green sunfish acclimated to different temperatures showed a greater partial compensation of membrane fluidity for the altered acclimation temperature than did liver microsomal membranes. The fatty acid composition of both membrane preparations generally became more unsaturated at lower acclimation temperatures, though the differences between 5°C and 25°C acclimated fish were more pronounced in the mitochondrial fraction than in the microsomal fraction.Differential polarised phase fluorimetric studies indicated that the rotations of diphenylhexatriene in mitochondrial and microsomal membranes were highly hindered, though the hindrance offered by membranes of 25°C acclimated green sunfish was far greater than that offered by the membranes of 5°C acclimated fish, thus supporting the concept of homeoviscous adaptation. The absolute rotational rate was not consistently affected by acclimation treatment.     

Contractions of holothurian muscles

Summary In response to quick stretch, contraction is elicited in longitudinal retractor muscles of five tested species of holothurians, and in the pharyngeal retractor ofCucumaria. The effects of amplitude of stretch and rate of stretch are additive. Rates of contraction and repetitiveness of response, and spontaneous rhythmicity (especially in muscles ofLeptosynapta), correlate with mode of life.Contractile responses to stretch are abolished by anesthesia with procaine or magnesium. Responses are enhanced by physostigmine or prostigmine, blocked by d-tubocurarine. Responses to electric shocks persist after block of responses to stretch and after block of spontaneous activity by anesthesia, by cholinergic blockers or by Na replacement. Responses to both stretch and shock are abolished by reducing calcium or by agents which block Ca-conductance.It is postulated (1) that quick stretch stimulates the terminals of cholinergic nerves, (2) that conduction in these nerve fibers is by Na but is TTX resistant, (3) that the nerve endings activate conductance increase for Ca⁺⁺ in muscle fibers which initiate contractions.No muscle potentials were recorded by suction or pressure electrodes and no nexal junctions were observed between muscle fibers. The muscles were well innervated and synaptic endings and some neural somata were seen in the nerve bundles.Thanks are due to Dennis Willows, director and to staff, University of Washington Laboratories, Friday Harbor; to C.L. Singla of the University of Victoria for preparing and examining electron micrographs; to J.L.S. Cobb for commenting on electron micrographs; to Richard Meiss for designing and constructing ramp stretching device. C. Ladd Prosser was supported by NIH grant 5-R01 AM 12768-10 and George O. Mackie by grant no. A 1427, Nat. Sci. and Eng. Res. Council of Canada.     

The neurofibromatosis type I pre-mRNA is a novel target of CELF protein-mediated splicing regulation

The CUG-BP and ETR-3 like factors (CELF) are a family of six highly conserved RNA-binding proteins that preferentially bind to UG-rich sequences. One of the key functions of these proteins is to mediate alternative splicing in a number of tissues, including brain, heart and muscle. To fully understand the function of CELF proteins, it is important to identify downstream targets of CELF proteins. In this communication, we report that neurofibromatosis type I (NF1) exon 23a is a novel target of CELF protein-mediated splicing regulation in neuron-like cells. NF1 regulates Ras signaling, and the isoform that excludes exon 23a shows 10 times greater ability to down-regulate Ras signaling than the isoform that includes exon 23a. Five of the six CELF proteins strongly suppress the inclusion of NF1 exon 23a. Over-expression or siRNA knockdown of these proteins in cell transfection experiments altered the levels of NF1 exon 23a inclusion. In vitro binding and splicing analyses demonstrate that CELF proteins block splicing through interfering with binding of U2AF⁶⁵. These studies, combined with our previous investigations demonstrating a role for Hu proteins and TIA-1/TIAR in controlling NF1 exon 23a inclusion, highlight the complex nature of regulation of this important alternative splicing event.     

Algal colonization in urchin barrens: defense by association during recruitment of the brown alga Agarum cribrosum

We investigated the process whereby juveniles of the kelp Agarum cribrosum escape grazing by the green sea urchin, Strongylocentrotus droebachiensis, on urchin barrens in the rocky subtidal zone in the Mingan Islands, northern Gulf of St. Lawrence. The highest recruitment of juvenile A. cribrosum occurred under the canopy of the large filamentous phaeophyte Desmarestia viridis, where urchin densities were markedly reduced, compared to the surrounding area. This pattern of distribution appeared to be related to the wave-induced sweeping motion of D. viridis, although currents may modify the back and forth motion of the alga by pushing the canopy towards a specific direction, thereby allowing urchins to invade the non-swept areas. The density of juveniles under D. viridis plants increased with plant size and increasing proximity to the holdfast. Living under D. viridis slightly reduced the growth rate of the A. cribrosum juveniles, but this loss in growth was clearly outweighed by the gain in protection from sea urchin grazing. The time scale over which D. viridis provides protection is in the order of months, as D. viridis is an annual alga that disappears in early autumn. This defensive association of juvenile A. cribrosum with D. viridis is possibly a successional step leading to the formation of mature stands of A. cribrosum.     

Invasive species contribute to biotic resistance: negative effect of caprellid amphipods on an invasive tunicate

As the number of introductions of non-indigenous species (NIS) continues to rise, ecologists are faced with new and unique opportunities to observe interactions between species that do not naturally co-exist. These interactions can have important implications on the invasion process, potentially determining whether NIS become widespread and abundant, survive in small numbers, or fail to establish and disappear. Although many studies have naturally focused on the interactions between NIS and native species to examine their effects and the biological resistance of the recipient community to invasion, few have examined the effects that NIS have on each other. In some cases, interactions can facilitate the invasion process of one or both species (i.e., “invasional meltdowns”), but competition or predation can lead to negative interactions as well. The introduction of the vase tunicate, Ciona intestinalis, in Prince Edward Island (Canada) has harmed mussel aquaculture via heavy biofouling of equipment and mussels. Through both a broad-scale survey and small-scale field experiments, we show that Ciona recruitment is drastically reduced by caprellid amphipods, including the NIS Caprella mutica. This study provides an exciting example of how established invasive species can negatively impact the recruitment of a secondary invader, highlighting the potential for non-additive effects of multiple invasions.     

Investigation of the Saturation Pulse Artifact in Non-Enhanced MR Angiography of the Lower Extremity Arteries at 7 Tesla

When performing non-enhanced time-of-flight MR angiography of the lower extremity arteries at 7 T with cardiac triggering, the acquisition time is a crucial consideration. Therefore, in previous studies, saturation RF pulses were applied only every second TR. In the axial source images a slight artifact with an appearance similar to aliasing could be observed. The purpose of this study was to investigate the origin of this artifact. The reason for the artifact is supposed to be related to the two effective TRs during acquisition caused by the sparsely applied saturation RF pulse. Several sequence variants were simulated and implemented within the sequence source code to examine this hypothesis. An adaptation of the excitation flip angles for each TR as well as a correction factor for the k-space data was calculated. Additionally, a different ordering of the k-space data during acquisition was implemented as well as the combination of the latter with the k-space correction factor. The observations from the simulations were verified using both a static and a flow phantom and, finally, in a healthy volunteer using the same measurement setup as in previous volunteer and patient studies. Of all implemented techniques, only the reordering of the k-space was capable of suppressing the artifact almost completely at the cost of creating a ringing artifact. The phantom measurements showed the same results as the simulations and could thus confirm the hypothesis regarding the origin of the artifact. This was additionally verified in the healthy volunteer. The origin of the artifact could be confirmed to be the periodic signal variation caused by two effective TRs during acquisition.