Description, validation, and performance characteristics of a new computer-automated sperm motility analysis system

A computer-automated sperm motility assay (CASMA) system has been developed that provides a rapid and accurate analysis of multiple sperm movement parameters and a new measure of linearity, the linear deviation angle. CASMA provides objective, unbiased sampling and accurate quantitation of the movement characteristics of 200 sperm cells in 20 min. It consists of a microscope-mounted video camera, a high-resolution video disk recorder, a video digitizer/memory board mounted in an IBM 9000 microcomputer, and newly developed software. After manual recording, at 60 frames/s, of the video sequences (takes) of sperm suspensions, each take is automatically played back frame by frame, digitized, and stored in video memory. The software searches each frame, recognizes sperm cells, randomly selects a preset number for analysis, and traces each cell through the sequence to generate sperm "tracks" that are then stored in disk memory. This process is repeated for each take. Analysis of the stored tracks of each take yields the mean +/- SEM of the standard sperm motility parameters: percent motile (%M), curvilinear velocity (VC), net velocity (Vn), position-averaged velocity (Va), linear index (Vn/Va), progressiveness ration (Vn/Vc), and curvilinear progressiveness ratio (Va/Vc). Additionally, CASMA allows measurement of the linear deviation angle, a more direct measure of the linearity of sperm movement. For statistical comparisons, multiple takes can be considered either as replicates or separate experimental determinations. Finally, for more detailed analysis, each individually stored track, with its associated parameters, or histogram distributions of all sperm for each parameter can be displayed and printed. The performance of CASMA was evaluated by comparison of CASMA-determined movement parameters with manually determined values derived from the same sperm cells in the same video sequence and by comparison with published values determined using microcinematographic techniques. In each case, the CASMA values were essentially identical to those determined by manual measurements. Finally, CASMA accurately quantitates the linearity of sperm movement, a characteristic previously determined only by much more time-consuming methods. CASMA is a rapid and accurate system for measuring washed bull sperm motility and has reliably analyzed monkey and elephant sperm. The system has the potential to quantitate motility equally well with sperm from any species that have similar sperm head size.     

Calcium-induced heterogeneous changes in membrane potential detected by flow cytofluorimetry

Ionophore-induced changes in the cell-associated fluorescence of samples of approx. 50 000 individual murine L1210 leukemia cells which had been incubated with the voltage-sensitive dye 3,3′-dihexyloctacarbocyanine iodide (DiOC6(3)) were monitored by flow cytometry. The K⁺ ionophore valinomycin (1 μM) produced homogeneous changes in the fluorescence of the entire population, the magnitude of which was dependent upon the concentration of extracellular K⁺. These changes allowed the estimation of the potassium equilibrium potential of the cells, by the null-point method, to be – 11.9 mV. The Ca²⁺ ionophore A23187 (500 nM) produced heterogeneous changes in fluorescence, with populations of both hyperpolarised and depolarised cells. In addition, the depolarised population underwent an apparent size change, with a reduction in cell volume. This heterogeneity of response resulted in a minimal change in the median fluorescence value for the whole population, which suggests that it would not have been detectable by methods dependent upon net population-averaged changes in fluorescence. Removal of extracellular Na⁺ or preincubation of cells with amiloride (500 μM) effectively eliminated the depolarised population. Removal of extracellular K⁺ increased the hyperpolarised population. These findings provide evidence for the presence of Ca²⁺-induced Na⁺ exchange and Ca²⁺-induced K⁺ efflux mechanisms in these cells which may be expressed simultaneously in the cell population.     

Enteric helminth coinfection enhances host susceptibility to neurotropic flaviviruses via a tuft cell-IL-4 receptor signaling axis

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Predators and invasive plants affect performance of amphibian larvae

Exotic ecosystem engineers induce structural and qualitative habitat changes in invaded landscapes, yet studies rarely examine the effects of both of these changes on native taxa. We used a factorial experiment in natural, predator‐containing environments to determine whether performance of amphibian larvae was affected by predators and/or changes in habitat structure or chemistry associated with the invasive shrub Lonicera maackii. Invertebrate predators significantly reduced survival of American toad Anaxyrus americanus larvae, whereas tadpole development was accelerated in pools inoculated with the chemical signature of L. maackii. The significant effect of L. maackii chemistry on A. americanus larvae suggests that invasive species may have non‐intuitive effects even on native taxa with which they share no trophic connection, and may represent cryptic components of the multiple, interactive drivers of biodiversity change.     

The amino acid linker between the endonuclease and helicase domains of adeno-associated virus type 5 Rep plays a critical role in DNA-dependent oligomerization

The adeno-associated virus (AAV) genome encodes four Rep proteins, all of which contain an SF3 helicase domain. The larger Rep proteins, Rep78 and Rep68, are required for viral replication, whereas Rep40 and Rep52 are needed to package AAV genomes into preformed capsids; these smaller proteins are missing the site-specific DNA-binding and endonuclease domain found in Rep68/78. Other viral SF3 helicases, such as the simian virus 40 large T antigen and the papillomavirus E1 protein, are active as hexameric assemblies. However, Rep40 and Rep52 have not been observed to form stable oligomers on their own or with DNA, suggesting that important determinants of helicase multimerization lie outside the helicase domain. Here, we report that when the 23-residue linker that connects the endonuclease and helicase domains is appended to the adeno-associated virus type 5 (AAV5) helicase domain, the resulting protein forms discrete complexes on DNA consistent with single or double hexamers. The formation of these complexes does not require the Rep binding site sequence, nor is it nucleotide dependent. These complexes have stimulated ATPase and helicase activities relative to the helicase domain alone, indicating that they are catalytically relevant, a result supported by negative-stain electron microscopy images of hexameric rings. Similarly, the addition of the linker region to the AAV5 Rep endonuclease domain also confers on it the ability to bind and multimerize on nonspecific double-stranded DNA. We conclude that the linker is likely a key contributor to Rep68/78 DNA-dependent oligomerization and may play an important role in mediating Rep68/78's conversion from site-specific DNA binding to nonspecific DNA unwinding.     

Melatonin biosynthesis: the structure of serotonin N-acetyltransferase at 2.5 A resolution suggests a catalytic mechanism

Conversion of serotonin to N-acetylserotonin, the precursor of the circadian neurohormone melatonin, is catalyzed by serotonin N-acetyltransferase (AANAT) in a reaction requiring acetyl coenzyme A (AcCoA). AANAT is a globular protein consisting of an eight-stranded beta sheet flanked by five alpha helices; a conserved motif in the center of the beta sheet forms the cofactor binding site. Three polypeptide loops converge above the AcCoA binding site, creating a hydrophobic funnel leading toward the cofactor and serotonin binding sites in the protein interior. Two conserved histidines not found in other NATs are located at the bottom of the funnel in the active site, suggesting a catalytic mechanism for acetylation involving imidazole groups acting as general acid/base catalysts.