The synaptic vesicle cycle: is kissing overrated?

In this issue of Neuron, Granseth et al. re-examine the mechanism of endocytosis at hippocampal synapses using a new optical reporter, sypHy. They conclude that only a single slow mode of endocytosis operates at this synapse and that retrieval after physiological stimuli is largely, if not solely, dominated by the clathrin-mediated pathway. These conclusions dispute previous assertions that "kiss-and-run" is a major mechanism of vesicle recycling at hippocampal synapses.     

Growth,survival, and metal content of marsh invertebrates fed diets of detritus from Spartina alterniflora Loisel. and Phragmites australis Cav. Trin. ex Steud. from metal-contaminated and clean sites

Marsh vegetation plays an important role in trophic ecology of estuaries. Once broken down to detritus, it is an important food source for manyorganisms. In Atlantic Coast marshes, the reed Phragmites australis hasbeen invading many areas once dominated by smooth cordgrass, Spartina alterniflora. In this study we evaluated the growth of and trophictransfer of metals to estuarine invertebrates when fed diets of detritus fromthese different plant species. Decaying leaves from populations of Phragmites, natural Spartina, and restored Spartina from boththe Hackensack Meadowlands, New Jersey, and the more pristineAccabonac Harbor of East Hampton, New York, were collected from themarsh surface in the spring. Decaying leaves were pureed and fed to thefiddler crabs Uca pugnax and U. pugilator, and to the grassshrimp Palaemonetes pugio. In fiddler crabs we monitored limbregeneration, molting and weight. U. pugilator regenerated limbs andmolted equally well on all six diets. Most of the U. pugnax arrestedgrowth midway through regeneration on all 6 diets. A repeat experimentwith smaller crabs, which did complete the process, found no consistentdifferences among the six diets and control food, although control food andPhragmites detritus had higher N concentrations than the Spartinadetritus. Grass shrimp fed all six diets did not survive beyond 3 weeks. Inanother experiment using HM sediments from each vegetation type(containing detritus, meiofauna, and microflora), survival was equally highamong treatments and the shrimp fed sediments from the restored Spartina site or control food grew better than those fed sediments fromthe Phragmites or natural Spartina sites. Although metalconcentrations in detritus varied between sites and plant species, the crabsof each group did not differ in metal concentrations after the feedingexperiment. Our data do not support the general assumption that Phragmites leaf detritus is of poorer nutritional quality than Spartinaalterniflora leaf detritus to estuarine consumers.     

PDGF-responsive progenitors persist in the subventricular zone across the lifespan

The SVZ (subventricular zone) contains neural stem cells and progenitors of various potentialities. Although initially parsed into A, B, and C cells, this germinal zone is comprised of a significantly more diverse population of cells. Here, we characterized a subset of postnatal PRPs (PDGF-AA-responsive precursors) that express functional PDGFα and β receptors from birth to adulthood. When grown in PDGF-AA, dissociated neonatal rat SVZ cells divided to produce non-adherent clusters of progeny. Unlike the self-renewing EGF/FGF-2-responsive precursors that produce neurospheres, these PRPs failed to self-renew after three passages; therefore, we refer to the colonies they produce as spheroids. Upon differentiation these spheroids could produce neurons, type 1 astrocytes and oligodendrocytes. When maintained in medium supplemented with BMP-4 they also produced type 2 astrocytes. Using lineage tracing methods, it became evident that there were multiple types of PRPs, including a subset that could produce neurons, oligodendrocytes, and type 1 and type 2 astrocytes; thus some of these PRPs represent a unique population of precursors that are quatropotential. Spheroids also could be generated from the newborn neocortex and they had the same potentiality as those from the SVZ. By contrast, the adult neocortex produced less than 20% of the numbers of spheroids than the adult SVZ and spheroids from the adult neocortex only differentiated into glial cells. Interestingly, SVZ spheroid producing capacity diminished only slightly from birth to adulthood. Altogether these data demonstrate that there are PRPs that persist in the SVZ that includes a unique population of quatropotential PRPs.     

Modeled production,oxidation, and transport processes of wetland methane emissions in temperate,boreal, and Arctic regions

Wetlands are the largest natural source of methane (CH₄) to the atmosphere. The eddy covariance method provides robust measurements of net ecosystem exchange of CH₄, but interpreting its spatiotemporal variations is challenging due to the co-occurrence of CH₄ production, oxidation, and transport dynamics. Here, we estimate these three processes using a data-model fusion approach across 25 wetlands in temperate, boreal, and Arctic regions. Our data-constrained model—iPEACE—reasonably reproduced CH₄ emissions at 19 of the 25 sites with normalized root mean square error of 0.59, correlation coefficient of 0.82, and normalized standard deviation of 0.87. Among the three processes, CH₄ production appeared to be the most important process, followed by oxidation in explaining inter-site variations in CH₄ emissions. Based on a sensitivity analysis, CH₄ emissions were generally more sensitive to decreased water table than to increased gross primary productivity or soil temperature. For periods with leaf area index (LAI) of ≥20% of its annual peak, plant-mediated transport appeared to be the major pathway for CH₄ transport. Contributions from ebullition and diffusion were relatively high during low LAI (<20%) periods. The lag time between CH₄ production and CH₄ emissions tended to be short in fen sites (3 ± 2 days) and long in bog sites (13 ± 10 days). Based on a principal component analysis, we found that parameters for CH₄ production, plant-mediated transport, and diffusion through water explained 77% of the variance in the parameters across the 19 sites, highlighting the importance of these parameters for predicting wetland CH₄ emissions across biomes. These processes and associated parameters for CH₄ emissions among and within the wetlands provide useful insights for interpreting observed net CH₄ fluxes, estimating sensitivities to biophysical variables, and modeling global CH₄ fluxes.     

Dissolved Inorganic Nitrogen Pools and Surface Flux under Different Brackish Marsh Vegetation Types,Common Reed (<Emphasis Type="Italic">Phragmites australis</Emphasis>) and Salt Hay (<Emphasis Type="Italic">Spartina patens</Emphasis>)

The current expansion of Phragmites australis into the high marsh shortgrass (Spartina patens, Distichlis spicata) communities of eastern U.S. salt marshes provided an opportunity to identify the influence of vegetation types on pools and fluxes of dissolved inorganic nitrogen (DIN). Two brackish tidal marshes of the National Estuarine Research Reserve system were examined, Piermont Marsh of the Hudson River NERR in New York and Hog Island in the Jacques Coustaeu NERR of New Jersey. Pools of DIN in porewater and rates of DIN surface flux were compared in replicated pairs of recently-expanded P. australis and neighboring S. patens-dominated patches on the high marsh surface. Both marshes generally imported nitrate (NO₃⁻) and exported ammonium (NH₄⁺), such that overall DIN was exported. No differences in surface exchange of NO₃⁻ or NH₄⁺ were observed between vegetation types. Depth-averaged porewater NH₄⁺ concentrations over the entire growing season were 56% lower under P. australis than under S. patens (average 1.4 vs. 3.2 mg NH₄⁺ L⁻¹) with the most profound differences in November. Porewater profiles showed an accumulation of NH₄⁺ at depth in S. patens and constant low concentrations in P. australis from the soil surface to 50 cm depth, with no significant differences in porewater salinity. Despite these profound differences in porewater, NH₄⁺ diffusion from soils of P. australis and S. patens were not measurably different, were similar to other published rates, and were well below estimated rates based on passive diffusion alone. Rapid adsorption and uptake by litter and microbes in surface soils of both communities may buffer NH₄⁺ loss to flooding tides in both communities, thereby reducing the impact of P. australis invasion on NH₄⁺ flux to flooding waters.     

Microscale Spatial Distribution of Phragmites australis (Common Reed) Invasion into Spartina patens (Salt Hay)-dominated Communities in Brackish Tidal Marsh

Over the last century, Phragmites australis (common reed) has been expanding rapidly from the marsh–upland boundary into Spartina patens (salt hay)-dominated high marsh communities of the eastern US coast. Whereas direct and indirect human disturbances and changes in hydrology or salinity are likely to influence rates of spread at the landscape scale, the susceptibility of specific plant communities to invasion also influence rates of Phragmites expansion at the local scale. I measured microscale (0.25 m²) spatial patterns of culms (emerging buds and mature stems) in October 1993 at both expanding and stable boundaries of Phragmites populations within a S. patens-dominant matrix. In both expanding and stable plots, Phragmites culms were observed more frequently than expected on hummocks that were created by S. patens tussock-forming root structure. Culm density within a plot was correlated with the percent hummock cover within a plot. Further, Phragmites culms, particularly mature stems, were concentrated along the perimeter of the hummocks. Because the culms were not evenly distributed between hummocks and hollows, I suggest that invasion rates of Phragmites are limited in S. patens communities by microscale differences in hummock availability. The pattern of emergence suggests that expanding rhizomes of Phragmites encounter both competition with S. patens roots on the hummocks and physiological stressors (salinity, anoxia, sulfide concentrations) in the hollows.     

Stable isotope signatures reflect dietary diversity in European forest moths

Background

Information on larval diet of many holometabolous insects remains incomplete. Carbon (C) and nitrogen (N) stable isotope analysis in adult wing tissue can provide an efficient tool to infer such trophic relationships. The present study examines whether moth feeding guild affiliations taken from literature are reflected in isotopic signatures.

Results

Non-metric multidimensional scaling and permutational analysis of variance indicate that centroids of dietary groups differ significantly. In particular, species whose larvae feed on mosses or aquatic plants deviated from those that consumed vascular land plants. Moth δ¹⁵N signatures spanned a broader range, and were less dependent on species identity than δ¹³C values. Comparison between moth samples and ostensible food sources revealed heterogeneity in the lichenivorous guild, indicating only Lithosia quadra as an obligate lichen feeder. Among root-feeding Agrotis segetum, some specimens appear to have developed on crop plants in forest-adjacent farm land. Reed-feeding stem-borers may partially rely on intermediary trophic levels such as fungal or bacterial growth.

Conclusion

Diagnostic partitioning of moth dietary guilds based on isotopic signatures alone could not be achieved, but hypotheses on trophic relationships based on often vague literature records could be assessed with high resolution. Hence, the approach is well suited for basic categorization of moths where diet is unknown or notoriously difficult to observe (i.e. Microlepidoptera, lichen-feeders).

     

Endocytosis at ribbon synapses

Unlike conventional synaptic terminals that release neurotransmitter episodically in response to action potentials, neurons of the visual, auditory and vestibular systems encode sensory information in graded signals that are transmitted at their synapses by modulating the rate of continuous release. The synaptic ribbon, a specialized structure found at the active zones of these neurons, is necessary to sustain the high rates of exocytosis required for continuous release. To maintain the fidelity of synaptic transmission, exocytosis must be balanced by high-capacity endocytosis, to retrieve excess membrane inserted during vesicle fusion. Capacitance measurements following vesicle release in ribbon-type neurons indicate two kinetically distinct phases of compensatory endocytosis, whose relative contributions vary with stimulus intensity. The two phases can be independently regulated and may reflect different underlying mechanisms operating on separate pools of recycling vesicles. Electron microscopy shows diversity among ribbon-type synapses in the relative importance of clathrin-mediated endocytosis versus bulk membrane retrieval as mechanisms of compensatory endocytosis. Ribbon synapses, like conventional synapses, make use of multiple endocytosis pathways to replenish synaptic vesicle pools, depending on the physiological needs of the particular cell type.     

Quantitative comparison of lipoprotein fractions derived from human plasma and serum by liquid chromatography-tandem mass spectrometry

Background  

Lipoproteins are complex, globular molecules which play essential roles in the transport and metabolism of cholesterol. Their implication in the development of cardiovascular diseases, such as atherosclerosis, has sustained a great deal of interest in these particles. Their various functions, and their contribution to the development of atherosclerosis, are often attributed to their protein constituents, which vary greatly among the different lipoprotein classes. Recent advances in the field of mass spectrometry have provided insight into the array of proteins associated with low-density lipoproteins (LDLs) and, even more so, with high-density lipoproteins (HDLs). Plasma and serum are the most commonly used samples for the analysis of lipoproteins. Although these lipoprotein sources are unique, it was our goal to determine whether or not their inherent differences would ultimately affect a quantitative analysis of the LDL and HDL proteomes. To this end, we isolated LDL and HDL fractions with fast protein liquid chromatography-size exclusion chromatography (FPLC-SEC) from both human plasma and serum samples from the same individuals. After delipidating these samples, we performed a quantitative proteomic analysis to compare the lipoprotein-associated proteins derived from both plasma and serum.     

Biogeochemical Implications of Episodic Impoundment in a Restored Tidal Marsh of San Francisco Bay,California

Impounded tidal conditions often compromise coastal marsh restoration goals, through vegetation loss and other biogeochemical feedbacks. To determine if episodic marsh impoundments could be partially responsible for the observed cordgrass (Spartina foliosa) dieback at Crissy Field, Golden Gate National Recreation Area, we examined sulfur chemistry and plant stress along transects between and during tidal inlet closure events from 2007 to 2008. During closures, porewater sulfide (PW S^2?) concentrations did not respond consistently among sites, nor did they increase to levels likely to cause stress damage to cordgrass (>1 mM). However, sediment solid‐phase total reduced sulfur (TRS) concentrations did respond strongly to closures both at surface and subsurface depth intervals, and they were greatest in sites with high organic matter content (>5%). The temporal patterns of both PW S^2? and TRS suggest that while sulfate reduction may be enhanced during closure events, the free sulfide produced is largely precipitated into solid‐phase minerals. Even without millimolar levels of PW S^2?, plant stress was observed during closures, as indicated by a buildup of ethanol in root tissues, a by‐product of fermentative respiration brought on by limited oxygen availability. Further, enhanced sulfate reduction may be related to the higher relative concentrations of methylmercury in low intertidal surface sediments observed during closure events. These data suggest that, in support of vegetated tidal marsh restoration goals, tidal flows should be maintained actively to reduce the impact of impoundment events on marsh biogeochemistry and productivity.