Life on the edge: the plankton and chemistry of Beaver Lake, an ultra-oligotrophic epishelf lake, Antarctica

1. Beaver Lake, a large epishelf lake in eastern Antarctica was sampled on two occasions during the austral summer of 2000. Two sites, one 1 km offshore and another 6 km offshore were sampled at intervals to depths of 40 and 110 m, respectively. 2. The lake is an end member of ultra‐oligotrophic lake systems with a very low carbon pool. Dissolved organic carbon concentrations ranged between 95 and 652 μg L^–1. Nutrient levels were generally low with soluble reactive phosphorus ranging from undetectable to 8.4 μg L^–1, ammonium ranged between 1.8 and 5.0 μg L^–1, nitrate from undetectable to 161 μg L^–1 and nitrite 1.1–5.3 μg L^–1. 3. Chlorophyll a concentrations (0.39–4.38 μg L^–1) showed an unusual distribution with the highest levels close to the lake bottom at the offshore site (110 m) where the phototrophic nanoflagellates (PNAN) displayed strong autofluorescence. 4. Bacterial concentrations were low, with a maximum of 7.60 × 10⁷ L^–1, as were the concentrations of heterotrophic nanoflagellates that exploit them. 5. Primary production ranged between 19.7 and 25.49 μg C L^–1 day^–1 and bacterial production from 0.32 to 1.15 μg C L^–1 day^–1. 6. In common with other continental Antarctic lakes, the system was dominated by a microbial plankton. However, a dwarf variety of the calanoid copepod, Boeckella poppei, occurred below 25 m at concentrations of 3–5 L^–1. 7. The data suggest that primary production and bacterial production were not limited by nutrient availability, but by other factors, e.g. in the case of bacterial production by organic carbon concentrations and primary production by low temperatures.     

Progranulin promotes neurite outgrowth and neuronal differentiation by regulating GSK-3β

Progranulin (PGRN) has recently emerged as a key player in a subset of frontotemporal dementias (FTD). Numerous mutations in the progranulin gene have been identified in patients with familial or sporadic frontotemporal lobar degeneration (FTLD). In order to understand the molecular mechanisms by which PGRN deficiency leads to FTLD, we examined activity of PGRN in mouse cortical and hippocampal neurons and in human neuroblastoma SH-SY5Y cells. Treatment of mouse neurons with PGRN protein resulted in an increase in neurite outgrowth, supporting the role of PGRN as a neurotrophic factor. PGRN treatment stimulated phosphorylation of glycogen synthase kinase-3 beta (GSK-3β) in cultured neurons. Knockdown of PGRN in SH-SY5Y cells impaired retinoic acid induced differentiation and reduced the level of phosphorylated GSK-3β. PGRN knockdown cells were also more sensitized to staurosporine- induced apoptosis. These results reveal an important role of PGRN in neurite outgrowth and involvement of GSK-3β in mediating PGRN activity. Identification of GSK-3β activation as a downstream event for PGRN signaling provides a mechanistic explanation for PGRN activity in the nervous system. Our work also suggest that loss of axonal growth stimulation during neural injury repair or deficits in axonal repair may contribute to neuronal damage or axonal loss in FTLD associated with PGRN mutations. Finally, our study suggests that modulating GSK-3β or similar signaling events may provide therapeutic benefits for FTLD cases associated with PGRN mutations.     

Characterisation of 5-HT3C, 5-HT3D and 5-HT3E receptor subunits: evolution, distribution and function

The 5‐HT₃ receptor is a member of the ‘Cys‐loop’ family of ligand‐gated ion channels that mediate fast excitatory and inhibitory transmission in the nervous system. Current evidence points towards native 5‐HT₃ receptors originating from homomeric assemblies of 5‐HT_3A or heteromeric assembly of 5‐HT_3A and 5‐HT_3B. Novel genes encoding 5‐HT_3C, 5‐HT_3D, and 5‐HT_3E have recently been described but the functional importance of these proteins is unknown. In the present study, in silico analysis (confirmed by partial cloning) indicated that 5‐HT_3C, 5‐HT_3D, and 5‐HT_3E are not human–specific as previously reported: they are conserved in multiple mammalian species but are absent in rodents. Expression profiles of the novel human genes indicated high levels in the gastrointestinal tract but also in the brain, Dorsal Root Ganglion (DRG) and other tissues. Following the demonstration that these subunits are expressed at the cell membrane, the functional properties of the recombinant human subunits were investigated using patch clamp electrophysiology. 5‐HT_3C, 5‐HT_3D, and 5‐HT_3E were all non‐functional when expressed alone. Co‐transfection studies to determine potential novel heteromeric receptor interactions with 5‐HT_3A demonstrated that the expression or function of the receptor was modified by 5‐HT_3C and 5‐HT_3E, but not 5‐HT_3D. The lack of distinct effects on current rectification, kinetics or pharmacology of 5‐HT_3A receptors does not however provide unequivocal evidence to support a direct contribution of 5‐HT_3C or 5‐HT_3E to the lining of the ion channel pore of novel heteromeric receptors. The functional and pharmacological contributions of these novel subunits to human biology and diseases such as irritable bowel syndrome for which 5‐HT₃ receptor antagonists have major clinical usage, therefore remains to be fully determined.     

Geographical variation in the periodicity of gypsy moth outbreaks

The existence of periodic oscillations in populations of forest Lepidoptera is well known. While information exists on how the periods of oscillations vary among different species, there is little prior evidence of variation in periodicity within the range of a single Lepidopteran species. The exotic gypsy moth is an introduced foliage-feeding insect in North America. Its populations are characterized by high amplitude oscillations between innocuously low densities and outbreak levels during which large regions of forests are defoliated. These outbreaks are recognized to arise periodically with considerable synchrony across much of the gypsy moth's North American range. Our analysis indicates that gypsy moth outbreaks in North America are periodic but they exhibit two dominant periodicities: a primary period of 8–10 yr (as previously reported) and a secondary period of 4–5 yr (a new finding in this study). The outbreak periodicity varied geographically and this variation was associated with forest type. We found that in the most susceptible forest types, those on xeric sites where oak is often mixed with pines, outbreak periodicity had a more dominant 5-yr period while in forest types characteristic of more mesic sites where oak was mixed with maples and other species, cycles were clearly operating on a 10-yr period.