Alzheimer disease models and human neuropathology: similarities and differences

Animal models aim to replicate the symptoms, the lesions or the cause(s) of Alzheimer disease. Numerous mouse transgenic lines have now succeeded in partially reproducing its lesions: the extracellular deposits of Aβ peptide and the intracellular accumulation of tau protein. Mutated human APP transgenes result in the deposition of Aβ peptide, similar but not identical to the Aβ peptide of human senile plaque. Amyloid angiopathy is common. Besides the deposition of Aβ, axon dystrophy and alteration of dendrites have been observed. All of the mutations cause an increase in Aβ 42 levels, except for the Arctic mutation, which alters the Aβ sequence itself. Overexpressing wild-type APP alone (as in the murine models of human trisomy 21) causes no Aβ deposition in most mouse lines. Doubly (APP × mutated PS1) transgenic mice develop the lesions earlier. Transgenic mice in which BACE1 has been knocked out or overexpressed have been produced, as well as lines with altered expression of neprilysin, the main degrading enzyme of Aβ. The APP transgenic mice have raised new questions concerning the mechanisms of neuronal loss, the accumulation of Aβ in the cell body of the neurons, inflammation and gliosis, and the dendritic alterations. They have allowed some insight to be gained into the kinetics of the changes. The connection between the symptoms, the lesions and the increase in Aβ oligomers has been found to be difficult to unravel. Neurofibrillary tangles are only found in mouse lines that overexpress mutated tau or human tau on a murine tau −/− background. A triply transgenic model (mutated APP, PS1 and tau) recapitulates the alterations seen in AD but its physiological relevance may be discussed. A number of modulators of Aβ or of tau accumulation have been tested. A transgenic model may be analyzed at three levels at least (symptoms, lesions, cause of the disease), and a reading key is proposed to summarize this analysis

Sustainable timber harvesting in Venezuela: a modelling approach

Reliable data on the growth and yield of logged-over forest to determinesustainable cutting cycles are widely missing for the tropics. We used theprocess-based model Formind2.0 to analyse the growth and yield of logged-overforest in Venezuela under different logging scenarios over a period of 240years and compared results with unlogged stands.The performance of the modelwas evaluated with a detailed stability and sensitivity analysis.In theabsence of further logging,the logged-over stand approached the standstructure of mature forest in terms of bole volume and basal area afterabout 50-100 years.Thirty-year cutting cycles with conventional loggingmethods and net extraction volumes of 45 and 60 m^3/(ha cycle) did not providesustainable yields under either of two minimum felling diameters(35 or 50 cm) that were applied. Only the 60-year cutting cycle providedsustainable yields under conventional and reduced-impact logging with thedi minimum felling diameters and a range of net volumes extracted(30-60 m^3/(ha cycle). With the longest cutting cycle (60 years), bole volumerecovered to levels similar to the mature unlogged stand, but the speciescomposition was very different.Scenarios with reduced-impact logging provideda significantly higher timber volume than under conventional logging. Theconservation of forest resources will only be possible with long cuttingcycles (at least 60 years) in combination with reduced-impact logging

Simulating logging scenarios in secondary forest embedded in a fragmented neotropical landscape

In publications on tropical forest fragmentation regrowing secondary forests(SF) on abandoned agricultural land, which are highly fragmented components inthe landscape matrix (generally less than 100 ha in size), are often overlooked.This forest type is found on privately owned land. These forest fragments arecontributing to on-farm income by logging and/or use of non-timber products.Hence, defining pure conservation goals for this forest type seems to beunrealistic. In this paper, we used the process-based model FORMIX3-Q tosimulate successional processes and logging scenarios in SF exposed todifferent degrees of fragmentation (either facing non-forest land on onlyone side or totally surrounded by agricultural land) in the subtropicaleastern part of Paraguay. We compared results with primary forest embeddedin a similar matrix. Under light fragmentation, bole volume of SF approachedto primary forest values after about 50 years of succession. Speciescomposition, however, was clearly distinct from primary forest over thefirst 200 years of succession. The development of bole volume and speciescomposition in severely fragmented SF was similar to the less fragmented SFover the initial 50 years. However, limited seed input, largely confined toon-site seed sources in the severely fragmented SF, led to a decline in bolevolume by about one-third compared to the reference value over the simulationperiod of 400 years. By applying a minimum felling diameter (MFD) of 35 cm inthe lightly fragmented SF, first logging was only possible after 30 years,harvesting a mere 3.7 m/ha. Highest timber yields were obtained with 10-yrcutting cycles and a maximum removal of 20 stems, though only providingsustainable yields by lowering the MFD to 30 cm. Logging in severelyfragmented SF accelerated the decline of bole volume. Overall, resultssuggest that timber production in SF without silvicultural treatmentsis not a viable option to diversify farm income. Only managed SF mayserve this function. Research focusing on the elaboration of managementconcepts for SF, integrating as much as possible of the originalvegetation, is needed