Research focus

​One of the goals of our group is to unravel the basic mechanisms of neuronal polarization, i.e. the position of axon and dendrites in a neuron. This process starts with the migration of the last post-mitotic neuroblasts from the germinal layer to their final destination in the cortical layer, the generation of short cytoplasmic extensions serving as sensors for the extracellular environment and as a stop-signal to determine its final positioning. The accuracy of this process is of fundamental importance for proper neuronal connectivity.

To address this issue, we have largely relied on neuron differentiation experiments with rodent embryonic hippocampal neurons in vitro. The current project will study this process in an in situ environment using wild-type and genetically modified mice and Drosophila melanogaster.
 
Generation and maintenance of plasma membrane asymmetry is essential for proper cell function in any polarized cell, including neurons. In neurons, action potentials are produced by a peculiar  molecular composition of the axonal membrane, whereas dendritic electronic potentials and growth factors’ uptake are determined by the segregated positioning of a series of specific receptors.

From studies in different cell types, it is today accepted that this initial step in membrane segregation occurs in the early secretory pathway (trans Golgi network and/or sorting endosomes). As protein over-expression studies cannot provide us with a realistic image of the situation, we are now generating neural-stem cells constitutively expressing GFP-tagged membrane proteins in order to study the neuronal membrane protein sorting and transport.

In addition to these projects, in which we try to unravel the basic mechanisms in neuronal development and differentiation, our research is also focusing on pathological indications in which neural differentiation is affected in early-life as well as late-life mental defects.

In the field of early-life mental defects we use Niemann Pick disease type A mice as an experimental model in which mutations in the acid sphingomyelinase (ASM) cause a biochemical defect in the turnover of sphygomyeline to ceramide in the lysosomes. This model is also used to mimic the human disease, where we study which type of neuronal functions are altered by this biochemical defect.

Plasma membranes of hippocampi of Alzheimer’s disease patients present a 25-30% reduction in cholesterol content compared to controls. In two experimental settings, lowering cholesterol content (biochemically reduced cholesterol in embryonic hippocampal neurons, and genetically reduced cholesterol in hippocampal neurons from mice lacking one copy of an enzyme involved in cholesterol synthesis), we have illustrated the displacement of raft-enriched proteins s.a. PrPc, BACE1 and flotillin and an increase in A beta production upon such raft disorganization.

Our main concern is to understand whether such higher A beta production could be taken as one of the causes leading to amyloid accumulation, or whether it is a result of the death/survival signaling of neurons defending themselves from an external/internal insult.

In addition, we generate transgenic flies expressing a modified version human APP under the control of a ubiquitous heat shock promoter. In these flies, APP is expressed as a chimeric protein with the transmembrane protein fragment of human APP fused to the yeast transcription factor GAL4 at its cytoplasmic domain. Upon gamma cleavage, Gal4 is transported to the nucleus and activates a GFP-reporter gene in a presenilin-dependent matter. This tool is used to study modulators of gamma secretase cleavage in vivo. In addition, using an in-vitro approach, we study gamma cleavage in stable S2 cell lines expressing the transmembrane protein fragment of human APP in which we manipulate gamma secretase activity by double-stranded RNA interference (RNAi).