Calcium-permeable neurotransmitter receptors are concentrated into structurally and biochemically isolated cellular compartments to localize calcium-mediated events during neurotransmission. The cytoplasmic membrane contains lipid microdomains called lipid rafts, which can gather into microscopically visible clusters, and thus the association of a particular protein with lipid rafts can result in its redistribution on the cell surface. The present study asks whether lipid rafts participate in the formation and maintenance of the calcium-permeable alpha7-subunit nicotinic acetylcholine receptor (alpha7nAChR) clusters found in somatic spines of ciliary neurons. Lipid rafts and alpha7nAChR become progressively colocalized within somatic spines during synaptogenesis. To determine whether these rafts are required for the maintenance of alpha7nAChR aggregates, cholesterol was extracted from dissociated ciliary neurons by treatment with methyl-beta-cyclodextrin. This treatment caused the dispersion of lipid rafts and the redistribution of alpha7nAChR into small clusters over the cell surface, suggesting that the integrity of lipid rafts is required to maintain the receptor clustering. However, lipid raft dispersion also caused the depolymerization of the F-actin cytoskeleton, which can also tether the receptor at specific sites. To assess whether interaction between rafts and alpha7nAChR is independent of F-actin filaments, the lipid raft patches were stabilized with a combination of the cholera toxin B subunit (CTX), which specifically binds to the raft component ganglioside GM1, and an antibody against CTX. The stabilized rafts were then treated with latrunculin-A to depolymerize F-actin. Under these conditions, large patches of CTX persisted and were colocalized with alpha7nAChR, indicating that the aggregates of receptors can be maintained independently of the underlying F-actin cytoskeleton. Moreover, it was found that the alpha7nAChR is resistant to detergent extraction at 4 degrees C and floats with the caveolin-containing lipid-rich fraction during density gradient centrifugation, properties that are consistent with a direct association between the receptor and the membrane microdomains.

Intermediate filaments (IFs) are a major component of the cytoskeleton in astrocytes. Their role is far from being completely understood. Immature astrocytes play a major role in neuronal migration and neuritogenesis, and their IFs are mainly composed of vimentin. In mature differentiated astrocytes, vimentin is replaced by the IF protein glial fibrillary acidic protein (GFAP). In response to injury of the CNS in the adult, astrocytes become reactive, upregulate the expression of GFAP, and reexpress vimentin. These modifications contribute to the formation of a glial scar that is obstructive to axonal regeneration. Nevertheless, astrocytes in vitro are considered to be the ideal substratum for the growth of embryonic CNS axons. In the present study, we have examined the potential role of these two major IF proteins in both neuronal survival and neurite growth. For this purpose, we cocultured wild-type neurons on astrocytes from three types of knock-out (KO) mice for GFAP or/and vimentin in a neuron-astrocyte coculture model. We show that the double KO astrocytes present many features of immaturity and greatly improve survival and neurite growth of cocultured neurons by increasing cell-cell contact and secreting diffusible factors. Moreover, our data suggest that the absence of vimentin is not a key element in the permissivity of the mutant astrocytes. Finally, we show that only the absence of GFAP is associated with an increased expression of some extracellular matrix and adhesion molecules. To conclude, our results suggest that GFAP expression is able to modulate key biochemical properties of astrocytes that are implicated in their permissivity.

The pituitary gland has long been considered to be a random patchwork of hormone-producing cells. By using pituitary-scale tridimensional imaging for two of the least abundant cell lineages, the corticotropes and gonadotropes, we have now uncovered highly organized and interdigitated cell networks that reflect homotypic and heterotypic interactions between cells. Although newly differentiated corticotrope cells appear on the ventral surface of the gland, they rapidly form homotypic strands of cells that extend from the lateral tips of the anterior pituitary along its ventral surface and into the medial gland. As the corticotrope network is established away from the microvasculature, cell morphology changes from rounded, to polygonal, and finally to cells with long cytoplasmic processes or cytonemes that connect corticotropes to the perivascular space. Gonadotropes differentiate later and are positioned in close proximity to corticotropes and capillaries. Blockade of corticotrope terminal differentiation produced by knockout of the gene encoding the transcription factor Tpit results in smaller gonadotropes within an expanded cell network, particularly in the lateral gland. Thus, pituitary-scale tridimensional imaging reveals highly structured cell networks of unique topology for each pituitary lineage. The sequential development of interdigitated cell networks during organogenesis indicate that extensive cell:cell interactions lead to a highly ordered cell positioning rather than random patchwork.reproduction | stress | systems biology | pro-opiomelanocortin