Very little is known about the biophysical properties and the lipid or protein composition of membrane domains presumably present in endocytic and biosynthetic organelles. Here we analyzed the membrane composition of late endosomes by suborganellar fractionation in the absence of detergent. We found that the internal membranes of this multivesicular organelle can be separated from the limiting membrane and that each membrane population exhibited a defined composition. Our data also indicated that internal membranes may consist of at least two populations, containing primarily phosphatidylcholine or lysobisphosphatidic acid as major phospholipid, arguing for the existence of significant microheterogeneity within late endosomal membranes. We also found that lysobisphosphatidic acid exhibited unique pH-dependent fusogenic properties, and we speculated that this lipid is an ideal candidate to regulate the dynamic properties of this internal membrane mosaic.

Ena/VASP proteins play important roles in axon outgrowth and guidance. Ena/VASP activity regulates the assembly and geometry of actin networks within fibroblast lamellipodia. In growth cones, Ena/VASP proteins are concentrated at filopodia tips, yet their role in growth cone responses to guidance signals has not been established. We found that Ena/VASP proteins play a pivotal role in formation and elongation of filopodia along neurite shafts and growth cone. Netrin-1-induced filopodia formation was dependent upon Ena/VASP function and directly correlated with Ena/VASP phosphorylation at a regulatory PKA site. Accordingly, Ena/VASP function was required for filopodial formation from the growth cone in response to global PKA activation. We propose that Ena/VASP proteins control filopodial dynamics in neurons by remodeling the actin network in response to guidance cues.

We report that lipids contribute to regulate the bidirectional motility of late endocytic compartments. Late endocytic vesicles loaded with cholesterol lose their dynamic properties, and become essentially immobile, including in cells from Niemann–Pick C patients. These vesicles then retain cytoplasmic dynein activity, but seem to be unable to acquire kinesin activity, eventually leading to paralysis. Our data suggest that this defect depends on the small GTPase Rab7, since the motility of vesicles loaded with cholesterol can be restored by the Rab7 inhibitory mutant N125I. Conversely, wild‐type Rab7 overexpression mimics the effects of cholesterol on motility in control cells. Consistently, cholesterol accumulation increases the amounts of membrane‐associated Rab7, and inhibits Rab7 membrane extraction by the guanine nucleotide dissociation inhibitor. Our observations thus indicate that cholesterol contributes to regulate the Rab7 cycle, and that Rab7 in turn controls the net movement of late endocytic elements. We conclude that motor functions can be regulated by the membrane lipid composition via the Rab7 cycle.

Serotonin (5-HT) has been shown to affect the development and patterning of the mouse barrelfield. We show that the dense transient 5-HT innervation of the somatosensory, visual, and auditory cortices originates in the thalamus rather than in the raphe: 5-HT is detected in thalamocortical fibers and most 5-HT cortical labeling disappears after thalamic lesions. Thalamic neurons do not synthesize 5-HT but take up exogenous 5-HT through 5-HT high affinity uptake sites located on thalamocortical axons and terminals. 3H-5-HT injected into the cortex is retrogradely transported to thalamic neurons. In situ hybridization shows a transient expression of the genes encoding the serotonin transporter and the vesicular monoamine transporter in thalamic sensory neurons. In these glutamatergic neurons, internalized 5-HT might thus be stored and used as a "borrowed transmitter" for extraneuronal signaling or could exert an intraneuronal control on thalamic maturation.

In the present study, we show that in human endothelial cells the tetraspanin CD63/lamp3 distributes predominantly to the internal membranes of multivesicular-multilamellar late endosomes, which contain the unique lipid lysobisphosphatidic acid. Some CD63/lamp3 is also present in Weibel-Palade bodies, the characteristic secretory organelle of these cells. We find that CD63/lamp3 molecules can be transported from late endosomes to Weibel-Palade bodies and thus that CD63/lamp3 cycles between endocytic and biosynthetic compartments; however, movement of CD63/lamp3 is much slower than that of P-selectin, which is known to cycle between plasma membrane and Weibel-Palade bodies. When cells are treated with U18666A, a drug that mimics the Niemann-Pick type C syndrome, both proteins accumulate in late endosomes and fail to reach Weibel-Palade bodies efficiently, suggesting that P-selectin, like CD63/lamp3, cycles via late endosomes. Our data suggest that CD63/lamp3 partitions preferentially within late endosome internal membranes, thus causing its accumulation, and that this mechanism contributes to CD63/lamp3 retention in late endosomes; however, our data also indicate that the protein can eventually escape from these internal membranes and recycle toward Weibel-Palade bodies to be reused. Our observations thus uncover the existence of a selective trafficking route from late endosomes to Weibel-Palade bodies.