Unlike in the adult brain, the newborn brain speci®cally takes up serum albumin during the postnatal period, coinciding with the stage of maximal brain development. Here we report that albumin stimulates oleic acid synthesis by astrocytes from the main metabolic substrates available during brain development. Oleic acid released by astrocytes is used by neurons for the synthesis of phospholipids and is speci®cally incorporated into growth cones. Oleic acid promotes axonal growth, neuronal clustering, and expression of the axonal growth-associated protein-43, GAP-43; all these observations indicating neuronal differentiation. The effect of oleic acid on GAP-43 synthesis is brought about by the activation of protein kinase C, since it was prevented by inhibitors of this kinase, such as H-7, polymyxin or sphingosine. The expression of GAP-43 was signi®cantly increased in neurons co-cultured with astrocytes by the presence of albumin indicating that neuronal differentiation takes place in the presence of oleic acid synthesized and released by astrocytes in situ. In conclusion, during brain development the presence of albumin could play an important role by triggering the synthesis and release of oleic acid by astrocytes, which induces neuronal differentiation.

We have recently reported that albumin, a serum protein present in the developing brain, stimulates the synthesis of oleic acid by astrocytes, which promotes neuronal differentiation. In this work, we gain insight into the mechanism by which albumin induces the synthesis of this neurotrophic factor. Our results show that astrocytes internalize albumin in vesicle-like structures by receptor-mediated endocytosis. Albumin uptake was followed by transcytosis, including passage through the endoplasmic reticulum, which was required to induce the synthesis of oleic acid. Oleic acid synthesis is feedback-regulated by the sterol regulatory element-binding protein-1, which induces the transcription of stearoylCoA 9-desaturase, the key rate-limiting enzyme for oleic acid synthesis. In our research, the presence of albumin activated the sterol regulatory element-binding protein-1 and increased stearoyl-CoA 9-desaturase mRNA. Moreover, when the activity of sterol regulatory element-binding protein-1 was inhibited by overexpression of a truncated form of this protein, albumin did not affect stearoyl-CoA 9-desaturase mRNA, indicating that the effect of albumin is mediated by this transcription factor. The effect of albumin was abolished when traffic to the endoplasmic reticulum was prevented or when albumin was accompanied with oleic acid. In conclusion, our results suggest that the transcytosis of albumin includes passage through the endoplasmic reticulum, where oleic acid is sequestrated, initiating the signal cascade leading to an increase in its own synthesis.Astrocytes, the main glial cell population in the central nervous system, play a major role in supporting the development of neurons. In fact, astrocytes synthesize and release extracellular matrix proteins and adhesion molecules, which participate not only in the migration of neurons but also in the formation of neuronal aggregates. In addition, astrocytes produce a broad spectrum of growth factors and cytokines, which can regulate the morphology, proliferation, differentiation, and survival of neurons (for a review, see Ref.

MicroRNAs (miRNAs) are small regulatory molecules suppressing mRNA activity in metazoans. Here we describe two new miRNAs cloned from brain tissue of mouse embryos. These miRNAs are expressed mainly during embryogenesis and specifically in the central nervous system. We also established the expression patterns of three recently identified miRNAs that were found in our short RNA library. All of them were expressed in the brain and spinal chord but while miR-410 and miR-431 were central nervous system specific, miR-500 was also expressed in limb buds. In addition, the expression of miR-500 in limb buds showed very strong asymmetry in favour of the left hand side.

Using the scrape-loading technique in cultured astrocytes, we show that sulfonylureas such as tolbutamide and glybenzcyclamide, which inhibit the ATP-sensitive K+ channel, prevent the inhibition of gap junction permeability caused by several structurally unrelated uncouplers such as oleic acid, arachidonic acid, endothelin-1, octanol, and alpha-glycyrrhetinic acid. When the intracellular level of Ca2+ was diminished, all the uncouplers tested were still able to inhibit gap junction communication, indicating that their inhibitory effect was not mediated by Ca2+. In addition, tolbutamide and glybenzcyclamide prevented the inhibitory effect of these uncouplers in Ca(2+)-depleted astrocytes, suggesting that the inhibition of the ATP-sensitive K+ channel increases gap junction permeability through a Ca(2+)-independent mechanism. The activation of the ATP-sensitive K+ channel caused by potassium channel openers such as diazoxide and pinacidil led to the inhibition of gap junction communication and overcame the effect of sulfonylureas. These results suggest that the ATP-sensitive K+ channel regulates gap junctional permeability.

Using the scrape-loading technique we show that tolbutamide and glybenzcyclamide, two inhibitors of the K + channel sensitive to ATP (K-ATP channel), partially prevent the inhibition of gap junction permeability promoted by Ca 2+ in cultured astrocytes. This effect was dose-dependent, reaching a maximum at 400 W WM and 1 W WM of tolbutamide and glybenzcyclamide, respectively. The presence of the Ca 2+ ionophore A-23187 strongly reduced the concentration of Ca 2+ required to block gap junction permeability but did not abolish the effect of tolbutamide and glybenzcyclamide. These results suggest that the effect of these two compounds are not brought about by control of the intracellular concentration of Ca 2+ but probably by the promotion of plasma membrane depolarization.z 1998 Federation of European Biochemical Societies.