Frank W. Pfrieger

Role of cholesterol in synapse formation and function.

Cholesterol is a multifaceted molecule, which serves as essential membrane component, as cofactor for signaling molecules and as precursor for steroid hormones. Consequently, defects in cholesterol metabolism cause devastating diseases. So far, the role of cholesterol in the nervous system is less well understood. Recent studies showed that cultured neurons from the mammalian central nervous system (CNS) require glia-derived cholesterol to form numerous and efficient synapses. This suggests that the availability of cholesterol in neurons limits the extent of synaptogenesis. Here, I will summarize the experimental evidence for this hypothesis, describe what is known about the structural and functional role of cholesterol at synapses, and discuss how cholesterol may influence synapse development and stability.

New roles for astrocytes: Regulation of CNS synaptogenesis

The notion that astrocytes have a profound influence on the function of synapses between CNS neurons implies that the development of synaptic connections and their glial neighbors are controlled by reciprocally acting signals. Currently, however, synaptogenesis is considered a purely neuronal affair. This article summarizes recent experimental evidence suggesting that this may not be the case. Astrocytes may indeed regulate the formation, maturation and maintenance of synapses. The recent advances caution that synapses cannot develop correctly without astrocytes. Further progress on this issue requires new experimental models to identify signaling pathways and to scrutinize the relevance of glia-synapse interactions in vivo.

Marked differences in cholesterol synthesis between neurons and glial cells from postnatal rats.

Neurons have a high demand for cholesterol to develop and maintain membrane‐rich structures like axons, dendrites and synapses, but it remains unclear, whether they can satisfy their need by costly de novo synthesis. To address this, we compared cholesterol synthesis in serum‐free cultures of highly purified CNS neurons and glial cells from postnatal rats. We observed marked cell‐specific differences: Compared with glial cells, neurons showed different profiles of biosynthetic enzymes, post‐squalene precursors and cholesterol metabolites, and they produced cholesterol less efficiently, possibly because of very low levels of lanosterol‐converting enzymes. Astrocytes responded to inhibition of cholesterol synthesis with a much stronger up‐regulation of biosynthetic enzymes than neurons. Our results support the idea that neurons cannot produce cholesterol efficiently and that they depend on an external source of this lipid.

Role of glia-derived cholesterol in synaptogenesis: new revelations in the synapse-glia affair

Brain development and function relies on the exchange of signals between neurons and glial cells. Here we review a series of recent studies on cultures of purified retinal ganglion cells (RGCs) that point to a new role of glial cells in the formation and plasticity of synaptic connections. The results suggest that neurons must import glia-derived cholesterol via lipoproteins to form numerous and efficient synaptic connections. This finding may explain why throughout the central nervous system (CNS) the main phase of synaptogenesis starts synchronously after glia differentiation and why astrocytes produce apolipoprotein E (apoE) and cholesterol-containing lipoproteins. Experimental tests of these hypotheses may further our understanding of the cholesterol metabolism in the brain and may help to explain neurologic symptoms resulting from defective cholesterol and lipoprotein metabolism.

Transgenic mice for conditional gene manipulation in astroglial cells

Astrocytes are thought to exert diverse functions in the brain, but it has been difficult to prove this in vivo because of a scarcity of tools to manipulate these cells. Here, we report the generation of new transgenic mouse lines that allow for conditional gene ablation in astrocytes using the tamoxifen‐ (TAM‐) inducible CreERT2/loxP system and bacterial artificial chromosome (BAC)‐based transgenesis. In adult transgenic mice, where CreERT2 expression is driven by the promoter of the sodium‐dependent glutamate/aspartate transporter (Glast/Slc1a3) or of connexin 30 (Cx30/Gjb6), intraperitoneal TAM‐injection induced Cre‐mediated recombination in astroglial cells throughout the brain. Targeting efficacies varied in a region‐specific manner from 20 to 90% as indicated by enzyme‐based reporter lines and immunohistochemical staining. In addition, the Glast‐line allowed to target retinal Müller cells and adult neural stem/progenitor cells in neurogenic regions of the adult brain. Transgenic mice expressing CreERT2 under the control of the apolipoprotein e (ApoE) or aquaporin 4 (Aqp4) promoter showed inducible recombination in different areas of the central nervous system (CNS) albeit at low levels. Transgenic lines showed TAM‐induced recombination in specific peripheral organs. These new mouse lines should help to further explore the relevance of astrocytes for brain function, as well as their contribution to pathological conditions because of aging, disease or injury.

Synaptic plasticity, astrocytes and morphological homeostasis

Recent discoveries suggest that astrocytes are an integral part of synaptic connections, as they sense and modulate synaptic activity. Moreover, there is evidence that astrocytes change the number of synaptic connections directly via synaptogenic signals or indirectly, by modifying the morphology of axons and dendrites. Here, we formulate the hypothesis that astrocytes mediate the morphological homeostasis of nerve cells, which is any adaptation of the morphology of a neuron to preserve its ability to respond to and generate synaptic activity during learning and memory-induced changes. We argue that astrocytes control neuronal morphology locally and across long-ranging assemblies of neurons and that on the other hand, astrocytes are part of the engram with plasticity-related changes affecting their morphology.

Glia-induced neuronal differentiation by transcriptional regulation

There is increasing evidence that different phases of brain development depend on neuron–glia interactions including postnatal key events like synaptogenesis. To address how glial cells influence synapse development, we analyzed whether and how glia‐derived factors affect gene expression in primary cultures of immunoisolated rat retinal ganglion cells (RGCs) by oligonucleotide microarrays. Our results show that the transcript pattern matched the developmental stage and characteristic properties of RGCs in vitro. Glia‐conditioned medium (GCM) and cholesterol up‐ and downregulated a limited number of genes that influence the development of dendrites and synapses and regulate cholesterol and fatty acid metabolism. The oligonucleotide microarrays detected the transcriptional regulation of neuronal cholesterol homeostasis in response to GCM and cholesterol treatment. Surprisingly, our study revealed neuronal expression and glial regulation of matrix gla protein (Mgp). Together, our results suggest that glial cells promote different aspects of neuronal differentiation by regulating transcription of distinct classes of genes.

Role of astrocytes in the formation, maturation and maintenance of synapses

Views on the functions of astrocytes have changed within the last few years and mainstream neuroscientists have begun to acknowledge that these cells, once considered as mere filling mass, have a profound influence on neurons (for reviews see Laming et al., 2000; Araque et al., 2001; Bezzi and Volterra, 2001; Castonguay et al., 2001; Haydon, 2001; Volterra et al., 2002). A look at recent reviews on synaptogenesis (Lee and Sheng, 2000; Zhang and Benson, 2000; Dresbach et al., 2001; Featherstone and Broadie, 2000; Sanes and Lichtman, 2001; Garner et al., 2002; Ahmari and Smith, 2002; Cohen-Cory, 2002; Jin, 2002; Okabe, 2002; Zito and Svoboda, 2002; Scheiffele, 2003) reveals, however, that this fundamental process is still considered to be merely a play between two neuronal partners. Here, we summarize evidence suggesting that this may not hold true, and that there is a third actor on stage — the astrocyte. As with other types of liaisons, the life of a synapse can be divided into three sequential phases: 1) the establishment of a physical contact, 2) a maturation process, which endows the synapse with its characteristic properties and 3) a stabilization or elimination phase, where only selected connections are maintained. Here, we discuss the contribution of astrocytes to each phase (Fig. 17.1). Their roles in other aspects of neuronal development have been summarized elsewhere (Peles and Salzer, 2000; Wang and Barres, 2000; Gomes et al., 2001; Klambt et al., 2001; Lemke, 2001; Campbell and Gotz, 2002; Du and Dreyfus, 2002; Mirsky et al., 2002; Nadarajah and Parnavelas, 2002; Oland and Tolbert, 2003).