FGF22 and its close relatives are presynaptic organizing molecules in the mammalian brain

Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis

The establishment of neural circuitry requires vast numbers of synapses to be generated during a specific window of brain development, but it is not known why the developing mammalian brain has a much greater capacity to generate new synapses than the adult brain. Here we report that immature but not mature astrocytes express thrombospondins (TSPs)-1 and -2 and that these TSPs promote CNS synaptogenesis in vitro and in vivo. TSPs induce ultrastructurally normal synapses that are presynaptically active but postsynaptically silent and work in concert with other, as yet unidentified, astrocyte-derived signals to produce functional synapses. These studies identify TSPs as CNS synaptogenic proteins, provide evidence that astrocytes are important contributors to synaptogenesis within the developing CNS, and suggest that TSP-1 and -2 act as a permissive switch that times CNS synaptogenesis by enabling neuronal molecules to assemble into synapses within a specific window of CNS development.

Patterning molecules; multitasking in the nervous system

Classical patterning molecules that were previously implicated in controlling cell fate choices in the nervous system have recently been shown to play additional roles in axon guidance and synaptogenesis. Bone morphogenetic proteins (BMPs), Sonic hedgehog (Shh), Wnts, and fibroblast growth factors (FGFs) all participate in multiple acts of controlling neuronal circuit assembly. Depending on the cellular context, they can provide instructive signals at the growth cone or synapse or alternatively can elicit responses in the nucleus initiating transcriptional changes. Differences in signal transduction pathways may contribute to the diversity of the functional repertoire of these versatile molecules.

Progress from the postsynaptic side: signaling in synaptic differentiation

The signals mediating synaptic differentiation are critical for our understanding of synapse formation and nervous system development. Recent progress highlights the roles of membrane-bound and soluble signaling pathways in this process. The postsynaptic adhesion molecules neuroligin-1, neuroligin-2, and SynCAM1 provide target-derived signals leading to the assembly of presynaptic terminals. In the reverse direction, neuroligins also relay signals into postsynaptic sites through interactions with their presynaptic partners, the beta-neurexins. This adhesion-based system helps to determine the neurotransmitter specificity of postsynaptic specializations. In addition, soluble molecules are released from target neurons and initiate presynaptic differentiation, as demonstrated for the fibroblast growth factor FGF-22. This Perspective summarizes new insights into early synaptic differentiation signals and discusses underlying principles.

Cellular signaling by fibroblast growth factor receptors

The 22 members of the fibroblast growth factor (FGF) family of growth factors mediate their cellular responses by binding to and activating the different isoforms encoded by the four receptor tyrosine kinases (RTKs) designated FGFR1, FGFR2, FGFR3 and FGFR4. Unlike other growth factors, FGFs act in concert with heparin or heparan sulfate proteoglycan (HSPG) to activate FGFRs and to induce the pleiotropic responses that lead to the variety of cellular responses induced by this large family of growth factors. A variety of human skeletal dysplasias have been linked to specific point mutations in FGFR1, FGFR2 and FGFR3 leading to severe impairment in cranial, digital and skeletal development. Gain of function mutations in FGFRs were also identified in a variety of human cancers such as myeloproliferative syndromes, lymphomas, prostate and breast cancers as well as other malignant diseases. The binding of FGF and HSPG to the extracellular ligand domain of FGFR induces receptor dimerization, activation and autophosphorylation of multiple tyrosine residues in the cytoplasmic domain of the receptor molecule. A variety of signaling proteins are phosphorylated in response to FGF stimulation including Shc, phospholipase-Cgamma, STAT1, Gab1 and FRS2alpha leading to stimulation of intracellular signaling pathways that control cell proliferation, cell differentiation, cell migration, cell survival and cell shape. The docking proteins FRS2alpha and FRS2beta are major mediators of the Ras/MAPK and PI-3 kinase/Akt signaling pathways as well as negative feedback mechanisms that fine-tune the signal that is initiated at the cell surface following FGFR stimulation.

Identification of candidate Purkinje cell-specific markers by gene expression profiling in wild-type and pcd(3J) mice

The identification of mRNAs that have restricted expression patterns in the brain represents powerful tools with which to characterize and manipulate the nervous system. Here, we describe a strategy using microarray technology (Affymetrix Mouse Genome 430 2.0 Arrays) to identify mRNA transcripts that are candidate markers of cerebellar Purkinje neurons. Initially, gene expression profiles were compared between cerebella of 4-month-old Purkinje cell degeneration (pcd(3J)) mice, in which most Purkinje cells had already degenerated and wild-type littermates with a normal complement of Purkinje neurons. Of 14,563 probe sets expressed in wild-type cerebellum, 797 showed a significant (p<0.0001) reduction in pcd(3J) mice. These probes could represent transcripts with varying levels of specificity for Purkinje cells as well as transcripts in other cell types that decline as a secondary consequence of Purkinje cell loss. Ranking of the probe signals revealed that well-known Purkinje cell-specific transcripts such as calbindin and L7/pcp2 clustered in a group that was <33% of wild-type levels. Therefore, to identify potentially new Purkinje cell-specific transcripts that cluster with the known markers, more stringent selection criteria were applied (<33% of wild-type signal and p<0.0001). With these criteria, 55 independent transcripts were identified of which 33 were annotated genes and 22 were ESTs and RIKEN cDNAs. A literature search revealed that 25 of the 33 annotated genes were expressed in Purkinje cells, with no data being available on the other 8. Thus, the additional 8 annotated and 22 un-annotated genes are clustered with many genes expressed in Purkinje cells making them candidate markers. To confirm the microarray data, eight representative annotated genes were selected including five reported to be in Purkinje neurons and three for which no data was available. Semi-quantitative RT-PCR demonstrated reduced expression of all eight transcripts in cerebella from pcd(3J) mice. The promoters of genes expressed selectively in subsets of neurons can be used to direct heterologous gene expression in transgenic mice and the more restricted the expression pattern the greater their utility. Therefore, microarray analysis was used to assess expression levels of all 55 transcripts in cerebral cortex, striatum, substantia nigra and ventral tegmental area. This permitted the identification of a set of genes whose promoters might have utility for selectively targeting gene expression to cerebellar Purkinje cells.