No abstract

Higher organisms rely on multiple modes of memory storage using the hippocampal network, which is built by precisely orchestrated mechanisms of axonal outgrowth, guidance and synaptic targeting. We demonstrate essential roles of the transcription factor serum response factor (SRF), a sensor of cytoskeletal actin dynamics, in all these processes. Conditional deletion of the mouse Srf gene reduced neurite outgrowth and abolished mossy fiber segregation, resulting in ectopic fiber growth inside the pyramidal layer. SRF-deficient mossy fibers aberrantly targeted CA3 somata for synapse formation. Axon guidance assays showed that SRF was a key mediator of ephrin-A and semaphorin guidance cues; in SRF-deficient neurons, these resulted in the formation of F-actin-microtubule rings rather than complete growth cone collapse. Dominant-negative variants of the SRF cofactor megakaryocytic acute leukemia (MAL) severely impeded neurite outgrowth and guidance. These data highlight essential links between SRF-mediated transcription and axon guidance and circuit formation in the hippocampus.

The central nervous system is fundamentally dependent on guided cell migration, both during development and in adulthood. We report an absolute requirement of the transcription factor serum response factor (SRF) for neuronal migration in the mouse forebrain. Conditional, late-prenatal deletion of Srf causes neurons to accumulate ectopically at the subventricular zone (SVZ), a prime neurogenic region in the brain. SRF-deficient cells of the SVZ exhibit impaired tangential chain migration along the rostral migratory stream into the olfactory bulb. SVZ explants display retarded chain migration in vitro. Regarding target genes, SRF deficiency impairs expression of the ␤-actin and gelsolin genes, accompanied by reduced cytoskeletal actin fiber density. At the posttranslational level, cofilin, a key regulator of actin dynamics, displays dramatically elevated inhibitory phosphorylation at Ser-3. Our studies indicate that SRF-controlled gene expression directs both the structure and dynamics of the actin microfilament, thereby determining cell-autonomous neuronal migration.actin cytoskeleton ͉ cofilin ͉ transcription P roper development and functionality of the mammalian brain require migration of postmitotic neurons from their site of origin to their ultimate destination in the brain (1-4). Chain migration of interneuron precursors through the rostral migratory stream (RMS) to the olfactory bulb (5) is a highly informative system for the study of neuronal migration, because it occurs throughout life.Transcriptional control mechanisms regulating migrationassociated gene activities are poorly understood. The transcription factor serum response factor (SRF) (6) regulates the expression of genes encoding cytoskeletal proteins (6-8). Previously, in a solely in vitro study, we observed impaired migration of SRF-deficient murine embryonic stem (ES) cells, accompanied by a disproportionate reduction of the F-actin versus G-actin content (8). To investigate whether SRF was also required for mammalian cell migration in vivo, we now specifically address neuronal migration in the mouse forebrain by using conditional, Cre-mediated, Srf knock-out mutagenesis.SRF (6-9) is broadly expressed and regulates various target genes in brain, muscle, and other tissues by cooperating with cell type-specific SRF accessory proteins (10-14). Different SRF complexes are controlled by various intracellular signaling pathways, including mitogen-activated protein kinase cascades (15), Rho-dependent signaling (12, 16), and Ca 2ϩ stimulation (17). SRF target genes can be classified into two types, i.e., those representing cellular immediate early genes (IEGs) and those that are activated more slowly and less transiently (18). The latter often encode structural cytoskeletal proteins, such as actins, myosins, tropomyosin, vinculin, and others. SRF is essential in murine embryogenesis, because SRF-deficient embryos do not gastrulate (9) and display enhanced cell death (19).In Drosophila melanogaster development, SRF, together with its cofactor DMRTF͞MAL-D...