Abstract
The stimulus-regulated transcription factor Serum Response Factor (SRF) plays an important role in diverse neurodevelopmental processes related to structural plasticity and motile functions, although its precise mechanism of action has not yet been established. To further define the role of SRF in neural development and distinguish between cell-autonomous and non cell-autonomous effects, we bidirectionally manipulated SRF activity through gene transduction assays that allow the visualization of individual neurons and their comparison with neighboring control cells. In vitro assays showed that SRF promotes survival and filopodia formation and is required for normal asymmetric neurite outgrowth, indicating that its activation favors dendrite enlargement versus branching. In turn, in vivo experiments demonstrated that SRF-dependent regulation of neuronal morphology has important consequences in the developing cortex and retina, affecting neuronal migration, dendritic and axonal arborization and cell positioning in these laminated tissues. Overall, our results show that the controlled and timely activation of SRF is essential for the coordinated growth of neuronal processes, suggesting that this event regulates the switch between neuronal growth and branching during developmental processes.
Highlights
From the early gastrulation defects observed in conventional knockout (KO) mice to the most specific phenotypes found in tissue-restricted conditional mutants, the deficits associated with Serum Response Factor (SRF)’s loss-of-function (LOF) have been consistently related to cytoskeletal perturbations[1,2,7,8]
For gain-of-function (GOF) studies, we expressed a constitutively active SRF variant that results from fusing the DNA binding domain (DBD) of SRF with the potent acidic transactivation domain of the viral protein viral protein 16 (VP16) (Fig. 1B)[17]
Consistent with this view, when the electroporation was conducted at E16.5, a time in which control neurons find its final location in layer 2, the soma of many constitutively active SRF variant (caSRF)-expressing neurons located even more superficially than when electroporation occurred at E14.5. In these animals we often observed dendrites that projected parallel to the pial surface (Fig. 3E). These results suggest that defects in dendritic growth/arborization can contribute to the strong lamination defect observed in caSRF-electroporated cortices, alternative interpretations are possible
Summary
From the early gastrulation defects observed in conventional knockout (KO) mice to the most specific phenotypes found in tissue-restricted conditional mutants, the deficits associated with SRF’s loss-of-function (LOF) have been consistently related to cytoskeletal perturbations[1,2,7,8]. Experiments in conditional KO (cKO) mice lacking SRF in neural progenitor cells revealed deficits in cortical axonal projections, including corticostriatal, corticospinal, and corticothalamic tracts, that were often associated with the loss of the corpus callosum[12]. Our experiments in neuronal cultures reveal that SRF controls the asymmetric outgrowth of neurites favoring primary dendrite enlargement versus branching. This cellular function influences different neurodevelopmental processes, including neuronal migration, tissue lamination and circuit assembly, which highlights the critical importance of fine-tuned SRF activity in the development of the nervous system
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