A Nuclear Actin Function Regulates Neuronal Motility by Serum Response Factor-Dependent Gene Transcription

Abstract

Neuronal motility relies on actin treadmilling. In addition to regulating cytoskeletal dynamics in the cytoplasm, actin modulates nuclear gene expression. We present a hitherto unappreciated cross talk of actin signaling with gene expression governing neuronal motility. Toward this end, we used a novel approach using mutant actins either favoring (G15S) or inhibiting (R62D) F-actin assembly. Overexpressing these mutant actins in mouse hippocampal neurons not only modulated growth-cone function but also neurite elongation, which was ambiguous by traditional pharmacological interference. G15S actin enhanced neurite outgrowth and filopodia number. In contrast, R62D reduced neurite length and impaired growth-cone filopodia formation. Growth-cone collapse induced by ephrin-As, a family of repulsive axon guidance molecules, is impaired upon R62D expression, resulting in perseverance of ring-shaped F-actin filaments. R62D-induced phenotypes strongly resemble neurons lacking SRF (Serum Response Factor). SRF controls gene transcription of various actin isoforms (e.g., Actb, Acta1) and actin-binding proteins (e.g., Gsn) and is the archetypical transcription factor to study actin interplay with transcription. We show that neuronal motility evoked by these actin mutants requires SRF activity. Further, constitutively active SRF partially rescues R62D-induced phenotypes. Conversely, actin signaling regulates neuronal SRF-mediated gene expression. Notably, a nucleus-resident actin (R62D NLS ) also regulates SRF's transcriptional activity. Moreover, R62D NLS decreases neuronal motility similar to the cytoplasmic R62D actin mutant although R62D NLS has no access to cytoplasmic actin dynamics. Thus, herein we provide first evidence that neuronal motility not only depends on cytoplasmic actin dynamics but also on the availability of actin to modulate nuclear functions such as gene transcription.

Serum Response Factor (SRF)-cofilin-actin signaling axis modulates mitochondrial dynamics

“…F-actin polymerization in growth cones is enhanced by BDNF (27,39). Notably, BDNF increased the number of mitochondrial objects in growth cones about threefold (Fig.…”

“…S5). Thus, actin R62D, inhibiting SRF gene regulation (27), phenocopies Srf mutant neurons with regard to mitochondrial dynamics (compare Figs. 2B and 5 C and N).…”

“…2N). This parameter normalizes for the reduced neurite length observed in SRF-deficient neurons (19,27,28). In addition to mitochondrial size and occupancy, we assessed transport parameters (Figs.…”

“…Actin R62D is excluded from F-actin, thereby increasing monomeric G-actin (36,37). In addition, actin G15S stimulates, whereas actin R62D reduces, SRF-mediated gene expression in neurons (27).…”

Actin on DNA—An ancient and dynamic relationship

“…It is interesting that in this context nuclear actin seems to have a negative role in regulating the transcription of specific genes, whereas the general role of actin in gene expression appears positive (see above). Because nuclear actin controlled SRF activity has been implicated in such important processes as regulation of cytoskeletal dynamics in cancer [Medjkane et al, 2009] and neuronal development [Stern et al, 2009] it will be interesting to elucidate how these opposing activities are resolved on different target genes.…”