Ssdps are required for LIM-complexes to induce transcriptionally active chromatin and specify spinal neuronal identities

Abstract

LIM homeodomain factors regulate the development of many cell types. However, transcriptional coactivators that mediate their developmental function remain poorly defined. To address these, we examined how two related NLI-dependent LIM complexes, which govern the development of spinal motor neurons and V2a interneurons, activate the transcription in the embryonic spinal cord. We found that single-stranded DNA-binding proteins are recruited to these LIM complexes via NLI, and enhance their transcriptional activation potential. Ssdp1 and Ssdp2 (Ssdp1/2) are highly expressed in the neural tube and promote motor neuron differentiation in the embryonic spinal cord and P19 stem cells. Inhibition of Ssdp1/2 activity in mouse and chick embryos suppresses the generation of motor neurons and V2a interneurons. Furthermore, Ssdp1/2 recruit histone-modifying enzymes to the motor neuronspecifying LIM complex and trigger acetylation and lysine 4 trimethylation of histone H3, which are well-established chromatin marks for active transcription. Our results suggest that Ssdp1/2 function as crucial transcriptional coactivators for LIM complexes to specify spinal neuronal identities during development.

Rotational symmetry of the structured Chip/LDB-SSDP core module of the Wnt enhanceosome

“…These phenotypic similarities indicate an intimate cooperation between Chip and SSDP in mediating enhancer function. Consistent with this, chicken Ssdp1 and Ssdp2 are required to confer transcriptionally active chromatin on loci controlled by Ldb1 (15), and murine Ssbp2 is required for hematopoietic stem cell maintenance, similarly to Lbd1 (18). The molecular basis underlying this intimate cooperation between Chip/LDB and SSDP is not known.…”

“…1 A and SI Appendix , Fig. S4) required for binding to SSDP (14, 15). It spans 2 predicted α-helices (α6 and α7), whereby α6 is separated from the DD by a linker of 9 conserved residues ( SI Appendix , Fig.…”

“…Chip/LDB binds to a single-stranded DNA binding protein (SSDP, also known as single-stranded binding protein, SSBP) through the LDB1/Chip conserved domain (LCCD), a short region downstream of the DD (14, 15). The loss-of-function phenotypes of Drosophila chip mutants closely resemble those of ssdp mutants in various developmental contexts, although the former tend to be stronger and more pleiotropic than the latter (14, 16), especially in the early embryo (17).…”

Disparate binding kinetics by an intrinsically disordered domain enables temporal regulation of transcriptional complex formation

“…Mechanisms other than timing of expression may contribute to LDB1:ISL1 sequestration. sMN progenitors express the E3 ubiquitin ligase RLIM, which targets LDB1 for proteasomal degradation, as well as ssDNA binding protein 1 (SSBP1), which protects LDB1:LIM-HD complexes from degradation and is required for full ternary complex activity (36)(37)(38). RLIM and SSBP1 could work together in these cells to protect nascent LDB1:ISL1 complexes and remove free LDB1 to prevent binary complex formation.…”

Islet Coordinately Regulates Motor Axon Guidance and Dendrite Targeting through the Frazzled/DCC Receptor

“…Hb9 likely indirectly promotes robo2 expression by repressing one or multiple intermediate targets, as its conserved Engrailed homology repressor domain is required for its function in motor axon guidance and for robo2 regulation (Santiago et al, 2014). In vertebrate motor neurons, Isl1 forms a complex with Lhx3 to directly activate several of its known targets (Cho et al, 2014; Lee et al, 2016; Thaler et al, 2002). A recent genome-wide DAM-ID analysis found that Isl binds to multiple regions within and near the fra locus in Drosophila embryos, suggesting it may directly activate fra (Wolfram et al, 2014) (Figure S1).…”