SSDP cofactors regulate neural patterning and differentiation of specific axonal projections

Planarian LDB and SSDP proteins scaffold transcriptional complexes for regeneration and patterning

Sequence-specific transcription factors often function as components of large regulatory complexes. LIM-domain binding protein (LDB) and single-stranded DNA-binding protein (SSDP) function as core scaffolds of transcriptional complexes in animals and plants. Little is known about potential partners and functions for LDB/SSDP complexes in the context of tissue regeneration. In this work, we find that planarian LDB1 and SSDP2 promote tissue regeneration, with a particular function in mediolateral polarity reestablishment. We find that LDB1 and SSDP2 interact with one another and with characterized planarian LIM-HD proteins Arrowhead, Islet1, and Lhx1/5-1. SSDP2 and LDB1 also function with islet1 in polarity reestablishment and with lhx1/5-1 in serotonergic neuron maturation. Finally, we show new roles for LDB1 and SSDP2 in regulating gene expression in the planarian intestine and parenchyma; these functions may be LIM-HD-independent. Together, our work provides insight into LDB/SSDP complexes in a highly regenerative organism. Further, our work provides a strong starting point for identifying and characterizing potential binding partners of LDB1 and SSDP2 and for exploring roles for these proteins in diverse aspects of planarian physiology.

SSBP1 mutations in dominant optic atrophy with variable retinal degeneration

OBJECTIVE

Autosomal dominant optic atrophy (ADOA) starts in early childhood with loss of visual acuity and color vision deficits. OPA1 mutations are responsible for the majority of cases, but in a portion of patients with a clinical diagnosis of ADOA, the cause remains unknown. This study aimed to identify novel ADOA-associated genes and explore their causality.

METHODS

Linkage analysis and sequencing were performed in multigeneration families and unrelated patients to identify disease-causing variants. Functional consequences were investigated in silico and confirmed experimentally using the zebrafish model.

RESULTS

We defined a new ADOA locus on 7q33-q35 and identified 3 different missense variants in SSBP1 (NM_001256510.1; c.113G>A [p.(Arg38Gln)], c.320G>A [p.(Arg107Gln)] and c.422G>A [p.(Ser141Asn)]) in affected individuals from 2 families and 2 singletons with ADOA and variable retinal degeneration. The mutated arginine residues are part of a basic patch that is essential for single-strand DNA binding. The loss of a positive charge at these positions is very likely to lower the affinity of SSBP1 for single-strand DNA. Antisense-mediated knockdown of endogenous ssbp1 messenger RNA (mRNA) in zebrafish resulted in compromised differentiation of retinal ganglion cells. A similar effect was achieved when mutated mRNAs were administered. These findings point toward an essential role of ssbp1 in retinal development and the dominant-negative nature of the identified human variants, which is consistent with the segregation pattern observed in 2 multigeneration families studied.

INTERPRETATION

SSBP1 is an essential protein for mitochondrial DNA replication and maintenance. Our data have established pathogenic variants in SSBP1 as a cause of ADOA and variable retinal degeneration. ANN NEUROL 2019;86:368-383.

Requirement for ssbp2 in hematopoietic stem cell maintenance and stress response

Transcriptional mechanisms governing hematopoietic stem cell (HSC) quiescence, self-renewal, and differentiation are not fully understood. Sequence-specific ssDNA-binding protein 2 (SSBP2) is a candidate acute myelogenous leukemia (AML) suppressor gene located at chromosome 5q14. SSBP2 binds the transcriptional adaptor protein Lim domain-binding protein 1 (LDB1) and enhances LDB1 stability to regulate gene expression. Notably, Ldb1 is essential for HSC specification during early development and maintenance in adults. We previously reported shortened lifespan and greater susceptibility to B cell lymphomas and carcinomas in Ssbp2(-/-) mice. However, whether Ssbp2 plays a regulatory role in normal HSC function and leukemogenesis is unknown. In this study, we provide several lines of evidence to demonstrate a requirement for Ssbp2 in the function and transcriptional program of hematopoietic stem and progenitor cells (HSPCs) in vivo. We found that hematopoietic tissues were hypoplastic in Ssbp2(-/-) mice, and the frequency of lymphoid-primed multipotent progenitor cells in bone marrow was reduced. Other significant features of these mice were delayed recovery from 5-fluorouracil treatment and diminished multilineage reconstitution in lethally irradiated bone marrow recipients. Dramatic reduction of Notch1 transcripts and increased expression of transcripts encoding the transcription factor E2a and its downstream target Cdkn1a also distinguished Ssbp2(-/-) HSPCs from wild-type HSPCs. Finally, a tendency toward coordinated expression of SSBP2 and the AML suppressor NOTCH1 in a subset of the Cancer Genome Atlas AML cases suggested a role for SSBP2 in AML pathogenesis. Collectively, our results uncovered a critical regulatory function for SSBP2 in HSPC gene expression and function.

Chondrolectin mediates growth cone interactions of motor axons with an intermediate target

The C-type lectin chondrolectin (chodl) represents one of the major gene products dysregulated in spinal muscular atrophy models in mice. However, to date, no function has been determined for the gene. We have identified chodl and other novel genes potentially involved in motor axon differentiation, by expression profiling of transgenically labeled motor neurons in embryonic zebrafish. To enrich the profile for genes involved in differentiation of peripheral motor axons, we inhibited the function of LIM-HDs (LIM homeodomain factors) by overexpression of a dominant-negative cofactor, thereby rendering labeled axons unable to grow out of the spinal cord. Importantly, labeled cells still exhibited axon growth and most cells retained markers of motor neuron identity. Functional tests of chodl, by overexpression and knockdown, confirm crucial functions of this gene for motor axon growth in vivo. Indeed, knockdown of chodl induces arrest or stalling of motor axon growth at the horizontal myoseptum, an intermediate target and navigational choice point, and reduced muscle innervation at later developmental stages. This phenotype is rescued by chodl overexpression, suggesting that correct expression levels of chodl are important for interactions of growth cones of motor axons with the horizontal myoseptum. Combined, these results identify upstream regulators and downstream functions of chodl during motor axon growth.

Expanding views of presynaptic terminals: new findings from Caenorhabditis elegans

The unique ability of chemical synapses to transmit information relies on the structural organization of presynaptic terminals. Empowered by forward genetics, research using Caenorhabditis elegans has continued to make pivotal contributions to discover conserved regulators and pathways for presynaptic development. Recent advances in microscopy have begun to pave the path for linking molecular dynamics with subsynaptic structures. Studies using diverse reporters for synapses further broaden the landscape of regulatory mechanisms underlying presynaptic differentiation. The identification of novel regulators at transcriptional and post-transcriptional levels raises new questions for understanding synapse formation at the genomic scale.