A three-component model of the spinal nerve ramification: Bringing together the human gross anatomy and modern Embryology

Due to its long history, the study of human gross anatomy has not adequately incorporated modern embryological findings; consequently, the current understanding has often been incompatible with recent discoveries from molecular studies. Notably, the traditional epaxial and hypaxial muscle distinction, and their corresponding innervation by the dorsal and ventral rami of the spinal nerve, do not correspond to the primaxial and abaxial muscle distinction, defined by the mesodermal lineages of target tissues. To resolve the disagreement between adult anatomy and embryology, we here propose a novel hypothetical model of spinal nerve ramification. Our model is based on the previously unknown developmental process of the intercostal nerves. Observations of these nerves in the mouse embryos revealed that the intercostal nerves initially had superficial and deep ventral branches, which is contrary to the general perception of a single ventral branch. The initial dual innervation pattern later changes into an adult-like single branch pattern following the retraction of the superficial branch. The modified intercostal nerves consist of the canonical ventral branches and novel branches that run on the muscular surface of the thorax, which sprout from the lateral cutaneous branches. We formulated the embryonic branching pattern into the hypothetical ramification model of the human spinal nerve so that the branching pattern is compatible with the developmental context of the target muscles. In our model, every spinal nerve consists of three components: (1) segmental branches that innervate the primaxial muscles, including the dorsal rami, and short branches and long superficial anterior branches from the ventral rami; (2) plexus-forming intramural branches, the serial homolog of the canonical intercostal nerves, which innervate the abaxial portion of the body wall; and (3) plexus-forming extramural branches, the series of novel branches located outside of the body wall, which innervate the girdle and limb muscles. The selective elaboration or deletion of each component successfully explains the reasoning for the standard morphology and variability of the spinal nerve. Therefore, our model brings a novel understanding of spinal nerve development and valuable information for basic and clinical sciences regarding the diverse branching patterns of the spinal nerve.

An expansion of the non-coding genome and its regulatory potential underlies vertebrate neuronal diversity

Proper assembly and function of the nervous system requires the generation of a uniquely diverse population of neurons expressing a cell-type-specific combination of effector genes that collectively define neuronal morphology, connectivity, and function. How countless partially overlapping but cell-type-specific patterns of gene expression are controlled at the genomic level remains poorly understood. Here we show that neuronal genes are associated with highly complex gene regulatory systems composed of independent cell-type- and cell-stage-specific regulatory elements that reside in expanded non-coding genomic domains. Mapping enhancer-promoter interactions revealed that motor neuron enhancers are broadly distributed across the large chromatin domains. This distributed regulatory architecture is not a unique property of motor neurons but is employed throughout the nervous system. The number of regulatory elements increased dramatically during the transition from invertebrates to vertebrates, suggesting that acquisition of new enhancers might be a fundamental process underlying the evolutionary increase in cellular complexity.

Critical roles of ARHGAP36 as a signal transduction mediator of Shh pathway in lateral motor columnar specification

During spinal cord development, Sonic hedgehog (Shh), secreted from the floor plate, plays an important role in the production of motor neurons by patterning the ventral neural tube, which establishes MN progenitor identity. It remains unknown, however, if Shh signaling plays a role in generating columnar diversity of MNs that connect distinct target muscles. Here, we report that Shh, expressed in MNs, is essential for the formation of lateral motor column (LMC) neurons in vertebrate spinal cord. This novel activity of Shh is mediated by its downstream effector ARHGAP36, whose expression is directly induced by the MN-specific transcription factor complex Isl1-Lhx3. Furthermore, we found that AKT stimulates the Shh activity to induce LMC MNs through the stabilization of ARHGAP36 proteins. Taken together, our data reveal that Shh, secreted from MNs, plays a crucial role in generating MN diversity via a regulatory axis of Shh-AKT-ARHGAP36 in the developing mouse spinal cord.

The Isl1-Lhx3 Complex Promotes Motor Neuron Specification by Activating Transcriptional Pathways that Enhance Its Own Expression and Formation

Motor neuron (MN) progenitor cells rapidly induce high expression of the transcription factors Islet-1 (Isl1), LIM-homeobox 3 (Lhx3), and the transcriptional regulator LMO4, as they differentiate. While these factors are critical for MN specification, the mechanisms regulating their precise temporal and spatial expression patterns are not well characterized. Isl1 and Lhx3 form the Isl1-Lhx3 complex, which induces the transcription of genes critical for MN specification and maturation. Here, we report that Isl1, Lhx3, and Lmo4 are direct target genes of the Isl1-Lhx3 complex. Our results show that specific genomic loci associated with these genes recruit the Isl1-Lhx3 complex to activate the transcription of Isl1, Lhx3, and Lmo4 in embryonic MNs of chick and mouse. These findings support a model in which the Isl1-Lhx3 complex amplifies its own expression through a potent autoregulatory feedback loop and simultaneously enhances the transcription of Lmo4. LMO4 blocks the formation of the V2 interneuron-specifying Lhx3 complex. In developing MNs, this action inhibits the expression of V2 interneuron genes and increases the pool of unbound Lhx3 available to incorporate into the Isl1-Lhx3 complex. Identifying the pathways that regulate the expression of these key factors provides important insights into the genetic strategies utilized to promote MN differentiation and maturation.

Chx10 Consolidates V2a Interneuron Identity through Two Distinct Gene Repression Modes

During development, two cell types born from closely related progenitor pools often express identical transcriptional regulators despite their completely distinct characteristics. This phenomenon implies the need for a mechanism that operates to segregate the identities of the two cell types throughout differentiation after initial fate commitment. To understand this mechanism, we investigated the fate specification of spinal V2a interneurons, which share important developmental genes with motor neurons (MNs). We demonstrate that the paired homeodomain factor Chx10 functions as a critical determinant for V2a fate and is required to consolidate V2a identity in postmitotic neurons. Chx10 actively promotes V2a fate, downstream of the LIM-homeodomain factor Lhx3, while concomitantly suppressing the MN developmental program by preventing the MN-specific transcription complex from binding and activating MN genes. This dual activity enables Chx10 to effectively separate the V2a and MN pathways. Our study uncovers a widely applicable gene regulatory principle for segregating related cell fates.

Redundant mechanisms are involved in suppression of default cell fates during embryonic mesenchyme and notochord induction in ascidians

During embryonic induction, the responding cells invoke an induced developmental program, whereas in the absence of an inducing signal, they assume a default uninduced cell fate. Suppression of the default fate during the inductive event is crucial for choice of the binary cell fate. In contrast to the mechanisms that promote an induced cell fate, those that suppress the default fate have been overlooked. Upon induction, intracellular signal transduction results in activation of genes encoding key transcription factors for induced tissue differentiation. It is elusive whether an induced key transcription factor has dual functions involving suppression of the default fates and promotion of the induced fate, or whether suppression of the default fate is independently regulated by other factors that are also downstream of the signaling cascade. We show that during ascidian embryonic induction, default fates were suppressed by multifold redundant mechanisms. The key transcription factor, Twist-related.a, which is required for mesenchyme differentiation, and another independent transcription factor, Lhx3, which is dispensable for mesenchyme differentiation, sequentially and redundantly suppress the default muscle fate in induced mesenchyme cells. Similarly in notochord induction, Brachyury, which is required for notochord differentiation, and other factors, Lhx3 and Mnx, are likely to suppress the default nerve cord fate redundantly. Lhx3 commonly suppresses the default fates in two kinds of induction. Mis-activation of the autonomously executed default program in induced cells is detrimental to choice of the binary cell fate. Multifold redundant mechanisms would be required for suppression of the default fate to be secure.

Single-stranded DNA binding proteins are required for LIM complexes to induce transcriptionally active chromatin and specify spinal neuronal identities

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 neuron-specifying 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.

Lhx3 and Lhx4 suppress Kolmer-Agduhr interneuron characteristics within zebrafish axial motoneurons

A central problem in development is how fates of closely related cells are segregated. Lineally related motoneurons (MNs) and interneurons (INs) express many genes in common yet acquire distinct fates. For example, in mouse and chick Lhx3 plays a pivotal role in the development of both cell classes. Here, we utilize the ability to recognize individual zebrafish neurons to examine the roles of Lhx3 and its paralog Lhx4 in the development of MNs and ventral INs. We show that Lhx3 and Lhx4 are expressed by post-mitotic axial MNs derived from the MN progenitor (pMN) domain, p2 domain progenitors and by several types of INs derived from pMN and p2 domains. In the absence of Lhx3 and Lhx4, early-developing primary MNs (PMNs) adopt a hybrid fate, with morphological and molecular features of both PMNs and pMN-derived Kolmer-Agduhr' (KA') INs. In addition, we show that Lhx3 and Lhx4 distinguish the fates of two pMN-derived INs. Finally, we demonstrate that Lhx3 and Lhx4 are necessary for the formation of late-developing V2a and V2b INs. In conjunction with our previous work, these data reveal that distinct transcription factor families are deployed in post-mitotic MNs to unequivocally assign MN fate and suppress the development of alternative pMN-derived IN fates.

STAT3 promotes motor neuron differentiation by collaborating with motor neuron-specific LIM complex

The motor neuron (MN)-hexamer complex consisting of LIM homeobox 3, Islet-1, and nuclear LIM interactor is a key determinant of motor neuron specification and differentiation. To gain insights into the transcriptional network in motor neuron development, we performed a genome-wide ChIP-sequencing analysis and found that the MN-hexamer directly regulates a wide array of motor neuron genes by binding to the HxRE (hexamer response element) shared among the target genes. Interestingly, STAT3-binding motif is highly enriched in the MN-hexamer-bound peaks in addition to the HxRE. We also found that a transcriptionally active form of STAT3 is expressed in embryonic motor neurons and that STAT3 associates with the MN-hexamer, enhancing the transcriptional activity of the MN-hexamer in an upstream signal-dependent manner. Correspondingly, STAT3 was needed for motor neuron differentiation in the developing spinal cord. Together, our studies uncover crucial gene regulatory mechanisms that couple MN-hexamer and STAT-activating extracellular signals to promote motor neuron differentiation in vertebrate spinal cord.