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

Insights into the mechanisms of neuron generation and specification in the zebrafish ventral spinal cord

The vertebrate nervous system is composed of a wide range of neurons and complex synaptic connections, raising the intriguing question of how neuronal diversity is generated. The spinal cord provides an excellent model for exploring the mechanisms governing neuronal diversity due to its simple neural network and the conserved molecular processes involved in neuron formation and specification during evolution. This review specifically examines two distinct progenitor domains present in the zebrafish ventral spinal cord: the lateral floor plate (LFP) and the p2 progenitor domain. The LFP is responsible for the production of GABAergic Kolmer-Agduhr neurons (KA″), glutamatergic V3 neurons, and intraspinal serotonergic neurons, while the p2 domain generates V2 precursors that subsequently differentiate into three unique subpopulations of V2 neurons, namely glutamatergic V2a, GABAergic V2b, and glycinergic V2s. Based on recent findings, we will examine the fundamental signaling pathways and transcription factors that play a key role in the specification of these diverse neurons and neuronal subtypes derived from the LFP and p2 progenitor domains.

Determinants of motor neuron functional subtypes important for locomotor speed

Locomotion requires precise control of the strength and speed of muscle contraction and is achieved by recruiting functionally distinct subtypes of motor neurons (MNs). MNs are essential to movement and differentially susceptible in disease, but little is known about how MNs acquire functional subtype-specific features during development. Using single-cell RNA profiling in embryonic and larval zebrafish, we identify novel and conserved molecular signatures for MN functional subtypes and identify genes expressed in both early post-mitotic and mature MNs. Assessing MN development in genetic mutants, we define a molecular program essential for MN functional subtype specification. Two evolutionarily conserved transcription factors, Prdm16 and Mecom, are both functional subtype-specific determinants integral for fast MN development. Loss of prdm16 or mecom causes fast MNs to develop transcriptional profiles and innervation similar to slow MNs. These results reveal the molecular diversity of vertebrate axial MNs and demonstrate that functional subtypes are specified through intrinsic transcriptional codes.

Fmrp regulates neuronal balance in embryonic motor circuit formation

Motor behavior requires the balanced production and integration of a variety of neural cell types. Motor neurons are positioned in discrete locations in the spinal cord, targeting specific muscles to drive locomotive contractions. Specialized spinal interneurons modulate and synchronize motor neuron activity to achieve coordinated motor output. Changes in the ratios and connectivity of spinal interneurons could drastically alter motor output by tipping the balance of inhibition and excitation onto target motor neurons. Importantly, individuals with Fragile X syndrome (FXS) and associated autism spectrum disorders often have significant motor challenges, including repetitive behaviors and epilepsy. FXS stems from the transcriptional silencing of the gene Fragile X Messenger Ribonucleoprotein 1 (FMR1), which encodes an RNA binding protein that is implicated in a multitude of crucial neurodevelopmental processes, including cell specification. Our work shows that Fmrp regulates the formation of specific interneurons and motor neurons that comprise early embryonic motor circuits. We find that zebrafish fmr1 mutants generate surplus ventral lateral descending (VeLD) interneurons, an early-born cell derived from the motor neuron progenitor domain (pMN). As VeLD interneurons are hypothesized to act as central pattern generators driving the earliest spontaneous movements, this imbalance could influence the formation and long-term function of motor circuits driving locomotion. fmr1 embryos also show reduced expression of proteins associated with inhibitory synapses, including the presynaptic transporter vGAT and the postsynaptic scaffold Gephyrin. Taken together, we show changes in embryonic motor circuit formation in fmr1 mutants that could underlie persistent hyperexcitability.

Establishing the Molecular and Functional Diversity of Spinal Motoneurons

Spinal motoneurons are a remarkably diverse class of neurons responsible for facilitating a broad range of motor behaviors and autonomic functions. Studies of motoneuron differentiation have provided fundamental insights into the developmental mechanisms of neuronal diversification, and have illuminated principles of neural fate specification that operate throughout the central nervous system. Because of their relative anatomical simplicity and accessibility, motoneurons have provided a tractable model system to address multiple facets of neural development, including early patterning, neuronal migration, axon guidance, and synaptic specificity. Beyond their roles in providing direct communication between central circuits and muscle, recent studies have revealed that motoneuron subtype-specific programs also play important roles in determining the central connectivity and function of motor circuits. Cross-species comparative analyses have provided novel insights into how evolutionary changes in subtype specification programs may have contributed to adaptive changes in locomotor behaviors. This chapter focusses on the gene regulatory networks governing spinal motoneuron specification, and how studies of spinal motoneurons have informed our understanding of the basic mechanisms of neuronal specification and spinal circuit assembly.

LIM Homeobox 4 (lhx4) regulates retinal neural differentiation and visual function in zebrafish

LIM homeobox 4 (LHX4) is expressed in the photoreceptors (PRs) of the outer nuclear layer (ONL) and bipolar cells (BCs) of the inner nuclear layer (INL) in mouse and chicken retina. It regulates the subtype-specific development of rod BCs and cone BCs in the mouse retina. However, no report has been published on its expression and function in the zebrafish retina. In this study, we assessed the expression of Lhx4 using in situ hybridization (ISH) technique and explored its role in zebrafish (Danio rerio) retinal development via morpholino (MO) technology. We found that the expression of lhx4 in the zebrafish retina begins 48 h post-fertilization (hpf) and is continuously expressed in the ONL and INL. A zebrafish model constructed with lhx4 knockdown in the eyes through vivo-MO revealed that: lhx4 knockdown inhibits the differentiation of Parvalbumin⁺ amacrine cells (ACs) and Rhodopsin⁺ rod photoreceptors (RPs), enhances the expression of visual system homeobox 2 (vsx2); and damages the responses of zebrafish to light stimulus, without affecting the differentiation of OFF-BCs and rod BCs, and apoptosis in the retina. These findings reveal that lhx4 regulates neural differentiation in the retina and visual function during zebrafish embryonic development.

The Genetic Programs Specifying Kolmer-Agduhr Interneurons

Kolmer-Agduhr (KA) cells are a subgroup of interneurons positioned adjacent to the neurocoele with cilia on the apical surface protruding into the central canal of the spinal cord. Although KA cells were identified almost a century ago, their development and functions are only beginning to be unfolded. Recent studies have revealed the characteristics of KA cells in greater detail, including their spatial distribution, the timing of their differentiation, and their specification via extrinsic signaling and a unique combination of transcription factors in zebrafish and mouse. Cell lineage-tracing experiments have demonstrated that two subsets of KA cells, named KA' and KA" cells, differentiate from motoneuronal progenitors and floor-plate precursors, respectively, in both zebrafish and mouse. Although KA' and KA" cells originate from different progenitors/precursors, they each share a common set of transcription factors. Intriguingly, the combination of transcription factors that promote the acquisition of KA' cell characteristics differs from those that promote a KA" cell identity. In addition, KA' and KA" cells exhibit separable neuronal targets and differential responses to bending of the spinal cord. In this review, we summarize what is currently known about the genetic programs defining the identities of KA' and KA" cell identities. We then discuss how these two subgroups of KA cells are genetically specified.

The Transcription Factor LHX1 Regulates the Survival and Directed Migration of POA-derived Cortical Interneurons

The delicate balance of excitation and inhibition is crucial for proper function of the cerebral cortex, relying on the accurate number and subtype composition of inhibitory gamma-aminobutyric (GABA)-expressing interneurons. Various intrinsic and extrinsic factors precisely orchestrate their multifaceted development including the long-range migration from the basal telencephalon to cortical targets as well as interneuron survival throughout the developmental period. Particularly expressed guidance receptors were described to channel the migration of cortical interneurons deriving from the medial ganglionic eminence (MGE) and the preoptic area (POA) along distinct routes. Hence, unveiling the regulatory genetic networks controlling subtype-specific gene expression profiles is key to understand interneuron-specific developmental programs and to reveal causes for associated disorders. In contrast to MGE-derived interneurons, little is known about the transcriptional networks in interneurons born in the POA. Here, we provide first evidence for the LIM-homeobox transcription factor LHX1 as a crucial key player in the post-mitotic development of POA-derived cortical interneurons. By transcriptional regulation of related genes, LHX1 modulates their survival as well as the subtype-specific expression of guidance receptors of the Eph/ephrin family, thereby affecting directional migration and layer distribution in the adult cortex.

Transcriptional mechanisms of motor neuron development in vertebrates and invertebrates

Across phylogeny, motor neurons (MNs) represent a single but often remarkably diverse neuronal class composed of a multitude of subtypes required for vital behaviors, such as eating and locomotion. Over the past decades, seminal studies in multiple model organisms have advanced our molecular understanding of the early steps of MN development, such as progenitor specification and acquisition of MN subtype identity, by revealing key roles for several evolutionarily conserved transcription factors. However, very little is known about the molecular strategies that allow distinct MN subtypes to maintain their identity- and function-defining features during the late steps of development and postnatal life. Here, we provide an overview of invertebrate and vertebrate studies on transcription factor-based strategies that control early and late steps of MN development, aiming to highlight evolutionarily conserved gene regulatory principles necessary for establishment and maintenance of neuronal identity.

Development, functional organization, and evolution of vertebrate axial motor circuits

Neuronal control of muscles associated with the central body axis is an ancient and essential function of the nervous systems of most animal species. Throughout the course of vertebrate evolution, motor circuits dedicated to control of axial muscle have undergone significant changes in their roles within the motor system. In most fish species, axial circuits are critical for coordinating muscle activation sequences essential for locomotion and play important roles in postural correction. In tetrapods, axial circuits have evolved unique functions essential to terrestrial life, including maintaining spinal alignment and breathing. Despite the diverse roles of axial neural circuits in motor behaviors, the genetic programs underlying their assembly are poorly understood. In this review, we describe recent studies that have shed light on the development of axial motor circuits and compare and contrast the strategies used to wire these neural networks in aquatic and terrestrial vertebrate species.

Massively Parallel Single Nucleus Transcriptional Profiling Defines Spinal Cord Neurons and Their Activity during Behavior

To understand the cellular basis of behavior, it is necessary to know the cell types that exist in the nervous system and their contributions to function. Spinal networks are essential for sensory processing and motor behavior and provide a powerful system for identifying the cellular correlates of behavior. Here, we used massively parallel single nucleus RNA sequencing (snRNA-seq) to create an atlas of the adult mouse lumbar spinal cord. We identified and molecularly characterized 43 neuronal populations. Next, we leveraged the snRNA-seq approach to provide unbiased identification of neuronal populations that were active following a sensory and a motor behavior, using a transcriptional signature of neuronal activity. This approach can be used in the future to link single nucleus gene expression data with dynamic biological responses to behavior, injury, and disease.

ISL1-based LIM complexes control Slit2 transcription in developing cranial motor neurons

LIM-homeodomain (HD) transcription factors form a multimeric complex and assign neuronal subtype identities, as demonstrated by the hexameric ISL1-LHX3 complex which gives rise to somatic motor (SM) neurons. However, the roles of combinatorial LIM code in motor neuron diversification and their subsequent differentiation is much less well understood. In the present study, we demonstrate that the ISL1 controls postmitotic cranial branchiomotor (BM) neurons including the positioning of the cell bodies and peripheral axon pathfinding. Unlike SM neurons, which transform into interneurons, BM neurons are normal in number and in marker expression in Isl1 mutant mice. Nevertheless, the movement of trigeminal and facial BM somata is stalled, and their peripheral axons are fewer or misrouted, with ectopic branches. Among genes whose expression level changes in previous ChIP-seq and microarray analyses in Isl1-deficient cell lines, we found that Slit2 transcript was almost absent from BM neurons of Isl1 mutants. Both ISL1-LHX3 and ISL1-LHX4 bound to the Slit2 enhancer and drove endogenous Slit2 expression in SM and BM neurons. Our findings suggest that combinations of ISL1 and LHX factors establish cell-type specificity and functional diversity in terms of motor neuron identities and/or axon development.