Evolution of motor innervation to vertebrate fins and limbs

Fish That Fish for Fish-A Peculiar Location of "Fishing Motoneurons" in the Striated Frogfish Antennarius striatus

In lophiform teleosts, the first dorsal fin has evolved as a specialized structure called the "illicium" equipped with the esca, which is a modified skin flap used to attract small fish for predation. The motor control system of the illicium, however, remained unknown. The present study investigated the innervation of muscles for the illicium and morphology of motoneurons innervating them in the striated frogfish Antennarius striatus. We found that the dorsal ramus of occipital nerve innervates the muscles. Motoneurons for the illicium are present in the dorsolateral zone of ventral horn at the medullo-spinal boundary level, forming a cluster somewhat distinct from other motoneurons of the ventral horn. Motoneurons for the second to fourth dorsal fins and pectoral fin were located in the ventrolateral and ventromedial zones of ventral horn, respectively, whereas those of the dorsal trunk muscle in the dorsomedial zone of ventral horn. Motoneurons for the first dorsal spine of white-spotted pygmy filefish were also investigated for species comparison and were found to locate in the ventrolateral zone of ventral horn, similarly to the motoneurons for the second to fourth dorsal fins of the frogfish. These results suggest that motoneurons for the illicium have become segregated from other motoneurons to be situated in an unusual dorsal position for a motoneuron pool of a dorsal fin, in concert with the evolution of specialized "fishing behavior" performed by the illicium.

Are superficial neuromasts proprioceptors underlying fast copulatory behavior?

In male Poeciliid fishes, the modified anal fin (i.e., gonopodium) and its axial and appendicular support are repositioned within the axial skeleton, creating a novel sexually dimorphic ano-urogenital region. During copulation, the relative location of the gonopodium is crucial for successful insemination. Therefore, the repositioning of these structures and organ relied on the reorganization of the efferent circuitry that controls spinal motor neurons innervating appendicular muscles critical for the movement of the gonopodium, including the fast and synchronous torque-trust motion during insemination attempts. Copulation occurs when a male positions himself largely outside a female's field of view, circumducts his gonopodium, and performs a rapid, complex maneuver to properly contact the female urogenital sinus with the distal tip of the gonopodium and transfers sperm. Although understanding of the efferent circuitry has significantly increased in the last 24 years, nothing is known about the cutaneous receptors involved in gonopodium movement, or how the afferent signals are processed to determine the location of this organ during copulation. Using Western mosquitofish, Gambusia affinis, as our model, we attempt to fill this gap in knowledge. Preliminary data showed cutaneous nerves and sensory neurons innervating superficial neuromasts surrounding the base of adult male gonopodium; those cutaneous nerves projected ventrally from the spinal cord through the 14th dorsal root ganglion and its corresponding ventral root towards the base and fin rays of the gonopodium. We asked what role the cutaneous superficial neuromasts play in controlling the positioning and timing of the gonopodium's fast and synchronous movements for effective sperm transfer. First, we found a greater number of superficial neuromasts surrounding the base of the male's gonopodium compared to the base of the female's anal fin. Second, we systemically removed superficial neuromasts surrounding the gonopodium base and observed significant impairment of the positioning and timing of gonopodial movements. Our findings provide a first step to supporting the following hypothesis: during radical reorganization of the Poeciliid body plan, superficial neuromasts have been partially co-opted as proprioceptors that allow the gonopodium to control precise positioning and timing during copulatory attempts.

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.

Development of the Pectoral Lobed Fin in the Australian Lungfish Neoceratodus forsteri

The evolutionary transition from paired fins to limbs involved the establishment of a set of limb muscles as an evolutionary novelty. In parallel, there was a change in the topography of the spinal nerves innervating appendicular muscles, so that distinct plexuses were formed at the bases of limbs. However, the key developmental changes that brought about this evolutionary novelty have remained elusive due to a lack of data on the development of lobed fins in sarcopterygian fishes. Here, we observed the development of the pectoral fin in the Australian lungfish Neoceratodus forsteri (Sarcopterygii) through synchrotron radiation X-ray microtomography. Neoceratodus forsteri is a key taxon for understanding the fin-to-limb transition due to its close phylogenetic relationships to tetrapods and well-developed lobed fins. At the onset of the fin bud in N. forsteri, there is no mesenchyme at the junction between the axial body wall and the fin bud, which corresponds to the embryonic position of the brachial plexus formed in the mesenchyme in tetrapods. Later, concurrent with the cartilage formation in the fin skeleton, the fin adductor and abductor muscles become differentiated within the surface ectoderm of the fin bud. Subsequently, the girdle muscle, which is homologous to the tetrapod serratus muscle, newly develops at the junction between the axial body wall and the fin. Our study suggests that the acquisition of embryonic mesenchyme at the junction between the axial body wall and the appendicular bud opened the door to the formation of the brachial plexus and the specialization of individual muscles in the lineage that gave rise to tetrapods.

Myelination during fracture healing in vivo in myelin protein zero (p0) transgenic medaka line

During the fracture healing process, osteoblasts and osteoclasts, as well as the nervous system are known to play important roles for signaling in the body. Glia cells contribute to the healing process by myelination, which can increase the speed of signals transmitted between neurons. However, the behavior of myelinating cells at a fracture site remains unclear. We developed a myelin protein zero (mpz)-EGFP transgenic medaka line for tracing myelinating cells. Mpz-enhanced green fluorescence protein (EGFP)-positive (mpz⁺) cells are driven by the 2.9-kb promoter of the medaka mpz gene, which is distributed throughout the nervous system, such as the brain, spinal cord, lateral line, and peripheral nerves. In the caudal fin region, mpz⁺ cells were found localized parallel with the fin ray (bone) in the adult stage. mpz⁺ cells were not distributed with fli-DsRed positive (fli⁺) blood vessels, but with some nerve fibers, and were dyed with the anti-acetylated tubulin antibody. We then fractured one side of the caudal lepidotrichia in a caudal fin of mpz-EGFP medaka and found a unique phenomenon, in that mpz⁺ cells were accumulated at 1 bone away from the fracture site. This mpz⁺ cell accumulation phenomenon started from 4 days after fracture of the proximal bone. Thereafter, mpz⁺ cells became elongated from the proximal bone to the distal bone and finally showed a crosslink connection crossing the fracture site to the distal bone at 28 days after fracture. Finally, the effects of rapamycin, known as a mTOR inhibitor, on myelination was examined. Rapamycin treatment of mpz-EGFP/osterix-DsRed double transgenic medaka inhibited not only the crosslink connection of mpz⁺ cells but also osterix⁺ osteoblast accumulation at the fracture site, accompanied with a fracture healing defect. These findings indicated that mTOR signaling plays important roles in bone formation and neural networking during fracture healing. Taken together, the present results are the first to show the dynamics of myelinating cells in vivo.

The Ancient Origins of Neural Substrates for Land Walking

Walking is the predominant locomotor behavior expressed by land-dwelling vertebrates, but it is unknown when the neural circuits that are essential for limb control first appeared. Certain fish species display walking-like behaviors, raising the possibility that the underlying circuitry originated in primitive marine vertebrates. We show that the neural substrates of bipedalism are present in the little skate Leucoraja erinacea, whose common ancestor with tetrapods existed ∼420 million years ago. Leucoraja exhibits core features of tetrapod locomotor gaits, including left-right alternation and reciprocal extension-flexion of the pelvic fins. Leucoraja also deploys a remarkably conserved Hox transcription factor-dependent program that is essential for selective innervation of fin/limb muscle. This network encodes peripheral connectivity modules that are distinct from those used in axial muscle-based swimming and has apparently been diminished in most modern fish. These findings indicate that the circuits that are essential for walking evolved through adaptation of a genetic regulatory network shared by all vertebrates with paired appendages. VIDEO ABSTRACT.

Divergent Hox Coding and Evasion of Retinoid Signaling Specifies Motor Neurons Innervating Digit Muscles

The establishment of spinal motor neuron subclass diversity is achieved through developmental programs that are aligned with the organization of muscle targets in the limb. The evolutionary emergence of digits represents a specialized adaptation of limb morphology, yet it remains unclear how the specification of digit-innervating motor neuron subtypes parallels the elaboration of digits. We show that digit-innervating motor neurons can be defined by selective gene markers and distinguished from other LMC neurons by the expression of a variant Hox gene repertoire and by the failure to express a key enzyme involved in retinoic acid synthesis. This divergent developmental program is sufficient to induce the specification of digit-innervating motor neurons, emphasizing the specialized status of digit control in the evolution of skilled motor behaviors. Our findings suggest that the emergence of digits in the limb is matched by distinct mechanisms for specifying motor neurons that innervate digit muscles.

The Molecular Apgar Score: A Key to Unlocking Evolutionary Principles

One of the first "tools" used for systematically evaluating successful newborn transitional physiology at birth was the Apgar Score, devised by Virginia Apgar in 1953. This objective assessment tool allowed clinicians to immediately gauge the relative success of a newborn infant making the transition from the in utero liquid immersive environment to the ex utero gas environment in the delivery room during the first minutes after birth. The scoring system, although eponymous, is generally summarized as an acronym based on Appearance, Pulse, Grimace, Activity, and Respiration, criteria evaluated and scored at 1 and 5 min after birth. This common clinical appraisal is a guide for determining the elements of integrated physiology involved as the infant makes the transition from a "sea water" environment of 3% oxygen to a "land" environment in 21% oxygen. Appearance determines the perfusion of the skin with oxygenated blood-turning it pink; Pulse is the rate of heart beat, reflecting successful oxygen delivery to organs; Grimace, or irritability, is a functional marker for nervous system integration; Activity represents locomotor capacity; and, of course, Respiration represents pulmonary function as well as the successful neuro-feedback-mediated drive to breathe, supplying oxygen by inspiring atmospheric gas. Respiration, locomotion, and metabolism are fundamental processes adapted for vertebrate evolution from a water-based to an atmosphere-based life and are reflected by the Apgar Score. These physiologic processes last underwent major phylogenetic changes during the water-land transition some 300-400 million years ago, during which specific gene duplications occurred that facilitated terrestrial adaptation, in particular the parathyroid hormone-related protein receptor, the β-adrenergic receptor, and the glucocorticoid receptor. All these genetic traits and the gene regulatory networks they comprise represent the foundational substructure of the Apgar Score. As such, these molecular elements can be examined using a Molecular Apgar evaluation of keystone evolutionary events that predict successful evolutionary adaptation of physiologic functions necessary for neonatal transition and survival.

Developmental Mechanism of Limb Field Specification along the Anterior-Posterior Axis during Vertebrate Evolution

In gnathostomes, limb buds arise from the lateral plate mesoderm at discrete positions along the body axis. Specification of these limb-forming fields can be subdivided into several steps. The lateral plate mesoderm is regionalized into the anterior lateral plate mesoderm (ALPM; cardiac mesoderm) and the posterior lateral plate mesoderm (PLPM). Subsequently, Hox genes appear in a nested fashion in the PLPM and provide positional information along the body axis. The lateral plate mesoderm then splits into the somatic and splanchnic layers. In the somatic layer of the PLPM, the expression of limb initiation genes appears in the limb-forming region, leading to limb bud initiation. Furthermore, past and current work in limbless amphioxus and lampreys suggests that evolutionary changes in developmental programs occurred during the acquisition of paired fins during vertebrate evolution. This review presents these recent advances and discusses the mechanisms of limb field specification during development and evolution, with a focus on the role of Hox genes in this process.

Functional Diversification of Motor Neuron-specific Isl1 Enhancers during Evolution

Functional diversification of motor neurons has occurred in order to selectively control the movements of different body parts including head, trunk and limbs. Here we report that transcription of Isl1, a major gene necessary for motor neuron identity, is controlled by two enhancers, CREST1 (E1) and CREST2 (E2) that allow selective gene expression of Isl1 in motor neurons. Introduction of GFP reporters into the chick neural tube revealed that E1 is active in hindbrain motor neurons and spinal cord motor neurons, whereas E2 is active in the lateral motor column (LMC) of the spinal cord, which controls the limb muscles. Genome-wide ChIP-Seq analysis combined with reporter assays showed that Phox2 and the Isl1-Lhx3 complex bind to E1 and drive hindbrain and spinal cord-specific expression of Isl1, respectively. Interestingly, Lhx3 alone was sufficient to activate E1, and this may contribute to the initiation of Isl1 expression when progenitors have just developed into motor neurons. E2 was induced by onecut 1 (OC-1) factor that permits Isl1 expression in LMCm neurons. Interestingly, the core region of E1 has been conserved in evolution, even in the lamprey, a jawless vertebrate with primitive motor neurons. All E1 sequences from lamprey to mouse responded equally well to Phox2a and the Isl1-Lhx3 complex. Conversely, E2, the enhancer for limb-innervating motor neurons, was only found in tetrapod animals. This suggests that evolutionarily-conserved enhancers permit the diversification of motor neurons.

During evolution, motor neurons became specialized to control movements of different body parts including head, trunk and limbs. Here we report that two enhancers of Isl1, E1 and E2, are active together with transcription factors in motor neurons. Surprisingly, E1 and its response to transcription factors has been conserved in evolution from the lamprey to man, whereas E2 is only found in animals with limbs. Our study provides an evolutionary example of how functional diversification of motor neurons is achieved by a dynamic interplay between enhancers and transcription factors.

Tetrapod axial evolution and developmental constraints; Empirical underpinning by a mouse model

The tetrapod vertebral column has become increasingly complex during evolution as an adaptation to a terrestrial life. At the same time, the evolution of the vertebral formula became subject to developmental constraints acting on the size of the cervical and thoraco-lumbar regions. In the course of our studies concerning the evolution of Hox gene regulation, we produced a transgenic mouse model expressing fish Hox genes, which displayed a reduced number of thoraco-lumbar vertebrae and concurrent sacral homeotic transformations. Here, we analyze this mutant stock and conclude that the ancestral, pre-tetrapodial Hox code already possessed the capacity to induce vertebrae with sacral characteristics. This suggests that alterations in the interpretation of the Hox code may have participated to the evolution of this region in tetrapods, along with potential modifications of the HOX proteins themselves. With its reduced vertebral number, this mouse stock violates a previously described developmental constraint, which applies to the thoraco-lumbar region. The resulting offset between motor neuron morphology, vertebral patterning and the relative positioning of hind limbs illustrates that the precise orchestration of the Hox-clock in parallel with other ontogenetic pathways places constraints on the evolvability of the body plan.

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A transgenic mouse line expressing fish Hox genes has anterior homeotic transformations.

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Fish Hox genes are capable of inducing tetrapod specific vertebral characters.

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A sacral Hox-code influences adult hindlimb position, yet not the position of limb budding.

Evolution of patterning systems and circuit elements for locomotion

Evolutionary modifications in nervous systems enabled organisms to adapt to their specific environments and underlie the remarkable diversity of behaviors expressed by animals. Resolving the pathways that shaped and modified neural circuits during evolution remains a significant challenge. Comparative studies have revealed a surprising conservation in the intrinsic signaling systems involved in early patterning of bilaterian nervous systems but also raise the question of how neural circuit compositions and architectures evolved within specific animal lineages. In this review, we discuss the mechanisms that contributed to the emergence and diversity of animal nervous systems, focusing on the circuits governing vertebrate locomotion.

Hypaxial muscle: controversial classification and controversial data?

Hypaxial muscle is the anatomical term commonly used when referring to all the ventrally located musculature in the body of vertebrates, including muscles of the body wall and the limbs. Yet these muscles had very humble beginnings when vertebrates evolved from their chordate ancestors, and complex anatomical changes and changes in underlying gene regulatory networks occurred. This review summarises the current knowledge and controversies regarding the development and evolution of hypaxial muscles.

Distinct limb and trunk premotor circuits establish laterality in the spinal cord

Movement coordination between opposite body sides relies on neuronal circuits capable of controlling muscle contractions according to motor commands. Trunk and limb muscles engage in distinctly lateralized behaviors, yet how regulatory spinal circuitry differs is less clear. Here, we intersect virus technology and mouse genetics to unravel striking distribution differences of interneurons connected to functionally distinct motor neurons. We find that premotor interneurons conveying information to axial motor neurons reside in symmetrically balanced locations while mostly ipsilateral premotor interneurons synapse with limb-innervating motor neurons, especially those innervating more distal muscles. We show that observed distribution differences reflect specific premotor interneuron subpopulations defined by genetic and neurotransmitter identity. Synaptic input across the midline reaches axial motor neurons preferentially through commissural axon arborization, and to a lesser extent, through midline-crossing dendrites capturing contralateral synaptic input. Together, our findings provide insight into principles of circuit organization underlying weighted lateralization of movement.

Evolving locomotion with Hoxc9

Limb innervation is established by opposing Hox-dependent activities. In this issue of Developmental Cell, Jung et al. (2014) show that Hoxc9 restriction of Foxp1, high levels of which specify limb-innervating motor neurons, first appeared in vertebrates concomitantly with paired appendages. Spatial control of this activity shapes neural networks controlling locomotion patterns.

Evolving Hox activity profiles govern diversity in locomotor systems

The emergence of limb-driven locomotor behaviors was a key event in the evolution of vertebrates and fostered the transition from aquatic to terrestrial life. We show that the generation of limb-projecting lateral motor column (LMC) neurons in mice relies on a transcriptional autoregulatory module initiated via transient activity of multiple genes within the HoxA and HoxC clusters. Repression of this module at thoracic levels restricts expression of LMC determinants, thus dictating LMC position relative to the limbs. This suppression is mediated by a key regulatory domain that is specifically found in the Hoxc9 proteins of appendage-bearing vertebrates. The profile of Hoxc9 expression inversely correlates with LMC position in land vertebrates and likely accounts for the absence of LMC neurons in limbless species such as snakes. Thus, modulation of both Hoxc9 protein function and Hoxc9 gene expression likely contributed to evolutionary transitions between undulatory and ambulatory motor circuit connectivity programs.

Zebrafish models of human motor neuron diseases: advantages and limitations

Motor neuron diseases (MNDs) are an etiologically heterogeneous group of disorders of neurodegenerative origin, which result in degeneration of lower (LMNs) and/or upper motor neurons (UMNs). Neurodegenerative MNDs include pure hereditary spastic paraplegia (HSP), which involves specific degeneration of UMNs, leading to progressive spasticity of the lower limbs. In contrast, spinal muscular atrophy (SMA) involves the specific degeneration of LMNs, with symmetrical muscle weakness and atrophy. Amyotrophic lateral sclerosis (ALS), the most common adult-onset MND, is characterized by the degeneration of both UMNs and LMNs, leading to progressive muscle weakness, atrophy, and spasticity. A review of the comparative neuroanatomy of the human and zebrafish motor systems showed that, while the zebrafish was a homologous model for LMN disorders, such as SMA, it was only partially relevant in the case of UMN disorders, due to the absence of corticospinal and rubrospinal tracts in its central nervous system. Even considering the limitation of this model to fully reproduce the human UMN disorders, zebrafish offer an excellent alternative vertebrate model for the molecular and genetic dissection of MND mechanisms. Its advantages include the conservation of genome and physiological processes and applicable in vivo tools, including easy imaging, loss or gain of function methods, behavioral tests to examine changes in motor activity, and the ease of simultaneous chemical/drug testing on large numbers of animals. This facilitates the assessment of the environmental origin of MNDs, alone or in combination with genetic traits and putative modifier genes. Positive hits obtained by phenotype-based small-molecule screening using zebrafish may potentially be effective drugs for treatment of human MNDs.

Generating spinal motor neuron diversity: a long quest for neuronal identity

Understanding how thousands of different neuronal types are generated in the CNS constitutes a major challenge for developmental neurobiologists and is a prerequisite before considering cell or gene therapies of nervous lesions or pathologies. During embryonic development, spinal motor neurons (MNs) segregate into distinct subpopulations that display specific characteristics and properties including molecular identity, migration pattern, allocation to specific motor columns, and innervation of defined target. Because of the facility to correlate these different characteristics, the diversification of spinal MNs has become the model of choice for studying the molecular and cellular mechanisms underlying the generation of multiple neuronal populations in the developing CNS. Therefore, how spinal motor neuron subpopulations are produced during development has been extensively studied during the last two decades. In this review article, we will provide a comprehensive overview of the genetic and molecular mechanisms that contribute to the diversification of spinal MNs.

The evolutionary history of the development of the pelvic fin/hindlimb

The arms and legs of man are evolutionarily derived from the paired fins of primitive jawed fish. Few evolutionary changes have attracted as much attention as the origin of tetrapod limbs from the paired fins of ancestral fish. The hindlimbs of tetrapods are derived from the pelvic fins of ancestral fish. These evolutionary origins can be seen in the examination of shared gene and protein expression patterns during the development of pelvic fins and tetrapod hindlimbs. The pelvic fins of fish express key limb positioning, limb bud induction and limb outgrowth genes in a similar manner to that seen in hindlimb development of higher vertebrates. We are now at a point where many of the key players in the development of pelvic fins and vertebrate hindlimbs have been identified and we can now readily examine and compare mechanisms between species. This is yielding fascinating insights into how the developmental programme has altered during evolution and how that relates to anatomical change. The role of pelvic fins has also drastically changed over evolutionary history, from playing a minor role during swimming to developing into robust weight-bearing limbs. In addition, the pelvic fins/hindlimbs have been lost repeatedly in diverse species over evolutionary time. Here we review the evolution of pelvic fins and hindlimbs within the context of the changes in anatomical structure and the molecular mechanisms involved.

Molecular and evolutionary basis of limb field specification and limb initiation

Specification of limb field and initiation of limb development involve multiple steps, each of which is tightly regulated both spatially and temporally. Recent developmental analyses on various vertebrates have provided insights into the molecular mechanisms that specify limb field and have revealed several genetic interactions of signals involved in limb initiation processes. Furthermore, new approaches to the study of the developmental mechanisms of the lateral plate mesoderm of amphioxus and lamprey embryos have given us clues to understand the evolutionary scenarios that led to the acquisition of paired appendages during evolution. This review highlights such recent findings and discusses the mechanisms of limb field specification and limb bud initiation during development and evolution.