Differential regulation of AChR clustering in the polar and equatorial region of murine muscle spindles

LRP4-related signalling pathways and their regulatory role in neurological diseases

The mechanism of action of low-density lipoprotein receptor related protein 4 (LRP4) is mediated largely via the Agrin-LRP4-MuSK signalling pathway in the nervous system. LRP4 contributes to the development of synapses in the peripheral nervous system (PNS). It interacts with signalling molecules such as the amyloid beta-protein precursor (APP) and the wingless type protein (Wnt). Its mechanisms of action are complex and mediated via interaction between the pre-synaptic motor neuron and post-synaptic muscle cell in the PNS, which enhances the development of the neuromuscular junction (NMJ). LRP4 may function differently in the central nervous system (CNS) than in the PNS, where it regulates ATP and glutamate release via astrocytes. It mayaffect the growth and development of the CNS by controlling the energy metabolism. LRP4 interacts with Agrin to maintain dendrite growth and density in the CNS. The goal of this article is to review the current studies involving relevant LRP4 signaling pathways in the nervous system. The review also discusses the clinical and etiological roles of LRP4 in neurological illnesses, such as myasthenia gravis, Alzheimer's disease and epilepsy. In this review, we provide a theoretical foundation for the pathogenesis and therapeutic application of LRP4 in neurologic diseases.

Proprioceptors in extraocular muscles

Proprioception is the sense that lets us perceive the location, movement and action of the body parts. The proprioceptive apparatus includes specialized sense organs (proprioceptors) which are embedded in the skeletal muscles. The eyeballs are moved by six pairs of eye muscles and binocular vision depends on fine-tuned coordination of the optical axes of both eyes. Although experimental studies indicate that the brain has access to eye position information, both classical proprioceptors (muscle spindles and Golgi tendon organ) are absent in the extraocular muscles of most mammalian species. This paradox of monitoring extraocular muscle activity in the absence of typical proprioceptors seemed to be resolved when a particular nerve specialization (the palisade ending) was detected in the extraocular muscles of mammals. In fact, for decades there was consensus that palisade endings were sensory structures that provide eye position information. The sensory function was called into question when recent studies revealed the molecular phenotype and the origin of palisade endings. Today we are faced with the fact that palisade endings exhibit sensory as well as motor features. This review aims to evaluate the literature on extraocular muscle proprioceptors and palisade endings and to reconsider current knowledge of their structure and function.

Intrafusal-fiber LRP4 for muscle spindle formation and maintenance in adult and aged animals

Proprioception is sensed by muscle spindles for precise locomotion and body posture. Unlike the neuromuscular junction (NMJ) for muscle contraction which has been well studied, mechanisms of spindle formation are not well understood. Here we show that sensory nerve terminals are disrupted by the mutation of Lrp4, a gene required for NMJ formation; inducible knockout of Lrp4 in adult mice impairs sensory synapses and movement coordination, suggesting that LRP4 is required for spindle formation and maintenance. LRP4 is critical to the expression of Egr3 during development; in adult mice, it interacts in trans with APP and APLP2 on sensory terminals. Finally, spindle sensory endings and function are impaired in aged mice, deficits that could be diminished by LRP4 expression. These observations uncovered LRP4 as an unexpected regulator of muscle spindle formation and maintenance in adult and aged animals and shed light on potential pathological mechanisms of abnormal muscle proprioception.

The effect of methocarbamol and mexiletine on murine muscle spindle function

INTRODUCTION/AIMS

The muscle relaxant methocarbamol and the antimyotonic drug mexiletine are widely used for the treatment of muscle spasms, myotonia, and pain syndromes. To determine whether these drugs affect muscle spindle function, we studied their effect on the resting discharge and on stretch-induced action potential frequencies of proprioceptive afferent neurons.

METHODS

Single unit action potential frequencies of proprioceptive afferents from muscle spindles in the murine extensor digitorum longus muscle of adult C57BL/6J mice were recorded under resting conditions and during ramp-and-hold stretches. Maximal tetanic force of the same muscle after direct stimulation was determined. High-resolution confocal microscopy analysis was performed to determine the distribution of Na_v 1.4 channels, a potential target for both drugs.

RESULTS

Methocarbamol and mexiletine inhibited the muscle spindle resting discharge in a dose-dependent manner with IC₅₀ values around 300 μM and 6 μM, respectively. With increasing concentrations of both drugs, the response to stretch was also affected, with the static sensitivity first followed by the dynamic sensitivity. At high concentrations, both drugs completely blocked muscle spindle afferent output. Both drugs also reversibly reduced the specific force of the extensor digitorum longus muscle after tetanic stimulation. Finally, we present evidence for the presence and specific localization of the voltage-gated sodium channel Na_v 1.4 in intrafusal fibers.

DISCUSSION

In this study we demonstrate that both muscle relaxants affect muscle spindle function, suggesting impaired proprioception as a potential side effect of both drugs. Moreover, our results provide additional evidence of a peripheral activity of methocarbamol and mexiletine.

Validation of terminal Schwann cell gene marker expression by fluorescent in situ hybridization using RNAscope

Recent RNA-seq studies have generated a new crop of putative gene markers for terminal Schwann cells (tSCs), non-myelinating glia that cap axon terminals at the vertebrate neuromuscular junction (NMJ). While compelling, these studies did not validate the expression of the novel markers using in situ hybridization techniques. Here, we use RNAscope technology to study the expression of top candidates from recent tSC and non-myelinating Schwann cell marker RNA-seq studies. Our results validate the expression of these markers at tSCs but also demonstrate that they are present at other sites in the muscle tissue, specifically, at muscle spindles and along intramuscular nerves.

Methodological advances for studying gamma motor neurons

The muscle spindle is an important sense organ for motor control and proprioception. Specialized intrafusal fibers are innervated by both stretch sensitive afferents and γ motor neurons that control the length of the spindle and tune the sensitivity of the muscle spindle afferents to both dynamic movement and static length. γ motor neurons share many similarities with other skeletal motor neurons, making it challenging to identify and specifically record or stimulate them. This short review will discuss recent advances in genetic and molecular biology techniques, electrophysiological recording, optical imaging, computer modelling, and stem cell culture techniques that have the potential to help answer important questions about fusimotor function in motor control and disease.

Muscle spindle function in healthy and diseased muscle

Almost every muscle contains muscle spindles. These delicate sensory receptors inform the central nervous system (CNS) about changes in the length of individual muscles and the speed of stretching. With this information, the CNS computes the position and movement of our extremities in space, which is a requirement for motor control, for maintaining posture and for a stable gait. Many neuromuscular diseases affect muscle spindle function contributing, among others, to an unstable gait, frequent falls and ataxic behavior in the affected patients. Nevertheless, muscle spindles are usually ignored during examination and analysis of muscle function and when designing therapeutic strategies for neuromuscular diseases. This review summarizes the development and function of muscle spindles and the changes observed under pathological conditions, in particular in the various forms of muscular dystrophies.

Impaired muscle spindle function in murine models of muscular dystrophy

KEY POINTS

Muscular dystrophy patients suffer from progressive degeneration of skeletal muscle fibres, sudden spontaneous falls, balance problems, as well as gait and posture abnormalities. Dystrophin- and dysferlin-deficient mice, models for different types of muscular dystrophy with different aetiology and molecular basis, were characterized to investigate if muscle spindle structure and function are impaired. The number and morphology of muscle spindles were unaltered in both dystrophic mouse lines but muscle spindle resting discharge and their responses to stretch were altered. In dystrophin-deficient muscle spindles, the expression of the paralogue utrophin was substantially upregulated, potentially compensating for the dystrophin deficiency. The results suggest that muscle spindles might contribute to the motor problems observed in patients with muscular dystrophy.

ABSTRACT

Muscular dystrophies comprise a heterogeneous group of hereditary diseases characterized by progressive degeneration of extrafusal muscle fibres as well as unstable gait and frequent falls. To investigate if muscle spindle function is impaired, we analysed their number, morphology and function in wildtype mice and in murine model systems for two distinct types of muscular dystrophy with very different disease aetiology, i.e. dystrophin- and dysferlin-deficient mice. The total number and the overall structure of muscle spindles in soleus muscles of both dystrophic mouse mutants appeared unchanged. Immunohistochemical analyses of wildtype muscle spindles revealed a concentration of dystrophin and β-dystroglycan in intrafusal fibres outside the region of contact with the sensory neuron. While utrophin was absent from the central part of intrafusal fibres of wildtype mice, it was substantially upregulated in dystrophin-deficient mice. Single-unit extracellular recordings of sensory afferents from muscle spindles of the extensor digitorum longus muscle revealed that muscle spindles from both dystrophic mouse strains have an increased resting discharge and a higher action potential firing rate during sinusoidal vibrations, particularly at low frequencies. The response to ramp-and-hold stretches appeared unaltered compared to the respective wildtype mice. We observed no exacerbated functional changes in dystrophin and dysferlin double mutant mice compared to the single mutant animals. These results show alterations in muscle spindle afferent responses in both dystrophic mouse lines, which might cause an increased muscle tone, and might contribute to the unstable gait and frequent falls observed in patients with muscular dystrophy.

Alternative Splicing and the Intracellular Domain Mediate TM-agrin's Ability to Differentially Regulate the Density of Excitatory and Inhibitory Synapse-like Specializations in Developing CNS Neurons

Agrin is a multi-domain protein best known for its essential function during formation of the neuromuscular junction. Alternative mRNA splicing at sites named y and z in the C-terminal part of agrin regulates its interaction with a receptor complex consisting of the agrin-binding low-density lipoprotein receptor-related protein 4 (Lrp4) and the muscle-specific kinase (MuSK). Isoforms with inserts at both splice sites bind to Lrp4, activate MuSK and are synaptogenic at the neuromuscular junction. Agrin is also expressed as a transmembrane protein in the central nervous system (CNS) but its function during interneuronal synapse formation is unclear. Recently we demonstrated that transfection of a full-length cDNA coding for transmembrane agrin (TM-agrin) in cultured embryonic cortical neurons induced an Lrp4-dependent but MuSK-independent increase in dendritic glutamatergic synapses and an Lrp4- and MuSK-independent reduction of inhibitory synapses. Here we show that presynaptic specializations were similarly affected by TM-agrin overexpression. In addition, we mapped the regions within TM-agrin responsible for TM-agrin's effects on dendritic aggregates of synapse-associated proteins. We show that the presence of a four amino acid insert at splice site y is essential for the increase in the density of puncta containing the postsynaptic density protein 95 kDa. This effect was independent of splice site z. The reduction of the gephyrin puncta density was independent of the entire extracellular part of TM-agrin but required a highly conserved serine residue in the intracellular domain of TM-agrin. These results provide further evidence for a function of TM-agrin during CNS synaptogenesis and demonstrate that different domains and alternative splicing of TM-agrin differentially affect excitatory and inhibitory synapse formation in cultured embryonic CNS neurons.

The effects of neuregulin-1β on intrafusal muscle fiber formation in neuromuscular coculture of dorsal root ganglion explants and skeletal muscle cells

Background

The formation of intrafusal muscle (IM) fibers and their contact with afferent proprioceptive axons is critical for construction, function, and maintenance of the stretch reflex. Many factors affect the formation of IM fibers. Finding new factors and mechanisms of IM fiber formation is essential for the reconstruction of stretch reflex arc after injury.

Methods

We established a coculture system of organotypic dorsal root ganglion (DRG) explants and dissociated skeletal muscle (SKM) cells. The formation of IM fibers was observed in this coculture system after neuregulin-1β (NRG-1β) incubation.

Results

We found that NRG-1β promoted outgrowth of neurites and migration of neurons from the organotypic DRG explants and that this correlated with an induction of growth-associated protein 43 (GAP-43) expression. NRG-1β also increased the amount of nuclear bag fibers and nuclear chain fibers by elevating the proportion of tyrosine kinase receptor C (TrkC) phenotypic DRG neurons. In addition, we found that the effects of NRG-1β could be blocked by inhibiting ERK1/2, PI3K/Akt, and JAK2/STAT3 signaling pathways.

Conclusion

These data imply that NRG-1β promoted neurite outgrowth and neuronal migration from the organotypic DRG explants and that this correlated with an induction of GAP-43 expression. The modulating effects of NRG-1β on TrkC DRG neuronal phenotype may link to promote IM fiber formation. The effects produced by NRG-1β in this neuromuscular coculture system provide new data for the therapeutic potential on IM fiber formation after muscle injury.

Electronic supplementary material

The online version of this article (10.1186/s13395-018-0175-9) contains supplementary material, which is available to authorized users.

Specific effects of neuregulin-1β on the communication between DRG neurons and skeletal muscle cells in vitro

The communication between primary afferent neuron and skeletal muscle (SKM) is one of the important factors on maintaining the structure and function of SKM cells. Neuregulin-1β (NRG-1β) signaling is essential for regulating synaptic neurotransmission. Here, we established a neuromuscular coculture model of dorsal root ganglion (DRG) sensory neurons and SKM cells to explore the nerve-muscle communication in the presence of exogenous NRG-1β. The expression of three distinct subtypes (TrkA, TrkB, and TrkC) of tyrosine kinase receptors was monitored for the phenotypical alterations of the neurons. The aggregation extent of acetylcholine receptor (AChR) represents the specific changes of SKM cells after NRG-1β incubation in this neuromuscular coculture model. The results showed that NRG-1β not only enhanced neurite outgrowth of DRG neurons but also increased the length and branches of SKM cells. NRG-1β treatment not only induced expression of all the three subtypes of Trk receptors in neurons but also promoted AChR aggregation on the surface of SKM cells. The effects of NRG-1β could be blocked by administration of ERK1/2 inhibitor PD98059, PI3K inhibitor LY294002, and JAK2 inhibitor AG490, respectively. These data imply that NRG-1β is essential for the nerve-muscle communication by enhancing growth and modifying phenotypes of the two different kinds of cells. The specific effects produced by NRG-1β add novel interpretation for nerve-muscle communication between sensory neurons and SKM cells.

Requirement for Dicer in Maintenance of Monosynaptic Sensory-Motor Circuits in the Spinal Cord

In contrast to our knowledge of mechanisms governing circuit formation, our understanding of how neural circuits are maintained is limited. Here, we show that Dicer, an RNaseIII protein required for processing microRNAs (miRNAs), is essential for maintenance of the spinal monosynaptic stretch reflex circuit in which group Ia proprioceptive sensory neurons form direct connections with motor neurons. In postnatal mice lacking Dicer in proprioceptor sensory neurons, there are no obvious defects in specificity or formation of monosynaptic sensory-motor connections. However, these circuits degrade through synapse loss and retraction of proprioceptive axonal projections from the ventral spinal cord. Peripheral terminals are also impaired without retracting from muscle targets. Interestingly, despite these central and peripheral axonal defects, proprioceptive neurons survive in the absence of Dicer-processed miRNAs. These findings reveal that Dicer, through its production of mature miRNAs, plays a key role in the maintenance of monosynaptic sensory-motor circuits.

Neuronal LRP4 regulates synapse formation in the developing CNS

The low-density lipoprotein receptor-related protein 4 (LRP4) is essential in muscle fibers for the establishment of the neuromuscular junction. Here, we show that LRP4 is also expressed by embryonic cortical and hippocampal neurons, and that downregulation of LRP4 in these neurons causes a reduction in density of synapses and number of primary dendrites. Accordingly, overexpression of LRP4 in cultured neurons had the opposite effect inducing more but shorter primary dendrites with an increased number of spines. Transsynaptic tracing mediated by rabies virus revealed a reduced number of neurons presynaptic to the cortical neurons in which LRP4 was knocked down. Moreover, neuron-specific knockdown of LRP4 by in utero electroporation of LRP4 miRNA in vivo also resulted in neurons with fewer primary dendrites and a lower density of spines in the developing cortex and hippocampus. Collectively, our results demonstrate an essential and novel role of neuronal LRP4 in dendritic development and synaptogenesis in the CNS.

A system for studying mechanisms of neuromuscular junction development and maintenance

The neuromuscular junction (NMJ), a cellular synapse between a motor neuron and a skeletal muscle fiber, enables the translation of chemical cues into physical activity. The development of this special structure has been subject to numerous investigations, but its complexity renders in vivo studies particularly difficult to perform. In vitro modeling of the neuromuscular junction represents a powerful tool to delineate fully the fine tuning of events that lead to subcellular specialization at the pre-synaptic and post-synaptic sites. Here, we describe a novel heterologous co-culture in vitro method using rat spinal cord explants with dorsal root ganglia and murine primary myoblasts to study neuromuscular junctions. This system allows the formation and long-term survival of highly differentiated myofibers, motor neurons, supporting glial cells and functional neuromuscular junctions with post-synaptic specialization. Therefore, fundamental aspects of NMJ formation and maintenance can be studied using the described system, which can be adapted to model multiple NMJ-associated disorders.

Degeneration of proprioceptive sensory nerve endings in mice harboring amyotrophic lateral sclerosis-causing mutations

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease that primarily targets the motor system. Although much is known about the effects of ALS on motor neurons and glial cells, little is known about its effect on proprioceptive sensory neurons. This study examines proprioceptive sensory neurons in mice harboring mutations associated with ALS, in SOD1(G93A) and TDP43(A315T) transgenic mice. In both transgenic lines, we found fewer proprioceptive sensory neurons containing fluorescently tagged cholera toxin in their soma five days after injecting this retrograde tracer into the tibialis anterior muscle. We asked whether this is due to neuronal loss or selective degeneration of peripheral nerve endings. We found no difference in the total number and size of proprioceptive sensory neuron soma between symptomatic SOD1(G93A) and control mice. However, analysis of proprioceptive nerve endings in muscles revealed early and significant alterations at Ia/II proprioceptive nerve endings in muscle spindles before the symptomatic phase of the disease. Although these changes occur alongside those at α-motor axons in SOD1(G93A) mice, Ia/II sensory nerve endings degenerate in the absence of obvious alterations in α-motor axons in TDP43(A315T) transgenic mice. We next asked whether proprioceptive nerve endings are similarly affected in the spinal cord and found that nerve endings terminating on α-motor neurons are affected during the symptomatic phase and after peripheral nerve endings begin to degenerate. Overall, we show that Ia/II proprioceptive sensory neurons are affected by ALS-causing mutations, with pathological changes starting at their peripheral nerve endings.

Mechanisms Regulating Neuromuscular Junction Development and Function and Causes of Muscle Wasting

The neuromuscular junction is the chemical synapse between motor neurons and skeletal muscle fibers. It is designed to reliably convert the action potential from the presynaptic motor neuron into the contraction of the postsynaptic muscle fiber. Diseases that affect the neuromuscular junction may cause failure of this conversion and result in loss of ambulation and respiration. The loss of motor input also causes muscle wasting as muscle mass is constantly adapted to contractile needs by the balancing of protein synthesis and protein degradation. Finally, neuromuscular activity and muscle mass have a major impact on metabolic properties of the organisms. This review discusses the mechanisms involved in the development and maintenance of the neuromuscular junction, the consequences of and the mechanisms involved in its dysfunction, and its role in maintaining muscle mass during aging. As life expectancy is increasing, loss of muscle mass during aging, called sarcopenia, has emerged as a field of high medical need. Interestingly, aging is also accompanied by structural changes at the neuromuscular junction, suggesting that the mechanisms involved in neuromuscular junction maintenance might be disturbed during aging. In addition, there is now evidence that behavioral paradigms and signaling pathways that are involved in longevity also affect neuromuscular junction stability and sarcopenia.