The pattern of avian intramuscular nerve branching is determined by the innervating motoneuron and its level of polysialic acid

Clinical, genetic, and pathological characterization of GNE myopathy in China

BACKGROUND

GNE myopathy is the most common distal myopathy in China. We summarized the clinical, genetic, and pathological characteristics of 125 Chinese patients with GNE myopathy.

METHODS

We collected clinical data of 21 patients diagnosed at our hospital and 104 patients from previous reports. Clinical, genetic, and pathological characteristics were summarized. According to the location of mutations, patients were classified into groups to analyze genotype-phenotype correlation. We reviewed the pathological features and studied the expressions of neural cell adhesion molecule.

RESULTS

The severity of involvement of lower limb muscles was in the following order: tibialis anterior > biceps femoris > gastrocnemius > iliopsoas > quadriceps femoris. Mutation p.D207V was the most common variant in China. Patients carrying p.D207V tended to show later disease onset. In the epimerase/epimerase group, men had earlier disease onset than women (p < 0.05). In other groups, age at disease onset in females was earlier than that in males. Protein analysis showed decreased sialylation of NCAM and upregulation of LC3 in patients with different mutations.

CONCLUSIONS

Mutation p.D207V is the most common GNE variant in China. Involvement of flexor muscles in lower limbs was more obvious than extensor muscles. NCAM expression in patients with various mutations may be a useful diagnostic biomarker in GNE myopathy.

Single-nucleus RNA-seq and FISH identify coordinated transcriptional activity in mammalian myofibers

Skeletal muscle fibers are large syncytia but it is currently unknown whether gene expression is coordinately regulated in their numerous nuclei. Here we show by snRNA-seq and snATAC-seq that slow, fast, myotendinous and neuromuscular junction myonuclei each have different transcriptional programs, associated with distinct chromatin states and combinations of transcription factors. In adult mice, identified myofiber types predominantly express either a slow or one of the three fast isoforms of Myosin heavy chain (MYH) proteins, while a small number of hybrid fibers can express more than one MYH. By snRNA-seq and FISH, we show that the majority of myonuclei within a myofiber are synchronized, coordinately expressing only one fast Myh isoform with a preferential panel of muscle-specific genes. Importantly, this coordination of expression occurs early during post-natal development and depends on innervation. These findings highlight a previously undefined mechanism of coordination of gene expression in a syncytium.

Expression of sialic acids in human adult skeletal muscle tissue

Investigations mostly in animal models have shown a role of sialic acid in the morphology and functionality of skeletal muscle during development and adult life. Several studies in humans have been performed regarding changes in sialic acid expression in a particular pathology, hereditary inclusion body myopathy, leading to muscular weakness and atrophy, with a similar phenomenon appearing also in sarcopenia of aging. In this study the expression of monomeric and polymeric sialic acids was evaluated in human skeletal muscle during adult life. Surgical biopsies of the Quadriceps femoris muscle from men aged 18-25 years (young group; n=8) and men aged 72-78 (elderly group; n=10) were collected for analysis. Expression of sialic acids was evaluated using lectin histochemistry, associated with enzymatic and chemical treatments to characterize monomeric and polymeric sialic acids. The polysialic acid expression was also evaluated by immunohistochemistry. Various types of sialic acid in the muscle tissue, in different amounts in the study groups, were detected. Monomeric sialic acids decreased in the elderly group compared with the young group, whereas polysialic acid increased. Sialic acid acetylation was present only in the young group. These findings demonstrated that changes in the expression of sialic acids in skeletal muscle tissue may be related to morphofunctional modifications occurring during aging.

A stem-cell based bioassay to critically assess the pathology of dysfunctional neuromuscular junctions

Pluripotent stem cells can be directed to differentiate into motor neurons and assessed for functionality in vitro. An emerging application of this technique is to model genetically inherited diseases in differentiated motor neurons and to screen for new therapeutic targets. The neuromuscular junction (NMJ) is essential to the functionality of motor neurons and its dysfunction is a primary hallmark of motor neuron disease. However, mature NMJs that possess the functional and morphological characteristics of those formed in vivo have so far not been obtained in vitro. Here we describe the generation and analysis of mature NMJs formed between embryonic stem cell-derived motor neurons (ESCMNs) and primary myotubes. We compared the formation and maturation of NMJs generated by wild-type (NCAM+/+) ESCMNs to those generated by neural cell adhesion molecule null (NCAM-/-) ESCMNs in order to definitively test the sensitivity of this assay to identify synaptic pathology. We find that co-cultures using NCAM-/- ESCMNs replicate key in vivo NCAM-/- phenotypes and reveal that NCAM influences neuromuscular synaptogenesis by controlling the mode of synaptic vesicle endocytosis. Further, we could improve synapse formation and function in NCAM-/- co-cultures by chronic treatment with nifedipine, which blocks an immature synaptic vesicle recycling pathway. Together, our results demonstrate that this ESCMN/myofiber co-culture system is a highly sensitive bioassay for examining molecules postulated to regulate synaptic function and for screening therapeutics that will improve the function of compromised NMJs.

Brief transvertebral electrical stimulation of the spinal cord improves the specificity of femoral nerve reinnervation

BACKGROUND

Functional outcomes are generally poor following peripheral nerve injury (PNI). The reason is multifactorial but includes the misdirection of regenerating axons to inappropriate end organs. It has been shown that brief electrical stimulation (Estim) of nerves has the potential to improve the accuracy and rate of peripheral axon regeneration.

OBJECTIVE

The present study explores a novel percutaneous transvertebral approach to Estim, which was tested in the mouse femoral nerve model.

METHODS

Inspired by the protocol of Gordon and colleagues (ie, 20 Hz, for 1 hour), we applied Estim to the cervicothoracic spinal cord (SC-Estim) to remotely activate lumbar motor neurons following transection and repair of the femoral nerve. Fluorescent dyes were applied to the distal nerve to label reinnervating cells. Sections of nerve were taken to quantify the numbers of reinnervating axons as well as to stain for a known femoral axon guidance molecule-polysialylated neural cell adhesion molecule (PSA-NCAM).

RESULTS

In comparison to sham treatment, SC-Estim led to significantly greater expression of PSA-NCAM as well as improved the specificity of motor reinnervation. Interestingly, although SC-Estim did not alter the number of early reinnervating (ie, pioneer) axons, there was a reduction in the number of retrogradely labeled neurons at 2 weeks postrepair. However, by 6 weeks postrepair, there was no difference in the number of neurons that had reinnervated the femoral nerve.

CONCLUSIONS

The present findings support the development of SC-Estim as a novel approach to enhance the specificity of reinnervation and potentially improve functional outcomes following PNI.

Neuronal Nogo-A regulates neurite fasciculation, branching and extension in the developing nervous system

Wiring of the nervous system is a multi-step process involving complex interactions of the growing fibre with its tissue environment and with neighbouring fibres. Nogo-A is a membrane protein enriched in the adult central nervous system (CNS) myelin, where it restricts the capacity of axons to grow and regenerate after injury. During development, Nogo-A is also expressed by neurons but its function in this cell type is poorly known. Here, we show that neutralization of neuronal Nogo-A or Nogo-A gene ablation (KO) leads to longer neurites, increased fasciculation, and decreased branching of cultured dorsal root ganglion neurons. The same effects are seen with antibodies against the Nogo receptor complex components NgR and Lingo1, or by blocking the downstream effector Rho kinase (ROCK). In the chicken embryo, in ovo injection of anti-Nogo-A antibodies leads to aberrant innervation of the hindlimb. Genetic ablation of Nogo-A causes increased fasciculation and reduced branching of peripheral nerves in Nogo-A KO mouse embryos. Thus, Nogo-A is a developmental neurite growth regulatory factor with a role as a negative regulator of axon-axon adhesion and growth, and as a facilitator of neurite branching.

Identification of novel spinal cholinergic genetic subtypes disclose Chodl and Pitx2 as markers for fast motor neurons and partition cells

Spinal cholinergic neurons are critical for motor function in both the autonomic and somatic nervous systems and are affected in spinal cord injury and in diseases such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy. Using two screening approaches and in situ hybridization, we identified 159 genes expressed in typical cholinergic patterns in the spinal cord. These include two general cholinergic neuron markers, one gene exclusively expressed in motor neurons, and nine genes expressed in unknown subtypes of somatic motor neurons. Further, we present evidence that chondrolectin (Chodl) is expressed by fast motor neurons and that estrogen-related receptor beta (ERRbeta) is a candidate marker for slow motor neurons. In addition, we suggest paired-like homeodomain transcription factor 2 (Pitx2) as a marker for cholinergic partition cells.

Neural cell adhesion molecule is required for stability of reinnervated neuromuscular junctions

Studies examining the etiology of motoneuron diseases usually focus on motoneuron death as the defining pathophysiology of the disease. However, impaired neuromuscular transmission and synapse withdrawal often precede cell death, raising the possibility that abnormalities in synaptic function contribute to disease onset. Although little is known about the mechanisms maintaining the synaptic integrity of neuromuscular junctions (NMJs), Drosophila studies suggest that Fasciclin II plays an important role. Inspired by these studies we used a reinnervation model of synaptogenesis to analyze neuromuscular function in mice lacking neural cell adhesion molecule (NCAM), the Fasciclin II vertebrate homolog. Our results showed that the recovery of contractile force was the same in wild-type and NCAM-/- mice at 1 month after nerve injury, indicating that endplates were appropriately reformed. This normality was only transient because the contractile force and myofiber number decreased at 3 months after injury in NCAM-/- mice. Both declined further 3 months later. Myofibers degenerated, not because motoneurons died but because synapses were withdrawn. Although neurotransmission was initially normal at reinnervated NCAM-/- NMJs, it was significantly compromised 3 months later. Interestingly, the selective ablation of NCAM from motoneurons, or muscle fibers, did not mimic the deficits observed in reinnervated NCAM-/- mice. Taken together, these results indicate that NCAM is required to maintain normal synaptic function at reinnervated NMJs, although its loss pre-synaptically or post-synaptically is not sufficient to induce synaptic destabilization. Consideration is given to the role of NCAM in terminal Schwann cells for maintaining synaptic integrity and how NCAM dysfunction may contribute to motoneuron disorders.

Dissecting polysialic acid and NCAM functions in brain development

The unique modification of the neural cell adhesion molecule (NCAM) by polysialic acid (polySia) is tightly associated with nervous system development and plasticity. The prevailing view that this large carbohydrate polymer acts as an anti-adhesive factor seems straightforward at first sight. However, during almost 25 years of polySia research it became increasingly clear that the impact of polySia on cell surface interactions can not be explained by one unifying mechanism. Recent progress in the generation of mouse models, which partially or completely lack polySia due to ablation of one or both of the two polySia synthesizing enzymes, provides novel insights into the function of this unique post-translational modification. The present review is focused on a phenotype comparison between the newly established mouse strains which combine polySia-deficiency with normal NCAM expression and the well-characterized NCAM negative mouse model. Analysis of shared and individual phenotypes allows a clear distinction between NCAM and polySia functions and revealed that polySia plays a vital role as a specific control element of NCAM-mediated interactions.

Injury to retinal ganglion cell axons increases polysialylated neural cell adhesion molecule (PSA-NCAM) in the adult rodent superior colliculus

The adult mammalian central nervous system (CNS) exhibits a limited regenerative response to injury. It is well established that polysialylated neural cell adhesion molecule (PSA-NCAM) contributes to nervous system plasticity. In the visual system, PSA-NCAM participates in retinal ganglion cell (RGC) axon growth during development and specifically influences RGC innervation of its principle target tissue, the superior colliculus (SC). The goals of this study were to determine whether PSA-NCAM is expressed in the normal adult mouse SC and to evaluate PSA-NCAM expression following RGC injury. In the normal rostral, but not caudal, SC we find that PSA-NCAM is present in the retinorecipient layers; however, PSA-NCAM and RGC axons do not co-localize. In the deeper collicular layers, PSA-NCAM is observed as several distinct patches that occur at the same depth along the medial-lateral axis throughout the colliculus. RGC axotomy denervates predominantly the contralateral colliculus, where increased PSA-NCAM levels are seen at 7 and 10 days after the injury. Further evaluation of the retinorecipient layers of the partially denervated SC reveals that some intact CTB-traced RGC axons (less than 5%) labeled from the ipsilateral eye do co-localize with PSA-NCAM. This study is the first characterization of PSA-NCAM expression in the normal and partially denervated adult SC and may indicate that PSA-NCAM is involved in attempted visual system remodeling after injury.

Six1 and Eya1 expression can reprogram adult muscle from the slow-twitch phenotype into the fast-twitch phenotype

Muscle fibers show great differences in their contractile and metabolic properties. This diversity enables skeletal muscles to fulfill and adapt to different tasks. In this report, we show that the Six/Eya pathway is implicated in the establishment and maintenance of the fast-twitch skeletal muscle phenotype. We demonstrate that the MEF3/Six DNA binding element present in the aldolase A pM promoter mediates the high level of activation of this promoter in fast-twitch glycolytic (but not in slow-twitch) muscle fibers. We also show that among the Six and Eya gene products expressed in mouse skeletal muscle, Six1 and Eya1 proteins accumulate preferentially in the nuclei of fast-twitch muscles. The forced expression of Six1 and Eya1 together in the slow-twitch soleus muscle induced a fiber-type transition characterized by the replacement of myosin heavy chain I and IIA isoforms by the faster IIB and/or IIX isoforms, the activation of fast-twitch fiber-specific genes, and a switch toward glycolytic metabolism. Collectively, these data identify Six1 and Eya1 as the first transcriptional complex that is able to reprogram adult slow-twitch oxidative fibers toward a fast-twitch glycolytic phenotype.

Neural cell adhesion molecule (NCAM) null mice do not show a deficit in odour discrimination learning

Polysialylated neural cell adhesion molecule (PSA-NCAM) is predominantly expressed during development where it regulates biological functions including axon targeting and neuronal precursor cell migration. Although dramatically down regulated after birth in most regions of the nervous system, PSA-NCAM remains highly expressed into adulthood in areas that have ongoing regeneration and plasticity such as in the olfactory bulb and hippocampus. Consequently, lack of PSA-NCAM in NCAM null mice results in distinct morphological changes to these areas. The functional correlates of these changes are not well defined although there have been reports that learning is impaired in NCAM null mice. In the present study, we assessed the ability of old and young NCAM null mice to learn an odour discrimination task. We tested male and female experimental and control animals of two different ages: 30-60 days and 12-15 months. During 4 days of training, NCAM null and C57BL/6J received trials where one odour (CS+) was paired with sugar while another odour (CS-) was not. In a subsequent preference test, conducted in the absence of sugar, all animals, regardless of strain or age, spent significantly more time digging in the CS+ odour than in the CS- odour. In addition, there was no significant difference in digging behaviour in the CS+ between the NCAM null and the control animals. These data indicate that deletion of the NCAM gene may change the morphology of the olfactory bulb but does not interfere with the ability to learn an odour discrimination task.

Distinct muscle targets do not vary in the developmental regulation of brain-derived neurotrophic factor. J Comp Neurol 470:317-329,2004

Developing neurons depend on many target-derived signals. One of these signals is the neurotrophin brain-derived neurotrophic factor (BDNF). Exogenous application of BDNF in vitro and in vivo rescues a population of lumbar motoneurons from programmed cell death. Given that BDNF does not rescue all motoneurons and that motoneurons differ in trophic factor receptor expression, subpopulations of motoneurons may have different sensitivities to the factor. These differences may be reflected in distinct target muscles specialized to produce different protein concentrations, or muscles may contain equal amounts of the factor and receptor expression determines motoneuron responsiveness. By using a sensitive electrochemiluminescent immunoassay (ECLIA), we measured normal developmental changes in BDNF protein concentration in anatomically and functionally distinct chick embryonic thigh muscles from E6 to E18. We found that there were no significant differences in BDNF protein concentration between muscles classified according to function (fast vs. slow) or anatomical position (flexor vs. extensor) and that the quantity of BDNF in the target did not appear to be activity dependent. These results suggest that, during development, the differences in the response of motoneurons to BDNF are not due to the anatomical or functional diversity of muscle targets. J. Comp. Neurol. 470:330-337, 2004.

Neuromuscular synapses mediate motor axon branching and motoneuron survival during the embryonic period of programmed cell death

The embryonic period of motoneuron programmed cell death (PCD) is marked by transient motor axon branching, but the role of neuromuscular synapses in regulating motoneuron number and axonal branching is not known. Here, we test whether neuromuscular synapses are required for the quantitative association between reduced skeletal muscle contraction, increased motor neurite branching, and increased motoneuron survival. We achieved this by comparing agrin and rapsyn mutant mice that lack acetylcholine receptor (AChR) clusters. There were significant reductions in nerve-evoked skeletal muscle contraction, increases in intramuscular axonal branching, and increases in spinal motoneuron survival in agrin and rapsyn mutant mice compared with their wild-type littermates at embryonic day 18.5 (E18.5). The maximum nerve-evoked skeletal muscle contraction was reduced a further 17% in agrin mutants than in rapsyn mutants. This correlated to an increase in motor axon branch extension and number that was 38% more in agrin mutants than in rapsyn mutants. This suggests that specializations of the neuromuscular synapse that ensure efficient synaptic transmission and muscle contraction are also vital mediators of motor axon branching. However, these increases in motor axon branching did not correlate with increases in motoneuron survival when comparing agrin and rapsyn mutants. Thus, agrin-induced synaptic specializations are required for skeletal muscle to effectively control motoneuron numbers during embryonic development.

Different isoforms of fasciclin II are expressed by a subset of developing olfactory receptor neurons and by olfactory-nerve glial cells during formation of glomeruli in the moth Manduca sexta

During development of the primary olfactory projection, olfactory receptor axons must sort by odor specificity and seek particular sites in the brain in which to create odor-specific glomeruli. In the moth Manduca sexta, we showed previously that fasciclin II, a cell adhesion molecule in the immunoglobulin superfamily, is expressed by the axons of a subset of olfactory receptor neurons during development and that, in a specialized glia-rich "sorting zone," these axons segregate from nonfasciclin II-expressing axons before entering the neuropil of the glomerular layer. The segregation into fasciclin II-positive fascicles is dependent on the presence of the glial cells in the sorting zone. Here, we explore the expression patterns for different isoforms of Manduca fasciclin II in the developing olfactory system. We find that olfactory receptor axons express transmembrane fasciclin II during the period of axonal ingrowth and glomerulus development. Fascicles of TM-fasciclin II+ axons target certain glomeruli and avoid others, such as the sexually dimorphic glomeruli. These results suggest that TM-fasciclin II may play a role in the sorting and guidance of the axons. GPI-linked forms of fasciclin II are expressed weakly by glial cells associated with the receptor axons before they reach the sorting zone, but not by sorting-zone glia. GPI-fasciclin II may, therefore, be involved in axon-glia interactions related to stabilization of axons in the nerve, but probably not related to sorting.

Antibody to the HNK-1 glycoepitope affects fasciculation and axonal pathfinding in the developing posterior lateral line nerve of embryonic zebrafish

The HNK-1 glycoepitope, carried by many cell recognition molecules, is present in the developing posterior lateral line nerve and on other primary axons of zebrafish. To elucidate the function of HNK-1 in vivo, the antibody 412 to HNK-1 was injected into zebrafish embryos at 16 h post fertilization (hpf). The injected antibody bound specifically to axons carrying HNK-1. This treatment selectively affected the growth of either one or both posterior lateral line nerves in 39% of the experimental cases (13 of 33 animals), which was significantly more (P<0.0002) than in uninjected, vehicle injected, and non-immune IgG injected controls (1.2% of the animals; one of 85 animals), as assessed at 27 or 33 hpf. Other HNK-1 immunoreactive nerves, such as the ventral motor nerves were unaffected, indicating that antibody binding per se did not interfere with axon growth. The primordium of the posterior lateral line was not affected in its caudal migration and in depositing differentiating neuromasts along the trunk, showing that injections did not retard development and that initial formation of lateral line organs is probably independent of contact with nerve fibers. We suggest that the HNK-1 glycoepitope is an important modulator of embryonic nerve growth.

Polysialylated neural cell adhesion molecule is involved in the neuroplasticity induced by axonal injury in the avian ciliary ganglion

We demonstrated previously in the quail ciliary ganglion, that the immunoreactivity for the neural cell adhesion molecule labeling the postsynaptic specializations of intraganglionic synapses decreases when synaptic remodeling is induced by crushing the postganglionic ciliary nerves. Here we show, in the same experimental conditions, that the immunolabeling for its polysialylated non-stabilizing isoform, which promotes cell plasticity, increases at these subcellular compartments. In control ganglia, poor immunolabeling for the polysialylated neural cell adhesion molecule was occasionally observed surrounding the soma of the ciliary neurons, in correspondence with the calyciform presynaptic ending and the perineuronal satellite cells sheath. At the electron microscope, several neuronal compartments, including some postsynaptic specializations, somatic spines and multivesicular bodies, were immunopositive. Three to six days after ciliary nerve crush, both the number of ciliary neurons labeled for the polysialylated neural cell adhesion molecule and the intensity of their immunolabeling increased markedly. Electron microscopy revealed that, in parallel to the injury-induced detachment of the preganglionic boutons, numerous postsynaptic specializations were found to be immunopositive. Twenty days later, when intraganglionic connections were re-established, polysialylated neural cell adhesion molecule immunoreactivity was comparable to that observed in control ganglia. The increase in immunolabeling also involved the other neuronal compartments mentioned above, the perineuronal satellite cells and the intercellular space between these and the ciliary neurons. From these results we suggest that the switch, at the postsynaptic specializations, between the neural cell adhesion molecule and its polysialylated form may be among the molecular changes occurring in axotomized neurons leading to injury-induced synaptic remodeling. Moreover, from the increase in polysialylated neural cell adhesion molecule immunolabeling, observed at the somatic spines and at the interface between neurons and perineuronal satellite cells, we suggest that this molecule may be involved not only in synaptic remodeling, but also in other more general aspects of injury-induced neuronal plasticity.

Development of the limb neuromuscular system

Appendages, such as wings of a fly or limbs of a vertebrate, are excellent models to study the principles of patterning and morphogenesis. In the adult these structures are used for a variety of behaviors, including locomotion. Although support structures of the adult vertebrate limb are generated within the limb bud, its dynamic elements are derived from the somitic mesoderm and neural tube. Recent studies show that regional patterns set up in the mesenchyme-filled limb bud guide muscle precursors and developing motor axons to their proper location within the limb. Subsequent development of the neuromuscular system is regulated by cell surface interactions between pre-specified muscle fibers and motor axons.

Promotion of motoneuron survival and branching in rapsyn-deficient mice

Inhibition of programmed cell death of motoneurons during embryonic development requires the presence of their target muscle and coincides with the initial stages of synaptogenesis. To evaluate the role of synapse formation on motoneuron survival during embryonic development, we counted the number of motoneurons in rapsyn-deficient mice. Rapsyn is a 43 kDa protein needed for the formation of postsynaptic specialisations at vertebrate neuromuscular synapses. Here we show that the rapsyn-deficient mice have a significant increase in the number of motoneurons in the brachial lateral motor column during the period of naturally occurring programmed cell death compared to their wild-type littermates. In addition, we observed an increase in intramuscular axonal branching in the rapsyn-deficient diaphragms compared to their wild-type littermates at embryonic day 18.5. These results suggest that deficits in the formation of the postsynaptic specialisation at the neuromuscular synapse, brought about by the absence of rapsyn, are sufficient to induce increases in both axonal branching and the survival of the innervating motoneuron. Moreover, these results support the idea that skeletal muscle activity through effective synaptic transmission and intramuscular axonal branching are major mechanisms that regulate motoneuron survival during development.

The pattern of avian intramuscular nerve branching is determined by the innervating motoneuron and its level of polysialic acid

Most skeletal muscles are composed of a heterogeneous population of fast and slow muscle fibers that are selectively innervated during development by fast and slow motoneurons, respectively. It is well recognized that, in both birds and mammals, fast and slow motoneurons have substantially different intramuscular branching patterns, a difference critical for proper motor function. However, the cellular mechanisms regulating these differences in motoneuron branching are unknown. In a previous study, we showed that the fast and slow pattern of intramuscular branching, in a chick muscle containing distinct fast and slow muscle regions, was remarkably similar to normal when formed by foreign motoneurons. Whether this was attributable to some property of the innervating "fast" or "slow" motoneurons or to some property of the developing fast-slow muscle fibers was not determined. To distinguish between these two possibilities, we performed chick-quail hindlimb chimeras to force slow chick plantaris motoneurons to innervate a fast quail plantaris muscle. The pattern of intramuscular nerve branching in the fast plantaris of these chimeras closely resembled the slow branching pattern normally observed in chick slow plantaris muscles. Enzymatic removal of polysialic acid (PSA) from nerve and muscle during normal quail plantaris development dramatically changed the normal fast pattern to more closely resemble a slow pattern. In contrast, removal of PSA from chick plantaris motoneurons and muscle fibers had little effect on the pattern of nerve branching. Together, these results indicate that the pattern of intramuscular nerve branching is determined by the level of PSA on the innervating motoneurons.