The MuSK-BMP pathway regulates synaptic Nav1.4 localization and muscle excitability

The neuromuscular junction (NMJ) is the linchpin of nerve-evoked muscle contraction. Broadly considered, the function of the NMJ is to transduce a nerve action potential into a muscle fiber action potential (MFAP). Efficient information transfer requires both cholinergic signaling, responsible for the generation of endplate potentials (EPPs), and excitation, the activation of postsynaptic voltage-gated sodium channels (Nav1.4) to trigger MFAPs. In contrast to the cholinergic apparatus, the signaling pathways that organize Nav1.4 and muscle fiber excitability are poorly characterized. Muscle-specific kinase (MuSK), in addition to its Ig1 domain-dependent role as an agrin-LRP4 receptor, is also a BMP co-receptor that binds BMPs via its Ig3 domain and shapes BMP-induced signaling and transcriptional output. Here we probed the function of the MuSK-BMP pathway at the NMJ using mice lacking the MuSK Ig3 domain ('ΔIg3-MuSK'). Synapses formed normally in ΔIg3-MuSK animals, but the postsynaptic apparatus was fragmented from the first weeks of life. Anatomical denervation was not observed at any age examined. Moreover, spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents were comparable to WT. However, trains of nerve-evoked MFAPs in ΔIg3-MuSK muscle were abnormal as revealed by increased jitter and blocking in single fiber electromyography. Further, nerve-evoked compound muscle action potentials (CMAPs), as well as twitch and tetanic muscle torque force production, were also diminished. Finally, Nav1.4 levels were reduced at ΔIg3-MuSK synapses but not at the extrajunctional sarcolemma, indicating that the observed excitability defects are the result of impaired localization of this voltage-gated ion channel at the NMJ. We propose that MuSK plays two distinct roles at the NMJ: as an agrin-LRP4 receptor necessary for establishing and maintaining cholinergic signaling, and as a BMP co-receptor required for maintaining proper Nav1.4 density, nerve-evoked muscle excitability and force production. The MuSK-BMP pathway thus emerges as a target for modulating excitability and functional innervation, which are defective in conditions such as congenital myasthenic syndromes and aging.

Hyperexcitability precedes motoneuron loss in the Smn2B/- mouse model of spinal muscular atrophy

Spinal motoneuron dysfunction and loss are pathological hallmarks of the neuromuscular disease spinal muscular atrophy (SMA). Changes in motoneuron physiological function precede cell death, but how these alterations vary with disease severity and motoneuron maturational state is unknown. To address this question, we assessed the electrophysiology and morphology of spinal motoneurons of presymptomatic Smn^2B/- mice older than 1 wk of age and tracked the timing of motor unit loss in this model using motor unit number estimation (MUNE). In contrast to other commonly used SMA mouse models, Smn^2B/- mice exhibit more typical postnatal development until postnatal day (P)11 or 12 and have longer survival (~3 wk of age). We demonstrate that Smn^2B/- motoneuron hyperexcitability, marked by hyperpolarization of the threshold voltage for action potential firing, was present at P9-10 and preceded the loss of motor units. Using MUNE studies, we determined that motor unit loss in this mouse model occurred 2 wk after birth. Smn^2B/- motoneurons were also larger in size, which may reflect compensatory changes taking place during postnatal development. This work suggests that motoneuron hyperexcitability, marked by a reduced threshold for action potential firing, is a pathological change preceding motoneuron loss that is common to multiple models of severe SMA with different motoneuron maturational states. Our results indicate voltage-gated sodium channel activity may be altered in the disease process.NEW & NOTEWORTHY Changes in spinal motoneuron physiologic function precede cell death in spinal muscular atrophy (SMA), but how they vary with maturational state and disease severity remains unknown. This study characterized motoneuron and neuromuscular electrophysiology from the Smn^2B/- model of SMA. Motoneurons were hyperexcitable at postnatal day (P)9-10, and specific electrophysiological changes in Smn^2B/- motoneurons preceded functional motor unit loss at P14, as determined by motor unit number estimation studies.

AAV1.NT-3 gene therapy attenuates spontaneous autoimmune peripheral polyneuropathy

The spontaneous autoimmune peripheral polyneuropathy (SAPP) model in B7-2 knockout non-obese diabetic mice shares clinical and histological features with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP). Secondary axonal loss is prominent in the progressive phase of this neuropathy. Neurotrophin 3 (NT-3) is an important autocrine factor supporting Schwann cell survival and differentiation and stimulates neurite outgrowth and myelination. The anti-inflammatory and immunomodulatory effects of NT-3 raised considerations of potential efficacy in the SAPP model that could be applicable to CIDP. For this study, scAAV1.tMCK.NT-3 was delivered to the gastrocnemius muscle of 25-week-old SAPP mice. Measurable NT-3 levels were found in the serum at 7-week postgene delivery. The outcome measures included functional, electrophysiological and histological assessments. At week 32, NT-3-treated mice showed increased hind limb grip strength that correlated with improved compound muscle action potential amplitude. Myelinated fiber density was 1.9 times higher in the NT-3-treated group compared with controls and the number of demyelinated axons was significantly lower. The remyelinated nerve fiber population was significantly increased. These improved histopathological parameters from scAAV1.tMCK.NT-3 treatment occurred in the setting of reduced sciatic nerve inflammation. Collectively, these findings suggest a translational application to CIDP.

Comparative effectiveness of antinociceptive gene therapies in animal models of diabetic neuropathic pain

Peripheral neuropathic pain is one of the most common and debilitating complications of diabetes. Several genes have been shown to be effective in reducing neuropathic pain in animal models of diabetes after transfer to the dorsal root ganglion using replication-defective herpes simplex virus (HSV)1-based vectors, yet there has never been a comparative analysis of their efficacy. We compared four different HSV1-based vectors engineered to produce one of two opioid receptor agonists (enkephalin or endomorphin), or one of two isoforms of glutamic acid decarboxylase (GAD65 or GAD67), alone and in combination, in the streptozotocin-induced diabetic rat and mouse models. Our results indicate that a single subcutaneous hindpaw inoculation of vectors expressing GAD65 or GAD67 reduced diabetes-induced mechanical allodynia to a degree that was greater than daily injections of gabapentin in rats. Diabetic mice that developed thermal hyperalgesia also responded to GAD65 or endomorphin gene delivery. The results suggest that either GAD65 or GAD67 vectors are the most effective in the treatment of diabetic pain. The vector combinations, GAD67+endomorphin, GAD67+enkephalin or endomorphin+enkephalin also produced a significant antinociceptive effect but the combination did not appear to be superior to single gene treatment. These findings provide further justification for the clinical development of antinociceptive gene therapies for the treatment of diabetic peripheral neuropathies.

Mutant small heat shock protein B3 causes motor neuropathy: utility of a candidate gene approach

OBJECTIVE

Idiopathic peripheral neuropathy is common and likely due to genetic factors that are not detectable using standard linkage analysis. We initiated a candidate gene approach to study the genetic influence of the small heat shock protein (sHSP) gene family on an axonal motor and motor/sensory neuropathy patient population.

METHODS

The promoter region and all exonic and intronic sequences of the 10 sHSP genes (HSPB1-HSPB10) were screened in a cohort of presumed nonacquired, axonal motor and motor/sensory neuropathy patients seen at the Ohio State University Neuromuscular Clinic.

RESULTS

A missense mutation in the gene encoding small heat shock protein B3 (HSPB3, also called HSP27, protein 3) was discovered in 2 siblings with an asymmetric axonal motor neuropathy. Electrophysiologic studies revealed an axonal, predominantly motor, length-dependent neuropathy. The mutation, HSPB3(R7S), is located in the N-terminal domain and involves the loss of a conserved arginine.

CONCLUSIONS

The discovery of an HSPB3 mutation associated with an axonal motor neuropathy using a candidate gene approach supports the notion that the small heat shock protein gene family coordinately plays an important role in motor neuron viability.

The bowel management tube: an effective means for controlling fecal incontinence

Regular bowel washout enemas have been used as a method of management of fecal incontinence. The effective administration of a washout enema to a child with weak anal sphincters is often a problem. Using a new silastic balloon-tipped enema catheter (bowel management tube [BMT]) of our design, we prospectively studied its effectiveness in a group of children who suffered fecal incontinence. Thirty-one children were studied over a 1-year period. Their diagnoses included meningomyelocele (19), postoperative Hirschsprung's disease or imperforate anus (10), and other (2). Before and after starting the BMT enema system, clinical assessment and a diary, which graded the degree of fecal incontinence and satisfaction with the system, were completed. Five patients failed to benefit because of noncompliance (3) or balloon extrusion (2). Three more patients discontinued the use of the tube system. Twenty-three patients achieved successful results with this system as evidenced by a significant amelioration in their fecal incontinence and their unwillingness to give up the use of the BMT. We conclude that the use of a regular washout enemas with BMT can be an effective method for control of fecal incontinence in children.

The effect of early weight-bearing on the stability of femoral neck fractures treated with Knowles pins

I compared the effects of early weight-bearing on the stability of femoral neck fractures following Knowles-pin fixation with data on a similar group of patients who were studied ten years previously, in which weight-bearing had been delayed until radiographic evidence of fracture-healing was apparent. The results concerning stability were essentially comparable, indicating that early weight-bearing did not adversely affect the stability of a properly reduced and pinned displaced fracture of the femoral neck. Factors that did adversely affect stability following fixation with Knowles pins included the degree of initial displacement of the fracture (Garden Type IV), failure to adequately reduce or fix the fracture, and severe demineralizing bone disease as measured in an iliac-crest biopsy specimen obtained at the time of surgery.

Untreated congenital hip dysplasia in the Navajo

Congenital hip dysplasia without dislocation was identified in 18 Navajo Indian children. Treatment had been declined and the children presented a unique opportunity to observe the natural course of hip dysplasia. In a follow-up period of three to 19 years (average, 11.2 years), none of the dysplastic hips were observed to progress to dislocation. In 15 of the children with dysplastic hips, the condition became roentgenographically normal in the course of normal growth and development. The other three children continued to show roentgenographic signs of hip dysplasia.

Simultaneous fractures of both femoral necks: review of the literature and report of two cases

Simultaneous fracture of both femoral necks is extremely rare. Delay in diagnosis may be avoided by noting the association of these fractures with two conditions: violent skeletal trauma and major motor seizures. Treatment of two patients is reported in addition to a review of seven cases previously reported. The author's experience indicates that accurate reduction and percutaneous Knowles pinning provide adequate fixation and permit early weight-bearing.

Haemophilic arthropathy in the joints of the hands and feet

Radiographs of the hands and feet of 72 haemophilic patients were reviewed for peripheral joint involvement. Fifty patients or 69% had changes in the small joints of their hands and/or feet with a total of 160 abnormal joints. In the hands the metacarpo-phalangeal joints were predominantly involved (42 of 50 joints), and in the feet, the metatarso-phalangeal joints (68 of 110 abnormal joints), as well as the posterior subtalar joint (36 of 110 joints). Röntgen abnormalities were characterized by irregularity and/or flattening of the articular cortices. The involvement of the small peripheral joint in haemophilic patients has not been a primary consideration in previous clinical and radiographic studies because of the more common and more debilitating changes in the large joints. The recognition of involvement of the small joints is described to avoid misinterpretation of the röntgen findings and to appreciate the incidence of involvement, especially with the increased availability of replacement therapy.

Osteoid osteomas in siblings. Case reports

Osteoid osteomas occurred in siblings with a nearly simultaneous onset of symptoms, and with some unusual neoplastic characteristics. The combination of simultaneous occurrence and neoplastic change suggests that a viral or other infectious etiology is possible.

Fractures in thalassemia

Fractures occur frequently in patients with homozygous beta-thalassemia. A study was made of the fractures noted in a group of patients who were followed at the Thalassemia Clinic at The New York Hospital-Cornell Medical Center. Results indicate that these patients often sustain fractures which are multiple and which frequently heal with resultant deformities.