Ermilov, Leonid G.

Neurotrophin brain-derived neurotrophic factor (BDNF) enhances neuromuscular transmission via activation of tropomyosin-related kinase receptor B (TrkB). Rat phrenic nerve-diaphragm muscle preparations were used to determine the rapid effect of BDNF on synaptic quantal release during repetitive stimulation. BDNF treatment significantly enhanced quantal release at all fiber types. BDNF increased synaptic vesicle cycling (measured using FM4-64 fluorescence uptake); conversely, inhibiting BDNF/TrkB signaling decreased FM4-64 uptake.

Retrograde transport of fluorescent dyes permits imaging of nearly the entire pool of motoneurons when administered in the target muscle or axon shaft, but the extent of dendritic labeling is usually limited. We used an intracellular injection of Lucifer Yellow CH (LY) into retrogradely labeled phrenic motoneurons (PMNs) to enhance dendritic structure. PMNs were specifically labeled by Alexa Fluor 488‐conjugated cholera toxin fragment B (CTB) injected into the pleural space of 18‐days old rats. After a 72‐hr survival period, animals were transcardially perfused with 4% paraformaldehyde and 4% sucrose in 0.1 M phosphate buffer. Cervical spinal cords were sectioned in coronal plane. Individual CTB‐labeled PMNs were intracellularly injected with LY under direct visual control and subsequently imaged with an Olympus FluoView 200 laser‐scanning confocal system. Image processing and analyses were performed using Metamorph 7.4 software. PMNs were extensively labeled and reconstructed in 3D. The range of visible primary, secondary and tertiary processes was 3 ‐ 7, 2 ‐ 4 and 0 ‐ 2 per PMN, respectively. Average length of a single dendritic tree was 218.0±20.1 μm and surface area was 1295±199 μm 2 . The described technique provides 3D quantitative morphometry of a large sample of retrogradely‐labeled PMNs in great detail. Supported by NIH grants HL37680 and AR51173.

We studied the effects of TNF on cell‐to‐cell communication in HeLa cells expressing vascular connexin 37, 40 or 43. HeLa cells were treated with TNF (20 ng/ml) for up to 2 h. In dye‐transfer experiments, carboxyfluroscein (HeLaCx40 and HeLaCx43) or Alexa Fluor‐480 (HeLaCx37) was injected into one cell for 10 s and cell transfer allowed to proceed for 10 m and the number of labeled cells counted. Cell lysates were prepared in Triton X‐100 lysate buffer and detergent‐soluble fractions collected. Cx37, 40 and 43 were detected by Western blot. After 1 hour, TNF treatment resulted in near total loss of dye‐coupling in HeLaCx37 and HeLaCx43 (p<0.02, n=14‐16) and remained constant up to 2 hours. Dye coupling in HeLaCx40 cells remained unchanged after 1 hour and decreased after 2 hours (p<0.05, n=10). Western blots indicated that TNF treatment did not affect detergent solubility of Cx40 and 43. However, TNF caused a significant increase in detergent solubility of Cx37. These results suggest that inflammatory mediators affect connexins differently, and that the loss of Cx37 function may be due to the loss of detergent resistance.

The age‐related mechanisms of diaphragm muscle (DIAm) sarcopenia are largely unknown. We hypothesize that sarcopenia may be related to neurotrophic signaling, specifically that of brain derived neurotrophic factor (BDNF) acting through the high‐affinity tropomyosin‐related kinase receptor subtype B (TrkB). In total 47, adult male TrkB F616A mice were examined at 6, 18, and 24 months of age (mo), representing survival rates of 100, 90, and 75%. At each age, ex vivo contractility of the DIAm was assessed to determine maximal force as well DIAm‐phrenic nerve neuromuscular transmission failure under the following preincubation conditions; control (vehicle) solution, BDNF, 1NMPP1 (inhibiting TrkB kinase activity), or co‐treatment of BDNF and 1NMPP1. Specific DIAm maximal twitch and isometric tetanic force, in addition to fatigue index were all negatively impacted at 24 mo (P蠄0.02). A significant interaction between age and neurotrophic treatment was found for DIAm‐phrenic nerve neuromuscular transmission failure (P蠄0.01). Specifically, at 6 and 18 mo BDNF decreased failure, while this was lost at 24 mo. Conversely, TrkB kinase inhibition increased failure at 6 mo, but this relationship was lost at both 18 and 24 mo. Taken together, neurotrophic signaling is key to the physiological function of the DIAm, however there is likely an age‐related loss of BDNF and TrkB signaling that detrimentally affects the DIAm. Grant Funding Source : Supported by R01‐AG044615, T32‐HL105355, and the Mayo Clinic.

Upper cervical spinal cord injury (SCI) disrupts descending inspiratory‐related drive to phrenic motoneurons paralyzing the diaphragm muscle. Over time following unilateral C2 hemisection (SH), there is gradual recovery of rhythmic diaphragm activity ipsilaterally, consistent with neuroplasticity. We recently reported that functional recovery after SH is enhanced by chronic intrathecal delivery of BDNF to the region of the phrenic motor neuron pool. BDNF also exerts acute effects on synaptic transmission that may enhance functional recovery. Accordingly, we hypothesized that intrathecal delivery of BDNF to the phrenic motor neuron pool acutely enhances functional recovery post‐SH. Rats were implanted with diaphragm electrodes for chronic EMG recordings post‐SH, verifying absence of ipsilateral eupneic diaphragm EMG activity by 3 and 28 days after SH. In spontaneously breathing, anesthetized animals, BDNF was administered intrathecally (30 μl) at C4 and ipsilateral rhythmic diaphragm EMG activity was restored (~120% of pre‐injury diaphragm root‐mean‐square [RMS] EMG amplitude) within ~30 min of administration. Diaphragm RMS EMG increased further following exposure to hypoxia (10% O 2 )‐hypercapnia (5% CO 2 ) and intranasal capsaicin. These results highlight the acute role of BDNF signaling in the phrenic motor neuron pool in chronically‐injured rats displaying no spontaneous recovery. Grant Funding Source : Supported by NIH grant R01 HL096750.

Regulation of gap junctional intercellular communication plays a very important role in many physiological and pathophysiological processes. Our hypothesis is that tumor necrosis factor (TNF) will decrease gap junction dependent cell‐to‐cell communication of vascular connexin by disturbing connexin‐cytoskeleton interactions. Transformed HeLa cells expressing vascular connexin 37 were used in these experiments. After 1 hour, TNF (20 ng/ml) treatment resulted in near total loss of dye‐coupling (Alexa Fluor‐480) in HeLaCx37 (p<0.02, n=16) and remained constant up to 2 hours. siRNA‐mediated knockdown of ZO‐1 restored dye coupling. TNF caused a significant increase in detergent solubility of Cx37. ZO‐1 was co‐IP with Cx37 only after TNF treatment, suggesting that TNF induces a ZO‐1 and Cx37 interaction. Actin was co‐IP with Cx37 but TNF did not affect this association. N‐cadherin was not co‐IP with Cx37. Immunofluorescence double labeling for Cx37/ZO‐1 and Cx37/actin confirm the co‐IP experiments. TNF reduces gap junction coupling of Cx37 when expressed alone in epithelial cells. TNF mediates Cx37 interaction with ZO‐1 but not actin. N‐cadherin does not interact with Cx37. TNF may affect Cx37/ZO‐1 interaction resulting in reduced dye coupling.

Myasthenia gravis is a disease of the neuromuscular junction (NMJ) associated with autoimmune destruction of the motor end‐plate. Inhibitors of acetylcholine (ACh) esterase are commonly used to mitigate symptoms by increasing ACh availability at the NMJ. Brain‐derived neurotrophic factor (BDNF) improves neuromuscular transmission in the rat diaphragm muscle (DIAm) by maintaining ACh release with repeated activation. We hypothesized that BDNF will mitigate the deficit of neuromuscular transmission in a passive transfer model of myasthenia gravis. Female Lewis rats were injected with vehicle, McAb1 (that does not induce myasthenia), and a myasthenogenic McAb3 antibody. Neuromuscular transmission failure (NMTF) was determined in DIAm‐phrenic nerve preparations by comparing the isometric force evoked by phrenic nerve stimulation and that evoked by superimposed direct muscle stimulation. After 2 min of repetitive stimulation, neuromuscular transmission was significantly impaired in myasthenic animals (NMTF: 80%) compared to vehicle and McAb1 animals (NMTF: ~60%). Exogenous BDNF treatment significantly improved neuromuscular transmission in all groups and NMTF in McAb3‐treated rats was no longer different from control or McAb1‐treated animals. We conclude that BDNF improves neuromuscular transmission in adult myasthenic rats. Supported by a Career Development Award from Mayo Clinic.

Neuregulin (NRG) is important for expression of acetylcholine (ACh) receptors during development of the neuromuscular junction (NMJ). NRG treatment significantly increases ACh‐induced currents in hippocampal neurons, but it is not known if NRG exerts a similar effect at NMJs. Diaphragm muscle (DIAm)‐phrenic nerve preparations from 7‐ and 14‐day old rats were used to determine the effects of NRG (heregulin β177–244) on neuromuscular transmission and DIAm contractile properties. Neuromuscular transmission failure (NMTF) was assessed by the difference in force elicited by repetitive phrenic nerve stimulation (0.2 ms pulses in 40 Hz, 330 ms trains) and superimposed, intermittent direct DIAm stimulation (1 ms pulses in 40 Hz, 330 ms trains applied every 15 seconds). NMTF was higher in 7‐day old compared to 14‐day old rats (76% vs. 65%; p<0.05). Exogenous NRG treatment significantly improved neuromuscular transmission in 7‐day old rats (NMTF: 52%; p<0.05), but not in 14‐day old rats (65%). Isometric DIAm contractile properties were not affected by NRG treatment in either age group. These results indicate that NRG has important synaptic effects during early postnatal development of the DIAm. Whether NRG effects on neuromuscular transmission result from acute changes in AChR expression or alternative mechanisms remains to be explored. Supported by NIH grant AR51173 and a Career Development Award from Mayo Clinic.

AbstractBackgroundNormal gastrointestinal motility depends on electrical slow‐wave activity generated by interstitial cells of Cajal (ICC) in the tunica muscularis of the gastrointestinal tract. A requirement for HCO3− in extracellular solutions used to record slow waves indicates a role for HCO3− transport in ICC pacemaking. The Slc4a4 gene transcript encoding the electrogenic Na+/HCO3− cotransporter, NBCe1, is enriched in mouse small intestinal myenteric region ICC (ICC‐MY) that generate slow waves. This study aimed to determine how extracellular HCO3− concentrations affect electrical activity in mouse small intestine and to determine the contribution of NBCe1 activity to these effects.MethodsImmunohistochemistry and sharp electrode electrical recordings were used.Key ResultsThe NBCe1 immunoreactivity was localized to ICC‐MY of the tunica muscularis. In sharp electrode electrical recordings, removal of HCO3‐ from extracellular solutions caused significant, reversible, depolarization of the smooth muscle and a reduction in slow‐wave amplitude and frequency. In 100 mM HCO3−, the muscle hyperpolarized and slow wave amplitude and frequency increased. The effects of replacing extracellular Na+ with Li+, an ion that does not support NBCe1 activity, were similar to, but larger than, the effects of removing HCO3‐. There were no additional changes to electrical activity when HCO3‐ was removed from Li+ containing solutions. The Na+/HCO3− cotransport inhibitor, S‐0859 (30µM) significantly reduced the effect of removing HCO3− on electrical activity.Conclusions & InferencesThese studies demonstrate a major role for Na+/HCO3− cotransport by NBCe1 in electrical activity of mouse small intestine and indicated that regulation of intracellular acid:base homeostasis contributes to generation of normal pacemaker activity in the gastrointestinal tract.