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M W Cohen

Publications and source records attributed to M W Cohen.

At least 19 recordsLinked to original sources
PubMed2001

A technique of occipitocervical arthrodesis in children using autologous rib grafts.

STUDY DESIGN: Description of an operative technique with an illustrative case report. OBJECTIVES: The technique is presented to provide an alternative to iliac crest graft procedures for achieving occipitocervical fusion in children. This technique is particularly useful in children with instability after extensive decompression or laminectomy and in children with a large protuberant occiput. SUMMARY OF BACKGROUND DATA: The majority of techniques previously described for occipitocervical fusion in children rely on corticocancellous iliac crest autograft. Results generally have been promising; however, it can be difficult to harvest enough graft to span large defects after extensive decompression or to contour an iliac crest graft to a protuberant occiput. Structural rib autograft is superior in terms of availability and its unique and modifiable contour. Theoretical benefits of rib graft include superior strength and lower donor site morbidity. METHODS: The surgical technique is described. A case of a 2-year-old boy with Down's syndrome and myelopathy secondary to cervical instability is reviewed. RESULTS: The patient underwent occipitocervical arthrodesis using the technique described. The child made a full neurologic recovery, and at the 2-years follow-up evaluation, the graft had incorporated and the spine was stable. CONCLUSION: A technique of occipitocervical arthrodesis in children is described using autologous rib graft. This procedure was designed to span large defects or to deal with a large protuberant occiput; however, it is also useful for less demanding cases and may offer several advantages compared with procedures relying on iliac crest graft.

Arthrodesis↗
PubMed2000

Active zones on motor nerve terminals contain alpha 3beta 1 integrin.

Active zones are the sites along nerve terminals where synaptic vesicles dock and undergo calcium-dependent exocytosis during synaptic transmission. Here we show, by immunofluorescent staining with antibodies generated against Xenopus laevis integrins, that alpha3beta1 integrin is concentrated at the active zones of Xenopus motor nerve terminals. Because integrins can link extracellular matrix molecules to cytoskeletal elements and participate in the formation of signaling complexes, the localization of integrin at active zones suggests that it may play a role in the adhesion of the nerve terminals to the synaptic basal lamina, in the formation and maintenance of active zones, and in some of the events associated with calcium-dependent exocytosis of neurotransmitter. Our findings also indicate that the integrin composition of the terminal Schwann cells differs from that of the motor nerve terminals, and this may account at least in part for differences in their adhesiveness to the synaptic basal lamina.

Animals↗
PubMed1997

Laminin-induced clustering of dystroglycan on embryonic muscle cells: comparison with agrin-induced clustering.

The effect of laminin on the distribution of dystroglycan (DG) and other surface proteins was examined by fluorescent staining in cultures of muscle cells derived from Xenopus embryos. Western blotting confirmed that previously characterized antibodies are reactive in Xenopus. In control cultures, alphaDG, betaDG, and laminin binding sites were distributed as microclusters (<1 microm2 in area) over the entire dorsal surface of the muscle cells. Treatment with laminin induced the formation of macroclusters (1-20 microm2), accompanied by a corresponding decline in the density of the microclusters. With 6 nM laminin, clustering was apparent within 150 min and near maximal within 1 d. Laminin was effective at 30 pM, the lowest concentration tested. The laminin fragment E3, which competes with laminin for binding to alphaDG, inhibited laminin-induced clustering but did not itself cluster DG, thereby indicating that other portions of the laminin molecule in addition to its alphaDG binding domain are required for its clustering activity. Laminin-induced clusters also contained dystrophin, but unlike agrin-induced clusters, they did not contain acetylcholine receptors, utrophin, or phosphotyrosine, and their formation was not inhibited by a tyrosine kinase inhibitor. The results reinforce the notion that unclustered DG is mobile on the surface of embryonic muscle cells and suggest that this mobile DG can be trapped by at least two different sets of molecular interactions. Laminin self binding may be the basis for the laminin-induced clustering.

Agrin↗
PubMed1996

The role of an agrin-growth factor interaction in ACh receptor clustering.

The clustering of acetylcholine receptors (AChRs) at the neuromuscular junction is mediated in part by the heparan-sulfate proteoglycan agrin. However, our previous studies have also suggested the role of heparin-binding growth-associated molecular (HB-GAM) in AChR clustering. Here the role of an agrin-HB-GAM interaction in this process was examined using cultured Xenopus muscle cells. Agrin-coated beads further treated with HB-GAM were highly effective in AChR cluster induction. Protein overlay assays showed specific binding of HB-GAM to agrin. In addition, agrin-enriched neuritic tracks bound HB-GAM in a manner that showed a high degree of colocalization between the neural agrin and the applied factor. Finally, the introduction of exogenous HB-GAM together with soluble agrin resulted in the appearance of AChR clusters on the dorsal surface of cells in an agrin isoform-dependent manner; a dramatic change from the characteristic ventral AChR clustering seen in response to agrin alone. These results suggest that agrin may mediate AChR clustering by interacting with muscle-bound heparin-binding growth factors such as HB-GAM.

Agrin↗
PubMed1996

Supplementation with selenium augments the functions of natural killer and lymphokine-activated killer cells.

This study examined the effects of dietary (2.0 ppm for 8 wk) and in vitro (1 x 10(-7)M) supplementation with selenium (Se, as sodium selenite) on the activity of spleen natural killer (NK) cells and plastic-adherent lymphokine-activated killer (A-LAK) cells from C57B1/6J male mice. Dietary supplementation with Se resulted in a significant increase in the lytic activity of activated NK cells, and cells from these highly lytic effector cell populations expressed significantly higher numbers of intermediate affinity interleukin-2 receptors (II-2R)/cell. In the presence of high concentrations of II-2 and 1 x 10(-7)M Se, resting populations of spleen NK cells developed into A-LAK cells that had a significantly enhanced ability to proliferate, as indicated by the significantly higher amounts of nuclear 3H-thymidine incorporation, and a significantly augmented cytolytic activity against both NK-sensitive and NK-resistant target cells. Se appears to enhance the lytic activity of activated NK cells and to augment the proliferation, expansion, and lytic activity of A-LAK cells in the presence of high concentrations of II-2 through its ability to enhance the expression of intermediate affinity II-2R on these cells.

Adjuvants, Immunologic↗
PubMed1995

Former neuritic pathways containing endogenous neural agrin have high synaptogenic activity.

When Xenopus spinal cord (SC) neurons are grown on an appropriate substrate of basal lamina molecules, the agrin they externalize along their neuritic outgrowth remains bound to the substrate even after the neurons are removed. Here we demonstrate that these former neuritic pathways containing substrate-bound, neural agrin cause an accumulation of acetylcholine receptors (AChR) and cholinesterase (ChE) at sites of contact with muscle cells and inhibit AChR aggregation over the rest of the muscle cell surface. These local and global synaptogenic effects were not triggered by former neuritic pathways that were agrin-negative. The length of AChR accumulation along the agrin pathways contacted by individual muscle cells corresponded to a saturation process, in agreement with the notion that muscle cells have a limited capacity to cluster AChR. The AChR accumulation caused by the agrin pathways was almost twice as extensive as that induced by living neurites. It is concluded that agrin and possibly other synaptogenic molecules externalized by competent SC neurons bind to the culture substrate in quantities which are more than sufficient to account fully for the local and global changes in AChR and ChE distribution associated with embryonic nerve-muscle synaptogenesis.

Agrin↗
PubMed1995

Distribution of alpha-dystroglycan during embryonic nerve-muscle synaptogenesis.

The distribution of alpha-dystroglycan (alpha DG) relative to acetylcholine receptors (AChRs) and neural agrin was examined by immunofluorescent staining with mAb IIH6 in cultures of nerve and muscle cells derived from Xenopus embryos. In Western blots probed with mAb IIH6, alpha DG was evident in membrane extracts of Xenopus muscle but not brain. alpha DG immunofluorescence was present at virtually all synaptic clusters of AChRs and neural agrin. Even microclusters of AChRs and agrin at synapses no older than 1-2 h (the earliest examined) had alpha DG associated with them. alpha DG was also colocalized at the submicrometer level with AChRs at nonsynaptic clusters that have little or no agrin. The number of large (> 4 microns) nonsynaptic clusters of alpha DG, like the number of large nonsynaptic clusters of AChRs, was much lower on innervated than on noninnervated cells. When mAb IIH6 was included in the culture medium, the large nonsynaptic clusters appeared fragmented and less compact, but the accumulation of agrin and AChRs along nerve-muscle contacts was not prevented. It is concluded that during nerve-muscle synaptogenesis, alpha DG undergoes the same nerve-induced changes in distribution as AChRs. We propose a diffusion trap model in which the alpha DG-transmembrane complex participates in the anchoring and recruitment of AChRs and alpha DG during the formation of synaptic as well as nonsynaptic AChR clusters.

Agrin↗
PubMed1995

Supplementation with selenium restores age-related decline in immune cell function.

This study examined the effect of dietary (2.00 ppm for 8 weeks) supplementation with selenium (as sodium selenite) on the ability of lymphocytes from aged (24-month-old), male, C57BL/6JNIA mice to respond to: (i) stimulation with mitogen (phytohemagglutinin) or alloantigen; (ii) develop into cytotoxic effector cells; and (iii) destroy tumor cells. Supplementation with selenium resulted in a significant increase in the ability of spleen lymphocytes from aged animals to undergo blastogenesis, as indicated by significantly higher amounts of nuclear incorporation of 3H-thymidine after stimulation with mitogen. The dietary regimen restored the age-related deficiency of the cells to respond to stimulation by nuclear DNA synthesis and cell proliferation, at least, to the level of cells from unsupplemented young adult animals. Furthermore, populations of in vivo, alloantigen-activated lymphocytes from Se-supplemented aged animals contained significantly higher numbers of cytotoxic lymphocytes than those from Se-normal aged animals, which resulted in an enhanced capacity to destroy tumor cells. The significant increase in the number of cytotoxic effector cells within these activated T-lymphocyte populations was probably the result of an enhanced clonal proliferation of cytotoxic precursors cells, followed by the differentiation of greater numbers of cytotoxic effector cells. This effect occurred in the absence of changes in the ability of the cells to produce IL-2, which confirmed our earlier observation that dietary supplementation with selenium does not affect the production of IL-2. The data suggested that selenium restores the age-related defect in cell proliferation through an increase in the number of high-affinity IL-2 receptors.

Aging↗
PubMed1994

Neuritic deposition of agrin on culture substrate: implications for nerve-muscle synaptogenesis.

Recent experiments have indicated that neural agrin is deposited at newly forming nerve-muscle synapses and has a primary synaptogenic role there. As a step toward assessing how the spatial arrangement of new synaptic sites is regulated, we compared the pattern of agrin deposition by Xenopus neurites on culture substrate and on muscle cells. The neurons were grown on a substrate that bound their externalized agrin so tightly that it remained bound even when the neurites retracted spontaneously or were eliminated experimentally. By contrast, the neural cell adhesion molecule, NCAM, was not left behind on the substrate when the neurites were eliminated. Agrin, visualized by immunofluorescent staining, was deposited on the culture substrate in a continuous fashion along virtually the entire neuritic arbor of many spinal cord (SC) neurites. The pattern of agrin deposition by the same neurites changed from continuous to discontinuous when the neurites contacted muscle cells, and it became continuous again when the neurites returned to the culture substrate. The sites of agrin deposition on muscle cells were also sites of accumulation of ACh receptors (AChRs). Dorsal root ganglion (DRG) neurons and some SC neurons did not deposit agrin along their neuritic outgrowth, either on the culture substrate or on the muscle cells, and did not induce AChR accumulation at sites of contact with muscle cells. Besides adding to the evidence in support of agrin's synaptogenic role, the findings indicate that muscle cells significantly influence how neural agrin and synaptic sites become distributed along paths of neurite-muscle contact.

Agrin↗
PubMed1993

Selenium supplementation enhances the expression of interleukin 2 receptor subunits and internalization of interleukin 2.

Selenium (Se) is an essential nutritional factor that was shown previously by us to alter the kinetics of expression of high affinity (p55/p75) interleukin 2 receptors (IL-2R). This study shows that dietary (2 ppm for 8 weeks) or in vitro (1 x 10(-7) M) supplementation with Se (as sodium selenite) results in a significant upregulation of the expression of both the p55 and p70/75 IL-2 binding sites on the surface of concanavalin A-stimulated lymphocytes from C57BL/6J mice. This resulted in the formation of significantly higher numbers of high affinity IL-2R/cell with preservation of the normal ratio of high affinity to total IL-2 binding sites/cell. The high affinity IL-2R on cells from Se-supplemented animals functioned normally in terms of ligand binding and kinetics of IL-2 internalization, but their greater numbers/cell resulted in the internalization of significantly larger amounts of IL-2/cell. As Se supplementation results in an earlier expression of greater numbers of high affinity IL-2R, the presence of Se in the cell environment can result in an accelerated clonal expansion of activated lymphocytes.

Animals↗
PubMed1992

Effect of selenium on the expression of high affinity interleukin 2 receptors.

Selenium (Se) is an essential nutritional factor that has been shown to affect the development and expression of cell-mediated immune responses. This study shows that dietary (2 ppm for 8 weeks) or in vitro (1 x 10(-7) M) supplementation with Se results in a significant increase in the number of high affinity interleukin (IL) 2-binding sites (Kd of 10(-11) M) on the surface of concanavalin A-stimulated lymphocytes from C57BL/6J mice, whereas Se deficiency (0.02 ppm for 8 weeks) has the opposite effect. Se supplementation or deficiency apparently alters the kinetics of IL-2 receptor expression. Supplementation with Se in vivo or in vitro resulted in an earlier expression of high affinity IL-2 receptors, whereas Se deficiency resulted in a delayed expression of lower numbers of receptors. To exert its effect on IL-2 receptor expression, Se must be present or absent in the cell environment 8-24 hr after stimulation, and it most likely affects processes in the cytoplasmic and/or nuclear compartments of activated lymphocytes. Thus, in the presence of continuous immunologic stimulation, the presence or absence of Se in the cell environment can result in an accelerated or delayed clonal expansion of immunocompetent lymphocytes, respectively.

Animals↗
PubMed1992

Early appearance of and neuronal contribution to agrin-like molecules at embryonic frog nerve-muscle synapses formed in culture.

Antibodies against chicken and Torpedo agrin were used for immunofluorescent staining in order to assess the spatial distribution and temporal appearance of agrin-like molecules at newly formed synaptic contacts in cultures of embryonic Xenopus nerve and muscle cells. The antibodies stained Xenopus neuromuscular junctions and removed ACh receptor (AChR)-aggregating activity from extracts of Xenopus brain. Immunofluorescence was observed at almost all nerve-induced AChR aggregates, even at microaggregates in cocultures as young as 7.5 hr and at nerve-muscle contacts less than 2 hr old. Microdeposits of immunofluorescence extended as far distally as, or farther than, the microaggregates of AChRs along young nerve-muscle contacts. They also occurred along portions of growing neurites that were not in contact with muscle. By contrast, immunofluorescence was rarely observed at the nonsynaptic aggregates of AChRs that form on noninnervated muscle cells. These results raise the possibility that neuronally derived microaggregates of agrin-like molecules may be primary sites of nerve-induced clustering of AChRs, and they indicate that these molecules are present at embryonic nerve-muscle synapses from the very onset of AChR aggregation. The cellular origin of the agrin-like molecules at synapses was examined in cross-species cocultures in which the neurons and muscle cells were obtained from embryos of Xenopus laevis and Rana pipiens. Immunofluorescent staining with anti-agrin antibodies reactive at both Rana and Xenopus neuromuscular junctions revealed immunofluorescence at AChR aggregates along nerve-muscle contacts involving both cross-species combinations. Immunofluorescent staining with an anti-agrin antibody reactive at Rana but not at Xenopus neuromuscular junctions was positive only at cross-species nerve-muscle contacts involving Rana neurons. These results provide the first demonstration that embryonic neurons supply agrin-like molecules to the synapses they form with embryonic muscle cells.

Agrin↗
PubMed1991

Distribution of Ca2+ channels on frog motor nerve terminals revealed by fluorescent omega-conotoxin.

Tetramethylrhodamine-conjugated omega-conotoxin was used as a fluorescent stain (Jones et al., 1989) to determine the spatial distribution of voltage-gated Ca2+ channels along frog motor nerve terminals. Like native omega-conotoxin, the fluorescent toxin blocked neuromuscular transmission irreversibly. The fluorescent staining was confined to the neuromuscular junction and consisted of a series of narrow bands (in face views) or dots (in side views) approximately 1 micron apart. This characteristic staining pattern was prevented by pretreatment with omega-conotoxin and by prior denervation for 5-7 d. Combined fluorescence and phase-contrast optics indicated that the stain was on the synaptic rather than the nonsynaptic side of the nerve terminal. The bands and dots of stain proved to be in spatial register with the postsynaptic junctional folds, as revealed by combined staining of ACh receptors. It is concluded that the voltage-gated Ca2+ channels on frog motor nerve terminals are concentrated at active zones. The findings are consistent with the suggestion (Heuser et al., 1974; Pumplin et al., 1981) that the large intramembraneous particles seen at freeze-fractured active zones are voltage-gated Ca2+ channels.

Animals↗
PubMed1990

Formation and survival of a postsynaptic specialization in cultures of embryonic Xenopus nerve and muscle cells.

The formation and survival of nerve-induced clusters of acetylcholine receptors (AChRs) was monitored over a synaptogenic period of several days in cultures of myotomal muscle cells and spinal cord neurons derived from embryos of Xenopus laevis. AChRs were labeled with fluorescent alpha-bungarotoxin so that neurite-associated receptor patches (NARPs) could be viewed at daily intervals throughout the neuritic arbor of selected neurons. To avoid bleaching the NARPs and damaging the neurons, the intensity of the fluorescence excitation was reduced to 3%. Images were digitized and NARPs were measured with a computer-based image analysis system. Virtually all newly formed NARPs (greater than 90%) were detected at the same time as neurite-muscle contact and in the same proximal-distal sequence as neuritic growth. Those which formed in 6- to 13-day-old cocultures had similar distributions with respect to length, area, intensity, and area X intensity to those which formed in 1- to 2-day-old cocultures. NARPs exhibited variable daily changes in these parameters but on average they grew and reached close to their ultimate values within 1-2 days. Almost all (greater than 95%) survived as long as their contacts. In cases where NARP formation occurred on the same muscle on 2 or more different days, the ones which formed first were the most extensive. Spontaneous neurite withdrawal occurred mainly from young NARPs and resulted in their rapid disappearance. It is suggested that during the period when neurons grow and make new contacts with muscle cells there is no substantial change in their capacity to trigger the formation of new synaptic sites and maintain preexisting ones, and that the first-forming synapses on a muscle cell tend to be the largest because muscle cells have a limited capacity to generate postsynaptic membrane. Additional implications of the findings for synapse formation and elimination are discussed.

Animals↗
PubMed1990

Developmental changes in the half-life of acetylcholine receptors in the myotomal muscle of Xenopus laevis.

1. Tail preparations, containing myotomal muscle and associated spinal cord, were isolated from embryos and tadpoles of Xenopus laevis between stages 25 and 49 (1.15-12 days) and were pulse-labelled with 125I-alpha-bungarotoxin (125I alpha BT) so that the half-life (T1/2) of their acetylcholine receptors (AChRs) could be estimated in organ culture. 2. For the entire population of AChRs, estimates of T1/2 based on a single exponential decline in radioactivity (but see item 4 below) increased from 53-55 h at stages 25-31 (1.15-1.56 days) to approximately 135 h at stage 47 (5.5 days). Beyond stage 47 T1/2 increased only slightly. 3. Radioautographic estimates of the T1/2 of extrajunctional AChRs at stages 47-48 (5.5-7.5 days) were 41-50 h. It follows that the developmental change in the T1/2 of the entire population of AChRs was due to the junctional AChRs. 4. At stages 47-49 (5.5-12 days) the decline in radioactivity for the entire population of AChRs was fitted well by a double exponential. Assuming a T1/2 of 50 h for the extrajunctional AChRs and 210 h for the junctional AChRs, the correlation coefficient (r) was 0.9947 +/- 0.0014 (mean +/- S.E.M.; n = 14) and junctional AChRs were estimated to comprise 80 +/- 3% of the entire population. Similar analysis, as well as experiments in which the degradation of junctional AChRs was assessed by pulse-labelling with fluorescent alpha-bungarotoxin, suggested that at earlier stages of development the junctional AChRs have a shorter T1/2 and comprise a smaller fraction of the entire population. 5. The developmental increase in T1/2 occurred even when animals were raised in the anaesthetic tricaine or in tetrodotoxin, conditions which abolished all motor activity. 6. Developmental increases in T1/2 also occurred in culture but were smaller than those in vivo. The increases in culture did not occur amongst those AChRs which were pre-labelled with 125I alpha BT. 7. It is concluded that in Xenopus myotomal muscle the T1/2 of junctional AChRs begins to increase within a day after the onset of innervation and that the increase does not require nerve or muscle impulse activity. We suggest, among other possibilities, that it may depend upon incorporation of a different molecular species of AChR into the postsynaptic membrane.

Aminobenzoates↗