Selective Vulnerability of Spinal and Cortical Motor Neuron Subpopulations in delta7 SMA Mice

Loss of the survival motor neuron gene (SMN1) is responsible for spinal muscular atrophy (SMA), the most common inherited cause of infant mortality. Even though the SMA phenotype is traditionally considered as related to spinal motor neuron loss, it remains debated whether the specific targeting of motor neurons could represent the best therapeutic option for the disease. We here investigated, using stereological quantification methods, the spinal cord and cerebral motor cortex of ∆7 SMA mice during development, to verify extent and selectivity of motor neuron loss. We found progressive post-natal loss of spinal motor neurons, already at pre-symptomatic stages, and a higher vulnerability of motor neurons innervating proximal and axial muscles. Larger motor neurons decreased in the course of disease, either for selective loss or specific developmental impairment. We also found a selective reduction of layer V pyramidal neurons associated with layer V gliosis in the cerebral motor cortex. Our data indicate that in the ∆7 SMA model SMN loss is critical for the spinal cord, particularly for specific motor neuron pools. Neuronal loss, however, is not selective for lower motor neurons. These data further suggest that SMA pathogenesis is likely more complex than previously anticipated. The better knowledge of SMA models might be instrumental in shaping better therapeutic options for affected patients.     

Structure and variability of mammalian peroxisomal membrane proteins.

Peroxisomal membrane proteins (PMPs) from the Swiss-Webster mouse are analyzed and compared to those of rats and humans using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblotting. A purification procedure for fresh mouse, rat, or human biopsy liver which enriches peroxisomal/mitochondrial marker enzyme ratios over 100-fold is characterized. When analyzed by SDS-PAGE, membranes of purified liver peroxisomes are shown to contain the same complement of 145-, 70-, 55-, 36-, and 22-kDa PMPs in rats, mice, and humans. A rabbit polyclonal antibody raised against mouse peroxisomal membranes demonstrates immunoreactivity to 145- and 70-kDa proteins in fresh liver homogenates from all three species and in control or Zellweger syndrome fibroblasts from humans. Human autopsy or placental tissues which were refrigerated before analysis exhibited 105-, 55-, and 36-kDa peptides which may be derived from the 145- and 70-kDa peptides. Such conversions, if related to degradation, may explain difficulties in purifying peroxisomes from human autopsy specimens. Variable amounts of the 55-kDa peptide also occurred in mouse adrenal and lung, and the conversion of higher to lower molecular weight PMPs could not be demonstrated by in vitro incubation of mouse liver. Further definition of the structure and variability of mammalian PMPs should be helpful in understanding polyenzymopathies such as Zellweger syndrome.     

Characterization of the Fc gamma receptor on human platelets

IgG-containing immune complexes may play a role in the immune destruction of human platelets by interacting with an Fc gamma receptor on the platelet surface. We studied the platelet Fc gamma receptor and characterized its interaction with IgG ligand and anti-Fc gamma receptor monoclonal antibodies. Oligomers of IgG, but not monomeric IgG, bound to platelets and the number of binding sites was significantly increased at low ionic strength. Ligand-binding studies indicated that normal human platelets express a single Fc gamma receptor (Fc gamma RII) with 8559 +/- 852 sites per cell, Kd = 12.5 +/- 1.7 X 10(-8) M using trimeric IgG. Results of studies with bivalent and Fab monoclonal anti-Fc gamma RII were consistent with each Fc gamma receptor expressing two epitopes recognized by the antibody. The number of Fc gamma binding sites and affinity of binding were unchanged by the presence of 2.0 mM Mg2+ or 10 micrograms/ml cytochalasin B. Platelet stimulation with thrombin or ADP in the presence of fibrinogen also did not alter the number of Fc gamma binding sites or the affinity of binding. However, platelets preincubated with 5 microM dexamethasone expressed a decreased number of Fc gamma binding sites as well as decreased IgG-dependent platelet aggregation. Platelets from patients with Glanzmann's thrombasthenia and from patients with the Bernard Soulier syndrome expressed a normal number and affinity of Fc gamma binding sites. The data suggest that platelet Fc gamma RII binding of trimeric IgG occurs independent of actin filament interaction, Mg2+, ADP, or thrombin and does not require GPIIb/IIIa or GPIIb/IIIa-fibrinogen interaction. Furthermore, this receptor appears to be normally expressed on GPIb-deficient platelets and susceptible to modulation by glucocorticoids. Finally, the Fc gamma-binding protein was isolated from whole platelets as a 220-kDa protein which upon reduction dissociates into 50,000 Mr subunits.     

Conformationally altered aortic myosin light chains

Aorta smooth myosin contains two types of light chain, LC20 and LC17, which fold together with the N-terminal region of each heavy chain to form the globular head region of myosin. We demonstrate an altered conformation of LC20 after its separation from heavy chain by high concentrations of urea, on the basis of the following evidende: 1) A polyclonal antibody against LC20 was not able to recognize this conformationally altered form; 2) Myosin reconstituted from heavy chains and urea-dissociated light chains exhibited extremely low ATPase activity. Circular dichroism unfolding profiles showed that light chains dissociated from heavy chains by SDS appeared to be more stable than those generated by urea dissociation.     

Properties of the oscillatory cAMP binding component of Dictyostelium discoideum cells and isolated plasma membranes.

The cAMP receptor on the surface of aggregation competent Dictyostelium discoideum cells specifically binds [3H]cAMP in an oscillatory manner with a periodicity of 2 min. The oscillatory cAMP-binding component is developmentallly regulated and has the nucleotide specificity expected for recognition of chemotactic signals. The concentration dependence of the peak amplitudes of cAMP binding exhibit an apparent threshold at 10(-8) M cAMP. The threshold concentration for cAMP binding that we measure is consistent with the concentration dependence of signal relay (cAMP secretion) and the chemotactic response. The kinetic data of binding and dissociation are very rapid, consistent with the time course of oscillations in receptor capacity (affinity). Specific binding oscillations are destroyed by heat or chymotrypsin but are insensitive to trypsin or glycosidase. A plasma membrane localization of receptor is supported by enrichment of cAMP binding in a plasma membrane preparation from differentiated cells. Receptor oscillations with a 2-min period are preserved in the membrane preparations, and the peak amplitudes are increased about 10-fold consistent with the enrichment of other plasma membrane markers. The alternating change in the receptor's binding capacity for cAMP may be the basis of the relay refractory period as well as the primary oscillator involved in the generation of postreceptor events such as stimulation of adenylate cyclase, cAMP secretion, and cellular movement, all of which have been previously shown to oscillate.     

Mapping of the adenosine 5'-triphosphate binding site of type II calmodulin-dependent protein kinase

The specificity of the ATP-binding site of the type II calmodulin-dependent protein kinase was probed with 25 analogues of ATP modified at various positions of the molecule. The analogues were compared by their ability to compete with ATP in the protein kinase reaction. The result of this comparison indicates that the enzyme is most sensitive to modifications at, or replacement of, the purine moiety. Changes at the triphosphate chain are much better tolerated, although the enzyme exhibited a selective sensitivity to changes in the conformation of this group. The smallest contribution to the specificity of ATP binding appears to be made by the ribose ring. The Ki values obtained for a subset of these analogues were compared to those previously reported for phosphorylase b kinase and the cyclic nucleotide dependent protein kinases [Flockhart, D. A., Freist, W., Hoppe, J., Lincoln, T. M., & Corbin, J. D. (1984) Eur. J. Biochem. 140, 289-295]. A striking similarity in the responses of these protein kinases to modifications of the ATP molecule suggests that the type II calmodulin-dependent protein kinase is related to these enzymes. Support for this conclusion was provided, recently, through comparisons of the deduced primary structures of the alpha and beta subunits of the type II calmodulin-dependent protein kinase with the protein sequences of the catalytic subunits of phosphorylase b kinase and cAMP-dependent protein kinase [Hanley, R. M., Means, A. R., Ono, T., Kemp, B. E., Burgin, K. E., Waxham, N., & Kelly, P. T. (1987) Science (Washington, D.C.) 237, 293-297; Bennett, M. K., & Kennedy, M. B. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 1794-1798], which indicated areas of extensive homology.