Early Effects of β,β'-Iminodipropionitrile on Tubulin Solubility and Neurofilament Phosphorylation in the Axon

Abstract: To elucidate the role of neurofilaments in microtubule stabilization in the axon, we studied the effects of β,β'-iminodipropionitrile (IDPN) on the solubility and transport of tubulin as well as neurofilament phosphorylation in the motor fibers of the rat sciatic nerve. IDPN is known to impair the axonal transport of neurofilaments, causing accumulation of neurofilaments in the proximal axon and segregation of neurofilaments to the peripheral axoplasm throughout the nerve. Administration of IDPN at various intervals after radioactive labeling of the spinal cord with l -[³⁵S]methionine revealed that transport inhibition occurred all along the nerve within 1–2 days. Transport of cold-insoluble tubulin, which accounts for 50% of axonal tubulin, was also affected. A significant increase in the proportion of cold-soluble tubulin was observed, reaching a maximum at 3 days after IDPN treatment and returning to the control level in the following weeks. Preceding this change in tubulin solubility, a transient decrease in the phosphorylation level of the 200-kDa neurofilament protein was detected in the ventral root using phosphorylation-dependent antibodies. These early changes agreed in timing with the onset of segregation and transport inhibition, suggesting that interaction between neurofilaments and microtubules possibly regulated by phosphorylation plays a significant role in microtubule stabilization.     

Nucleotide sequence of 5S ribosomal RNA from rainbow trout (Salmo gairdnerii) liver.

Staring from low molecular weight RNA obtained from rainbow trout (Salmo gairdnerii) liver, 5S ribosomal RNA (rRNA) was highly purified by successive chromatography on columns of DEAE-Sephadex A50 and Sephadex G100. Products of complete and partial digestions on this RNA with pancreatic ribonuclease (RNase A) [EC 3.1.4.22] and RNase T [EC 3.1.4.8] were isolated and sequenced by conventional and high-performance liquid chromatography (HPLC) procedures. The nucleotide sequence of this RNA thus established was compared with those of five other vertebrae 5S rRNAs, and the rates of base substitution per site per year were found to be nearly constant in these RNAs. The analyses of the partial digests of the trout 5S rRNA revealed several sites susceptible to RNase attack, which could be accounted for by the secondary structure model for eukaryotic 5S rRNAs proposed by Nishikawa and Takemura (1).     

Organization and slow axonal transport of cytoskeletal proteins under normal and regenerating conditions

The organization of the axonal cytoskeleton was investigated by analyzing the solubility and transport profile of the major cytoskeletal proteins in motor axons of the rat sciatic nerve under normal and regenerating conditions. When extracted with the Triton-containing buffer at low temperature, 50% of tubulin and 30% of actin were recovered in the insoluble form resistant to further depolymerizing treatments. Most of this cold-insoluble form was transported in slow component a (SCa), the slower of the two subcomponents of slow axonal transport, whereas the cold-soluble form showed a biphasic distribution between SCa and SCb (slow component b). Changes in slow transport during regeneration were studied by injuring the nerve either prior to (experiment I) or after (experiment II) radioactive labeling. In experiment I where the transport of proteins synthesized in response to injury was examined, selective acceleration of SCb was detected together with an increase in the relative proportion of this component. In experiment II where the response of the preexisting cytoskeleton was examined, a shift from SCa to SCb of the cold-soluble form was observed. The differential distribution and response of the two forms of tubulin and actin suggest that the cold-soluble form may be more directly involved in axonal transport.     

Endogenous TGF-beta signaling suppresses maturation of osteoblastic mesenchymal cells

Transforming growth factor-beta (TGF-beta), one of the most abundant cytokines in bone matrix, has positive and negative effects on bone formation, although the molecular mechanisms of these effects are not fully understood. Bone morphogenetic proteins (BMPs), members of the TGF-beta superfamily, induce bone formation in vitro and in vivo. Here, we show that osteoblastic differentiation of mouse C2C12 cells was greatly enhanced by the TGF-beta type I receptor kinase inhibitor SB431542. Endogenous TGF-beta was found to be highly active, and induced expression of inhibitory Smads during the maturation phase of osteoblastic differentiation induced by BMP-4. SB431542 suppressed endogenous TGF-beta signaling and repressed the expression of inhibitory Smads during this period, possibly leading to acceleration of BMP signaling. SB431542 also induced the production of alkaline phosphatase and bone sialoprotein, and matrix mineralization of human mesenchymal stem cells. Thus, signaling cross-talk between BMP and TGF-beta pathways plays a crucial role in the regulation of osteoblastic differentiation, and TGF-beta inhibitors may be invaluable for the treatment of various bone diseases by accelerating BMP-induced osteogenesis.     

Defining the roles of beta-catenin and plakoglobin in cell-cell adhesion: isolation of beta-catenin/plakoglobin-deficient F9 cells

F9 teratocarcinoma cells in which beta-catenin and/or plakoglobin genes are knocked-out were generated and investigated in an effort to define the role of beta-catenin and plakoglobin in cell adhesion. Loss of beta-catenin expression only did not affect cadherin-mediated cell adhesion activity. Loss of both beta-catenin and plakoglobin expression, however, severely affected the strong cell adhesion activity of cadherin. In beta-catenin-deficient cells, the amount of plakoglobin associated with E-cadherin dramatically increased. In beta-catenin/plakoglobin-deficient cells, the level of E-cadherin and alpha-catenin markedly decreased. In these cells, E-cadherin formed large aggregates in cytoplasm and membrane localization of alpha-catenin was barely detected. These data confirmed that beta-catenin or plakoglobin is required for alpha-catenin to form complex with E-cadherin. It was also demonstrated that plakoglobin can compensate for the absence of beta-catenin. Moreover it was suggested that beta-catenin or plakoglobin is required not only for the cell adhesion activity but also for the stable expression and cell surface localization of E-cadherin.