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To study microtubule (MT) dynamics in nerve cells, we microinjected biotin-labeled tubulin into the cell body of chemically fused and differentiated PC12 cells and performed the immunofluorescence or immunogold procedure using an anti-biotin antibody followed by secondary antibodies coupled to fluorescent dye or colloidal gold. Incorporation of labeled subunits into the cytoskeleton of neurites was observed within minutes after microinjection. Serial electron microscopic reconstruction revealed that existing MTs in PC12 neurites incorporated labeled subunits mainly at their distal ends and the elongation rate of labeled segments was estimated to be less than 0.3 micron/min. Overall organization of MTs in the nerve cells was different from that in undifferentiated cells such as fibroblasts. Namely, we have not identified any MT-organizing centers from which labeled MTs are emanating in the cell bodies of the injected cells. Stereo electron microscopy revealed that some fully labeled segments seemed to start in the close vicinity of electron dense material within the neurites. This suggests new nucleation off some structures in the neurites. We have also studied the overall pattern of the incorporation of labeled subunits which extended progressively from the proximal part of the neurites toward their tips. To characterize the mechanism of tubulin incorporation, we have measured mean density of gold labeling per unit length of labeled segments at different parts of the neurites. The results indicate access of free tubulin subunits into the neurites and local incorporation into the neurite cytoskeleton. Our results lead to the conclusion that MTs are not static polymers but dynamic structures that continue to elongate even within the differentiated nerve cell processes.

The renal tubular epithelial antigen (Tub-Ag) of rats was solublized by Pronase and purified by gel filtration and acrylamide gel electrophoresis. Purified Tub-Ag was a glycoprotein with S20,W value of 8.4. Utilizing radiolabeled Tug-Ag, a sensitive radioimmunoassay for Tub-Ag and homologous antibody (anti-Tub-Ag) was developed. Tub-Ag activity associated with a protein of the same molecular size was demonstrated in the serum, as well as in Pronase extracts of all the organs tested, including kidney, liver, lung, spleen, intestine, stomach, and heart. The physiochemical properties of the Tub-Ag of rats and its distribution were essentially the same as the Tub-Ag of humans, which had been found in immune deposits in the kidney of some patients with idiopathic membranous glomerulonephritis. Rats were immunized with the purified Tub-Ag emulsified in Freund's complete adjuvant and followed for Tub-Ag and anti-Tub-Ag in the serum, as well as for proteinuria and immunohistological changes in the kidney. Serum Tub-Ag dropped sharply after 20 days, when anti-Tub-Ag appeared in the circulation. Persistent, massive proteinuria appeared still later, more than 30 days after injection, when anti-Tub-Ag disappeared and Tub-Ag reappeared in the serum of some of those rats. In others, anti-Tub-Ag in the serum persisted throughout the observation period of 90 days. The pathology of the kidney of the rats with proteinuria was that of a typical membranous glomerulonephritis; thickening of glomerular capillary walls with granular deposits of gamma-globulin and Tub-Ag was observed. On the basis of these results, Tub-Ag in the serum, probably released from cellular membranes of various organs as a physiological metabolite, is considered to maintain the pathological process in the kidney by providing the antigen continuously to form immune complexes.

The elaborate cross-connections among membranous organelles (MO), microtubules (MT), and neurofilaments (NF) were demonstrated in unifixed axons by the quick-freeze, deep-etch, and rotary-shadowing method. They were categorized into three groups: NF-associated cross-linker, MT-associated cross-bridges, and long cross-links in the subaxolemmal space. Other methods were also employed to make sure that the observed cross-connections in the unfixed axons were not a result of artifactual condensation or precipitation of soluble components or salt during deep-etching. Axolemma were permeablized either chemically (0.1% saponin) or physically (gentle homogenization), to allow egress of their soluble components from the axon; or else the axons were washed with distilled water after fixation. After physical rupture of the axolemma or saponin treatment, most of the MO remained intact. MT were stabilized by adding taxol in the incubation medium. Axons prepared by these methods contained many longitudinally oriented NF connected to each other by numerous fine cross-linkers (4-6 nm in diameter, 20-50 nm in length). Two specialized regions were apparent within the axons: one composed of fascicles of MT linked with each other by fine cross-bridges; the other was in the subaxolemmal space and consisted of actinlike filaments and a network of long cross-links (50-150 nm) which connected axolemma and actinlike filaments with NF and MT. F-actin was localized to the subaxolemmal space by the nitrobenzooxadiazol phallacidin method. MO were located mainly in these two specialized regions and were intimately associated with MT via fine short (10-20 nm in length) cross-bridges. Cross-links from NF to MO and MT were also common. All these cross-connections were observed after chemical extraction or physical rupture of the axon; however, these procedures removed granular materials which were attached to the filaments in the fresh unextracted axons. The cross-connections were also found in the axons washed with distilled water after fixation. I conclude that the cross- connections are real structures while the granular material is composed of soluble material, probably protein in nature.