Role of cholesterol in developing T-tubules: analogous mechanisms for T-tubule and caveolae biogenesis.
Recent work has suggested that caveolae biogenesis and transverse-tubule (T-tubule) formation in muscle cells share similar underlying features. We compared the properties of caveolin-1 (cav-1)-positive caveolae, in epithelial cells, with caveolin-3 (cav-3)-positive precursor T-tubules, in differentiating C2C12 muscle cells, using the cholesterol-binding drug, Amphotericin B (AmphB). Treatment of MDCK epithelial cells with acute high doses or chronic low doses of AmphB caused a loss of surface caveolae and the rapid redistribution of cav-1, and exogenously expressed cav-3, from the cell surface into modified endosomes. This effect was reversible and specific, as the GPI-anchored protein, alkaline phosphatase, was largely unaffected by the treatment unless it had been previously partitioned into caveolar domains. In differentiating C2C12 mouse myotubes, AmphB also caused a complete redistribution of cav-3 from precursor T-tubule elements into enlarged endosomes, morphologically very similar to those seen in MDCK cells. This was accompanied by redistribution of a T-tubule marker and a dramatic reduction in the extent of surface-connected tubular elements. We propose that cholesterol-enriched glycolipid 'raft' domains are involved in the formation and maintenance of diverse membrane systems including caveolae and the T-tubule system of muscle.