Macroendocytosis and endosome processing in snake motor boutons

J Physiol. 2007 Jul 1;582(Pt 1):243-62. doi: 10.1113/jphysiol.2007.130989. Epub 2007 May 3.

Abstract

We have examined the processing of endosomes formed by macroendocytosis (ME), or bulk membrane retrieval, in active motor terminal boutons at the snake nerve-muscle synapse. Endocytic probes were imaged at light (FM1-43) and electron (horseradish peroxidase (HRP)) levels over stimulus frequencies representing low, intermediate and high levels of use. Endosomes formed rapidly (1-2 s) at all frequencies, concomitant with clathrin-mediated vesicular endocytosis (CME). Endosomes dissipated rapidly into vesicles (approximately 10 s). The dissipation rate was not influenced by activity. Many endosomes split into clusters of 2-20 smaller endosomes of varying size. Vesicles budded from these smaller endosomes, from large endosomes that had not undergone fission into smaller ones, and from precursor membrane infoldings that had not yet internalized. In snake, exocytosed vesicular membrane is not competent for reuse until after a delay (> 3 min). We found that time required for endosome processing is not responsible for this delay. Endosome processing might, however, limit availability of some vesicles for release at very high levels of use. Generally, endosome processing paralleled that of vesicles internalized directly from the plasma membrane via CME, regardless of stimulus frequency. There was no evidence for differential recruitment of ME versus CME depending upon level of use.

Publication types

  • Comparative Study
  • Research Support, N.I.H., Extramural

MeSH terms

  • Abdominal Muscles / innervation*
  • Animals
  • Cell Membrane / metabolism
  • Clathrin-Coated Vesicles / metabolism*
  • Clathrin-Coated Vesicles / ultrastructure
  • Colubridae
  • Electric Stimulation / methods
  • Endocytosis*
  • Endosomes / metabolism*
  • Endosomes / ultrastructure
  • Fluorescent Dyes
  • Horseradish Peroxidase
  • In Vitro Techniques
  • Membrane Fusion
  • Microscopy, Confocal
  • Microscopy, Electron, Transmission
  • Motor Neurons / metabolism*
  • Motor Neurons / ultrastructure
  • Neuromuscular Junction / cytology
  • Neuromuscular Junction / metabolism*
  • Presynaptic Terminals / metabolism*
  • Presynaptic Terminals / ultrastructure
  • Pyridinium Compounds
  • Quaternary Ammonium Compounds
  • Time Factors

Substances

  • FM1 43
  • Fluorescent Dyes
  • Pyridinium Compounds
  • Quaternary Ammonium Compounds
  • Horseradish Peroxidase