Internalization of hormone receptor complexes: route and significance

Insulin and alpha 2-macroglobulin-methylamine undergo endocytosis by different mechanisms in rat adipocytes: II. Comparison of intracellular events

A previous ultrastructural study showed that gold-labeled insulin (Au-I) and the non-hormonal ligand gold-labeled alpha-2-macroglobulin-methylamine (Au-alpha 2MGMA) underwent endocytosis by dissimilar cell surface structures on rat adipocytes. The present ultrastructural study compared the intracellular routes taken by these two ligands in adipocytes. Intracellular Au-alpha 2MGMA was initially found within apparent coated vesicles but Au-I was not, consistent with the previous demonstration that Au-alpha 2MGMA underwent endocytosis by coated pits whereas Au-I was internalized by uncoated micropinocytotic invaginations. Early in the endocytic pathway, the two ligands were segregated within separate small vesicles and tubulovesicles. Au-alpha 2MGMA was concentrated in a small number of these structures whereas Au-I was sparsely distributed among a relatively large number. Subsequently, the two endocytic pathways converged as the ligands intermingled within pale multivesicular bodies and lysosome-like structures. Au-I was less efficiently transferred to lysosomes than Au-alpha 2MGMA since a greater proportion of intracellular Au-I remained associated with small vesicles and tubulovesicles. This study indicates that early intracellular events in the endocytic pathways of insulin and alpha 2MGMA are distinct. These findings are discussed in light of the fundamentally dissimilar biological roles of these two molecules and the possible involvement of the endocytic pathway in the insulin signaling mechanism.

Internalization of (125I) alpha-bungarotoxin into rat suprachiasmatic nucleus neurons and dendrites

The distribution and rate of internalization of (125I) alpha-bungarotoxin (alpha-BuTX) was evaluated by quantitative electron microscopic autoradiography in rat suprachiasmatic nucleus. The ultrastructural distribution of silver grains was studied by both line source and 50% probability circle analyses. Line source analysis demonstrated that with increasing time following intracerebroventricular infusion, ligand is internalized from neuronal membranes. At all time points, real grain distribution was significantly different from randomly generated hypothetical grains. Quantification of silver grain localization by probability circle analysis indicated that membrane bound sources were mainly associated with axo-dendritic appositions, regardless of the length of time the tissue was exposed to radioligand. The compartment containing synaptic terminals was the most enriched when comparing real to hypothetical grains. By eight hours after intracerebroventricular infusion of specifically labelled alpha-BuTX binding sites were most likely to be within neurons and dendrites. These studies demonstrate that the majority of alpha-BuTX binding sites remain membrane bound with respect to time and may be associated with synaptic transmission; a significant proportion of silver grains are internalized within SCN neurons and dendrites.