Figure 3

The signaling endosome hypothesis of long-distance axonal signal transmission. Soluble protein trophic factors released by the target (1) bind to transmembrane receptors on the presynaptic axon terminal (2), inducing receptor activation and internalization via clathrin-coated membranes or other endocytic structures (3). These endocytic vesicles give rise to transport endosomes that bear the receptor and associated signaling molecules as well as molecular motors (shown in turquoise) (4) that utilize microtubules (shown in pink) within the axon to carry the endosome toward the cell body (5). Upon arrival at the neuron cell body the endosome-associated signals may either initiate additional local signals or may directly translocate (6) into the nucleus to elicit transcriptional changes (7).