Neurotrophin-4, Alone or Heterodimerized with Brain-derived Neurotrophic Factor, Is Sorted to the Constitutive Secretory Pathway

Abstract

Nerve growth factor and neurotrophin-3 (NT-3) are processed within the constitutive secretory pathway of neurons and neuroendocrine cells and are released continuously in an activity-independent fashion. In contrast, brain-derived neurotrophic factor (BDNF) is processed in the regulated secretory pathway, stored in vesicles, and released in response to neuronal activity, consistent with its role in modulating synaptic plasticity. In this study, we used vaccinia virus infection and transfection methods to monitor the processing and sorting of neurotrophin-4 (NT-4) in AtT-20 cells, which have been used as a model for the sorting of secretory proteins in neurons. Our data show that NT-4 is processed in the constitutive secretory pathway. The molecule is diffusely distributed within the cells and released, soon after being synthesized, in a manner that is not affected by cell depolarization. We further show that NT-4 and BDNF, when co-expressed, can form heterodimers that are constitutively released. In contrast, heterodimers of NT-3 and BDNF have been shown to be released through the regulated secretory pathway. Thus, NT-4, alone or when co-expressed with BDNF, is processed within and secreted by the constitutive secretory pathway.

Differential Vesicular Targeting and Time Course of Synaptic Secretion of the Mammalian Neurotrophins

“…In contrast, NT-3 and especially BDNF are targeted almost exclusively to the regulated pathway of secretion. Thus, our data corroborate a targeting of NGF and NT-4 to the constitutive pathway in central neurons (Mowla et al, 1999;Hibbert et al, 2003). However, this was not an all-or-none phenomenon, and a considerable fraction of neurons displayed regulated secretion of NGF and NT-4 from distal dendritic granules (Figs.…”

“…Data from two laboratories suggested a similar intracellular targeting and release of all NTs (Heymach et al, 1996;Griesbeck et al, 1999;Wu et al, 2004). In contrast, NT-3, NT-4, and NGF were described by others to be released predominantly via the constitutive pathway (Mowla et al, 1999;Farhadi et al, 2000, Hibbert et al, 2003.…”

“…Overexpressed NGF and NT-3 have been described to be either targeted to secretory granules (Blochl and Thoenen, 1996;Wu et al, 2004) or were found in the constitutive pathway of secretion (Mowla et al, 1999;Farhadi et al, 2000). For NT-4, targeting to only the constitutive pathway was suggested by one previous study (Hibbert et al, 2003). These disparate findings now prompted us to evaluate neuronal targeting and synaptic secretion of the four mammalian neurotrophins under identical conditions.…”

Trk signaling regulates neural precursor cell proliferation and differentiation during cortical development

“…E13/E14 CD1 mice were used, injecting a nuclear EGFP expression plasmid driven from the Ef1␣ (Eef1a -Mouse Genome Informatics) promoter (pEF-EGFP) (as above) at a 1:3 ratio, or in some cases as indicated, 1:2 ratio with pEF-GM (empty vector), pEFdnTrkB, pEF-dnTrkC, shRNA negative control, TrkB shRNA-1 (shTrkB-1), TrkB shRNA-2 (shTrkB-2), dnAkt or pcDNA3.1-BDNF-HA (Hibbert et al, 2003) for a total of 4 or 3 g DNA per embryo, and 0.05% Trypan Blue as a tracer. When both dnTrk constructs were co-electroporated, DNA was mixed at a ratio of 1 pEF-EGFP:2 pEF-dnTrkC:2 pEF-dnTrkB for a total of 5 g DNA per embryo.…”

Neurotrophins and B-cell malignancies

“…This occurs either intracellularly (e.g., by furin) or extracellularly (e.g., by matrix metalloproteinases) [43]. They may be secreted from cells in either form [44][45][46]. Neurotrophins all share a similar structure, with a cysteine knot between beta sheets which are linked by disulphide bridges [47].…”