Munc18 and Munc13 regulate early neurite outgrowth.
BACKGROUND
During development, growth cones of outgrowing neurons express proteins involved in vesicular secretion, such as SNARE (soluble N-ethylmaleimide-sensitive fusion protein-attachment protein receptor) proteins, Munc13 and Munc18. Vesicles are known to fuse in growth cones prior to synapse formation, which may contribute to outgrowth.


RESULTS
We tested this possibility in dissociated cell cultures and organotypic slice cultures of two release-deficient mice (Munc18-1 null and Munc13-1/2 double null). Both types of release-deficient neurons have a decreased outgrowth speed and therefore have a smaller total neurite length during early development [DIV1-4 (day in vitro 1-4)]. In addition, more filopodia per growth cone were observed in Munc18-1 null, but not WT (wild-type) or Munc13-1/2 double null neurons. The smaller total neurite length during early development was no longer observed after synaptogenesis (DIV14-23).


CONCLUSIONS
These data suggest that the inability of vesicle fusion in the growth cone affects outgrowth during the initial phases when outgrowth speed is high, but not during/after synaptogenesis. Overall, the outgrowth speed is probably not rate-limiting during neuronal network formation, at least in vitro. In addition, Munc18, but not Munc13, regulates growth cone filopodia, potentially via its previously observed effect on filamentous actin.
Broeke
Jurjen H P
JH
Department of Functional Genomics, Vrije Universiteit (VU) and VU Medical Center (VUmc), Amsterdam, The Netherlands.
Roelandse
Martijn
M
Luteijn
Maartje J
MJ
Boiko
Tatiana
T
Matus
Andrew
A
Toonen
Ruud F
RF
Verhage
Matthijs
M
D000818:Animals; D020439:Growth Cones / Q000502:physiology; D047908:Intracellular Signaling Peptides and Proteins / Q000172:deficiency / Q000502:physiology*; D051379:Mice; D008822:Mice, Transgenic; D051938:Munc18 Proteins / Q000172:deficiency / Q000502:physiology*; D009419:Nerve Tissue Proteins / Q000172:deficiency / Q000502:physiology*; D016501:Neurites / Q000502:physiology*; D011554:Pseudopodia / Q000378:metabolism
D047908:Intracellular Signaling Peptides and Proteins; D051938:Munc18 Proteins; D009419:Nerve Tissue Proteins; C497938:Stxbp1 protein, mouse; C503321:Unc13a protein, mouse; C485549:Unc13b protein, mouse