Cholinesterases and the cholinergic system in development
Acetylcholinesterase (AChE) degrading the neurotransmitter acetylcholine (ACh) is a most remarkable protein, not only because it is one of the fastest enzymes in nature, but also since it appears in many molecular forms and is regulated by elaborate genetic networks. Moreover, specific inhibitors of cholinesterase (ChEs) play major roles in everyday life as medical therapeutics, as pesticides in agriculture and households, and as chemical warfare agents (nerve gases).
As revealed by sensitive histochemical procedures, AChE is expressed specifically in many tissues during development and in many mature organisms, as well as in healthy and diseased states. Therefore it is not surprising that there has been a long-standing search for additional, “non-classical” functions of cholinesterases (ChEs). In principle, AChE could either act non-enzymatically, e.g. exerting cell adhesive roles, or, alternatively, it could work within the frame of classic cholinergic systems, but in non-neural tissues. AChE might be considered a highly co-opting protein, since possibly it combines such various functions within one molecule.
Our research into ChEs functioning has focused upon i) the expression of ChEs in the neural tube and their close relation to cell proliferation and differentiation, ii) that AChE expression reflects a polycentric brain development, iii) the retina as a model for AChE functioning in neural network formation, and iv) non-neural ChEs in limb development and mature bones. Also, possible roles of AChE in neuritic growth and of cholinergic regulations in stem cells.
AChE functions in proper formation of the mouse retina. An AChE knockout mouse (right) presents disturbed sublayer formation of the inner plexiform layer (IPL; a synaptic layer). Staining: calretinin. Details in Bytyqi et al. 2004.
Possible schemes of developmental actions of ACh and AChE in neurite growth, cell contact formation and cell signaling. (a) secreted ACh from growth cone stops its further advancement („deceleration“). (b) approaching an AChE+ target cell, secreted ACh is degraded and growth cone is further attracted („attraction“), (c) „adhesion“ between two cells could be further stabilised by heterotypic interaction of ChED proteins (e.g. neuroligins, AChE) with neurexins. (d) Alternatively, AChE can bind to laminin of the ECM, which in turn can bind to integrin-2, enabling „adhesion“ and „signaling“ into cell interior. Further see Vogel-Höpker et al. 2010