Geneeskundige Stichting Koningin Elisabeth verslag - GSKE - FMRE

Role and regulation of actin binding proteins of the profilin, cofilin, β-thymosin and
Ena/VASP families in actin filament dynamics during neuronal outgrowth.
Context and aim
Actin based cell motility is essential for cell migration. During development, neural cells
extend processes that are guided to their destination by short and longe range repulsive or
attractive guidance cues (Mueller, 1999). Formation of these processes is critically dependent
on dynamic turn over of actin filaments and on transient formation of adhesive structures
resembling focal contacts. Since the actin system is located in the periphery of the cell and in
the filopodia of the growth cones, it is thought that the microfilament is the machinery that
receives the transduced guidance information (reviewed by Suter and Forscher, 1998). An
emerging picture from the last years is that the balance of actin
polymerization/depolymerization is important (neither too little is good, nor too much). This
balance is dictated by the activities of several actin binding proteins. The actin binding
proteins studied here : Ena/VASP-proteins, profilins, cofilins and thymosin β-members, each
modulate a different point of the actin polymerization cycle (Ampe and Vandekerckhove,
1999). This also enables cells to regulate distinct steps of the cycle differentially. It is also
evident that the various actin binding proteins act in concert (Carlier 1998, see also below)
necessitating to study combined effects of actin binding proteins. For instance, relevant to this
project, we previously demonstrated that EVL and profilins, both actin binding proteins are
also partner proteins. The simultaneous up-regulation of EVL and profilin IIa expression in brain
in mouse embryos (Lanier et al., 1999, Lambrechts et al., 2000b) suggests an important role
for the interaction of these proteins at this stage of neuronal development. EVL, an Ena/VASPfamily
member, nucleates actin polymerization in vitro (Lambrechts et al., 2000a) and profilins
promote actin filament elongation if free polymerizing filament ends are available (if filaments
are capped profilins reduce the length of filaments) (Pantaloni and Carlier, 1993; Lambrechts
et al., 2000b). The combined action of Ena/VASP proteins and profilins on actin dynamics and
on neurite formation is subject of our research, as well as the effects of cofilin and thymosin
β family members. With the exception of the latter family all actin binding proteins are
regulated by a variety of signal transduction mechanisms. For this project we focus on the
regulation of 1) EVL by Protein kinase A and possibly by interaction with n-Src kinase and FE65,
of 2) profilins and cofilins by phosphoinositides, and 3) of cofilin by LIM-kinases or other
kinases.
The correct balance of actin polymerization/depolymerization appears critical for correct
neuronal outgrowth. We chose actin binding proteins known or suspected from genetic or
biochemical studied to be involved in neurite formation and that act at different steps of the
actin polymerization cycle. Ena/VASP-proteins and profilins may work synergistically to
promote filament formation. Profilins and cofilins are dynamizers of filament turn-over and
profilins and mammalian β-thymosins work antagonistically. We investigate the role of these
various key actin binding proteins in neurite extensions. Our long term goal is to understand
the interplay (and the way it is regulated) of the actin binding proteins during neuronal
outgrowth.
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