Introduction

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Actin is the most abundant protein in eukaryotic cells (Bamburg and Wiggan 2002). Cytokinesis, endocytosis, chemotaxis and neurite growth are examples of cellular processes that depend on polymerization of actin filaments (Carlier et al 1999). In order for cell motility to occur, there has to be a combination of elongation and increase in number of actin filaments to push the leading edge of the cell causing plasma membrane protrusion, lamellipodia formation and cell motility (Pollard and Borisy 2003). New filament formation and growth of existing filaments which favours dissociation at the pointed end and association at barbed end (i.e. treadmilling process) is regulated by signalling pathways and several actin binding proteins including cofilin, capping protein, ARP 2/3 complex and profilin (Pollard and Borisy 2003).
Cofilin was discovered more than 30 years ago (Troy et al 2008). It is part of a family of small (15-18kDa) actin binding proteins (ADF/cofilin family) that is present in all eukaryotic cells (Carlier et al 1999). Cofilin plays several important roles in the regulation of actin dynamics and cell motility.

Review

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Cellular motility occurs via formation of new actin filaments, branching of available actin filaments and elongation of actin filaments. Formation of new filaments can occur either by with nucleation of actin monomer (G-actin), or severing of available actin filaments (F-actin)(Pollard and Borisy 2003). Cofilin activity is implicated in multiple steps in this process. As reviewed by Troys et al (2008), cofilin binds preferentially to ADP actin monomers and enhances actin filament disassociation from the pointed end. This increases the pool of actin monomers available for association at the barbed end of actin filaments. At low concentration, cofilin has severing action while high concentrations of cofilin can lead to actin monomer nucleation (Troys et al 2008). Regulation of cofilin in vivo is via phosphorylation by LIMK-1 (LIM Kinase-1) leading to inactivation and dephosphorylation to re-activate it (Troys et al 2008). A discussion of studies carried out on cofilin in vitro and in vivo follows.
In vitro studies develop the understanding of physical and functional properties of cofilin. Experiments on recombinant ADF1 protein from A thaliana found that ADF only partially depolymerize actin filaments and achieve a steady state between actin polymerization and depolymerization, contrary to suggestions of its sequestering effects by previous in vitro studies (Carlier et al 1997). This study carried out by Carlier et al 1997 found that at low concentrations, ADF1 was found to bind G-actin (presumably from F-actin depolymerization) and at higher concentrations, ADF1 would start to also bind F-actin. ADF has a higher affinity to bind ADP form G-actin compared to the ATP form, encouraging depolymerization form pointed end (Carlier et al 1997). ADF can increase the rate of treadmilling process by up to 25 times and it does not affect actin depolymerization from barbed end of F-actin (Carlier et al 1997). The authors however argued against the proposed severing activity ADF but suggested further studies to explain discrepancies that lead to this hypothesis (Carlier et al 1997). ADF and cofilin shares similar properties, therefore the findings apply to cofilin.
Another study of cofilin in vitro demonstrated using fluorescent microscopy the severing action of cofilin on fluorescent labelled F-actin(Andrianantoandro and Pollard 2006). It occurs with low cofilin concentration while no severing of F-actin was seen when high concentration of cofilin was used (Andrianantoandro and Pollard 2006). The authors suggest that cofilin binding on actin filaments will have an effect on filament twist causing severing action. This study also demonstrated that cofilin enhances pointed end depolymerization in keeping with findings by Carlier et al 1997, and inhibition of barbed end depolymerization (Andrianantoandro and Pollard 2006). More actin filaments are observed in experiments with high cofilin concentration and actin where filament severing does not occur compared to actin alone. These filaments are formed by de novo nucleation (Andrianantoandro and Pollard 2006).
Lappalainen and Drubin (1997), using yeast with temperature sensitive cofilin alleles found that after incubation in temperature non-permissive for growth, these cells had multiple nuclei and unusually large actin patches, suggesting that this might be due to the actin turnover defects in the cell. By treating these mutant cells and wild type cells with monomer sequestering drug and measuring rate of actin filament loss, they found that mutant cells had slower rate of actin depolymerisation compared to wild type cells. This demonstrated the in vivo actions of cofilin in promoting actin polymerisation and depolymerisation (Lappalainen and Drubin 1997).
Caged mutant cofilin that is activated by photoirradiation is microinjected into cells is used by Ghosh et al (2004) in their study of cofilin. The cells that had cofilin mutants activated by irradiation had an increase in F-actin and barbed end levels compared to cells with inactive cofilin (Ghosh et al 2004). By using localized photorelease of cofilin, the authors demonstrated that cofilin induces lamellipodia protrusion and also sets the direction of cell migration (Ghosh et al 2004). In my opinion, this research, allowing the authors to control the activation of cofilin and focus its action in specific regions of the cell, has managed to demonstrate very well several different functions of cofilin in vivo.
Rat metastatic mammary adenocarcinomas cells with 86% of its cofilin phosphorylated with LIMK-1 had inhibition of barbed end formation compared to cells with inactive LIMK-1 and non-phosphorylated cofilin (Zebda et al 2000). In this study, Zebda et. al. (2000) also found that epidermal growth factor (EGF) failed to induce lamellipod extension in cells with inactive phosphorylated (Zebda et al 2000). This suggests a correlation between failures of barbed end formation due to loss of cofilin activity, therefore actin filament polymerization and lamellipod extension does not occur.
To investigate the effects of cofilin on actin monomer pool in cells, Kiuchi et al (2007) used methods based on similar principles to a study by Zebda et al (2000) comparing cells with normal cofilin and cofilin inactivated by LIMK-1. The authors found that cofilin action contributes to the production of more than half of actin monomers in the cells. They also demonstrated the severing activity of cofilin and its contribution to actin monomer pool in cells (Kiuchi et al 2007). Although the paper suggested that severing activity of cofilin plays a dominant role in increasing the actin monomer pool in cytoplasm, there has been no comparison between cells where both activities are present, therefore I feel this has not been proven. Other findings in this paper include a correlation between actin monomer assembly at the tip of lamellipodium and cofilin activity, and the requirement of cofilin for EGF-induced actin assembly in cells (Kiuchi et al 2007) in keeping with findings from Zebda et. al. (2000).
The suggestion that cofilin is the main provider of free actin filament barbed ends for polymerization after EGF however was questioned by DesMarais et al (2004).  After carcinoma cells are stimulated with EGF, cofilin (within 40s) followed by Arp2/3 (after 50s) are recruited to the cell’s leading edge and extension of broad lamellipods, indicating presence of actin polymerization occurred (DesMarais 2004). When this experiment was repeated with microinjection of antibody against cofilin or Arp2/3, there was no accumulation of actin monomers and no broad lamellipod extension at the leading edge (DesMarais 2004). When either cofilin or Arp2/3 is blocked, only small accumulation of barbed ends were observed. The sum of barbed ends with only Arp2/3 or cofilin is much smaller then in models with both Arp2/3 and cofilin functioning, suggesting a synergistic action between the cofilin severing and Arp branching activities (DesMarais 2004).

Conclusion(s)

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With the development of new and improved research techniques as observed above, a better understanding of actions cofilin has on actin regulation has been achieved. Cofilin’s role in actin regulation and cell motility is diverse with suggestions that these actions are affected by factors like cell EGF and Arp2/3 (DesMarais et al 2004, Kiuchi et al 2007, Zebda et al 2000). More research is still required before a clear understanding of the actions of this cellular protein can be achieved. Several diseases including Alzheimer’s disease, ischemic kidney disease, infertility and cancer have been suggested to be associated with altered production, regulation or localization of cofilin (Bamburg and Wiggan 2002). There is therefore promising therapeutic potential in this field of study.

References

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1. Bamburg,J. and Wiggan,O.P.(2002) ADF/cofilin and actin dynamics in disease. TRENDS in Cell Biology. 12;598-605.
2. Carlier,M.F., Laurent,V., Santolini,J., Melki,R., Didry,D., Xia,G.X., Hong,Y., Chua,N.H. and Pantaloni,D. (1997) Actin Depolymerizing Factor (ADF/Cofilin) Enhances The Rate of Filament Turnover: Implication in Actin-based Motility. Journal of Cell Biology. 136(6);1307-1322.
3. Carlier,M.F., Ressad,F. and Pantaloni,D.(1999) Control of Actin Dynamics in Cell Motility. The Journal of Biological Chemistry. 274(48);33827-33830.
4. Delorme,V., Machacek,M., DerMardirossian,C., Anderson,K.L., Wittman,T., Hanein,D., Waterman-Storer,C., Danuse,G. and  Bokoch,G.M. (2007) Cofilin activity downstream of Pak1 regulates cell protrusion efficiency by organizing lamellipodium and lamella actin networks. Developmental cell. 13(5);646-662.
5. Ghosh,M., Song,X., Mouneimne,G., Sidani,M., Lawrence,D.S. and Condeelis,J.S.(2004) Cofilin Promotes Actin Polymerization and Defines the Direction of Cell Motility. Science. 304;743-746.
6. Kiuchi,T., Ohashi,K., Kurita,S and Mizuno,K. (2007) Cofilin promotes stimulus-induced lamellipodium formation by generating an abundant supply of actin monomers. The Journal of Cell Biology. 177(3);465-476.
7. Lappalainen,P. and Drubin,D.G.(1997) Cofilin promotes rapid actin filament turnover in vivo. Nature. 388;78-82.
8. Pollard,T.D. and Borisy,G.G.(2003)  Cellular Motility Driven by Assembly and Disassembly of Actin Filaments. Cell. 112;453-465.
9. Pollard,T.D. and Andrianantoandro,E.(2006) Mechanism of Actin Filament Turnover by Severing and Nucleation at Different Concentrations of ADF/Cofilin. Molecular Cell. 24(1);13-23.
10. Van Troys,M., Huyck,L., Leyman,S., Dhaese,S., Vandekerkhove,J. and Ampe,C. (2008) Ins and outs of ADF/cofilin activity and regulation. European Journal of Cell Biology. 87;649-667.
11. Zebda,N., Bernard, O., Bailly,M., Welti,S. and Lawrence,D.S. (2000) Phosphorylation of ADF/Cofilin Abolishes EGF-induced Actin Nucleation at the Leading Edge and Subsequent Lamellipod Extension. The Journal of Cell Biology. 151(5);1119-1127.

Source(s) of Funding

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No funding was received for this article.

Competing Interests

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The author has no competing interests.

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