Thymosin

See also: Actin

van Kesteren, et al., Smit • 2006 • JNeuro • FullText

Local Synthesis of Actin-Binding Protein β-Thymosin Regulates Neurite Outgrowth.

Local protein synthesis plays an essential role in the regulation of various aspects of axonal and dendritic function in adult neurons. At present, however, there is no direct evidence that local protein translation is functionally contributing to neuronal outgrowth. Here, we identified the mRNA encoding the actin-binding protein β-thymosin as one of the most abundant transcripts in neurites of outgrowing neurons in culture. β-Thymosin mRNA is not evenly distributed in neurites, but appears to accumulate at distinct sites such as turning points and growth cones. Using double-stranded RNA knockdown, we show that reducing β-thymosin mRNA levels results in a significant increase in neurite outgrowth, both in neurites of intact cells and in isolated neurites. Together, our data demonstrate that local synthesis of β-thymosin is functionally involved in regulating neuronal outgrowth.

Articles

Honkura, Matsuzaki, Noguchi, Ellis-Davies, Kasai • 2008 • Cell • FullText

The subspine organization of actin fibers regulates the structure and plasticity of dendritic spines.

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Fischer, Kaech, Knutti, Matus • 1998 • Cell • FullText

Rapid actin-based plasticity in dendritic spines.

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Lang • 2004 • PNAS • FullText

Transient expansion of synaptically connected dendritic spines upon induction of hippocampal long-term potentiation.

We find that induction of long-term potentiation (LTP) of synaptic transmission in acute hippocampal slices of adult mice evokes a reliable, transient expansion in spines that are synaptically activated, as determined with calcium imaging. Similar to LTP, transient spine expansion requires N-methyl- D-aspartate (NMDA) receptor-mediated Ca2 influx and actin polymerization. Moreover, like the early phase of LTP induced by the stimulation protocol, spine expansion does not require Ca2 influx through L-type voltage-gated Ca2 channels nor does it require protein synthesis. Thus, transient spine expansion is a characteristic feature of the initial phases of plasticity at mature synapses and so may contribute to synapse remodeling important for LTP.

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Fujiwara, Vavylonis, Pollard • 2007 • PNAS • FullText

Polymerization kinetics of ADP- and ADP-Pi-actin determined by fluorescence microscopy

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Koskinen and Hotulainen • 2014 • Frontiers • FullText

Measuring F-actin properties in dendritic spines

Measurements of actin turnover in dendritic spines: Fitting the data from individual measurements resulted in a mean stable component size of 18% as well as mean time constants of 51 sec for the dynamic component and 840 sec for the stable component.

Bindschadler, Osborn, Dewey, McGrath • 2004 • BiophysicalJ • PMC

A Mechanistic Model of the Actin Cycle

Actin polymerization proceeds until only a small concentration (~0.1 µM) of unpolymerized actin (Gactin) remains. This ‘‘critical concentration’’ is also the minimum concentration required to form filaments (F-actin).

Both regulated and unregulated actin binding proteins modify the actin cycle in cells (Fig. 1). Barbed-end binding proteins block the assembly of G-actin at filament-barbed ends. The most abundant barbed-end binding proteins, capping protein (CP) and gelsolin (Isenberg et al., 1980; Yin et al., 1981), are inactivated by PIP2 and other polyphosphoinositides (Heiss and Cooper, 1991; Janmey and Stossel, 1987). Gelsolin, which also severs actin filaments (Yin and Stossel, 1979), requires micromolar calcium for its activity. CP, gelsolin, and Arp2/3 complex (Mullins et al., 1998), can nucleate new actin filaments. The processes of severing and nucleation help determine the number and length of actin filaments. Arp2/3 complex can also cap pointed ends (Mullins et al., 1998). Arp2/ 3 complex activities are greatly enhanced by the GTPase binding protein N-WASp (Machesky et al., 1999; Yarar et al., 1999). Inhibited by phosphorylation (Morgan et al., 1993), the ADF/cofilin family proteins bind preferentially to ADP containing subunits (Carlier et al., 1997). Cofilin destabilizes filaments by severing them (Maciver et al., 1991), by accelerating the rate of ADP subunit disassembly (Carlier et al., 1997), and by enhancing the rate of Pi release (Blanchoin and Pollard, 1999). Unregulated proteins of the b4-thymosin family bind actin monomers to maintain unpolymerized actin at hundreds of times the critical concentration (Safer et al., 1990). Unlike b4-thymosin, the monomer binding protein profilin has catalytic functions. Profilin accelerates the exchange of ADP for ATP on actin monomer 140-fold (Selden et al., 1999). Also unlike actin complexed with b4-thymosin, profilin-bound G-actin assembles at barbed ends but not pointed ends (Pollard and Cooper, 1984), releasing unbound profilin (Pantaloni and Carlier, 1993).