Cytoskeleton plays a vital role in both establishing as well as maintaining polarity in neurons. Basically actin filaments and microtubules have the functional properties which make them uniquely suited to determine and regulate polarity in neurons as well as in other polarizing cells.
Two basic characteristic features of Actin filament and microtubules in polarity:
- The Microtubules rapidly convert the molecular signals into structural changes modulating cell shape.
- They possess an intrinsic polarity.
Growth cones support the growth by sensing environmental cues and transducing those signals to the cytoskeletons.
The structure (axonal growth cone) is basically composed of a central region filled with organelles and microtubules, whereas the pheripheral region is highly dynamic, actin rich area containing lamellipodia and filopodia.
it’s shown here that the barbed ends of the actin filament which is oriented towards the rim and latter pointed ends are towards the base of the growth cone. We can find that G-actin subunits are continuously incorporated into the barbed end while the other pointed end is found to be dissociated which is basically resulting in a “treadmilling” of F actin and regulation of growth cone dynamics.
In the initial establishment of the polarity, rearrangement of the actin cytoskeleton and microtubules is very important. There is an enhanced growth cone dynamics shown by the future axon before the occurrence of morphological polarization (circled in black). Even the future dendrites are not growing properly at that stage, therefore it have a static growth cone with a rigid actin cytoskeleton (circled in red). If we pharmacologically depolymerase the actin cytoskeleton, then the non growing dendrites will be transformed into growing axons. It suggests that the actin filaments of the future dendritic growth cones form a barrier for the protrusion of microtubules, whereas the axonal growth cone contains an actin structure permissive for the microtubule protrusion.
Microtubules also actively participate in the neuronal polarization and they are more stable in the axonal region compared to the minor neurites. The stabilization of microtubules is far more sufficient to induce axon formation.
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