Forces from the rear: deformed microtubules in neuronal growth cones influence retrograde flow and advancement

Abstract

The directed motility of growth cones at the tip of neuronal processes is a key function in neuronal path-finding and relies on a complex system of interacting cytoskeletal components. Despite intensive research in this field, many aspects of the mechanical roles of actin structures and, in particular, of microtubules throughout this process remain unclear. Mostly, force generation is ascribed to actin-myosin-based structures such as filopodia bundles and the dynamic polymer gel within the lamellipodium. Our analysis of microtubule buckling and deformation in motile growth cones reveals that extending microtubule filaments contribute significantly to the overall protrusion force. In this study, we establish a relationship of the local variations in stored bending energy and deformation characteristics to growth cone morphology and retrograde actin flow. This implies the relevance of microtubule pushing and deformation for general neurite advancement as well as steering processes.

Measurement of Subcellular Force Generation in Neurons

“…It is well accepted that growth cones pull (24)(25)(26)(27)55) and axons are under tension (9,24,34,46), and it is likewise clear that pushing forces are associated with microtubules (13,30,56,57). What has never been thoroughly analyzed or quantified is whether the net forces generated across the rear of the growth cone and along the axon are contractile or expansive.…”

“…Likewise, it is equally clear that microtubule assembly and the microtubule-associated motors Kinesin-1 and dynein generate pushing forces (13,29,30). What has never been directly addressed is whether tension along the axon is generated as the result of local contractile forces or if it occurs as the result of the forward pull of the growth cone.…”

“…In addition, Kinesin-1 has been demonstrated to drive microtubule sliding from the neuronal cell body (29). Likewise, because microtubule assembly generates a pushing force (30), it seems very reasonable that the net forces generated in these regions are expansive. Nonetheless, the C-domain of the growth cone (31,32) and the axon (33) both contain an organized actin cytoskeleton.…”

Biomimetic Liposome Model Systems to Study Cell Shape Control by the Cytoskeleton

“…Disruption of microtubules in fibroblasts results in lamellipodium retraction and an irregular polygonal cell shape [19]. Microtubules can also drive cellular protrusions such as neurite extensions in nerve cells [21][22][23][24][25]. Microtubules can also drive cellular protrusions such as neurite extensions in nerve cells [21][22][23][24][25].…”

Focus on soft mesoscopics: physics for biology at a mesoscopic scale

“…Next to membranes, the cytoskeleton is the crucial meso-scale structure where mechanical self-organization takes place. A number of studies therefore address the formation or relaxation of forces and spatial patterns in the cytoskeleton, as well as in cells and tissues under geometric constraints [16][17][18][19][20][21][22][23]. Finally, the interaction and orchestration of various organelles or functional modules, such as the cytoskeleton and the cell membrane are considered [24][25][26][27].Sometimes experimental methods commonly used to study soft matter systems need to be adapted or redesigned to target mesoscopic phenomena in biology.…”