Neurite Branch Retraction Is Caused by a Threshold-Dependent Mechanical Impact

Abstract

Recent results indicate that, in addition to chemical cues, mechanical stimuli may also impact neuronal growth. For instance, unlike most other cell types, neurons prefer soft substrates. However, the mechanisms responsible for the neuronal affinity for soft substrates have not yet been identified. In this study, we show that, in vitro, neurons continuously probe their mechanical environment. Growth cones visibly deform substrates with a compliance commensurate with their own. To understand the sensing of stiff substrates by growth cones, we investigated their precise temporal response to well-defined mechanical stress. When the applied stress exceeded a threshold of 274 +/- 41 pN/microm(2), neurons retracted and re-extended their processes, thereby enabling exploration of alternative directions. A calcium influx through stretch-activated ion channels and the detachment of adhesion sites were prerequisites for this retraction. Our data illustrate how growing neurons may detect and avoid stiff substrates--as a mechanism involved in axonal branch pruning--and provide what we believe is novel support of the idea that mechanics may act as guidance cue for neuronal growth.

The mechanical control of nervous system development

“…Once a neurite is connected to its target, tension promotes its stabilization; at the same time, it causes retraction or elimination of collateral neurites (Anava et al, 2009). Thus, tension might serve as a signal for axonal and dendritic survival, and reduced tension might, therefore, contribute to branch pruning (Franze et al, 2009). Accordingly, the orientation of apical dendrites of pyramidal neurons in the cortex and the degree of their dendritic and axonal arborization depends on their location relative to the curvature of the tissue (Welker, 1990), and thus probably on local tension (Xu et al, 2010).…”

“…The actin cytoskeleton is also coupled to the substrate via point contacts, which are made up of protein complexes containing integrins, vinculin, talin and many others (Renaudin et al, 1999). These point contacts form molecular 'clutches' (Suter and Forscher, 1998), which allow growth cones to transmit forces to their substrate, which may lead to its deformation (Franze et al, 2009;Betz et al, 2011;Koch et al, 2012). Accordingly, inhibition of actin polymerization leads to a reduction in the maximum force and velocity of growth cone protrusion, and a reduction in membrane stiffness results in larger forces and increased velocity (Amin et al, 2012).…”

“…Netrin 1, in turn, positively regulates traction forces via Pak1-mediated shootin1 phosphorylation, thus promoting actin-substrate coupling, force generation and axon outgrowth (Toriyama et al, 2013). Finally, calcium influx through mechanosensitive ion channels, which also may affect talin, is involved in the neuronal response to mechanical stimuli (Franze et al, 2009;Kerstein et al, 2013).It is likely that, similar to chemical signaling pathways, more than one mechanism is involved in cellular mechanotransduction. Furthermore, individual mechanical and chemical cues might activate similar or the same downstream signaling pathways and thus interact with each other.…”

Characterization of Embryonic Stem (ES) Neuronal Differentiation Combining Atomic Force, Confocal and DIC Microscopy Imaging

“…The fragmentation can be related to growth cones pruning that occurs during the early development of the central nervous system, where an excessive outgrowth of projections need to be refined to achieve precise connectivity (Faulkner et al, 2007). The selective elimination of neuronal cell processes, or neurites, is an essential step during normal development and occurs through retraction, degeneration, or a combination of both (Franze et al, 2009). This phenomenon resembles the growth cone collapse induced by several factors like mercury (Leong et al, 2001), X-ray (Al-Jahdari et al, 2008) or semaphorin in the absence of growth factors (Tamagnone & Comoglio, 2004).…”

Acute and Chronic Neural Stimulation via Mechano-Sensitive Ion Channels

“…99,100 Excessive mechanical tension can also lead to neurite retraction due to deformations of mechano-sensitive ion channels. 101 Interestingly, it has also been proposed that mechanical tension and a critical neurite length (~10 µm longer than other neurites) can specify axonal fate, potentially establishing neurons with two axons (Note: usually neurons have only one axon). 7 The growing neurites can also form communication networks which increase the tensile forces on each neuron.…”