Microtechnologies for studying the role of mechanics in axon growth and guidance

Abstract

The guidance of axons to their proper targets is not only a crucial event in neurodevelopment, but also a potential therapeutic target for neural repair. Axon guidance is mediated by various chemo- and haptotactic cues, as well as the mechanical interactions between the cytoskeleton and the extracellular matrix (ECM). Axonal growth cones, dynamic ends of growing axons, convert external stimuli to biochemical signals, which, in turn, are translated into behavior, e.g., turning or retraction, via cytoskeleton–matrix linkages. Despite the inherent mechanical nature of the problem, the role of mechanics in axon guidance is poorly understood. Recent years has witnessed the application of a range of microtechnologies in neurobiology, from microfluidic circuits to single molecule force spectroscopy. In this mini-review, we describe microtechnologies geared towards dissecting the mechanical aspects of axon guidance, divided into three categories: controlling the growth cone microenvironment, stimulating growth cones with externally applied forces, and measuring forces exerted by the growth cones. A particular emphasis is given to those studies that combine multiple techniques, as dictated by the complexity of the problem.

Force-Mediating Magnetic Nanoparticles to Engineer Neuronal Cell Function

“…Technically, we can quantify forces exerted by a cell through traction force microscopy, (Sniadecki et al, 2007 ; Style et al, 2014 ; Kilinc et al, 2015 ), atomic force microscopy (Baumgartner et al, 2003 ; Elkin et al, 2007 ; Kuznetsova et al, 2007 ; Kirmizis and Logothetidis, 2010 ; Azeloglu and Costa, 2011 ), or laser ablation (Campàs, 2016 ). These methods capture changes in cell shape formation, force dynamics of filopodia at growth cones, at the terminal end of neurites, or reveal shootin1–cortactin interactions within the promotion of traction forces at growth cones at high subcellular precision in single cells (Chan and Odde, 2008 ; Kubo et al, 2015 ).…”

“…The dimensions of the channel and micropipette are the determining factor for precision. Alternatively, optical, magnetic, thermal, or electric tweezers are tools that allow for direct force manipulation depending on the physical properties of the force-mediating object (Thoumine et al, 2000 ; Baumgartner et al, 2003 ; Jeney et al, 2004 ; Neuman and Nagy, 2008 ; Kilinc et al, 2015 ; Allen Liu, 2016 ; Tay et al, 2016b ; Timonen and Grzybowski, 2017 ). An external magnetic, optical, or electrical field is required to direct and accelerate the internalized object (Figure 2A ).…”

Acute and Chronic Neural Stimulation via Mechano-Sensitive Ion Channels

“…33 The substrate can also be patterned with proteins to create protein gradients 34,35 with different stiffness 36 and topographies such as grooves to influence neuronal growth. 37,13,38 This method offers much higher throughput compared to the aforementioned techniques which assess mechanically evoked responses one cell at a time. Using this technique, scientists have found that mechanical stresses can mediate synaptic transmission and that applied tensions on neurites can preferentially favor the fate of neurites to become the axons which are typically longer than dendrites.…”

“…However, these methods have extremely low throughput, require expensive equipment, and are not compatible with repeated and long term observation of the same cell/population ( Table 1). 13 2 Advances in micro-fabrication have made engineering micro-and nano-technologies possible for electrophysiological recordings on soft substrates 14 and likewise to apply biomechanical force on neurons. 13 Micro-and nano-technologies making use of micro-channels or functionalized nanoparticles can offer high spatial and temporal resolution for applying biomechanical forces in a high throughput fashion.…”

“…13 2 Advances in micro-fabrication have made engineering micro-and nano-technologies possible for electrophysiological recordings on soft substrates 14 and likewise to apply biomechanical force on neurons. 13 Micro-and nano-technologies making use of micro-channels or functionalized nanoparticles can offer high spatial and temporal resolution for applying biomechanical forces in a high throughput fashion. 15 In addition, these technologies match the length scales of cells, require smaller number of cells and less reagents, and have efficient nutrient and waste transport to enhance neuronal viability.…”