Mechanotransduction in neuronal cell development and functioning

Abstract

Although many details remain still elusive, it became increasingly evident in recent years that mechanosensing of microenvironmental biophysical cues and subsequent mechanotransduction are strongly involved in the regulation of neuronal cell development and functioning. This review gives an overview about the current understanding of brain and neuronal cell mechanobiology and how it impacts on neurogenesis, neuronal migration, differentiation, and maturation. We will focus particularly on the events in the cell/microenvironment interface and the decisive extracellular matrix (ECM) parameters (i.e. rigidity and nanometric spatial organisation of adhesion sites) that modulate integrin adhesion complex-based mechanosensing and mechanotransductive signalling. It will also be outlined how biomaterial approaches mimicking essential ECM features help to understand these processes and how they can be used to control and guide neuronal cell behaviour by providing appropriate biophysical cues. In addition, principal biophysical methods will be highlighted that have been crucial for the study of neuronal mechanobiology.

Keywords: Bioengineering; Biomaterials; Biophysics; Mechanobiology; Neuronal differentiation.

Fig. 1

Compositional, mechanical, and structural features of the brain extracellular matrix. (a) The cartoon illustrates principal components of the brain extracellular matrix (ECM). Image with permission from Lau et al. (2013), Copyright (2013) Springer Nature. (b) Two examples of the mechanical properties (elastic modulus) in different brain compartments (top: mouse hippocampus, bottom: rat cerebellum, scale bar: 400 μm) are shown that were obtained by atomic force microscopy-based recordings. Images from Antonovaite et al. (2018), and with permission from Christ et al. (2010), Copyright (2010) Elsevier. (c) The graphic highlights the general softness of the brain tissue in comparison with other body tissues and indicates the stiffening in case of brain tumours. Image with permission from Barnes et al. (2017), Copyright (2017) Company of Biologists LTD. (d) The upper image demonstrates a stochastic optical reconstruction microscopy super-resolution recording of perineuronal nets (chondroitin sulphate proteoglycans stained with Wisteria floribunda agglutinin-Dy749P1). Image courtesy of Xiaowei Zhuang (Harvard University, Cambridge, MA, USA) (Sigal et al. 2019). The lower image shows a scanning electron microscopic recording of the configuration of decellularised hippocampal ECM. Image from Tajerian et al. (2018).

Fig. 2

Integrin-mediated mechanosensing and mechanotransductive sequence with influencing parameters of the extracellular matrix. (a) The cartoon illustrates the initial ECM-integrin-talin-actin linkage in the nascent adhesions and how the force loading within this molecular clutch determines whether this structure disassembles (in case of too low force loading) or reinforces (in case of sufficient force loading) and (b) matures into integrin adhesion complexes (IAC) by recruitment of further proteins. (c) In this graphic, the stratified nanoarchitecture of mature IAC with its different layers is shown. (d) The extent of force loading and IAC maturation is determined by biophysical cues of the extracellular matrix, in particular, the rigidity and the spatial organisation of the integrin adhesion sites (in terms of spacing, distribution, (dis)order, (an)isotropy and nanotopography). Further details on IAC maturation are outlined in "Mechanosensing and mechanotransduction". The figure contains adapted elements of images with permission from Case et al. (2015) and Case and Waterman (2015), Copyright (2015) Springer Nature; Barnes et al. (2017), Copyright (2017) Company of Biologists LTD; and an adapted element with permission from Borghi et al. (2018), Copyright (2018) American Chemical Society.

Fig. 3

Mechanotransductive processes in neuronal development and functioning. (a) The graphic illustrates the different phases of neuronal cell development (in this case during cortex formation) starting from self-renewal and neurogenesis, passing to neuronal migration, neuritogenesis, and ending with terminal differentiation and maturation with synaptogenesis and network integration (VZ: Ventricular zone, SVZ: Subventricular zone, IZ: Intermediate zone, CP: Cortical plate). Examples of extracellular matrix and cellular proteins related to mechanotransductive processes that are known to influence different phases of these events are indicated in bold and underlined. Further details can be found in "Mechanotransductive processes and signalling in neuronal cell development and functioning". The figure contains elements of an image from Schulte et al. (2016b). (b) The panel shows in vivo mechanosensitivity of a growing axon in the Xenopus brain. The colour code indicates (a) the stiffness (elastic modulus) of the brain tissue measured by atomic force microscopy-based recordings or (b) the stiffness changes over time in the same region. The fluorescently labelled axon was tracked and outlined in blue and documents the directed axon movement towards the softer region (Scale bars = 100 μm). The image has been reproduced from Thompson et al. (n.d.). (c) The panel demonstrates the modulations along the mechanotransductive sequence induced by the interaction of neuron-like PC12 cells with ECM-mimicking nanotopographical zirconia substrates produced by the nanofabrication technique supersonic cluster beam deposition, compared with flat zirconia surfaces. In the transmission electron images, it can be seen that the cells interact only with the apical part of the nanotopographical asperities which restricts the dimension of the nanometric adhesion sites (indicated by the white arrows) to smaller sizes with respect to the situation on flat zirconia. Also, the integrin adhesion complexes (vinculin staining in green recorded by TIRF microscopy) remain of small dimensions (focal contact/point contact size, see white arrows with dashed lines) whereas mature focal adhesions form only on the flat substrate (see white arrows). Consequently, on the nanostructured zirconia, no stress fibre formation (epifluorescence of phalloidin staining in red) can be noted, while there are abundant stress fibres on the flat zirconia (examples marked by white asterisks). The cells on the nanotopographical substrate are softer than on the flat surface (quantified by atomic microscopy-based analysis, the colour code indicates the Young’s modulus (YM)) and neurite outgrowth was visible (black arrow). The image was adapted with permission from Schulte et al. (2017), Copyright (2017) American Chemical Society.

Fig. 4

Biomaterial and biophysical approaches to study and/or control mechanotransductive processes that regulate neuronal cell development and functioning. (a) The scheme illustrates important stages during neuronal cell development. (b) The graphic outlines schematically principal (extra)cellular structures and processes of interest within the integrin adhesion complex-mediated mechanotransductive sequence in neuronal cells (on the right the axon/neurite growth cone is highlighted) and (c) some biophysical methods (AFM: atomic force microscopy, TFM: traction force microscopy) that are used to study them (see also "BIOPHYSICAL METHODS TO STUDY NEURONAL MECHANOBIOLOGY"). (d) Examples of different bioengineering approaches are listed that are used for the production of biomaterials which mimic biophysical extracellular matrix (ECM) features, such as hydrogels derived from decellularised brain ECM (Jin et al. 2018), hydrogels made of polymers (in this case, polyacrylamide gels with different stiffness ranges, Young’s modulus: left; < 14.5 kPa, middle: 14.5–29 kPa, right: > 29 kPa) (Hadden et al. 2017), electrospinning, lithographic methods, and pattern transfer, as well as reactive-ion etching (RIE) (Chen et al. 2014), carbon nanotubes (Cellot et al. 2011), and assembly of zirconia nanocluster by supersonic cluster beam deposition (Schulte et al. 2017). Various approaches are referenced throughout the review in which these types of substrates are applied and exploited to study and guide neuronal cell mechanotransduction and development. (a–d) Together, these mechanobiological approaches can contribute to a better understanding of how mechanotransductive processes impact on neuronal cell development and functioning. The figure contains adapted elements of images with permission from Jin et al. (2018), Copyright (2018) Springer Nature; Chen et al. (2014), Copyright (2014) Elsevier; as well as elements from Hadden et al. (2017), Cellot et al. (2011), Schulte et al. (2016b, c), and Maffioli et al. (2017).