From stress and strain to spikes: mechanotransduction in spider slit sensilla

This review focuses on the structure and function of a single mechanoreceptor organ in the cuticle of spiders. Knowledge emerging from the study of this organ promises to yield general principles that can be applied to mechanosensation in a wide range of animal systems. The lyriform slit sense organ on the antero-lateral leg patella of the spider Cupiennius salei is unusual in possessing large sensory neurons, whose cell bodies are close to the sites of sensory transduction, and accessible to intracellular recording during mechanotransduction. This situation, combined with recent technical developments, has made it possible to observe and experiment with all the major stages of mechanosensation. Important findings include the approximate size, number and ionic selectivity of the ion channels responsible for mechanotransduction, the types of voltage-activated ion channels responsible for action potential encoding, and the mechanisms controlling the dynamic properties of transduction and encoding. Most recently, a complex efferent system for peripheral modulation of mechanosensation has been discovered and partially characterized. Much remains to be learned about mechanosensation, but the lyriform slit sense organ system continues to offer important opportunities to advance our understanding of this crucial sense.     

Structural correlates of mechanosensory transduction and adaptation in identified neurons of spider slit sensilla

We used isolated but functionally intact preparations of the lyriform slit-sense organ VS-3 from the leg of the spider, Cupiennius salei Keys, to examine the role of prominent fine-structural elements for mechanosensory transduction and adaptation. Slit sensilla act as strain sensors in the cuticular exoskeleton; each slit is innervated by two mechanosensitive neurons. Punctate mechanical deformation at four points along the dendrites demonstrated that mechanical excitability is confined to membrane sites at the extreme dendrite tips that are enclosed by cuticular slit structures. Depletion of microtubules in VS-3 neurons by prolonged mechanical stimulation and application of 5 mmol l(-1) colchicine did not disrupt the generation of a receptor potential. Hence, putative gating mechanisms of the mechanically activated membrane channels at the dendrite tips appear to be largely independent of microtubular structures. The discrete adaptation pattern in each of the two partner neurons, rapidly adapting versus slowly adapting, did not depend on the distinct mode of dendrite attachment to cuticular slit structures, and even persisted in isolated neurons after their dendrite tips and auxiliary structures were lost. We suggest that the two discrete adaptation patterns are based on intrinsic differences in the action potential encoding process rather than differences in stimulus transformation or mechanotransduction.     

Model studies on the mechanical significance of grouping in compound spider slit sensilla (Chelicerata,Araneida)

Summary The mechanical implications of various types of slit arrangements found among the strain-sensitive slit sensilla in the arachnid exoskeleton (Fig. 3) were studied by measuring the deformation of model slits, cut into plastic discs, under static load applied in the plane of the disc and from varying directions (Figs. 1, 2).1. Close parallel, lyriform arrangements. Compression of slits (adequate stimulus) reaches much higher values than dilatation. It is highest with the load direction at right angle to the slit axes. Also, in the majority of slits the range of load angles resulting in compression is considerably larger than that leading to dilatation. Length distribution and lateral shift of slits in the models have a pronounced effect on slit deformability (Figs. 4-5): (a) In the oblique bar arrangement with slits of equal length and regular lateral shift (Fig. 4A) deformation of all slits is very similar at all load directions. In all slits compression results from a range of load angles larger than 120°. (b) In arrangements with a regular increase in slit length and a triangular outline shape deformability differs greatly among the slits at all load angles (Fig. 4B). (c) The slit configuration with a heartshaped outline (Fig. 4C) is peculiar for the large spread of load angles at which the compression of the different slits is highest. — These properties recommend different arrangements for the solution of different strain measuring problems, with for instance, the particular need of a wide angular working range (arrangement a), of a large spectrum of absolute sensitivities (b), or of the analysis of load direction (c).2. Angle and distance between slits. Due to the mechanical directionality inherent in an elongated slit the divergence of slit axes within a group of slits is likely to indicate the importance of the analysis of strain direction (Fig. 6). The mechanical interaction between closely neighbouring slits decreases with their distance from each other. In a parallel arrangement of equally long slits it disappears if the distance is about 1.5 times the slit length (Fig. 7).3. Aiming towards a mechanical model which would explain the complex deformation found in a lyriform organ, we consider the outline of the organ as a hole traversed by beams of material. Slit deformation can be calculated from the elastic lines of the beams which separate the slits and information drawn from photoelastic experiments (Figs. 8-11).     

Lyriform slit sense organs and muscle reflexes in the spider leg

Journal of Comparative Physiology A -     

Arthropod touch reception: spider hair sensilla as rapid touch detectors

Wandering spiders like Cupiennius salei are densely covered by tactile hairs. In darkness Cupiennius uses its front legs as tactile feelers. We selected easily identifiable hairs on the tarsus and metatarsus which are stimulated during this behavior to study tactile hair properties. Both the mechanical and electrophysiological hair properties are largely independent of the direction of hair displacement. Restoring torques measure 10(-9) to 10(-8) Nm. The torsional restoring constant S changes non-linearly with deflection angle. It is of the order of 10(-8) Nm/rad, which is about 10,000 times larger than for trichobothria. Angular thresholds for the generation of action potentials are ca.1 degrees. Electrophysiology reveals a slow and a fast sensory cell, differing in adaptation time. Both cells are movement detectors mainly responding to the dynamic phase (velocity) of a stimulus. When applying behaviorally relevant stimulus velocities (up to 11 cm s(-1)) threshold deflection for the elicitation of action potentials and maximum response frequency are reached as early as 1.2 ms after stimulus onset and followed by a rapid decline of impulse frequency. Obviously these hairs inform the spider on the mere presence of a stimulus but not on details of its time-course and spatial orientation.     

Mechanotransduction in Caenorhabditis elegans

One of the looming mysteries in signal transduction today is the question of how mechanical signals, such as pressure or mechanical force delivered to a cell, are interpreted to direct biological responses. All living organisms, and probably all cells, have the ability to sense and respond to mechanical stimuli. At the single-cell level, mechanical signaling underlies cell-volume control and specialized responses such as the prevention of poly-spermy in fertilization. At the level of the whole organism, mechanotransduction underlies processes as diverse as stretch-activated reflexes in vascular epithelium and smooth muscle; gravitaxis and turgor control in plants; tissue development and morphogenesis; and the senses of touch, hearing, and balance. Intense genetic, molecular, and elecrophysiological studies in organisms ranging from nematodes to mammals have highlighted members of the recently discovered DEG/ENaC family of ion channels as strong candidates for the elusive metazoan mechanotransducer. Here, we discuss the evidence that links DEG/ENaC ion channels to mechanotransduction and review the function of Caenorhabiditis elegans members of this family called degenerins and their role in mediating mechanosensitive behaviors in the worm.     

In search of differences between the two types of sensory cells innervating spider slit sensilla (<Emphasis Type="Italic">Cupiennius salei</Emphasis> Keys.)

The metatarsal lyriform organ of the spider Cupiennius salei is a vibration detector consisting of 21 cuticular slits supplied by two sensory cells each, one ending in the outer and the other at the inner slit membrane. In search of functional differences between the two cell types due to differences in stimulus transmission, we analyzed (1) the adaptation of responses to electrical stimulation, (2) the thresholds for mechanical stimulation and (3) the representation of male courtship vibrations using intracellular recording and staining techniques. Single- and multi-spiking receptor neurons were found among both cell types, which showed high-pass filter characteristics. Below 100-Hz threshold, tarsal deflections were between 1° and 10°. At higher frequencies, they decreased down to values as small as 0.05°, corresponding to 4.5-nm tarsal deflection in the most sensitive cases. Different slits in the organ and receptor cells with slow or fast adaptation did not differ in this regard. When stimulated with male courtship vibrations, both types of receptor cells again did not differ significantly regarding number of action potentials, latency and synchronization coefficients. Surprisingly, the differences in dendrite coupling were not reflected by the physiological responses of the two cell types innervating the slits.     

Mechanotransduction in gravisensing cells

Plant organs can re-orientate themselves with respect to gravity. Gravisensing cells (statocytes) contain movable amyloplasts whose potential energy is apparently used to activate calcium channels by exerting tension on the actin network and/or pressure on the cytoskeleton elements lining the plasma membrane. The chain of events that follows remains to be further analysed but includes transient pH changes in the cytosol and sustained pH changes in the cell wall. Transduction ends with relocation of the auxin efflux carriers responsible for the lateral transport of auxin, which reorients the root tip in the direction of gravity. Many questions remain to be solved but recent studies now herald a better understanding of the molecular events involved in gravisensing.     

Mechanotransduction in embryonic vascular development

A plethora of biochemical signals provides spatial and temporal cues that carefully orchestrate the complex process of vertebrate embryonic development. The embryonic vasculature develops not only in the context of these biochemical cues, but also in the context of the biomechanical forces imparted by blood flow. In the mature vasculature, different blood flow regimes induce distinct genetic programs, and significant progress has been made toward understanding how these forces are perceived by endothelial cells and transduced into biochemical signals. However, it cannot be assumed that paradigms that govern the mature vasculature are pertinent to the developing embryonic vasculature. The embryonic vasculature can respond to the mechanical forces of blood flow, and these responses are critical in vascular remodeling, certain aspects of sprouting angiogenesis, and maintenance of arterial–venous identity. Here, we review data regarding mechanistic aspects of endothelial cell mechanotransduction, with a focus on the response to shear stress, and elaborate upon the multifarious effects of shear stress on the embryonic vasculature. In addition, we discuss emerging predictive vascular growth models and highlight the prospect of combining signaling pathway information with computational modeling. We assert that correlation of precise measurements of hemodynamic parameters with effects on endothelial cell gene expression and cell behavior is required for fully understanding how blood flow-induced loading governs normal vascular development and shapes congenital cardiovascular abnormalities.     

Mechanotransduction pathways in skeletal muscle hypertrophy

In the last decade, molecular biology has contributed to define some of the cellular events that trigger skeletal muscle hypertrophy. Recent evidence shows that insulin like growth factor 1/phosphatidyl inositol 3-kinase/protein kinase B (IGF-1/PI3K/Akt) signaling is not the main pathway towards load-induced skeletal muscle hypertrophy. During load-induced skeletal muscle hypertrophy process, activation of mTORC1 does not require classical growth factor signaling. One potential mechanism that would activate mTORC1 is increased synthesis of phosphatidic acid (PA). Despite the huge progress in this field, it is still early to affirm which molecular event induces hypertrophy in response to mechanical overload. Until now, it seems that mTORC1 is the key regulator of load-induced skeletal muscle hypertrophy. On the other hand, how mTORC1 is activated by PA is unclear, and therefore these mechanisms have to be determined in the following years. The understanding of these molecular events may result in promising therapies for the treatment of muscle-wasting diseases. For now, the best approach is a good regime of resistance exercise training. The objective of this point-of-view paper is to highlight mechanotransduction events, with focus on the mechanisms of mTORC1 and PA activation, and the role of IGF-1 on hypertrophy process.     

Hygro- and thermoreceptors in tip-pore sensilla of the tarsal organ of the spider Cupiennius salei: innervation and central projection

The hygro- and thermoreceptive tarsal organ in the wandering spider Cupiennius salei is located on the tarsus of each walking leg and pedipalp, and consists of a tiny air-filled capsule in the cuticle. This capsule communicates with the outside world through a small aperture and contains seven nipple-shaped sensilla, each with a pore at its tip. In both their external morphology and internal structure, the sensilla are indistinguishable, although one sensillum is innervated by only two sensory cells, whereas the other six sensilla contain three sensory cells. Their dendrites are unbranched and terminate at the tip-pore, where they are enveloped by amorphous material that appears to limit their exposure to the atmosphere. Cobalt fillings reveal that each tarsal organ projects to three different areas within the suboesophageal ganglionic mass: (1) the sensory longitudinal tract 3 and 4; (2) the corresponding pedipalpal or leg ganglion; (3) a structured neuropil (here termed the Blumenthal neuropil) beneath the oesophagus. The multiple representation of sensory afferents from each tarsal organ in different regions of the suboesophageal ganglionic mass suggests parallel processing of hygro-/thermoreceptive information.     

Mechanotransduction at cadherin-mediated adhesions

Cell-to-cell junctions are crucial mechanical and signaling hubs that connect cells within tissues and probe the mechanics of the surrounding environment. Although the capacity of cell-to-extracellular-matrix (ECM) adhesions to sense matrix mechanics and proportionally modify cell functions is well established, cell-cell adhesions only recently emerged as a new class of force sensors. This finding exposes new pathways through which force can instruct cell functions. This review highlights recent findings, which demonstrate that protein complexes associated with classical cadherins, the principal architectural proteins at cell-cell junctions in all soft tissues, are mechanosensors. We further discuss the current understanding of the rudiments of a cadherin-based mechanosensing and transduction pathway, which is distinct from the force sensing machinery of cell-ECM adhesions.     

Finite element modeling of arachnid slit sensilla: II. Actual lyriform organs and the face deformations of the individual slits

Arachnid slit sensilla respond to minute strains in the exoskeleton. After having applied Finite Element (FE) analysis to simplified arrays of five straight slits (Hößl et al. J Comp Physiol A 193:445–459, 2007) we now present a computational study of the effects of more subtle natural variations in geometry, number and arrangement of slits on the slit face deformations. Our simulations show that even minor variations in these parameters can substantially influence a slit’s directional response. Using white-light interferometric measurements of the surface deformations of a lyriform organ, it is shown that planar FE models are capable of predicting the principal characteristics of the mechanical responses. The magnitudes of the measured and calculated slit face deformations are in good agreement. At threshold, they measure between 1.7 and 43 nm. In a lyriform organ and a closely positioned loose group of slits, the detectable range of loads increases to approximately 3.5 times the range of the lyriform organ alone. Stress concentration factors (up to ca. 29) found in the vicinity of the slits were evaluated from the models. They are mitigated due to local thickening of the exocuticle and the arrangement of the chitinous microfibers that prevents the formation of cracks under physiological loading conditions.