In vivo imaging of C. elegans mechanosensory neurons demonstrates a specific role for the MEC-4 channel in the process of gentle touch sensation

Genetic Models of Mechanotransduction: The NematodeCaenorhabditis elegans

Mechanotransduction, the conversion of a mechanical stimulus into a biological response, constitutes the basis for a plethora of fundamental biological processes such as the senses of touch, balance, and hearing and contributes critically to development and homeostasis in all organisms. Despite this profound importance in biology, we know remarkably little about how mechanical input forces delivered to a cell are interpreted to an extensive repertoire of output physiological responses. Recent, elegant genetic and electrophysiological studies have shown that specialized macromolecular complexes, encompassing mechanically gated ion channels, play a central role in the transformation of mechanical forces into a cellular signal, which takes place in mechanosensory organs of diverse organisms. These complexes are highly efficient sensors, closely entangled with their surrounding environment. Such association appears essential for proper channel gating and provides proximity of the mechanosensory apparatus to the source of triggering mechanical energy. Genetic and molecular evidence collected in model organisms such as the nematode worm Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse highlight two distinct classes of mechanically gated ion channels: the degenerin (DEG)/epithelial Na+channel (ENaC) family and the transient receptor potential (TRP) family of ion channels. In addition to the core channel proteins, several other potentially interacting molecules have in some cases been identified, which are likely parts of the mechanotransducing apparatus. Based on cumulative data, a model of the sensory mechanotransducer has emerged that encompasses our current understanding of the process and fulfills the structural requirements dictated by its dedicated function. It remains to be seen how general this model is and whether it will withstand the impiteous test of time.

Genetic methods for illuminating the function of neural circuits

Guided by the notion that biology itself offers some of the most incisive tools for studying biological systems, neurophysiologists rely increasingly on cell biological mechanisms and materials encoded in DNA to visualize and control the activity of neurons in functional circuits. Optical reporter proteins can broadcast the operational states of genetically designated cells and synapses; remote-controlled effectors can suppress or induce electrical activity. Many challenges, however, remain. These include the development of novel gene expression systems that target reporters and effectors to functionally relevant neuronal ensembles, the capacity to monitor and manipulate multiple populations of neurons in parallel, the ability to observe and elicit precisely timed action potentials, and the power to communicate with genetically designated target neurons through electromagnetic signals other than light.

In vivo monitoring of Ca(2+) uptake into mitochondria of mouse skeletal muscle during contraction

Although the importance of mitochondria in patho-physiology has become increasingly evident, it remains unclear whether these organelles play a role in Ca(2+) handling by skeletal muscle. This undefined situation is mainly due to technical limitations in measuring Ca(2+) transients reliably during the contraction-relaxation cycle. Using two-photon microscopy and genetically expressed "cameleon" Ca(2+) sensors, we developed a robust system that enables the measurement of both cytoplasmic and mitochondrial Ca(2+) transients in vivo. We show here for the first time that, in vivo and under highly physiological conditions, mitochondria in mammalian skeletal muscle take up Ca(2+) during contraction induced by motor nerve stimulation and rapidly release it during relaxation. The mitochondrial Ca(2+) increase is delayed by a few milliseconds compared with the cytosolic Ca(2+) rise and occurs both during a single twitch and upon tetanic contraction.

Hemolytic disease due to membrane ion channel disorders

PURPOSE OF REVIEW

To summarize recent findings in the study of the 'hereditary stomatocytoses and allied disorders', diseases in which the red cell membrane leaks Na and K, disturbing the osmotic homeostasis of the cell.

RECENT FINDINGS

Recent work has emphasized the diversity of these conditions, especially evident in the variations in temperature dependence of the cation leak. The association between the dehydrated, xerocytic form that maps to chromosome 16, with perinatal ascites is confirmed. Two cases that may represent a new hematoneurologic syndrome have been recognized.

SUMMARY

These leaky-membrane diseases fall into three main categories. The 'dehydrated' or xerocytic form maps to chromosome 16 and shows a minimal leak, and can show an excess of phosphatidylcholine in the membrane. Some of these xerocytic cases show a syndrome of self-limiting perinatal ascites of unknown cause. A second group shows very variable temperature dependence in the cation leak. The most severe 'overhydrated' form shows very leaky cells and the 32 kD stomatin protein is missing, although the gene is not mutated. This deficiency seems to be the result of a trafficking problem. The protein is associated with cholesterol and sphingomyelin-rich 'rafts' and may be some kind of partner protein for a membrane-bound proteolytic system.

Functional fluorescent Ca2+ indicator proteins in transgenic mice under TET control

Genetically encoded fluorescent calcium indicator proteins (FCIPs) are promising tools to study calcium dynamics in many activity-dependent molecular and cellular processes. Great hopes—for the measurement of population activity, in particular—have therefore been placed on calcium indicators derived from the green fluorescent protein and their expression in (selected) neuronal populations. Calcium transients can rise within milliseconds, making them suitable as reporters of fast neuronal activity. We here report the production of stable transgenic mouse lines with two different functional calcium indicators, inverse pericam and camgaroo-2, under the control of the tetracycline-inducible promoter. Using a variety of in vitro and in vivo assays, we find that stimuli known to increase intracellular calcium concentration (somatically triggered action potentials (APs) and synaptic and sensory stimulation) can cause substantial and rapid changes in FCIP fluorescence of inverse pericam and camgaroo-2.

Winfred Denk and colleagues succeed in generating transgenic mice that express one of two calcium indicators in their cells, creating a valuable tool to study neuronal activity

The fragile X-related Gene Affects the Crawling Behavior of Drosophila Larvae by Regulating the mRNA Level of the DEG/ENaC Protein Pickpocket1

BACKGROUND

Fragile X syndrome is caused by loss-of-function mutations in the fragile X mental retardation 1 (FMR1) gene. How FMR1 affects the function of the central and peripheral nervous systems is still unclear. FMR1 is an RNA binding protein that associates with a small percentage of total mRNAs in vivo. It remains largely unknown what proteins encoded by mRNAs in the FMR1-messenger ribonuclear protein (mRNP) complex are most relevant to the affected physiological processes.

RESULTS

Loss-of-function mutations in the Drosophila fragile X-related (dfmr1) gene, which is highly homologous to the human fmr1 gene, decrease the duration and percentage of time that crawling larvae spend on linear locomotion. Overexpression of DFMR1 in multiple dendritic (MD) sensory neurons increases the time percentage and duration of linear locomotion; this phenotype is similar to that caused by reduced expression of the MD neuron subtype-specific degenerin/epithelial sodium channel (DEG/ENaC) family protein Pickpocket1 (PPK1). Genetic analyses indicate that PPK1 is a key component downstream of DFMR1 in controlling the crawling behavior of Drosophila larvae. DFMR1 and ppk1 mRNA are present in the same mRNP complex in vivo and can directly bind to each other in vitro. DFMR1 downregulates the level of ppk1 mRNA in vivo, and this regulatory process also involves Argonaute2 (Ago2), a key component in the RNA interference pathway.

CONCLUSIONS

These studies identify ppk1 mRNA as a physiologically relevant in vivo target of DFMR1. Our finding that the level of ppk1 mRNA is regulated by DFMR1 and Ago2 reveals a genetic pathway that controls sensory input-modulated locomotion behavior.

Knockout of the ASIC2 channel in mice does not impair cutaneous mechanosensation, visceral mechanonociception and hearing

Mechanosensitive cation channels are thought to be crucial for different aspects of mechanoperception, such as hearing and touch sensation. In the nematode C. elegans, the degenerins MEC-4 and MEC-10 are involved in mechanosensation and were proposed to form mechanosensitive cation channels. Mammalian degenerin homologues, the H(+)-gated ASIC channels, are expressed in sensory neurones and are therefore interesting candidates for mammalian mechanosensors. We investigated the effect of an ASIC2 gene knockout in mice on hearing and on cutaneous mechanosensation and visceral mechanonociception. However, our data do not support a role of ASIC2 in those facets of mechanoperception.

Regulation of the apoptosis-necrosis switch

Execution of the apoptotic program involves a relatively limited number of pathways. According to a general view, these would converge to activate the caspase family of proteases. However, there is increasing evidence that apoptotic-like features can also be found when caspases are inhibited. Moreover, under pathological conditions, apoptosis and nonapoptotic death paradigms are often interwined, which suggest that, in vivo, cells may use diverging execution pathways. Molecular switches between apoptosis and necrosis include adenosine triphosphate-dependent steps in the activation of caspases or steps sensitive to reactive oxygen/nitrogen species. In turn, caspase activation can cause necrosis by promoting ion overload.

Patch clamp study of the UNC-105 degenerin and its interaction with the LET-2 collagen in Caenorhabditis elegans muscle

Degenerins have emerged from genetic studies in Caenorhabditis elegans as candidate mechanically gated amiloride-sensitive ion channels for transducing mechanical stimuli into cellular responses. In C. elegans muscle, the existence of a genetic interaction between the unc-105 degenerin gene and let-2, a gene encoding an alpha2(IV) collagen, raised the possibility that UNC-105 may function as a mechanically gated channel in a stretch receptor complex. However, to date, ion channel activity of UNC-105 has only been recorded in a gain-of-function mutant form in heterologous expression systems. In this study we investigated the in situ properties of UNC-105 using the whole cell configuration of the patch clamp technique on body wall muscle cells from acutely dissected C. elegans. Amiloride was found to be without effect on membrane potential of wild-type muscle cells, suggesting that the UNC-105 degenerin is electrically silent in resting muscle. Hypo-osmotic shocks induced a reversible depolarization of muscle cells but which was not affected by amiloride. Deformation of the cells by applying tension to the filamentous complex on which muscle cells remained attached or by ejecting external solution under pressure failed to induce any change of membrane potential. In gain-of-function unc-105(n506) mutant cells, an amiloride-sensitive inward Na(+) current was found to be constitutively active, leading to maintained muscle depolarization. An associated mutation in the alpha2(IV) collagen LET-2 led to the closure of the mutant UNC-105(n506) channel while a collagenase treatment of these double mutant cells caused it to re-open, giving evidence for a functional interaction between LET-2 collagen and mutant UNC-105 channel.

Serotonin and Go modulate functional states of neurons and muscles controlling C. elegans egg-laying behavior

From nematodes to humans, animals employ neuromodulators like serotonin to regulate behavioral patterns and states. In the nematode C. elegans, serotonin has been shown to act in a modulatory fashion to increase the rate and alter the temporal pattern of egg laying. Though many candidate effectors and regulators of serotonin have been identified in genetic studies, their effects on specific neurons and muscles in the egg-laying circuitry have been difficult to determine. Using the genetically encoded Ca(2+) indicator cameleon, we found that serotonin acts directly on the vulval muscles to increase the frequency of Ca(2+) transients. In contrast, we found that the spontaneous activity of the egg-laying motorneurons was silenced by serotonin. Mutations in G protein alpha subunit genes altered the responses of both vulval muscles and egg-laying neurons to serotonin; specifically, mutations in the G(q)alpha homolog egl-30 blocked serotonin stimulation of vulval muscle Ca(2+) transients, while mutations in the G(o)alpha homolog goa-1 prevented the silencing of motorneuron activity by serotonin. These data indicate that serotonin stimulates egg laying by directly modulating the functional state of the vulval muscles. In addition, serotonin inhibits the activity of the motorneurons that release it, providing a feedback regulatory effect that may contribute to serotonin adaptation.

Mechanosensation and pain

The ability of cells to detect and transduce mechanical stimuli impinging on them is a fundamental process that underlies normal cell growth, hearing, balance, touch, and pain. Surprisingly, little research has focused on mechanotransduction as it relates to the sensations of somatic touch and pain. In this article we will review data on the wealth of different mechanosensitive sensory neurons that innervate our main somatic sense organ the skin. The role of different types of mechanosensitive sensory neurons in pain under physiological and pathophysiological conditions (allodynia and hyperalgesia) will also be reviewed. Finally, recent work on the cellular and molecular mechanisms by which mechanoreceptive sensory neurons signal both innocuous and noxious sensation is evaluated in the context of pain.

Invertebrate nociception: Behaviors, neurons and molecules

Genetic analysis of nociceptive behaviors in the nematode Caenorhabditis elegans and the fruit fly Drosophila melanogaster has led to the discovery of conserved sensory transduction channels and signaling molecules. These are embedded in neurons and circuits that generate responses to noxious signals. This article reviews the neurons and molecular mechanisms that underlie invertebrate nociception. We begin with the neurobiology of invertebrate nociception, and then focus on molecules with conserved functions in vertebrate nociception and sensory biology.

Epithelial Na+ channels are activated by laminar shear stress

The degenerin/epithelial Na+ channel (ENaC) superfamily is a group of structurally related ion channels that are involved in diverse biological processes, including responses to mechanical stimuli. In renal cortical collecting ducts, changes in rates of perfusion affect Na+ reabsorption through an amiloride-sensitive pathway, suggesting that ENaC may be a mechanosensitive channel. In this study, we examined whether ENaC expressed in oocytes is regulated by laminar shear stress (LSS). A 1.8-mm (internal diameter) perfusion pipette was placed within 0.5-1.0 mm of the oocyte to provide laminar flow across the oocyte surface. LSS induced a dose-dependent and reversible increase in benzamil-sensitive whole cell Na+ currents in oocytes expressing alphabetagamma ENaC. The half-time for activation by LSS was approximately 5 s. Repetitive stimulation by LSS of oocytes expressing ENaC was associated with a reduction in the response to LSS. Oocytes expressing alphabetaS518Kgamma, a pore region mutant with a high open probability, were insensitive to LSS. We demonstrated previously that channels with a Cys residue introduced at position alphaSer-580 had a low open probability, but, following modification by [2-(trimethylammonium)ethyl]methanethiosulfonate bromide (MTSET), channels exhibited a high open probability. Oocytes expressing alphaS580Cbetagamma ENaC respond to LSS similar to wild type; however, covalent modification by MTSET largely eliminated the response to LSS. Our results suggest that shear stress activates ENaC by modifying the gating properties of the channel.