Mechanotransduction in Caenorhabditis elegans: the role of DEG/ENaC ion channels

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 Caenorhabditis elegans members of this family called degenerins and their role in mediating mechanosensitive behaviors in the worm.

Necrotic cell death in C. elegans requires the function of calreticulin and regulators of Ca(2+) release from the endoplasmic reticulum

In C. elegans, a hyperactivated MEC-4(d) ion channel induces necrotic-like neuronal death that is distinct from apoptosis. We report that null mutations in calreticulin suppress both mec-4(d)-induced cell death and the necrotic cell death induced by expression of a constitutively activated Galpha(S) subunit. RNAi-mediated knockdown of calnexin, mutations in the ER Ca(2+) release channels unc-68 (ryanodine receptor) or itr-1 (inositol 1,4,5 triphosphate receptor), and pharmacological manipulations that block ER Ca(2+) release also suppress death. Conversely, thapsigargin-induced ER Ca(2+) release can restore mec-4(d)-induced cell death when calreticulin is absent. We conclude that high [Ca(2+)](i) is a requirement for necrosis in C. elegans and suggest that an essential step in the death mechanism is release of ER-based Ca(2+) stores. ER-driven Ca(2+) release has previously been implicated in mammalian necrosis, suggesting necrotic death mechanisms may be conserved.

Degenerins. At the core of the metazoan mechanotransducer?

Mechanosensory signaling, believed to be mediated by mechanically gated ion channels, constitutes the basis for the senses of touch and hearing, and contributes fundamentally to the development and homeostasis of all organisms. Despite this profound importance in biology, little is known of the molecular identities or functional requirements of mechanically gated ion channels. Genetic analyses of touch sensation and locomotion in Caenorhabditis elegans have implicated a new class of ion channels, the degenerins (DEG) in nematode mechanotransduction. Related fly and vertebrate proteins, the epithelial sodium channel (ENaC) family, have been implicated in several important processes, including transduction of mechanical stimuli, pain sensation, gametogenesis, sodium reabsorption, and blood pressure regulation. Still-to-be-discovered DEG/ENaC proteins may compose the core of the elusive human mechanotransducer.

Analysis of programmed cell death in the nematode Caenorhabditis elegans

The nematode Caenorhabditis elegans has been shown to be an excellent model organism with which to study the mechanisms of programmed cell death because of its powerful genetics and the ability to study cell death with single-cell resolution. In this chapter, we describe methods that are commonly used to examine various aspects of programmed cell death in C. elegans. These methods, in combination with genetic analyses, have helped identify and characterize many components of the C. elegans cell death pathway, illuminating the mechanisms by which these components affect programmed cell death.

A common set of engulfment genes mediates removal of both apoptotic and necrotic cell corpses in C. elegans

Similar to mammalian excitotoxic cell death, necrotic-like cell death (NCD) in Caenorhabditis elegans can be initiated by hyperactive ion channels. Here we investigate the requirements for genes that execute and regulate programmed cell death (PCD) in necrotic-like neuronal death caused by a toxic MEC-4 channel. Neither the kinetics of necrosis onset nor the total number of necrotic corpses generated is altered by any C. elegans mutation known to block PCD, which provides genetic evidence that the activating mechanisms for NCD and apoptotic cell death are distinct. In contrast, all previously reported ced genes required for phagocytotic removal of apoptotic corpses, as well as ced-12, a new engulfment gene we have identified, are required for efficient elimination of corpses generated by distinct necrosis-inducing stimuli. Our results show that a common set of genes acts to eliminate cell corpses irrespective of the mode of cell death, and provide the first identification of the C. elegans genes that are required for orderly removal of necrotic cells. As phagocytotic mechanisms seem to be conserved from nematodes to humans, our findings indicate that injured necrotic cells in higher organisms might also be eliminated before lysis through a controlled process of corpse removal, a hypothesis that has significant therapeutic implications.

Healthcare provider characteristics and perceived confidence from HIV/AIDS education

This study reports findings from six training projects designed to keep health providers up-to-date on emerging developments and approaches in HIV/AIDS care. Participants were 3,779 individuals who described themselves, their professional background, and their specific experience in the HIV/AIDS field. These characteristics were compared with their self-reported confidence in managing clients, counseling clients, providing services, and the training topics. A repeated-measures design examining level and change of confidence showed little support for links between provider characteristics and confidence due to HIV/AIDS training experience. Thus, knowing a provider's background does not necessarily provide diagnostic information about who might most benefit in improved confidence from HIV/AIDS educational training. These results suggest that HIV/AIDS training programs may be targeted broadly-to a wide range of healthcare providers of diverse backgrounds-with little or no impact on overall levels and changes in provider confidence.

Do characteristics of HIV/AIDS education and training affect perceived training quality? Lessons from the evaluation of seven projects

Initial and continuing HIV/AIDS education and training has been a critical way to bring the nation's health providers up to date on emerging developments and approaches. This study reports cross-cutting findings from seven HIV/AIDS education and training projects. Trainers described over 600 training sessions from these projects in terms of their structural characteristics and design elements, while trainees described these sessions on several dimensions related to training quality. Training characteristics were compared to trainee assessments of training quality. Using a decision-tree analytic approach for major training attributes, considerable support emerged for links between training characteristics and perceived quality of the HIV/AIDS training experience. More favorable quality ratings were associated with certain projects, the training setting, the types of trainees served by the training, the intended training impact, discussion of special populations, and training methods involving interactive learning. With increased knowledge regarding how these educational experiences relate to the ways they are perceived and processed, more targeted approaches to training design on HIV/AIDS can be developed.

Trainee characteristics and perceptions of HIV/AIDS training quality

HIV/AIDS education and training have played a vital role in keeping health providers up to date on emerging developments and approaches. This study reports findings from seven HIV/AIDS education and training projects. Participants in more than 600 training sessions described themselves, their professional background, and their general reasons for taking the training. Immediately following the training, they also rated the quality of their educational experience along several dimensions. Trainee characteristics were related to assessments of training quality, using a regression decision-tree analytic approach. Although effect sizes were generally small, quality ratings of the HIV/AIDS training experiences were associated with certain projects, basic trainee demographic characteristics, professional background, and experience in the HIV field. Greater understanding about participant characteristics can provide clues about how these training experiences are perceived and processed and may inform decision making about instructional HIV/AIDS curricula.

Effects of HIV/AIDS education and training on patient care and provider practices: a cross-cutting evaluation

Initial and continuing training in HIV/AIDS service provision is a critical way to enable the nation's health providers to use state-of-the-art developments and perspectives. Typically, the efficacy of HIV/AIDS training programs is evaluated using assessments administered to trainees immediately following the training. This study reports cross-cutting findings from telephone interviews conducted with 218 trainees an average of 8 months after training. Long-term training effects are examined in three domains: (a) general perspectives on HIV/AIDS; (b) health care provider service provision; and (c) changes in procedures and operations at the health care system level. The findings show the different ways that the training experience had long-term positive and observable effects in these three domains. In some cases, background characteristics and job positions predicted the specific type of reported training effects. The pattern of results suggests ways in which training methods can be targeted to specific audiences.

In vivo structure-function analyses of Caenorhabditis elegans MEC-4, a candidate mechanosensory ion channel subunit

Mechanosensory signaling mediated by mechanically gated ion channels constitutes the basis for the senses of touch and hearing and contributes fundamentally to the development and homeostasis of all organisms. Despite this profound importance in biology, little is known of the molecular identities or functional requirements of mechanically gated ion channels. We report a genetically based structure-function analysis of the candidate mechanotransducing channel subunit MEC-4, a core component of a touch-sensing complex in Caenorhabditis elegans and a member of the DEG/ENaC superfamily. We identify molecular lesions in 40 EMS-induced mec-4 alleles and further probe residue and domain function using site-directed approaches. Our analysis highlights residues and subdomains critical for MEC-4 activity and suggests possible roles of these in channel assembly and/or function. We describe a class of substitutions that disrupt normal channel activity in touch transduction but remain permissive for neurotoxic channel hyperactivation, and we show that expression of an N-terminal MEC-4 fragment interferes with in vivo channel function. These data advance working models for the MEC-4 mechanotransducing channel and identify residues, unique to MEC-4 or the MEC-4 degenerin subfamily, that might be specifically required for mechanotransducing function. Because many other substitutions identified by our study affect residues conserved within the DEG/ENaC channel superfamily, this work also provides a broad view of structure-function relations in the superfamily as a whole. Because the C. elegans genome encodes representatives of a large number of eukaryotic channel classes, we suggest that similar genetic-based structure-activity studies might be generally applied to generate insight into the in vivo function of diverse channel types.

Epithelial Na+ channels and stomatin are expressed in rat trigeminal mechanosensory neurons

Caenorhabditis elegans MEC-4 and MEC-10 are subunits of the degenerin/epithelial Na+ channel (DEG/ENaC) ion channel superfamily thought to be associated with MEC-2 (a stomatin-like protein) in a mechanotransducing molecular complex in specialized touch sensory neurons. A key question is whether analogous molecular complexes in higher organisms transduce mechanical signals. To address this question, we selected mechanoreceptors of the rat vibrissal follicle-sinus complex in the mystacial pad and the trigeminal ganglia for an immunocytochemical and molecular biological study. RT-PCR of poly(A+) mRNA of rat trigeminal ganglia indicated that alpha-, beta-, and gamma-ENaC and stomatin mRNA are expressed in rat trigeminal ganglia. Using immunocytochemistry, we found that alpha-, beta-, and gamma-ENaC subunits and stomatin are localized in the perikarya of the trigeminal neurons and in a minor fraction of their termination site in the vibrissal follicle-sinus complex, where longitudinal lanceolate endings are immunopositive. We conclude that alpha-, beta-, and gamma-ENaC subunits as well as the candidate interacting protein stomatin are coexpressed in a mammalian mechanoreceptor, a location consistent with a possible role in mechanotransduction.

Heritable and inducible genetic interference by double-stranded RNA encoded by transgenes

Double-stranded RNA interference (RNAi) is an effective method for disrupting expression of specific genes in Caenorhabditis elegans and other organisms. Applications of this reverse-genetics tool, however, are somewhat restricted in nematodes because introduced dsRNA is not stably inherited. Another difficulty is that RNAi disruption of late-acting genes has been generally less consistent than that of embryonically expressed genes, perhaps because the concentration of dsRNA becomes lower as cellular division proceeds or as developmental time advances. In particular, some neuronally expressed genes appear refractory to dsRNA-mediated interference. We sought to extend the applicability of RNAi by in vivo expression of heritable inverted-repeat (IR) genes. We assayed the efficacy of in vivo-driven RNAi in three situations for which heritable, inducible RNAi would be advantageous: (i) production of large numbers of animals deficient for gene activities required for viability or reproduction; (ii) generation of large populations of phenocopy mutants for biochemical analysis; and (iii) effective gene inactivation in the nervous system. We report that heritable IR genes confer potent and specific gene inactivation for each of these applications. We suggest that a similar strategy might be used to test for dsRNA interference effects in higher organisms in which it is feasible to construct transgenic animals, but impossible to directly or transiently introduce high concentrations of dsRNA.

Caenorhabditis elegans degenerins and vertebrate ENaC ion channels contain an extracellular domain related to venom neurotoxins

The DEG/ENaC (DEGenerin/Epithelial Na+ Channel) superfamily includes closely related ion channel subunits from divergent species ranging from the simple nematode Caenorhabditis elegans to humans. Members of this protein group play roles in several important processes including transduction of mechanical stimuli, sodium re-absorption and blood pressure regulation. Structure/function relationships in members of this superfamily are just beginning to be elaborated. Using a bio-informatics approach, we identified a novel structural element in the extracellular region of DEG/ENaC proteins that exhibits significant similarity to venom neurotoxins. Since venom neurotoxins bind to sodium channels at high affinity, we suggest that the related domain embedded in DEG/ENaC channels may interact with other regions of the channel or channel complex to modulate channel function.

Systems change resulting from HIV/AIDS education and training. A cross-cutting evaluation of nine innovative projects

An evaluation of nine diverse HIV/AIDS training programs assessed the degree to which the programs produced changes in the ways that health care systems deliver HIV/AIDS care. Participants were interviewed an average of 8 months following completion of training and asked for specific examples of a resulting change in their health care system. More than half of the trainees gave at least one example of a systems change. The examples included the way patient referrals are made, the manner in which agency collaborations are organized, and the way care is delivered.

UNC-4/UNC-37-dependent repression of motor neuron-specific genes controls synaptic choice in Caenorhabditis elegans

The UNC-4 homeoprotein and the Groucho-like corepressor UNC-37 specify synaptic choice in the Caenorhabditis elegans motor neuron circuit. In unc-4 mutants, VA motor neurons are miswired with inputs from interneurons normally reserved for their lineal sisters, the VB motor neurons. Here we show that UNC-4 and UNC-37 function together in VA motor neurons to repress VB-specific genes and that this activity depends on physical contact between UNC-37 and a conserved Engrailed-like repressor domain (eh1) in UNC-4. Missense mutations in the UNC-4 eh1 domain disrupt interactions between UNC-4 and UNC-37 and result in the loss of UNC-4-dependent repressor activity in vivo. A compensatory amino acid substitution in UNC-37 suppresses specific unc-4 alleles by restoring physical interactions with UNC-4 as well as UNC-4-dependent repression of VB-specific genes. We propose that repression of VB-specific genes by UNC-4 and UNC-37 is necessary for the creation of wild-type inputs to VA motor neurons. The existence of mammalian homologs of UNC-4 and UNC-37 indicates that a similar mechanism could regulate synaptic choice in the vertebrate spinal cord.

DEG/ENaC channels: a touchy superfamily that watches its salt

To the surprise of many, studies of molecular mechanisms of touch transduction and analyses of epithelial Na+ transport have converged to define a new class of ion channel subunits. Based on the names of the first two identified subfamilies, the Caenorhabditis elegans degenerins and the vertebrate epithelial amiloride-sensitive Na+ channel, this ion channel class is called the DEG/ENaC superfamily. Members of the DEG/ENaC superfamily have been found in nematodes, flies, snails, and vertebrates. Family members share common topology, such that they span the membrane twice and have intracellular N- and C-termini; a large extracellular loop includes a conserved cysteine-rich region. DEG/ENaC channels have been implicated a broad spectrum of cellular functions, including mechanosensation, proprioception, pain sensation, gametogenesis, and epithelial Na+ transport. These channels exhibit diverse gating properties, ranging from near constitutive opening to rapid inactivation. We discuss working understanding of DEG/ENaC functions, channel properties, structure/activity correlations and possible evolutionary relationship to other channel classes.

A nationwide survey of financing health-related services for special education students

The Individuals with Disability Education Act (IDEA) requires state educational systems to provide school-based, health related services (RS). This survey determined the financing arrangements used by states for health-related services for school-aged children with disabilities. A survey was sent to directors of special education, Medicaid, and public health departments in each of the 50 states. Financial patterns for RS were sought at the state level for children ages 3-21 with disabilities for the 1993-1994 school year, the most recent year for which complete financial data were available. Univariate analyses probed the relationship between systems' variables and the extent of Medicaid usage by local education agencies. Respondents reported that schools tapped traditional health resources to supplement educational dollars in paying for related services in schools. Medicaid was by far the most common source with 29 states reporting established mechanisms for recouping Medicaid dollars and 10 states reporting phase-in activities. Seventeen states reported that departments of public health played some role in administration, training, and demonstrations, but only six states provided specific dollars for related services through the department. Use of private insurance was reported sporadically with only one state indicating a specific state-level program. Correlates of increased Medicaid usage were presence of interagency agreements (IAAs) (OR 11.1, p = 0.002), having specific personnel for school-based medical assistance (OR 17.7, p = 0.001), and utilizing school nursing services as a Medicaid optional service (OR 4.2, p = 0.048).

The maternal and child health sites' practices regarding HIV education, counseling, and testing of women of reproductive age in Chicago: barriers to universal implementation

OBJECTIVES

Women of reproductive age are increasingly at risk for human immunodeficiency virus (HIV) infection. Recent advances in reducing perinatal transmission have resulted in official guidelines on universal HIV education, counseling, and voluntary testing of women of reproductive age, especially pregnant women. This study assesses to what extent the maternal child health (MCH) sites are implementing these guidelines with their female patient population (including pregnant women) and examines the barriers that prevent them from implementing these guidelines.

METHOD

The study uses survey data from 92 hospitals and community health centers offering MCH services in Chicago regarding their providers' practices on HIV education, counseling, and testing, implementation of zidovudine (ZDV) therapy to reduce perinatal transmission, and the barriers to implementing these services. In addition, 20 taped in-depth interviews were conducted with experts to examine the barriers to universal implementation.

RESULTS

Almost half (45% of perinatal care and 50% of family planning providers) of the institutions are not consistently offering HIV testing. One-third of those institutions that offer testing are not offering pretest counseling. Thirty-nine percent of the perinatal care providers in these institutions are not providing posttest counseling to HIV-negative women. Over one-third (35%) of these institutions reported that they are not set up to implement ZDV therapy during labor and delivery. Almost half (49%) had no protocols for ZDV therapy in place. Barriers to implementation included lack of provider training, limited staff time, physician resistance, unavailability or avoidance to seek perinatal care by high-risk women, cost, absence of a statewide and hospital-specific plan, lack of reproductive choice focus in posttest counseling, lack of provider knowledge about the administration of ZDV or its availability during labor, and lack of consumer education on perinatal risk reduction.

CONCLUSIONS

MCH sites and their providers need assistance to overcome many barriers they face to implement universal HIV education, counseling, and testing of women of reproductive age.