The nematode Caenorhabditis elegans provides numerous experimental advantages for defining the Keywords mechanosensing osmoregulation; osmotic stress; organic osmolytes; Ste20 kinases; TR
Trang 1Molecular and genetic characterization of osmosensing and signal transduction in the nematode Caenorhabditis elegans
Keith P Choe and Kevin Strange
Departments of Anesthesiology, Pharmacology and Molecular Physiology and Biophysics, Vanderbilt University, Nashville, TN, USA
Introduction
Regulation of intracellular and extracellular solute and
water balance is a fundamental requirement for
meta-zoans The volume of animal cells is regulated by
accu-mulation and loss of inorganic ions, primarily Na+,
K+ and Cl–, and specialized organic solutes termed
organic osmolytes [1,2] The effector mechanisms that
mediate volume regulatory changes in the intracellular
levels of these solutes are generally well-defined [1]
However, little is known about how animal cells detect
volume changes and transduce those signals into
regu-latory responses
Extracellular solute and water balance in animals is maintained by behavioral responses and by the func-tion of the kidney and kidney-like organs, and extra-renal organs, such as the insect hindgut, fish and crustacean gills and the mammalian intestine As with cell volume perturbations, the molecular mechanisms
by which animals detect extracellular osmotic and ionic disturbances are not fully defined In addition, little is known about how the activities of various osmoregulatory solute and water transport pathways are coordinately regulated during osmotic challenges The nematode Caenorhabditis elegans provides numerous experimental advantages for defining the
Keywords
mechanosensing osmoregulation; osmotic
stress; organic osmolytes; Ste20 kinases;
TRP channels; WNK kinases
Correspondence
K Strange, Vanderbilt University Medical
Center, T-4202 Medical Center North,
Nashville, TN 37232-2520, USA
Fax: +1 615 343 3916
E-mail: kevin.strange@vanderbilt.edu
(Received 2 July 2007, accepted 30 August
2007)
doi:10.1111/j.1742-4658.2007.06098.x
Osmotic homeostasis is a fundamental requirement for life In general, the effector mechanisms that mediate cellular and extracellular osmoregulation
in animals are reasonably well defined However, at the molecular level, little is known about how animals detect osmotic and ionic perturbations and transduce them into regulatory responses The nematode Caenorhabd-itis elegansprovides numerous powerful experimental advantages for defin-ing the genes and integrated gene networks that underlie basic biological processes These advantages include a fully sequenced and well-annotated genome, forward and reverse genetic and molecular tractability, and a rela-tively simple anatomy C elegans normally inhabits soil environments where it is exposed to repeated osmotic stress In the laboratory, nema-todes readily acclimate to and recover from extremes of hypertonicity We review recent progress in defining the molecular mechanisms that underlie osmosensing and associated signal transduction in C elegans Some of these mechanisms are now known to be highly conserved Therefore, studies of osmosensing in nematodes have provided, and will undoubtedly continue to provide, new insights into similar processes in more complex organisms including mammals
Abbreviations
dsRNA, double stranded RNA; GCK-3, germinal center kinase-3; GFP, green fluorescent protein; OSR1, oxidative-responsive 1; PASK, proline-alanine-rich Ste20-related kinase (also known as SPAK); RNAi, RNA interference; Ste20, sterile-20; TRP, transient receptor potential; TRPV, TRP-vanilloid; WNK, with-no-lysine (K).
Trang 2molecular bases of physiological processes including
osmoregulation [3,4] Worms have a short life cycle (2–
3 days at 25C), produce large numbers of offspring by
sexual reproduction and can be cultured easily and
inex-pensively in the laboratory Sexual reproduction occurs
by self-fertilization in hermaphrodites or by mating with
males Self-fertilization allows homozygous animals to
breed true and greatly facilitates the isolation and
main-tenance of mutant strains whereas mating with males
allows mutations to be moved between strains The
reproductive and laboratory culture characteristics of
C elegansmake it an exceptionally powerful model
sys-tem for forward genetic analysis Mutagenesis and
genetic screening allows the unbiased identification of
genes underlying a biological process of interest, allows
genes to be ordered into pathways, and can provide
important and novel mechanistic insights into the
molecular structure and function of proteins
In addition to forward genetic tractability, C
ele-gans also has a fully sequenced and well-annotated
genome The genomic sequence and virtually all other
biological data on this organism are assembled in
read-ily accessible public databases (e.g WormBase; http://
www.wormbase.org) Numerous reagents including
mutant worm strains and cosmid and YAC clones
spanning the genome are freely available through
pub-lic resources Creation of transgenic worms is relatively
easy, inexpensive and rapid, requiring little more than
injection of transgenes into the animal’s gonad or
bombardment with DNA-coated microparticles C
ele-gans gene expression can be specifically and potently
targeted for knockdown using RNA interference
(RNAi), either at the single worm level by injection of
double stranded RNA (dsRNA), or at the population
level by feeding worms dsRNA-producing bacteria
Libraries of dsRNA feeding bacteria are now available
that allow for over 90% of the genome to be screened
for a particular phenotype Finally, C elegans is a
highly differentiated animal but is comprised of
< 1000 somatic cells This relatively simple anatomy
greatly facilitates the study of biological processes and
has made it possible to trace the lineage of every adult
cell beginning with the first cell division
C elegansinhabits surface soil and decaying organic
matter that undergoes periodic desiccation Under
lab-oratory conditions, worms can readily survive and
acclimate to extreme osmotic stress [5,6] In addition,
these animals have well developed sensory mechanisms
that allow them to detect and avoid hypertonic
envi-ronments [7] Our goal in this minireview is to
summa-rize what is currently known about the molecular
mechanisms of osmotic stress resistance, osmosensing
and signal transduction in C elegans
Osmoregulatory organs in C elegans
C elegans has simple ‘kidney’ that is comprised of three cells, the excretory cell, the duct cell and the pore cell [8] Destruction of any of these cells by laser abla-tion causes the animal to swell with fluid and die [9] The excretory cell is a large, H-shaped cell that sends out processes both anteriorly and posteriorly from the cell body A fluid-filled excretory canal is sur-rounded by the cell cytoplasm The basal pole of the cell faces either the pseudocoel or hypodermis whereas the apical membrane faces the excretory canal lumen
An excretory duct connects the excretory canal to the outside surface of the worm The duct is formed from cuticle that is continuous with the animal’s exoskele-ton A duct cell surrounds the upper two-thirds of the duct and a pore cell surrounds the lower third
The excretory cell is a single-cell ‘epithelium’ that secretes solutes and water into the excretory canal Duct cells may also play an important role in solute and water transport The apical surface area of duct cells is greatly amplified by extensive invaginations and the cytoplasm is filled with mitochondria, suggesting that it may be involved in solute transport, possibly selective solute reabsorption [8]
The worm ‘skin’ or hypodermis is an epithelium that underlies a thick cuticle composed of collagens Gap junctions connect the excretory cell to the hypodermis, suggesting an interaction between the two cell types important for whole animal osmoregulation In addi-tion, several studies have suggested a role for the hypodermis in osmoregulation For example, a recent study suggests that fibroblast growth factor signaling
in the hypodermis regulates whole animal fluid balance [10]
The intestine of adult C elegans is comprised of 20 epithelial cells that function in digestion and nutrient absorption In addition, the intestine is in direct con-tact with the external environment and thus could readily mediate osmoregulatory exchanges of solutes and water Recent studies support the notion that the intestine plays an important role in whole animal osmoregulation [11,12]
Behavioral avoidance of osmotic stress
Nematodes sense the external environment in part through a pair of openings in the cuticle on their head termed amphids Eight neurons associate directly with each amphid pore and contact the external environ-ment via dendrites that terminate in sensory cilia Another four sensory neurons have dendrites that associate with a support cell termed the amphid sheath
Trang 3cell Axons extend from the cell bodies of the amphid
neurons to the central nervous system where they
make synaptic contacts with other neurons Laser
abla-tion studies have demonstrated that the amphid
neu-rons function in thermosensation, chemosensation and
mechanosensation [13]
C elegansis attracted to low concentrations of salts,
sugars and other chemicals However, at high
concen-trations, worms initiate an avoidance response to these
substances Culotti and Russell [7] concluded that the
repellent effect of these solutes is due to hypertonicity
rather than to the solutes themselves Using forward
genetic analysis, Culotti and Russell [7] identified osm
mutants that were osmotic avoidance defective
osm-9 mutants are defective in their ability to detect
not only hypertonic media, but also mechanical
pertur-bation (nose touch), volatile repellents and chemical
attractants [14,15] The OSM-9 protein is a 937-amino
acid member of the transient receptor potential (TRP)
family of cation channels TRP channels are divided
into TRPC, TRPV, TRPM, TRPML, TRPP, TRPN
and TRPA subfamilies All TRP channels are
com-prised of six predicted transmembrane domains and
intracellular N- and C-termini Functional TRP
chan-nels are formed from homomeric or heteromeric
asso-ciation of four TRP subunits TRP channels function
in diverse physiological processes, including sensory
transduction, epithelial transport of Ca2+ and Mg2+,
Ca2+signaling and modulation of membrane potential
[16,17]
OSM-9 was the first TRP-vanilloid (TRPV) channel
to be identified at the molecular level Green
fluores-cent protein (GFP) reporter studies demonstrated that
OSM-9 localizes in the superficial sensory cilia of
amphid neurons where it could directly detect
envi-ronmental hypertonicity, mechanical force, and
chemi-cal attractants and repellents [14] Unfortunately,
OSM-9 has not yet been successfully expressed in a
heterologous system where it can be functionally
characterized However, consistent with the role of
osm-9 in osmosensation and mechanosensation,
mam-malian TRPV4 has been shown to be activated by
hypotonic stress when expressed heterologously [18]
TRPV4 is expressed in circumventricular organs of
the mammalian central nervous system where it
appears to play a role in detecting plasma osmolality
and regulating the secretion of the systemic
osmoregu-latory hormone vasopressin [19] Interestingly, even
though OSM-9 and TRPV4 only share 26% amino
acid identity, TRPV4 rescues the defects in osmotic
avoidance and nose touch behaviors when it is
expressed in the amphid neurons of osm-9 mutant
worms [20] Mutations in TRPV4 that affect its
functioning as an ion channel eliminate or reduce its ability to rescue, indicating that cation flux through the channel is likely the proximal signal that mediates osmosensing and mechanosensing [20] The mechanism
by which OSM-9 and TRPV4 detect hypertonic media and nose touch are unclear However, it is possible that the channels are regulated via mechanical forces transmitted directly through the lipid bilayer and⁄ or cytoskeletal attachments
ocr-2, odr-3 and osm-10 are also expressed in amp-hid sensory neurons and are essential for the osmotic avoidance behavior OCR-2 is another TRPV family member and it colocalizes with OSM-9 in amphid neuron sensory cilia [21] Interestingly, both proteins must be present for either of them to localize prop-erly, suggesting that OSM-9 and OCR-2 form a het-eromeric channel and⁄ or function in a multiprotein signaling complex [21] odr-3 encodes a Ga protein and null mutations in this gene prevent detection of nose touch, volatile repellents and hypertonicity [22] These phenotypes are very similar to those of osm-9 mutants, suggesting that ODR-3 may function with the channel to regulate the response to external stim-uli osm-10 encodes a novel cytosolic protein that is essential for detection of hypertonicity, but not nose touch or volatile repellents [23], suggesting that it may be involved in discriminating osmotic from other stimuli that activate OSM-9 and⁄ or OCR-2 Although more work is needed to understand if and how OSM-9, OCR-2, ODR-3 and OSM-10 function together to mediate osmotic avoidance, studies on these proteins illustrate the power of forward genetic analysis in C elegans as a means to identify rapidly and in an unbiased manner genes involved in osmo-sensing
Protein damage triggers organic osmolyte accumulation
Measurements of internal osmolality have not been made on C elegans because of the animal’s small size However, other nematodes that have been stud-ied are hyper-osmoconformers that maintain an inter-nal osmolality slightly higher than that of the environment [24] This is not surprising considering that nematodes lack a rigid skeleton Instead, internal turgor or hydrostatic pressure inflates a flexible cuticle and gives the animal rigidity that is necessary for locomotion
The soil and decaying organic environments of
C elegans are osmotically unstable and the animal undoubtedly experiences periods of desiccation and rehydration in its native habitat In the laboratory,
Trang 4worms readily acclimate to extreme hypertonic stress
[5,6] When exposed to hypertonic media, C elegans
rapidly loses water and up to 50% of its body volume
If the shrinkage is severe, worms become paralyzed
due to loss of turgor pressure [5,25] (Fig 1A) Worms
exposed to nonlethal hypertonic stress recover their
volume within a few hours by yet to be characterized
mechanisms During long-term exposure to
hypertonic-ity, C elegans synthesizes and accumulates large
quan-tities of the compatible organic osmolyte glycerol
(Fig 1B) [5,11]
Signaling pathways that regulate organic osmolyte
accumulation in animal cells are poorly defined To
begin identifying the signaling mechanisms that
regu-late glycerol synthesis, we performed a genome-wide
RNA interference screen for genes that regulate
osmosensitive gene expression [11] Expression of the gene gpdh-1, encoding glycerol-3-phosphate dehydro-genase-1, an enzyme that catalyzes a rate-limiting step of glycerol synthesis, increases dramatically in
C elegans following exposure to hypertonic stress [5] (Fig 1B) To assess gpdh-1 expression in vivo, we generated a strain of worms that expresses GFP dri-ven by the gene’s promoter GFP expression in this strain is virtually undetectable unless the worms are exposed to hypertonic media Using this reporter strain and a library of RNAi feeding bacteria [26],
we screened for gene knockdowns that activate gpdh-1 This screen identified 122 genes whose knockdown induced gpdh-1 expression in the absence
of hypertonic stress These genes are termed regula-tors of glycerol-3-phosphate-dehydrogenase (rgpd) expression [11]
rgpd gene functions fell into six defined categories; extracellular matrix, signaling, metabolism, protein trafficking, transcriptional regulation and protein homeostasis, as well as a group of genes with unas-signed functions Interestingly, genes predicted to func-tion in cellular protein homeostasis are the largest class of rgpd genes (54 out of 122 total rgpd genes) These include genes that encode proteins required for RNA processing, protein synthesis, protein folding and protein degradation Knockdown of these genes is expected to increase intracellular levels of damaged and denatured proteins Damaged proteins in turn act
as a signal to activate glycerol accumulation (Fig 2) Glycerol functions to stabilize protein structure and its accumulation would allow cells to lower intracellular ionic strength, which can disrupt protein synthesis and folding [11]
Interestingly, protein damage caused by other stres-sors, such as heat shock and oxidative stress, do not activate gpdh-1 expression, demonstrating that osmotic stress causes a form of protein damage that selectively induces glycerol accumulation [11] Osmotic stress has been shown previously to disrupt new protein synthesis [27,28], which likely results in the accumulation of incomplete and aberrantly folded polypeptides in the cytoplasm The majority of the protein homeostasis genes identified in our RNAi screen function in RNA processing, protein synthesis and cotranslational pro-tein folding [11] These observations are consistent with a model in which gpdh-1 expression is specifically activated by osmotically induced disruption of new protein synthesis and cotranslational folding rather than by denaturation of existing proteins (Fig 2) Such
a mechanism would allow cells to discriminate between osmotically induced and other forms of stress-induced protein damage [11]
Hours in hypertonic media
365 m M NaCl
51 m M NaCl
A
B
0 6 12 18 24 30 36 42 48
0
2
4
6
8
10
0 100 200 300 400 500 600 700 800
Hours in hypertonic media
Fig 1 Response of C elegans to acute and chronic hypertonic
stress (A) Images of a single worm on agar containing 51 m M NaCl
and after acute transfer to agar containing 365 m M NaCl Note that
the worm initially shrinks and becomes paralyzed Complete
vol-ume recovery occurs within 2–3 h and full mobility is regained.
Scale bar ¼ 200 lm (B) Changes in gpdh-1 expression and whole
animal glycerol levels in worms exposed to hypertonic stress.
gpdh-1 expression was assessed using a GFP reporter driven by
the gene’s promoter Data are replotted from Lamitina et al [5,11].
Trang 5Role of cuticle collagens in regulating
organic osmolyte accumulation
Four of the genes identified in our RNAi screen
encode the collagens DPY-7, DPY-8, DPY-9 and
DPY-10, which play important roles in formation of
the C elegans cuticle Loss-of-function mutations in
these genes induce gpdh-1 expression [11] and glycerol
accumulation [11,29] Mutations in dpy genes also
cause a short and fat, dumpy phenotype, which is
thought to reflect changes in cuticle structure [30]
DPY collagens are secreted proteins and given
their role in cuticle formation, they almost certainly
function extracellularly to regulate glycerol
accumula-tion Interestingly, our RNAi screen of gpdh-1
expression also identified ten genes that are predicted
to encode secreted proteins [11] Mutant alleles of
three of these genes, osr-1, osm-7 and osm-11, have
been characterized and shown to cause constitutive
accumulation of glycerol [25,29] osr-1 and osm-7 are
expressed in the hypodermis [25,29] Epistasis
analy-sis suggests that OSR-1 functions with
calmodulin-dependent protein-kinase II and a mitogen-activated
protein kinase cascade to regulate glycerol
accumula-tion [25,29]
Collagens and other secreted proteins are essential
components of a C elegans mechanosensorory
com-plex that detects tactile stimuli [31] Similarly, DPY
collagens and OSR-1, OSM-7 and OSM-11 could
func-tion in the cuticle to detect and transduce hypertonic
stress-induced mechanical signals that regulate glycerol
accumulation Further characterization of secreted
proteins that regulate gpdh-1 expression will help define their role in osmosensing
With-no-lysine (K) (WNK) and Ste20 kinases regulate hypertonic stress responses
C elegansgerminal center kinase-3 (GCK-3) is a mem-ber of the GCK-VI subfamily of sterile-20 (Ste20) ser-ine-threonine protein kinases that includes vertebrate oxidative-responsive 1 (OSR1) and proline-alanine-rich Ste20-related kinase (PASK; also known as SPAK) [32] GCK-VI kinases are expressed in transporting epithelia [33] We recently demonstrated that GCK-3 binds to and regulates a cell volume sensitive ClC Cl– channel in C elegans [34] Mammalian OSR1 and PASK bind to and phosphorylate members of the SLC12 cation-Cl– cotransporter family in response to cell volume changes [35] Taken together, these data suggest that GCK-VI kinases may play a role in cell and systemic osmoregulation [36]
We recently examined the role of GCK-3 in whole animal osmotic homeostasis [12] GFP reporter analy-sis demonstrated that gck-3 is expressed in multiple locations including osmoregulatory tissues Systemic RNAi of gck-3 almost completely prevents acute vol-ume recovery and chronic survival in 400 mm NaCl, demonstrating that the kinase is essential for systemic osmoregulation Using tissue-specific RNAi, we also demonstrated that GCK-3 functions in the hypodermis and intestine to mediate acute volume recovery and survival during hypertonic stress These two tissues are
Fig 2 Model for regulation of gpdh-1 expression by disruption of protein homeo-stasis Hypertonic stress induced water loss causes elevated cytoplasmic ionic strength, which in turn disrupts new protein synthesis and cotranslational protein folding Damaged proteins function as a signal that activates gpdh-1 expression and glycerol synthesis Glycerol replaces inorganic ions in the cyto-plasm and functions as a chemical chaper-one that aids in the refolding of misfolded proteins Loss of function of protein homeo-stasis genes also causes accumulation of damaged proteins and activation of gpdh-1 expression Green arrows and red lines indi-cate activation and inhibition, respectively.
Trang 6in contact with the environment and likely mediate
osmoregulatory exchanges of ions and water We
pro-pose that GCK-3 regulates ion and water uptake
mechanisms in these two tissues to mediate acute
sys-temic volume recovery following water loss and
shrink-age (Fig 3) After volume recovery, accumulation of
the compatible osmolyte glycerol (see above) replaces
inorganic ions absorbed during acute volume recovery
[11]
Interestingly, survival of gck-3(RNAi) worms is
much lower in animals exposed to high NaCl versus
high sorbitol [12] Acute volume recovery was similar
in the presence of the two solutes This suggests that
in addition to regulating solute and water uptake
mechanisms required for acute volume recovery,
GCK-3 may also regulate transport processes
responsi-ble for excretion of a chronic NaCl load
Using a yeast two-hybrid screen, we identified
WNK-1 as a protein that physically interacts with
GCK-3 [12] WNK protein kinases are serine⁄
threo-nine protein kinases that lack a conserved lysine
resi-due in the catalytic domain [37] Humans have four
WNK kinases and rare mutations in WNK1 and
WNK4 cause an autosomal dominant form of
hyper-tension [38] WNK1 and WNK4 control blood
pressure by regulating the activity of ion transport pathways that mediate salt transport in distal renal tubules [39] Several recent studies have demonstrated that WNK1 and WNK4 bind to, phosphorylate, and activate PASK and OSR1 [39] In C elegans, systemic RNAi of WNK-1 decreases acute volume recovery and survival in a manner qualitatively similar to GCK-3 RNAi The effects of RNAi for WNK-1 and GCK-3
in the same worms are not additive, suggesting that the kinases function in a common pathway to regulate systemic osmoregulation We hypothesize that WNK-1 functions upstream from GCK-3 in a manner similar
to that proposed for its mammalian homologues (Fig 3) [39] Almost nothing is known about what functions upstream from WNK kinases to sense osmo-tic stress in animals Our study provides the founda-tion for genetic analysis of the WNK-1⁄ GCK-3 pathway that regulates hypertonic stress responses
Conclusions and future perspectives
C elegans has proven to be an exceptionally powerful model system for defining the genes and gene networks that underlie basic biological processes such as devel-opment, neural function and sensory physiology The worm normally inhabits osmotically unstable soil environments and is thus well-suited for studies of osmosensing and associated signal transduction mecha-nisms Little is known about the molecular bases of these processes in animals The forward and reverse genetic and molecular tractability of C elegans has already provided unique insights into TRP channel physiology, regulation of organic osmolyte accumula-tion and WNK and Ste20 kinase signaling C elegans will undoubtedly continue to provide new understand-ing of how animals detect and protect themselves from osmotic stress Given the fundamental and conserved nature of osmotic stress resistance, studies on C ele-gans will clearly provide new and important insights that are applicable to all animals
Acknowledgements
This work was supported by NIH grants DK61168 and DK51610 K.C was supported by NIH NRSA GM77904
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