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and -1ß Perform Nonredundant Roles in Early Vertebrate Development
From the Urological Diseases Research Center,* and the Department of Orthopaedic Surgery, Children’s Hospital Boston, Harvard Medical School; Massachusetts General Hospital; and the Department of Cytokine Biology, The Forsyth Institute, and Department of Developmental Biology, Harvard School of Dental Medicine, Boston, Massachusetts
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and -1ß cDNAs were functionally characterized in the zebrafish. Cav-1 and -1ß mRNAs exhibited overlapping but distinct expression patterns throughout embryogenesis. Targeted depletion of either Cav-1 isoform, using antisense morpholino oligomers, resulted in a substantial loss of caveolae and dramatic neural, eye, and somite defects by 12 hours after fertilization, the time at which mRNA levels of both isoforms substantially increased in wild-type animals. Morphant phenotypes were rescued by injection of homotypic (cav-1/cav-1) but not heterotypic (cav-1/cav-1ß) zebrafish and human cav-1 cRNAs, revealing nonredundant and evolutionarily conserved functions for the individual Cav-1 isoforms. Mutation of a known Cav-1 phosphorylation site unique to Cav-1 (Y14
F) resulted in a failure to rescue the cav-1 morphant phenotype, verifying an essential role for Cav-1 specifically and implicating this residue in early developmental functions. Cav-1 and -1ß morphants also exhibited disruption in the actin cytoskeleton. These results support important and previously unanticipated roles for the Caveolin-1 isoforms in vertebrate organogenesis.
50- to 100-nm plasma membrane invaginations visible in electron micrographs.9
Caveolae are predominantly found in the plasma membrane but also exist as detached vesicles and/or racemose structures that penetrate deep into the cell, forming transcytotic channels.10,11
Caveolae are abundant in endothelia, adipocytes, and muscle cells and have been implicated in diverse cellular processes, including polarized sorting of membrane proteins and regulation of signal transduction.12-14
The invaginated morphology characteristic of caveolae is conferred by homo- or hetero-oligomerization of members of the caveolin family of structural proteins.15,16
Four caveolins have been identified in vertebrates: Cav-1 and -1ß, which arise from differential splicing, and Cav-2 and -3, encoded by separate genes. Cav-1, -1ß, and -2 are present in nonmuscle cells, whereas myocytes express all four caveolins. Caveolins interact with and functionally regulate a number of signaling proteins via direct protein/protein interactions and, consequently, are believed to mediate many processes attributed to caveolar rafts.8
Despite extensive study, the in vivo roles of the caveolin proteins are not well understood. Surprisingly, cav-1-null mice, which lack most caveolae, and cav-1/3-null mice, which have been reported to lack all caveolae, are viable and fertile.17,18
However, cav-null mice do exhibit a variety of moderate pathologies, including hyperplasia of the genitourinary tract, cardiomyopathy and pulmonary hypertension, vascular abnormalities, and reduced lifespan.7,19,20
Very recently, cav-1-null mouse mammary epithelium was found to exhibit defects in three-dimensional acinar architecture21
and, when deprived of exogenous extracellular matrix, to undergo spontaneous epithelial-mesenchymal transition and reorganization of the actin cytoskeleton.21
Targeted depletion of the muscle-specific Cav-3 was found to result in severe defects in muscle differentiation and function.22
The fact that caveolin genes exhibit significant evolutionary conservation suggests important biological functions for these proteins.23-27
To investigate possible roles for the two Cav-1 isoforms in vertebrate development, zebrafish cav-1 and -1ß cDNAs were isolated, the expression profiles of cav-1 mRNAs were defined, and Cav-1 proteins were functionally characterized using a targeted protein depletion approach. Our studies revealed unique developmental functions for zebrafish Cav-1 and -1ß isoforms, including roles in actin cytoskeletal organization, which were rescued by expression of human Cav-1 and -1ß isoforms. We also demonstrate, for the first time in an in vivo context, that the Cav-1 Y14 phosphorylation site, which is a substrate of Src in other vertebrates, is required for Cav-1 function in early zebrafish development.
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Wild-type and heterozygous fli-enhanced green fluorescent protein (EGFP) transgenic28 zebrafish (Danio rerio) were maintained in The Forsyth Institute Zebrafish Facility. Embryos were generated by natural pairwise crosses, raised at 28.5°C, and staged as described previously.29,30
Cloning of Zebrafish and Human cav-1 and -1ß cDNAs
Full-length zebrafish cav-1 and -1ß cDNAs were isolated using degenerate oligonucleotide primers, polymerase chain reaction, and rapid amplification of cDNA ends (DOP-PCR/RACE). In brief, degenerate cav-1 primers were chosen from conserved amino acid sequences of human, bovine, mouse, rat, and chick caveolin-1 genes. The cav-1 forward primer was 5'-ATHGAYYTIGTIAAYMGIGAYCC-3', and the cav-1 reverse primer was 5'-AIGGIAYIAIIRYCCADATRTG-3'. Total RNA was isolated from adult zebrafish using the TRIzol Reagent method (Invitrogen, Carlsbad, CA). Reverse transcription (RT) reactions were performed using the SuperScript First-Strand Synthesis System (Invitrogen) followed by PCR amplification. 5'- and 3'-RACE was performed using the GeneRacer kit (Invitrogen), and the resulting PCR products were cloned into the pCR4-TOPO TA cloning vector. Nucleotide sequence comparison with the previously identified zebrafish cav-1 genomic clone confirmed the identity of full-length zebrafish cav-1 and -1ß cDNAs, whose nucleotide sequences were submitted to GenBank (accession nos. AY124572 and AY124573).
Human cav-1 cDNAs were generated as follows. Wild-type human cav-1 was PCR amplified from a construct kindly provided by Dr. Michael Lu,31
adding BamHI and EcoRI sites to 5' and 3' ends, respectively (sense primer, 5'-CAGGATCCATGTCTGGGGGCAAATACGTG-3'; and antisense primer, 5'-CAGAATTCTTATATTTCTTTCTGCAAGTTGATGCG-3'), and cloned into pcDNA3. The human cav-1a-Y14F mutant cDNA was generated by introducing an A to T base change at position 41, using the Stratagene QuickChange Site-Directed Mutagenesis kit (Stratagene, La Jolla, CA) and the following primers: sense primer, 5'-CGGAGGGACATCTCTTCACCGTTCCCATC-3'; and antisense primer, 5'-GATGGGAACGGTGAAGAGATGTCCCTCCG-3'. Human cav-1ß cDNA was cloned using the sense primer 5'-CAGGATCCATGGCAGACGAGCTGAGCGAGAAG-3' and the above antisense primer used to clone human cav-1, using a cDNA library from human prostate cancer cell (PC3) as template. The PCR product was cloned into pcDNA3 at BamHI/EcoRI sites. The nucleotide sequence of generated human cav-1 isoform expression constructs was confirmed by DNA sequence analyses.
Expression of Zebrafish Cav-1 and -1ß in Mammalian Cell Lines
Full-length zebrafish cav-1 and -1ß coding regions were cloned into pcDNA3.1/Myc-HisB (Invitrogen), generating the eukaryotic expression vectors pcDNA3.1-zfcav-1 and pcDNA3.1-zfcav-1ß, respectively. LNCaP and human embryonic kidney (HEK) 293 cells (American Type Culture Collection, Rockville, MD) were cultured and transfected using standard procedures. Protein extracts were generated from transfected cell lines and subjected to immunoblotting with the anti-human caveolin-1 polyclonal antibody (1:1000 dilution; Transduction Labs, San Diego, CA).
RT-PCR
Developmentally staged zebrafish were collected, frozen in liquid nitrogen, and stored at –80°C. Total RNA was isolated using TRIzol Reagent (Invitrogen), and 5 µg of each sample was used in RT reactions using the SuperScript III First-Strand Synthesis System for RT-PCR (Invitrogen). One seventh of each RT reaction was used as template in 50-µl PCRs, which were performed per the manufacturer’s specifications using the Platinum TaqDNA Polymerase High Fidelity (Invitrogen) and the following reaction conditions: denaturing at 95°C for 5 minutes; followed by 35 cycles at 95°C for 40 seconds, 60°C for 1 minute, and 72°C for 2 minutes; and final extension at 72°C for 7 minutes. Ten-microliter aliquots of each PCR reaction were resolved by agarose gel electrophoresis, stained with ethidium bromide, and digitally photographed. PCR primers were ß-actin upstream, 5'-GGAGAAGATCTGGCATCACACCTTCTAC-3'; ß-actin downstream, 5'-TGGTCTCGTGGATACCGCAAGATTCCAT-3'; cav-1 specific upstream, 5'-AATATCCCCACGCCTCCCACACTG-3'; cav-1ß specific upstream, 5'-CTCCCACTCCACGCTCATGAACTTC-3'; and cav-1 and -1ß common downstream, 5'-GCTGTCAGCAGCCTGTAGCACC-3'.
Lysate Preparation and Immunoblotting of Embryonic and Adult Zebrafish
Staged zebrafish embryos were collected, quick frozen in liquid nitrogen, and stored at –80°C. Frozen embryos were solubilized in immunoprecipitation assay buffer (50 mmol/L Tris-HCl, pH 7.4, 1% Nonidet P-40, 0.25% sodium deoxycholate, 150 mmol/L NaCl, 1 mmol/L NaF, and 1 mmol/L sodium orthovanadate) and COMPLETE protease inhibitor cocktail (Roche, Mannheim, Germany). After a 30-minute incubation on ice, insoluble material was removed by centrifugation at 18,000 x g for 15 minutes at 4°C, and the supernatants were collected as whole cell lysates. Adult zebrafish lysates were prepared from anesthetized zebrafish quick frozen in liquid nitrogen and ground with mortar and pestle. Protein concentrations were determined using the Bio-Rad Dc protein assay kit (Bio-Rad Laboratories, Hercules, CA). Equal amounts of total cell lysate were solubilized in 3x cell lysis buffer [6 mol/L urea (7.28 g/20 ml), 0.5 mol/L Tris, pH 6.8, 9 mmol/L ethylenediamine tetraacetic acid, pH 8.0, 3.6% ß-mercaptoethanol, and 9% sodium dodecyl sulfate (SDS), aliquoted, and stored at –20°C], boiled for 15 minutes, and fractionated by SDS-polyacrylamide gel electrophoresis using 20% gels. Ponceau S staining was used to confirm equal loading of protein extracts. Adult zebrafish lysates were diluted 1:20 relative to developmentally staged extracts, due to the very high Cav-1 expression in adults. Fractionated proteins were electrotransferred to a nitrocellulose membrane (Bio-Rad Laboratories), blocked with 5% milk, and immunoblotted with the anti-human Cav-1 polyclonal antibody (1:1000 dilution; Transduction Labs). Cav-1 proteins were visualized using horseradish peroxidase-conjugated secondary antibody and the Supersignal Chemiluminescence System (Pierce, Rockford, IL).
Whole-Mount in Situ Hybridization
Digoxigenin-labeled sense and antisense riboprobes (Roche Applied Science, Indianapolis, IN) were generated from unique 5'- and shared 3'-untranslated regions of cav-1 and -1ß. Whole-mount in situ hybridization (WISH) was performed as previously described.32,33
Targeted Depletion and Rescue of Cav-1 and -1ß
Antisense morpholino oligomers (MOs) targeted to the ATG start sites of cav-1 and -1ß mRNAs were designed and synthesized by Gene Tools, LLC (Eugene, OR). Antisense and 4-mismatch negative control MOs at 1, 2, and 4 mg/ml were injected at 1- to 3-picoliter volumes into single-cell staged embryos, which were then allowed to develop at 28.5°C with periodic examination using Leica MZ12 (Wetzlar, Germany) and Zeiss M2Bio (Thornwood, NY) dissecting microscopes. The lowest MO concentration resulting in a consistent phenotype, 1 mg/ml, was used in all subsequent experiments. The specificity of each cav-1 isoform morphant phenotype was determined using human cav-1 (hcav-1) isoform cRNAs that do not contain the antisense MO binding site. Translation-competent cav-1 and -1ß cRNAs were transcribed with mMESSAGE mMACHINE (Ambion, Austin, TX) and co-injected with antisense MOs, as detailed in Figure 6
. We injected 100 pg per embryo of cav-1 cRNA, 100 pg per embryo of cav-1 Y14F cRNA, and 200 pg per embryo of cav-1ß cRNA.
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At 12 hours after fertilization (hpf), embryos were fixed in 3.7% paraformaldehyde (PFA) in actin stabilizing buffer (ASB) (1 mmol/L phenylmethylsulfonyl fluoride, 10 mmol/L EGTA, 10 mmol/L PIPES, pH 7.3, 5 mmol/L MgCl2, and 900 mmol/L KCl), washed extensively in ASB, blocked with 1% fetal bovine serum (FBS)/3% bovine serum albumin in ASB overnight, and labeled with 66 nmol/L Alexa 546 phalloidin (Molecular Probes, Eugene, OR) for 1 hour. At 24 hpf, embryos were fixed in sucrose/PFA for 3 hours and washed in PBS-Tween (PBT) and then in distilled water. Washed embryos were placed in prechilled –20°C acetone for 5 minutes and washed in PBT again. Embryos were then blocked with either PBT-2% dimethylsulfoxide (PBTD) and 10% goat serum or 5% goat serum and 5 mg/ml bovine serum albumin in PBTD for 2 hours and labeled with 165 nmol/L Alexa 546 phalloidin in PBDT and 1% goat serum for 2 hours and washed in PBTD. Labeled embryos were analyzed using a Leica TCS SP2 AOBS Confocal Laser Scanning Microscope.
Transmission Electron Microscopy Analysis of Zebrafish Caveolae
Control and cav-1/-1ß morphant 24-hpf embryos were dechorionated and fixed for 3 hours at room temperature and then overnight at 4°C in 2% PFA, 2.5% glutaraldehyde, and 0.1 mol/L sodium cacodylate, pH 7.4. Fixed embryos were treated with osmium collidine for 1 hour at room temperature, washed in 0.05 mol/L sodium maleate, pH 5.2, and dehydrated in successive incubations of 50, 70, 85, 95, and 100% (2x) EtOH, for 15 minutes each. Dehydrated embryos were incubated in acetone for 3 minutes and infiltrated in a 1:1 araldite/acetone mixture overnight at 4°C, followed by overnight incubation in 100% araldite at 4°C. Embryos were then incubated in fresh 100% araldite for 2 hours at room temperature and in fresh araldite overnight at 68°C. Embedded embryos were thin sectioned for electron microscopy analysis. Transmission electron microscopy (TEM) was performed at the Massachusetts General Hospital East Core Facility (Charlestown, MA).
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and -1ß
Two full-length zebrafish cav-1 isoform cDNAs, cav-1 and -1ß, were cloned by DOP-PCR/RACE (GenBank accession nos. AY124572 and AY124573, respectively). Zebrafish cav-1 and -1ß encode proteins of 181 and 148 amino acids, with predicted molecular weights of 21 and 17 kd, respectively. Comparison of zebrafish cav-1 and -1ß nucleotide sequences to a zebrafish genomic cav-1 clone (GenBank clone 66I19, accession no. AC087105), revealed that zebrafish cav-1 and -1ß represent alternatively spliced transcripts, as is the case for mammalian cav-1 transcripts.25,34
The predicted amino acid sequence of zebrafish Cav-1 exhibits 72 and 71% identity to human and mouse Cav-1, respectively, whereas the predicted amino acid sequence of zebrafish Cav-1ß exhibits 77 and 76% identity to human and mouse Cav-1ß, respectively (Figure 1B)
. Immunoblot analysis of the zebrafish proteins expressed in mammalian cells demonstrated that zebrafish Cav-1 and -1ß migrate as expected in SDS-polyacrylamide gel electrophoresis gels (Figure 1C)
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Isoform-specific primers were used to assess cav-1 and -1ß mRNA expression by RT-PCR at discrete stages of zebrafish development (Figure 2A)
. Both genes were weakly expressed as maternal mRNAs, whereas zygotic cav-1 mRNA expression increased dramatically between 6 and 12 hpf. Caveolin-1 protein (Cav-1 and -1ß) expression in developing zebrafish was examined by Western blot analysis. Using an antibody that detects both mammalian Cav-1 and -1ß isoforms,35
zebrafish Cav-1 was first detectable as a faint band at 18 hpf (Figure 2B)
, exhibited steady expression levels between 20 to 28 hpf, appeared up-regulated between 28 and 48 hpf, and was detected at even higher levels in adults. The discrepancy between the developmental times where cav-1 mRNAs (0.7 hpf) and Cav-1 proteins (18 to 24 hpf) are first detected is likely due to the relative difference in sensitivity of the two assays (RT-PCR versus Western blot). The developmental expression profile of zebrafish Cav-1ß was similar to that of Cav-1 but at reduced levels. The zebrafish Cav-1-to-Cav-1ß expression ratios are similar to those observed in mammalian cells.36
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and -ß isoform mRNAs or a probe specific for cav-1ß mRNA only (Figure 3)
. We were unable to generate a cav-1-specific probe suitable for WISH, despite repeated attempts. The common probe detected cav-1 mRNAs in the notochord, somites, and skin of 24-hpf embryos (Figure 3, A–C)
, whereas at 48 hpf, expression was also detected in cranial and trunk sensory neuromasts (Figure 3, E versus F)
. At 90 hpf, the common probe detected cav-1 mRNAs in the pharyngeal vasculature, intestinal epithelium, and skin (Figure 3G)
, whereas the cav-1ß-specific probe detected expression in all of these tissues except intestinal epithelium (Figure 3, H versus I)
. Together, these data indicate that both cav-1 and -1ß isoform mRNAs are expressed as maternal transcripts (Figure 2
; data not shown) and are subsequently expressed in heart, pharyngeal vasculature, notochord, somites, skin, and neuromast tissues. In contrast, only cav-1 mRNA was detectable in intestinal epithelium.
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To identify putative developmental functions for individual Cav-1 isoforms, targeted protein depletion studies were performed using antisense MOs designed to hybridize to the unique translation start site of individual cav-1 isoform mRNAs and block their translation.37
Obvious defects in cav-1 morphant embryos were first evident at 12 hpf (not shown) and were quite distinct at 24 hpf, when neural, eye, and somite defects were observed, and morphant embryos appeared shortened along the anteroposterior (AP) axis relative to control embryos (Figure 4, E and I)
. The specificity of targeted Cav-1 isoform protein depletions was confirmed by Western blot analysis of 28-hpf morphant embryos (Figure 4A
, inset). At this time, Cav-1-depleted embryos exhibited obvious defects in retinal pigmented epithelium (RPE) differentiation, vacuolated neural ventricles, deformed notochord, and misshapen somites present in shortened tails composed of enlarged ventral tail tissue [Figure 4, E–H
, versus wild type (wt) A–D]. Cav-1ß-depleted embryos exhibited flattened heads, neural ventricle defects, abnormal RPE, and irregularly shaped somites in otherwise fairly well-patterned tail tissue (Figure 4, I–L)
. Analyses of thin-sectioned morphant embryo specimens confirmed abnormal cellular organization of eye, neural, RPE, and somite tissues (not shown). Molecular analyses revealed that 24-hpf cav-1 and -1ß morphant embryos exhibited reduced or lack of expression of neural markers pax2.1,38
engr3,39
fgf8,40
and otx2,41
examples of which are shown in Figure 5
, and aberrant expression of the muscle differentiation marker myoD42
in somite tissue, consistent with the observed neural and somite tissue defects. These data illustrate the dramatic effects on the embryo of the selective knockdown of either Cav-1 isoform.
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-depleted embryos were phenotypic, compared with 2.3% (9 of 384) of cav-1 4-mismatch control MO-injected embryos. Likewise, 100% (178 of 178) of Cav-1ß-depleted embryos were phenotypic at 24 hpf, compared with 2.6% (5 of 192) of control MO-injected embryos. Specific depletion of only the targeted and not the alternate Cav-1 isoform was confirmed by Western blot analyses of 28-hpf morphant embryos (Figure 4A
, inset). The specificity of our antisense MO-targeted protein depletion studies was confirmed by demonstrated rescue of morphant phenotypes with co-injected full-length cRNAs corresponding to human (Figure 6)
and zebrafish (not shown) sequences. Criteria for rescue included proper neural fold formation, normal eye development including RPE differentiation, chevron-shaped somites, and proper vascular development. Human cav-1 and -1ß cDNAs were isolated, sequenced, cloned into expression vectors, and used to generate translation-competent cRNAs. Human cav-1 and -1ß cRNAs were tested for their ability to rescue isoform-specific and alternative isoform, morphant phenotypes as shown in Figure 6
. These experiments demonstrated that injection of human cav-1 cRNA reproducibly rescued the zebrafish cav-1 morphant phenotype (98.6%, 275 of 279), and co-injection of human cav-1ß cRNA reproducibly rescued the zebrafish cav-1ß morphant phenotype (96.6%, 86 of 89). In contrast, zebrafish morphant phenotypes were not rescued by co-injection of alternative human cav-1 isotype cRNAs. The rescue results were essentially identical using zebrafish instead of human cav-1 cRNAs (not shown). These results suggest functionally nonredundant roles for individual Cav-1 and -1ß isoforms in early zebrafish development. Importantly, they also demonstrate that human Cav-1 isoforms are capable of performing the functions of zebrafish Cav-1 isoforms within the early developmental time frame studied here.
The Cav-1 Y14 Phosphorylation Site Is Required for Early Zebrafish Development
An interesting difference between Cav-1 and -1ß is the presence of a Src kinase phosphorylation site at Y14 in the unique amino-terminal region of Cav-1. The Cav-1 Y14 Src phosphorylation site has been implicated in Cav-1 function using mammalian cell culture models.43
This site is conserved in the zebrafish Cav-1 isoform, suggesting that it performs a functional role. We took advantage of the zebrafish cav-1 morphant phenotypes to determine whether a conservative but nonphosphorylatable human Cav-1 mutant (Y14F) was capable of rescuing the cav1- morphant phenotype. We reasoned that if Y14 phosphorylation is not required for early developmental functions of Cav-1, then co-injection of cav-1 Y14F cRNA would rescue the cav-1 morphant phenotype. We also examined whether the human Y14F Cav-1 isoform could rescue the cav-1ß morphant phenotype, allowing for the possibility that the Y14F site is the primary functional difference between the two Cav isoforms and that cav-1 Y14F might functionally mimic Cav-1ß. If so, cav-1 Y14F cRNA would rescue the cav-1ß morphant phenotype. Our results, summarized in Figure 6
, demonstrated that cav-1 Y14F cRNAs could not rescue either the cav-1 or cav-1ß morphant phenotype. Together, these results suggest that, although the Y14 phosphorylation site is required for Cav-1 function in early zebrafish development, this difference in the two proteins does not by itself define the functional difference between the Cav-1 and -1ß isoforms.
Cav-1 and -1ß Are Required for Caveolae Formation, Actin Cytoskeletal Organization, and Vascular Development
Transmission electron microscopy (TEM) was used to determine the effect of Cav-1 depletion on the formation of caveolae in cav-1/1ß double morphant embryos (Figure 7)
. At 24 hpf, numerous caveolae were clearly identifiable in notochord tissues of control MO-injected embryos, whereas few caveolae were evident in cav-1/-1ß double morphant notochord tissue (Figure 7, A
versus B, arrows). Caveolae present in notochord tissue sections obtained from similar AP positions of 24-hpf cav-1/-1ß morphant and control embryos were quantified. Cav-1/-1ß morphant notochord tissue contained only 16% of the caveolae present in control embryos (0.6 versus 3.8 caveolae/µm). Similarly, although caveolae were found in neural and somite tissues of 24-hpf wt and control MO-injected embryos, none were found in cav-1/-1ß morphant neural tissues (not shown). Interestingly, caveolae appear to be transient structures in the developing zebrafish notochord, with numerous caveolae present at 24 hpf and few to none detectable in 4-day postfertilization wt embryos (not shown). Together, these results demonstrate that targeted depletion of Cav-1 and -1ß results in reduced numbers of caveolae. TEM analyses also revealed additional characteristics that distinguished wt and cav-1/-1ß morphant tissues. Control 24-hpf embryos exhibited abundant, polarized somitic myofibrils, which were surrounded by sarcoplasmic reticulum (SR) transverse systems (Figure 7C)
. In contrast, 24-hpf cav-1/-1ß morphant somites contained only disorganized cytoplasmic structures, in which myofibrils were not readily identifiable, and organized SR was not evident (Figure 7D)
.
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/-1ß morphants (Figure 7)
. Actin-specific phalloidin staining46
of head and tail epidermis of 12-hpf Cav-1 and -1ß-depleted embryos revealed highly irregular cell borders and punctate fluorescence patterns, indicative of disrupted actin microfilaments in these tissues (Figure 8)
. The actin microfilament networks in Cav-1ß-depleted somites appeared less disrupted than those of cav-1 morphant embryos (Figure 8
, 24-hpf tail panels), consistent with the relatively intact vasculature observed in Cav-1ß-depleted embryos in comparison with cav-1 morphant embryos (Figure 9)
. At 24 hpf, somite defects were clearly evident in phalloidin-stained cav-1 morphant embryos (Figure 8
, bottom panels). Cav-1 morphants exhibited little to no organized somite tissues, whereas cav-1ß morphant somites lacked a midline myoseptum and the classic chevron shapes were shortened between each intersegmental boundary and exhibited thinner and disorganized myofibrils relative to that of control embryos. We also observed aberrant pigment granule movements in retinal pigment epithelium (RPE) and melanocytes of cav-1 and -1ß morphants (data not shown), suggestive of disrupted cytoskeletal organization and consistent with purported roles for actin microfilaments in intracellular pigment granule trafficking.47-49
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. In contrast, Cav-1-depleted embryos exhibited substantial vascular patterning defects in both head and tail tissues (Figure 9, C, C', D, and D')
, whereas Cav-1ß morphant embryos exhibited extensively disrupted head but fairly well-patterned tail tissue vasculature (Figure 9, E, E', F, and F')
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and -ß isoforms during vertebrate development. We demonstrate here that targeted depletion of either isoform results in embryonic lethality in the zebrafish and that the heterotypic isoform is incapable of substituting for the alternative form of the protein. These results are in marked contrast to Cav-1 knockout phenotypes observed in mice, where loss of both Cav-1 isoforms results in animals that are viable and fertile, live well into adulthood, and exhibit rather modest functional defects. The results in the mouse are surprising because, in the Cav-1 knockout animals, a prominent and presumably important class of subcellular organelles (caveolae) is completely absent in essential nonmuscle tissues, such as the vasculature. In the zebrafish, targeted depletion of either Cav-1 isoform resulted in morphological defects evident at 12 hpf and distinct neural, somitic, and vascular tissue defects evident by 24 hpf. The inability of one isoform to substitute for the other is an unanticipated observation because the proteins are very similar: Cav-1ß lacks the first 33 amino acids of Cav-1 but is otherwise identical. In the present study, we demonstrate that early morphological defects observed in Cav-1 isoform-targeted morphant embryos coincided with a failure to form caveolae, a breakdown in actin cytoskeletal organization, and extensive disruption of vascular endothelial tissue formation, consistent with published functions for caveolins (and caveolae) in each of these processes. Embryonic lethality of Cav-1 depletion in the zebrafish embryo, without regard to isoform specificity, was recently reported by another group, and our conclusion about the necessity for Cav-1 functions in early zebrafish development is consistent with theirs.51
Cav-1 and -1ß isoforms, initially identified as alternative translation products from a single mRNA,36
were subsequently demonstrated to form by differential RNA splicing.25
Collectively, our studies indicate that the two Cav-1 isoforms perform nonoverlapping roles during zebrafish development. We arrived at this conclusion from several independent observations. 1) Defects arising from targeted depletion of one Cav-1 isoform were not rescued by the heterotypic isoform but were rescued by the homotypic isoform. The ability of the human cav-1 isoform cRNAs to rescue defects arising from depletion of the zebrafish proteins suggests strong functional conservation across a long evolutionary timeline. 2) Targeted depletion of Cav-1 resulted in severe neural and tail tissue defects, whereas targeted depletion of Cav-1ß more selectively affected neural tissues. Isoform-characteristic patterning defects of cav-1 and -1ß morphant tissues were also clearly evident 3) with phalloidin-staining of the actin cytoskeleton and 4) in the vasculature of fli-EGFP, cav-1 morphant zebrafish. 5) WISH analyses revealed that only cav-1 mRNA was detected in intestinal epithelium, consistent with a recent report of Cav-1/Annexin 2 heterocomplex formation in intestinal sterol transport.51
6) Mutation of the Y14 Src phosphorylation site to the nonphosphorylatable residue phenylalanine disrupted the ability of human cav-1 cRNA to rescue the zebrafish cav-1 morphant defect. This mutation also did not convert Cav-1 into a Cav-1ß mimic. These results suggest that this tyrosine residue is essential for Cav-1 function and therefore is an important molecular difference between the two proteins. 7) Finally, the genomic organization (including intron/exon boundaries) specifying the zebrafish and mammalian cav-1 isoforms are conserved, implying strong evolutionarily constraints on the expression of the Cav-1 splice variants. We submitted the zebrafish cav-1 and -1ß cDNA sequence data to GenBank in 2003 (accession nos. AY124572 and AY124573). After GenBank released this information to the public, the existence of two zebrafish cav-1 isoform mRNAs was reported by another group.22
To date, very little published evidence exists supporting discrete functions for individual Cav-1 isoform proteins. One recent study, reporting Cav-1 isoform-specific interactions with bone morphogenetic protein (BMP) receptors RIa and RII, suggests that Cav-1ß inhibits BMP signaling, whereas Cav-1 does not.52
Another study used immunofluorescence and Western blotting to show that, although both Cav-1 isoforms were initially co-distributed in cardiac tissue at birth, at later times, Cav-1 exhibited restricted expression in endothelial tissues.53
At the present time, it is not clear whether developmental defects observed in Cav-1-depleted embryos are a primary or secondary effect of cytoskeletal disruption. Actin microfilament defects preceded and mirrored later tissue specific defects, suggesting a functional link between these observations. This study is the first in vivo demonstration that down-regulation of Cav-1 isoforms results in disruption of the actin cytoskeleton, consistent with mammalian cell culture studies demonstrating that actin microfilament defects arise from loss of Cav-1.54
Izumi et al55
first demonstrated that caveolae present in endothelial membranes could be decorated with phalloidin, which binds to actin at the junction between subunits.56-58
More recently, filamin, an F-actin cross-linking protein and Cav-1 ligand, was shown to directly link Cav-1 to the actin cytoskeleton, and Cav-1-positive membranes were demonstrated to redistribute to filamin-decorated stress fibers in response to the low-molecular weight GTPase RhoB, known to promote cytoskeletal reorganization in vitro.44
That study also used a quantitative yeast two hybrid approach to demonstrate that Cav-1ß, and not Cav-1, might be the primary ligand for filamin. Further confirmation of this result would provide additional evidence for discrete biochemical functions for the two Cav-1 isoforms. Additional evidence for an actin-caveolin interaction includes the demonstration that Cav-1 resides in structures at the plasma membrane that appear to be assembled into the cortical actin cytoskeleton59,60
and that disruption of filamentous actin by cytochalasin D treatment results in enhanced mobility and clustering of cell surface caveolae.59
Finally, numerous reports have shown that a variety of proteins known to communicate with or participate in the formation of cytoskeletal structures localize to lipid rafts and/or caveolae.8,61-69
The biological function(s) of caveolae is incompletely understood, despite a substantial body of literature linking these specialized organelles to multiple signal transduction processes and membrane trafficking. Consistent with observations from the Cav mouse knockout studies, we have shown that simultaneous targeted depletion of Cav-1 and -1ß in zebrafish resulted in dramatically reduced numbers of caveolae, in accord with the view that caveolins are integral membrane proteins required for the distinctive caveolar architecture.7,44
The reason for the disparity in the requirement for Cav-1 during development between mouse and zebrafish is unknown. It is possible that Cav-1 serves specialized functions in zebrafish that can be compensated for by redundant system(s) in the mouse. Another possibility is that the difference may arise from features characteristic of placental versus nonplacental embryonic development. Further elucidation of the molecular mechanisms of mammalian and zebrafish Cav-1 signaling pathways will clarify this issue.
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and -1ß, key structural proteins of caveolar lipid rafts, in the developing zebrafish. Our primary conclusions are that 1) expression of both Cav-1 isoforms is required for embryonic development in the zebrafish, because targeted depletion of either Cav-1 isoform results in tissue patterning defects evident as early as 12 hpf; 2) morphological characterization of Cav 1 morphants, tissue distribution analysis, and rescue experiments indicate that Cav-1 and -1ß exhibit distinct and nonredundant roles in early zebrafish development; 3) human Cav-1 and -1ß rescued the early developmental functions of zebrafish Cav-1 isoforms; 4) the Cav-1 Y14 Src phosphorylation site is required for the developmental function(s) of Cav-1; and 5) targeted depletion of either Cav-1 or -1ß results in disrupted actin cytoskeletal organization, which likely contributes to later vascular and developmental defects. Together, these findings suggest important and previously unrecognized developmental roles for Cav-1 and, by implication, for caveolar lipid rafts in vertebrate organogenesis.
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Related Commentary on page 1910
Supported by National Institutes of Health grants R01 CA101046 (to K.R.S.), P50 DK65298, R37DK47556, and R01 CA112303 (to M.R.F.), and R01 DE01613 (to P.C.Y.).
The sequences of zebrafish caveolin-1 and caveolin-1ß have been deposited in the GenBank database (accession numbers: AY124572 and AY124573).
Accepted for publication August 17, 2006.
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