The new name RCAN1 (regulator of calcineurin) has recently been accepted by the HUGO Gene Nomenclature Committee and the Mouse Genomic Nomenclature Committee (MGNC) for the gene previously known as DSCR1 or Adapt78. Similarly, RCAN1 is the new name for its protein product, which was previously called calcipressin1 or MCIP1.
K. J. A. Davies, Andrus Gerontology Center, University of Southern California, 3715 McClintock Avenue, Los Angeles, CA 90089–0191, USA Fax: +1 213 740646 Tel: +1 213 7408959 E-mail: email@example.com
The RCAN1 protein (previously called calcipressin 1 or MCIP1) binds to calcineurin, a serine/threonine phosphatase (PP2B), and inhibits its activity. Here we demonstrate that regulated overexpression of an RCAN1transgene (this gene was previously called DSCR1 or Adapt78) also stimulates expression of the GSK-3β kinase, which can antagonize the action of calcineurin. We also show that GSK-3β is regulated by RCAN1 at a post-transcriptional level. In humans, high RCAN1 expression is found in the brain, where at least two mRNA isoforms have been reported. Therefore, we further investigated expression of the various RCAN1 isoforms, resulting from differential splicing and alternative promotors in human brain. We detected at least three distinct RCAN1s: RCAN1-1 Short at 31 kDa (RCAN1-1S), RCAN1-1 Long at 38 kDa (RCAN1-1 L), and RCAN1-4. Furthermore, the levels of RCAN1-1S, but not RCAN1-1 L or RCAN1-4 correlated with the levels of GSK-3β. This suggests that RCAN1-1S might induce production of GSK-3βin vivo. While RCAN1s can regulate calcineurin and GSK-3β, it has also been shown that calcineurin and GSK-3β can regulate RCAN1s. Here we propose a new model (incorporating all these findings) in which cells maintain an equilibrium between RCAN1s, calcineurin, and GSK-3β.
The RCAN1 (regulator of calcineurin) gene was previously known as DSCR1 or Adapt78. DSCR1 or Adapt78 was discovered by two independent laboratories as a chromosome 21 Down syndrome critical region (DSCR1) gene , and as a gene transiently induced during cellular adaptation (Adapt78) to oxidative stress . The yeast homolog was named RCN1 and the Drosophila homolog was named NEBULA. Several groups simultaneously reported that DSCR1 or Adapt78 protein can specifically bind to, and down-regulate the activity of calcineurin [3,5–7]. Some groups therefore named this protein calcipressin 1, while others called it calcineurin binding protein (CBP 1)  or myocyte-enriched calcineurin interacting protein (MCIP1) . In yeast, the RCAN1 protein was called Rcn1p , and in Drosophila, this protein was named sarah . The new name RCAN1 (regulator of calcineurin) has recently been accepted by the HUGO Gene Nomenclature Committee for the protein product of the RCAN1 gene.
RCAN1 is thought to play a role in number of physiological processes. It has been associated with Down's syndrome and Alzheimer's disease [5,9]. It has also been demonstrated to inhibit cardiac hypertrophy  and to attenuate angiogenesis and cancer . Exactly how RCAN1 may contribute to human pathologies, however, is a subject of intense continuing study. So far, the only established function of the RCAN1 protein is the inhibition and regulation of calcineurin, which is protein phosphatase 2B (PP2B). Calcineurin, however, works together with (or against) kinases such as glycogen synthase kinase-3β (GSK-3β) to regulate protein phosphorylation/dephosphorylation, and GSK-3β can antagonize calcineurin. Therefore, here we have investigated whether GSK-3β can also be regulated by RCAN1.
One of the major cell types expressing RCAN1 in the human body is neurons , suggesting important roles for the protein in brain function. The RCAN1 (DSCR1, Adapt78) gene consists of seven exons, four of which (exons 1–4) can be alternatively spliced to produce a number of different mRNA isoforms. Since exons 5, 6, and 7 are likely to be common to all mRNA isoforms, for nomenclature simplification, it was proposed to assign them (and the proteins they encode) numbers which correspond to the first exons they contain . Expression of RCAN1 mRNA in human brain was previously investigated , but not expression of the protein isoforms. Therefore, we have now identified which RCAN1s are expressed in human brain, and we have questioned which isoforms (if any) may regulate GSK-3β.
It has recently been demonstrated, both in vitro and in vivo, that RCAN1 can be phosphorylated [13–16]. Phosphorylation may alter RCAN1's ability to inhibit calcineurin, as well as RCAN1 degradation rate [14–16]. Therefore, phosphorylation may represent a major mechanism for regulating RCAN1. Remarkably, RCAN1 can be phosphorylated by GSK-3β and dephosphorylated by calcineurin [13,16] and we now demonstrate that RCAN1 can also, reversibly, regulate both enzymes.
Transgenic RCAN1 stimulates expression of GSK-3βin vitro
To test whether RCAN1 might regulate GSK-3β we developed a tet-off regulated gene expression system, in which RCAN1 expression can be regulated by doxycycline [12,17]. To keep the transgene ‘silent’, cells were continuously grown in the presence of doxycycline, and inducible RCAN1 transgene expression was achieved by doxycycline withdrawal from the cell culture medium. RCAN1 was maximally overexpressed about 6 h after doxycycline withdrawal from the cell medium, but its levels then declined over the next 48 h (Fig. 1A). Remarkably, GSK-3 protein levels followed the levels of RCAN1; they increased following RCAN1 overexpression and declined at 48 h Fig. 1A,B. It seems that both GSK-3β and GSK-3α levels were elevated. However, the level of GSK-3β was increased much more significantly (∼ 2.3-fold after 24 h of RCAN1 overexpression) than was the level of GSK-3α.
GSK-3 activity is regulated by phosphorylation. Particularly, its activity is inhibited by phosphorylation at the S9 position . Therefore, we also tested whether the phosphorylated form of GSK-3 (pGSK-3) was elevated following RCAN1 overexpression. We found no significant increase in pGSK-3 levels until 48 h after doxycycline withdrawal. These results indicate that RCAN1 stimulates production of active GSK-3, which then phosphorylates the tau protein and promotes accumulation of hyperphosphorylated tau. It seems, however, that cells might have a feedback mechanism that controls GSK-3 activity and, after 48 h, GSK-3 was increasingly phosphorylated (Fig. 1C).
As hypothesized, the levels of phosphorylated tau correlated with RCAN1 (Fig. 1A). Tau phosphorylation increased during RCAN1 overexpression (Fig. 1D), whereas the levels of total tau protein were unchanged. When combined, these results further strengthen our proposal that RCAN1 can induce the active form of GSK-3. It is interesting that the levels of GSK-3β were modulated. GSK-3β is closely associated with pTau of tangle-bearing neurons , and it is specifically the GSK-3β isoform that is increased in pretangle neurons . GSK-3β levels did not perfectly correlate with pTau levels; the highest pTau levels were observed about 6 h after RCAN1 overexpression, and although GSK-3β levels were increased almost two-fold at this time point, they did not peak until 24 h (Fig. 1). This might be due to competition between calcineurin and GSK-3β, or to activation of other phosphatases after the 6-h time point. It is also possible that other kinases, that phosphorylate the tau protein, are also activated following RCAN1 overexpression but, unlike GSK-3β, they are activated maximally at the 6 h time point.
GSK-3β appears to be regulated by RCAN1 at a post-transcriptional level
We next tested whether RCAN1 induces transcription of the GSK-3β gene or if it regulates GSK-3β expression at a post-transcriptional level. RCAN1 was overexpressed as described in Fig. 1 and cells were split into two portions: one portion was used for western blot analysis and the other for northern blot analysis. Levels of GSK-3β and RCAN1 mRNA were evaluated by northern blot analysis (Fig. 2). To verify that each sample was equally loaded, the membrane was probed with GAPDH (a housekeeping gene) probe. Results revealed that RCAN1 mRNA was elevated as early as 3 h and was clearly overexpressed at each time point: 6, 24, and 48 h. GSK-3β mRNA levels, however, remained unchanged at all time points. These results are also confirmed by our previous data obtained using microarray analysis of genes that are regulated by RCAN1 . In these previous experiments we did not observe any GSK-3β mRNA transcription level changes following RCAN1 down-regulation. Altogether these results indicate that RCAN1 does not affect the levels of transcription of GSK-3β, but it rather regulates production of the GSK-3β protein at a post-transcriptional level. This might be due to either accelerated GSK-3β translation and/or slower GSK-3β degradation; the exact mechanism(s) remain(s) to be addressed.
It is interesting that RCAN1 mRNA levels were not auto-down-regulated at any time point, while levels of the RCAN1 protein were slightly down-regulated 24 h after overexpression, and then returned to basal levels in 48 h (Fig. 1A, top panel). These results indicate that RCAN1 expression might be feed-back regulated at a post-transcriptional level, most likely, by accelerated degradation of RCAN1.
At least three RCAN1s are expressed in human brain
Western blot analysis of RCAN1 expression in human brain revealed that at least three different RCAN1 proteins are expressed (Fig. 3). The RCAN1 gene consists of seven exons that can be alternately spliced to produce a large number of isoforms . Two mRNA isoforms were previously shown to be expressed in adult human brain: isoform 1 (encoded by exons 1, 5, 6, and 7) and isoform 4 (encoded by exons 4, 5, 6, and 7). To determine the structure of each of these RCAN1s we have developed three different antibodies: an antibody that recognizes RCAN1 encoded by exon 1 (RCAN1-1), an antibody that recognizes RCAN1 encoded by exon 4 (RCAN1-4), and an antibody that recognizes all potential RCAN1 protein isoforms (directed against the invariant exon 7). Surprisingly, we found that not one but two RCAN1-1 proteins are expressed in human brain (Fig. 3A,C): RCAN1-1 ‘Long’ at 38 kDa (RCAN1-1 L) and RCAN1-1 ‘Short’ at 31 kDa (RCAN1-1S). Using the complete sequence of chromosome 21, we analyzed for RCAN1 potential transcription initiation sites and translation codons. Computer analysis revealed that exon 1 has an alternative initiation translation codon, located further upstream than previously thought, indicating that, indeed, two RCAN1-1 isoforms can be expressed. This possibility was also confirmed by our experiments, in which RCAN1 was overexpressed using the tet-off system (please see above). The construct used to overexpress RCAN1 in these experiments carried a DNA fragment which encodes only RCAN1-1S, and this fragment was translated only into a 31-kDa protein (Fig. 1).
Unexpectedly, RCAN1-4 was detected as a 70 kDa protein in human brain (Fig. 3A–C). Based on the sequence of the RCAN1 gene, the size of the 1–4 isoform was expected to be roughly equal to RCAN1-1S, which is about 31 kDa. This raises questions about the specificity of our antibodies; however, all three RCAN1 antibodies, including the RCAN1-4 antibody, were carefully tested as shown in Fig. 4. The RCAN1-4 antibody always showed one clear band. In addition, we further tested our antibodies in substrate competition experiments, in which the peptide used to develop each antibody was added to the antibody solutions used for western blots. In each case, the addition of the correct peptide significantly reduced the appropriate signal levels (not shown).
It seems that different RCAN1 isoforms can be produced depending on the cell type, the model system used, and the particular conditions employed. We have tested RCAN1 expression in several cell lines and found that some of them strongly express RCAN1s of various sizes, while others express very low levels of RCAN1s (e.g. Fig. 5). In most cases, RCAN1-1L appears to be the predominant isoform expressed.
GSK-3β correlates with the levels of RCAN1-1S, but not RCAN1-1L or RCAN1-4 in vivo
We next tried to address whether, similar to our in vitro results, RCAN1 might influence GSK-3β expression in vivo. We analyzed 27 randomly chosen human brain samples from various brain regions: cerebral cortex, cerebellum and hippocampus (Fig. 6). Interestingly, GSK-3β levels in all samples was tightly correlated (r = 0.7, P < 0.001) with the expression of only one RCAN1 isoform, RCAN1-1S, but not RCAN1-1L or RCAN1-4. This correlation was independent of the brain region analyzed.
RCAN1-1S was the isoform overexpressed in the experiments described in Fig. 1above. These results therefore suggest that, similar to our in vitro observations (Fig. 1), GSK-3β may also be induced by expression of RCAN1-1S in human brain in vivo. This study was not designed to address whether induction of RCAN1 can actually cause Alzheimer's disease. The brain samples examined in our studies were all from patients ∼ 80 years old, some of whom suffered from Alzheimer's disease and some of whom did not. Alzheimer's disease, however, is clearly an age-related disorder. Almost half of all humans suffer some degree of Alzheimer's disease by this age, and a large proportion of our population might have developed the disease without displaying overt symptoms . Therefore, older patients might have increased expression of RCAN1 and GSK-3β even if they are not yet diagnosed with Alzheimer's disease. It has actually been shown that GSK-3β is induced in pretangle neurons . For these reasons, a completely different approach will now be needed to address whether induction of RCAN1 can actually cause Alzheimer's disease.
Previously it has been shown that RCAN1 can down-regulate the activity of calcineurin, the serine-threonine phosphatase PP2B [3,5–7]. Here we demonstrate that, in addition, RCAN1 can also induce GSK-3β, a kinase that antagonizes calcineurin. Calcineurin and GSK-3β work in opposition to regulate the phosphorylation/dephosphorylation of proteins and, thus, to regulate their activities. Because RCAN1 inhibits calcineurin, its overexpression will lead to protein phosphorylation and, as RCAN1 induces GSK-3β, its overexpression should lead to further protein phosphorylation. Thus, in both cases, phosphorylated forms of proteins will be produced. The relationship between RCAN1 and calcineurin is, however, extremely complex. It has been shown, for example, that RCAN1 can inhibit, activate, or have no effect on calcineurin, depending on its concentration and its phosphorylation status [13,14,16]. Therefore, since almost any change in calcineurin activity could be interpreted in any convenient (or trivial) way, it was not measured in our study.
A number of vital proteins are regulated by GSK-3β and calcineurin. One of these is tau, a microtubule-associated protein that is important for cytoskeleton stabilization and cell growth. Accumulation of hyperphosphorylated tau can lead to the formation of neurofibrillary tangles, and to neurodegeneration associated with tauopathies. Hyperphosphorylated forms of tau are also involved in pathologies such as Down's syndrome and Alzheimer's disease. Importantly, we observed accumulation of phosphorylated tau protein following overexpression of RCAN1 in our experiments.
RCAN1 is a stress-inducible gene, and neurodegenerative diseases are associated with stresses such as chronic inflammation. For example, it has been shown that mechanical stress and traumatic brain injury are robust factors for Alzheimer's disease [22–25]. Interestingly, RCAN1 expression has been shown to be inducible by mechanical stimulus , thus providing a possible link between injury and Alzheimer's disease. We have also observed RCAN1 overexpression following mechanical injury in cell culture models (not shown). Of course, any event causing inflammation would be expected to induce RCAN1 expression, since the gene was identified as an oxidative stress adaptive factor . Thus, it is possible that chronic stress may contribute to neurodegeneration through induction of RCAN1, which in turn leads to inhibition of calcineurin and activation of GSK-3β, both resulting in tau protein hyperphosphorylation.
Besides regulation of tau, both calcineurin and GSK-3β are key regulators of nuclear factors of activated T-cells (NFATs), which are important players in a number of physiological processes. The best characterized role of NFATs is in T-cell activation. When NFATs are dephosphorylated (due to the induced activity of calcineurin and/or reduced activity of GSK-3β) they are transported from the cytoplasm into the nucleus, and activate T-cells. Therefore, overexpression of RCAN1 could lead to phosphorylation of NFATs and T-cell inactivation. NFATs are also important players in musculoskeletal development, cardiovascular development and functions , and neuronal development . Thus, NFATs and RCAN1 may contribute to the pathologies observed in Down's syndrome, such as mental retardation, heart defects, skeletal muscle abnormalities, and immune system deficiencies. Indeed, it has been previously shown that activated GSK-3β phosphorylates NFAT proteins and suppresses cardiac hypertrophy . On the other hand, it was also shown that overexpression of RCAN1 can inhibit cardiac hypertrophy , which is consistent with our hypothesis that overexpression of RCAN1 can activate GSK-3β, promoting NFATs phosphorylation and suppression of cardiac hypertrophy.
GSK-3β has a number of substrates and is a multifunctional kinase. Besides Alzheimer's disease and cardiac development, its function is closely linked to type II diabetes, various forms of cancer and other human pathologies . Our finding that RCAN1 can modulate the levels of this enzyme uncovers a possible new link between stress and human disease. Similarly, calcineurin is involved in many vital pathways [27,31] and inhibition of this phosphatase by RCAN1 may be expected to have serious repercussions. Thus, our results raise the possibility that RCAN1 may play important roles in a variety of chronic stress, or chronic inflammation-related diseases including (but not limited to) Alzheimer's disease, Down's syndrome, and other neurodegenerative disorders.
It would seem that RCAN1 overexpression can lead to dramatic physiological changes, and our results suggest that cells cope with RCAN1 overexpression quickly and efficiently. In our experiments, cells recognized high levels of RCAN1-1S and down-regulated its forced expression to basal levels within 48 h (Figs 1 and 2). We wish to know how cells do this. Current data suggest that RCAN1 regulation is accomplished by phosphorylation. While RCAN1 can regulate calcineurin and GSK-3β, calcineurin and GSK-3β can also regulate the phosphorylation of RCAN1 [13,16,29]. It has been demonstrated that phosphorylation of RCAN1s increases their degradation rates . Therefore, both activation of GSK-3β and inactivation of calcineurin should lead to the same results: increased RCAN1 phosphorylation and faster degradation (Fig. 7). Thus, overexpression of RCAN1 will eventually lead to RCAN1 phosphorylation and removal. It appears that cells maintain an equilibrium between RCAN1, calcineurin, and GSK-3β, and serious problems may arise when this balance is distorted. This model, however, remains to be tested.
Doxycycline-regulated RCAN1 expression system
An RCAN1 DNA fragment corresponding to the RCAN1–1S protein was produced by long and accurate (LA) RT-PCR as previously described . The fragment was inserted into a pTRE vector from Clontech Laboratories, Inc. (Paolo Alto, CA) PC-12 tet-off cells from Clontech Laboratories, Inc., stably transfected with ‘regulator plasmid’, were next transfected with the pTRE carrying RCAN1 fragment to finally produce a double-stable cell line in which the RCAN1 transgene could be turned off by doxycycline addition. All procedures were performed as described in the tet-off gene expression user manual from Clontech Laboratories, Inc.
PC-12 cell culture
PC-12 cells were cultivated in media containing 85% Dulbecco's modified Eagle's medium (DMEM), 10% horse serum, 5% antibiotic free fetal bovine serum (Clontech Laboratories, Inc.), 2 mm l-glutamine, 100 U·mL−1 penicillin, and 100 µg·mL−1 streptomycin sulfate. PC-12 tet-off cells, stably transfected with ‘regulator plasmid’, were maintained in media additionally containing 50 µg·mL−1 G418. The double-stable cell line carrying both the ‘regulator plasmid’ and the RCAN1 transgene was maintained in media additionally containing 50 µg·mL−1 G418 and 50 µg·mL−1 hygromycin. Cells were cultivated in a humidified 10% CO2 atmosphere at 37°C. Expression of the RCAN1 transgene was turned off by application of 0.1 µg·mL−1 doxycycline to the media.
Post mortem human brain tissue
Brain tissue/CSF used in this project was provided by the Alzheimer's Disease Research Center, University of Southern California. Samples were frozen at −70°C until use. Every sample was accompanied by a full medical history, and by detailed neuropathology summaries that included microscopic exams. Protocols using frozen samples of human brain were approved by Institutional Review Board.
The following three RCAN1 antibodies were designed using the predicted open reading frame sequence of RCAN1: (1) an antibody that recognizes RCAN1 encoded by exon 1 (RCAN1-1) was developed using a hypothetical peptide encoded by the MEEVDLQDLPSAT portion of exon 1; (2) an antibody that recognizes RCAN1 encoded by exon 4 (RCAN1-4) was developed using a hypothetical peptide encoded by the VANSDIFSESETR portion of exon 4; and (3) an antibody that recognizes all potential RCAN1 protein isoforms was developed using a hypothetical peptide encoded by the KIIQTRRPEYTPIHLS portion of the exon 7. We added a cysteine to the N-terminus of these peptides and conjugated them to keyhole limpet hemocyanin. The peptides were then used to immunize rabbits and polyclonal antibodies were raised commercially (ProSci, Inc., Poway, CA). Serum from the immunized rabbits was affinity purified on columns with the covalently attached RCAN1 peptide used for immunization.
Western blot analysis
Western blot analysis was performed following standard protocols, using an ECL-detection system from Amersham Biosciences (Piscataway, NJ). Final dilution of the RCAN1 antibodies for western blot analysis ranged between 1 : 500 and 1 : 1000. Phosphorylated GSK-3 protein (pGSK-3) was detected using an antibody that specifically recognizes the GSK-3 protein, phosphorylated at serine 9. Phosphorylated tau protein (pTau) was detected using an antibody that specifically recognizes the tau protein, phosphorylated at threonine 231 (pT). It has been demonstrated that tau is phosphorylated at threonine 231 by GSK-3 and it can be dephosphorylated by calcineurin .
Anti-tau antibody was obtained from Upstate Biotechnology (catalog #05–348). Anti-tau [pT]-phosphospecific antibody was from Biosource International (catalog #44–746) (Camarillo, CA). Anti-GSK-3 antibody was from Santa Cruz Biotechnology, Inc. (Santa Cruz, CA) Anti-GSK-3 [pS] phosphospecific antibody was from Santa Cruz Biotechnology, Inc. Tubulin antibody was from Santa Cruz Biotechnology, Inc. Commercial antibodies were diluted as suggested by the manufacturers.
Total RNA was extracted using the TRIzol reagent (Life Technologies, Inc.). RNA concentration was quantified spectrophotometrically, and relative content was further confirmed on ethidium bromide-stained gels. Integrity of the RNA was estimated by agarose gel electrophoresis; only RNA samples displaying discrete 28S and 18S RNA bands were used in our experiments.
Northern blot analysis
Samples containing 30 µg of total RNA were subjected to electrophoresis through 1% agarose formaldehyde gels, blotted to nylon membranes (Intergen, Purchase, NY) with HETS (Tel-Test Inc., Friendswood, TX), and cross-linked by ultraviolet radiation. Three probes were used in this study: RCAN1, GAPDH, and GSK-3β. RCAN1 and GAPDH were synthesized by PCR, and to detect GSK-3β we used synthetic oligonucleotides. Therefore, hybridization conditions for these probes were different. For RCAN1 and GAPDH detection, the membranes were prehybridized for 4 h and hybridized for 15 h in Hybrizol I (Oncor) at 42°C. After hybridization they were washed with 2 × NaCl/Cit plus 0.1% SDS at room temperature for 1 and 10 min, then with 0.1 × NaCl/Cit plus 0.1% SDS at 60°C for 10 and 30 min. For hybridization with GSK-3β, we used ULTRAhyb–Oligo solution, Ambion Inc. (Austin, TX) followed by protocol from manufacturer. The membranes were exposed, developed, and scanned using the phosphorimager system (Molecular Dynamics, Sunnyvale, CA).
To rehybridize RNA blots, hybridized and labeled probes were removed by washing the membranes in a solution of 0.1 × NaCl/Cit, 0.1% SDS, and 10 mm Tris/HCl (pH 7.0) at 90°C for 10 min. To quantify the level of DSCR1 (Adapt78) mRNA expression the membranes were scanned and the hybridization signal measured using imagequant software (Molecular Dynamics). Each signal was recalculated according to the amount of RNA actually loaded on the gels. The amount of RNA loaded was controlled by hybridization with a GAPDH probe.
RCAN1 and GAPDH probes were labeled using [32P]dCTP[αP] and the High Prime system (Boehringer Mannheim, Indianapolis, IN). A fully cloned hamster Adapt78 fragment was used to prepare DSCR1 (Adapt78) probes; and a PCR-fragment consisting of exons 7 and 8 was used to prepare GAPDH probes as we previously described [9, 17]. To prepare GSK-3β probe we synthesized the following oligonucleotide: 5′-CTGTGGCCTGTCAGGACCCTGTCCAGGAGT-3′.
The reverse complementary oligonucleotide was used as a negative control for hybridization. This oligonucleotide was labeled using T4 polynucleotide kinase from Invitrogen Inc. (Chicago, IL) following a protocol from the manufacturer.
Tissues for this study were obtained from the Alzheimer's Disease Research Center Neuropathology Core, University of Southern California School of Medicine, Los Angeles, CA, USA. This work was supported by PHS grant # AG16256 to K.J.A.D and G.E. from the NIH/NIA.