Enhanced cardiomyocyte DNA synthesis during myocardial hypertrophy in mice expressing a modified TSC2 transgene

Musings on intrinsic cardiomyocyte cell cycle activity and myocardial regeneration

Although the myocardial renewal rate in the adult mammalian heart is quite low, recent studies have identified genetic variants which can impact the degree of cardiomyocyte cell cycle reentry. Here we use the compound interest law to model the level of regenerative growth over time in mice exhibiting different rates of cardiomyocyte cell cycle reentry following myocardial injury. The modeling suggests that the limited ability of S-phase adult cardiomyocytes to progress through cytokinesis, rather than the ability to reenter the cell cycle per se, is a major contributor to the low levels of intrinsic regenerative growth in the adult myocardium.

Cardiac Troponin I-Interacting Kinase Affects Cardiomyocyte S-Phase Activity but Not Cardiomyocyte Proliferation

BACKGROUND

Identifying genetic variants that affect the level of cell cycle reentry and establishing the degree of cell cycle progression in those variants could help guide development of therapeutic interventions aimed at effecting cardiac regeneration. We observed that C57Bl6/NCR (B6N) mice have a marked increase in cardiomyocyte S-phase activity after permanent coronary artery ligation compared with infarcted DBA/2J (D2J) mice.

METHODS

Cardiomyocyte cell cycle activity after infarction was monitored in D2J, (D2J×B6N)-F1, and (D2J×B6N)-F1×D2J backcross mice by means of bromodeoxyuridine or 5-ethynyl-2'-deoxyuridine incorporation using a nuclear-localized transgenic reporter to identify cardiomyocyte nuclei. Genome-wide quantitative trait locus analysis, fine scale genetic mapping, whole exome sequencing, and RNA sequencing analyses of the backcross mice were performed to identify the gene responsible for the elevated cardiomyocyte S-phase phenotype.

RESULTS

(D2J×B6N)-F1 mice exhibited a 14-fold increase in cardiomyocyte S-phase activity in ventricular regions remote from infarct scar compared with D2J mice (0.798±0.09% versus 0.056±0.004%; P<0.001). Quantitative trait locus analysis of (D2J×B6N)-F1×D2J backcross mice revealed that the gene responsible for differential S-phase activity was located on the distal arm of chromosome 3 (logarithm of the odds score=6.38; P<0.001). Additional genetic and molecular analyses identified 3 potential candidates. Of these, Tnni3k (troponin I-interacting kinase) is expressed in B6N hearts but not in D2J hearts. Transgenic expression of TNNI3K in a D2J genetic background results in elevated cardiomyocyte S-phase activity after injury. Cardiomyocyte S-phase activity in both Tnni3k-expressing and Tnni3k-nonexpressing mice results in the formation of polyploid nuclei.

CONCLUSIONS

These data indicate that Tnni3k expression increases the level of cardiomyocyte S-phase activity after injury.

ERO1α promotes testosterone secretion in hCG-stimulated mouse Leydig cells via activation of the PI3K/AKT/mTOR signaling pathway

ER oxidoreduclin 1α (ERO1α) is an oxidase, participating in formation of secretory and membrane proteins. However, the other physiological functions ERO1α is not well known. We found that ERO1α is high in the Leydig cells of the testis. Therefore, the purposes of the current study are to explore the role of ERO1α and the possible mechanisms in regulating cell proliferation, apoptosis, and testosterone secretion of Leydig cells. ERO1α was mainly localized in Leydig cells in the adult mice testes by immunofluorescence staining. Western blot analysis showed that ERO1α was higher in Leydig cells than that in the seminiferous tubules. The effect of ERO1α on cell proliferation, apoptosis, and testosterone secretion was detected by transducing ERO1α overexpression and knockdown lentiviruses into cultured primary Leydig cells (PLCs) together with hCG exposure. Flow cytometry analysis showed that ERO1α promoted cell proliferation by increasing cell distribution at the S phase and decreasing that at the G0/G1 phase. Western bolt analysis showed that ERO1α increased CDK2 and CDK6 expression. Cell apoptosis determination found that ERO1α inhibited PLC apoptosis. Western bolt analysis showed that ERO1α increased the ratio of BCL-2/BAX, and decreased BAD and Caspase-3 expression. Enzyme-linked immunosorbent assay analysis demonstrated that ERO1α enhanced testosterone secretion. Western bolt analysis found that ERO1α increased StAR, 3β-HSD, and CYP17A1 expression. Furthermore, ERO1α could activate the PI3K/AKT/mTOR signaling pathway. In summary, these results suggest that ERO1α might play proliferation promotion and antiapoptotic roles and enhance testosterone secretion in PLC, at least partly, via activation of the PI3K/AKT/mTOR signaling pathway.

The Tuberin and Cyclin B1 complex functions as a novel G2/M sensor of serum conditions and Akt signaling

A great deal of ground breaking work has determined that the Tuberin and Hamartin Complex function as a negative regulator of protein synthesis and cell cycle progression through G1/S. This is largely attributed to the GTPase activity of Tuberin that indirectly inhibits the mammalian target of rapamycin (mTOR). During times of ample nutrition Tuberin is inhibited by growth factor signaling, including direct phosphorylation by Akt/PKB, allowing for activation of mTOR and subsequent protein synthesis. It is well rationalized that maintaining homeostasis requires communication between cell growth (mTOR signaling) and cell division (cell cycle regulation), however how this occurs mechanistically has not been resolved. This work demonstrates that in the presence of high serum, and/or Akt signaling, direct binding between Tuberin and the G2/M cyclin, Cyclin B1, is stabilized and the rate of mitotic entry is decreased. Importantly, we show that this results in an increase in cell size. We propose that this represents a novel cell cycle checkpoint linking mitotic onset with the nutritional status of the cell to control cell growth.

Upregulation of C/EBPβ and TSC2 by an HDAC inhibitor CG200745 protects heart from DOCA-induced hypertrophy

Histone deacetylases (HDACs) are a vast family divided into four major classes: class I (1, 2, 3, and 8), class II (4, 5, 6, 7, 9 and 10), class III (sirtuin family) and class IV (HDAC11). HDAC inhibition attenuates cardiac hypertrophy through suppression of the mechanistic target of rapamycin complex1 (mTORC1) signaling. HDAC inhibitors upregulate the expression of tuberous sclerosis complex 2 (TSC2), an mTORC1 inhibitor. However, the molecular mechanism underlying HDAC inhibitor-mediated upregulation of TSC2 is unclear. We hypothesized that an HDAC inhibitor, CG200745 (CG), ameliorates cardiac hypertrophy through the inhibition of mTORC1 signaling by upregulating of the CCAAT/enhancer-binding protein-β (C/EBP-β)/TSC2 pathway. To establish a cardiac hypertrophy model, deoxycorticosterone acetate (DOCA, 40 mg/kg/wk) was subcutaneously injected for 4 weeks into Sprague-Dawley rats. All rats were unilaterally nephrectomized and had free access to drinking water containing 1% NaCl with or without CG of different concentrations. The expression level of TSC2 and C/EBP-β was measured by quantitative real-time PCR (qRT-PCR) and western blot analysis. Acetylation of C/EBP-β was analyzed by immunoprecipitation. The recruitment of C/EBP-β and polymerase II (Pol II) on TSC2 promoter region was analyzed by chromatin immunoprecipitation (ChIP). CG treatment increased the expression of TSC2. In addition, CG treated rats showed an increased in the expression and acetylation of C/EBP-β, owing to the increase in the recruitment of C/EBP-β and Pol II at Tsc2 gene promoter. Thus, CG ameliorates cardiac hypertrophy through the inhibition of mTORC1 signaling via upregulation of the C/EBP-β/TSC2 pathway in DOCA-induced hypertensive rats.

Cardiomyocyte proliferation prevents failure in pressure overload but not volume overload

Induction of the cell cycle is emerging as an intervention to treat heart failure. Here, we tested the hypothesis that enhanced cardiomyocyte renewal in transgenic mice expressing cyclin D2 would be beneficial during hemodynamic overload. We induced pressure overload by transthoracic aortic constriction (TAC) or volume overload by aortocaval shunt in cyclin D2-expressing and WT mice. Although cyclin D2 expression dramatically improved survival following TAC, it did not confer a survival advantage to mice following aortocaval shunt. Cardiac function decreased following TAC in WT mice, but was preserved in cyclin D2-expressing mice. On the other hand, cardiac structure and function were compromised in response to aortocaval shunt in both WT and cyclin D2-expressing mice. The preserved function and improved survival in cyclin D2-expressing mice after TAC was associated with an approximately 50% increase in cardiomyocyte number and exaggerated cardiac hypertrophy, as indicated by increased septum thickness. Aortocaval shunt did not further impact cardiomyocyte number in mice expressing cyclin D2. Following TAC, cyclin D2 expression attenuated cardiomyocyte hypertrophy, reduced cardiomyocyte apoptosis, fibrosis, calcium/calmodulin-dependent protein kinase IIδ phosphorylation, brain natriuretic peptide expression, and sustained capillarization. Thus, we show that cyclin D2-induced cardiomyocyte renewal reduced myocardial remodeling and dysfunction after pressure overload but not after volume overload.

Inhibition of class I histone deacetylases blunts cardiac hypertrophy through TSC2-dependent mTOR repression

Altering chromatin structure through histone posttranslational modifications has emerged as a key driver of transcriptional responses in cells. Modulation of these transcriptional responses by pharmacological inhibition of class I histone deacetylases (HDACs), a group of chromatin remodeling enzymes, has been successful in blocking the growth of some cancer cell types. These inhibitors also attenuate the pathogenesis of pathological cardiac remodeling by blunting and even reversing pathological hypertrophy. The mechanistic target of rapamycin (mTOR) is a critical sensor and regulator of cell growth that, as part of mTOR complex 1 (mTORC1), drives changes in protein synthesis and metabolism in both pathological and physiological hypertrophy. We demonstrated through pharmacological and genetic methods that inhibition of class I HDACs suppressed pathological cardiac hypertrophy through inhibition of mTOR activity. Mice genetically silenced for HDAC1 and HDAC2 had a reduced hypertrophic response to thoracic aortic constriction (TAC) and showed reduced mTOR activity. We determined that the abundance of tuberous sclerosis complex 2 (TSC2), an mTOR inhibitor, was increased through a transcriptional mechanism in cardiomyocytes when class I HDACs were inhibited. In neonatal rat cardiomyocytes, loss of TSC2 abolished HDAC-dependent inhibition of mTOR activity, and increased expression of TSC2 was sufficient to reduce hypertrophy in response to phenylephrine. These findings point to mTOR and TSC2-dependent control of mTOR as critical components of the mechanism by which HDAC inhibitors blunt pathological cardiac growth. These results also suggest a strategy to modulate mTOR activity and facilitate the translational exploitation of HDAC inhibitors in heart disease.

Recombinant neuregulin 1 does not activate cardiomyocyte DNA synthesis in normal or infarcted adult mice

Objectives

Neuregulin 1 signaling plays an important role in cardiac trabecular development, and in sustaining functional integrity in adult hearts. Treatment with neuregulin 1 enhances adult cardiomyocyte differentiation, survival and/or function in vitro and in vivo. It has also been suggested that recombinant neuregulin 1β1 (NRG1β1) induces cardiomyocyte proliferation in normal and injured adult hearts. Here we further explore the impact of neuregulin 1 signaling on adult cardiomyocyte cell cycle activity.

Methods and Results

Adult mice were subjected to 9 consecutive daily injections of recombinant NRG1β1 or vehicle, and cardiomyocyte DNA synthesis was quantitated via bromodeoxyuridine (BrdU) incorporation, which was delivered using mini-osmotic pumps over the entire duration of NRG1β1 treatment. NRG1β1 treatment inhibited baseline rates of cardiomyocyte DNA synthesis in normal mice (cardiomyocyte labelling index: 0.019±0.005% vs. 0.003±0.001%, saline vs. NRG1β1, P<0.05). Acute NRG1β1 treatment did result in activation of Erk1/2 and cardiac myosin regulatory light chain (down-stream mediators of neuregulin signalling), as well as activation of DNA synthesis in non-cardiomyocytes, validating the biological activity of the recombinant protein. In other studies, mice were subjected to permanent coronary artery occlusion, and cardiomyocyte DNA synthesis was monitored via tritiated thymidine incorporation which was delivered as a single injection 7 days post-infarction. Daily NRG1β1 treatment had no impact on cardiomyocyte DNA synthesis in the infarcted myocardium (cardiomyocyte labelling index: 0.039±0.011% vs. 0.027±0.021%, saline vs. NRG1β1, P>0.05).

Summary

These data indicate that NRG1β1 treatment does not increase cardiomyocyte DNA synthesis (and consequently does not increase the rate of cardiomyocyte renewal) in normal or infarcted adult mouse hearts. Thus, any improvement in cardiac structure and function observed following neuregulin treatment of injured hearts likely occurs independently of overt myocardial regeneration.

FMRP S499 is phosphorylated independent of mTORC1-S6K1 activity

Hyperactive mammalian target of rapamycin (mTOR) is associated with cognitive deficits in several neurological disorders including tuberous sclerosis complex (TSC). The phosphorylation of the mRNA-binding protein FMRP reportedly depends on mTOR complex 1 (mTORC1) activity via p70 S6 kinase 1 (S6K1). Because this phosphorylation is thought to regulate the translation of messages important for synaptic plasticity, we explored whether FMRP phosphorylation of the S6K1-dependent residue (S499) is altered in TSC and states of dysregulated TSC-mTORC1 signaling. Surprisingly, we found that FMRP S499 phosphorylation was unchanged in heterozygous and conditional Tsc1 knockout mice despite significantly elevated mTORC1-S6K1 activity. Neither up- nor down-regulation of the mTORC1-S6K1 axis in vivo or in vitro had any effect on phospho-FMRP S499 levels. In addition, FMRP S499 phosphorylation was unaltered in S6K1-knockout mice. Collectively, these data strongly suggest that FMRP S499 phosphorylation is independent of mTORC1-S6K1 activity and is not altered in TSC.

Recent advances in heart regeneration

Although cardiac stem cells (CSCs) and tissue engineering are very promising for cardiac regenerative medicine, studies with model organisms for heart regeneration will provide alternative therapeutic targets and opportunities. Here, we present a review on heart regeneration, with a particular focus on the most recent work in mouse and zebrafish. We attempt to summarize the recent progresses and bottlenecks of CSCs and tissue engineering for heart regeneration; and emphasize what we have learned from mouse and zebrafish regenerative models on discovering crucial genetic and epigenetic factors for stimulating heart regeneration; and speculate the potential application of these regenerative factors for heart failure. A brief perspective highlights several important and promising research directions in this exciting field.

Synaptic plasticity and learning in animal models of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is caused by a mutation of either the Tsc1 or Tsc2 gene. As these genes work in concert to negatively regulate the mammalian target of rapamycin (mTOR) kinase which is involved in protein translation, mutations of these genes lead to a disinhibited mTOR activity. Both the clinical appearance of this condition including tumors, cognitive decline, and epileptic seizures and the molecular understanding of the mTOR signaling pathway, not only involved in cell growth, but also in neuronal functioning, have inspired numerous studies on learning behavior as well as on synaptic plasticity which is the key molecular mechanism of information storage in the brain. A couple of interesting animal models have been established, and the data obtained in these animals will be discussed. A special focus will be laid on differences among these models, which may be in part due to different background strains, but also may indicate pathophysiological variation in different mutations.

Metabotropic glutamate receptor-dependent long-term depression is impaired due to elevated ERK signaling in the ΔRG mouse model of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) and fragile X syndrome (FXS) are caused by mutations in negative regulators of translation. FXS model mice exhibit enhanced metabotropic glutamate receptor-dependent long-term depression (mGluR-LTD). Therefore, we hypothesized that a mouse model of TSC, ΔRG transgenic mice, also would exhibit enhanced mGluR-LTD. We measured the impact of TSC2-GAP mutations on the mTORC1 and ERK signaling pathways and protein synthesis-dependent hippocampal synaptic plasticity in ΔRG transgenic mice. These mice express a dominant/negative TSC2 that binds to TSC1, but has a deletion and substitution mutation in its GAP-domain, resulting in inactivation of the complex. Consistent with previous studies of several other lines of TSC model mice, we observed elevated S6 phosphorylation in the brains of ΔRG mice, suggesting upregulated translation. Surprisingly, mGluR-LTD was not enhanced, but rather was impaired in the ΔRG transgenic mice, indicating that TSC and FXS have divergent synaptic plasticity phenotypes. Similar to patients with TSC, the ΔRG transgenic mice exhibit elevated ERK signaling. Moreover, the mGluR-LTD impairment displayed by the ΔRG transgenic mice was rescued with the MEK-ERK inhibitor U0126. Our results suggest that the mGluR-LTD impairment observed in ΔRG mice involves aberrant TSC1/2-ERK signaling.

Impaired social interactions and motor learning skills in tuberous sclerosis complex model mice expressing a dominant/negative form of tuberin

Tuberous sclerosis complex (TSC) is a genetic disorder characterized by the development of hamartomas in multiple organs. Neurological manifestation includes cortical dysplasia, epilepsy, and cognitive deficits such as mental impairment and autism. We measured the impact of TSC2-GAP mutations on cognitive processes and behavior in, ΔRG transgenic mice that express a dominant/negative TSC2 that binds to TSC1, but has mutations affecting its GAP domain and its rabaptin-5 binding motif, resulting in inactivation of the TSC1/2 complex. We performed a behavioral characterization of the ΔRG transgenic mice and found that they display mild, but significant impairments in social behavior and rotarod motor learning. These findings suggest that the ΔRG transgenic mice recapitulate some behavioral abnormalities observed in human TSC patients.

Increased levels of anxiety-related behaviors in a Tsc2 dominant negative transgenic mouse model of tuberous sclerosis

Tuberous sclerosis (TSC) is a single-gene disorder caused by heterozygous mutations in the TSC1 or TSC2 gene. TSC is often associated with neurological (e.g., epilepsy), cognitive (intellectual disabilities, specific neuropsychological impairments) and behavioral pathologies (e.g., autism, attention deficit hyperactivity disorder). In addition, there is a high prevalence of psychiatric problems in TSC populations, including anxiety and mood disorders. To date, little is known about the pathogenetic bases of these associated psychiatric symptoms; for instance, it is unclear whether they are rooted in TSC-associated neurobiological alterations or whether they are secondary psychological phenomena (e.g., because individuals have to cope with the burden of the disease). Here, we report elevated levels of anxiety-related behaviors and mild deficits in two hippocampal-dependent learning tasks in a Tsc2 dominant negative transgenic mouse model of TSC. These findings establish a mouse model for TSC-related anxiety phenotypes and suggest that anxiety disorders in TSC have a biological foundation.

Epigenetic control of cardiomyocyte production in response to a stress during the medaka heart development

The size and morphology of organs are largely determined by a genetic program. However in some cases, an epigenetic mechanism influences the process of organ development. Particularly, epigenetic factors such as hemodynamic stress and blood pressure affect the morphogenesis of cardiac chambers and valves. Here, we report that the epigenetic influences affect the cardiomyocyte production. Taking advantage of longer developmental period of medaka fish, we could examine the later emerging tissue responses to the defect of ventricular beating, which occurred in the hozuki (hoz) mutant that harbors the mutated ventricular myosin heavy chain (vmhc) gene. The mutant showed a remarkable ventricular enlargement, and we showed that this enlargement was due to an excess production of ventricular cardiomyocytes in addition to the lack of concentric chamber growth. By experimental blockade of blood flow, we demonstrated that an elevated cardiac pressure was responsible for the aberrant cardiomyocyte production. From these data, we propose that the epigenetic tissue response to a stressed situation controls the production of cardiomyocytes to attain a fine tuning of heart formation.

Expression of a transgene encoding mutant p193/CUL7 preserves cardiac function and limits infarct expansion after myocardial infarction

BACKGROUND

Transgenic mice expressing the dominant interfering p193 protein in cardiomyocytes (MHC-1152stop mice) exhibit an induction of cell cycle activity and altered remodelling after experimental myocardial infarction (MI).

OBJECTIVE

To determine whether the altered remodelling results in improved cardiac function in the MHC-1152stop mice after MI, as compared with non-transgenic mice.

METHODS

MHC-1152stop mice and non-transgenic littermates were subjected to experimental MI via permanent occlusion of the coronary artery. Infarct size was determined at 24 h and at 4 weeks after MI, and left ventricular pressure-volume measurements were performed at 4 weeks after MI in infarcted and sham-operated animals.

RESULTS

Infarct size in MHC-1152stop mice and non-transgenic littermates was not statistically different at 24 h after MI, as measured by tetrazolium staining. Morphometric analysis showed that infarct scar expansion at 4 weeks after MI was reduced by 10% in the MHC-1152stop mice (p<0.05). No differences in cardiac function were detected between sham-operated MHC-1152stop mice and their non-transgenic littermates. However, at 4 weeks after MI, the ventricular isovolumic relaxation time constant (tau) was decreased by 19% (p<0.05), and the slope of the dP/dt(max)-EDV relationship was increased 99% (p<0.05), in infarcted MHC-1152stop mice as compared with infarcted non-transgenic littermates.

CONCLUSION

Expression of the dominant interfering p193 transgene results in a decrease in infarct scar expansion and preservation of myocardial function at 4 weeks after MI. Antagonism of p193 activity may represent an important strategy for the treatment of MI.

Cardiomyocyte cell cycle activation improves cardiac function after myocardial infarction

AIMS

Cardiomyocyte loss is a major contributor to the decreased cardiac function observed in diseased hearts. Previous studies have shown that cardiomyocyte-restricted cyclin D2 expression resulted in sustained cell cycle activity following myocardial injury in transgenic (MHC-cycD2) mice. Here, we investigated the effects of this cell cycle activation on cardiac function following myocardial infarction (MI).

METHODS AND RESULTS

MI was induced in transgenic and non-transgenic mice by left coronary artery occlusion. At 7, 60, and 180 days after MI, left ventricular pressure-volume measurements were recorded and histological analysis was performed. MI had a similar adverse effect on cardiac function in transgenic and non-transgenic mice at 7 days post-injury. No improvement in cardiac function was observed in non-transgenic mice at 60 and 180 days post-MI. In contrast, the transgenic animals exhibited a progressive and marked increase in cardiac function at subsequent time points. Improved cardiac function in the transgenic mice at 60 and 180 days post-MI correlated positively with the presence of newly formed myocardial tissue which was not apparent at 7 days post-MI. Intracellular calcium transient imaging indicated that cardiomyocytes present in the newly formed myocardium participated in a functional syncytium with the remote myocardium.

CONCLUSION

These findings indicate that cardiomyocyte cell cycle activation leads to improvement of cardiac function and morphology following MI and may represent an important clinical strategy to promote myocardial regeneration.

Endogenous cardiac stem cells

In the past few years it has been established that the heart contains a reservoir of stem and progenitor cells. These cells are positive for various stem/progenitor cell markers (Kit, Sca-1, Isl-1, and Side Population (SP) properties). The relationship between the various cardiac stem cells (CSC) and progenitor cells described awaits clarification. Furthermore, they may open a new therapeutic strategies of cardiac repair based on the regeneration potential of cardiac stem cells. Currently, cellular cardiomyoplasty is actively explored as means of regenerating damaged myocardium using several different cell types. CSCs seem a logical cell source to exploit for cardiac regeneration therapy. Their presence into the heart, the frequent co-expression of early cardiac progenitor transcription factors, and the capability for ex vivo and in vivo differentiation toward the cardiac lineages offer promise of enhanced cardiogenicity compared to other cell sources. CSCs, when isolated from various animal models by selection based on c-Kit, Sca-1, and/or MDR1, have shown cardiac regeneration potential in vivo following injection in the infracted myocardium. Recently, we have successfully isolated CSCs from small biopsies of human myocardium and expanded them ex vivo by many folds without losing differentiation potential into cardiomyocytes and vascular cells, bringing autologous transplantation of CSCs closer to clinical evaluation. These cells are spontaneously shed from human surgical specimens and murine heart samples in primary culture. This heterogeneous population of cells forms multi-cellular clusters, dubbed cardiospheres (CSs), in suspension culture. CSs are composed of clonally-derived cells, consist of proliferating c-Kit positive cells primarily in their core and differentiating cells expressing cardiac and endothelial cell markers on their periphery. Although the intracardiac origin of adult myocytes has been unequivocally documented, the potential of an extracardiac source of cells, able to repopulate the lost CSCs in pathological conditions (infarct) cannot be excluded and will be discussed in this review. The delivery of human CSs or of CSs-derived cells into the injured heart of the SCID mouse resulted in engraftment, migration, myocardial regeneration and improvement of left ventricular function. Our method for ex vivo expansion of resident CSCs for subsequent autologous transplantation back into the heart, may give these cell populations, the resident and the transplanted one, the combined ability to mediate myocardial regeneration to an appreciable degree, and may change the way in which cardiovascular disease will be approached in the future.

Cytotoxic effect of 5-aminoimidazole-4-carboxamide-1-beta-4-ribofuranoside (AICAR) on childhood acute lymphoblastic leukemia (ALL) cells: implication for targeted therapy

Background

Acute lymphoblastic leukemia (ALL) is the most common hematological malignancy affecting children. Despite significant progress and success in the treatment of ALL, a significant number of children continue to relapse and for them, outcome remains poor. Therefore, the search for novel therapeutic approaches is warranted. The aim of this study was to investigate the AMP activated protein kinase (AMPK) as a potential target in childhood acute lymphoblastic leukemia (ALL) subtypes characterized by non-random translocation signature profiles. We evaluated the effects of the AMPK activator AICAR on cell growth, cell cycle regulators and apoptosis of various childhood ALL cells.

Results

We found that treatment with AICAR inhibited cell proliferation, induced cell cycle arrest in G1-phase, and apoptosis in CCRF-CEM (T-ALL), NALM6 (Bp-ALL), REH (Bp-ALL, TEL/AML1) and SupB15 (Bp-ALL, BCR/ABL) cells. These effects were abolished by treatment with the adenosine kinase inhibitor 5'-iodotubericidin prior to addition of AICAR indicating that AICAR's cytotoxicity is mediated through AMPK activation. Moreover, we determined that growth inhibition exerted by AICAR was associated with activation of p38-MAPK and increased expression of the cell cycle regulators p27 and p53. We also demonstrated that AICAR mediated apoptosis through the mitochondrial pathway as revealed by the release of cytochrome C and cleavage of caspase 9. Additionally, AICAR treatment resulted in phosphorylation of Akt suggesting that activation of the PI3K/Akt pathway may represent a compensatory survival mechanism in response to apoptosis and/or cell cycle arrest. Combined treatment with AICAR and the mTOR inhibitor rapamycin resulted in additive anti-proliferative activity ALL cells.

Conclusion

AICAR-mediated AMPK activation was found to be a proficient cytotoxic agent in ALL cells and the mechanism of its anti-proliferative and apoptotic effect appear to be mediated via activation of p38-MAPK pathway, increased expression of cell cycle inhibitory proteins p27 and p53, and downstream effects on the mTOR pathway, hence exhibiting therapeutic potential as a molecular target for the treatment of childhood ALL. Therefore, activation of AMPK by AICAR represents a novel approach to targeted therapy, and suggests a role for AICAR in combination therapy with inhibitors of the PI3K/Akt/mTOR pathways for the treatment of childhood in ALL.

Cardiac myocyte cell cycle control in development, disease, and regeneration

Cardiac myocytes rapidly proliferate during fetal life but exit the cell cycle soon after birth in mammals. Although the extent to which adult cardiac myocytes are capable of cell cycle reentry is controversial and species-specific differences may exist, it appears that for the vast majority of adult cardiac myocytes the predominant form of growth postnatally is an increase in cell size (hypertrophy) not number. Unfortunately, this limits the ability of the heart to restore function after any significant injury. Interest in novel regenerative therapies has led to the accumulation of much information on the mechanisms that regulate the rapid proliferation of cardiac myocytes in utero, their cell cycle exit in the perinatal period, and the permanent arrest (terminal differentiation) in adult myocytes. The recent identification of cardiac progenitor cells capable of giving rise to cardiac myocyte-like cells has challenged the dogma that the heart is a terminally differentiated organ and opened new prospects for cardiac regeneration. In this review, we summarize the current understanding of cardiomyocyte cell cycle control in normal development and disease. In addition, we also discuss the potential usefulness of cardiomyocyte self-renewal as well as feasibility of therapeutic manipulation of the cardiac myocyte cell cycle for cardiac regeneration.

Cell-based approaches for cardiac repair

Many forms of cardiovascular disease are associated with cardiomyocyte loss via necrosis and/or apoptosis. The cumulative loss of contractile cells ultimately results in diminished cardiac function. Numerous approaches have been employed to reduce the rate of cardiomyocyte loss, or alternatively, to repopulate the heart with new cardiomyocytes. Strategies aimed at repopulating the heart include cardiomyocyte cell therapy, myogenic stem cell therapy, and cell cycle activation therapy. All three approaches are based on the assumption that the de novo cardiomyocytes will participate in a functional syncytium with the surviving myocardium. This review will discuss the current status of interventions aimed at repopulating the heart with functional cardiomyocytes.

Akt regulates nuclear/cytoplasmic localization of tuberin

The autosomal dominantly inherited disease tuberous sclerosis (TSC) affects approximately 1 in 6000 individuals and is characterized by the development of tumors, named hamartomas, in different organs. TSC1, encoding hamartin, and TSC2, encoding tuberin are tumor suppressor genes responsible for TSC. Hamartin and tuberin form a complex, of which tuberin is assumed to be the functional component. The TSC proteins have been implicated in the control of cell cycle by activating the cyclin-dependent kinase inhibitor p27 and in cell size regulation by inhibiting the mammalian target of rapamycin (mTOR)/p70S6K cascade. Phosphorylation of S939 and T1462 by Akt downregulates tuberin's potential to inhibit mTOR/p70S6K. Here, we show that this tuberin phosphorylation by Akt does not affect tuberin-mediated control of p27 protein amounts. This demonstrates that regulating p27 protein amounts and mTOR/p70S6K are separable functions of tuberin. Furthermore, we found that phosphorylation by Akt triggers upregulation of cytoplasmic and downregulation of nuclear tuberin. In cycling cells with high Akt activity, tuberin is predominantly localized to the cytoplasm. In arrested G0 cells with downregulated Akt activity, a significant proportion of tuberin is localized to the nucleus. Upon re-entry into the normal ongoing cell cycle, nuclear localization of tuberin is downregulated parallel to the activation of Akt. Recently, the mTOR/p70S6K cascade has been demonstrated to exist in both the cytoplasm and nucleus. We here also found that tuberin harbors the potential to regulate p70S6K activity in both the cytoplasm and nucleus. This description of functional tuberin in the cytoplasm and the nucleus together with our observation of Akt-controlled and cell cycle-regulated tuberin localization are of particular interest for a further understanding of tuberin's function as a gate keeper of the G0 cell status as well as of Akt's activity to control cell proliferation.

Cardiac regeneration: repopulating the heart

Many forms of pediatric and adult heart disease result from a deficiency in cardiomyocyte number. Through repopulation of the heart with new cardiomyocytes (that is, induction of regenerative cardiac growth), cardiac disease potentially can be reversed, provided that the newly formed myocytes structurally and functionally integrate in the preexisting myocardium. A number of approaches have been utilized to effect regenerative growth of the myocardium in experimental animals. These include interventions aimed at enhancing the ability of cardiomyocytes to proliferate in response to cardiac injury, as well as transplantation of cardiomyocytes or myogenic stem cells into diseased hearts. Here we review efforts to induce myocardial regeneration. We also provide a critical review of techniques currently used to assess cardiac regeneration and functional integration of de novo cardiomyocytes.

Tuberin negatively affects BCL-2’s cell survival function

Uncontrolled cell cycle progression and cell growth are key properties of tumor cells. The tumor suppressor genes responsible for the autosomal dominantly inherited disease tuberous sclerosis (TSC) have been demonstrated to control both, cell cycle and cell size regulation. Hamartin, encoded by TSC1, and tuberin, encoded by TSC2, form a complex, of which tuberin is assumed to be the functional component. Loss of TSC genes function triggers hamartoma development in TSC patients. However, in vivo mostly tumor cell development is rapidly terminated via apoptosis. BCL-2, the founding member of the BCL-2 family of proteins, is well known for its anti-apoptotic properties. Here we show that pro-apoptotic actinomycin D cannot interfere with BCL-2's cell survival functions. However, we found tuberin to negatively regulate BCL-2's anti-apoptotic effects on low serum-induced apoptosis. These findings warrant further investigations to elucidate the molecular mechanism underlying tuberin's negative effects on cell survival.

Transgenic Expression of Dominant Negative Tuberin through a Strong Constitutive Promoter Results in a Tissue-specific Tuberous Sclerosis Phenotype in the Skin and Brain

Tuberous sclerosis (TS) is a common autosomal dominant disorder caused by loss or malfunction of hamartin (tsc1) or tuberin (tsc2). Many lesions in TS do not demonstrate loss of heterozygosity for these genes, implying that dominant negative forms of these genes may account for some hamartomas and neoplasms in TS. To test this hypothesis, we expressed a dominant negative allele of tuberin (DeltaRG) behind the cytomegalovirus promoter in NIH3T3 cells and transgenic mice. This allele binds hamartin but has a deletion in the C terminus of tuberin, leading to constitutive activation of rap1 and rab5/rabaptin. Expression of DeltaRG in NIH3T3 cells led to a strong induction of reactive oxygen species, induction of vascular endothelial growth factor, and malignant transformation in vivo. Expression of DeltaRG driven by the constitutive cytomegalovirus promoter led to high level expression in all murine tissues examined, including skin, kidney, liver, and brain. Surprisingly, mice expressing the DeltaRG transgene developed a fibrovascular collagenoma in the dermis, which closely resembles the Shagreen patch observed in human patients with TS. In addition, numerous small subpial collections of external granule cells in the cerebellum were observed, which may be the murine equivalent of subependymal giant cell astrocytomas or tubers commonly seen in TS patients. Thus, expression of a dominant negative tuberin in multiple tissues can lead to a tissue-specific phenotype resembling some of the findings in human TS. Our data are the first to demonstrate that specific signaling abnormalities underlie specific hamartomas in a model of a human genetic disorder.

Modulation of the Cardiomyocyte Cell Cycle in Genetically Altered Animals

Many forms of cardiovascular disease are associated with cardiomyocyte loss via apoptosis and/or necrosis. Although there is currently debate regarding the level at which adult cardiomyocytes can reenter the cell cycle and proliferate, it is clear that the intrinsic regenerative growth capacity is insufficient to reverse the progression to failure in badly injured hearts. The ability to reactivate cardiomyocyte proliferation in damaged hearts might permit regenerative growth, provided that the nascent cells are able to participate in a functional syncytium with the surviving myocardium. In this review, techniques commonly used to monitor cardiomyocyte cell cycle activity in normal and injured hearts are discussed. In addition, several genetic models are described wherein the expression of fundamental cell cycle regulatory proteins has been altered in cardiomyocytes.

Autism: a target of pharmacotherapies?

Autism is reaching epidemic proportions. The diagnosis can be made as early as 2 years of age, and autistic patients are expected to have a normal life span. Thus, in terms of the number of 'patient years', autism spectrum disorder (ASD) represents a market that is as large as that of the biggest neurological indication, Alzheimer's disease. However, despite the clear unmet medical need no effective treatment is yet available. This could be because the biology of ASD is not clearly understood and thus proper drug treatment has not been possible. However, significant advances are being made toward understanding the mechanisms of the disease. Here, we review the most recent preclinical advances in the hope that they will lead to a breakthrough in the near future.

Thyroid-stimulating hormone/cAMP and glycogen synthase kinase 3beta elicit opposing effects on Rap1GAP stability

Beyond regulating Rap activity, little is known regarding the regulation and function of the Rap GTPase-activating protein Rap1GAP. Tuberin and E6TP1 protein levels are tightly regulated through ubiquitin-mediated proteolysis. A role for these RapGAPs, along with SPA-1, as tumor suppressors has been demonstrated. Whether Rap1GAP performs a similar role was investigated. We now report that Rap1GAP protein levels are dynamically regulated in thyroid-stimulating hormone (TSH)-dependent thyroid cells. Upon TSH withdrawal, Rap1GAP undergoes a net increase in phosphorylation followed by proteasome-mediated degradation. Sequence analysis identified two putative destruction boxes in the Rap1GAP C-terminal domain. Glycogen synthase kinase 3beta (GSK3beta) phosphorylated Rap1GAP immunoprecipitated from thyroid cells, and GSK3beta inhibitors prevented phosphorylation and degradation of endogenous Rap1GAP. Co-expression of GSK3beta and Rap1GAP in human embryonic kidney 293 cells stimulated proteasome-dependent Rap1GAP turnover. Mutational analysis established a role for serine 525 in the regulation of Rap1GAP stability. Overexpression of Rap1GAP in thyroid cells impaired TSH/cAMP-stimulated p70S6 kinase activity and cell proliferation. These data are the first to show that Rap1GAP protein levels are tightly regulated and are the first to support a role for Rap1GAP as a tumor suppressor.

Autism: a large unmet medical need and a complex research problem

Autism has been becoming the focus of attention as its apparently increasing prevalence is better appreciated. According to some estimates, the frequency of children with autistic spectrum disorder (ASD) can be as high as 1 in 150. The diagnosis can be made as early as 2 years of age, and autistic patients often have a normal life span. Thus, in terms of the number of "patient years," ASD represents a market that is as large as that of the biggest neurological indication, Alzheimer's disease. Despite the clear unmet medical need, no effective treatment is available. This may be because the mechanism of ASD is not understood. The aim of the present paper is to review recent advances in autism research and to discuss some of the most stressing problems mainly from a preclinical research standpoint. We hope to draw attention to the need to study this devastating disease that places an enormous burden on the society in general and the relatives and caregivers of autistic patients in particular.

Tumour suppressors hamartin and tuberin: intracellular signalling

Tumour suppressors hamartin and tuberin, encoded by tuberous sclerosis complex 1(TSC1) and TSC2 genes, respectively, are critical regulators of cell growth and proliferation. Mutations in TSC1 and TSC2 genes are the cause of an autosomal dominant disorder known as tuberous sclerosis complex (TSC). Another genetic disorder, lymphangioleiomyomatosis (LAM), is also associated with mutations in the TSC2 gene. Hamartin and tuberin control cell growth by negatively regulating S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1), potentially through their upstream modulator mammalian target of rapamycin (mTOR). Growth factors and insulin promote Akt/PKB-dependent phosphorylation of tuberin, which in turn, releases S6K1 from negative regulation by tuberin and results in the activation of S6K1. Although much has been written regarding the molecular genetics of TSC and LAM, which is associated with either the loss of or mutation in the TSC1 and TSC2 genes, few reviews have addressed the intracellular signalling pathways regulated by hamartin and tuberin. The current review will fill the gap in our understanding of their role in cellular signalling networks, and by improving this understanding, an integrated picture regarding the normal function of tuberin and hamartin is beginning to emerge.

Cell size regulation by the human TSC tumor suppressor proteins depends on PI3K and FKBP38

TSC1 and TSC2 are responsible for the tumor suppressor gene syndrome tuberous sclerosis (TSC). Mammalian TSC genes have been shown to be involved in cell cycle regulation. Recently, in Drosophila, these data have been confirmed and TSC genes have further been demonstrated to affect cell size control. Here we provide supporting data for the fact that the latter function is conserved in mammals. Human TSC1 and TSC2 trigger mammalian cell size reduction and a dominant-negative TSC2 mutant induces increased size. These effects occur in all cell cycle phases, are dependent on the activity of the phosphoinositide-3-kinase and are abolished by co-overexpression of a dominant-negative Akt mutant. Two independent naturally occurring and disease-causing mutations within the TSC2 gene eliminate tuberin's capacity to affect cell size control, emphasizing the relevance of this function for the development of the disease. The same mutations have earlier been shown not to affect tuberin's antiproliferative capacity. That the consequences of modulated TSC gene expression on cell proliferation and on cell size can be assigned to separable functions is further supported by two findings: A mutation within the TSC1 gene, earlier shown to still harbor anti-proliferative effects, was found to eliminate the cell size regulating functions. An important mammalian cell size regulator, c-Myc, was found to inhibit tuberin's antiproliferative capacity, but to have no effects on tuberin-dependent cell size control. To obtain further mechanistical insights, microarray screens for genes involved in TSC1- or TSC2-mediated cell size effects were performed. Antisense experiments revealed that the so observed regulation of the FK506-binding protein, FKBP38, plays a role in TSC gene-dependent cell size regulation. These data provide new insights into mammalian cell size regulation and allow a better understanding of the function of human TSC genes.

Autologous stem cells for functional myocardial repair

Recent experimental studies based on innovative hypothesis utilizing cell therapy for the damaged myocardium are recently becoming increasingly promising. The naturally occurring myocardial reparative process is apparently complex and relatively inefficient. It consists of up-regulation of progenitor cell release from the bone marrow after myocardial infarction, homing of these cells to the injured tissue, and differentiation of these progenitor cells into vascular cells and cardiomyocytes within the infarcted tissue. Accordingly, there are two main strategies to regenerate myocardium with autologous stem cells: (1) Extracting stem cells from the bone marrow and injecting these cells into the damaged area, (2) Increasing the efficiency of the naturally occurring reparative process by increasing the mobilization of bone marrow-derived stem cells after myocardial infarction. This review summarizes the growing field of autologous stem cell utilization over the past decade and outlines scientific and clinical hurdles that need to be overcome before this therapy can fully reach its clinical potential.

Cell cycle regulation to repair the infarcted myocardium

Lower vertebrates such as newt and zebrafish are able to reactivate high levels of cardiomyocyte cell cycle activity in response to experimental injury resulting in apparent regeneration. In contrast, damaged myocardium is replaced by fibrotic scar tissue in higher vertebrates. This process compromises the contractile function of the surviving myocardium, ultimately leading to heart failure. Various strategies are being pursued to augment myocyte number in the diseased hearts. One approach entails the reactivation of cell cycle in surviving cardiomyocytes. Here, we provide a summary of methods to monitor cell cycle activity, and interventions demonstrating positive cell cycle effects in cardiomyocytes as well as discuss the potential utility of cell cycle regulation to augment myocyte number in diseased hearts.

Nongenomic estrogen action regulates tyrosine phosphatase activity and tuberin stability

Estrogen action and tuberin function has been suggested to play a crucial role in the proliferation of lung smooth muscle-like cells and/or myofibroblasts in pulmonary lymphangioleiomyomatosis (LAM). Tuberin is a tumor suppressor phosphoprotein, which also regulates fluid phase endocytosis. Its activity, turnover and complex association with hamartin depends on its phosphorylation status. We have recently reported that nongenomic estrogen action regulates the phosphorylation status of several cytoplasmic proteins. Herein, we demonstrate that estrogen increases tyrosine phosphatase activity, which can be abrogated by antiestrogen ICI 182780 and tyrosine phosphatase inhibitor bpV(phen), but not by the protein synthesis inhibitor cyclohexamide. Furthermore, we show that estrogen transiently enhances the turnover of tuberin, which follows an inverse pattern to that observed for tyrosine phosphatase and endocytosis activity. We showed that tuberin phosphorylation protects it from degradation and induces its accumulation in female human lung fibroblasts and myofibroblasts. Our results suggest that nongenomic estrogen action induces tyrosine phosphatase activity that regulates stability of tyrosine phosphorylated proteins, including tuberin, which may play a crucial role in cellular specific functions such as endocytosis.

Tuberous sclerosis as an underlying basis for infantile spasm

The study of the molecular pathogenesis of epilepsy in tuberous sclerosis has taken on a new dimension with the identification of the TSC1 and TSC2 genes. While the development of seizures is ultimately related to mutations in one of the two genes, the mechanism underlying the genotype-phenotype relationship remains a puzzle. This chapter, presented arguments in favor of the hypothesis that abnormal cortical excitability originates in and around focal areas of structural malformations (i.e., cortical tubers and dysplasia) and that these "lesions" are the biologic consequences of tuberin and/or hamartin dysfunction. This model relies on the concept of a multistep process occurring early in cortical development whereby certain progenitor cells in the germinal layer of the ventricular zone destined for the cortex undergo inactivation of the TSC1 or TSC2 locus (Fig. 2). Immature neuroepithelial cells carrying "two-hit" mutations at either locus are believed to proliferate, migrate, and differentiate abnormally, resulting in the formation of "dysplastic" cells that are heterotopic in distribution. The pathology of the classic tuber suggests a clonal expansion of the bizarre-appearing giant cells that display incomplete, multilineage, and often ambiguous phenotype. Further, they infiltrate the six-layered structure of the cortex to form a poorly circumscribed area containing a mixture of cell types to create a highly disorganized region of a neuronal and glial network. Whether arising from the dysplastic "two-hit" target cells themselves or adjacent "innocent" bystander neurons as a result of aberrant cell-cell interaction, abnormal epileptic discharges originate from these structural abnormalities. The mechanism of how TSC1 and TSC2 inactivation causes tuber to develop is not known, but emerging experimental evidence suggests a disruption of the hamartin-tuberin "haloenzyme" in the regulation of cell size and number via the insulin signaling pathway and a p27/CDK-dependent mechanism. Biochemically, TSC1/TSC2 may associate with cytoskeletal components and vesicular adaptors in regulating sorting and trafficking of newly synthesized and recycling proteins in the post-Golgi compartments. As such, spatial and temporal localization of proteins may be affected in tuberin or hamartin-deficient neuronal cells where proper synaptic delivery of neurotransmitters plays an important role in normal cerebral function. We are in the earliest stages of understanding the role of TSC genes in epileptogenesis. To test the hypothesis outlined earlier, there is a need to create in vitro and in vivo models, as direct human experimentation is not feasible. To date, there are several rodent models of TSC, both spontaneous and recombinant strains. Unfortunately, none has consistently developed spontaneous cortical tubers, although one example was reported in an otherwise asymptomatic Eker rat (Mizuguchi et al., 2000). If the "two-hit" hypothesis is operational in tubers, as seen in other TSC lesions, it follows that radiation and chemical carcinogens should have a quantitative and qualitative effect on the development of these cerebral malformations. In preliminary experiments, we have found evidence of areas of cortical dysplasia in Eker rats irradiated early in life (Fig. 3). These dysplastic [figure: see text] cells stained positively with NeuN, consistent with the immunophenotype of cells in tubers. Alternatively, one can analyze the in vivo and in vitro characteristics of neuroprogenitor cells that are deficient of hamartin or tuberin. While homozygous mutants of TSC1 and TSC2 are lethal during midgestation, one of several techniques can be used to derive mutant neuroepithelial cells, including the procurement of -/- cells prior to embryonic deaths and subsequent cortical transplantation into syngeneic animals, development of conditional "knock outs," or chimeric mutants. These approaches, with their unique advantages and disadvantages, will be helpful in gaining insights into the development of cortical tubers and their electrophysiologic consequences.

Tuberous sclerosis in a 19-week fetus: immunohistochemical and molecular study of hamartin and tuberin

Tuberous sclerosis complex (TSC) is a genetically heterogeneous disease caused by mutations of TSC1 or TSC2 genes. It involves multiple organ systems resulting in mild to lethal hamartoma formation due to gene mutation in the germ line and loss of heterozygosity (LOH) in somatic cells. Hamartin (TSC1) and tuberin (TSC2) are expressed broadly. However, little is known about tissue susceptibility to hamartomas when equal or similar amounts of TSC gene expression are present. In this study, we present a 19-week gestational age fetus with pathological features of TSC, which was confirmed by finding LOH of TSC2 in a cardiac rhabdomyoma. Developmental expression of hamartin and tuberin in the TSC fetus, an age-matched non-TSC fetus, and a 26-week gestational age non-TSC fetus were analyzed by immunohistochemistry. We found that in addition to the differential expression of the TSC genes in some normal tissues compared with that in the TSC-affected fetus, the cellular localization and distribution of hamartin and tuberin were dramatically different in different tissues. In general, hamartin and tuberin are mainly expressed in epithelial cells, myocytes, and neural tissues. By comparing the incidence of the hamartomas in early childhood and gene expression in tissues, it appears that tissues with co-expression of hamartin and tuberin are prone to a higher incidence of hamartomas than those expressing only one protein, or two proteins but in different patterns of cellular localization.

Reduction of expression of tuberin, the tuberous-sclerosis-complex-gene-2 product in tuberous sclerosis complex associated connective tissue nevi and sporadic squamous and basal cell carcinomas

BACKGROUND

Patients affected with tuberous sclerosis complex (TSC) are prone to the development of multiple benign tumors of the skin and other organs. Tuberin, the protein product of the tuberous-sclerosis-complex-2 tumor suppressor gene (TSC2) has been shown to inhibit cell proliferation. In TSC associated kidney tumors and sporadic brain tumors the loss/reduction of tuberin has been shown.

METHODS

Specimens of nine squamous cell carcinomas (SCC) and five basal cell carcinomas (BCC) from patients without TSC and six biopsies of connective tissue nevi (CTN) of patients with TSC were obtained. Specimens were analyzed by immunoblotting for the expression of tuberin.

RESULTS

Absent or reduced levels of tuberin were detected in the dermal parts of three of six shagreen patches, two of five BCC, and four of nine SCC.

CONCLUSIONS

In tumors/hamartomas of patients with TSC the complete loss of TSC2 and tuberin is a mechanism which could be shown for CTN, thereby excluding the possibility of haploinsufficiency of TSC2. In a substantial number of cutaneous BCC and SCC the loss or downregulation of tuberin seems to be epigenetic, as alterations of TSC2 are not known in these tumors. The absence or reduction of tuberin might contribute to their proliferation.

The role of stem cells in skeletal and cardiac muscle repair

In postnatal muscle, skeletal muscle precursors (myoblasts) can be derived from satellite cells (reserve cells located on the surface of mature myofibers) or from cells lying beyond the myofiber, e.g., interstitial connective tissue or bone marrow. Both of these classes of cells may have stem cell properties. In addition, the heretical idea that post-mitotic myonuclei lying within mature myofibers might be able to re-form myoblasts or stem cells is examined and related to recent observations for similar post-mitotic cardiomyocytes. In adult hearts (which previously were not considered capable of repair), the role of replicating endogenous cardiomyocytes and the recruitment of other (stem) cells into cardiomyocytes for new cardiac muscle formation has recently attracted much attention. The relative contribution of these various sources of precursor cells in postnatal muscles and the factors that may enhance stem cell participation in the formation of new skeletal and cardiac muscle in vivo are the focus of this review. We concluded that, although many endogenous cell types can be converted to skeletal muscle, the contribution of non-myogenic cells to the formation of new postnatal skeletal muscle in vivo appears to be negligible. Whether the recruitment of such cells to the myogenic lineage can be significantly enhanced by specific inducers and the appropriate microenvironment is a current topic of intense interest. However, dermal fibroblasts appear promising as a realistic alternative source of exogenous myoblasts for transplantation purposes. For heart muscle, experiments showing the participation of bone marrow-derived stem cells and endothelial cells in the repair of damaged cardiac muscle are encouraging.

Tuberous sclerosis-associated lesions of the kidney, brain, and skin are angiogenic neoplasms

BACKGROUND

Tuberous sclerosis is an autosomal dominant condition characterized by the development of benign neoplasms of the brain, kidney, and skin. Progressive growth and malignant transformation of brain and kidney lesions constitute the major cause of morbidity and mortality in adults with tuberous sclerosis. In addition, growth of skin lesions may be disfiguring to patients.

OBJECTIVE

The purpose of this study was to determine whether benign tumors in patients with tuberous sclerosis are angiogenic.

METHODS

Brain, kidney, and skin tumors from patients with tuberous sclerosis were stained with CD31, a specific marker of vascular endothelium. In addition, we used Northern blot analysis to demonstrate that renal angiomyolipoma cells express the potent angiogenesis stimulator vascular endothelial growth factor (VEGF).

RESULTS

Brain, kidney, and skin neoplasms from patients with tuberous sclerosis are highly angiogenic. Renal angiomyolipoma cells produce the potent angiogenic factor VEGF.

CONCLUSION

Benign neoplasms of patients with tuberous sclerosis are highly vascular. Our results provide a rationale for antiangiogenic therapy in the treatment and prevention of tuberous sclerosis-associated neoplasms.

Evidence for separable functions of tuberous sclerosis gene products in mammalian cell cycle regulation

Tuberous sclerosis is an autosomal dominant disease affecting approximately 1 in 6,000 individuals. It is caused by mutations in either TSC1 on chromosome 9q34, which encodes hamartin, or TSC2 on chromosome 16p13.3, which encodes tuberin. The growths, named hamartomas, characteristically occur in different organs of patients and are speculated to result from defects in proliferation control. The observation that hamartin and tuberin can interact in vivo suggests that they might function in the same complex. Here we show that hamartin can affect proliferation control independent of the presence of functional tuberin and that binding to hamartin is not essential for tuberin to affect proliferation. Ectopic expression of hamartin negatively regulates proliferation to a similar extent in tuberin-positive and tuberin-negative cells; this is accompanied by binding to tuberin and upregulation of endogenous p27 in tuberin-positive cells and is without effects on p27 expression in the latter. Our data show for the first time that TSC proteins possess separable functions. We further demonstrate that hamartin can deregulate proliferation control by different mechanisms depending on the presence of tuberin. Besides an overlap in many features of patients with TSC1 and TSC2 mutations, data has accumulated that provides evidence for specific clinical differences. This study provides new insights into the cellular roles of TSC proteins and initiates a discussion of whether separable functions of these proteins might be associated with the clinical differences of TSC1- and TSC2-associated disease.

Molecular genetics and animal models in autistic disorder

Autistic disorder is a behavioural syndrome beginning before the age of 3 years and lasting over the whole lifetime. It is characterised by impaired communication, impaired social interactions, and repetitive interests and behaviour. The prevalence is about 7/10,000 taking a restrictive definition and more than 1/500 with a broader definition, including all the pervasive developmental disorders. The importance of genetic factors has been highlighted by epidemiological studies showing that autistic disorder is one of the most genetic neuropsychiatric diseases. The relative risk of first relatives is about 100-fold higher than the risk in the normal population and the concordance in monozygotic twin is about 60%. Different strategies have been applied on the track of susceptibility genes. The systematic search of linked loci led to contradictory results, in part due to the heterogeneity of the clinical definitions, to the differences in the DNA markers, and to the different methods of analysis used. An oversimplification of the inferred model is probably also cause of our disappointment. More work is necessary to give a clearer picture. One region emerges more frequently: the long arm of chromosome 7. Several candidate genes have been studied and some gave indications of association: the Reelin gene and the Wnt2 gene. Cytogenetical abnormalities are frequent at 15q11-13, the region of the Angelman and Prader-Willi syndrome. Imprinting plays an important role in this region, no candidate gene has been identified in autism. Biochemical abnormalities have been found in the serotonin system. Association and linkage studies gave no consistent results with some serotonin receptors and in the transporter, although it seems interesting to go further in the biochemical characterisation of the serotonin transporter activity, particularly in platelets, easily accessible. Two monogenic diseases have been associated with autistic disorder: tuberous sclerosis and fragile X. A better knowledge of the pathophysiology of these disorders can help to understand autism. Different other candidate genes have been tested, positive results await replications in other samples. Animal models have been developed, generally by knocking out the different candidate genes. Behaviour studies have mainly focused on anxiety and learning paradigms. Another group of models results from surgical or toxic lesions of candidate regions in the brain, in general during development. The tools to analyse these animals are not yet standardised, and an important effort needs to be undertaken.

Tuberous sclerosis causing mutants of the TSC2 gene product affect proliferation and p27 expression

The autosomal dominant disease tuberous sclerosis (TSC) is caused by mutations in either TSC1 on chromosome 9q34, encoding hamartin, or TSC2 on chromosome 16p13.3, encoding tuberin. TSC is characterized by hamartomas that occur in many organs of affected patients and these have been considered to likely result from defects in proliferation control. Although the true biochemical functions of the two TSC proteins have not been clarified, a series of independent investigations demonstrated that modulated hamartin or tuberin expression cause deregulation of proliferation/cell cycle in human, rodent and Drosophila cells. In support of tuberin acting as a tumor suppressor, ectopic overexpression of TSC2 has been shown to decrease proliferation rates of mammalian cells. Furthermore, overexpression of TSC2 has been demonstrated to trigger upregulation of the cyclin-dependent kinase inhibitor p27. We report that three different naturally occurring and TSC causing mutations within the TSC2 gene eliminate neither the anti-proliferative capacity of tuberin nor tuberin's effects on p27 expression. For the first time these data provide strong evidence that deregulation of proliferation and/or upregulation of p27 are not likely to be the primary/only mechanisms of hamartoma development in TSC. These results demand reassessment of previous hypotheses of the pathogenesis of TSC.

Hamartin and tuberin interaction with the G2/M cyclin-dependent kinase CDK1 and its regulatory cyclins A and B

Tuberous sclerosis (TSC) is a multi-system disorder characterized by hamartomatous tumors and abnormal brain development, with multiple foci of disrupted neuronal migration and giant dysmorphic neurons within cortical tubers. TSC is associated with mutations in 2 genes, TSC1 and TSC2, which encode hamartin and tuberin, respectively. The functions of these proteins have yet to be determined. Recently, the Drosophila homologue of TSC2, gigas, has been shown to be required for the G2/M transition of the cell cycle. However, the mechanism of this action remains unknown. Because the cyclin-dependent kinase CDK1 forms a complex with cyclin B1 to trigger the G2/M transition, we hypothesized that tuberin interacts with CDK1 to regulate its activity. In the study reported in this paper, we have used co-immunoprecipitation and confocal microscopy to demonstrate that tuberin interacts with and co-localizes with CDK1 and its binding partner cyclin B1 in multiple cell types. We also demonstrate that hamartin interacts with CDK1 and cyclin B1. We further present evidence that tuberin interacts with the other regulatory subunit of CDK1, cyclin A. These findings suggest a direct role for tuberin and hamartin in modulating the activity of CDK1 during G2 and the G2/M transition. This is the first description of a role for both tuberin and hamartin in a common cellular function, providing a potential mechanism for the identical clinicopathologic manifestations that result when either of these proteins are inactivated.

Tuberous sclerosis gene products in proliferation control

Two genes, TSC1 and TSC2, have been shown to be responsible for tuberous sclerosis (TSC). The detection of loss of heterozygosity of TSC1 or TSC2 in hamartomas, the growths characteristically occurring in TSC patients, suggested a tumor suppressor function for their gene products hamartin and tuberin. Studies analyzing ectopically modulated expression of TSC2 in human and rodent cells together with the finding that a homolog of TSC2 regulates the Drosophila cell cycle suggest that TSC is a disease of proliferation/cell cycle control. We discuss this question including very recent data obtained from analyzing mice expressing a modulated TSC2 transgene, and from studying the effects of deregulated TSC1 expression. Elucidation of the cellular functions of these proteins will form the basis of a better understanding of how mutations in these genes cause the disease and for the development of new therapeutic strategies.

Tumors and the heart: molecular genetic advances

Recent molecular genetic investigations of primary cardiac tumors (myxomas, lipomas, rhabdomyomas, and fibromas) have provided insight into fundamental mechanisms of cardiac cell growth. Myxomas are the most common adult cardiac tumor, and familial cardiac myxomas are now appreciated to be caused by mutations in the PRKAR1alpha gene that encodes a regulatory subunit of protein kinase A. Cytogenetic studies have targeted candidate chromosomal loci that may be perturbed during cardiac lipoma pathogenesis. Rhabdomyomas, the most common pediatric cardiac neoplasm, are frequently associated with tuberous sclerosis, caused by mutations in the TSC-1 and TSC-2 genes. The study of Gorlin syndrome has shed light on the etiology of cardiac fibromas. This disorder is caused by mutation of the PTC gene, which regulates cell growth, commitment and differentiation. In the future, manipulation of PRKAR1alpha-, TSC-, and PTC-dependent pathways may foster new strategies to regenerate myocardium in the ischemic or myopathic heart.