Regulation of Cell Cycle to Stimulate Adult Cardiomyocyte Proliferation and Cardiac Regeneration.

Human diseases are often caused by loss of somatic cells that are incapable of re-entering the cell cycle for regenerative repair. Here, we report a combination of cell-cycle regulators that induce stable cytokinesis in adult post-mitotic cells. We screened cell-cycle regulators expressed in proliferating fetal cardiomyocytes and found that overexpression of cyclin-dependent kinase 1 (CDK1), CDK4, cyclin B1, and cyclin D1 efficiently induced cell division in post-mitotic mouse, rat, and human cardiomyocytes. Overexpression of the cell-cycle regulators was self-limiting through proteasome-mediated degradation of the protein products. In vivo lineage tracing revealed that 15%-20% of adult cardiomyocytes expressing the four factors underwent stable cell division, with significant improvement in cardiac function after acute or subacute myocardial infarction. Chemical inhibition of Tgf-ß and Wee1 made CDK1 and cyclin B dispensable. These findings reveal a discrete combination of genes that can efficiently unlock the proliferative potential in cells that have terminally exited the cell cycle.

Syndecan-4 signaling at a glance.

Syndecan-4, a ubiquitous cell surface proteoglycan, mediates numerous cellular processes through signaling pathways that affect cellular proliferation, migration, mechanotransduction and endocytosis. These effects are achieved through syndecan-4 functioning as both a co-receptor for the fibroblast growth factor receptors (FGFR1-FGFR4) and its ability to independently activate signaling pathways upon ligand binding. As an FGFR co-receptor, syndecan-4 strengthens the duration and intensity of downstream signaling upon ligand binding; this is particularly evident with regard to mitogen-activated protein kinase (MAPK) signaling. In contrast, syndecan-4 also functions as an independent receptor for heparin-binding growth factors, such as fibroblast growth factors (FGFs), vascular endothelial growth factors (VEGFs) and platelet-derived growth factors (PDGFs). These signaling cascades affect canonical signaling components, such as the mammalian target of rapamycin (mTOR), AKT1 and the Rho family of GTPases. In combination with the integrin family of proteins, syndecan-4 is also able to form physical connections between the extracellular matrix (ECM) and cytoskeletal signaling proteins, and it has a key role in regulation of integrin turnover. This unique versatility of the interactions of syndecan-4 is characterized in this Cell Science at a Glance article and illustrated in the accompanying poster.

Selective regulation of arterial branching morphogenesis by synectin.

Branching morphogenesis is a key process in the formation of vascular networks. To date, little is known regarding the molecular events regulating this process. We investigated the involvement of synectin in this process. In zebrafish embryos, synectin knockdown resulted in a hypoplastic dorsal aorta and hypobranched, stunted, and thin intersomitic vessels due to impaired migration and proliferation of angioblasts and arterial endothelial cells while not affecting venous development. Synectin(-/-) mice demonstrated decreased body and organ size, reduced numbers of arteries, and an altered pattern of arterial branching in multiple vascular beds while the venous system remained normal. Murine synectin(-/-) primary arterial, but not venous, endothelial cells showed decreased in vitro tube formation, migration, and proliferation and impaired polarization due to abnormal localization of activated Rac1. We conclude that synectin is involved in selective regulation of arterial, but not venous, growth and branching morphogenesis and that Rac1 plays an important role in this process.

Myocardial hypertrophy in the absence of external stimuli is induced by angiogenesis in mice.

Although studies have suggested a role for angiogenesis in determining heart size during conditions demanding enhanced cardiac performance, the role of EC mass in determining the normal organ size is poorly understood. To explore the relationship between cardiac vasculature and normal heart size, we generated a transgenic mouse with a regulatable expression of the secreted angiogenic growth factor PR39 in cardiomyocytes. A significant change in adult mouse EC mass was apparent by 3 weeks following PR39 induction. Heart weight; cardiomyocyte size; vascular density normalization; upregulation of hypertrophy markers including atrial natriuretic factor, beta-MHC, and GATA4; and activation of the Akt and MAP kinase pathways were observed at 6 weeks post-induction. Treatment of PR39-induced mice with the eNOS inhibitor L-NAME in the last 3 weeks of a 6-week stimulation period resulted in a significant suppression of heart growth and a reduction in hypertrophic marker expression. Injection of PR39 or another angiogenic growth factor, VEGF-B, into murine hearts during myocardial infarction led to induction of myocardial hypertrophy and restoration of myocardial function. Thus stimulation of vascular growth in normal adult mouse hearts leads to an increase in cardiac mass.

Non-canonical fibroblast growth factor signalling in angiogenesis.

Whereas fibroblast growth factors (FGFs) classically transmit their signals via high-affinity tyrosine kinase receptors (FGFR1-4), recent evidence strongly implicates non-tyrosine kinase receptors (NTKR) or cell-surface FGFR-interacting proteins as important players in FGF signalling. Although NTKR have lower affinity for FGFs in comparison with cognate tyrosine kinase receptors, because of their high abundance they can effectively bind FGFs and produce unique biological effects independent of FGFRs. A prime example of such NTKR is the syndecan family of plasma membrane proteoglycans and, in particular, syndecan-4, which transmits FGF signalling via a protein kinase Calpha pathway. Another NTKR, alpha(v)beta(3) integrin, functions as an FGF signalling modulator by binding both FGF2 and FGFR1. Yet another NTKR, neural cell adhesion molecule (NCAM), can serve as an FGFR ligand and assemble an FGFR signalling complex in the absence of FGFs. Furthermore, N-cadherin, which has been reported to associate with FGFR, appears to activate FGFR in both ligand (FGF)-dependent and ligand-independent manners. Finally, gangliosides are implicated as a co-receptor system of FGFs. The biological consequence of non-canonical FGF signalling tends to be less discernable compared to the canonical FGFR activation because of the overlap between these two pathways; nevertheless, non-canonical signalling is important and sometimes essential for cellular functions. Given the diversity of FGF activities through embryonic development to adult physiology, the existence of the non-canonical signalling system may account for the different cellular response to the FGF input in different biological contexts. In this review, we will discuss recent findings related to non-canonical FGF signalling with emphasis on the endothelial biology and angiogenesis.

Oligogenic inheritance of a human heart disease involving a genetic modifier.

Complex genetic mechanisms are thought to underlie many human diseases, yet experimental proof of this model has been elusive. Here, we show that a human cardiac anomaly can be caused by a combination of rare, inherited heterozygous mutations. Whole-exome sequencing of a nuclear family revealed that three offspring with childhood-onset cardiomyopathy had inherited three missense single-nucleotide variants in the MKL2, MYH7, and NKX2-5 genes. The MYH7 and MKL2 variants were inherited from the affected, asymptomatic father and the rare NKX2-5 variant (minor allele frequency, 0.0012) from the unaffected mother. We used CRISPR-Cas9 to generate mice encoding the orthologous variants and found that compound heterozygosity for all three variants recapitulated the human disease phenotype. Analysis of murine hearts and human induced pluripotent stem cell-derived cardiomyocytes provided histologic and molecular evidence for the NKX2-5 variant's contribution as a genetic modifier.

Syndecan-4 clustering induces cell migration in a PDZ-dependent manner.

Cell migration is a dynamic process involving formation of a leading edge in the direction of migration and adhesion points from which tension is generated to move the cell body forward. At the same time, disassembly of adhesion points occurs at the back of the cell, a region known as the trailing edge. Syndecan-4 (S4) is a transmembrane proteoglycan thought to be involved in the formation of focal adhesions. Recent studies have shown that its cytoplasmic domain can engage in signal transduction, making S4 a bona fide receptor. Here, we show that ligand clustering of cell surface S4 on endothelial cells initiates a signaling cascade that results in activation of Rac1, induction of cell polarization, and stimulation of cell migration that depends on S4 interaction with its PDZ-binding partner. Expression of an S4 mutant lacking its PDZ-binding region (S4-PDZ(-)) leads to decreased cell motility and a failure to form a trailing edge. On clustering S4, but not S4-PDZ(-), targets activated Rac1 to the leading edge of live cells. Cells lacking synectin, a PDZ domain containing protein that interacts with S4, fail to migrate in response to S4 clustering. Both S4-PDZ(-)-expressing and synectin(-/-) endothelial cells exhibit elevated basal levels of Rac1. Thus, our data suggest that S4 promotes endothelial cell migration in response to ligand binding by activating Rac1 and localizing it to the leading edge, and that these processes are dependent on its PDZ-binding domain interaction with synectin.

Spontaneous Closure of a Secundum Atrial Septal Defect.

Spontaneous closure of an atrial septal defect (ASD) is well described in pediatric cardiology but may be less familiar to adult internists and cardiologists. We report a moderately sized 6-mm ASD that closed spontaneously without intervention. A literature review found that a smaller defect size and an early age of diagnosis are the most important predictors of closure. Possible mechanisms of a spontaneous ASD closure include adaptive endothelial migration, limited myocardial proliferation, and fibroblast migration with extracellular matrix deposition.

Minocycline decreases in vitro microglial motility, beta1-integrin, and Kv1.3 channel expression.

Minocycline is a semisynthetic, tetracycline derivative that exerts anti-inflammatory and neuroprotective effects unrelated to its anti-microbial action. We have previously shown that minocycline prevented peripheral nerve injury-induced mechanical allodynia. Minocycline's mechanisms of action as a neuroprotective and anti-allodynic agent are unknown. In response to injury, microglia become activated, proliferate, and migrate. Resting microglia express voltage-dependent inward K(+) currents and blocking Kv1.3 channels has been shown to inhibit microglial-mediated neuronal death. We investigated the effect of minocycline on the expression of Kv channels, cell motility, and beta-integrin expression using primary rat cortical microglia, transwell assays, and by flow cytometry. Minocycline significantly reduced microglial migration to cellular debris, astrocyte-conditioned medium, ADP, and algesic mediators and significantly reduced the expression of CD29 (beta(1)-integrin) but not CD18 (beta(2)-integrin). Minocycline reduced the effect of extracellular potassium and later decreased microglial Kv1.3 expression. In summary, we uncovered a novel effect of minocycline that demonstrates this agent decreases microglial beta(1)-integrin expression, which leads to inhibition of motility. We propose an in vivo model whereby reduced microglial trafficking to injured neurons following nerve injury decreases the release of proinflammatory mediators into the synaptic milieu, preventing neuronal sensitization, the pathological correlate to chronic pain.

Syndecan 4 regulates FGFR1 signaling in endothelial cells by directing macropinocytosis.

Fibroblast growth factor 2 (FGF2) induces endothelial cell migration and angiogenesis through two classes of receptors: receptor tyrosine kinases, such as FGF receptor 1 (FGFR1), and heparan sulfate proteoglycans, such as syndecan 4 (S4). We examined the distinct contributions of FGFR1 and S4 in shaping the endothelial response to FGF2. S4 determined the kinetics and magnitude of FGF2-induced mitogen-activated protein kinase (MAPK) signaling by promoting the macropinocytosis of the FGFR1-S4-FGF2 signaling complex. Internalization of the S4 receptor complex was independent of clathrin and dynamin, proceeded from lipid raft-enriched membranes, and required activation of the guanosine triphosphatases RhoG and Rab5. Genetic knockout of S4, disruption of S4 function, or inhibition of Rab5 led to increased endocytosis and MAPK signaling. These data define the mechanism by which FGFR1 and S4 coordinate downstream signaling upon FGF2 stimulation: FGFR1 initiates MAPK signaling, whereas S4-dependent FGFR1 macropinocytosis modulates the kinetics of MAPK activation. Our studies identify S4 as a regulator of MAPK signaling and address the question of how distinct classes of FGFRs individually contribute to signal transduction in endothelial cells.

Auxiliary and autonomous proteoglycan signaling networks.

Proteoglycans represent a structurally heterogeneous family of proteins that typically undergo extensive posttranslational modification with sulfated sugar chains. Although historically believed to affect signaling pathways exclusively as growth factor coreceptors, proteoglycans are now understood to initiate and modulate signal transduction cascades independently of other receptors. From within the extracellular matrix, proteoglycans are able to shield protein growth factors from circulating proteases and establish gradients that guide cell migration. Extracellular proteoglycans are also critical in the maintenance of growth factor stores and are thus instrumental in modulating paracrine signaling. At the cell membrane, proteoglycans stabilize ligand-receptor interactions, creating potentiated ternary signaling complexes that regulate cell proliferation, endocytosis, migration, growth factor sensitivity, and matrix adhesion. In some cases, proteoglycans are able to independently activate various signaling cascades, attenuate the signaling of growth factors, or orchestrate multimeric intracellular signaling complexes. Signaling between cells is also modulated by proteoglycan activity at the cell membrane, as exemplified by the proteoglycan requirement for effective synaptogenesis between neurons. Finally, proteoglycans are able to regulate signaling from intracellular compartments, particularly in the context of storage granule formation and maintenance. These proteoglycans are also major determinants of exocytic vesicle fate and other vesicular trafficking pathways. In contrast to the mechanisms underlying classical ligand-receptor signaling, proteoglycan signaling is frequently characterized by ligand promiscuity and low-affinity binding; likewise, these events commonly do not exhibit the same degree of reliance on intermolecular structure or charge configurations as other ligand-receptor interactions. Such unique features often defy conventional mechanisms of signal transduction, and present unique challenges to the study of their indispensable roles within cell signaling networks.

Suppression of RhoG activity is mediated by a syndecan 4-synectin-RhoGDI1 complex and is reversed by PKCalpha in a Rac1 activation pathway.

Fibroblast growth factor 2 (FGF2) is a major regulator of developmental, pathological, and therapeutic angiogenesis. Its activity is partially mediated by binding to syndecan 4 (S4), a proteoglycan receptor. Angiogenesis requires polarized activation of the small guanosine triphosphatase Rac1, which involves localized dissociation from RhoGDI1 and association with the plasma membrane. Previous work has shown that genetic deletion of S4 or its adapter, synectin, leads to depolarized Rac activation, decreased endothelial migration, and other physiological defects. In this study, we show that Rac1 activation downstream of S4 is mediated by the RhoG activation pathway. RhoG is maintained in an inactive state by RhoGDI1, which is found in a ternary complex with synectin and S4. Binding of S4 to synectin increases the latter's binding to RhoGDI1, which in turn enhances RhoGDI1's affinity for RhoG. S4 clustering activates PKCalpha, which phosphorylates RhoGDI1 at Ser(96). This phosphorylation triggers release of RhoG, leading to polarized activation of Rac1. Thus, FGF2-induced Rac1 activation depends on the suppression of RhoG by a previously uncharacterized ternary S4-synectin-RhoGDI1 protein complex and activation via PKCalpha.

Direct allosteric regulation between the GAF domain and catalytic domain of photoreceptor phosphodiesterase PDE6.

Photoreceptor cGMP phosphodiesterase (PDE6) is the central enzyme in the visual transduction cascade. The PDE6 catalytic subunit contains a catalytic domain and regulatory GAF domains. Unlike most GAF domain-containing cyclic nucleotide phosphodiesterases, little is known about direct allosteric communication of PDE6. In this study, we demonstrate for the first time direct, inter-domain allosteric communication between the GAF and catalytic domains in PDE6. The binding affinity of PDE6 for pharmacological inhibitors or for the C-terminal region of the inhibitory gamma subunit (Pgamma), known to directly inhibit PDE6 catalysis, was increased approximately 2-fold by ligands binding to the GAF domain. Binding of the N-terminal half of Pgamma to the GAF domains suffices to induce this allosteric effect. Allosteric communication between GAF and catalytic domains is reciprocal, in that drug binding to the catalytic domain slowed cGMP dissociation from the GAF domain. Although cGMP hydrolysis was not affected by binding of Pgamma1-60, Pgamma lacking its last seven amino acids decreased the Michaelis constant of PDE6 by 2.5-fold. Pgamma1-60 binding to the GAF domain increased vardenafil but not cGMP affinity, indicating that substrate- and inhibitor-binding sites do not totally overlap. In addition, prolonged incubation of PDE6 with vardenafil or sildenafil (but not 3-isobutyl-1-methylxanthine and zaprinast) induced a distinct conformational change in the catalytic domain without affecting the binding properties of the GAF domains. We conclude that although Pgamma-mediated regulation plays the dominant role in visual excitation, the direct, inter-domain allosteric regulation described in this study may play a feedback role in light adaptational processes during phototransduction.