In key C. elegans adult tissues, the lin-4 miRNA may act to suppress the translation of lin-14, preventing lin-14 from affecting the transcription of a yet unidentified factor that regulates or interacts with the daf-2 insulin/ IGF-1 pathway. By demonstrating that lin-4 and lin-14, two key temporal regulators of development, also influence the rate of aging, we provide support for the theory that life span is affected by an innate, programmed timing mechanism. However, our data are also consistent with an alternative theory of aging, antagonistic pleiotropy, which posits that genes with primary roles in development can later secondarily influence life span (27). miRNAs are important regulators of development, apoptosis, and metabolism (28-31), and our work demonstrates that a miRNA can regulate aging, possibly through the insulin-like signaling pathway. It is possible that the mammalian lin-4 miRNA homologs, the miR-125 family, may regulate processes responsible for lifespan determination in vertebrates.

The zinc-finger transcription factors GATA4 and GATA6 play critical roles in embryonic development. Mouse embryos lacking GATA4 die at embryonic day (E) 8.5 because of failure of ventral foregut closure and cardiac bifida, whereas GATA6 is essential for development of the visceral endoderm. Although mice that are heterozygous for either a GATA4 or GATA6 null allele are normal, we show that compound heterozygosity of GATA4 and GATA6 results in embryonic lethality by E13.5 accompanied by a spectrum of cardiovascular defects, including thin-walled myocardium, ventricular and aortopulmonary septal defects, and abnormal smooth muscle development. Myocardial hypoplasia in GATA4͞GATA6 double heterozygous mutant embryos is associated with reduced proliferation of cardiomyocytes, diminished expression of the myogenic transcription factor MEF2C (myocyte enhancer factor 2C), and down-regulation of ␤-myosin heavy chain expression, a key determinant of cardiac contractility. These findings reveal a threshold of GATA4 and GATA6 activity that is required for gene expression in the developing cardiovascular system and underscore the potential of recessive mutations to perturb the delicate regulation of cardiovascular development.GATA factors ͉ heart development ͉ ventricular septal defect ͉ heart defects ͉ cardiogenesis T he GATA family of transcription factors plays an important role in differentiation, growth, and survival of diverse cell types (1, 2). Six GATA family members have been identified in vertebrates, all of which contain two zinc-finger domains that bind the consensus site (A͞T)GATA(A͞G) and mediate cofactor interactions. GATA1, 2, and 3 are primarily expressed in hematopoietic lineages (2), and GATA4, 5, and 6 are expressed in mesoderm-and endoderm-derived tissues such as the heart, liver, lung, and gut (1).GATA4 regulates the expression of genes that are critical for cardiac contraction, as well as the expression of cardiac transcription factors such as Nkx2.5, Hand2, and MEF2C (myocyte enhancer factor 2C) (3-11). GATA4 null mice display defects in heart morphogenesis and ventral foregut closure (12), resulting in embryonic lethality by embryonic day (E) 8.5. Tetraploid rescue experiments with GATA4 null embryonic stem cells give rise to embryos with abnormal looping of the heart tube and thin-walled myocardium (13). Early cardiac-specific deletion of GATA4 also results in myocardial thinning, abnormal endocardial cushion development, and right ventricular hypoplasia (14), whereas cardiac-specific deletion at later time points results in reduced cardiac function and an inability to undergo hypertrophy after pressure overload or exercise (15). Mice that are homozygous for a hypomorphic GATA4 mutation display a variety of heart defects, including double outlet right ventricle and hypoplasia of the compact myocardium (16). Heterozygous mutations in GATA4 are also associated with congenital heart defects in humans (17).GATA6 null mice die after implantation because of defects in visceral endoderm function and extrae...

MEF2 is a MADS-box transcription factor required for muscle development in Drosophila. Here, we show that the bHLH transcription factor Twist directly regulates Mef2 expression in adult somatic muscle precursor cells via a 175-bp enhancer located 2245 bp upstream of the transcriptional start site. Within this element, a single evolutionarily conserved E box is essential for enhancer activity. Twist protein can bind to this E box to activate Mef2 transcription, and ectopic expression of twist results in ectopic activation of the wild-type 175-bp enhancer. By use of a temperature-sensitive mutant of twist, we show that activation of Mef2 transcription via this enhancer by Twist is required for normal adult muscle development, and reduction in Twist function results in phenotypes similar to those observed previously in Mef2 mutant adults. The 175-bp enhancer is also active in the embryonic mesoderm, indicating that this enhancer functions at multiple times during development, and its function is dependent on the same conserved E box. In embryos, a reduction in Twist function also strongly reduced Mef2 expression. These findings define a novel transcriptional pathway required for skeletal muscle development and identify Twist as an essential and direct regulator of Mef2 expression in the somatic mesoderm.

Neonatal mouse hearts fully regenerate after ventricular resection similar to adult zebrafish. We established cryoinjury models to determine if different types and varying degrees of severity in cardiac injuries trigger different responses in neonatal mouse hearts. In contrast to ventricular resection, neonatal mouse hearts fail to regenerate and show severe impairment of cardiac function post transmural cryoinjury. However, neonatal hearts fully recover after non-transmural cryoinjury. Interestingly, cardiomyocyte proliferation does not significantly increase in neonatal mouse hearts after cryoinjuries. Epicardial activation and new coronary vessel formation occur after cryoinjury. The profibrotic marker PAI-1 is highly expressed after transmural but not non-transmural cryoinjuries, which may contribute to the differential scarring. Our results suggest that regenerative medicine strategies for heart injuries should vary depending on the nature of the injury.