Neural stem cells (NSCs) generate new granule cells throughout life in the mammalian hippocampus. Canonical Wnt signaling regulates the differentiation of NSCs towards the neuronal lineage. Here we identified the prospero-related homeodomain transcription factor Prox1 as a target of β-catenin–TCF/LEF signaling in vitro and in vivo. Prox1 overexpression enhanced neuronal differentiation whereas shRNA-mediated knockdown of Prox1 impaired the generation of neurons in vitro and within the hippocampal niche. In contrast, Prox1 was not required for survival of adult-generated granule cells after they had matured, suggesting a role for Prox1 in initial granule cell differentiation but not in the maintenance of mature granule cells. The data presented here characterize a molecular pathway from Wnt signaling to a transcriptional target leading to granule cell differentiation within the adult brain and identify a stage-specific function for Prox1 in the process of adult neurogenesis.

Newborn granule cells become functionally integrated into the synaptic circuitry of the adult dentate gyrus after a morphological and electrophysiological maturation process. The molecular mechanisms by which immature neurons and the neurites extending from them find their appropriate position and target area remain largely unknown. Here we show that single-cell–specific knockdown of cyclin-dependent kinase 5 (cdk5) activity in newborn cells using a retrovirus-based strategy leads to aberrant growth of dendritic processes, which is associated with an altered migration pattern of newborn cells. Even though spine formation and maturation are reduced in cdk5-deficient cells, aberrant dendrites form ectopic synapses onto hilar neurons. These observations identify cdk5 to be critically involved in the maturation and dendrite extension of newborn neurons in the course of adult neurogenesis. The data presented here also suggest a mechanistic dissociation between accurate dendritic targeting and subsequent synapse formation.

Germline removal provokes longevity in several species and shifts resources towards survival and repair. Several Caenorhabditis elegans transcription factors regulate longevity arising from germline removal; yet, how they work together is unknown. Here we identify a Myc-like HLH transcription factor network comprised of Mondo/Max-like complex (MML-1/MXL-2) to be required for longevity induced by germline removal, as well as by reduced TOR, insulin/IGF signalling and mitochondrial function. Germline removal increases MML-1 nuclear accumulation and activity. Surprisingly, MML-1 regulates nuclear localization and activity of HLH-30/TFEB, a convergent regulator of autophagy, lysosome biogenesis and longevity, by downregulating TOR signalling via LARS-1/leucyl-transfer RNA synthase. HLH-30 also upregulates MML-1 upon germline removal. Mammalian MondoA/B and TFEB show similar mutual regulation. MML-1/MXL-2 and HLH-30 transcriptomes show both shared and preferential outputs including MDL-1/MAD-like HLH factor required for longevity. These studies reveal how an extensive interdependent HLH transcription factor network distributes responsibility and mutually enforces states geared towards reproduction or survival.

Physiological homeostasis becomes compromised during ageing, as a result of impairment of cellular processes, including transcription and RNA splicing1–4. However, the molecular mechanisms leading to the loss of transcriptional fidelity are so far elusive, as are ways of preventing it. Here we profiled and analysed genome-wide, ageing-related changes in transcriptional processes across different organisms: nematodes, fruitflies, mice, rats and humans. The average transcriptional elongation speed (RNA polymerase II speed) increased with age in all five species. Along with these changes in elongation speed, we observed changes in splicing, including a reduction of unspliced transcripts and the formation of more circular RNAs. Two lifespan-extending interventions, dietary restriction and lowered insulin–IGF signalling, both reversed most of these ageing-related changes. Genetic variants in RNA polymerase II that reduced its speed in worms5 and flies6 increased their lifespan. Similarly, reducing the speed of RNA polymerase II by overexpressing histone components, to counter age-associated changes in nucleosome positioning, also extended lifespan in flies and the division potential of human cells. Our findings uncover fundamental molecular mechanisms underlying animal ageing and lifespan-extending interventions, and point to possible preventive measures.

Aging impairs cellular homeostasis, thereby compromising multiple cellular processes, including transcription and splicing. However, the molecular mechanisms at work, and hence ways of preventing loss of transcriptional fidelity, are so far elusive. We analyzed changes in genome-wide, transcription-coupled processes with age in Caenorhabditis elegans, Drosophila melanogaster, Mus musculus, Rattus norvegicus and Homo sapiens. Using total RNA profiling, we quantified transcriptional elongation speed (Pol-II speed). Genomeaveraged Pol-II speed increased with age in all five species. Lifespan-extending dietary restriction and lowered insulin signaling both rescued these age-related trends.Experimentally reducing Pol-II speed in worms and flies increased lifespan. These findings uncover fundamental molecular mechanisms driving animal aging and underlying lifespanextending interventions, and point to possible preventative measures. One-sentence summaryGenome-wide analysis of total RNA during aging reveals increased transcriptional elongation speed and decreased splicing efficiency, with an impact on longevity. Main TextAging impairs a wide range of cellular processes, many of which affect the quality and concentration of proteins. Among these, transcription is particularly important, because it is the main regulator of protein levels (1-3). Transcriptional elongation is critical for proper mRNA synthesis, due to its association with pre-mRNA processing steps such as splicing, editing, and 3' end formation (4, 5). Indeed, dysregulation of transcriptional elongation can lead to a number of diseases (6). During aging, animal transcriptomes undergo extensive remodeling, with large-scale changes in expression of transcripts involved in signaling pathways, DNA damage response, protein homeostasis (e.g. the ubiquitin-proteasome pathway), immune response, and stem cell plasticity (7). Further, some studies detected an age-related increase in variability and error of gene expression (8-10). This prior work has provided insights into how the transcriptome adapts to, and is affected by, aging-associated cellular stress. However, it is not known if, or to what extent, the transcription process itself is affected by aging. In this study, we have investigated how the kinetics of transcription is affected by aging, how such changes can affect mRNA biosynthesis, and the role of these changes in age-related loss of function in the whole organism.The translocation speed of RNA polymerase II (Pol-II) during transcription can be monitored using total RNA sequencing (RNA-seq), because the distribution of reads in introns is affected by the elongation speed of Pol-II. Co-transcriptional splicing results in a characteristic saw-tooth pattern of the read coverage, observable from total RNA-seq or nascent RNA-seq measurements (11,12). The read coverage generally decreases 5' to 3' along an intron and the extent of this decrease depends on Pol-II speed: the faster the elongation, the shallower the slope (13-15), i.e. fast elongation of Pol-II r...