p53 is required for brain growth but is dispensable for resistance to nutrient restriction during Drosophila larval development

Validation of Enhancer Regions in Primary Human Neural Progenitor Cells using Capture STARR-seq

Genome-wide association studies (GWAS) and expression analyses implicate noncoding regulatory regions as harboring risk factors for psychiatric disease, but functional characterization of these regions remains limited. We performed capture STARR-sequencing of over 78,000 candidate regions to identify active enhancers in primary human neural progenitor cells (phNPCs). We selected candidate regions by integrating data from NPCs, prefrontal cortex, developmental timepoints, and GWAS. Over 8,000 regions demonstrated enhancer activity in the phNPCs, and we linked these regions to over 2,200 predicted target genes. These genes are involved in neuronal and psychiatric disease-associated pathways, including dopaminergic synapse, axon guidance, and schizophrenia. We functionally validated a subset of these enhancers using mutation STARR-sequencing and CRISPR deletions, demonstrating the effects of genetic variation on enhancer activity and enhancer deletion on gene expression. Overall, we identified thousands of highly active enhancers and functionally validated a subset of these enhancers, improving our understanding of regulatory networks underlying brain function and disease.

The Serine Protease Homolog, Scarface, Is Sensitive to Nutrient Availability and Modulates the Development of the Drosophila Blood-Brain Barrier

The adaptable transcriptional response to changes in food availability not only ensures animal survival but also lets embryonic development progress. Interestingly, the CNS is preferentially protected from periods of malnutrition, a phenomenon known as “brain sparing.” However, the mechanisms that mediate this response remain poorly understood. To get a better understanding of this, we used Drosophila melanogaster as a model, analyzing the transcriptional response of neural stem cells (neuroblasts) and glia of the blood–brain barrier (BBB) from larvae of both sexes during nutrient restriction using targeted DamID. We found differentially expressed genes in both neuroblasts and glia of the BBB, although the effect of nutrient deficiency was primarily observed in the BBB. We characterized the function of a nutritional sensitive gene expressed in the BBB, the serine protease homolog, scarface (scaf). Scaf is expressed in subperineurial glia in the BBB in response to nutrition. Tissue-specific knockdown of scaf increases subperineurial glia endoreplication and proliferation of perineurial glia in the blood–brain barrier. Furthermore, neuroblast proliferation is diminished on scaf knockdown in subperineurial glia. Interestingly, reexpression of Scaf in subperineurial glia is able to enhance neuroblast proliferation and brain growth of animals in starvation. Finally, we show that loss of scaf in the blood–brain barrier increases sensitivity to drugs in adulthood, suggesting a physiological impairment. We propose that Scaf integrates the nutrient status to modulate the balance between neurogenesis and growth of the BBB, preserving the proper equilibrium between the size of the barrier and the brain.

SIGNIFICANCE STATEMENT The Drosophila BBB separates the CNS from the open circulatory system. The BBB glia are not only acting as a physical segregation of tissues but participate in the regulation of the metabolism and neurogenesis during development. Here we analyze the transcriptional response of the BBB glia to nutrient deprivation during larval development, a condition in which protective mechanisms are switched on in the brain. Our findings show that the gene scarface reduces growth in the BBB while promoting the proliferation of neural stem, assuring the balanced growth of the larval brain. Thus, Scarface would link animal nutrition with brain development, coordinating neurogenesis with the growth of the BBB.

Ionizing radiation reduces larval brain size by inducing premature differentiation of Drosophila neural stem cells

DNA damaging agents, such as ionizing radiation (IR), induce cell cycle arrest, senescence, differentiation, or cell death of stem cells, which may affect tissue homeostasis. The specific response of stem cells upon irradiation seems to vary depending on the cell type and their developmental stages. Drosophila larval brain contains neural stem cells called neuroblasts (NBs) and maintaining an appropriate number of NBs is critical to maintain brain size. Irradiation of larvae at early larval stage results in microcephaly, whereas the DNA damage response of NBs that could explain this small brain size is not clearly understood. We observed that the irradiation of larvae in the second instar retarded brain growth, accompanied by fewer NBs. The IR-induced microcephaly does not seem to result from apoptosis since the irradiated larval brain was not stained with activated Caspase nor was the microcephaly affected by the ectopic expression of the apoptosis inhibitor. When analyzed for the percentage of mitotic cells, irradiated NBs recovered their proliferative potential within 6 h post-irradiation after transient cell cycle arrest. However, IR eventually reduced the proliferation of NBs at later time points and induced the premature differentiation of NBs. In summary, IR-induced microcephaly occurs by NB loss due to premature differentiation, rather than apoptotic cell death.

Drosophila p53 directs nonapoptotic programs in postmitotic tissue

TP53 is the most frequently mutated gene in human cancers, and despite intensive research efforts, genome-scale studies of p53 function in whole animal models are rare. The need for such in vivo studies is underscored by recent challenges to established paradigms, indicating that unappreciated p53 functions contribute to cancer prevention. Here we leveraged the Drosophila system to interrogate p53 function in a postmitotic context. In the developing embryo, p53 robustly activates important apoptotic genes in response to radiation-induced DNA damage. We recently showed that a p53 enhancer (p53RE^rpr) near the cell death gene reaper forms chromatin contacts and enables p53 target activation across long genomic distances. Interestingly, we found that this canonical p53 apoptotic program fails to activate in adult heads. Moreover, this failure to exhibit apoptotic responses was not associated with altered chromatin contacts. Instead, we determined that p53 does not occupy the p53RE^rpr enhancer in this postmitotic tissue as it does in embryos. Through comparative RNA-seq and chromatin immunoprecipitation-seq studies of developing and postmitotic tissues, we further determined that p53 regulates distinct transcriptional programs in adult heads, including DNA repair, metabolism, and proteolysis genes. Strikingly, in the postmitotic context, p53-binding landscapes were poorly correlated with nearby transcriptional effects, raising the possibility that p53 enhancers could be generally acting through long distances.