Cone photoreceptors are required for color discrimination and highresolution central vision and are lost in macular degenerations, cone and cone/rod dystrophies. Cone transplantation could represent a therapeutic solution. However, an abundant source of human cones remains difficult to obtain. Work performed in model organisms suggests that anterior neural cell fate is induced 'by default' if BMP, TGFβ and Wnt activities are blocked, and that photoreceptor genesis operates through an S-cone default pathway. We report here that Coco (Dand5), a member of the Cerberus gene family, is expressed in the developing and adult mouse retina. Upon exposure to recombinant COCO, human embryonic stem cells (hESCs) differentiated into S-cone photoreceptors, developed an inner segment-like protrusion, and could degrade cGMP when exposed to light. Addition of thyroid hormone resulted in a transition from a unique S-cone population toward a mixed M/S-cone population. When cultured at confluence for a prolonged period of time, COCOexposed hESCs spontaneously developed into a cellular sheet composed of polarized cone photoreceptors. COCO showed dosedependent and synergistic activity with IGF1 at blocking BMP/TGFβ/ Wnt signaling, while its cone-inducing activity was blocked in a dosedependent manner by exposure to BMP, TGFβ or Wnt-related proteins. Our work thus provides a unique platform to produce human cones for developmental, biochemical and therapeutic studies and supports the hypothesis that photoreceptor differentiation operates through an S-cone default pathway during human retinal development.

Late-onset sporadic Alzheimer's disease (AD) is the most prevalent form of dementia, but its origin remains poorly understood. The Bmi1/Ring1 protein complex maintains transcriptional repression of developmental genes through histone H2A mono-ubiquitination, and Bmi1 deficiency in mice results in growth retardation, progeria, and neurodegeneration. Here, we demonstrate that BMI1 is silenced in AD brains, but not in those with early-onset familial AD, frontotemporal dementia, or Lewy body dementia. BMI1 expression was also reduced in cortical neurons from AD patient-derived induced pluripotent stem cells but not in neurons overexpressing mutant APP and PSEN1. BMI1 knockout in human post-mitotic neurons resulted in amyloid beta peptide secretion and deposition, p-Tau accumulation, and neurodegeneration. Mechanistically, BMI1 was required to repress microtubule associated protein tau (MAPT) transcription and prevent GSK3beta and p53 stabilization, which otherwise resulted in neurodegeneration. Restoration of BMI1 activity through genetic or pharmaceutical approaches could represent a therapeutic strategy against AD.

Glioblastoma multiforme (GBM) is an incurable primary brain tumor containing a sub-population of cancer stem cells (CSCs). Polycomb Repressive Complex (PRC) proteins BMI1 and EZH2 are enriched in CSCs, promoting clonogenic growth and resistance to genotoxic therapies. We report here that when used at appropriate concentrations, pharmaceutical inhibitors of BMI1 could efficiently prevent GBM colony growth and CSC self-renewal in vitro and significantly extend lifespan in terminally ill tumor-bearing mice. Notably, molecular analyses revealed that the commonly used PTC596 molecule targeted both BMI1 and EZH2, possibly providing beneficial therapeutic effects in some contexts. On the other hand, treatment with PTC596 resulted in instant reactivation of EZH2 target genes and induction of a molecular program of epithelial-mesenchymal transition (EMT), possibly explaining the modified phenotype of some PTC596-treated tumors. Treatment with a related but more specific BMI1 inhibitor resulted in tumor regression and maintenance of cell identity. We conclude that inhibition of BMI1 alone is efficient at inducing GBM regression, and that dual inhibition of BMI1 and EZH2 using PTC596 may be also beneficial but only in specific contexts.

Background: BMI1 silences the expression of genes located at the facultative heterochromatin. Results: BMI1 is abundant at repetitive genomic regions, including the pericentromeric heterochromatin (PCH), where it is required for compaction and silencing. Conclusion: BMI1 is essential for PCH formation. Significance: BMI1 function at PCH is important to understand how BMI1 regulates genomic stability.