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Gene regulatory divergence is thought to play a central role in determining human-specific traits. However, our ability to link divergent regulation to divergent phenotypes is limited. Here, we utilized human-chimpanzee hybrid induced pluripotent stem cells to study gene expression separating these species. The tetraploid hybrid cells allowed us to separate cis- from trans-regulatory effects, and to control for non-genetic confounding factors. We differentiated these cells into cranial neural crest cells (CNCCs), the primary cell type giving rise to the face. We discovered evidence of lineage-specific selection on the hedgehog signaling pathway, including a human-specific 6-fold down-regulation of EVC2 (LIMBIN), a key hedgehog gene. Inducing a similar down-regulation of EVC2 substantially reduced hedgehog signaling output. Mice and humans lacking functional EVC2 show striking phenotypic parallels to human-chimpanzee craniofacial differences, suggesting that the regulatory divergence of hedgehog signaling may have contributed to the unique craniofacial morphology of humans.

Among primates, humans display a unique trajectory of development responsible for the many traits specific to our species. However, the inaccessibility of human and chimpanzee primary tissues has limited our ability to study human evolution. Comparative in vitro approaches using primate-derived induced pluripotent stem cells have begun to reveal species differences on the cellular and molecular levels(1,2). In particular, brain organoids have emerged as a promising platform to study primate neural development in vitro(3-5), although cross-species comparisons of organoids are complicated by differences in developmental timing and variability of differentiation(6,7). Here, we developed a new platform to address these limitations. We first generated a panel of tetraploid hybrid stem cells by fusing human and chimpanzee induced pluripotent stem cells. We next applied this approach to study species divergence in cerebral cortical development by differentiating them into neural organoids. We found that hybrid organoids provide a controlled system for disentangling cis- and trans-acting gene expression divergence across cell types and developmental stages, revealing a signature of selection on astrocyte-related genes. In addition, we identified an up-regulation of human somatostatin receptor 2 (SSTR2), which regulates neuronal calcium signaling and is associated with neuropsychiatric disorders(8,9). We discovered a human-specific response to modulation of SSTR2 function in cortical neurons, underscoring the potential of this unique platform to reveal the molecular basis of human evolution.

Myogenic cells have the ability to adopt two divergent fates upon exit from the cell cycle: differentiation or self-renewal. The Notch signaling pathway is a well-known negative regulator of myogenic differentiation. Using mouse primary myoblasts cultured in vitro or C2C12 myogenic cells, we find that Notch activity is essential for maintaining the expression of Pax7, a transcription factor associated with the self-renewal lineage, in quiescent undifferentiated myoblasts after they exit the cell cycle. Stimulation of the Notch pathway by expression of a constitutively active Notch 1 or co-culture of myogenic cells with Delta like-1 (Dll1)-transfected CHO cells increases the level of Pax7. Dll1, a ligand for Notch receptor, is shed by ADAM metalloproteases in a pool of Pax7-positive C2C12 reserve cells, but it remains intact in differentiated myotubes. Dll1 shedding changes the receptor/ligand ratio and modulates the level of Notch signaling. Inhibition of Dll1 cleavage by a soluble, dominant-negative mutant form of ADAM12 leads to elevation of Notch signaling, inhibition of differentiation, and expansion of the pool of self-renewing Pax7-positive/MyoD-negative cells. These results suggest that ADAM-mediated shedding of Dll1 in a subset of cells during myogenic differentiation in vitro contributes to down-regulation of Notch signaling in neighboring cells and facilitates their progression into differentiation. We propose that the proteolytic processing of Dll1 helps achieve an asymmetry in Notch signaling in initially equivalent myogenic cells and helps sustain the balance between differentiation and self-renewal.

Neurodegenerative diseases, such as frontotemporal dementia (FTD), are often associated with behavioral deficits, but the underlying anatomical and molecular causes remain poorly understood. Here we show that forebrain-specific expression of FTD-associated mutant CHMP2B in mice causes several age-dependent neurodegenerative phenotypes, including social behavioral impairments. The social deficits were accompanied by a change in AMPA receptor (AMPAR) composition, leading to an imbalance between Ca(2+)-permeable and Ca(2+)-impermeable AMPARs. Expression of most AMPAR subunits was regulated by the brain-enriched microRNA miR-124, whose abundance was markedly decreased in the superficial layers of the cerebral cortex of mice expressing the mutant CHMP2B. We found similar changes in miR-124 and AMPAR levels in the frontal cortex and induced pluripotent stem cell-derived neurons from subjects with behavioral variant FTD. Moreover, ectopic miR-124 expression in the medial prefrontal cortex of mutant mice decreased AMPAR levels and partially rescued behavioral deficits. Knockdown of the AMPAR subunit Gria2 also alleviated social impairments. Our results identify a previously undescribed mechanism involving miR-124 and AMPARs in regulating social behavior in FTD and suggest a potential therapeutic avenue.

Phagophore maturation is a key step in the macroautophagy pathway, which is critical in many important physiological and pathological processes. Here we identified Drosophila N-ethylmaleimide-sensitive fusion protein 2 (dNSF2) and soluble NSF attachment protein (Snap) as strong genetic modifiers of mutant CHMP2B, an ESCRT-III component that causes frontotemporal dementia and autophagosome accumulation. Among several SNAP receptor (SNARE) genes, Drosophila syntaxin 13 (syx13) exhibited a strong genetic interaction with mutant CHMP2B. Knockdown of syntaxin 13 (STX13) or its binding partner Vti1a in mammalian cells caused LC3-positive puncta to accumulate and blocks autophagic flux. STX13 was present on LC3-positive phagophores induced by rapamycin and was highly enriched on multilamellar structures induced by dysfunctional ESCRT-III. Loss of STX13 also caused the accumulation of Atg5-positive puncta and the formation of multilamellar structures. These results suggest that STX13 is a genetic modifier of ESCRT-III dysfunction and participates in the maturation of phagophores into closed autophagosomes.

ADAM12 has recently emerged as a Candidate Cancer Gene in a comprehensive genetic analysis of human breast cancers. Three somatic mutations in ADAM12 were observed at significant frequencies in breast cancers: D301H, G479E and L792F. The first 2 of these mutations involve highly conserved residues in ADAM12, and our computational sequence analysis confirms that they may be cancer-related. We show that the corresponding mutations in mouse ADAM12 inhibit the proteolytic processing and activation of ADAM12 in NIH3T3, COS-7, CHO-K1 cells and in MCF-7 breast cancer cells. The D/H and G/E ADAM12 mutants exert a dominant-negative effect on the processing of the wild-type ADAM12. Immunofluorescence analysis and cell surface biotinylation experiments demonstrate that the D/H and G/E mutants are retained inside the cell and are not transported to the cell surface. Consequently, the D/H and G/E mutants, unlike the wild-type ADAM12, are not capable of shedding Delta-like l, a ligand for Notch receptor, at the cell surface, or of stimulating cell migration. Our results suggest that the breast cancer-associated mutations interfere with the intracellular trafficking of ADAM12 and result in loss of the functional ADAM12 at the cell surface. © 2008 Wiley-Liss, Inc.

Delta-like 1 (Dll1) is a mammalian ligand for Notch receptors. Interactions between Dll1 and Notch in trans activate the Notch pathway, whereas Dll1 binding to Notch in cis inhibits Notch signaling. Dll1 undergoes proteolytic processing in its extracellular domain by ADAM10. In this work we demonstrate that Dll1 represents a substrate for several other members of the ADAM family. In co-transfected cells, Dll1 is constitutively cleaved by ADAM12, and the N-terminal fragment of Dll1 is released to medium. ADAM12-mediated cleavage of Dll1 is cell density-dependent, takes place in cis orientation, and does not require the presence of the cytoplasmic domain of ADAM12. Full-length Dll1, but not its N- or C-terminal proteolytic fragment, co-immunoprecipitates with ADAM12. By using a Notch reporter construct, we show that Dll1 processing by ADAM12 increases Notch signaling in a cell-autonomous manner. Furthermore, ADAM9 and ADAM17 have the ability to process Dll1. In contrast, ADAM15 does not cleave Dll1, although the two proteins still co-immunoprecipitate with each other. Asn-353 present in the catalytic motif of ADAM12 and other Dll1-processing ADAMs, but absent in ADAM15, is necessary for Dll1 cleavage. Dll1 cleavage is reduced in ADAM9/12/15(-/-) mouse embryonic fibroblasts (MEFs), suggesting that the endogenous ADAM9 and/or ADAM12 present in wild type MEFs contribute to Dll1 processing. Finally, the endogenous Dll1 present in primary mouse myoblasts undergoes cleavage in confluent, differentiating myoblast cultures, and this cleavage is decreased by ADAM12 small interfering RNAs. Our findings expand the role of ADAM proteins in the regulation of Notch signaling.