Regulation of chromatin accessibility and gene expression in the developing hippocampal primordium by LIM-HD transcription factor LHX2.

In the mammalian cerebral cortex, the hippocampal primordium (Hcp) occupies a discrete position in the dorsal telencephalic neuroepithelium adjacent to the neocortical primordium (Ncp). We examined transcriptomic and chromatin-level features that distinguish the Hcp from the Ncp in the mouse during the early neurogenic period, embryonic day (E)12.5. ATAC-seq revealed that the Hcp was more accessible than the Ncp at this stage. Motif analysis of the differentially accessible loci in these tissues revealed LHX2 as a candidate transcription factor for modulating gene regulatory networks (GRNs). We analyzed LHX2 occupancy profiles and compared these with transcriptomic data from control and Lhx2 mutant Hcp and Ncp at E12.5. Our results revealed that LHX2 directly regulates distinct genes in the Hcp and Ncp within a set of common pathways that control fundamental aspects of development namely pluripotency, axon pathfinding, Wnt, and Hippo signaling. Loss of Lhx2 caused a decrease in accessibility, specifically in hippocampal chromatin, suggesting that this factor may play a unique role in hippocampal development. We identified 14 genes that were preferentially enriched in the Hcp, for which LHX2 regulates both chromatin accessibility and mRNA expression, which have not thus far been examined in hippocampal development. Together, these results provide mechanistic insight into how LHX2 function in the Hcp may contribute to the process by which the hippocampus acquires features distinct from the neocortex.

Dentate gyrus morphogenesis is regulated by ß-catenin function in hem-derived fimbrial glia.

The dentate gyrus, a gateway for input to the hippocampal formation, arises from progenitors in the medial telencephalic neuroepithelium adjacent to the cortical hem. Dentate progenitors navigate a complex migratory path guided by two cell populations that arise from the hem, the fimbrial glia and Cajal-Retzius (CR) cells. As the hem expresses multiple Wnt genes, we examined whether ß-catenin, which mediates canonical Wnt signaling and also participates in cell adhesion, is necessary for the development of hem-derived lineages. We report that, in mice, the fimbrial glial scaffold is disorganized and CR cells are mispositioned upon hem-specific disruption of ß-catenin. Consequently, the dentate migratory stream is severely affected, and the dentate gyrus fails to form. Using selective Cre drivers, we further determined that ß-catenin function is required in the fimbrial glial scaffold, but not in the CR cells, for guiding the dentate migration. Our findings highlight a primary requirement for ß-catenin for the organization of the fimbrial scaffold and a secondary role for this factor in dentate gyrus morphogenesis.

BAF (mSWI/SNF) complex regulates mediolateral cortical patterning in the developing forebrain.

Early forebrain patterning entails the correct regional designation of the neuroepithelium, and appropriate specification, generation, and distribution of neural cells during brain development. Specific signaling and transcription factors are known to tightly regulate patterning of the dorsal telencephalon to afford proper structural/functional cortical arealization and morphogenesis. Nevertheless, whether and how changes of the chromatin structure link to the transcriptional program(s) that control cortical patterning remains elusive. Here, we report that the BAF chromatin remodeling complex regulates the spatiotemporal patterning of the mouse dorsal telencephalon. To determine whether and how the BAF complex regulates cortical patterning, we conditionally deleted the BAF complex scaffolding subunits BAF155 and BAF170 in the mouse dorsal telencephalic neuroepithelium. Morphological and cellular changes in the BAF mutant forebrain were examined using immunohistochemistry and in situ hybridization. RNA sequencing, Co-immunoprecipitation, and mass spectrometry were used to investigate the molecular basis of BAF complex involvement in forebrain patterning. We found that conditional ablation of BAF complex in the dorsal telencephalon neuroepithelium caused expansion of the cortical hem and medial cortex beyond their developmental boundaries. Consequently, the hippocampal primordium is not specified, the mediolateral cortical patterning is compromised, and the cortical identity is disturbed in the absence of BAF complex. The BAF complex was found to interact with the cortical hem suppressor LHX2. The BAF complex suppresses cortical hem fate to permit proper forebrain patterning. We provide evidence that BAF complex modulates mediolateral cortical patterning possibly by interacting with the transcription factor LHX2 to drive the LHX2-dependent transcriptional program essential for dorsal telencephalon patterning. Our data suggest a putative mechanistic synergy between BAF chromatin remodeling complex and LHX2 in regulating forebrain patterning and ontogeny.

Constitutive activation of canonical Wnt signaling disrupts choroid plexus epithelial fate.

The choroid plexus secretes cerebrospinal fluid and is critical for the development and function of the brain. In the telencephalon, the choroid plexus epithelium arises from the Wnt- expressing cortical hem. Canonical Wnt signaling pathway molecules such as nuclear ß-CATENIN are expressed in the mouse and human embryonic choroid plexus epithelium indicating that this pathway is active. Point mutations in human ß-CATENIN are known to result in the constitutive activation of canonical Wnt signaling. In a mouse model that recapitulates this perturbation, we report a loss of choroid plexus epithelial identity and an apparent transformation of this tissue to a neuronal identity. Aspects of this phenomenon are recapitulated in human embryonic stem cell derived organoids. The choroid plexus is also disrupted when ß-Catenin is conditionally inactivated. Together, our results indicate that canonical Wnt signaling is required in a precise and regulated manner for normal choroid plexus development in the mammalian brain.

Using multi-organ culture systems to study Parkinson's disease.

In recent years, it has been revealed that Parkinson's disease pathology may begin to manifest in the gastrointestinal track at a much earlier time point than in the brain. This paradigm shift has been suggested following evidence in humans that has been reproduced in animal models. Since rodent models cannot recapitulate many of the human disease features, human induced pluripotent stem cells derived from Parkinson's patients have been used to generate brain organoids, greatly contributing to our understanding of the disease pathophysiology. To understand the multifaced aspects of Parkinson's disease, it may be desirable to expand the complexity of these models, to include different brain regions, vasculature, immune cells as well as additional diverse organ-specific organoids such as gut and intestine. Furthermore, the contribution of gut microbiota to disease progression cannot be underestimated. Recent biotechnological advances propose that such combinations may be feasible. Here we discuss how this need can be met and propose that additional brain diseases can benefit from this approach.

Modeling human neuronal migration deficits in 3D.

During the past few decades, we have witnessed an impressive gain in the knowledge regarding the basic mechanisms underlying human neuronal migration disorders by the usage of mouse models. Nevertheless, despite the remarkable conservation both in the genetic encoded information and the developmental processes, there are still numerous important differences between human and mouse. This may explain the vast excitement following the realization that technological breakthroughs enabled generating tissue-like human-based organoids for modeling human neuronal migration diseases. This review will provide a short introduction on human and mouse neuronal migration processes, and highlight human brain organoid models of neuronal migration diseases.

RNAi-Mediated Reverse Genetic Screen Identified Drosophila Chaperones Regulating Eye and Neuromuscular Junction Morphology.

Accumulation of toxic proteins in neurons has been linked with the onset of neurodegenerative diseases, which in many cases are characterized by altered neuronal function and synapse loss. Molecular chaperones help protein folding and the resolubilization of unfolded proteins, thereby reducing the protein aggregation stress. While most of the chaperones are expressed in neurons, their functional relevance remains largely unknown. Here, using bioinformatics analysis, we identified 95 Drosophila chaperones and classified them into seven different classes. Ubiquitous actin5C-Gal4-mediated RNAi knockdown revealed that ∼50% of the chaperones are essential in Drosophila Knocking down these genes in eyes revealed that ∼30% of the essential chaperones are crucial for eye development. Using neuron-specific knockdown, immunocytochemistry, and robust behavioral assays, we identified a new set of chaperones that play critical roles in the regulation of Drosophila NMJ structural organization. Together, our data present the first classification and comprehensive analysis of Drosophila chaperones. Our screen identified a new set of chaperones that regulate eye and NMJ morphogenesis. The outcome of the screen reported here provides a useful resource for further elucidating the role of individual chaperones in Drosophila eye morphogenesis and synaptic development.