Pax6 Lengthens G1 Phase and Decreases Oscillating Cdk6 Levels in Murine Embryonic Cortical Progenitors

Transcriptional regulation in the development and dysfunction of neocortical projection neurons

Glutamatergic projection neurons generate sophisticated excitatory circuits to integrate and transmit information among different cortical areas, and between the neocortex and other regions of the brain and spinal cord. Appropriate development of cortical projection neurons is regulated by certain essential events such as neural fate determination, proliferation, specification, differentiation, migration, survival, axonogenesis, and synaptogenesis. These processes are precisely regulated in a tempo-spatial manner by intrinsic factors, extrinsic signals, and neural activities. The generation of correct subtypes and precise connections of projection neurons is imperative not only to support the basic cortical functions (such as sensory information integration, motor coordination, and cognition) but also to prevent the onset and progression of neurodevelopmental disorders (such as intellectual disability, autism spectrum disorders, anxiety, and depression). This review mainly focuses on the recent progress of transcriptional regulations on the development and diversity of neocortical projection neurons and the clinical relevance of the failure of transcriptional modulations.

Molecular biology of microRNA-342 during tumor progression and invasion

Cancer is considered as one of the main causes of human deaths and health challenges in the world. Various factors are involved in the high death rate of cancer patients, including late diagnosis and drug resistance that result in treatment failure and tumor recurrence. Invasive diagnostic methods are one of the main reasons of late tumor detection in cancer patients. Therefore, it is necessary to investigate the molecular tumor biology to introduce efficient non-invasive markers. MicroRNAs (miRNAs) are involved in regulation of the cellular mechanisms such as cell proliferation, apoptosis, and migration. MiRNAs deregulations have been also frequently shown in different tumor types. Here, we discussed the molecular mechanisms of miR-342 during tumor growth. MiR-342 mainly functions as a tumor suppressor by the regulation of transcription factors and signaling pathways such as WNT, PI3K/AKT, NF-kB, and MAPK. Therefore, miR-342 mimics can be used as a reliable therapeutic strategy to inhibit the tumor cells growth. The present review can also pave the way to introduce the miR-342 as a non-invasive diagnostic/prognostic marker in cancer patients.

The impact of maternal immune activation on embryonic brain development

The adult brain is a complex structure with distinct functional sub-regions, which are generated from an initial pool of neural epithelial cells within the embryo. This transition requires a number of highly coordinated processes, including neurogenesis, i.e., the generation of neurons, and neuronal migration. These take place during a critical period of development, during which the brain is particularly susceptible to environmental insults. Neurogenesis defects have been associated with the pathogenesis of neurodevelopmental disorders (NDDs), such as autism spectrum disorder and schizophrenia. However, these disorders have highly complex multifactorial etiologies, and hence the underlying mechanisms leading to aberrant neurogenesis continue to be the focus of a significant research effort and have yet to be established. Evidence from epidemiological studies suggests that exposure to maternal infection in utero is a critical risk factor for NDDs. To establish the biological mechanisms linking maternal immune activation (MIA) and altered neurodevelopment, animal models have been developed that allow experimental manipulation and investigation of different developmental stages of brain development following exposure to MIA. Here, we review the changes to embryonic brain development focusing on neurogenesis, neuronal migration and cortical lamination, following MIA. Across published studies, we found evidence for an acute proliferation defect in the embryonic MIA brain, which, in most cases, is linked to an acceleration in neurogenesis, demonstrated by an increased proportion of neurogenic to proliferative divisions. This is accompanied by disrupted cortical lamination, particularly in the density of deep layer neurons, which may be a consequence of the premature neurogenic shift. Although many aspects of the underlying pathways remain unclear, an altered epigenome and mitochondrial dysfunction are likely mechanisms underpinning disrupted neurogenesis in the MIA model. Further research is necessary to delineate the causative pathways responsible for the variation in neurogenesis phenotype following MIA, which are likely due to differences in timing of MIA induction as well as sex-dependent variation. This will help to better understand the underlying pathogenesis of NDDs, and establish therapeutic targets.

Pax6 mutant cerebral organoids partially recapitulate phenotypes of Pax6 mutant mouse strains

Cerebral organoids show great promise as tools to unravel the complex mechanisms by which the mammalian brain develops during embryogenesis. We generated mouse cerebral organoids harbouring constitutive or conditional mutations in Pax6, which encodes a transcription factor with multiple important roles in brain development. By comparing the phenotypes of mutant organoids with the well-described phenotypes of Pax6 mutant mouse embryos, we evaluated the extent to which cerebral organoids reproduce phenotypes previously described in vivo. Organoids lacking Pax6 showed multiple phenotypes associated with its activity in mice, including precocious neural differentiation, altered cell cycle and an increase in abventricular mitoses. Neural progenitors in both Pax6 mutant and wild type control organoids cycled more slowly than their in vivo counterparts, but nonetheless we were able to identify clear changes to cell cycle attributable to the absence of Pax6. Our findings support the value of cerebral organoids as tools to explore mechanisms of brain development, complementing the use of mouse models.

Pax6 limits the competence of developing cerebral cortical cells to respond to inductive intercellular signals

The development of stable specialized cell types in multicellular organisms relies on mechanisms controlling inductive intercellular signals and the competence of cells to respond to such signals. In developing cerebral cortex, progenitors generate only glutamatergic excitatory neurons despite being exposed to signals with the potential to initiate the production of other neuronal types, suggesting that their competence is limited. Here, we tested the hypothesis that this limitation is due to their expression of transcription factor Pax6. We used bulk and single-cell RNAseq to show that conditional cortex-specific Pax6 deletion from the onset of cortical neurogenesis allowed some progenitors to generate abnormal lineages resembling those normally found outside the cortex. Analysis of selected gene expression showed that the changes occurred in specific spatiotemporal patterns. We then compared the responses of control and Pax6-deleted cortical cells to in vivo and in vitro manipulations of extracellular signals. We found that Pax6 loss increased cortical progenitors’ competence to generate inappropriate lineages in response to extracellular factors normally present in developing cortex, including the morphogens Shh and Bmp4. Regional variation in the levels of these factors could explain spatiotemporal patterns of fate change following Pax6 deletion in vivo. We propose that Pax6’s main role in developing cortical cells is to minimize the risk of their development being derailed by the potential side effects of morphogens engaged contemporaneously in other essential functions.

The development of stable specialized cell types in multicellular organisms relies on mechanisms controlling inductive intercellular signals and the competence of cells to respond. This study shows that cortical development is stabilized by the protective actions of the transcription factor Pax6, which adjusts the ability of cortical cells to respond to potentially destabilizing signals present in their local environment.

YAP prevents premature senescence of astrocytes and cognitive decline of Alzheimer's disease through regulating CDK6 signaling

Senescent astrocytes accumulate with aging and contribute to brain dysfunction and diseases such as Alzheimer's disease (AD), however, the mechanisms underlying the senescence of astrocytes during aging remain unclear. In the present study, we found that Yes‐associated Protein (YAP) was downregulated and inactivated in hippocampal astrocytes of aging mice and AD model mice, as well as in D‐galactose and paraquat‐induced senescent astrocytes, in a Hippo pathway‐dependent manner. Conditional knockout of YAP in astrocytes significantly promoted premature senescence of astrocytes, including reduction of cell proliferation, hypertrophic morphology, increase in senescence‐associated β‐galactosidase activity, and upregulation of several senescence‐associated genes such as p16, p53 and NF‐κB, and downregulation of Lamin B1. Further exploration of the underlying mechanism revealed that the expression of cyclin‐dependent kinase 6 (CDK6) was decreased in YAP knockout astrocytes in vivo and in vitro, and ectopic overexpression of CDK6 partially rescued YAP knockout‐induced senescence of astrocytes. Finally, activation of YAP signaling by XMU‐MP‐1 (an inhibitor of Hippo kinase MST1/2) partially rescued the senescence of astrocytes and improved the cognitive function of AD model mice and aging mice. Taken together, our studies identified unrecognized functions of YAP‐CDK6 pathway in preventing astrocytic senescence in vitro and in vivo, which may provide further insights and new targets for delaying brain aging and aging‐related neurodegenerative diseases such as AD.

Oncogenic PAX6 elicits CDK4/6 inhibitor resistance by epigenetically inactivating the LATS2-Hippo signaling pathway

Intrinsic resistance to CDK4/6 inhibitors hinders their clinical utility in cancer treatment. Furthermore, the predictive markers of CDK4/6 inhibitors in gastric cancer (GC) remain incompletely described. Here, we found that PAX6 expression was negatively correlated with the response to palbociclib in vitro and in vivo in GC. We observed that the PAX6 expression level was negatively correlated with the overall survival of GC patients and further showed that PAX6 can promote GC cell proliferation and the cell cycle. The cell cycle is regulated by the interaction of cyclins with their partner serine/threonine cyclin-dependent kinases (CDKs), and the G1/S-phase transition is the main target of CDK4/6 inhibitors. Therefore, we tested whether PAX6 expression was correlated with the GC response to palbociclib. We found that PAX6 hypermethylates the promoter of LATS2 and inactivates the Hippo pathway, which upregulates cyclin D1 (CCND1) expression. This results in a suppressed response to palbociclib in GC. Furthermore, we found that the induction of the Hippo signaling pathway or treatment with a DNA methylation inhibitor could overcome PAX6-induced palbociclib resistance in GC. These findings uncover a tumor promoter function of PAX6 in GC and establish overexpressed PAX6 as a mechanism of resistance to palbociclib.