Understanding microcephaly through the study of centrosome regulation in Drosophila neural stem cells

Lateral expansion of the mammalian cerebral cortex is related to anchorage of centrosomes in apical neural progenitors

The centrosome is the main microtubule organizing center in stem cells, and its mother centriole, anchored to the cell membrane, serves as the basal body of the primary cilium. Prolonged anchorage of centrosomes and primary cilia to the apical segment of the membrane of apical neural progenitor cells is considered vital for interkinetic nuclear translocation and repetitive cycling in the ventricular zone. In contrast, the basolateral anchorage of primary cilia has been regarded as the first step in delamination and conversion of apical to basal neural progenitor cells or neurons. Using electron microscopy analysis of serial sections, we show that centrosomes, in a fraction of cells, anchor to the basolateral cell membrane immediately after cell division and before development of cilia. In other cells, centrosomes situate freely in the cytoplasm, increasing their probability of subsequent apical anchorage. In mice, anchored centrosomes in the cells shortly after mitosis predominate during the entire cerebral neurogenesis, whereas in macaque monkeys, cytoplasmic centrosomes are more numerous. Species-specific differences in the ratio of anchored and free cytoplasmic centrosomes appear to be related to prolonged neurogenesis in the ventricular zone that is essential for lateral expansion of the cerebral cortex in primates.

Cep120 is essential for kidney stromal progenitor cell growth and differentiation

Mutations in genes that disrupt centrosome structure or function can cause congenital kidney developmental defects and lead to fibrocystic pathologies. Yet, it is unclear how defective centrosome biogenesis impacts renal progenitor cell physiology. Here, we examined the consequences of impaired centrosome duplication on kidney stromal progenitor cell growth, differentiation, and fate. Conditional deletion of the ciliopathy gene Cep120, which is essential for centrosome duplication, in the stromal mesenchyme resulted in reduced abundance of interstitial lineages including pericytes, fibroblasts and mesangial cells. These phenotypes were caused by a combination of delayed mitosis, activation of the mitotic surveillance pathway leading to apoptosis, and changes in both Wnt and Hedgehog signaling that are key for differentiation of stromal cells. Cep120 ablation resulted in small hypoplastic kidneys with medullary atrophy and delayed nephron maturation. Finally, Cep120 and centrosome loss in the interstitium sensitized kidneys of adult mice, causing rapid fibrosis after renal injury via enhanced TGF-β/Smad3-Gli2 signaling. Our study defines the cellular and developmental defects caused by loss of Cep120 and aberrant centrosome biogenesis in the embryonic kidney stroma.

PCDH12 loss results in premature neuronal differentiation and impeded migration in a cortical organoid model

Protocadherins (PCDHs) are cell adhesion molecules that regulate many essential neurodevelopmental processes related to neuronal maturation, dendritic arbor formation, axon pathfinding, and synaptic plasticity. Biallelic loss-of-function variants in PCDH12 are associated with several neurodevelopmental disorders (NDDs). Despite the highly deleterious outcome resulting from loss of PCDH12, little is known about its role during brain development and disease. Here, we show that PCDH12 loss severely impairs cerebral organoid development, with reduced proliferative areas and disrupted laminar organization. 2D models further show that neural progenitor cells lacking PCDH12 prematurely exit the cell cycle and differentiate earlier when compared with wild type. Furthermore, we show that PCDH12 regulates neuronal migration and suggest that this could be through a mechanism requiring ADAM10-mediated ectodomain shedding and/or membrane recruitment of cytoskeleton regulators. Our results demonstrate a critical involvement of PCDH12 in cortical organoid development, suggesting a potential cause for the pathogenic mechanisms underlying PCDH12-related NDDs.

The neurological and non-neurological roles of the primary microcephaly-associated protein ASPM

Primary microcephaly (MCPH), is a neurological disorder characterized by small brain size that results in numerous developmental problems, including intellectual disability, motor and speech delays, and seizures. Hitherto, over 30 MCPH causing genes (MCPHs) have been identified. Among these MCPHs, MCPH5, which encodes abnormal spindle-like microcephaly-associated protein (ASPM), is the most frequently mutated gene. ASPM regulates mitotic events, cell proliferation, replication stress response, DNA repair, and tumorigenesis. Moreover, using a data mining approach, we have confirmed that high levels of expression of ASPM correlate with poor prognosis in several types of tumors. Here, we summarize the neurological and non-neurological functions of ASPM and provide insight into its implications for the diagnosis and treatment of MCPH and cancer.

The impact of TP53 activation and apoptosis in primary hereditary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a constellation of disorders that share significant brain size reduction and mild to moderate intellectual disability, which may be accompanied by a large variety of more invalidating clinical signs. Extensive neural progenitor cells (NPC) proliferation and differentiation are essential to determine brain final size. Accordingly, the 30 MCPH loci mapped so far (MCPH1-MCPH30) encode for proteins involved in microtubule and spindle organization, centriole biogenesis, nuclear envelope, DNA replication and repair, underscoring that a wide variety of cellular processes is required for sustaining NPC expansion during development. Current models propose that altered balance between symmetric and asymmetric division, as well as premature differentiation, are the main mechanisms leading to MCPH. Although studies of cellular alterations in microcephaly models have constantly shown the co-existence of high DNA damage and apoptosis levels, these mechanisms are less considered as primary factors. In this review we highlight how the molecular and cellular events produced by mutation of the majority of MCPH genes may converge on apoptotic death of NPCs and neurons, via TP53 activation. We propose that these mechanisms should be more carefully considered in the alterations of the sophisticated equilibrium between proliferation, differentiation and death produced by MCPH gene mutations. In consideration of the potential druggability of cell apoptotic pathways, a better understanding of their role in MCPH may significantly facilitate the development of translational approaches.

Impaired centrosome biogenesis in kidney stromal progenitors reduces abundance of interstitial lineages and accelerates injury-induced fibrosis

Defective centrosome function can disrupt embryonic kidney development, by causing changes to the renal interstitium that leads to fibrocystic disease pathologies. Yet, it remains unknown how mutations in centrosome genes impact kidney interstitial cells. Here, we examined the consequences of defective centrosome biogenesis on stromal progenitor cell growth, differentiation and fate. Conditional deletion of Cep120 , a ciliopathy gene essential for centrosome duplication, in the stromal mesenchyme resulted in reduced abundance of pericytes, interstitial fibroblasts and mesangial cells. This was due to delayed mitosis, increased apoptosis, and changes in Wnt and Hedgehog signaling essential for differentiation of stromal lineages. Cep120 ablation resulted in hypoplastic kidneys with medullary atrophy and delayed nephron maturation. Finally, centrosome loss in the interstitium sensitized kidneys of adult mice, causing rapid fibrosis via enhanced TGF-β/Smad3-Gli2 signaling after renal injury. Our study defines the cellular and developmental defects caused by centrosome dysfunction in embryonic kidney stroma.

Highlights

Defective centrosome biogenesis in kidney stroma causes:Reduced abundance of stromal progenitors, interstitial and mesangial cell populationsDefects in cell-autonomous and paracrine signalingAbnormal/delayed nephrogenesis and tubular dilationsAccelerates injury-induced fibrosis via defective TGF-β/Smad3-Gli2 signaling axis.

In vivo models to study neurogenesis and associated neurodevelopmental disorders-Microcephaly and autism spectrum disorder

The genesis and functioning of the central nervous system are one of the most intricate and intriguing aspects of embryogenesis. The big lacuna in the field of human CNS development is the lack of accessibility of the human brain for direct observation during embryonic and fetal development. Thus, it is imperative to establish alternative animal models to gain deep mechanistic insights into neurodevelopment, establishment of neural circuitry, and its function. Neurodevelopmental events such as neural specification, differentiation, and generation of neuronal and non-neuronal cell types have been comprehensively studied using a variety of animal models and in vitro model systems derived from human cells. The experimentations on animal models have revealed novel, mechanistic insights into neurogenesis, formation of neural networks, and function. The models, thus serve as indispensable tools to understand the molecular basis of neurodevelopmental disorders (NDDs) arising from aberrations during embryonic development. Here, we review the spectrum of in vivo models such as fruitfly, zebrafish, frog, mice, and nonhuman primates to study neurogenesis and NDDs like microcephaly and Autism Spectrum Disorder. We also discuss nonconventional models such as ascidians and the recent technological advances in the field to study neurogenesis, disease mechanisms, and pathophysiology of human NDDs. This article is categorized under: Cancer > Stem Cells and Development Congenital Diseases > Stem Cells and Development Neurological Diseases > Stem Cells and Development Congenital Diseases > Genetics/Genomics/Epigenetics.

Impaired migration and premature differentiation underlie the neurological phenotype associated with PCDH12 loss of function

Protocadherins (PCDHs) are cell adhesion molecules that regulate many essential neurodevelopmental processes related to neuronal maturation, dendritic arbor formation, axon pathfinding, and synaptic plasticity. Bi-allelic loss-of-function variants in PCDH12 are associated with several neurodevelopmental disorders (NDDs) such as diencephalic-mesencephalic dysplasia syndrome, cerebral palsy, cerebellar ataxia, and microcephaly. Despite the highly deleterious outcome resulting from loss of PCDH12, little is known about its role during brain development and disease. Here, we show that PCDH12 loss severely impairs cerebral organoid development with reduced proliferative areas and disrupted laminar organization. 2D models further show that neural progenitor cells lacking PCDH12 prematurely exit cell cycle and differentiate earlier when compared to wildtype. Furthermore, we show that PCDH12 regulates neuronal migration through a mechanism requiring ADAM10-mediated ectodomain shedding and membrane recruitment of cytoskeleton regulators. Our data demonstrate a critical and broad involvement of PCDH12 in cortical development, revealing the pathogenic mechanisms underlying PCDH12-related NDDs.

The international exchange of Drosophila melanogaster strains

Drosophila melanogaster has been a model organism for experimental research for more than a century, and the knowledge and associated genetic technologies accumulated around this species make it extremely important to contemporary biomedical research. A large international community of highly collaborative scientists investigate a remarkable diversity of biological problems using genetically characterised strains of Drosophila, and frequently exchange these strains across borders. Despite its importance to the study of fundamental biological processes and human disease-related cellular mechanisms, and the fact that it presents minimal health, agricultural or environmental risks, Drosophila can be difficult to import. The authors argue that streamlined regulations and practices would benefit biomedical research by lowering costs and increasing efficiencies.

Zika Virus Neuropathogenesis: The Different Brain Cells, Host Factors and Mechanisms Involved

Zika virus (ZIKV), despite being discovered six decades earlier, became a major health concern only after an epidemic in French Polynesia and an increase in the number of microcephaly cases in Brazil. Substantial evidence has been found to support the link between ZIKV and neurological complications in infants. The virus targets various cells in the brain, including radial glial cells, neural progenitor cells (NPCs), astrocytes, microglial and glioblastoma stem cells. It affects the brain cells by exploiting different mechanisms, mainly through apoptosis and cell cycle dysregulation. The modulation of host immune response and the inflammatory process has also been demonstrated to play a critical role in ZIKV induced neurological complications. In addition to that, different ZIKV strains have exhibited specific neurotropism and unique molecular mechanisms. This review provides a comprehensive and up-to-date overview of ZIKV-induced neuroimmunopathogenesis by dissecting its main target cells in the brain, and the underlying cellular and molecular mechanisms. We highlighted the roles of the different ZIKV host factors and how they exploit specific host factors through various mechanisms. Overall, it covers key components for understanding the crosstalk between ZIKV and the brain.

Dynamic Changes of Brain Cilia Transcriptomes across the Human Lifespan

Almost all brain cells contain primary cilia, antennae-like microtubule sensory organelles, on their surface, which play critical roles in brain functions. During neurodevelopmental stages, cilia are essential for brain formation and maturation. In the adult brain, cilia play vital roles as signaling hubs that receive and transduce various signals and regulate cell-to-cell communications. These distinct roles suggest that cilia functions, and probably structures, change throughout the human lifespan. To further understand the age-dependent changes in cilia roles, we identified and analyzed age-dependent patterns of expression of cilia's structural and functional components across the human lifespan. We acquired cilia transcriptomic data for 16 brain regions from the BrainSpan Atlas and analyzed the age-dependent expression patterns using a linear regression model by calculating the regression coefficient. We found that 67% of cilia transcripts were differentially expressed genes with age (DEGAs) in at least one brain region. The age-dependent expression was region-specific, with the highest and lowest numbers of DEGAs expressed in the ventrolateral prefrontal cortex and hippocampus, respectively. The majority of cilia DEGAs displayed upregulation with age in most of the brain regions. The transcripts encoding cilia basal body components formed the majority of cilia DEGAs, and adjacent cerebral cortices exhibited large overlapping pairs of cilia DEGAs. Most remarkably, specific α/β-tubulin subunits (TUBA1A, TUBB2A, and TUBB2B) and SNAP-25 exhibited the highest rates of downregulation and upregulation, respectively, across age in almost all brain regions. α/β-tubulins and SNAP-25 expressions are known to be dysregulated in age-related neurodevelopmental and neurodegenerative disorders. Our results support a role for the high dynamics of cilia structural and functional components across the lifespan in the normal physiology of brain circuits. Furthermore, they suggest a crucial role for cilia signaling in the pathophysiological mechanisms of age-related psychiatric/neurological disorders.