Strength of signal: a fundamental mechanism for cell fate specification

Summary:  How equipotent cells develop into complex tissues containing many diverse cell types is still a mystery. However, evidence is accumulating from different tissue systems in multiple organisms that many of the specific receptor families known to regulate cell fate decisions target conserved signaling pathways. A mechanism for preserving specificity in the cellular response that has emerged from these studies is one in which quantitative differences in receptor signaling regulate the cell fate decision. A signal strength model has recently gained support as a means to explain αβ/γδ lineage commitment. In this review, we compare the αβ/γδ fate decision with other cell fate decisions that occur outside of the lymphoid system to attain a better picture of the quantitative signaling mechanism for cell fate specification.     

Human aneuploidy: lessons from achiasmate segregation in Drosophila melanogaster

Aneuploidy is a crucial issue in human reproductive biology, accounting for both a significant proportion of miscarriages and, among liveborns, multiple congenital malformation syndromes such as Down Syndrome. Although the etiology of human aneuploidy remains poorly understood, recent studies have elucidated certain fundamental correlates of meiotic nondisjunction, such as altered recombination. These features are extraordinarily similar to those associated with chromosome misbehavior in Drosophila melanogaster females. Furthermore, these organisms also share a significant level of achiasmate chromosome nondisjunction. Here we describe in detail the processes of achiasmate chromosome segregation in Drosophila and discuss how they may be most effectively applied to our understanding of the etiology of human aneuploidy. In particular, we examine the possibility that similar "backup" mechanisms of chromosome segregation might function in mammalian meiosis, particularly mammalian females. Drawing upon observations made in flies, we also propose a new model for the segregation of achiasmate chromosomes in humans.     

Direct derivation of neural rosettes from cloned bovine blastocysts: a model of early neurulation events and neural crest specification in vitro

Embryonic stem cells differentiate into neuroectodermal cells under specific culture conditions. In primates, these cells are organized into rosettes expressing Pax6 and Sox1 and are responsive to inductive signals such as Sonic hedgehog (Shh) and retinoic acid. However, direct derivation of organized neuroectoderm in vitro from preimplantation mammalian embryos has never been reported. Here, we show that bovine inner cell masses from nuclear transfer and fertilized embryos, grown on feeders in serum-free medium, form polarized rosette structures expressing nestin, Pax6, Pax7, Sox1, and Otx2 and exhibiting interkinetic nuclear migration activity and cell junction distribution as in the developing neural tube. After in vitro expansion, neural rosettes give rise to p75-positive neural crest precursor cell lines capable of long-term proliferation and differentiation in autonomic and sensory peripheral neurons, glial cells, melanocytes, smooth muscle cells, and chondrocytes, recapitulating in vitro the unique plasticity of the neural crest lineage. Challenging the rosette dorsal fate by early exposure to Shh induces the expression of ventral markers Isl1, Nkx2.2, and Nkx6.1 and differentiation of mature astrocytes and neurons of central nervous system ventral identity, demonstrating appropriate response to inductive signals. All together, these findings indicate that neural rosettes directly derived from cloned and fertilized bovine embryos represent an in vitro model of early neural specification and differentiation events. Moreover, this study provides a source of highly proliferative neural crest precursor cell lines of wide differentiation potential for cell therapy and tissue engineering applications.     

Asymmetric localization of Numb:EGFP in dividing neuroepithelial cells during neurulation in Danio rerio.

In the neural plate and tube of the zebrafish embryo, cells divide with their mitotic spindles oriented parallel to the plane of the neuroepithelium, whilst in the neural keel and rod, the spindle is oriented perpendicular to it. This change is achieved by a 90 degrees rotation of the mitotic spindle. We cloned zebrafish homologues of the gene for the Drosophila cell fate determinant Numb, and analyzed the localization of EGFP fusion proteins in vivo in dividing neuroepithelial cells during neurulation. Whereas Numb isoform 3 and the related protein Numblike are localized in the cytoplasm, Numb isoform 1 is localized to the cell membrane. Time-lapse analyses showed that Numb 1 is distributed uniformly around the cell cortex in dividing cells during plate and keel stages, but becomes localized at the basolateral membrane of some dividing cells during the transition from neural rod to tube. Using in vitro mutagenesis and Numb:EGFP deletion constructs, we showed that the first 196 amino acids of Numb are sufficient for this localization. Furthermore, we found that an 11-amino acid insertion in the PTB domain is essential for localization to the cortex, whereas amino acids 2-12 mediate the basolateral localization in the neural tube stage.     

Numb and Numbl are required for maintenance of cadherin-based adhesion and polarity of neural progenitors

The polarity and adhesion of radial glial cells (RGCs), which function as progenitors and migrational guides for neurons, are critical for morphogenesis of the cerebral cortex. These characteristics largely depend on cadherin-based adherens junctions, which anchor apical end-feet of adjacent RGCs to each other at the ventricular surface. Here, we show that mouse numb and numb-like are required for maintaining radial glial adherens junctions. Numb accumulates in the apical end-feet, where it localizes to adherens junction-associated vesicles and interacts with cadherins. Numb and Numbl inactivation in RGCs decreases proper basolateral insertion of cadherins and disrupts adherens junctions and polarity, leading to progenitor dispersion and disorganized cortical lamination. Conversely, overexpression of Numb prolongs RGC polarization, in a cadherin-dependent manner, beyond the normal neurogenic period. Thus, by regulating RGC adhesion and polarity, Numb and Numbl are required for the tissue architecture of neurogenic niches and the cerebral cortex.     

MicroRNA-9a ensures the precise specification of sensory organ precursors in Drosophila

MicroRNAs (miRNAs) have been implicated in regulating various aspects of animal development, but their functions in neurogenesis are largely unknown. Here we report that loss of miR-9a function in the Drosophila peripheral nervous system leads to ectopic production of sensory organ precursors (SOPs), whereas overexpression of miR-9a results in a severe loss of SOPs. We further demonstrate a strong genetic interaction between miR-9a and senseless (sens) in controlling the formation of SOPs in the adult wing imaginal disc. Moreover, miR-9a suppresses Sens expression through its 3' untranslated region. miR-9a is expressed in epithelial cells, including those adjacent to SOPs within proneural clusters, suggesting that miR-9a normally inhibits neuronal fate in non-SOP cells by down-regulating Sens expression. These results indicate that miR-9a ensures the generation of the precise number of neuronal precursor cells during development.     

The spindle checkpoint: a quality control mechanism which ensures accurate chromosome segregation

The centromere defines where on a chromosome the kinetochores assemble. Kinetochores, large protein structures, mediate chromosome segregation during mitosis and meiosis by performing three key functions. Firstly, kinetochores attach chromosomes to the microtubule spindle apparatus. Secondly, kinetochores co-ordinate microtubule dynamics to allow chromosomes to move along the spindle. Lastly, kinetochores generate the 'wait' signal which prevents anaphase onset until all the chromosomes are correctly aligned on the spindle. This signal forms part of the spindle checkpoint mechanism, a highly conserved cell cycle checkpoint which maintains the accuracy of the chromosome segregation process. This article provides a brief historical overview before focusing on some of the outstanding issues and more recent developments in the field.     

White spotting phenotype induced by targeted REST disruption during neural crest specification to a melanocyte cell lineage

Neural crest cells (NCCs) emerge from the dorsal region of the neural tube of vertebrate embryos and have the pluripotency to differentiate into both neuronal and non‐neuronal lineages including melanocytes. Rest, also known as NRSF (neuro‐restrictive silencer factor), is a regulator of neuronal development and function and suggested to be involved in the lineage specification of NCCs. However, further investigations of Rest gene functions in vivo have been hampered by the fact that Rest null mice show early embryonic lethality. To investigate the function of Rest in NCC development, we recently established NCC‐specific Rest conditional knockout (CKO) mice and observed their neonatal death. Here, we have established viable heterozygous NCC‐specific Rest CKO mice to analyze the function of Rest in an NCC‐derived melanocyte cell lineage and found that the white spotting phenotype was associated with the reduction in the number of melanoblasts in the embryonic skin. The Rest deletion induced after the specification to melanocytes did not reduce the number of melanoblasts; therefore, the expression of REST during the early neural crest specification stage was necessary for the normal development of melanoblasts to cover all of the skin.     

Microsites for immunoglobulin switch recombination breakpoints from Xenopus to mammals

Immunoglobulin (Ig) heavy chain class switch recombination has been studied at the DNA level in a non-mammalian vertebrate, the amphibian Xenopus. A switch (S) region of about 5 kb has been identified in the J_H-C_μ intron of the Ig heavy chain locus in Xenopus. Sμ contains 23 repeats approximately 150 bp long. Each repeat consists of internal shorter repeats and palindromic sequences, such as AGCT, which they share with mammalian switch regions. A deletion of the μ gene and the joining of the S regions of μ and χ occurs in B cells expressing IgX, one of the two non-μ isotypes in Xenopus. Sx shows no sequence homology with Sμ and is characterized by 16 and 121 bp repeats and a high frequency of CATG, AGCA and TGCA palindromes. Both IgM and IgX S regions are AT rich and not GC rich like mammalian S regions. Recombination occurs, most of the time, at positions (microsites) where a single-stranded DNA folding program predicts the transition from a stem to a loop structure. This feature is conserved in most mammalian switch junctions which points to the general existence and involvement of microsites at one step of the determination of the recombination breakpoint. The recombinogenic nature of the switch regions is therefore linked to its structure rather than to its base composition, the repetitive occurrence of palindromes being essential at creating many microsites.     

The neural organization of semantic memory: Electrophysiological activity suggests feature-based segregation

Despite decades of research, it remains controversial whether semantic knowledge is anatomically segregated in the human brain. To address this question, we recorded event-related potentials (ERPs) while participants viewed pictures of animals and tools. Within the 200-600-ms epoch after stimulus presentation, animals (relative to tools) elicited an increased anterior negativity that, based on previous ERP studies, we interpret as associated with semantic processing of visual object attributes. In contrast, tools (relative to animals) evoked an enhanced posterior left-lateralized negativity that, according to prior research, might reflect accessing knowledge of characteristic motion and/or more general functional properties of objects. These results support the hypothesis of the neuroanatomical knowledge organization at the level of object features: the observed neurophysiological activity was modulated by the features that were most salient for object recognition. The high temporal resolution of ERPs allowed us to demonstrate that differences in processing animals and tools occurred specifically within the time-window encompassing semantic analysis.     

The topographical N170: Electrophysiological evidence of a neural mechanism for human spatial navigation

We recently demonstrated that the latency of a component of the event-related brain potential, the topographical N170 (NT170), is sensitive to the spatial location of reward-related stimuli in a virtual maze environment, occurring earlier for rewards found following rightward turns compared to leftward turns. We suggested that this NT170 latency effect may result from phase reset of an ongoing theta rhythm by a parahippocampal system for spatial navigation. Here we tested several predictions that follow from this proposal, namely, that the effect is observed only when the rewards are presented in a spatial environment, that it is sensitive to individual differences in spatial ability, that it is localizable to the right parahippocampal region, and that it is consistent with partial phase resetting of an ongoing theta rhythm. These results hold promise for integrating ERP measures of spatial navigation with extensive animal, human, and computational literatures on parahippocampal function.     

Theta oscillations reflect a putative neural mechanism for human sensorimotor integration

Hippocampal theta oscillations (3-12 Hz) may reflect a mechanism for sensorimotor integration in rats (Bland BH. Prog Neurobiol 26: 1-54, 1986); however, it is unknown whether cortical theta activity underlies sensorimotor integration in humans. Rather, the mu rhythm (8-12 Hz) is typically found to desynchronize during movement. We measured oscillatory EEG activity for two conditions of an instructed delayed reaching paradigm. Conditions 1 and 2 were designed to differentially manipulate the contribution of the ventral visuomotor stream during the response initiation phase. We tested the hypothesis that theta activity would reflect changes in the relevant sensorimotor network: condition 2 engaged ventral stream mechanisms to a greater extent than condition 1. Theta oscillations were more prevalent during movement initiation and execution than during periods of stillness, consistent with a sensorimotor relevance for theta activity. Furthermore, theta activity was more prevalent at temporal sites in condition 2 than condition 1 during response initiation, suggesting that theta activity is present within the necessary sensorimotor network. Mu activity desynchronized more during condition 2 than condition 1, suggesting mu desynchronization is also specific to the sensorimotor network. In summary, cortical theta synchronization and mu desynchronization may represent broadly applicable rhythmic mechanisms for sensorimotor integration in the human brain.     

Induced neural stem cells from Macaca fascicularis show potential of dopaminergic neuron specification and efficacy in a mouse Parkinson's disease model

Induced neural stem cells (iNSCs) can be reprogrammed from somatic cells and have shown potentials in treatment of various neurological diseases/disorders. Obtaining iNSCs of nonhuman primates serves as an important bridge for clinical translation using iNSCs. In the current study, cynomolgus (Macaca fascicularis) bone marrow mesenchymal stromal cells (MSCs) were reprogrammed into iNSCs by transduction of non-integrative Sendai virus encoding transgenes OCT4, SOX2, KLF4 and C-MYC. The obtained iNSCs showed characteristics of normal neural stem cells (NSCs) and could differentiate into neurons, astrocytes and oligodendrocytes. Furthermore, iNSCs could give rise to dopaminergic neural cells in vitro, which showed safety and efficacy after transplantation into the striatum of an immunodeficient mouse Parkinson’s disease (PD) model.     

社交互动的杏仁核神经回路研究进展：从微回路到输入输出回路

社交互动是动物必不可少的复杂行为,对动物的生存和繁衍至关重要。诸如杏仁核(AMG)、内侧前额叶皮层(mPFC)、海马等社交脑区域,以及它们相互间形成的神经回路均参与了社交互动行为调控。神经微回路、神经回路构成复杂大脑的连接单元,虽然社交互动相关神经回路已被多方报道,但依旧缺乏其微回路、输入输出回路的系统报道。因此,本文基于杏仁核脑区,从分子、细胞和网络层面解析其社交互动的微回路和输入/输出回路,旨在为社交互动的神经机制提供回路证据,为社交互动障碍提供神经回路诊疗依据。