Luis C. Fuentealba

Regional Astrocyte Allocation Regulates CNS Synaptogenesis and Repair

Born to Stay Together For as many neurons as there are in the brain, there are many more astrocytes. These backstage workers perform a variety of functions, such as sustaining the blood-brain barrier and providing a stabilized environment for neurons. Diversity of astrocyte function is reflected in different molecular expression profiles. Tsai et al. (p. 358, published online 28 June) selectively labeled astrocytes that originated from different domains of the mouse spinal cord and found that not all astrocytes are created equal: Neighborhoods of astrocytes were defined by shared birthplaces. In the mouse brain, astrocytes are not as interchangeable as previously thought. Astrocytes, the most abundant cell population in the central nervous system (CNS), are essential for normal neurological function. We show that astrocytes are allocated to spatial domains in mouse spinal cord and brain in accordance with their embryonic sites of origin in the ventricular zone. These domains remain stable throughout life without evidence of secondary tangential migration, even after acute CNS injury. Domain-specific depletion of astrocytes in ventral spinal cord resulted in abnormal motor neuron synaptogenesis, which was not rescued by immigration of astrocytes from adjoining regions. Our findings demonstrate that region-restricted astrocyte allocation is a general CNS phenomenon and reveal intrinsic limitations of the astroglial response to injury.

Embryonic origin of postnatal neural stem cells

Adult neural stem/progenitor (B1) cells within the walls of the lateral ventricles generate different types of neurons for the olfactory bulb (OB). The location of B1 cells determines the types of OB neurons they generate. Here we show that the majority of mouse B1 cell precursors are produced between embryonic days (E) 13.5 and 15.5 and remain largely quiescent until they become reactivated postnatally. Using a retroviral library carrying over 100,000 genetic tags, we found that B1 cells share a common progenitor with embryonic cells of the cortex, striatum, and septum, but this lineage relationship is lost before E15.5. The regional specification of B1 cells is evident as early as E11.5 and is spatially linked to the production of neurons that populate different areas of the forebrain. This study reveals an early embryonic regional specification of postnatal neural stem cells and the lineage relationship between them and embryonic progenitor cells.

Adult neural stem cells in distinct microdomains generate previously unknown interneuron types

Throughout life, neural stem cells (NSCs) in different domains of the ventricular-subventricular zone (V-SVZ) of the adult rodent brain generate several subtypes of interneurons that regulate the function of the olfactory bulb. The full extent of diversity among adult NSCs and their progeny is not known. Here, we report the generation of at least four previously unknown olfactory bulb interneuron subtypes that are produced in finely patterned progenitor domains in the anterior ventral V-SVZ of both the neonatal and adult mouse brain. Progenitors of these interneurons are responsive to sonic hedgehog and are organized into microdomains that correlate with the expression domains of the Nkx6.2 and Zic family of transcription factors. This work reveals an unexpected degree of complexity in the specification and patterning of NSCs in the postnatal mouse brain.

Myb promotes centriole amplification and later steps of the multiciliogenesis program.

The transcriptional control of primary cilium formation and ciliary motility are beginning to be understood, but little is known about the transcriptional programs that control cilium number and other structural and functional specializations. One of the most intriguing ciliary specializations occurs in multiciliated cells (MCCs), which amplify their centrioles to nucleate hundreds of cilia per cell, instead of the usual monocilium. Here we report that the transcription factor MYB, which promotes S phase and drives cycling of a variety of progenitor cells, is expressed in postmitotic epithelial cells of the mouse airways and ependyma destined to become MCCs. MYB is expressed early in multiciliogenesis, as progenitors exit the cell cycle and amplify their centrioles, then switches off as MCCs mature. Conditional inactivation of Myb in the developing airways blocks or delays centriole amplification and expression of FOXJ1, a transcription factor that controls centriole docking and ciliary motility, and airways fail to become fully ciliated. We provide evidence that MYB acts in a conserved pathway downstream of Notch signaling and multicilin, a protein related to the S-phase regulator geminin, and upstream of FOXJ1. MYB can activate endogenous Foxj1 expression and stimulate a cotransfected Foxj1 reporter in heterologous cells, and it can drive the complete multiciliogenesis program in Xenopus embryonic epidermis. We conclude that MYB has an early, crucial and conserved role in multiciliogenesis, and propose that it promotes a novel S-like phase in which centriole amplification occurs uncoupled from DNA synthesis, and then drives later steps of multiciliogenesis through induction of Foxj1.

Wide Dispersion and Diversity of Clonally Related Inhibitory Interneurons

The mammalian neocortex is composed of two major neuronal cell types with distinct origins: excitatory pyramidal neurons and inhibitory interneurons, generated in dorsal and ventral progenitor zones of the embryonic telencephalon, respectively. Thus, inhibitory neurons migrate relatively long distances to reach their destination in the developing forebrain. The role of lineage in the organization and circuitry of interneurons is still not well understood. Utilizing a combination of genetics, retroviral fate mapping, and lineage-specific retroviral barcode labeling, we find that clonally related interneurons can be widely dispersed while unrelated interneurons can be closely clustered. These data suggest that migratory mechanisms related to the clustering of interneurons occur largely independent of their clonal origin.

A Dorsal SHH-Dependent Domain in the V-SVZ Produces Large Numbers of Oligodendroglial Lineage Cells in the Postnatal Brain

Summary Neural stem cells in different locations of the postnatal mouse ventricular-subventricular zone (V-SVZ) generate different subtypes of olfactory bulb (OB) interneurons. High Sonic hedgehog (SHH) signaling in the ventral V-SVZ regulates the production of specific subtypes of neurons destined for the OB. Here we found a transient territory of high SHH signaling in the dorsal V-SVZ beneath the corpus callosum (CC). Using intersectional lineage tracing in neonates to label dorsal radial glial cells (RGCs) expressing the SHH target gene Gli1, we demonstrate that this region produces many CC cells in the oligodendroglial lineage and specific subtypes of neurons in the OB. The number of oligodendroglial cells generated correlated with the levels of SHH signaling. This work identifies a dorsal domain of SHH signaling, which is an important source of oligodendroglial cells for the postnatal mammalian forebrain.

Heregulin and forskolin-induced cyclin D3 expression in Schwann cells: Role of a CCAAT promoter element and CCAAT enhancer binding protein

Heregulin, a polypeptide growth factor, and forskolin, an adenylyl cyclase activator, synergistically stimulate expression of cyclin D3 and cell division in Schwann cells. Heregulin induces expression in Schwann cells of a luciferase reporter gene linked to the cyclin D3 promoter. Forskolin markedly augments reporter expression in the presence of heregulin. Deletion analysis identified several promoter sites that contribute to high‐level reporter expression in heregulin‐ and forskolin‐treated Schwann cells. A promoter fragment that contains 103 bp of 5′‐flanking sequence produced significant reporter expression in heregulin‐ and forskolin‐stimulated cells. Deletion of a consensus CCAAT site within this promoter fragment caused a nearly complete loss of reporter expression. Similar results were obtained when CCAAT site mutations were introduced into the promoter. Heregulin and forskolin increased steady‐state levels of CCAAT/enhancer binding protein‐β (C/EBPβ) in Schwann cells. Mobility shift assays identified proteins in Schwann cell nuclear extracts that formed stable complexes with the cyclin D3 CCAAT promoter element and were disrupted by anti‐C/EBPβ antibody. Transfection of Schwann cells with C/EBPβ cDNA increased cyclin D3 reporter expression. In contrast to these results, mutation of a cAMP response element in the cyclin D3 promoter had only a modest effect on heregulin‐ and forskolin‐stimulated reporter expression. These findings demonstrate that C/EBPβ plays a key role in the heregulin and cAMP‐dependent regulation of cyclin D3 expression in Schwann cells.

Neurogenesis in the Postnatal VZ-SVZ and the Origin of Interneuron Diversity

In order to understand how the brain is assembled into functional circuits, it is essential to understand how the extraordinarily diverse cell types that comprise these circuits are generated. An attractive system for studying this process of neural specification is the adult mammalian ventricular–subventricular zone (VZ–SVZ), which continuously generates different types of neurons throughout an animals lifetime. Newborn neurons (Type A cells) are generated in the VZ–SVZ by neural stem cells (NSCs) or primary progenitors (astroglial cells known as Type B1 cells) via intermediate progenitor cells (Type C cells). These newborn neurons use chain migration to move from the VZ–SVZ through the rostral migratory stream (RMS) and into the olfactory bulb (OB), where they differentiate into at least six different types of OB interneurons. These new neurons are generated in the VZ–SVZ that covers an extensive area, including regions in the lateral, dorsal, and medial walls of the lateral ventricles, as well as the RMS itself. Recent work has revealed that Type B1 cells in different regions of the VZ–SVZ generate specific subtypes of new neurons, suggesting that there is a spatial code to postnatal OB neurogenesis. This suggests that, in vivo , NSCs form a heterogeneous population of restricted primary progenitors organized into different spatial domains, contradicting the long-held view that they are a homogeneous population of progenitors with a broader potential. Identifying the mechanisms of neuronal specification in different regions of the postnatal VZ–SVZ will likely shed light on conserved mechanisms of cell-type specification and provide insight into how adult neurogenesis affects brain function in health and disease.