Stephen B. Daniels

The aging neurogenic subventricular zone.

In the adult mouse brain, the subventricular zone (SVZ) is a neurogenic stem cell niche only 4–5 cell diameters thick. Within this narrow zone, a unique microenvironment supports stem cell self‐renewal, gliogenesis or neurogenesis lineage decisions and tangential migration of newly generated neurons out of the SVZ and into the olfactory bulb. However, with aging, SVZ neurogenesis declines. Here, we examine the dynamic interplay between SVZ cytoarchitecture and neurogenesis through aging. Assembly of high‐resolution electron microscopy images of corresponding coronal sections from 2‐, 10‐ and 22‐month‐old mice into photomontages reveal a thinning of the SVZ with age. Following a 2‐h BrdU pulse, we detect a significant decrease in cell proliferation from 2 to 22 months. Neuroblast numbers decrease with age, as do transitory amplifying progenitor cells, while both SVZ astrocytes and adjacent ependymal cells remain relatively constant. At 22 months, only residual pockets of neurogenesis remain and neuroblasts become restricted to the anterior dorsolateral horn of the SVZ. Within this dorsolateral zone many key components of the younger neurogenic niche are maintained; however, in the aged SVZ, increased numbers of SVZ astrocytes are found interposed within the ependyma. These astrocytes co‐label with markers to ependymal cells and astrocytes, form intercellular adherens junctions with neighboring ependymal cells, and some possess multiple basal bodies of cilia within their cytoplasm. Together, these data reveal an age‐related, progressive restriction of SVZ neurogenesis to the dorsolateral aspect of the lateral ventricle with increased numbers of SVZ astrocytes interpolated within the ependyma.

Noncontact stimulation from estrous females evokes penile erection in rats

Five experiments demonstrated that noncontact stimulation from estrous females evokes penile erection in a high proportion of sexually experienced male rats. In Experiment 1, 23 of 24 males (96%) displayed erections while separated from estrous females by a wire-mesh barrier, compared with 8% when no female was present. In Experiment 2, inaccessible estrous females stimulated erection in 100% of males, whereas only 38% responded to inaccessible unfamiliar males and 0% to inaccessible preferred food or an empty cage (n = 8/group). These data suggest that nonsexual arousing stimuli do not readily evoke erections. Experiments 3 and 4 demonstrated that bedding collected from estrous females is highly attractive to males, but is ineffective in promoting erections even when the males can burrow in the bedding. Therefore, estrous odors alone are apparently insufficient to stimulate erection. In Experiment 5, the percentage of males (n = 18) responding with erection did not vary significantly as a function of their exposure to ovariectomized females (67%), receptive but nonproceptive females (83%), or proceptive females (89%), but these stimuli were progressively more effective in reducing erection latency and increasing the number of erections displayed, suggesting that behavioral cues emitted by females promote erection. The display of erection by rats under the conditions used in these studies satisfies conventional criteria for recognition as psychogenic erections, which we have provisionally defined as erections that occur without concurrent somesthetic stimulation. The availability of a rodent model of psychogenic erection should foster analysis of its physiological mediation.

Densovirus associated with sea-star wasting disease and mass mortality

Significance Sea stars inhabiting the Northeast Pacific Coast have recently experienced an extensive outbreak of wasting disease, leading to their degradation and disappearance from many coastal areas. In this paper, we present evidence that the cause of the disease is transmissible from disease-affected animals to apparently healthy individuals, that the disease-causing agent is a virus-sized microorganism, and that the best candidate viral taxon, the sea star-associated densovirus (SSaDV), is in greater abundance in diseased than in healthy sea stars. Populations of at least 20 asteroid species on the Northeast Pacific Coast have recently experienced an extensive outbreak of sea-star (asteroid) wasting disease (SSWD). The disease leads to behavioral changes, lesions, loss of turgor, limb autotomy, and death characterized by rapid degradation (“melting”). Here, we present evidence from experimental challenge studies and field observations that link the mass mortalities to a densovirus (Parvoviridae). Virus-sized material (i.e., <0.2 μm) from symptomatic tissues that was inoculated into asymptomatic asteroids consistently resulted in SSWD signs whereas animals receiving heat-killed (i.e., control) virus-sized inoculum remained asymptomatic. Viral metagenomic investigations revealed the sea star-associated densovirus (SSaDV) as the most likely candidate virus associated with tissues from symptomatic asteroids. Quantification of SSaDV during transmission trials indicated that progression of SSWD paralleled increased SSaDV load. In field surveys, SSaDV loads were more abundant in symptomatic than in asymptomatic asteroids. SSaDV could be detected in plankton, sediments and in nonasteroid echinoderms, providing a possible mechanism for viral spread. SSaDV was detected in museum specimens of asteroids from 1942, suggesting that it has been present on the North American Pacific Coast for at least 72 y. SSaDV is therefore the most promising candidate disease agent responsible for asteroid mass mortality.

NG2 Glial Cells Provide a Favorable Substrate for Growing Axons

NG2 cells (polydendrocytes) comprise an abundant glial population that is widely and uniformly distributed throughout the developing and mature CNS and are identified by the expression of the NG2 proteoglycan at the cell surface. Although recent electrophysiological studies suggest that they are capable of receiving signals from axon terminals, other studies, based on the finding that the NG2 molecule itself induces growth cone collapse, have led to a widely held speculation that NG2 cells themselves also repel and inhibit growing axons. In this study, we have examined the effects of rat NG2 cells on growing hippocampal and neocortical axons in vitro and in vivo. NG2 cells did not repel growing axons but promoted their growth in vitro, and axonal growth cones formed extensive contacts with NG2 cells both in vitro and in the developing corpus callosum. Punctate immunoreactivity for fibronectin and laminin was found to be colocalized with NG2 on the surface of NG2 cells. Altering the level of cell surface NG2 expression had no effect on the growth-promoting effects of NG2 cells on growing axons. Thus, our study indicates that NG2 cells are not inhibitory to growing axons but provide an adhesive substrate for axonal growth cones and promote their growth even in the presence of elevated levels of the NG2 proteoglycan. These findings suggest a novel role for NG2 cells in facilitating axonal growth during development and regeneration.

Subventricular Zone-Mediated Ependyma Repair in the Adult Mammalian Brain

The subventricular zone (SVZ) of the adult mouse brain is a narrow stem cell niche that lies along the length of the lateral wall of the lateral ventricles. The SVZ supports neurogenesis throughout adulthood; however, with increasing age, the ventral SVZ deteriorates and only the dorsolateral SVZ remains neurogenic. Associated with the elderly dorsolateral SVZ, we reported previously an increased number of astrocytes interposed within the adjacent ependymal lining. Here, we show that astrocytes integrated within the ependyma are dividing, BrdU-labeled astrocytes that share cellular adherens with neighboring ependymal cells. By tracking BrdU-labeled astrocytes over time, we observed that, as they incorporated within the ependyma, they took on antigenic and morphologic characteristics of ependymal cells, suggesting a novel form of SVZ-supported “regenerative” repair in the aging brain. A similar form of SVZ-mediated ependyma repair was also observed in young mice after mild ependymal cell denudation with low dosages of neuraminidase. Together, this work identifies a novel non-neuronal mechanism of regenerative repair by the adult SVZ.

The distribution of P element sequences in Drosophila: the willistoni and saltans species groups

SummaryThis report describes the distribution of P-element sequences among members of the closely relatedwillistoni andsaltans species groups of the subgenusSophophora. Gel-blotting analyses showed that many, but not all, species from each of these groups possess sequences with homology to the P transposable element ofDrosophila melanogaster, a sophophoran species belonging to themelanogaster species group. Furthermore, P-homologous fragments are present in lower numbers inwillistoni- andsaltans-group species than inD. melanogaster P strains, and, in some species of those two groups, exhibit species-characteristic hybridization patterns. On the basis of these results, it is proposed that P elements have had a long evolutionary history in thewillistoni andsaltans lineages.

Synaptic and extrasynaptic GABAA receptor and gephyrin clusters.

Publisher Summary In the brain, most interneurons are GABAergic. Glycinergic neurons are mainly localized in the spinal cord. Apart from the neurotransmittermediated signaling that occurs between interneurons and between interneurons and principal neurons, interneurons can be electrically coupled to each other via gap junctions. Powerful tools are being used to reveal the molecular organization of excitatory and inhibitory synapses and the various signaling pathways involved in these synapses. Recent developments in genomics and proteomics, gene knockout animal models, protein-protein interaction assays (such as yeast two-hybrid), and the production and application of novel specific antibodies in combination with immunofluorescence microscopy (confocal and two-photon) and electron microscopy Immunogold techniques are advancing at a very rapid pace our knowledge of the organization of the molecular machinery of chemical synapses.

Midbrain Dopamine Neurons Associated with Reward Processing Innervate the Neurogenic Subventricular Zone

Coordinated regulation of the adult neurogenic subventricular zone (SVZ) is accomplished by a myriad of intrinsic and extrinsic factors. The neurotransmitter dopamine is one regulatory molecule implicated in SVZ function. Nigrostriatal and ventral tegmental area (VTA) midbrain dopamine neurons innervate regions adjacent to the SVZ, and dopamine synapses are found on SVZ cells. Cell division within the SVZ is decreased in humans with Parkinsons disease and in animal models of Parkinsons disease following exposure to toxins that selectively remove nigrostriatal neurons, suggesting that dopamine is critical for SVZ function and nigrostriatal neurons are the main suppliers of SVZ dopamine. However, when we examined the aphakia mouse, which is deficient in nigrostriatal neurons, we found no detrimental effect to SVZ proliferation or organization. Instead, dopamine innervation of the SVZ tracked to neurons at the ventrolateral boundary of the VTA. This same dopaminergic neuron population also innervated the SVZ of control mice. Characterization of these neurons revealed expression of proteins indicative of VTA neurons. Furthermore, exposure to the neurotoxin MPTP depleted neurons in the ventrolateral VTA and resulted in decreased SVZ proliferation. Together, these results reveal that dopamine signaling in the SVZ originates from a population of midbrain neurons more typically associated with motivational and reward processing.

Neuroblast Protuberances in the Subventricular Zone of the Regenerative MRL/MpJ Mouse

The MRL mouse is unique in its capacity for regenerative healing of wounds. This regenerative ability includes complete closure, with little scarring, of wounds to the ear pinna and repair of cardiac muscle, without fibrosis, following cryoinjury. Here, we examine whether neurogenic zones within the MRL brain show enhanced regenerative capacity. The largest neurogenic zone in the adult brain, the subventricular zone (SVZ), lies adjacent to the lateral wall of the lateral ventricle and is responsible for replacement of interneuron populations within the olfactory bulb. Initial gross observation of the anterior forebrain in MRL mice revealed enlarged lateral ventricles; however, little neurodegeneration was detected within the SVZ or surrounding tissues. Instead, increased proliferation within the SVZ was observed, based on incorporation of the thymidine analogue bromodeoxyuridine. Closer examination using electron microscopy revealed that a significant number of SVZ astrocytes interpolated within the ependyma and established contact with the ventricle. In addition, subependymal, protuberant nests of cells, consisting primarily of neuroblasts, were found along the anterior SVZ of MRL mice. Whole mounts of the lateral wall of the lateral ventricle stained for the neuroblast marker doublecortin revealed normal formation of chains of migratory neuroblasts along the entire wall and introduction of enhanced green fluorescent protein‐tagged retrovirus into the lateral ventricles confirmed that newly generated neuroblasts were able to track into the olfactory bulb. J. Comp. Neurol. 498:747–761, 2006.

Molecular analysis of P element behavior in Drosophila simulans transformants.

SummaryIn this report we describe the successful transformation of Drosophila simulans with an autonomous P element from Drosophila melanogaster without the use of a selectable marker. This result demonstrates that there is no species barrier for P element transposition. Utilizing gel blotting and in situ hybridization techniques, we have monitored the behavior of newly-introduced P elements in several D. simulans transformed lines over twelve generations. In most instances, an overall increase in the number of P elements was observed. An examination of the frequency of P-element-bearing individuals in one line revealed the rapid spread of P elements through the population. Analysis of well-characterized sublines confirmed that P elements increase in number by transposition to new genomic sites. The formation of degenerate elements occurred in at least one case. These observations suggest that P elements may behave similarly in D. melanogaster and D. simulans.