This Week in The Journal

Cellular/Molecular

Short Hairpin RNAs Off-Target

Veronica A. Alvarez, Dennis A. Ridenour, and Bernardo L. Sabatini

Even for scientists it often takes a while to appreciate that the latest and greatest new technique has its limitations. In this week’s Journal, Alvarez et al. provide an example in the case of gene silencing with short hairpin RNA (shRNA) sequences. In rat organotypic hippocampal slices, the authors expressed an shRNA sequence with homology to luciferase (shLUCI), a gene not present in the mammalian genome. shLUCI expression substantially reduced dendritic branch complexity, spine density, and miniature synaptic currents, consistent with a loss of synaptic number. These effects required the double-stranded RNA-dependent protein kinase that activates interferon target genes. The authors also transfected cells with two forms of shRNA against a chromatin-regulating protein, one known to activate the interferon-like response, the other not. The one with the off-target interferon-like response also reduced dendritic complexity confirming an off-target, and thus nonspecific, action on dendritic and synaptic structures.

Development/Plasticity/Repair

Making Oligodendrocytes in the SVZ

Bénédicte Menn, Jose Manuel Garcia-Verdugo, Cynthia Yaschine, Oscar Gonzalez-Perez, David Rowitch, and Arturo Alvarez-Buylla

(see pages 7907–7918)

Type B precursor cells in the adult subventricular zone (SVZ) express glial fibrillary acidic protein (GFAP) and are known to produce new neurons as well as astrocytes. Oligodendrocyte progenitor cells (OPCs), characterized by NG2 chrondrotin sulfate expression, are rather widely distributed in cortex and can generate mature oligodendrocytes. However, this week, Menn et al. show that type B cells in the SVZ can also produce limited numbers of oligodendrocytes in vivo. The authors used several approaches to identify cells. For example, using electron microscropy, the authors characterized the subset of cells in the SVZ that expressed Olig2, a transcription factor and marker of OPCs. About one-half looked like transit-amplifying type C cells, whereas the other half resembled astrocytes. After a demyelinating lesion, a much higher proportion of oligodendrocytes were derived from type B cells. Thus SVZ astrocytes, in addition to the parenchymal OPC progenitors, may participate in myelin repair.

Behavioral/Systems/Cognitive

A Touching Story from the Leech

Eric E. Thomson and William B. Kristan

(see pages 8009–8016)

Sensory information is encoded in the spiking pattern of sensory neurons, but how, or if, this temporal information is actually decoded in the process of sensory discrimination is not obvious. This week, Thomson and Kristan recorded from mechanosensory neurons in the medicinal leech while applying direct mechanical stimuli. A touch near the ventral midline evoked the local bend response, in which the animal bends away from the stimulus and activates two ventral pressure (P) cells. The difference between spike counts in the two cells encoded stimulus location to some degree, whereas relative latency better indicated location. The authors next asked whether leeches decoded these firing characteristics to determine a behavioral response. They stimulated P cells with arbitrary spike trains and determined the bend response. For decoding, the profile was reversed from encoding. Spike count difference was the major determinant of behavior, whereas latency difference had a smaller effect.

Neurobiology of Disease

A Case of Rescue by Neural Stem Cells

Jianxue Li, Jaime Imitola, Evan Y. Snyder, Richard L. Sidman

(see pages 7839–7848)

Li et al. show this week that the mere presence of neural stem cells (NSCs) may in some cases rescue ailing neurons rather than just replacing them. In nervous (nr) mice, the majority of Purkinje neurons (PNs) degenerate between postnatal day 21 and 35. This is thought to be because of mitochondrial damage related to overexpression of tissue plasminogen activator (tPA). In untreated nr adult mice, PNs were severely reduced. NSC implants rescued the number of PNs and improved motor performance. Surprisingly however, the survivors were host cells rather than newly differentiated NSCs. PNs were also rescued by NSCs in organotypic slice cultures. Direct contact between transplanted NSCs and host cells was required for survival and for normal mitochondrial morphology. Expression of tPA was normalized in nr mice with transplanted NSCs, probably through transcription factor regulation. ⇓

The reduction in PNs in nr mice (wild-type, right panel; nr mice, middle panel) was rescued by NSC (right panel). The decrease in PN dendrites is most apparent in the molecular layer (ML). See the article by Li et al. for details.