Mammalian Pumilio 2 regulates dendrite morphogenesis and synaptic function.

In Drosophila, Pumilio (Pum) is important for neuronal homeostasis as well as learning and memory. We have recently characterized a mammalian homolog of Pum, Pum2, which is found in discrete RNA-containing particles in the somatodendritic compartment of polarized neurons. In this study, we investigated the role of Pum2 in developing and mature neurons by RNA interference. In immature neurons, loss of Pum2 led to enhanced dendritic outgrowth and arborization. In mature neurons, Pum2 down-regulation resulted in a significant reduction in dendritic spines and an increase in elongated dendritic filopodia. Furthermore, we observed an increase in excitatory synapse markers along dendritic shafts. Electrophysiological analysis of synaptic function of neurons lacking Pum2 revealed an increased miniature excitatory postsynaptic current frequency. We then identified two specific mRNAs coding for a known translational regulator, eIF4E, and for a voltage-gated sodium channel, Scn1a, which interacts with Pum2 in immunoprecipitations from brain lysates. Finally, we show that Pum2 regulates translation of the eIF4E mRNA. Taken together, our data reveal a previously undescribed role for Pum2 in dendrite morphogenesis, synapse function, and translational control.

A loss of function allele for murine Staufen1 leads to impairment of dendritic Staufen1-RNP delivery and dendritic spine morphogenesis.

The dsRNA-binding protein Staufen was the first RNA-binding protein proven to play a role in RNA localization in Drosophila. A mammalian homolog, Staufen1 (Stau1), has been implicated in dendritic RNA localization in neurons, translational control, and mRNA decay. However, the precise mechanisms by which it fulfills these specific roles are only partially understood. To determine its physiological functions, the murine Stau1 gene was disrupted by homologous recombination. Homozygous stau1(tm1Apa) mutant mice express a truncated Stau1 protein lacking the functional RNA-binding domain 3. The level of the truncated protein is significantly reduced. Cultured hippocampal neurons derived from stau1(tm1Apa) homozygous mice display deficits in dendritic delivery of Stau1-EYFP and beta-actin mRNA-containing ribonucleoprotein particles (RNPs). Furthermore, these neurons have a significantly reduced dendritic tree and develop fewer synapses. Homozygous stau1(tm1Apa) mutant mice are viable and show no obvious deficits in development, fertility, health, overall brain morphology, and a variety of behavioral assays, e.g., hippocampus-dependent learning. However, we did detect deficits in locomotor activity. Our data suggest that Stau1 is crucial for synapse development in vitro but not critical for normal behavioral function.

High-efficiency transfection of short hairpin RNAs-encoding plasmids into primary hippocampal neurons.

The transfection of expression constructs encoding a variety of transgenes is a widely used method to study gene function in cultured cells. Especially when the efficiency of the knock-down of target proteins via small interfering RNAs (siRNAs) is to be determined by quantitative Western blotting, large proportions of untransfected cells compromise the analysis. Achieving high transfection efficiencies in postmitotic cells, such as neurons, poses a particular problem in that these cells cannot be selected for the expression of the transgene following transfection. It is therefore important to develop protocols that allow for the highly efficient transfection of these cells. In the present study, we identify three important parameters that prove especially useful for chronically difficult to transfect short hairpin RNA (shRNA)-encoding plasmids: the amount and quality of the plasmid DNA used and the use of new nucleofection programs. Combining those changes increases the rate of transfected cells from less than 5% to up to approximately 80%. Importantly, these high transfection efficiencies can be obtained while maintaining good cell viability and normal cellular development. Taken together, these improvements allow for a detailed biochemical and phenotypical analysis of neurons that have been nucleoporated with a wide variety of shRNAs.