MicroRNAs (miRNAs) are evolutionarily conserved, 18-25 nucleotide non-protein coding transcripts that play an important function in post-transcriptional regulation of gene expression during development (1-4). However, the significance of miRNAs in postmitotic cells, such as neurons in the mammalian CNS, is less well characterized. Here we investigate the role of miRNAs in the terminal differentiation, function, and survival of mammalian midbrain dopaminergic neurons (DNs). We identify a miRNA, miR-133b, that is specifically expressed in midbrain DNs and deficient in Parkinson's disease midbrain tissue that has lost midbrain DNs. MiR-133b regulates the maturation and function of midbrain DNs within a negative feedback circuit that includes the paired-like homeodomain transcription factor Pitx3. KeywordsmiRNA; miR-133b; midbrain dopamine neurons; Parkinson's disease; Dicer miRNAs are derived from long primary transcripts through sequential processing by the Drosha ribonuclease (5) and the Dicer enzyme (1,6). In the context of an RNA-induced silencing complex (RISC), miRNAs guide the cleavage of target mRNAs and/or inhibit their translation (2). miRNAs were first characterized in invertebrates, where they function to regulate developmental cell fate decisions in the nervous system (7,8) and elsewhere (9).Midbrain dopamine neurons (DNs) play a central role in complex behaviors such as reward and addiction, and these cells are lost in Parkinson's disease. Furthermore, a number of transcription factors have been identified that regulate midbrain DN development, function, and survival (16). However, the role of post-transcriptional mechanisms in these processes is uncharacterized. We sought to establish a role for miRNAs in mammalian dopamine neuron differentiation, function, and survival. To facilitate a kinetic analysis, we first used an in vitro model system: the differentiation of murine ES cells into DNs (17,18). An ES cell line was obtained that expresses Dicer enzyme containing LoxP recombinase sites that flank both chromosomal copies of the Dicer gene (floxed Dicer alleles)(19). Introduction of Cre recombinase into these cells by lentiviral transduction leads to the deletion of Dicer in nearly 100% of cells (Supplementary Figure 1A). Floxed Dicer ES cultures were differentiated to a midbrain DN phenotype using the embryoid body protocol (EB; Supplementary Figure 1B) (18). Briefly, cells were initially grown in non-adherent conditions in the context of defined media, including growth factors, to generate neuronal precursors (stage 2); subsequently, neuronal precursors were expanded in the presence of basic fibroblast growth factor (bFGF; stages 3 and 4); and finally, the bFGF was withdrawn to obtain mature DNs (stage 5), which constitute 10-25% of the cells in these cultures (18). Cre-mediated deletion of the floxed Dicer alleles at stage 4, when postmitotic dopamine neurons first arise, led to a nearly complete loss of dopamine neuron accumulation at stage 5, as quantified by the expression of markers i...

Phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) has an important function in cell regulation both as a precursor of second messenger molecules and by means of its direct interactions with cytosolic and membrane proteins. Biochemical studies have suggested a role for PtdIns(4,5)P2 in clathrin coat dynamics, and defects in its dephosphorylation at the synapse produce an accumulation of coated endocytic intermediates. However, the involvement of PtdIns(4,5)P2 in synaptic vesicle exocytosis remains unclear. Here, we show that decreased levels of PtdIns(4,5)P2 in the brain and an impairment of its depolarization-dependent synthesis in nerve terminals lead to early postnatal lethality and synaptic defects in mice. These include decreased frequency of miniature currents, enhanced synaptic depression, a smaller readily releasable pool of vesicles, delayed endocytosis and slower recycling kinetics. Our results demonstrate a critical role for PtdIns(4,5)P2 synthesis in the regulation of multiple steps of the synaptic vesicle cycle.

Synaptic dysfunction caused by oligomeric assemblies of amyloid-β peptide (Aβ) has been linked to cognitive deficits in Alzheimer's disease. Here we found that incubation of primary cortical neurons with oligomeric Aβ decreases the level of phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P 2 ), a phospholipid that regulates key aspects of neuronal function. The destabilizing effect of Aβ on PtdIns (4,5)P 2 metabolism was Ca 2+ -dependent and was not observed in neurons that were derived from mice that are haploinsufficient for Synj1. This gene encodes synaptojanin 1, the main PtdIns(4,5) P 2 phosphatase in the brain and at the synapses. We also found that the inhibitory effect of Aβ on hippocampal long-term potentiation was strongly suppressed in slices from Synj1 +/− mice, suggesting that Aβ-induced synaptic dysfunction can be ameliorated by treatments that maintain the normal PtdIns(4,5)P 2 balance in the brain.

SUMMARY Phosphatidylinositol-4,5-bisphosphate [PI(4,5)P2] plays a fundamental role in clathrin-mediated endocytosis. However, precisely how PI(4,5)P2 metabolism is spatially and temporally regulated during membrane internalization and the functional consequences of endocytosis-coupled PI(4,5)P2 dephosphorylation remain to be explored. Using cell free assays with liposomes of varying curvatures, we show that the major synaptic phosphoinositide phosphatase, synaptojanin 1 (Synj1), acts with membrane curvature generators/sensors, such as the BAR protein endophilin, to preferentially remove PI(4,5)P2 from curved membranes as opposed to relatively flat ones. Moreover, in vivo recruitment of Synj1’s inositol 5-phosphatase domain to endophilin-induced membrane tubules results in fragmentation and condensation of these structures largely in a dynamin-dependent fashion. Our study raises the possibility that geometry-based mechanisms may contribute to spatially restricting PI(4,5)P2 elimination during membrane internalization and suggests that the PI(4,5)P2-to-PI4P conversion achieved by Synj1 at sites of high curvature may cooperate with dynamin to achieve membrane fission.

Phosphatidylinositol-4,5-bisphosphate [PtdIns(4,5)P2] is a signaling phospholipid implicated in a wide variety of cellular functions. At synapses, where normal PtdIns(4,5)P 2 balance is required for proper neurotransmission, the phosphoinositide phosphatase synaptojanin 1 is a key regulator of its metabolism. The underlying gene, SYNJ1, maps to human chromosome 21 and is thus a candidate for involvement in Down's syndrome (DS), a complex disorder resulting from the overexpression of trisomic genes. Here, we show that PtdIns(4,5)P2 metabolism is altered in the brain of Ts65Dn mice, the most commonly used model of DS. This defect is rescued by restoring Synj1 to disomy in Ts65Dn mice and is recapitulated in transgenic mice overexpressing Synj1 from BAC constructs. These transgenic mice also exhibit deficits in performance of the Morris water maze task, suggesting that PtdIns(4,5)P 2 dyshomeostasis caused by gene dosage imbalance for Synj1 may contribute to brain dysfunction and cognitive disabilities in DS.Alzheimer's disease ͉ inositol 5-phosphatase ͉ phosphatidylinositol-4,5-bisphosphate ͉ phosphatidylinositol phosphate kinase ͉ synapse

Phosphatidylinositol 4,5-bisphosphate (PIP2) is an important cellular effector whose functions include the regulation of ion channels and membrane trafficking. Aberrant PIP 2 metabolism has also been implicated in a variety of human disease states, e.g., cancer and diabetes. Here we report that familial Alzheimer's disease (FAD)-associated presenilin mutations cause an imbalance in PIP 2 metabolism. We find that the transient receptor potential melastatin 7 (TRPM7)-associated Mg 2؉ -inhibited cation (MIC) channel underlies ion channel dysfunction in presenilin FAD mutant cells, and the observed channel deficits are restored by the addition of PIP 2, a known regulator of the MIC/TRPM7 channel. Lipid analyses show that PIP 2 turnover is selectively affected in FAD mutant presenilin cells. We also find that modulation of cellular PIP 2 closely correlates with 42-residue amyloid ␤-peptide (A␤42) levels. Our data suggest that PIP 2 imbalance may contribute to Alzheimer's disease pathogenesis by affecting multiple cellular pathways, such as the generation of toxic A␤42 as well as the activity of the MIC/TRPM7 channel, which has been linked to other neurodegenerative conditions. Thus, our study suggests that brain-specific modulation of PIP 2 may offer a therapeutic approach in Alzheimer's disease.␤-amyloid precursor protein ͉ channel ͉ secretase ͉ transient receptor potential melastatin 7 (TRPM7) ͉ capacitative calcium entry

The interaction of talin with phosphatidylinositol(4) phosphate 5 kinase type Iγ (PIPKIγ) regulates PI(4,5)P2 synthesis at synapses and at focal adhesions. Here, we show that phosphorylation of serine 650 (S650) within the talin-binding sequence of human PIPKIγ blocks this interaction. At synapses, S650 is phosphorylated by p35/Cdk5 and mitogen-activated protein kinase at rest, and dephosphorylated by calcineurin upon stimulation. S650 is also a substrate for cyclin B1/Cdk1 and its phosphorylation in mitosis correlates with focal adhesion disassembly. Phosphorylation by Src of the tyrosine adjacent to S650 (Y649 in human PIPKIγ) was shown to enhance PIPKIγ targeting to focal adhesions (Ling, K., R.L. Doughman, V.V. Iyer, A.J. Firestone, S.F. Bairstow, D.F. Mosher, M.D. Schaller, and R.A. Anderson. 2003. J. Cell Biol. 163:1339–1349). We find that Y649 phosphorylation does not stimulate directly PIPKIγ binding to talin, but may do so indirectly by inhibiting S650 phosphorylation. Conversely, S650 phosphorylation inhibits Y649 phosphorylation by Src. The opposite effects of the phosphorylation of Y649 and S650 likely play a critical role in regulating synaptic function as well as the balance between cell adhesion and cell motility.

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