Target of rapamycin (TOR) signaling is a regulator of cell growth. TOR activity can also enhance cell death, and the TOR inhibitor rapamycin protects cells against proapoptotic stimuli. Autophagy, which can protect against cell death, is negatively regulated by TOR, and disruption of autophagy by mutation of Atg5 or Atg7 can lead to neurodegeneration. However, the implied functional connection between TOR signaling, autophagy, and cell death or degeneration has not been rigorously tested. Using the Drosophila melanogaster visual system, we show in this study that hyperactivation of TOR leads to photoreceptor cell death in an age- and light-dependent manner and that this is because of TOR's ability to suppress autophagy. We also find that genetically inhibiting TOR or inducing autophagy suppresses cell death in Drosophila models of Huntington's disease and phospholipase C (norpA)–mediated retinal degeneration. Thus, our data indicate that TOR induces cell death by suppressing autophagy and provide direct genetic evidence that autophagy alleviates cell death in several common types of neurodegenerative disease.

Rett syndrome (RTT) is a severe neurodevelopmental disorder caused by mutations in the X chromosomal gene Methyl-CpG-binding Protein 2 (MECP2) (1). RTT treatment so far is symptomatic. Mecp2 disruption in mice phenocopies major features of the syndrome (2) that can be reversed upon re-expression of Mecp2 (3. It has recently been reported that transplantation of wild type (WT) bone marrow (BMT) into lethally irradiated Mecp2tm1.1Jae/y mice prevented neurologic decline and early death by restoring microglial phagocytic activity against apoptotic targets (4). Based on this report, clinical trials of BMT for patients with RTT have been initiated (5). We aimed to replicate and extend the BMT experiments in three different RTT mouse models but found that despite robust microglial engraftment, BMT from WT donors did not rescue early death or ameliorate neurologic deficits. Furthermore, early and specific genetic expression of Mecp2 in microglia did not rescue Mecp2-deficient mice. In conclusion our experiments do not support BMT as therapy for RTT.

Rett syndrome (RS) is a debilitating neurological disorder affecting mostly girls with heterozygous mutations in the gene encoding the methyl-CpG-binding protein MeCP2 on the X chromosome. Because restoration of MeCP2 expression in a mouse model reverses neurologic deficits in adult animals, reactivation of the wild-type copy of MeCP2 on the inactive X chromosome (Xi) presents a therapeutic opportunity in RS. To identify genes involved in MeCP2 silencing, we screened a library of 60,000 shRNAs using a cell line with a MeCP2 reporter on the Xi and found 30 genes clustered in seven functional groups. More than half encoded proteins with known enzymatic activity, and six were members of the bone morphogenetic protein (BMP)/TGF-β pathway. shRNAs directed against each of these six genes down-regulated X-inactive specific transcript (XIST), a key player in X-chromosome inactivation that encodes an RNA that coats the silent X chromosome, and modulation of regulators of this pathway both in cell culture and in mice demonstrated robust regulation of XIST. Moreover, we show that Rnf12, an X-encoded ubiquitin ligase important for initiation of X-chromosome inactivation and XIST transcription in ES cells, also plays a role in maintenance of the inactive state through regulation of BMP/TGF-β signaling. Our results identify pharmacologically suitable targets for reactivation of MeCP2 on the Xi and a genetic circuitry that maintains XIST expression and X-chromosome inactivation in differentiated cells.XIST | X inactivation | MeCP2 | Rett syndrome | BMP/TGF-β