The Society for Neuroscience (SfN) held its annual meeting in Chicago in October 17-21, 2009. The meeting attracted over 30,000 researchers worldwide to present and discuss the latest in neuroscience research. Talks and poster presentations were categorized into eight themes, which included Disorders of the Nervous System.
The ALS Forum previously presented several ALS relevant stories that emerged at the meeting: Chicago: New Technologies Help Drugs Cross Blood-Brain Barrier, Chicago: Tau and a-Synuclein Oligomers Follow Aβ Footsteps, Chicago: Axonal Transport Not So Fast in Neurodegenerative Disease. Here we expand our coverage of two other themes relevant to the ALS community: 1) the therapeutic potential of histone deacetylase (HDAC) regulation in ALS, and 2) newly generated TDP-43 animal models of disease.
HDACs: Therapeutic Targets for ALS
Several groups presented their studies reporting the neuroprotective effects of inhibition of HDACs. Maurizio Grimaldi and colleagues at the Southern Research Institute showed that the non-specific HDAC inhibitor, Lixte 2-1, protected rat cortical neurons challenged by environmental stressors. Lixte 2-1, which acts on both Class I and II HDACs, was also shown to promote neurite outgrowth in a cortical cell culture model. CLICK HERE FOR ABSTRACT:
Meanwhile, Mark Rivieccio, Brett Langley, and colleagues at the Burke/Cornell Medical Research Institute focused their attention on HDAC6, a microtubule-associated deacetylase with non-chromatin substrates, including tubulin. They utilized pharmacological compounds and siRNA to inhibit HDAC6 and showed that selective inhibition of HDAC6 conferred neuroprotection against oxidative stress in rat primary cortical neurons. CLICK HERE FOR ABSTRACT
Further studies presented by Kathryn McLaughlin from the Langley lab delved deeper to investigate a potential mechanism for HDAC6 mediated protection and explored the relationship between HDAC6, the chaperone protein Hsp90, and the MAPK pathway. While Hsp90 had been previously identified as a HDAC6 substrate, McLaughlin identified Lysine 294 in Hsp90 as the target of HDAC6 mediated deacetylation. Furthermore, selective inhibition of HDAC6 resulted in accumulation of acetylated Hsp90 and decreased interaction with one of its key target proteins, Raf. Disruption of the Hsp90-Raf interaction prevented the activation of cell death by Raf-mediated MAPK signals, which was thought to confer the observed neuroprotective effects. This somewhat counter-intuitive finding that deacetylated Hsp90 promotes a pathway leading to cell death appears contrary to the conventional view that chaperones confer neuroprotection in neurodegenerative diseases. However, as this work indicates, the acetylation status and nature of the client protein determine whether Hsp90 functions to promote or inhibit cell viability.CLICK HERE FOR ABSTRACT
An intriguing connection between HDAC6 and TDP-43, a gene associated with familial ALS, was presented in a seminar by Fabienne Fiesel and subsequently published in the J. EMBO [2009 Nov 12 issue]. Fiesel is a researcher in the laboratory of Philipp Kahle at the Hertie Institute for Clinical Brain Research, University Clinics Tuebingen in Germany. Unlike the studies discussed above, these researchers report that HDAC6 inhibition correlated with cytotoxicity, rather than the neuroprotective effects that others had described.
TDP-43 encodes a RNA binding protein with several proposed functions, including regulation of RNA metabolism. Fiesel and colleagues profiled gene expression changes in HEK293E cells, in which they had down-regulated TDP-43 using siRNA, to better understand the mechanism of TDP-43 associated pathologies. While several genes associated with ALS/FTD were not found to be significantly altered, they did find a two-fold down regulation of HDAC6 expression levels. They subsequently showed that TDP-43 binds directly to HDAC6 mRNA and regulates HDAC6 at both the RNA and protein levels in HEK293E and neuronal SH-SY5Y cells. The in vivo significance of this interaction was confirmed in Drosophila TDP43 mutant flies. While deletion of TDP43 in flies resulted in embryonic lethality, analysis of heterozygous TDP+/- adult flies showed a decrease in TDP43 expression and a corresponding decrease in HDAC6 protein levels. Moreover, analysis of embryonic TDP43 -/- tissue also showed diminished HDAC6 mRNA expression.
In light of previous studies implicating HDAC6 in aggresome formation, Fiesel and colleagues also examined aggresome formation in TDP-43 silenced cells that also expressed the aggregation prone polyQ-expanded Atx3 protein. They found that these cells contained fewer aggresomes. Importantly, they also noticed that impaired aggresome formation was associated with increased cell death. These findings were surprising given that aggregation of misfolded protein has historically been implicated to play a causative role in ALS. However, recent studies have suggested that formation of aggresomes may serve a neuroprotective function by sequestering toxic misfolded proteins for subsequent clearance by autophagy. This idea is supported by the findings of Fiesel and colleagues and suggests that inhibiting HDAC6 may be cytotoxic in cells that are accumulating misfolded proteins. CLICK HERE FOR ABSTRACT
TDP-43 animal models
TDP-43 animal models are critical investigational tools to understand disease pathology and develop therapeutics that either prevent and/or intervene in disease progression. Following multiple failed attempts to generate transgenic mice (see also ARF related news story), three groups presented new mouse and rat TDP-43 models at the SfN meeting. Only days prior to the start of SfN, the first published report of a TDP-43 mouse model was posted on-line in PNAS. This model generated by Robert Baloh and colleagues at Washington University in St. Louis is discussed in the ALS Forum article here.
Virginia Lee and colleagues at the University of Pennsylvania reported generation of a conditional mouse model that expresses a cytoplasmically localized form of TDP-43 harboring a mutation in its nuclear localization signal (TDP-43-NLS). Using a tetracycline inducible system and CaMKIIa promoter, expression of TDP-43-NLS was temporally and spatially restricted to the forebrain where it caused degeneration of neurons. Closer examination showed pathologies characteristically seen in ALS diseased tissues, including neuronal loss, astrocytic activation and inclusions. They compared the phenotypes of mice from three founder lines and noticed similar pathological alterations, but noted that each line exhibited different kinetics of disease progression. CLICK HERE FOR ABSTRACT
Jeffrey Elliott and colleagues from UT Southwestern Medical Center generated three transgenic mouse lines in which the human prion promoter was used to drive expression of either the wildtype or mutant (A315T or M337V) forms of human TDP-43. Initial observations of the founder mice indicated that all three lines developed severe motor phenotypes. These lines are currently being characterized. CLICK HERE FOR ABSTRACT
Xugang Xia and colleagues at Thomas Jefferson University have generated two transgenic rat models that allow tetracycline-inducible expression of either wildtype hTDP-43 or a mutated hTDP-43 (hTDP43-M337V). They observed that constitutive expression of mutant hTDP-43, but not wildtype hTDP-43, caused early death in transgenic founder rats. However, conditional expression of mutant hTDP-43-M337V in postnatal rats caused progressive paralysis with core pathologies associated with ALS. At disease onset, no motor neuron loss was observed and axon terminals appeared to be the primary target of degeneration. At endstage, some motor neuron loss as well as glial cell activation was observed in several brain regions including the cortex, cerebellum and hippocampus. Importantly, Xia’s group showed that turning off expression of the mutant transgene after initiation of disease was able to prevent disease progression. Over time, these rats exhibited improved motor function. These results indicate that suppressing TDP-43 expression may be an effective therapeutic approach for TDP-43 associated FALS. CLICK HERE FOR ABSTRACT
-Sheila Menzies, PhD, Scientific Program Officer at Prize4Life