At the Society for Neuroscience Annual Meeting held November 15-19 in Washington, DC, over 30,000 neuroscientists convened to discuss cutting-edge research on development, genetics, molecular mechanisms, tools and therapeutics in all areas of neuroscience. In this report, we bring you highlights of the nanosymposium on “Motor Neuron Disease Mechanisms and Models”, where researchers gathered to share their latest findings about mechanisms and models of ALS and other neuromuscular diseases.
Ke Zhang from the laboratory of Thomas Lloyd at Johns Hopkins University in Baltimore, Maryland presented recent findings on the role of RanGAP and nuclear transport defects in the pathogenesis of ALS, focusing on pathogenesis of C9ORF72 mutations. Hexanucleotide repeat expansions (HRE) in the C9ORF72 gene account for approximately 40% of familial ALS cases and 25% of familial frontotemporal dementia (FTD), but the precise mechanism of HRE toxicity is still a matter of debate (see Dec 2014 news story; Nov 2014 news story). Due to accumulating evidence for a toxic gain-of-function cause of HRE toxicity, the researchers set out to identify genetic supporessors of C9ORF72 toxicity. Interestingly, in a genetic screen performed in a fly model of C9ORF72-ALS (Xu et. al., 2013), genes that increased nuclear import rescued the HRE-mediated neurodegeneration. In particular, a gene called RanGAP, which encodes the Ran GTPase activating protein, is a potent suppressor of HRE toxicity. RanGAP interacts with Ran proteins, which regulate import and export of proteins from the nucleus. In C9ORF72-ALS, the HRE binds and sequesters RanGAP, thereby impairing nuclear import of proteins targeted to the nucleus. In both C9ORF72 flies and induced pluripotent stem cells (iPSC) derived from C9ORF72-ALS patients, RanGAP is mislocalized and this defect can be rescued with antisense oligonucleotides targeting the HRE. In addition, KPT-276, an inhibitor of exportin-1 (an analog of which is in Phase I clinical trials for hematological cancers), also rescues transport defects in these models. In line with the role of nuclear transport defects in ALS pathophysiology, stated Zhang, TDP-43 also accumulates in the cytoplasm in the vast majority of ALS cases, including in ALS due to C9ORF72 mutations.
Later in the session, Mahmood Kiaei from the University of Arkansas in Little Rock, Arkansas presented early findings from development of a mouse model for ALS based on profilin-1 (PFN1) mutations. The PFN1 gene, which was recently identified as an ALS-causing gene (see Nov 2013 news story; Sept 2012 news story), codes for an actin-binding protein, which also associates with stress granules. In order to further comprehend the mechanism by which PFN1 mutations cause ALS, as well as to develop a novel rodent model of ALS, the researchers generated a mouse overexpressing the human mutant PFN1 gene. At low levels, no phenotype was observed, and at intermediate levels the mice exhibited a subtle phenotype. The mouse selected for in-depth analysis expressed PFN1 at high levels and develop hindlimb skeletal muscle atrophy, and loss of motor neurons in the lumbar spinal cord. After onset of symptoms, these mice decline rapidly, with progressive weight loss and decline in motor function as well mitochondrial fragmentation. The high-copy expressing mouse model dies by 180 days. The researchers are continuing to characterize the mouse models in greater depth, in addition to generating additional mouse lines to control for insertion site of the transgene. We look forward to reading more about these new ALS mouse models as they are further characterized!
Another exciting presentation was given by Yvette Wong from Erica Holzbaur’s laboratory at the University of Pennsylvania in Philadelphia. Dr. Wong presented recent findings on the interaction between optineurin protein and parkin, mutations in which cause ALS and Parkinson’s, respectively. To read our detailed report on this work, read the full Oct 2014 news story.
P. Hande Ozdinler from Northwestern University in Chicago, Illinois gave an intriguing talk about a new approach to investigating innate and adaptive immune responses in ALS using a new triple transgenic mouse model developed in her laboratory. These mice express human mutant superoxide dismutase 1 (mSOD1), mutations in which account for approximately 2% of familial ALS, as well as genetically-encoded fluorescent proteins in cells expressing monocyte chemoattractant protein-1 (MCP-1) and its receptor CC chemokine receptor 2 (CCR2). MCP-1 is a chemokine that is important for monocyte recruitment to sites of inflammation, and its expression levels are elevated in cerebrospinal fluid of ALS patients (see Nov 2009 news story). In the hSOD1G93A-MCP1-CCR2 triple transgenic mice, MCP-1 and CCR2-expressing cells are labeled with two distinct fluorescent proteins, which facilitates tracking cellular expression of these proteins in the central nervous system of ALS mice throughout the progression of the disease. Surprisingly, high levels of MCP1-expressing cells were observed at presymptomatic stages in the motor cortex of ALS mice, prior to their appearance in the spinal cord (see related Nov 2014 news story). In addition, by using fluorescent-activated-cell-sorting (FACS) to isolate MCP1+ and CCR2+ cells from the brain and spinal cord, the researchers were able to investigate how gene expression changes in those cells over time. Preliminary analyses indicate that cortical and spinal motor neurons express distinct cytokine profiles, suggesting that mechanisms responsible for degeneration may differ between these neuronal populations in ALS. The researchers are now delving further into the transcriptional changes that underlie progression of the immune response in this ALS model.
Lee Martin from the Johns Hopkins University School of Medicine in Baltimore, Maryland gave the final presentation of the session. Dr. Martin presented promising results of a novel candidate therapeutic for ALS, which specifically targets the mitochondrial permeability transition pore (mPTP). Although mitochondrial dysfunction has been implicated in ALS pathogenesis, it remains unclear whether mitochondria are key drivers or rather bystanders affected downstream of other causes (see June 2014 news story; Jan 2012 news story). In order to investigate whether mPTP is a new therapeutic target for ALS, Martin collaborated with an Italian company called Congenia to test their small molecule modulator of the mPTP call Gnx4728. The drug prevents opening of the mPTP and increases retention of calcium in the mitochondria. In a transgenic model of ALS expressing the G37R mutation in SOD1, Gnx4728 crossed the blood-brain-barrier and increased lifespan nearly 2-fold. The treated mice exhibited improved mitochondrial calcium retention capacity in vivo, increased motor neuron survival and neuromuscular junction integrity. In addition, treatment reduced the prominent inflammatory response in the spinal cord of transgenic mice. These findings suggest that mPTP may be a novel therapeutic target for ALS, and support further testing of this drug in mouse models harboring other ALS-causing mutations to examine the broader applicability of these findings. Click here to read more about these studies published in the December 19 issue of Frontiers of Cellular Neuroscience.