The Demise of Neurons
One of the main aims of our research is to elucidate the molecular basis of neurodegeneration. In the absence of unbiased genetic screens in model organisms identifying neurodegenerative phenotypes, we were inspired to embark on such a screen utilizing fly photoreceptors. This extensive screening effort isolated 700 mutations corresponding to 165 genes (Yamamoto et al., 2014). The results of this screen have provided a rich resource of novel mutants for the fly community and have permitted us to dissect mechanisms for a variety of diseases.
Alzheimer's Disease (AD)
By performing TEM on eye clones of numerous mitochondrial mutants isolated in this screen, we discovered a dramatic increase in Lipid Droplets (LD) in glial cells (pigment cells) of some mutants. These mutants each exhibit elevated ROS with accumulated peroxidated lipids in glia. This elevated ROS associated with mitochondrial dysfunction triggers c-Jun-N-terminal Kinase (JNK) and Sterol Regulatory Element Binding Protein (SREBP) activity in neurons, leading to the synthesis of lipids. These lipids are then shuttled to glia to form LD rich in peroxidated lipids, which lowers ROS in neurons and delays the onset of neurodegeneration. Our model suggests that delay of neuronal demise is likely the result of ROS sequestration in LD. This phenomenon is evolutionarily conserved as it is also observed in mice that lack Ndufs4 (a subunit of Complex 1). Just as in the fly model, these mice accumulate LD in astrocytes and microglia prior to neural loss and death (Liu et al., 2015).
The neuronally produced lipids are transferred to extracellular apolipoproteins like ApoD and ApoE, which are critical for glial LD formation (Liu et al., 2017). Human ApoE3 can substitute for the fly glial apolipoprotein ApoD. However, ApoE4, the most prominent Alzheimer's Disease (AD) susceptibility allele, impairs lipid transport and LD formation because it likely cannot be properly lipidated. In contrast, ApoE2, a neuroprotective allele, leads to an efficient accumulation of peroxidated LD in glial cells and protects against neurodegeneration. We are currently studying numerous genes identified in Genome Wide Association Studies for AD in an attempt to pinpoint where each fits into the pathway that promotes the formation of peroxidated LD in glia.
Parkinson's Disease (PD)
We embarked on the study of iPLA2-VIA, a gene whose loss causes infantile axonal dystrophy (early death) as well as early-onset parkinsonism (death in midlife). Loss of the fly homolog, PLA2G6, reduces lifespan to less than 30 days (normally ~90 days), impairs synaptic transmission, and causes the demise of the CNS and the visual system. Phospholipases typically hydrolyze glycerol phospholipids, but loss of iPLA2-VIA does not affect phospholipid composition in the brain but rather causes an elevation in ceramides. We discovered that PLA2G6 binds two retromer subunits, Vps35 and Vps26, and is required to maintain proper retromer function. Loss of PLA2G6 causes a progressive loss of retromer function and leads to a deficit in the recycling of proteins and membranes from endosomes to plasma membrane, thereby increasing trafficking from the endosome to the lysosome. This leads to progressive lysosomal expansion and ceramide accumulation caused by lysosomal dysfunction. As ceramide increases and stiffens the membrane a feedback loop then impairs endocytosis and retromer function, further expanding lysosomes and elevating ceramide. A severe reduction in the levels of Vps35 and Vps26, enlargement of lysosomes, and an increase in ceramides is also observed upon overexpression of a-synuclein. These defects may be shared in Parkinson disease (PD) as synuclein accumulation is a common hallmark in PD patients. We are currently exploring how this pathway interacts with other genes associated with PD.
Other Neurological Diseases
We have a keen interest in other neurological diseases which include Friedreich Ataxia (FA), Amyotrophic Lateral Sclerosis (ALS), and Multiple Sclerosis (MS).
The Yamamoto et al. (2014) screen allowed us to isolate the first null allele of the fly gene whose human ortholog causes Freidreich Ataxia. Our work on FA revealed that the loss of the fly FA gene induces an iron accumulation in the nervous system which enhances the synthesis of sphingolipids. This in turn activates a phosphoinositide dependent protein kinase-1 (Pdk1) as well as a myocyte enhancer factor-2 (Mef2). The increased phosphorylation and activity of the Mef2 transcription factor is toxic in neurons and causes a hypertrophy of the heart. The latter phenotype is the cause of most deaths in FA patients. We have shown that this pathway is activated in flies, mice, and human hearts.
The 2014 screen also identified mutations in Ubiquilin, the fly homolog of four human Ubiquilins, two of which were previously shown to cause ALS. The role of Ubiquilins in proteasomal degradation has previously been established, but we discovered a crosstalk between the stress induced by the loss of Ubiquilin in the endoplasmic reticulum, mTOR signalling and autophagic flux in Drosophila as well as mammalian cells that lack ubiquilins. Indeed, loss of Ubiquilins impairs mTORC1 activity, promotes autophagy and causes the demise of neurons. ubiquilin mutants display defective autophagic flux due to reduced lysosome acidification. Indeed, they are required to maintain proper levels of the V0a/V100 subunit of the vacuolar H+-ATPase and lysosomal pH. Interestingly, feeding flies acidic nanoparticles alleviates defective autophagic flux in ubiquilin mutants. Our studies revealed a conserved role for Ubiquilins as regulators of autophagy by controlling vacuolar H+-ATPase activity and mTOR signalling.
Currently we are trying to model MS in flies and mice based on a gene that was submitted to us by the Undiagnosed Diseases Network (see below).
Yamamoto S, Jaiswal M, Charng WL, Gambin T, Karaca E, Mirzaa G, Wiszniewski W, Sandoval H,Haelterman NA, Xiong B, Zhang K, Bayat V, David G, Li T, Chen K, Gala U, Harel T, Pehlivan D, Penney S, Vissers L, de Ligt J, Jhangiani SN, Xie Y, Tsang SH, Parman Y, Sivaci M, Battaloglu E, Muzny D, Wan YW, Liu Z, Lin-Moore AT, Clark RD, Curry CJ, Link N, Schulze KL, Boerwinkle E, Dobyns WB, Allikmets R, Gibbs RA, Chen R, Lupski JR, Wangler MF, Bellen HJ (2014) A Drosophila genetic resource of mutants to study mechanisms underlying human genetic diseases. Cell 159:200-214. PMCID: PMC4298142. Recommended by F1000. Covered in Nature Methods 11(12):1197, Comment.
Liu L, MacKenzie KR, Putluri N, Maletic-Savatic M, Bellen HJ (2017) The glia-neuron lactate shuttle and elevated ROS promote lipid synthesis in neurons and lipid droplet accumulation in glia via APOE/D. Cell Metabolism 26:719-737. PMCID: PMC5677551. Covered in Cell Metabolism 26(5):701-2, Preview, and Science Translational Medicine 9(412), eaap8170, Editor's Choice.
Liu L, Zhang K, Sandoval H, Yamamoto S, Jaiswal M, Sanz E, Li Z, Hui J, Graham BH, Quintana A, Bellen HJ (2015) Glial lipid droplets and ROS induced by mitochondrial defects promote neurodegeneration. Cell 160:177-190. PMCID: PMC4377295.
Lin G, Lee PT, Chen K, Mao D, Tan KL, Zuo Z, Lin W-W, Wang L, Bellen HJ (2018) Phospholipase PLA2G6, a parkinsonism-associated gene, affects Vps26 and Vps35, retromer function, and ceramide levels, similar to a-Synuclein gain. Cell Metabolism 28:605-618. PMID: 29909971.
Senturk M, Lin G, Zuo, Z, Mao D, Watson E, Mikos AG, Bellen HJ (2019) Ubiquilins regulate autophagic flux through mTOR signaling and lysosomal acidification. Nature Cell Biology 21:384-396. PMID: 30804504.
Mao D, Lin G, Tepe B, Zuo Z, Tan KL, Senturk M, Zhang S, Arenkiel BR, Sardiello M, Bellen HJ (2019) VAMP associated proteins are required for autophagic and lysosomal degradation by promoting a PtdIns4-P-mediated endosomal pathway. Autophagy 11:1-20. PMID: 30741620.
Chen K, Ho TS, Lin G, Tan KL, Rasband MN, Bellen HJ (2016) Loss of Frataxin activates the iron/sphingolipid/PDK1/Mef2 pathway in mammals. eLife 5:e20732. PMCID: PMC5130293.
Chen K, Lin G, Haelterman NA, Ho TS, Li T, Li Z, Duraine L, Graham BH, Jaiswal M, Yamamoto S, Rasband MN, Bellen HJ (2016) Loss of Frataxin induces iron toxicity, sphingolipid synthesis, and Pdk1/Mef2 activation, leading to neurodegeneration. eLife 5:e16043. PMCID: PMC4956409.