Molecular Mechanisms of Neurodegeneration
We use a forward genetic approach to identify the molecular pathways associated with loss of neurons in the aging mammalian brain. Specifically, we study chemically induced and spontaneous mouse mutants with adult-onset neurodegeneration leading to progressive movement abnormalities associated with cerebellar ataxia. Because these mutants often have additional sites of neuron loss, pursuing this phenotype gives us access to genes affecting survival of multiple types of neurons.
The advantage of this forward genetic approach is that it allows the identification, without a priori assumptions, of molecules critical to survival of terminally differentiated neurons in the mammalian central nervous system (CNS). Analysis of these mutants and determination of the underlying molecular defects, combined with identification of single-locus suppressor/enhancer genes of these mutations, will allow identification of the molecular mechanisms that underlie neuron death in the aging CNS and enhance progress toward prevention and development of effective therapies.
Oligomerization and the formation of aggregates of misfolded proteins are common to many genetic and sporadic forms of neurodegenerative diseases. Although some of these misfolded proteins are due to mutations directly within disease-related proteins, such as in the polyglutamine expansion diseases and some forms of familial Alzheimer's disease, the mechanisms underlying protein misfolding in many sporadic forms of neurodegenerative diseases remain unknown. Using a forward genetic approach, we have identified novel genes that, when their function is disrupted, cause the accumulation of misfolded proteins in neurons prior to their death.
ER stress and neurodegeneration. Mice homozygous for the woozy (wz) mutation develop ataxia between 3 and 4 months of age concomitant with Purkinje cell loss. Ubiquitinated protein accumulations are found in the endoplasmic reticulum (ER) and nucleus in these neurons prior to their degeneration. These abnormal protein accumulations induce the cellular response known as the unfolded protein response, which helps restore ER homeostasis. By positional cloning, we identified the wz mutation in the Sil1 gene, which encodes a cochaperone of the ER chaperone and ER-stress transducer, BiP. As with other HSP70 proteins, binding of substrates to BiP and their subsequent release are controlled by the cycle of ATP hydrolysis and exchange.
In vivo alterations of genes that influence the BiP ATP/ADP cycle support suggest that neurodegeneration in Sil1–/– mice is due to alterations in this cycle. We observed aggravation or suppression of neurodegeneration in Sil1-deficient mice when the gene dosage of Hyou1, which encodes an atypical HSP70 protein that functions as a BiP nucleotide exchange factor, was reduced or transgenically overexpressed, respectively. Deletion of the Dnajc3 gene, which stimulates the ATPase activity of BiP, greatly attenuates neurodegeneration in Sil1–/– mice, consistent with the opposing functions of these genes. Sil1 mutations have now been found in several families with Marinesco-Sjögren syndrome, a disorder associated with cerebellar ataxia; thus the wz mutant mouse will be an excellent model for this syndrome.
Mistranslation and neurodegeneration. Purkinje cell loss in mice homozygous for the spontaneous sticky (sti) mutation is associated with accumulation of ubiquitinated proteins in the cytoplasm, ER, and nucleolus. We have determined that the sti molecular defect is a point mutation in the editing domain of alanyl transfer RNA (tRNA) synthetase (AlaRS). The aminoacyl tRNA synthetases establish the genetic code that links each amino acid to its cognate tRNA that bears the anticodon triplet of the code. The high accuracy of protein translation is largely due to the precision of these aminoacylation reactions, and much of this accuracy resides in the editing domains of these synthetases that clear misactivated amino acids or mischarged tRNAs. In collaboration with Paul Schimmel (Scripps Research Institute), we demonstrated that the sti mutation causes an increase in mischarged tRNAAla. This likely leads to random misincorporation of amino acids at Ala codons, ultimately causing production of unfolded, heterogeneous proteins. The loss of translational fidelity in sti mutant mice is an exciting new mechanism underlying neurodegeneration.
We have recently created a mouse with an AlaRS conditional knockin mutation that causes a more severe disruption of AlaRS editing than is caused by the sti mutation. Using various Cre lines to induce expression of this defective molecule, we are examining the effects of mistranslation on different neuronal populations. In addition, by positional cloning we have identified a modifier gene that suppresses protein inclusion formation and neurodegeneration in sti/sti Purkinje cells in a gene-dosage-dependent manner. We are working to determine the function of this gene and its role in other neuron populations.
In addition to these mutations that cause mistranslation, we have recently identified additional neurodegenerative mutations that disrupt other aspects of neuronal translation. We are investigating the role of these genes in neuron survival, as well as the role of a novel modifier gene that suppresses neuron loss in these mutant strains.
Splicing and neurodegeneration. Mutations in RNA-binding proteins have been associated with familial and sporadic neurodegenerative disorders, and these mutations have been proposed to disrupt pre-mRNA splicing and other DNA and RNA metabolic functions.
We have recently identified an ethyl methanesulfonate (EMS)-induced mutation in one member of the multigene, small nuclear RNA (snRNA) family that results in degeneration of cerebellar and hippocampal neurons. Consistent with the essential role of these snRNAs in the spliceosome, our ongoing work suggests that this mutation leads to global abnormalities of pre-mRNA (messenger RNA) splicing, demonstrating that disruptions in this process can cause neurodegeneration. Our results also show that the genes encoding these RNAs may have important gene-dosage-dependent functions that may act in a cell-type-specific manner.
Continued work on this project, as well as the investigation of other genes identified in our screen, should continue to yield exciting insights into the mechanisms underlying neurodegeneration.
Portions of these projects were supported by grants from the National Institutes of Health.
As of May 30, 2012