| Nervous tissue is highly specialized and is dependent on expression of a distinctive subset of proteins and on high energy generation. Neurons are especially sensitive to changes in homeostasis and are, therefore, more vulnerable than other cell types to degenerative disease. Amyotrophic Lateral Sclerosis (ALS) is one such disease in which the motor neurons of the brain and spinal cord are exclusively affected. In this study we show that mitochondria and nuclear abnormalities contribute to disease etiology, onset, and progression. By using a transgenic mouse model for the disease, coupled with immuno-based techniques, we show that motor neuron mitochondria highly express the components of a multiprotein mitochondrial complex, the permeability transition pore, which upon activation induces cell death. We further show that inhibiting complex formation, by crossing the ALS mouse model with a complex component knockout mouse, we can abrogate the mitochondrial abnormalities seen in this ALS mouse model and increase time to disease onset and lifespan, although they still die from motor neuron degeneration. These results would indicate that mitochondria abnormalities contribute to disease pathophysiology but not etiology. We further show that in spinal cord early in disease progression the human mutant protein localizes to the nucleus exclusively in motor neurons. Additionally, using immunohistochemistry and immunofluorescence, we show that motor neurons in the transgenic mice lack the SMN complex/gemins, an integral complex involved in the mRNA splicing pathway. By six weeks, when mice are still presymptomatic, more than half the motor neurons of the spinal cord have disruption of this complex. Our studies also explored a possible role for epigenetic modification of mitochondria DNA. We found that mitochondria of nervous tissue contain a DNA methylating enzyme, Dnmt3a and, using immunofluorescence, significant levels of 5methylcytosine present in a subset of mitochondria. We further show that these same mitochondria also contain an autophagocyte marker, LC3a. These results would indicate that methylation of mitochondrial DNA is associated with mitochondria] autophagosomes, mitosomes. Our data, taken together, shows that the neurons of the central nervous system have a unique biology, exquisitely sensitive to mitochondria and nuclear homeostatic abnormalities. Changes in the organelles, that would have little or no effect on the cells of other tissues, could have serious detrimental effects on the cells of nervous tissue leading to degeneration. Further elucidation on the unique role of these organelles in nervous tissue biology could shed light on a number of degenerative diseases. |
