Studies on the molecular pathogenesis of neuronal ceroid lipofuscinoses in the animal models of CLN8 and CLN10
Year: 2012
Type: dissertation
Abstract: The neuronal ceroid lipofuscinoses (NCLs) are a group of pediatric neurodegenerative syndromes. They are characterized by epilepsy, mental and motor regression, loss of vision and early death. Pathological findings include accumulation of lysosomal storage material together with neuron loss and glial activation in affected brains. Mutations in at least ten genes are reported to cause NCL disease. Apart from human disease, NCL phenotypes are widely present in the animal kingdom. However, the events leading to NCL disease onset and progression remain elusive and without a possibility to intervene. In this study these events were assessed in two forms of NCLs, the CLN8 and CLN10 diseases. Late-infantile onset CLN8 disease and the more protracted progressive epilepsy with mental retardation are caused by mutations in the CLN8 gene. CLN8 is a resident of the endoplasmic reticulum membrane and linked to a lipid metabolism-related protein family. Mutations in the cathepsin D (CTSD) gene cause the most severe form of NCL, CLN10, which typically leads to death shortly after birth. The lysosomal aspartic protease CTSD is involved in cellular protein degradation and apoptosis. Two animal models were used in this study, the Cln8mnd mouse carrying a spontaneous mutation in the mouse Cln8 gene, and the previously generated cathepsin D deficient Drosophila, cathD1. Both model the respective human NCL diseases with intracellular storage and neuronal death. The Drosophila cathD1 was characterized with degenerative changes in the retina. This phenotype could be utilized to study the genetic pathways of cathD in a hypothesis-based modifier screen using Drosophila genetics and histological techniques. Seven candidate modifiers involved in lipid metabolism regulation, endocytosis and oxidative stress were identified as enhancers of the retinal degeneration of cathD1. Similar processes have been described affected in other Drosophila models for NCLs, suggesting that Drosophila NCL proteins may act in overlapping genetic pathways. While the importance of these pathways needs to be assessed in the human disease, their overlap suggests a possibility for a common mechanism of neurodegeneration. Similarly to previously characterized NCL models, the CLN8 disease model Cln8mnd showed neuron loss and glial activation in sensory thalamocortical pathways. By using stereological methods and histology, neuron loss in the visual thalamocortical pathway was shown to appear relatively late compared to the previously described retinal degeneration in Cln8mnd. This suggests that the visual areas of the brain are spared even with diminished or absent input from the retina. The somatosensory thalamocortical pathway was affected first. While the cerebral cortical sensory areas were relatively spared, neuron loss progressed rapidly in the thalamic component of these pathways. Thalamic sensory nuclei have shown particular vulnerability in the majority of NCL models, indicating a common pathological pathway proceeding from thalamus to the cortex. In the future, knowledge of the differentially affected components of distinct pathways may be utilized in therapies, which should be targeted to the primary sites of pathology. In CLN8 disease these appear to be the retina and somatotosensory thalamus. Glial cells were observed to contribute to the Cln8mnd neuropathology. Especially microglial activation preceded neuron loss in Cln8mnd brain. Decreased axonal myelination, studied by stereology, light and electron microscopy, was observed even before microglial activation, yet myelination reached normal levels by the time of major brain pathology. By protein and gene expression profiling we were able to show a delayed maturation of oligodendrocytes in the Cln8mnd mouse brain and in vitro. These results suggest that perturbations in the glia-neuron-glia signaling occur well in advance of the neurodegeneration in Cln8mnd. With increasing information on the mechanisms of these interactions it may be possible to find a specific target for therapies for CLN8 or NCLs in general. The myelination defect in Cln8mnd was observed through large-scale brain lipid analysis, where decreased amounts of myelin-specific galactolipids were found in Cln8mnd cortex, especially in the early stages of Cln8mnd disease. Subsequent analyses showed a persistent defect in the galactolipid synthesis by the UDP-galactose:ceramide galactosyltransferase enzyme. The connection of CLN8 to lipid synthesis regulation is in agreement with the sequence-based hypothesis of a role for CLN8 in lipid metabolism regulation, while further studies are required to indicate a specific function. In conclusion, this study resulted in increased knowledge on the CLN8 and CLN10 disease pathogenesis and on the cellular functions of the affected proteins. Results from both cathD1 Drosophila and Cln8mnd mouse suggest both similarities and differences to other NCL models. In the future, increased knowledge on the molecular changes may take us towards targeting these diseases with therapies.
Source: Työväentutkimus Vuosikirja
Author: Mervi Kuronen
Language: English
Subfield: Genetics (clinical)
Field: Medicine
Domain: Health Sciences
Open Access status: green
OpenAlex ID: W2239449006