Changes in cortical and striatal neurons predict behavioral and electrophysiological abnormalities in a transgenic murine model of Huntington\u27s disease

Neurons in Huntington\u27s disease exhibit selective morphological and subcellular alterations in the striatum and cortex. The link between these neuronal changes and behavioral abnormalities is unclear. We investigated relationships between essential neuronal changes that predict motor impairment and possible involvement of the corticostriatal pathway in developing behavioral phenotypes. We therefore generated heterozygote mice expressing the N-terminal one-third of huntingtin with normal (CT18) or expanded (HD46, HD100) glutamine repeats. The HD mice exhibited motor deficits between 3 and 10 months. The age of onset depended on an expanded polyglutamine length; phenotype severity correlated with increasing age. Neuronal changes in the striatum (nuclear inclusions) preceded the onset of phenotype, whereas cortical changes, especially the accumulation of huntingtin in the nucleus and cytoplasm and the appearance of dysmorphic dendrites, predicted the onset and severity of behavioral deficits. Striatal neurons in the HD mice displayed altered responses to cortical stimulation and to activation by the excitotoxic agent NMDA. Application of NMDA increased intracellular Ca(2+) levels in HD100 neurons compared with wild-type neurons. Results suggest that motor deficits in Huntington\u27s disease arise from cumulative morphological and physiological changes in neurons that impair corticostriatal circuitry

The molecular pathology of Rett syndrome: synopsis and update

Genetic mutations of the X-linked gene MECP2, encoding methyl-CpG-binding protein 2, cause Rett syndrome (RTT) and other neurological disorders. It is increasingly recognized that MECP2 is a multifunctional protein, with at least four different functional domains: (1) a methyl-CpG-binding domain; (2) an arginine-glycine repeat RNA-binding domain; (3) a transcriptional repression domain; and (4) an RNA splicing factor binding region (WW group II binding domain). There is evidence that MECP2 is important for large-scale reorganization of pericentromeric heterochromatin during differentiation. Studies in MECP2-deficient mouse brain have identified a diverse set of genes with altered levels of mRNA expression or splicing. It is still unclear how altered MECP2 function ultimately results in neuronal disease after a period of grossly normal development. However, mounting evidence suggests that neuronal health and development depend on precise regulation of MECP2 expression. In genetically engineered mice, both increased and decreased levels of MECP2 result in a neurological phenotype. Furthermore, it was recently discovered that MECP2 gene duplications underlie a small number of atypical Rett cases and mental retardation syndromes. The finding that MECP2 levels are tightly regulated in neurons has important implications for the design of gene replacement or reactivation strategies for treatment of RTT, because affected individuals typically are somatic mosaics with one set of cells expressing a mutated MECP2 from the affected X, and another set expressing normal MECP2 from the unaffected X. Further studies are necessary to elucidate the molecular pathology of both loss-of-function and gain-of-function mutations in MECP2

Are there multiple pathways in the pathogenesis of Huntington\u27s disease

Studies of huntingtin localization in human post-mortem brain offer insights and a framework for basic experiments in the pathogenesis of Huntington\u27s disease. In neurons of cortex and striatum, we identified changes in the cytoplasmic localization of huntingtin including a marked perinuclear accumulation of huntingtin and formation of multivesicular bodies, changes conceivably pointing to an altered handling of huntingtin in neurons. In Huntington\u27s disease, huntingtin also accumulates in aberrant subcellular compartments such as nuclear and neuritic aggregates co-localized with ubiquitin. The site of protein aggregation is polyglutamine-dependent, both in juvenile-onset patients having more aggregates in the nucleus and in adult-onset patients presenting more neuritic aggregates. Studies in vitro reveal that the genesis of these aggregates and cell death are tied to cleavage of mutant huntingtin. However, we found that the aggregation of mutant huntingtin can be dissociated from the extent of cell death. Thus properties of mutant huntingtin more subtle than its aggregation, such as its proteolysis and protein interactions that affect vesicle trafficking and nuclear transport, might suffice to cause neurodegeneration in the striatum and cortex. We propose that mutant huntingtin engages multiple pathogenic pathways leading to neuronal death