Mutant Huntingtin Impairs Axonal Trafficking in Mammalian Neurons In Vivo and In Vitro

Abstract

Recent data in invertebrates demonstrated that huntingtin (htt) is essential for fast axonal trafficking. Here, we provide direct and functional evidence that htt is involved in fast axonal trafficking in mammals. Moreover, expression of full-length mutant htt (mhtt) impairs vesicular and mitochondrial trafficking in mammalian neurons in vitro and in whole animals in vivo. Particularly, mitochondria become progressively immobilized and stop more frequently in neurons from transgenic animals. These defects occurred early in development prior to the onset of measurable neurological or mitochondrial abnormalities. Consistent with a progressive loss of function, wild-type htt, trafficking motors, and mitochondrial components were selectively sequestered by mhtt in human Huntington's disease-affected brain. Data provide a model for how loss of htt function causes toxicity; mhtt-mediated aggregation sequesters htt and components of trafficking machinery leading to loss of mitochondrial motility and eventual mitochondrial dysfunction.Huntington's disease (HD) is a progressive neurodegenerative disorder caused by a CAG repeat expansion mutation in the coding region of a novel gene. The mechanism of HD is unknown. However, most data suggest that polyglutamine-mediated aggregation contributes to the pathology (32). Studies of human brain (14), mouse models (48), and cells (8, 28) demonstrate that mutant huntingtin (mhtt) binds and sequesters its normal counterpart as well as many cellular proteins (41). But whether pathophysiology results from a loss of normal function or a gain of a new function in adult neurons is not well understood.A major gap in our understanding of the disease mechanism is the absence of a known function for normal huntingtin (htt). Emerging evidence suggests that htt is likely to be a multifunctional protein that can mediate transactions in both the nucleus and the cytoplasm. Transcriptional dysfunction caused by mhtt has been proposed to lead to toxicity. The mutation in full-length htt prevents its normal ability to bind and sequester a repressor of brain-derived neurotrophic factor expression, reducing the availability of brain-derived neurotrophic factor to striatal neurons (54). The N-terminal, truncated form of mhtt can bind to and interfere with nuclear factors such as CREB (51), CREB binding protein (30, 39), corepressor (22), and transcriptional activator Sp1 (12,23).Cytoplasmic dysfunction has also been implicated as a toxic mechanism. Recently, novel data obtained with Drosophila (17) and isolated squid axoplasm (42) have provided direct evidence that htt is an essential protein involved in fast axonal trafficking. Additionally, these data demonstrate that the mutation in htt causes trafficking abnormalities. Reduction of htt expression in Drosophila caused axonal transport defects in larval nerves and the same neurodegenerative phenotype in adult eyes as expression of mutant dynein or p150 Glued (17). In invertebrate models for HD, expression of truncated proteins with an expanded gluta...

In vivo characterization of white matter pathology in premanifest huntington's disease

“…Although we cannot compare these 2 sets of findings statistically and so must proceed with caution, the NODDI and DTI analyses combined do suggest that the widespread increased diffusivity in pre‐HD can in principle be explained by reductions in the number of axons within the fibers of white matter pathways. This is highly compatible with the evidence of axonal degeneration in animal studies of pre‐HD 12, 13. It should be noted, however, that although it is biologically plausible that reduced NDI is underscored by changes in axonal density, there may also be an effect of myelin loss.…”

Dendritic spine loss and neurodegeneration is rescued by Rab11 in models of Huntington's disease

“…22 Synaptic components are also retrieved from the plasma membrane by endocytosis into REs and can be either reinserted or degraded. 23 As vesicle trafficking events have been implicated in HD, [24][25][26][27][28][29] we decided to investigate whether aggregation of a mutant htt exon-1 fragment in the cytoplasm would affect spine maintenance by interfering with endosomal trafficking. Previous reports have shown a disruption of endocytic trafficking in cells expressing a mutant htt fragment, 28,29 which appears to be mediated by Rab11, a GTPase involved in RE function.…”

p53 tumor suppressor protein regulates the levels of huntingtin gene expression

“…The normal function of the huntingtin protein is not fully understood. In addition to its possible functions in the cell nucleus, recent studies indicate that normal huntingtin protein is required for fast axonal trafficking (Trushina et al, 2004). Reduction of normal huntingtin expression, or expression of the polyglutamine track expanded mutant huntingtin proteins, which interact with the normal huntingtin proteins, impairs vesicular and mitochondrial trafficking in mammalian neurons, leading to mitochondrial dysfunction and toxicity (Trushina et al, 2004).…”

“…In addition to its possible functions in the cell nucleus, recent studies indicate that normal huntingtin protein is required for fast axonal trafficking (Trushina et al, 2004). Reduction of normal huntingtin expression, or expression of the polyglutamine track expanded mutant huntingtin proteins, which interact with the normal huntingtin proteins, impairs vesicular and mitochondrial trafficking in mammalian neurons, leading to mitochondrial dysfunction and toxicity (Trushina et al, 2004). Although the regulation of huntingtin gene expression has not been well studied, both genetic and environmental stress factors could affect huntingtin gene expression (Dixon et al, 2004), and that in turn could impact upon the onset and prognosis of Huntington's disease (Georgiou et al, 1999;Anca et al, 2004).…”