Selected vulnerability of neurons in Huntington’s disease (HD) suggests alterations in a cellular process particularly critical for neuronal function. Supporting this idea, pathogenic Htt (polyQ-Htt) inhibits fast axonal transport (FAT) in various cellular and animal HD models (mouse and squid), but the molecular basis of this effect remains unknown. Here we show that polyQ-Htt inhibits FAT through a mechanism involving activation of axonal JNK. Accordingly, increased activation of JNK was observed in vivo in cellular and animal HD models. Additional experiments indicate that polyQ-Htt effects on FAT are mediated by the neuron-specific JNK3, and not ubiquitously expressed JNK1, providing a molecular basis for neuron-specific pathology in HD. Mass spectrometry identified a residue in the kinesin-1 motor domain phosphorylated by JNK3, and this modification reduces kinesin-1 binding to microtubules. These data identify JNK3 as a critical mediator of polyQ-Htt toxicity and provides a molecular basis for polyQ-Htt-induced inhibition of FAT.

Expansion of the polyglutamine (polyQ) stretch in the androgen receptor (AR) protein leads to spinal and bulbar muscular atrophy (SBMA), a neurodegenerative disease characterized by lower motor neuron degeneration. The pathogenic mechanisms underlying SBMA remain unknown, but recent experiments show that inhibition of fast axonal transport (FAT) by polyQ-expanded proteins, including polyQ-AR, represents a new cytoplasmic pathogenic lesion. Using pharmacological, biochemical and cell biological experiments, we found a new pathogenic pathway that is affected in SBMA and results in compromised FAT. PolyQ-AR inhibits FAT in a human cell line and in squid axoplasm through a pathway that involves activation of cJun N-terminal kinase (JNK) activity. Active JNK phosphorylated kinesin-1 heavy chains and inhibited kinesin-1 microtubule-binding activity. JNK inhibitors prevented polyQ-AR-mediated inhibition of FAT and reversed suppression of neurite formation by polyQ-AR. We propose that JNK represents a promising target for therapeutic interventions in SBMA.

J. Neurochem. (2010) 113, 1073–1091. Abstract Abnormal expansion of a polyglutamine tract in huntingtin (Htt) protein results in Huntington’s disease (HD), an autosomal dominant neurodegenerative disorder involving progressive loss of motor and cognitive function. Contrasting with the ubiquitous tissue expression of polyglutamine‐expanded Htt, HD pathology is characterized by the increased vulnerability of specific neuronal populations within the striatum and the cerebral cortex. Morphological, biochemical, and functional characteristics of neurons affected in HD that might render these cells more vulnerable to the toxic effects of polyglutamine‐Htt are covered in this review. The differential vulnerability of neurons observed in HD is discussed in the context of various major pathogenic mechanisms proposed to date, and in line with evidence showing a ‘dying‐back’ pattern of degeneration in affected neuronal populations.

Conventional kinesin is a major microtubule-based motor protein responsible for anterograde transport of various membrane-bounded organelles (MBO) along axons. Structurally, this molecular motor protein is a tetrameric complex composed of two heavy (kinesin-1) chains and two light chain (KLC) subunits. The products of three kinesin-1 (kinesin-1A, -1B, and -1C, formerly KIF5A, -B, and -C) and two KLC (KLC1, KLC2) genes are expressed in mammalian nervous tissue, but the functional significance of this subunit heterogeneity remains unknown. In this work, we examine all possible combinations among conventional kinesin subunits in brain tissue. In sharp contrast with previous reports, immunoprecipitation experiments here demonstrate that conventional kinesin holoenzymes are formed of kinesin-1 homodimers. Similar experiments confirmed previous findings of KLC homodimerization. Additionally, no specificity was found in the interaction between kinesin-1s and KLCs, suggesting the existence of six variant forms of conventional kinesin, as defined by their gene product composition. Subcellular fractionation studies indicate that such variants associate with biochemically different MBOs and further suggest a role of kinesin-1s in the targeting of conventional kinesin holoenzymes to specific MBO cargoes. Taken together, our data address the combination of subunits that characterize endogenous conventional kinesin. Findings on the composition and subunit organization of conventional kinesin as described here provide a molecular basis for the regulation of axonal transport and delivery of selected MBOs to discrete subcellular locations.Molecular motors of the kinesin and dynein superfamilies are responsible for microtubule-(MT-) based motility in cells. Approximately 40−45 kinesin-related polypeptides have been identified in mouse and human (1), with 25 or more being expressed in the developing nervous system (2). From these, conventional kinesin is the most abundant kinesin family member in the adult nervous system (3). Biochemical (4) and electron microscopic studies (5) indicated that the native conventional kinesin holoenzyme exists as a tetramer consisting of two kinesin light chain (KLCs) 1 and two kinesin heavy chain (kinesin-1, KHC, KIF5s) subunits (6). † This work was supported by grants from the Huntington's Disease Society of America (HDSA) and ALSA (to G.M.), grants from NINDS (NS23868, NS23320, NS41170 and NS43408), MDA, and ALSA (to S.T.B.), and the Fitz-Thyssen Foundation and DFG (to S.K.).* To whom correspondence should be addressed. Phone: (312) 996−6791. Fax: (312) 413−0354. E-mail: gmorfini@uic.edu.. ‡ These authors contributed equally to this paper. § University of Illinois at Chicago. ∥ University of Heidelberg.1 Abbreviations: KHC, kinesin heavy chain; KLC, kinesin light chain; TR, tandem repeats; PIPES, 1,4-piperazinediethanesulfonic acid; HEPES, N-(2-hydroxyethyl)piperazine-N′-2-ethanesulfonic acid; AMP-PNP, adenosine 5′-(β,γ-imino)triphosphate; GST, glutathione Stransferase; SDS-PAGE, sodium d...