A Series of N-terminal Epitope Tagged Hdh Knock-In Alleles Expressing Normal and Mutant Huntingtin: Their Application to Understanding the Effect of Increasing the Length of Normal Huntingtin’s Polyglutamine Stretch on CAG140 Mouse Model Pathogenesis

Background: Huntington’s disease (HD) is an autosomal dominant neurodegenerative disease that is caused by the expansion of a polyglutamine (polyQ) stretch within Huntingtin (htt), the protein product of the HD gene. Although studies in vitro have suggested that the mutant htt can act in a potentially dominant negative fashion by sequestering wild-type htt into insoluble protein aggregates, the role of the length of the normal htt polyQ stretch, and the adjacent proline-rich region (PRR) in modulating HD mouse model pathogenesis is currently unknown. Results: We describe the generation and characterization of a series of knock-in HD mouse models that express versions of the mouse HD gene (Hdh) encoding N-terminal hemaglutinin (HA) or 3xFlag epitope tagged full-length htt with different polyQ lengths (HA7Q-, 3xFlag7Q-, 3xFlag20Q-, and 3xFlag140Q-htt) and substitution of the adjacent mouse PRR with the human PRR (3xFlag20Q- and 3xFlag140Q-htt). Using co-immunoprecipitation and immunohistochemistry analyses, we detect no significant interaction between soluble full-length normal 7Q- htt and mutant (140Q) htt, but we do observe N-terminal fragments of epitope-tagged normal htt in mutant htt aggregates. When the sequences encoding normal mouse htt’s polyQ stretch and PRR are replaced with non-pathogenic human sequence in mice also expressing 140Q-htt, aggregation foci within the striatum, and the mean size of htt inclusions are increased, along with an increase in striatal lipofuscin and gliosis. Conclusion: In mice, soluble full-length normal and mutant htt are predominantly monomeric. In heterozygous knock-in HD mouse models, substituting the normal mouse polyQ and PRR with normal human sequence can exacerbate some neuropathological phenotypes

The long and the short of aberrant ciliogenesis in Huntington disease

Huntington disease (HD) is a dominantly inherited neurodegenerative disorder that is caused by a mutant huntingtin (HTT) gene encoding a version of the Htt protein with an expanded polyglutamine stretch. Although the HTT gene was discovered more than 18 years ago, the functions of normal Htt and the mechanisms by which mutant Htt causes disease are not well defined. In this issue of the JCI, Keryer et al. uncovered a novel function for normal Htt in ciliogenesis and report that mutant Htt causes hypermorphic ciliogenesis and ciliary dysfunction. These observations suggest that it is now critical to understand the extent to which ciliary dysfunction contributes to the different symptoms of HD and to determine whether therapeutic strategies designed to normalize ciliary function can ameliorate the disease

Columnar-Intrinsic Cues Shape Premotor Input Specificity in Locomotor Circuits

Control of movement relies on the ability of circuits within the spinal cord to establish connections with specific subtypes of motor neuron (MN). Although the pattern of output from locomotor networks can be influenced by MN position and identity, whether MNs exert an instructive role in shaping synaptic specificity within the spinal cord is unclear. We show that Hox transcription-factor-dependent programs in MNs are essential in establishing the central pattern of connectivity within the ventral spinal cord. Transformation of axially projecting MNs to a limb-level lateral motor column (LMC) fate, through mutation of the Hoxc9 gene, causes the central afferents of limb proprioceptive sensory neurons to target MNs connected to functionally inappropriate muscles. MN columnar identity also determines the pattern and distribution of inputs from multiple classes of premotor interneurons, indicating that MNs broadly influence circuit connectivity. These findings indicate that MN-intrinsic programs contribute to the initial architecture of locomotor circuits

Hox Proteins Coordinate Motor Neuron Differentiation and Connectivity Programs through Ret/Gfrα Genes

The accuracy of neural circuit assembly relies on the precise spatial and temporal control of synaptic specificity determinants during development. Hox transcription factors govern key aspects of motor neuron (MN) differentiation; however, the terminal effectors of their actions are largely unknown. We show that Hox/Hox cofactor interactions coordinate MN subtype diversification and connectivity through Ret/Gfrα receptor genes. Hox and Meis proteins determine the levels of Ret in MNs and define the intrasegmental profiles of Gfrα1 and Gfrα3 expression. Loss of Ret or Gfrα3 leads to MN specification and innervation defects similar to those observed in Hox mutants, while expression of Ret and Gfrα1 can bypass the requirement for Hox genes during MN pool differentiation. These studies indicate that Hox proteins contribute to neuronal fate and muscle connectivity through controlling the levels and pattern of cell surface receptor expression, consequently gating the ability of MNs to respond to limb-derived instructive cues