The expression of Huntingtin-associated protein (HAP1) mRNA in developing, adult and ageing rat CNS: implications for Huntington's disease neuropathology

Huntingtin Interacting Proteins and Pathological Implications

Huntington's disease (HD) is caused by an expansion of a CAG repeat in the gene that encodes the huntingtin protein (HTT). The exact function of HTT is still not fully understood, and previous studies have mainly focused on identifying proteins that interact with HTT to gain insights into its function. Numerous HTT-interacting proteins have been discovered, shedding light on the functions and structure of HTT. Most of these proteins interact with the N-terminal region of HTT. Among the various HTT-interacting proteins, huntingtin-associated protein 1 (HAP1) and HTT-interacting protein 1 (HIP1) have been extensively studied. Recent research has uncovered differences in the distribution of HAP1 in monkey and human brains compared with mice. This finding suggests that there may be species-specific variations in the regulation and function of HTT-interacting proteins. Understanding these differences could provide crucial insights into the development of HD. In this review, we will focus on the recent advancements in the study of HTT-interacting proteins, with particular attention to the differential distributions of HTT and HAP1 in larger animal models.

Huntington-associated protein 1 inhibition contributes to neuropathic pain by suppressing Cav1.2 activity and attenuating inflammation

ABSTRACT

Although pain dysfunction is increasingly observed in Huntington disease, the underlying mechanisms still unknown. As a crucial Huntington-associated protein, Huntington-associated protein 1 (HAP1) is enriched in normal spinal dorsal horn and dorsal root ganglia (DRG) which are regarded as "primary sensory center," indicating its potential functions in pain process. Here, we discovered that HAP1 level was greatly increased in the dorsal horn and DRG under acute and chronic pain conditions. Lack of HAP1 obviously suppressed mechanical allodynia and hyperalgesia in spared nerve injury (SNI)-induced and chronic constriction injury-induced pain. Its deficiency also greatly inhibited the excitability of nociceptive neurons. Interestingly, we found that suppressing HAP1 level diminished the membrane expression of the L-type calcium channel (Cav1.2), which can regulate Ca 2+ influx and then influence brain-derived neurotrophic factor (BDNF) synthesis and release. Furthermore, SNI-induced activation of astrocytes and microglia notably decreased in HAP1-deficient mice. These results indicate that HAP1 deficiency might attenuate pain responses. Collectively, our results suggest that HAP1 in dorsal horn and DRG neurons regulates Cav1.2 surface expression, which in turn reduces neuronal excitability, BDNF secretion, and inflammatory responses and ultimately influences neuropathic pain progression.

Neurochemical phenotypes of huntingtin-associated protein 1 in reference to secretomotor and vasodilator neurons in the submucosal plexuses of rodent small intestine

Huntingtin-associated protein 1(HAP1) is an immunohistochemical marker of the stigmoid body (STB). Brain and spinal cord regions with lack of STB/HAP1 immunoreactivity are always neurodegenerative targets, whereas STB/HAP1 abundant regions are usually spared from neurodegeneration. In addition to the brain and spinal cord, HAP1 is abundantly expressed in the excitatory and inhibitory motor neurons in myenteric plexuses of the enteric nervous system (ENS). However, the detailed expression of HAP1 and its neurochemical characterization in submucosal plexuses of ENS are still unknown. In this study, we aimed to clarify the expression and neurochemical characterization of HAP1 in the submucosal plexuses of the small intestine in adult mice and rats. HAP1 was highly expressed in the submucosal plexuses of both rodents. The percentage of HAP1-immunoreactive submucosal neurons was not significantly varied between the intestinal segments of these rodents. Double immunofluorescence results revealed that almost all the cholinergic secretomotor neurons containing ChAT/ CGRP/ somatostatin/ calretinin, non-cholinergic secretomotor neurons containing VIP/NOS/TH/calretinin, and vasodilator neurons containing VIP/calretinin expressed HAP1. Our current study is the first to clarify that STB/HAP1 is expressed in secretomotor and vasodilator neurons of submucosal plexuses, suggesting that STB/HAP1 might modulate or protect the secretomotor and vasodilator functions of submucosal neurons in ENS.

Visualization of Three-Dimentional Stigmoid Body in FFPE and Ultrathin Sections of Mouse

Huntingtin-associated protein 1 (HAP1) is abundantly expressed in the neurons of the central nervous system and forms unique intracytoplasmic inclusions of unknown function called "stigmoid bodies" (STBs). Transmission electron microscopy has revealed that the STBs are aggregates of granules containing cavities with a diameter of 0.5-3 µm. Small STBs fuse to form larger STBs, the size of which is said to vary depending on the developmental growth stage and brain region. Light microscopy can only reveal that these STBs have similar circular shapes, due to its limited resolution. Therefore, light microscopy is only fit for the study of the STB distribution and quantitative changes. We, herein, suggest the adoption of correlative light and electron microscopy, that combines confocal laser scanning microscopy and scanning electron microscopy, as the method allowing us to identify the HAP1-positive STBs in formalin-fixed paraffin-embedded (FFPE) sections. This approach allows us to study the three-dimensional morphology of immunolabeled objects in histopathological specimens. The STBs in FFPE sections of murine hypothalami reflected the transmission electron microscopic images of Epon-embedded STBs, although we were not able to observe any organelle covering the STBs of the FFPE sections. Furthermore, we were able to reconstruct the three-dimensional structure of the STB and we identified it to be of spherical form, covered with mitochondria and rough endoplasmic reticulum, and bearing a cluster of cavities in the center. In the future, we might gain new insights by comparing the 3D structure of the STB between different neurons and under a variety of conditions.

Immunohistochemical expression and neurochemical phenotypes of huntingtin-associated protein 1 in the myenteric plexus of mouse gastrointestinal tract

Huntingtin-associated protein 1 (HAP1) is a neural huntingtin interactor and being considered as a core molecule of stigmoid body (STB). Brain/spinal cord regions with abundant STB/HAP1 expression are usually spared from neurodegeneration in stress/disease conditions, whereas the regions with little STB/HAP1 expression are always neurodegenerative targets. The enteric nervous system (ENS) can act as a potential portal for pathogenesis of neurodegenerative disorders. However, ENS is also a neurodegenerative target in these disorders. To date, the expression of HAP1 and its neurochemical characterization have never been examined there. In the current study, we determined the expression of HAP1 in the ENS of adult mice and characterized the morphological relationships of HAP1-immunoreactive (ir) cells with the markers of motor neurons, sensory neurons, and interneurons in the myenteric plexus using Western blotting and light/fluorescence microscopy. HAP1-immunoreaction was present in both myenteric and submucosal plexuses of ENS. Most of the HAP1-ir neurons exhibited STB in their cytoplasm. In myenteric plexus, a large number of calretinin, calbindin, NOS, VIP, ChAT, SP, somatostatin, and TH-ir neurons showed HAP1-immunoreactivity. In contrast, most of the CGRP-ir neurons were devoid of HAP1-immunoreactivity. Our current study is the first to clarify that HAP1 is highly expressed in excitatory motor neurons, inhibitory motor neurons, and interneurons but almost absent in sensory neurons in myenteric plexus. These suggest that STB/HAP1-ir neurons are mostly Dogiel type I neurons. Due to lack of putative STB/HAP1 protectivity, the sensory neurons (Dogiel type II) might be more vulnerable to neurodegeneration than STB/HAP1-expressing motoneurons/interneurons (Dogiel type I) in myenteric plexus.

Loss of Hap1 selectively promotes striatal degeneration in Huntington disease mice

Huntington disease (HD) is an ideal model for investigating selective neurodegeneration, as expanded polyQ repeats in the ubiquitously expressed huntingtin (HTT) cause the preferential neurodegeneration in the striatum of the HD patient brains. Here we report that adeno-associated virus (AAV) transduction-mediated depletion of Hap1, the first identified huntingtin-associated protein, in adult HD knock-in (KI) mouse brains leads to selective neuronal loss in the striatum. Further, Hap1 depletion-mediated neuronal loss via AAV transduction requires the presence of mutant HTT. Rhes, a GTPase that is enriched in the striatum and sumoylates mutant HTT to mediate neurotoxicity, binds more N-terminal HTT when Hap1 is deficient. Consistently, more soluble and sumoylated N-terminal HTT is presented in HD KI mouse striatum when HAP1 is absent. Our findings suggest that both Rhes and Hap1 as well as cellular stress contribute to the preferential neurodegeneration in HD, highlighting the involvement of multiple factors in selective neurodegeneration.

Huntingtin-Associated Protein 1 in Mouse Hypothalamus Stabilizes Glucocorticoid Receptor in Stress Response

Huntingtin-associated protein 1 (Hap1) was initially identified as a brain-enriched protein that binds to the Huntington’s disease protein, huntingtin. Unlike huntingtin that is ubiquitously expressed in the brain, Hap1 is enriched in the brain with the highest expression level in the hypothalamus. The selective enrichment of Hap1 in the hypothalamus suggests that Hap1 may play a specific role in hypothalamic function that can regulate metabolism and stress response. Here we report that Hap1 is colocalized and interacts with the glucocorticoid receptor (GR) in mouse hypothalamic neurons. Genetic depletion of Hap1 reduced the expression level of GR in the hypothalamus. Dexamethasone, a GR agonist, treatment or fasting of mice induced stress, resulting in increased expression of Hap1 in the hypothalamus. However, when Hap1 was absent, these treatments promoted GR reduction in the hypothalamus. In cultured cells, loss of Hap1 shortened the half-life of GR. These findings suggest that Hap1 stabilizes GR in the cytoplasm and that Hap1 dysfunction or deficiency may alter animal’s stress response.

Huntingtin-Associated Protein 1A Regulates Store-Operated Calcium Entry in Medium Spiny Neurons From Transgenic YAC128 Mice, a Model of Huntington's Disease

Huntington's disease (HD) is a hereditary neurodegenerative disease that is caused by polyglutamine expansion within the huntingtin (HTT) gene. One of the cellular activities that is dysregulated in HD is store-operated calcium entry (SOCE), a process by which Ca²⁺ release from the endoplasmic reticulum (ER) induces Ca²⁺ influx from the extracellular space. HTT-associated protein-1 (HAP1) is a binding partner of HTT. The aim of the present study was to examine the role of HAP1A protein in regulating SOCE in YAC128 mice, a transgenic model of HD. After Ca²⁺ depletion from the ER by the activation of inositol-(1,4,5)triphosphate receptor type 1 (IP₃R1), we detected an increase in the activity of SOC channels when HAP1 protein isoform HAP1A was overexpressed in medium spiny neurons (MSNs) from YAC128 mice. A decrease in the activity of SOC channels in YAC128 MSNs was observed when HAP1 protein was silenced. In YAC128 MSNs that overexpressed HAP1A, an increase in activity of IP₃R1 was detected while the ionomycin-sensitive ER Ca²⁺ pool decreased. 6-Bromo-N-(2-phenylethyl)-2,3,4,9-tetrahydro-1H-carbazol-1-amine hydrochloride (C₂₀H₂₂BrClN₂), identified in our previous studies as a SOCE inhibitor, restored the elevation of SOCE in YAC128 MSN cultures that overexpressed HAP1A. The IP₃ sponge also restored the elevation of SOCE and increased the release of Ca²⁺ from the ER in YAC128 MSN cultures that overexpressed HAP1A. The overexpression of HAP1A in the human neuroblastoma cell line SK-N-SH (i.e., a cellular model of HD (SK-N-SH HTT138Q)) led to the appearance of a pool of constitutively active SOC channels and an increase in the expression of STIM2 protein. Our results showed that HAP1A causes the activation of SOC channels in HD models by affecting IP₃R1 activity.

Immunohistochemical analysis of huntingtin-associated protein 1 in adult rat spinal cord and its regional relationship with androgen receptor

Huntingtin-associated protein 1 (HAP1) is a neuronal interactor with causatively polyglutamine (polyQ)-expanded huntingtin in Huntington's disease and also associated with pathologically polyQ-expanded androgen receptor (AR) in spinobulbar muscular atrophy (SBMA), being considered as a protective factor against neurodegenerative apoptosis. In normal brains, it is abundantly expressed particularly in the limbic-hypothalamic regions that tend to be spared from neurodegeneration, whereas the areas with little HAP1 expression, including the striatum, thalamus, cerebral neocortex and cerebellum, are targets in several neurodegenerative diseases. While the spinal cord is another major neurodegenerative target, HAP1-immunoreactive (ir) structures have yet to be determined there. In the current study, HAP1 expression was immunohistochemically evaluated in light and electron microscopy through the cervical, thoracic, lumbar, and sacral spinal cords of the adult male rat. Our results showed that HAP1 is specifically expressed in neurons through the spinal segments and that more than 90% of neurons expressed HAP1 in lamina I-II, lamina X, and autonomic preganglionic regions. Double-immunostaining for HAP1 and AR demonstrated that more than 80% of neurons expressed both in laminae I-II and X. In contrast, HAP1 was specifically lacking in the lamina IX motoneurons with or without AR expression. The present study first demonstrated that HAP1 is abundantly expressed in spinal neurons of the somatosensory, viscerosensory, and autonomic regions but absent in somatomotor neurons, suggesting that the spinal motoneurons are, due to lack of putative HAP1 protectivity, more vulnerable to stresses in neurodegenerative diseases than other HAP1-expressing neurons probably involved in spinal sensory and autonomic functions.

Huntingtin-associated protein-1 is a synapsin I-binding protein regulating synaptic vesicle exocytosis and synapsin I trafficking

Huntingtin-associated protein-1 (HAP1) is involved in intracellular trafficking, vesicle transport, and membrane receptor endocytosis. However, despite such diverse functions, the role of HAP1 in the synaptic vesicle (SV) cycle in nerve terminals remains unclear. Here, we report that HAP1 functions in SV exocytosis, controls total SV turnover and the speed of vesicle fusion in nerve terminals and regulates glutamate release in cortical brain slices. We found that HAP1 interacts with synapsin I, an abundant neuronal phosphoprotein that associates with SVs during neurotransmitter release and regulates synaptic plasticity and neuronal development. The interaction between HAP1 with synapsin I was confirmed by reciprocal co-immunoprecipitation of the endogenous proteins. Furthermore, HAP1 co-localizes with synapsin I in cortical neurons as discrete puncta. Interestingly, we find that synapsin I localization is specifically altered in Hap1(-/-) cortical neurons without an effect on the localization of other SV proteins. This effect on synapsin I localization was not because of changes in the levels of synapsin I or its phosphorylation status in Hap1(-/-) brains. Furthermore, fluorescence recovery after photobleaching in transfected neurons expressing enhanced green fluorescent protein-synapsin Ia demonstrates that loss of HAP1 protein inhibits synapsin I transport. Thus, we demonstrate that HAP1 regulates SV exocytosis and may do so through binding to synapsin I. The Proposed mechanism of synapsin I transport mediated by HAP1 in neurons. HAP1 interacts with synapsin I, regulating the trafficking of synapsin I containing vesicles and/or transport packets, possibly through its engagement of microtubule motors. The absence of HAP1 reduces synapsin I transport and neuronal exocytosis. These findings provide insights into the processes of neuronal trafficking and synaptic signaling.

Huntingtin-associated protein 1 regulates exocytosis, vesicle docking, readily releasable pool size and fusion pore stability in mouse chromaffin cells

Huntingtin-associated protein 1 (HAP1) was initially established as a neuronal binding partner of huntingtin, mutations in which underlie Huntington's disease. Subcellular localization and protein interaction data indicate that HAP1 may be important in vesicle trafficking and cell signalling. In this study, we establish that HAP1 is important in several steps of exocytosis in adrenal chromaffin cells. Using carbon-fibre amperometry, we measured single vesicle exocytosis in chromaffin cells obtained from HAP1(-/-) and HAP1(+/+) littermate mice. Numbers of Ca(2+)-dependent and Ca(2+)-independent full fusion events in HAP1(-/-) cells are significantly decreased compared with those in HAP1(+/+) cells. We observed no change in the frequency of 'kiss-and-run' fusion events or in Ca(2+) entry. Whereas release per full fusion event is unchanged in HAP1(-/-) cells, early fusion pore duration is prolonged, as indicated by the increased duration of pre-spike foot signals. Kiss-and-run events have a shorter duration, indicating opposing roles for HAP1 in the stabilization of the fusion pore during full fusion and transient fusion, respectively. We use electron microscopy to demonstrate a reduction in the number of vesicles docked at the plasma membrane of HAP1(-/-) cells, where membrane capacitance measurements reveal the readily releasable pool of vesicles to be reduced in size. Our study therefore illustrates that HAP1 regulates exocytosis by influencing the morphological docking of vesicles at the plasma membrane, the ability of vesicles to be released rapidly upon stimulation, and the early stages of fusion pore formation.

Huntingtin-associated protein 1 regulates postnatal neurogenesis and neurotrophin receptor sorting

Defective neurogenesis in the postnatal brain can lead to many neurological and psychiatric disorders, yet the mechanism behind postnatal neurogenesis remains to be investigated. Huntingtin-associated protein 1 (HAP1) participates in intracellular trafficking in neurons, and its absence leads to postnatal death in mice. Here, we used tamoxifen-induced (TM-induced) Cre recombination to deplete HAP1 in mice at different ages. We found that HAP1 reduction selectively affects survival and growth of postnatal mice, but not adults. Neurogenesis, but not gliogenesis, was affected in HAP1-null neurospheres and mouse brain. In the absence of HAP1, postnatal hypothalamic neurons exhibited reduced receptor tropomyosin-related kinase B (TRKB) levels and decreased survival. HAP1 stabilized the association of TRKB with the intracellular sorting protein sortilin, prevented TRKB degradation, and promoted its anterograde transport. Our findings indicate that intracellular sorting of neurotrophin receptors is critical for postnatal neurogenesis and could provide a therapeutic target for defective postnatal neurogenesis.

A huntingtin-HAP1-PCM1 pathway in ciliogenesis

Huntington's disease (HD) is caused by expansion of a polyglutamine repeat in the N-terminal region of huntingtin (htt), a large protein that has been found to interact with a variety of proteins. It remains to be determined how the interactions of htt with other proteins are involved in the pathogenesis of HD. A recent publication by Keryer et al. demonstrates that htt regulates ciliogenesis by interacting with PCM1 through HAP1. This recent study shows that htt and HAP1 are essential for protein trafficking to the centrosome, as well as normal ciliogenesis, and that mutant htt causes abnormal ciliogenesis, providing a novel insight into the pathogenesis of HD.

Characterization of the "sporadically lurking HAP1-immunoreactive (SLH) cells" in the hippocampus, with special reference to the expression of steroid receptors, GABA, and progenitor cell markers

Huntingtin-associated protein 1 (HAP1) is a neural huntingtin interactor that is widely expressed as a core molecule of the stigmoid body (a neurocytoplasmic inclusion) in the limbic and hypothalamic regions and has putative protective functions against some neurodegenerative diseases (HAP1 protection hypothesis). Although HAP1 has been reported to be intimately associated with several steroid receptors, HAP1-immunoreactive (HAP1-ir) cells remain to be identified in the hippocampus, which is one of the major steroidal targets. In this study, we determined the distribution of hippocampal HAP1-ir cells in light and fluorescence microscopy and characterized their morphological relationships with steroid receptors, markers of adult neurogenesis, and the GABAergic system in adult male and female Wistar rats. HAP1-ir cells, which were sporadically distributed particularly in the subgranular zone (SGZ) of the dentate gyrus and in the interface between the stratum lacunosum-moleculare and stratum radiatum of Ammon's horn, were identified as the "sporadically lurking HAP1-ir (SLH)" cells. The SLH cells showed no clear association with neural progenitor/proliferating or migrating cell markers of adult neurogenesis, such as Ki-67, proliferating cell nuclear antigen, doublecortin, and glial fibrillary acidic protein in the SGZ, whereas all the SLH cells expressed a neuronal specific nuclear protein (NeuN). More than 90% of the SLH cells expressed nuclear estrogen receptor (ER) α but neither ERβ nor the androgen receptor, whereas glucocorticoid receptor was differently stained in the SLH cells depending on the antibodies. More than 60% of them exhibited GABA immunoreactivity in the SGZ, suggestive of basket cells, but they were distinct from the ones expressing cholecystokinin or parvalbumin. We conclude that SLH cells, which should be stable against apoptosis due to putative HAP1 protectivity, might be involved in estrogen-dependent maturation, remodeling and activation of hippocampal memory and learning functions via ERα and partly through GABAergic regulation.

Impaired alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor trafficking and function by mutant huntingtin

Emerging evidence from studies of Huntington disease (HD) pathophysiology suggests that huntingtin (htt) and its associated protein HAP1 participate in intracellular trafficking and synaptic function. However, it is largely unknown whether AMPA receptor trafficking, which is crucial for controlling the efficacy of synaptic excitation, is affected by the mutant huntingtin with polyglutamine expansion (polyQ-htt). In this study, we found that expressing polyQ-htt in neuronal cultures significantly decreased the amplitude and frequency of AMPAR-mediated miniature excitatory postsynaptic current (mEPSC), while expressing wild-type huntingtin (WT-htt) increased mEPSC. AMPAR-mediated synaptic transmission was also impaired in a transgenic mouse model of HD expressing polyQ-htt. The effect of polyQ-htt on mEPSC was mimicked by knockdown of HAP1 and occluded by the dominant negative HAP1. Moreover, we found that huntingtin affected mESPC via a mechanism depending on the kinesin motor protein, KIF5, which controls the transport of GluR2-containing AMPARs along microtubules in dendrites. The GluR2/KIF5/HAP1 complex was disrupted and dissociated from microtubules in the HD mouse model. Together, these data suggest that AMPAR trafficking and function is impaired by mutant huntingtin, presumably due to the interference of KIF5-mediated microtubule-based transport of AMPA receptors. The diminished strength of glutamatergic transmission could contribute to the deficits in movement control and cognitive processes in HD conditions.

Selective expression of Huntingtin-associated protein 1 in {beta}-cells of the rat pancreatic islets

Huntingtin-associated protein-1 (HAP1) was initially identified as a binding partner of huntingtin, the Huntington's disease protein. Based on its preferred distribution among neurons and endocrine cells, HAP1 has been suggested to play roles in vesicular transportation in neurons and hormonal secretion of endocrine cells. Given that HAP1 is selectively expressed in the islets of rat pancreas, in this study, we analyzed the expression pattern of HAP1 in the islets. In rats injected intraperitoneally with streptozotocin, which can selectively destroy beta-cells of the pancreatic islets, the number of HAP1 immunoreactive cells was dramatically decreased and was accompanied by a parallel decrease in the number of insulin-immunoreactive cells. Immunofluorescent double staining of pancreas sections showed that, in rat islets, HAP1 is selectively expressed in the insulin-immunoreactive beta-cells but not in the glucagon-immunoreactive alpha-cells and somatostatin immunoreactive delta-cells. In isolated rat pancreatic islets, approximately 80% of cells expressed both HAP1 and insulin. Expression of HAP1 in the INS-1 rat insulinoma cell line was also demonstrated by immunofluorescent staining. Western blotting further revealed that HAP1 in both the isolated rat pancreatic islets and the INS-1 cells also has two isoforms, HAP1A and HAP1B, which are the same as those in the hypothalamus. These results demonstrated that HAP1 is selectively expressed in beta-cells of rat pancreatic islets, suggesting the involvement of HAP1 in the regulation of cellular trafficking and secretion of insulin.

Huntingtin associated protein 1 and its functions

Huntington disease (HD) is caused by a polyglutamine expansion in the protein huntingtin (Htt). Several studies suggest that Htt and huntingtin associated protein 1 (HAP1) participate in intracellular trafficking and that polyglutamine expansion affects vesicular transport. Understanding the function of HAP1 and its related proteins could help elucidate the pathogenesis of HD. The present review focuses on HAP1, which has proved to be involved in intracellular trafficking. Unlike huntingtin, which is expressed ubiquitously throughout the brain and body, HAP1 is enriched in neurons, suggesting that its dysfunction could contribute to the selective neuropathology in HD. We discuss recent evidence for the involvement of HAP1 and its binding proteins in potential functions.

Huntingtin-associated protein 1 interacts with Ahi1 to regulate cerebellar and brainstem development in mice

Joubert syndrome is an autosomal recessive disorder characterized by congenital malformation of the cerebellum and brainstem, with abnormal decussation in the brain. Mutations in the Abelson helper integration site 1 gene, which encodes the protein AHI1, have been shown to cause Joubert syndrome. In this study, we found that mouse Ahi1 formed a stable complex with huntingtin-associated protein 1 (Hap1), which is critical for neonatal development and involved in intracellular trafficking. Hap1-knockout mice showed significantly reduced Ahi1 levels, defective cerebellar development, and abnormal axonal decussation. Suppression of Ahi1 also decreased the level of Hap1; and truncated Ahi1, which corresponds to the mutations in Joubert syndrome, inhibited neurite outgrowth in neuronal culture. Reducing Hap1 expression suppressed the level and internalization of TrkB, a neurotrophic factor receptor that mediates neurogenesis and neuronal differentiation, which led to decreased TrkB signaling. These findings provide insight into the pathogenesis of Joubert syndrome and demonstrate the critical role of the Ahi1-Hap1 complex in early brain development.

A C. elegans homolog of huntingtin-associated protein 1 is expressed in chemosensory neurons and in a number of other somatic cell types

Huntingtin-associated protein 1 (HAP1) is a binding partner for huntingtin, the protein responsible for Huntington's disease. In mammals, HAP1 is mostly found in brain where it is expressed in neurons. Although several functions have been proposed for HAP1, its role has not yet been clearly established. In this paper, we report on the identification of a HAP1 Caenorhabditis elegans homolog called T27A3.1. T27A3.1 shows conservation with rat and human HAP1, as well as with Milton, a Drosophila HAP1 homolog. To determine the cellular expression of T27A3.1 (multiple isoforms; a-e), we generated several transgenic worm lines expressing a fluorescent reporter protein [green fluorescent protein (GFP) and DsRed2] under the control of the promoter for T27A3.1. We have found that T27A3.1 is expressed in many cell types including a subset of chemosensory neurons in the head and tail. These include the amphid chemosensory neurons ASKL and R, ASIL and R, ADFL and ASEL, the phasmid neurons PHBL and R, and the CAN neurons that are required for worm survival.

HAP1 can sequester a subset of TBP in cytoplasmic inclusions via specific interaction with the conserved TBP(CORE)

BACKGROUND

Huntington's disease, spinal and bulbar muscular atrophy, and spinocerebellar ataxia 17 (SCA17) are caused by expansions in the polyglutamine (polyQ) repeats in Huntingtin protein (Htt), androgen receptor protein (AR), and TATA-binding protein (TBP), respectively. Htt-associated protein 1 (HAP1), a component of neuronal cytoplasmic stigmoid bodies (STBs), can sequester polyQ-expanded Htt and AR in STBs, thereby antagonizing formation of the nuclear aggregates associated with apoptotic neuron loss and disease progression.

RESULTS

Clones of HAP1 were isolated from unbiased two-hybrid screens for proteins that interact with TBP. Domain mapping showed that regions between amino acids 157 and 261 and between amino acids 473 and 582 of mouse HAP1 both bind specifically to the conserved C-terminal TBP(CORE) domain, away from the TBP N-terminal polyQ region. When fluorescently tagged versions of HAP1 or TBP were expressed independently in COS-7, 293, or Neuro-2a cells, all TBP localized to the nucleus and all HAP1 assembled into cytoplasmic stigmoid-like bodies (STLBs). When co-expressed, a portion of the TBP was assembled into the HAP1 STLBs while the remainder was localized to the nucleus. Although the TBP N terminus, including the polyQ region, was unnecessary for TBP-HAP1 interaction, in mammalian cells, removal of the TBP Q(repeat) reduced the proportion of TBP that assembled into STLBs, whereas expansion of the Q(repeat) had no significant affect on TBP subcellular localization.

CONCLUSION

HAP1 can sequester a subset of TBP protein away from the nucleus; extranuclear TBP sequestration is quantitatively influenced by the TBP polyQ repeat. These results suggest HAP1 could provide protection from SCA17 neuropathology.

Anti-human placental antigen complex X-P2 (hPAX-P2) anti-serum recognizes C-terminus of huntingtin-associated protein 1A common to 1B as a determinant marker for the stigmoid body

The anti-serum against an unknown human placental antigen complex X-P2 (hPAX-P2) immunohistochemically recognizes three putative molecules (hPAX-P2S, hPAX-P2N, and hPAX-P2R), each of which is associated with the stigmoid bodies (STBs), necklace olfactory glomeruli (NOGs), or reticulo-filamentous structures (RFs) in the rat brain. The STBs also contain huntingtin-associated protein 1 (HAP1), and the HAP1-cDNA transfection induces STB-like inclusions in cultured cells. In order to clarify the relationship between hPAX-P2S and HAP1 isoforms (A/B), we performed Western blotting, immuno-histo/cytochemistry for light- and electron-microscopy and pre-adsorption tests with HAP1 deletion fragments. The results showed that the anti-hPAX-P2 anti-serum recognizes HAP1(474-577) of HAP1A/B in Western blotting and strongly immunostains HAP1A-induced STB-like inclusions but far weakly detects HAP1B-induced diffuse structures in HAP1-transfected HEK 293 cells. In the rat brain, immunoreactivity of the anti-hPAX-P2 anti-serum for the STBs was eliminated by pre-adsorption with HAP1(474-577), whereas no pre-adsorption with any different HAP1 fragments can suppress immunoreactivity for the NOGs and RFs, which were not immunoreactive to anti-HAP1 anti-serum. These findings indicate that hPAX-P2S, which is distinct from hPAX-P2N and hPAX-P2R, is identical with STB-constituted HAP1 and that the HAP1-induced/immunoreactive inclusions correspond to the hPAX-P2-immunoreactive STBs previously identified in the brain.

Polyglutamine expansion mutation yields a pathological epitope linked to nucleation of protein aggregate: determinant of Huntington's disease onset

Polyglutamine (polyQ) expansion mutation causes conformational, neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases. These diseases are characterized by the aggregation of misfolded proteins, such as amyloid fibrils, which are toxic to cells. Amyloid fibrils are formed by a nucleated growth polymerization reaction. Unexpectedly, the critical nucleus of polyQ aggregation was found to be a monomer, suggesting that the rate-limiting nucleation process of polyQ aggregation involves the folding of mutated protein monomers. The monoclonal antibody 1C2 selectively recognizes expanded pathogenic and aggregate-prone glutamine repeats in polyQ diseases, including Huntington's disease (HD), as well as binding to polyleucine. We have therefore assayed the in vitro and in vivo aggregation kinetics of these monomeric proteins. We found that the repeat-length-dependent differences in aggregation lag times of variable lengths of polyQ and polyleucine tracts were consistently related to the integration of the length-dependent intensity of anti-1C2 signal on soluble monomers of these proteins. Surprisingly, the correlation between the aggregation lag times of polyQ tracts and the intensity of anti-1C2 signal on soluble monomers of huntingtin precisely reflected the repeat-length dependent age-of-onset of HD patients. These data suggest that the alterations in protein surface structure due to polyQ expansion mutation in soluble monomers of the mutated proteins act as an amyloid-precursor epitope. This, in turn, leads to nucleation, a key process in protein aggregation, thereby determining HD onset. These findings provide new insight into the gain-of-function mechanisms of polyQ diseases, in which polyQ expansion leads to nucleation rather than having toxic effects on the cells.

Huntingtin-associated protein 1 (HAP1) interacts with androgen receptor (AR) and suppresses SBMA-mutant-AR-induced apoptosis

Huntingtin-associated protein 1 (HAP1), an interactor of huntingtin, has been known as an essential component of the stigmoid body (STB) and recently reported to play a protective role against neurodegeneration in Huntington's disease (HD). In the present study, subcellular association between HAP1 and androgen receptor (AR) with a long polyglutamine tract (polyQ) derived from spinal-and-bulbar-muscular-atrophy (SBMA) was examined using HEp-2 cells cotransfected with HAP1 and/or normal ARQ25, SBMA-mutant ARQ65 or deletion-mutant AR cDNAs. The results provided the first clear evidence that HAP1 interacts with AR through its ligand-binding domain in a polyQ-length-dependent manner and forms prominent inclusions sequestering polyQ-AR, and that addition of dihydrotestosterone reduces the association strength of HAP1 with ARQ25 more dramatically than that with ARQ65. Furthermore, SBMA-mutant-ARQ65-induced apoptosis was suppressed by cotransfection with HAP1. Our findings strongly suggest that HAP1/STB is relevant to polyQ-length-dependent modification on subcellular AR functions and critically involved in pathogenesis of not only HD but also SBMA as an important intrinsic neuroprotectant determining the threshold for cellular vulnerability to apoptosis. Taking together with previous reports that HAP1/STB is selectively expressed in the brain regions spared from degenerative targets in HD and SBMA, the current study might explain the region-specific occurrence of neurodegeneration in both diseases, shedding light on common aspects of their molecular pathological mechanism and yet-to-be-uncovered diagnostic or therapeutic applications for HD and SBMA patients.

Hypothalamic huntingtin-associated protein 1 as a mediator of feeding behavior

The hypothalamus responds to circulating leptin and insulin in the control of food intake and body weight. A number of neurotransmitters in the hypothalamus, including gamma-aminobutyric acid (GABA), also have key roles in feeding. Huntingtin-associated protein 1 (Hap1) is expressed more abundantly in the hypothalamus than in other brain regions, and lack of Hap1 in mice leads to early postnatal death. Hap1 is also involved in intracellular trafficking of the GABA(A) receptor. Here, we report that fasting upregulates the expression of Hap1 in the rodent hypothalamus, whereas intracerebroventricular administration of insulin downregulates Hap1 by increasing its degradation through ubiquitination. Decreasing the expression of mouse hypothalamic Hap1 by siRNA reduces the level and activity of hypothalamic GABA(A) receptors and causes a decrease in food intake and body weight. These findings provide evidence linking hypothalamic Hap1 to GABA in the stimulation of feeding and suggest that this mechanism is involved in the feeding-inhibitory actions of insulin in the brain.

Immunohistochemical localization of huntingtin-associated protein 1 in endocrine system of the rat

Huntingtin-associated protein 1 (HAP1) was originally found to be localized in neurons and is thought to play an important role in neuronal vesicular trafficking and/or organelle transport. Based on functional similarity between neuron and endocrine cell in vesicular trafficking, we examined the expression and localization of HAP1 in the rat endocrine system using immunohistochemistry. HAP1-immunoreactive cells are widely distributed in the anterior lobe of the pituitary, scattered in the wall of the thyroid follicles, or clustered in the interfollicular space of the thyroid gland, exclusively but diffusely distributed in the medullae of adrenal glands, and selectively located in the pancreas islets. HAP1-containing cells were also found in the mucosa of stomach and small intestine with a distributive pattern similar to that of gastrointestinal endocrine cells. However, no HAP1-immunoreactive cell was found in the cortex of the adrenal gland, the testis, and the ovary. In the posterior lobe of the pituitary, HAP1-immunoreactive products were not detected in the cell bodies but in many stigmoid bodies, one kind of non-membrane-bound cytoplasmic organelle with a central or eccentric electron-lucent core. HAP1-immunoreactive stigmoid bodies were also found in the cytoplasm of endocrine cells in the thyroid gland, the medullae of adrenal gland, the pancreas islets, the stomach, and small intestine. The present study demonstrates that HAP1 is selectively expressed in part of the small peptide-, protein-, and amino-acid analog and derivative-secreting endocrine cells but not in steroid hormone-secreting cells, suggesting that HAP1 is also involved in intracellular trafficking in certain types of endocrine cells.

Huntingtin-associated protein 1 (Hap1) mutant mice bypassing the early postnatal lethality are neuroanatomically normal and fertile but display growth retardation

Huntingtin-associated protein 1 (Hap1) is the first huntingtin interacting protein identified in a yeast two-hybrid screen. Although Hap1 expression has been demonstrated in neuronal and non-neuronal tissues, its molecular role is poorly understood. Recently, it has been shown that targeted disruption of Hap1 in mice results in early postnatal death as a result of depressed feeding behavior. Although this result clearly demonstrates an essential role of Hap1 in postnatal feeding, the mechanisms leading to this deficiency, as well as the role of Hap1 in adults, remain unclear. Here we show that Hap1 null mutants display suckling defects and die within the first days after birth due to starvation. Upon reduction of the litter size, some mutants survive into adulthood and display growth retardation with no apparent brain or behavioral abnormalities, suggesting that Hap1 function is essential only for early postnatal feeding behavior. Using a conditional gene repair strategy, we also show that the early lethality can be rescued if Hap1 expression is restored in neuronal cells before birth. Furthermore, no synergism was observed between Hap1 and huntingtin mutation during mouse development. Our results demonstrate that Hap1 has a fundamental role in regulating postnatal feeding in the first 2 weeks after birth and a non-essential role in the adult mouse.

HAP1 facilitates effects of mutant huntingtin on inositol 1,4,5-trisphosphate-induced Ca2+ release in primary culture of striatal medium spiny neurons

Huntington's disease is caused by polyglutamine expansion (exp) in huntingtin (Htt). Htt-associated protein-1 (HAP1) was the first identified Htt-binding partner. The type 1 inositol (1,4,5)-trisphosphate receptor (InsP3R1) is an intracellular Ca2+ release channel that plays an important role in neuronal function. Recently, we identified a InsP3R1-HAP1A-Htt ternary complex in the brain and demonstrated that Httexp, but not normal Htt, activates InsP3R1 in bilayers and facilitates InsP3R1-mediated intracellular Ca2+ release in medium spiny striatal neurons [MSN; T.-S. Tang et al. (2003) Neuron, 39, 227-239]. Here we took advantage of mice with targeted disruption of both HAP1 alleles (HAP1 -/-) to investigate the role of HAP1 in functional interactions between Htt and InsP3R1. We determined that: (i) HAP1 is expressed in the MSN; (ii) HAP1A facilitates functional effects of Htt and Htt(exp) on InsP3R1 in planar lipid bilayers; (iii) HAP1 is required for changes in MSN basal Ca2+ levels resulting from Htt or Htt(exp) overexpression; (iv) HAP1 facilitates potentiation of InsP3R1-mediated Ca2+ release by Htt(exp) in mouse MSN. Our present results indicate that HAP1 plays an important role in functional interactions between Htt and InsP3R1.

Neuroanatomical distribution of huntingtin-associated protein 1-mRNA in the male mouse brain

Huntingtin-associated protein 1 (HAP1) was identified as an interactor of the gene product (Huntingtin) responsible for Huntington's disease and found to be a core component of the stigmoid body. Even though HAP1 is highly expressed in the brain, detailed information on HAP1 distribution has not been fully described. Focusing on the neuroanatomical analysis of HAP1-mRNA expression using in situ hybridization histochemistry, the present study clarified its detailed regional distribution in the entire mouse brain. Mouse HAP1 (Hap1)-mRNAs were abundantly expressed in the limbic-related forebrain regions and midline/periventricular brainstem regions including the olfactory bulb, limbic-associated cortices, hippocampus, septum, amygdala, bed nucleus of the stria terminalis, preoptico-hypothalamic regions, central gray, raphe nuclei, locus coeruleus, parabrachial nuclei, nucleus of the solitary tract, and area postrema. In contrast, little expression was detected in the striatum and thalamus, implying that Hap1 is associated with neurodegeneration-sparing regions rather than target lesions in Huntington's disease. The distribution pattern, resembling that of the stigmoid body, suggests that HAP1 and the stigmoid body are implicated in protection from neuronal death rather than induction of neurodegeneration in Huntington's disease, and that they play an important role in integrating instinct behaviors and underlying autonomic, visceral, arousal, drive, memory, and neuroendocrinergic functions, particularly during extensive homeostatic or emotional processes. These data will provide an important morphological base for a future understanding of functions of HAP1 and the stigmoid body in the brain.

Lack of huntingtin-associated protein-1 causes neuronal death resembling hypothalamic degeneration in Huntington's disease

Huntington's disease (HD) is caused by a polyglutamine expansion in the disease protein huntingtin. The polyglutamine expansion causes huntingtin to interact abnormally with a number of proteins. However, it is unclear whether, and how, huntingtin-associated proteins are involved in the neurodegeneration in HD. Here, we show that huntingtin-associated protein-1 (HAP1), which is involved in intracellular trafficking of epidermal growth factor receptor (EGFR), is highly expressed in the hypothalamus. Mice lacking HAP1 die after birth because of depressed feeding activity. Terminal deoxynucleotidyl transferase-mediated biotinylated UTP nick end labeling staining and electron microscopic examination revealed the degeneration in hypothalamic regions that control feeding behavior. Hypothalamic degeneration was also observed in HD transgenic mice that have a significant loss of body weight. Inhibition of HAP1 expression decreases EGFR signaling and cell viability, whereas overexpression of HAP1 enhances this signaling activity and inhibits mutant huntingtin-mediated cytotoxicity. These results suggest that the effect of mutant huntingtin on HAP1 and EGFR signaling may contribute to the hypothalamic neurodegeneration and loss of body weight in HD.

Huntingtin and huntingtin-associated protein 1 influence neuronal calcium signaling mediated by inositol-(1,4,5) triphosphate receptor type 1

Huntington's disease (HD) is caused by polyglutamine expansion (exp) in huntingtin (Htt). The type 1 inositol (1,4,5)-triphosphate receptor (InsP3R1) is an intracellular calcium (Ca2+) release channel that plays an important role in neuronal function. In a yeast two-hybrid screen with the InsP3R1 carboxy terminus, we isolated Htt-associated protein-1A (HAP1A). We show that an InsP3R1-HAP1A-Htt ternary complex is formed in vitro and in vivo. In planar lipid bilayer reconstitution experiments, InsP3R1 activation by InsP3 is sensitized by Httexp, but not by normal Htt. Transfection of full-length Httexp or caspase-resistant Httexp, but not normal Htt, into medium spiny striatal neurons faciliates Ca2+ release in response to threshold concentrations of the selective mGluR1/5 agonist 3,5-DHPG. Our findings identify a novel molecular link between Htt and InsP3R1-mediated neuronal Ca2+ signaling and provide an explanation for the derangement of cytosolic Ca2+ signaling in HD patients and mouse models.

A protocol for isolation and biochemical characterization of stigmoid bodies from rat brain

Stigmoid bodies (SBs) are structures present in the cytoplasm of neurons. Many brain regions including hypothalamus, thalamus, amygdala, septum, hippocampus, colliculi, and brainstem contain neurons with at least one SB. Despite this widespread distribution their function remains unknown. SBs contain a brain protein called huntingtin-associated protein 1 (HAP1) and have more recently been found to contain the apolipoprotein E receptor LR11 (Lipoprotein Receptor containing 11 LDL binding domains, also called SorLA for sorting protein-related receptor containing LDLR class A repeats) and sortilin. To provide a first step towards further identification of their components and perhaps shed some light on their neurobiological role, we have developed a method for isolating SBs from rat brain. The protocol relies on a combination of centrifugational forces, sucrose gradient, and immunoisolation. Samples enriched in SBs were incubated with antibodies to HAP1B or to LR11 followed by incubation with FITC conjugated secondary antibodies. Anti-FITC coated beads were incubated with samples and SB-bead complexes formed were separated by magnetic sorting without pelleting the complexes during the isolation procedure. Immunopurified SBs, visualized by light and electron microscopy, show similar ultrastructure to those present in neurons.

Early degenerative changes in transgenic mice expressing mutant huntingtin involve dendritic abnormalities but no impairment of mitochondrial energy production

Mitochondrial defects, which occur in the brain of late-stage Huntington's disease (HD) patients, have been proposed to underlie the selective neuronal loss in the disease. To shed light on the possible role of mitochondrial energy impairment in the early phases of HD pathophysiology, we carried out Golgi impregnation and quantitative histochemical/biochemical studies in HD full-length cDNA transgenic mice that were symptomatic but had not developed to a stage in which neuronal loss could be documented. Golgi staining showed morphologic abnormalities that included a significant decrease in the number of dendritic spines and a thickening of proximal dendrites in striatal and cortical neurons. In contrast, measurements of mitochondrial electron transport Complexes I-IV did not reveal changes in the striatum and cerebral cortex in these mice. Examination of the neostriatum and cerebral cortex in human presymptomatic and pathological Grade 1 HD cases also showed no change in the activity of mitochondrial Complexes I-IV. These data suggest that dendritic alterations precede irreversible cell loss in HD, and that mitochondrial energy impairment is a consequence, rather than a cause, of early neuropathological changes.

Human huntingtin-associated protein (HAP-1) gene: genomic organisation and an intragenic polymorphism

The huntingtin-associated protein (HAP-1) interacts with the Huntington disease gene product, huntingtin. It is predominantly expressed in the brain and shows an increased affinity for mutant huntingtin. We have sequenced an 18,656bp genomic region encompassing the entire human HAP-1 gene and determined its genomic organisation, with 11 exons spanning 12.1kb. We have also found an intragenic polymorphism within intron 6 of HAP-1. We have recently shown that HAP-1 maps to a region of the genome which has been implicated in a variety of neurological conditions, including progressive supranuclear palsy (PSP), a late-onset atypical parkinsonian disorder. The detailed characterisation of the genomic organisation of HAP-1 and the presence of an intragenic polymorphism will be helpful in evaluating its role in different disorders, using candidate gene approaches.

Expression of huntingtin-associated protein-1 in neuronal cells implicates a role in neuritic growth

Huntingtin-associated protein-1 (HAP1) binds more tightly to mutant huntingtin, but its function and distribution in neurites are not clear. Using PC12 cells and cultured hippocampal neurons, we found that two HAP1 isoforms, HAP1-A and HAP1-B, have different subcellular localizations. While most HAP1-B is diffusely distributed within the cytoplasm, HAP1-A is enriched in the growth cones and neuritic puncta of developing neurons. In mature neurons and adult brain neurons, however, HAP1-A is concentrated in axon terminals and associated with synaptic vesicles. Transfection of HAP1-A in PC12 cells significantly promotes neurite extension, with HAP1-A distributed in the distal regions of the growing neurites. Cotransfection of mutant huntingtin with HAP1-A in PC12 cells results in the accumulation of HAP1-A on huntingtin aggregates and the inhibition of neurite promotion by HAP1-A. This study suggests that HAP1-A has a function in neuritic development and synaptic function and that mutant huntingtin may alter this function.

Expression of the Huntingtin-associated protein 1 gene in the developing and adult mouse

Huntingtin-associated protein 1 (HAP1) interacts with the product of the Huntington's disease gene. To investigate the function of Hap1 in development and in the adult mouse, we have examined the expression of Hap1 by northern analysis and in situ hybridization histochemistry. Hap1 expression is first detected in the embryonic day 8.5 (E8.5) neuroepithelium. Expression persists throughout development, predominantly in the brain and spinal cord, and to a lesser extent in enteric neurons and abdominal sympathetic ganglia. In the adult, Hap1 expression is detected not only in the brain but also in the ovary, testis, and the intermediate lobe of the pituitary.

Rapid aggregate formation of the huntingtin N-terminal fragment carrying an expanded polyglutamine tract

Huntington's disease (HD) is caused by an expansion of the CAG repeat in the HD gene. The repeat is translated to the polyglutamine tract as huntingtin, the product of HD gene. Several studies showed that the expansion of polyglutamine tract leads to formation of cytoplasminc and/or intranuclear aggregates in vivo or in vitro. To understand the molecular mechanism of the aggregate formation, we studied the transient expression of HD exon 1-GFP fusion proteins in COS-7 cells. The fusion protein carrying 77 glutamine repeats aggregated in a time-dependent manner, while the fusion protein carrying 25 glutamine tract remained to be distributed diffusely in the cytoplasm even 72 hours after transfection. Initially, fluorescent signals were diffusely distributed in the COS-7 cells that were transfected with the construct containing the 77 CAG repeats. Approximately 40 hours later after the transfection, large aggregates grew very rapidly in those cells and the diffuse cytoplasmic fluorescence faded out. This process was completed within 40 minutes from the appearance of small aggregates in the perinuclear regions. The addition of cycloheximide reduced the frequencies of aggregate formation. A possibility was discussed that the aggregate formation was via nucleation. The focal concentration of mutated proteins in neurons may trigger the aggregate formation.

Striatal transplantation in a transgenic mouse model of Huntington's disease

Striatal grafts have been proposed as a potential strategy for striatal repair in Huntington's disease, but it is unknown whether the diseased brain will compromise graft survival. A transgenic mouse line has recently been described in which hemizygotes with an expanded CAG repeat in exon 1 of the HD gene exhibit a progressive neurological phenotype similar to the motor symptoms of Huntington's disease. We have therefore evaluated the effects of the transgenic brain environment on the survival, differentiation, and function of intrastriatal striatal grafts and undertaken a preliminary analysis of the effects of the grafts on the development of neurological deficits in the host mice. Hemizygote transgenic and wild-type littermate female mice received striatal grafts at 10 weeks of age and were allowed to survive 6 weeks. Normal healthy grafts were seen to survive and differentiate within the striatum of transgenic mice in a manner comparable to that seen in control mice. The transgenic mice exhibited a progressive decline in body weight from 9 weeks of age and a progressive hypoactivity in an open field test of general locomotor behavior. Although striatal grafts exerted a statistically significant influence on several indices of this impairment, all behavioral effects were small and did not exert any clinically relevant effect on the profound neurological deficiency of the transgenic mice.

Molecular aspects of Huntington's disease

Huntington's disease (HD) is a progressive neurodegenerative disease striking principally medium spiny GABAergic neurons of the caudate nucleus of the basal ganglia. It affects about one in 10,000 individuals and is transmitted in an autosomal dominant fashion. The molecular basis of the disease is expansion of the trinucleotide CAG in the first exon of a gene on chromosome four. The CAG repeats are translated to polyglutamine repeats in the expressed protein, huntingtin. The normal function of huntingtin remains incompletely characterized, but based upon recently defined protein-protein interactions, it appears to be associated with the cytoskeleton and required for neurogenesis. Huntingtin has been demonstrated to interact with such proteins as HAP1, HIP1, microtubules, GADPH, calmodulin, and an ubiquitin-conjugating enzyme. Polyglutamine expansion alters many of these interactions and leads to huntingtin aggregation and the formation of neuronal nuclear inclusions, ultimately culminating in cell death. In this review, we discuss the molecular aspects of HD, including the present understanding of huntingtin-protein interactions, studies with transgenic mice, and postulated mechanisms of huntingtin aggregation.