ng more than 40 accounts for full penetrance, and the polyQ length partially links to the onset of the condition. The mean age of onset is 40 years; an early-onset, juvenile form can be manifested with a polyQ of 75 or more repeats. Death occurs in ~20 years after onset. HTT is essential during embryonic development and is required for both the developing and adult organism. It is constitutively expressed, found in brain areas that degenerate in HD and others that are unaffected, as well as in other parts of the body. HTT is subject to many post-translation modifications and numerous interacting partners, particularly at its N-terminus. Mostly cytoplasmic, it has other cellular localizations. The molecular functions of HTT are derived from the functional landscape of interactors, and the processes and pathways impaired in HD. Given the ubiquitous expression pattern, its role in HD is rather puzzling. As in other neurodegenerative diseases, HD exhibits selective loss of neurons and the presence of protein aggregates in inclusion bodies. In HD, the caudate-putamen striatal neurons are particularly vulnerable; other areas, such as the projecting cortical neurons are also affected and there is a decrease in size. mHTT is found in inclusion bodies; in particular, intranuclear inclusions containing toxic N-terminal fragments. The caudate-putamen in the dorsal striatum receives excitatory glutamatergic inputs from thalamic and cortical areas and modulatory dopaminergic inputs from the substantia nigra pars compacta (SNc). It is largely composed of GABAergic medium spiny neurons (MSNs), also known as spiny projection neurons (SPNs). Striatal SPNs consist of two populations: one projects directly to output nuclei in the substantia nigra pars reticulata (SNr)/globus pallidus internal (GPi) and expresses the D1-like dopamine receptor type - the direct 'go' pathway, promoting action selection (D1 SPNs); the other, which expresses the D2-like dopamine receptor type, projects indirectly via the globus pallidus external (GPe) and subthalamic nucleus (STN) - the indirect 'no-go' pathway, with the opposite effect (D2 SPNs). These striatal neurons are selectively lost in HD; those in the indirect pathway appear to be particularly vulnerable and the first to be dysfunctional.

Huntingtin (HTT) and mutant huntingtin (mHTT)
Huntingtin (HTT) is a large gene comprising 67 exons translating into a standard protein of 3,144 amino acids. It has two mRNA transcripts, and more recently it has been shown that it is subject to alternative splicing. The polyQ stretch, preceded by a stretch of 17 amino acids and followed by a stretch of prolines (proline-rich domain PRD), resides in exon 1. The N-terminal 17 amino acid stretch folds into an amphipathic helix, mediates membrane association, has multiple modification sites and may play a role in the aggregation of HTT. The PRD may serve as a mediator of protein-protein interactions. PolyQ can adopt several conformations, with the toxic polyQ likely harboring beta-sheet features and a proneness to aggregate/form fibrillar structures. The rest of HTT (exon2-67) consists of HEAT repeats, antiparallel alpha helices separated by a non-helical region, clustering within fewer larger domains separated by disordered regions. HEATs act as scaffolds for numerous interactions. PEST sequences, largely within disordered regions, contain proteolytic sites; both wild-type and mHTT can be subject to proteolysis. Of note are the toxic N-terminal mHTT fragments in intranuclear inclusions. It is generally agreed that large(r) aggregates may be neuroprotective. HTT is subject to many types of post-translational modifications (PTM) - phosphorylation, acetylation, palmitoylation, SUMOylation, ubiquitination, and likely the reverse modifications; most have been studied in the context of mHTT. SUMOylation of mHTT by Rhes/RASD2 (Ras homolog enriched in striatum), decreases aggregation and increases cytotoxicity. Rhes preferentially binds mHTT and the lysine substrates within the N-terminal 17aa region (K6, K9, K15), are also ubiquitination (Ub) sites. Phosphorylation of serines within this stretch (S13, S16) may impact on SUMO/Ub and play a role in pathogenicity. Cleavage at position 586, likely by caspase6 (Casp6), is thought to generate toxic mHTT fragments. HTT can establish various intra-molecular interactions, yielding a range of conformers. Recent work indicates that HTT adopts a spherical alpha-helical solenoid structure whose termini fold over a central cavity. The length of polyQ does not affect the core structure but it impacts on the intramolecular interactions and folding of termini. Though subtle, such changes could have far-reaching impacts on HTT structural propensities and functional attributes, possibly underlying the dominant effect of mHTT. HTT is thought to act as a scaffold; its size and the range of interactions support this view. It is on the basis of these interactions that possible HTT functions have been hypothesized. They include cellular trafficking and cellular homeostasis, autophagy, transcription, mitochondria function, metabolism and signaling. HTT and mHTT have been shown to differentially couple to a large set of interactors in striatal cells; the bias in preferences reflected in distinct functions/pathways as mHTT 'favored' interactors associate with mitochondrial function and cell death.

HD, mHTT and brain-derived neurotrophic factor (BDNF)
Brain-derived neurotrophic factor (BDNF) is essential for the proper development and function of striatal neurons. BDNF signaling through the Ntrk2 (TrkB) receptor prompts several intracellular pathways that include Raf/Mek/Erk, PI3k-AKt and protein kinase C signaling, and calcium-dependent events. The striatum does not express BDNF and relies on its anterograde transport for delivery. HTT plays a role in both the expression and transport of BDNF. It sequesters Rest (REST/NRSF) transcriptional repressor in the cytoplasm and promotes BDNF transcription; via interaction with HAP1 (huntingtin-associated-protein-1) and its recruitment of Dctn1, the largest dynactin subunit also known as p150^glued, it promotes BDNF vesicular trafficking. In the context of mHTT, BDNF expression and transport are inhibited or diminished. Although the mechanisms of mHTT inhibitory effects are not well understood, differential interactions could play a role. As mentioned above, there is differential coupling of HTT and mHTT to interacting partners and there are subtle, yet globally impacting, structural changes which are polyQ length-dependent. HTT may also promote the retrograde transport of Ntrk2/TrkB. Ntrk2 is preferentially expressed in the striatal population of the indirect pathway, which might explain why these neurons are particularly vulnerable in HD. Deletion of BDNF or its receptor leads to loss of striatal neurons, and selectively, of those of the indirect pathway (D2 SPNs).

HD, mHTT and Ras homolog enriched in striatum (RASD2, known as RHES)
RASD2 GTP-binding protein can interact with the alpha and beta subunits of heterotrimeric G proteins suggesting a possible role in modulating signaling via G protein-coupled receptors (GPCR). It appears to inhibit the activation of adenylyl cyclase (AC) and cAMP production, whether by interfering with the Galphas stimulation of AC or by activating the Galphai/o inhibitory effect (not shown). This may be particularly interesting as the two populations of SPNs - D1 SPNs (the direct 'go; pathway) and D2 SPNs (the indirect 'no-go' pathway), so named because of the selective expression of D1-like and D2-like dopamine receptors, couple to Galphas and Galphai signaling, respectively. RASD2 can bind HTT, but has a much higher affinity for mHTT. It has been postulated that the tight binding could sequester RASD2 and interfere with its function. RASD2 promotes mHTT SUMOylation leading to mHTT dispersal and cytotoxicity. RASD2 is reported to promote activation of the mTOR pathway; inhibition of mTOR signaling via RASD2 sequestration could add another dimension to mHTT mediated toxicity in the striatum.

HD, mHTT and mitochondria dynamics and function
The fission and fusion of mitochondria are features of mitochondria dynamics along with their transport. Together, they assure the provision of an adequate mitochondria pool, as dictated by the metabolic needs of the cells, the growth of the mitochondria network and/or the removal of defective organelles. Fission is thought to occur both in a symmetric fashion that allows mitochondria to divide and grow/expand or asymmetrically, promoting isolation of defective mitochondria. Fission may occur shortly after a fusion event (in transient fusion) to allow segregation of mitochondria with healthy membrane potential from those with less than optimal membranes. The main players in both the fission and the fusion pathways are dynamin-related GTPases, aided by a selected assortment of other proteins that could act as receptors/scaffolds, or adaptors/effectors. mHTT interacts with Dnml1 GTPase, the main effector of mitochondria fission. In HD-affected neurons, there are elevated levels of Dnml1 (known as Drp1), increased Dnml1 enzymatic activity, increased fission and decreased fusion. Other mitochondria-related alterations in HD include decreased activity of several complexes of the electron transport chain (ET) pathway and aberrant ATP/ADP levels, defective mitochondria transport and calcium buffering capacity.

HD, mHTT and autophagy
HTT is likely to function in autophagy, due to its scaffold size and role in trafficking. Autophagy is a principal mediator of clearance of damaged organelles and aggregated proteins. It involves the sequestration of cargo within autophagosomes and cargo delivery to lysosomes for degradation. The formation of autophagosomes in HD is maintained, if not increased, but these structures are devoid of cargo. The mechanisms whereby cargo loading is impaired are not known, but such defects lead to accumulation of toxic material. HTT interacts with Ulk1 autophagy protein and Sqstm1/p62 mitophagy (mitochondrial autophagy) protein. HTT is involved in trafficking of autophagosomes, which in neurons undergo retrograde transport to cell bodies and along the route, mature into degradative autolysosomes. mHTT interferes with HTT trafficking function, further adding to the impaired autophagy in neuronal cells. It is thought that HTT may have evolved from autophagy proteins, such as the yeast Atg proteins. These proteins contain LC3-interacting repeats (LIR), also present in mammalian autophagy receptors; HTT contains 11 predicted LIR motifs.

HD, mHTT and epigenetics
mHTT physically interacts with Crebbp and interferes with its function. The transcriptional regulatory Crebbp acts as a repressor or as a co-activator, and also has histone acetylase (HAT) activity. Sequestration of Crebbp by mHTT leads to transcriptional deregulation. It also affects chromatin modification with histone hypo-acetylation and hypermethylation, further adding to the transcriptional deregulation. Acetylation of histones weakens their interaction with DNA leading to a more permissive transcriptional state. Mono-, di- and tri-methylation of histones is probably the most extensive modification type which provides an essential epigenetic mark. It is believed that Crebbp represses the expression of Setdb1 methyltransferase to maintain adequate levels of tri-methylated histone 3 at the K9 position (H3K9me3). Sequestration of Crebbp would lead to elevated levels of Setdb1 and H3K9 hypermethylation. This is indeed observed in the striatal neurons of HD patients and HD animal models. Non-coding RNA - microRNA (miRNA) and long non-coding RNA (lncRNA), have been shown to be dysregulated in models of HD and HD patients. miRNAs post-transcriptionally repress target genes while lncRNAs exert a broader regulatory role on gene expression.Down- and up-regulation in the expression of several miRNAs and lncRNAs are reported.

HD, mHHT and kynurenine metabolism
Metabolism of kynurenine, a product of L-tryptophan degradation, gives rise to distinct classes of metabolites. Downstream of kynurenine 3-monooxygenase (KMO), the three products - 3-hydroxykynurenine (3HK), 3-hydroxyanthranilic acid (3-HANA), and quinolinic acid (QUIN), can exert neurotoxic effects. On the other hand, kynurenic acid (KYNA), metabolized by kynurenine aminotransferase (KAT), has antioxidant effects. Increased levels of QUIN and decreased levels of KYNA are observed in the brains of HD patients. QUIN can selectively activate NMDA receptors and striatal injection of QUIN into rodent brains is excitotoxic. The two arms of kynurenine metabolism take place in different populations as the KMO and KAT enzymes are expressed in microglia and astrocytes, respectively. Microglia are primary mediators of immune responses in the nervous system; neuroinflammatory responses could activate microglia, lead to QUIN overproduction and overactivation of NMDA receptors. Immune activation and enhanced expression of inflammatory proteins are observed in HD. Microglia activation links to loss of striatal neurons and the severity of HD.

HD, mHTT, and glutamate, NMDA receptors and excitotoxicity
Excitotoxicity in SPNs is thought to be attributable to the ionotropic N-methyl-D-aspartic acid (NMDA) glutamatergic receptors (NMDAR). The excitatory glutamate activates many ionotropic and metabotropic receptors, but the NMDAR may be those involved in excitotoxicity due to their high Ca2+ permeability and slow deactivation and desensitization. The NMDAR are tetrameric complexes largely formed by two Grin1 (GLUN1) and two Grin2 (GLUN2) subunits, mostly Grin2a and Grin2b isoforms. Grin2b is more expressed in striatal SNPs than in other parts of the brain and Grin2b-containing NMDAR contribute more to total NMDA-evoked current in D2 SPNs than in D1 SPNs. In addition, Grin2b, rather than Grin2a, is found extrasynaptically. Synaptic NMDA signaling promotes cell survival while extrasynaptic signaling promotes cell death via activation or inactivation of Creb, respectively. The cAMP-responsive element binding protein (CREB) is a master transcriptional regulator. It may be the extrasynaptic redistribution of Grin2b that promotes the toxic effect, rather than an actual increase in extracellular glutamate. And, as in other instances, it is the differential interaction of mHTT with scaffolding proteins such as Dlg4 (PSD-95) or with Zdhhc17 (HIP14) palmitoyl transferase (palmitoylation of PSD-95 and Grin2b regulates their trafficking) that may play a role in the redistribution/increased stability of extrasynaptic Grin2b. HTT itself is a substrate of Zdhhc17 and in turn, modulates Zdhhc17 auto-palmitoylation.

HD, mHTT and loss of striatal SPNs neurons
Striatal SPNs in the caudate-putamen are selectively lost in HD with the D2 population being particularly vulnerable. What makes these neurons prone to degeneration in the context of mHTT? Like other proteins associated with neurodegenerative diseases, mHTT tends to aggregate, with the expanded polyQ stretch likely to exhibit beta-sheet features and a readiness to aggregate/form fibrillar structures. Yet, the exact role of aggregates in HD toxicity, particularly the larger ones, is far from clear. Results indicate that the shorter, N-terminal polyQ-bearing fragments are the more toxic elements. The length of polyQ induces subtle but widespread alterations in the HTT structure and its intramolecular interactions, such changes being length-dependent. While the details of these changes at the atomic level are not known, it is possible that they impact on the many interactions the protein establishes and the post-translational modifications to which it is subject. Normal versus expanded polyQs differentially couple to many interactors, which play important roles in pathways and processes of relevance not only to normal cellular homeostasis but specifically to SPN cells in general and those in the D2 population in particular.

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Note that disruption(s) in pathways/processes described are in the context of mHTT and not the result of defective components within....(less)