<\/p>\n
Since the Huntington’s Study Group first identified the mutation<\/a> responsible for Huntington’s disease<\/a> (HD) in 1993, there have been many studies conducted seeking to understand how this defective gene<\/div> Although wild-type<\/a> huntingtin is expressed throughout the human body, it is most concentrated in the male testes as well as the neurons of the central nervous system<\/a> (especially those in the neocortex<\/a>, cerebellar cortex<\/a>, striatum<\/a> and hippocampus<\/a>). Huntingtin is a large protein<\/a> with a mass<\/a> of 347 kilo-Daltons that is made up of 3,144 amino acids. Near the beginning of its amino acid<\/a> chain, the protein<\/a> contains a repeating region of 11 to 34 glutamine<\/a> residues that are encoded from a region of CAG repeats on DNA<\/div> Studies comparing the sequence of human huntingtin to that of other organisms revealed that huntingtin appears to be quite an old protein<\/a> in evolution. The gene<\/div> Determining the present-day function of huntingtin in humans remains elusive, although progress has been made in characterizing the wild-type<\/a> protein<\/a>’s actions. Experiments that analyzed huntingtin’s sequence of amino acids, also known as its primary structure<\/a>, have found that it does not seem to share much of its sequence with other proteins in the human body. Nevertheless, several sites of interest have been found in the proteins sequence. Researchers have determined that huntingtin contains multiple regions of so-called HEAT repeats<\/a>, a sequence of ~40 amino acids named after the first four proteins in which it was discovered:H<\/strong>untingtin, E<\/strong>longation factor 3, a subunit of protein<\/a> phosphatase 2A<\/strong> and the lipid<\/a> kinase<\/a> T<\/strong>OR. Although the exact function of HEAT repeats<\/a> are currently unclear, studies have suggested that these domains play a role in a variety of interactions between proteins, including transportation in the cytoplasm<\/a> and nucleus<\/a>, microtubule<\/a> dynamics and chromosome segregation. The identification of 37 potential HEAT repeats<\/a> in huntingtin suggests that the normal function of huntingtin may involve some of these protein<\/a>-protein<\/a> interactions.<\/p>\n Further analyses of the sequence of huntingtin have revealed that the protein<\/a> contains several other regions of interest. Huntingtin has been shown to contain a functional nuclear export signal (NES)<\/a>, a sequence of amino acids that allows the protein<\/a> to travel out of the nucleus<\/a> The nucleus<\/a> of mammalian cells are separated from the cytoplasm<\/a> by the nuclear envelope<\/a>, a membrane<\/a> consisting of two lipid<\/a> layers. Small molecules can move through pores in the nuclear envelope<\/a>, but large molecules do not fit through these openings. In order for large proteins such as huntingtin to travel across this membrane<\/a>, they first need their NES bound by helper proteins known as exportin, which can then facilitate movement across the nuclear envelope<\/a> through specialized channels. This ability of huntingtin to exit the nucleus<\/a> seems to also depend on a 17 amino acid<\/a> sequence near its polyglutamine<\/a> region that interacts with a nuclear pore protein<\/a> on the nuclear envelope<\/a>. Removing this sequence causes huntingtin to accumulate in the nucleus<\/a>.<\/p>\n The NES in human huntingtin has been shown to be highly conserved in the huntingtin of other vertebrate<\/a> species, with only minor changes in the NES sequence that do not significantly change the function of this domain<\/a>. Generally, the more conserved a sequence of DNA<\/div> Huntingtin also contains several sites are that are known to be targeted by caspases, proteins that cleave<\/a> other proteins into smaller fragments. Experiments have observed that both wild-type<\/a> and mutant huntingtin<\/a> are cleaved by caspases, although the mutant protein<\/a> seems to be more susceptible to cleavage and its fragments are more likely to be found in the cytoplasm<\/a> and nucleus<\/a>. There is now strong evidence that the fragments resulting from the cleavage of mutant huntingtin<\/a> are key to the progression of HD (For more information on the significance of huntingtin cleavage in HD, click here<\/a>). However, the importance of cleaving in wild-type<\/a> huntingtin has yet to be determined.<\/p>\n The primary structure<\/a> of huntingtin gives some clues to its three-dimensional structure. Analyses of other proteins with HEAT repeats<\/a> show that these regions form alpha-helices, coiled cylindrical structures that are hydrophobic<\/a>, or repel water. For proteins with HEAT repeats<\/a>, over 50% of their final structures have been shown to be made up of alpha-helices, which tend to form superhelices<\/a>, helices that are themselves twisted into helices. Because of the way that they are arranged, superhelices<\/a> have a core that is very hydrophobic<\/a>. An experimental study that examined copies of laboratory-engineered huntingtin found that the folded protein<\/a>’s structure is dominated by alpha-helices that stack to form a superhelix with a hydrophobic<\/a> core. Indeed, the authors of this study suggest that the folded structure of huntingtin is a long superhelix in the shape of a solenoid.<\/p>\n Analyses of the sequence and structure of huntingtin have provided hints to the protein<\/a>’s functions in the human body. These findings are supported by various experimental studies that directly tested for huntingtin’s function and determined that huntingtin is involved in a number of important processes at both the cellular<\/a> and the organismal level. These processes include transport of molecules within a cell<\/a> (intracellular<\/a> transport), regulation of transcription<\/a>, inhibition<\/a> of programmed cell<\/a> death (apoptosis<\/a>), and embryonic development. These functions will be discussed in detail below.<\/p>\n Huntingtin’s many roles in intracellular<\/a> transport have been well-documented in scientific research (intra = “within” so intracellular<\/a> literally means “within cells”). Experiments have consistently observed that huntingtin is involved in the intracellular<\/a> trafficking of vesicles<\/a>, spherical containers made up of lipids that transport molecules around the cell<\/a>. One mechanism for this action appears to be huntingtin’s interaction with microtubules, the rod-like components of the cytoskeleton<\/a> that aid in the transport of vesicles<\/a>. Microtubules are made up of stacks of proteins called tubulin<\/a>, which constantly add on to and fall off of the ends of these stacks to lengthen or contract the length of these structures as necessary. This dynamic growth and dispersion is necessary for many important cell<\/a> processes, such as the separation of chromosomes<\/a> during mitosis<\/a> (For more information about the role of microtubules in dividing cells, click here<\/a>). Microtubules also provide the scaffolding for specialized molecules known as the microtubule motor proteins<\/a> to move across the cytoplasm<\/a>. These motor proteins can be thought of as cellular<\/a> deliverymen, moving along the microtubule<\/a> highways to carry their vesicle cargo from one place in the cell<\/a> to another.<\/p>\n Huntingtin has been shown to interact with a variety of different components involved in the action of microtubules, including tubulin<\/a> and dynein<\/a>, a microtubule<\/a> motor protein<\/a> that usually moves towards the center of cells. Mapping studies that examined the sequence of huntingtin found a binding site for dynein<\/a>, suggesting that the two proteins can interact within cells. This finding has been corroborated by evidence that huntingtin forms complexes with dynein<\/a> in cell<\/a> extracts from mouse neurons. In addition to the direct action of huntingtin, its binding partners have also been proposed to affect microtubule<\/a> dynamics. Huntingtin-associated protein<\/a> 1 (HAP1) is one such partner that interacts with dynactin<\/a>, a protein<\/a> that is essential<\/a> for dynein<\/a> function, and kinesin<\/a>, a microtubule<\/a> motor protein<\/a> that usually moves towards the cell<\/a> periphery (the opposite direction of dynein<\/a> along microtubules). Indeed, both huntingtin and HAP1 are transported in axons at a speed consistent with the hypothesis<\/a> that they move along microtubules with microtubule motor proteins<\/a>. In addition, huntingtin and HAP1 are capable of moving towards both the cell<\/a> nucleus<\/a> and the periphery, as would be expected if these proteins associate with dynein<\/a> and kinesin<\/a>. The direction in which huntingtin moves depends on the signals it receives from other interacting proteins. Based on observations such as these, scientists have suggested that huntingtin acts as a molecular scaffold, allowing different proteins to come together and interact.<\/p>\n Huntingtin’s interactions with microtubules, dynein<\/a>, and other elements of intracellular<\/a> transport can help explain its effect on vesicle trafficking. For example, huntingtin appears to be important in the intracellular<\/a> transport of brain-derived neuronal factor (BDNF<\/a>), a neurotrophic factor<\/a> important for striatal cells to develop and survive (For more information on the relationship between HD and BDNF<\/a>, click here<\/a>). BDNF<\/a> is initially produced by neurons in the cortex<\/a> and substantia nigra<\/a> and then moved to the striatum<\/a>, the major site of neurodegeneration<\/a> in HD. Like all secreted proteins, BDNF<\/a> must first be synthesized within cells before being released. After BDNF<\/a> is translated from its messenger RNA<\/div> Although the transport of BDNF<\/a> is a well-studied example, huntingtin has been shown to affect a broad range of the cellular<\/a> machinery involved in intracellular<\/a> trafficking. For example, one study found that the knockdown of huntingtin in zebrafish<\/a>, a popular model organism<\/a> in scientific experimentation, caused them to become iron<\/a> deficient. Although zebrafish<\/a> embryos with decreased levels of huntingtin could take in iron<\/a>, their cells were incapable of transporting iron<\/a> to where it was needed. Future studies investigating the role of huntingtin in intracellular<\/a> trafficking and transport are likely to be important in understanding the molecular mechanisms that underlie HD.<\/p>\n Huntingtin appears to help regulate transcription<\/a> by acting as part of an on\/off switch that tells cells when to begin expressing certain genes. One mechanism through which huntingtin regulates transcription<\/a> is by binding to transcription<\/a> factors, proteins that regulate the transcription<\/a> of DNA<\/div> Huntingtin’s role in regulating transcription<\/a> is consistent with its ability to shuttle between the nucleus<\/a> and the cytoplasm<\/a>. Although huntingtin was once thought to be present only in the cytoplasm<\/a> of neurons, both the full-length protein<\/a> and its fratments have been detected in the nucleus<\/a>. Since huntingtin is found in the nucleus<\/a>, it must be able to cross the nuclear envelope<\/a> that separates the nucleus<\/a> from the cytoplasm<\/a>. Most large proteins that enter the nucleus<\/a> have a nuclear localization signal<\/a> (NLS), which has the opposite function as NES and enables proteins to enter the nucleus<\/a> (see above section for an explanation). However, analyses of huntingtin’s structure found that it does not contain a classical NLS, suggesting that the protein<\/a> has an alternative means of crossing the nuclear envelope<\/a>. The ability of huntingtin to both enter and exit the nucleus<\/a> has led researchers to suggest that huntingtin is involved in a complex of proteins that shuttle molecules between the nucleus<\/a> and the cytoplasm<\/a>. Scientists hypothesize that this shuttling may be involved in regulating which transcription<\/a> factors are allowed into the nucleus<\/a> at certain times. However, as most studies have focused on the effects of mutant huntingtin<\/a> on transcription<\/a>, this role of wild-type<\/a> huntingtin remains speculative.<\/p>\n A role of huntingtin in neuroprotection<\/a> was suggested by the finding that turning off the gene<\/div> While huntingtin’s various cellular<\/a> functions are usually attributed to its actions in neurons, the protein<\/a> appears to also be essential<\/a> for the development of embryos. Studies that have knocked out the gene<\/div> Given huntingtin’s many important functions in the adult brain, scientists have suggested that wild-type<\/a> huntingtin may affect the development and progression of HD. This hypothesis<\/a> is supported by studies that have observed that humans and mice that are homozygous<\/a> for the CAG expansion<\/a> (and thus do not produce any wild-type<\/a> huntingtin) appear to have a more severe disease progression. These observations have been supported by experiments that have manipulated the levels of huntingtin in animal models<\/a> of HD. One study used a strain of HD mice with a mutant huntingtin<\/a> gene<\/div> There is also evidence suggesting that over-expressing wild-type<\/a> huntingtin may be beneficial in HD. Experiments examining the effects of mutant huntingtin<\/a> in mice testes found that adding wild-type<\/a> huntingtin reduced the extent of cell<\/a> death usually seen in these HD mice. Similar results have been observed in the central nervous system<\/a> of HD mice that were genetically engineered to also over-express wild-type<\/a> huntingtin. However, the benefits of increased levels of wild-type<\/a> huntingtin were deemed to be modest, suggesting that simply adding more of the wild-type<\/a> protein<\/a> will not be sufficient in treating HD.<\/p>\n Although there is still considerable uncertainty about the functions of wild-type<\/a> huntingtin, scientists have identified several important roles that the protein<\/a> plays in the cell<\/a> as well as in the body as a whole. Further research into wild-type<\/a> huntingtin would be valuable not only because it can give a better picture of what goes wrong in the mutant version of the protein<\/a>, but may even offer clues for developing future treatments and cures.<\/p>\n Notes:<\/strong><\/p>\n 1) Unless otherwise specified, when this article uses the term “huntingtin” it is referring to the wild-type<\/a> form. The mutated form of this protein<\/a> is explicitly referred to as “mutant huntingtin<\/a>.”<\/p>\n 2) In other articles, HOPES has opted to refer to the gene<\/div> Borrell-Pagès, M. et al. (2006). Huntington’s disease<\/a>: from huntingtin function and dysfunction to therapeutic strategies. Cellular<\/a> and Molecular Life Sciences<\/strong> 63: 2642-2660.<\/p>\n In this review article, the authors summarize the functions of wild-type<\/a> huntingtin and what goes wrong when it is mutated in HD. The writing is very technical.<\/em><\/p>\n Cattaneo, E. et al. (2005). Normal huntingtin<\/a> function: An alternative approach to Huntington’s disease<\/a>. Nature Reviews Neuroscience<\/strong> 6: 919-930.<\/p>\n This comprehensive review summarizes the different functions of wild-type<\/a> huntingtin. While it does get technical when talking about specific experiments, the article is generally quite readable.<\/em><\/p>\n Caviston, J. and E. Holzbaur (2009). Huntingtin is an essential<\/a> integrator of intracellular<\/a> vesicular trafficking. Trends in Cell<\/a> Biology<\/strong> 19(4): 147-155.<\/p>\n In this technical review article, the authors discuss the importance of huntingtin as a “scaffold protein<\/a>” that is needed in the intrascellular transport of vesicles<\/a>.<\/em><\/p>\n Colin, E. et al. (2008). Huntingtin phosphorylation<\/a> acts as a molecular switch for anterograde\/retrograde transport in neurons. The EMBO Journal<\/strong> 27: 2124-2134.<\/p>\n In this very technical primary source article, the authors provide evidence that huntingtin affects the movement of vesicles<\/a> in neurons by directly interacting with various components of the transport machinery.<\/em><\/p>\n Gauthier et al. (2004). Huntingtin controls neurotrophic support and survival of neurons by enhancing BDNF<\/a> vesicular transport along microtubules. Cell<\/a> <\/strong>118: 127-138.<\/p>\n This study presents convincing evidence that huntingtin affects the transport of BDNF<\/a> along microtubules. As a primary source article, the subject matter is very specific and the language is quite technical.<\/em><\/p>\n Gissi et al. (2006). Huntingtin gene<\/div> This primary source article provides a detailed (and technical) genomic analysis of the evolution of the huntingtin gene<\/div> Harjes, P. and E. Wanker (2003). The hunt for huntingtin function: interaction partners tell many different stories. Trends in Biochemical Sciences <\/strong>28(8): 425-433.<\/p>\n In this review article, the authors present the evidence suggesting that huntingtin interacts with many other proteins to execute its cellular<\/a> functions. The language is very technical.<\/em><\/p>\n Imarisio, S. et al. (2008). Huntington’s disease<\/a>: from pathology<\/a> and genetics<\/a> to potential therapies. Biochemical Journal<\/strong> 412: 191-209.<\/p>\n In this technical but readable article, the authors review the role of wild-type<\/a> huntingtin and discusses possible molecular mechanisms for HD.<\/em><\/p>\n Li et al. (2006). Expression and characterization of full-length huntingtin, an elongated HEAT repeat protein<\/a>. The Journal of Biological Chemistry<\/strong> 281: 15916-15922.<\/p>\n This primary source article presents evidence that huntingtin’s HEAT repeat regions causes it to fold into a structure dominated by alpha-helixes that form a superhelix.<\/em><\/p>\n Sandou, F. and S. Humbert (2009). Huntington’s disease<\/a>: Function and dysfunction of huntingtin in axonal transport. In P. St. George-Hyslop et al. (eds.) Intracellular<\/a> Traffic and Neurodegenerative<\/a> Disorders. <\/strong>115-123.<\/p>\n This chapter of a very specific and technical book provides evidence that huntingtin affects the transport of vesicles<\/a> in axons.<\/em><\/p>\n Truant, R. et al. (2007). Nucleocytoplasmic trafficking and transcription<\/a> effects of huntingtin in Huntington’s disease<\/a>. Progress in Neurobiology<\/strong> 83(4): 211-277.<\/p>\n In this technical review article, the authors discuss the evidence suggesting that huntingtin is involved in regulating transcription<\/a>.<\/em><\/p>\n Xia, J. et al. (2003). Huntingtin contains a highly conserved nuclear export signal<\/a>. Human Molecular Genetics<\/a> <\/strong>12(12): 1393-1403.<\/p>\n The authors of this primary source article presents evidence for a nuclear export signal<\/a> in huntingtin that is highly conserved in evolution.<\/em><\/p>\n Zuccato, C. et al. (2010). Molecular mechanisms and potential therapeutic targets in Huntington’s disease<\/a>. Physiological<\/a> Reviews<\/strong> 90(3): 905-981.<\/p>\n This recent review gives a nice overview about the molecular biology of HD, including the genetics<\/a> of model organisms, the function of wild-type<\/a> huntingtin, the mechanisms of neurodegeneration<\/a>, and potential targets for therapies. Even with this breadth, the article is quite technical.<\/em><\/p>\n Yuan, J. and B. Yanker (2000). Apoptosis<\/a> in the nervous system<\/a>. Nature <\/strong>407: 802-809.<\/p>\n<\/a>Sequence and Structure^<\/a><\/h2>\n
![\"3](\"http:\/\/web.stanford.edu\/group\/hopes\/cgi-bin\/hopes_test\/wp-content\/uploads\/2011\/07\/3-exportin-revised-1024x574.jpg\")
<\/a><\/p>\n<\/a>Potential Functions^<\/a><\/h2>\n
<\/a>Intracellular transport^<\/a><\/h3>\n
![\"vesicle_transport\"](\"http:\/\/web.stanford.edu\/group\/hopes\/cgi-bin\/hopes_test\/wp-content\/uploads\/2011\/07\/vesicle_transport-1024x791.jpg\")
<\/a><\/p>\n<\/a>Transcriptional regulation<\/strong>^<\/a><\/h3>\n
<\/a>Inhibiting apoptosis<\/strong>^<\/a><\/h3>\n
<\/a>Embryonic development <\/strong>^<\/a><\/h3>\n
<\/a>Wild-Type Huntingtin and HD<\/strong>^<\/a><\/h2>\n
![\"2](\"http:\/\/web.stanford.edu\/group\/hopes\/cgi-bin\/hopes_test\/wp-content\/uploads\/2011\/07\/2-mouse-model-edited.jpg\")
<\/a><\/p>\n<\/a>Sources<\/strong>:^<\/a><\/h2>\n