The huntingtin (HTT) protein is found in cells throughout the human body, with its presence most pronounced in the brain. Here, it plays a fundamental role in the health and function of neurons. The HTT gene is considered essential for mammalian development. The wild-type version of HTT is a scaffolding protein, serving as a central hub that interacts with hundreds of other cellular components to regulate numerous biological processes, particularly within the nervous system.
The Structure and Location of Huntingtin
The HTT gene is located on the short arm of chromosome 4. It codes for one of the largest known proteins, typically consisting of 3,144 amino acids and weighing approximately 350 kilodaltons. The three-dimensional structure of HTT is predominantly alpha-helical, organized into repeating units known as HEAT repeats. These repeats form a solenoid-like shape and allow HTT to act as a scaffold for protein-protein interactions.
A key feature is a trinucleotide repeat sequence of cytosine-adenine-guanine (CAG) located near the protein’s N-terminus. This sequence codes for a variable stretch of the amino acid glutamine, referred to as the polyglutamine (polyQ) tract. In healthy individuals, this polyQ tract is typically short, ranging from about seven to 35 repeats. HTT concentration is highest in the brain, especially in the neurons of the cortex and the striatum.
Inside the cell, HTT is found across several compartments, including the cytoplasm, the nucleus, and the mitochondria. It is frequently associated with cellular transport systems, specifically microtubules and various vesicles. This localization reflects its diverse roles in moving materials within the cell and regulating cellular signaling.
Essential Roles in Healthy Cells
The wild-type huntingtin protein performs functions essential for neuronal survival and normal brain activity. One primary role is coordinating long-distance transport within the neuron, known as axonal transport. HTT acts as a linker, stabilizing cellular cargoes—such as vesicles and organelles—onto motor proteins like dynein and kinesin. This ensures necessary materials, including proteins and lipids, are delivered efficiently to distant synapses.
HTT also acts as a neuroprotective agent, maintaining the health and survival of neurons. It regulates the transcription of key survival factors, most notably Brain-Derived Neurotrophic Factor (BDNF). Wild-type HTT binds to the transcriptional repressor protein REST/NRSF, sequestering it in the cytoplasm. This prevents REST/NRSF from entering the nucleus and shutting down the production of BDNF and other genes necessary for neuronal function.
HTT is integral to synaptic function. It is involved in endocytosis, the mechanism cells use to internalize molecules. HTT is also thought to assist in anchoring receptors at the postsynaptic membrane, the receiving side of the synapse. The loss of this normal function can severely impair a neuron’s ability to communicate and maintain its energy balance.
The Pathological Transformation
The huntingtin protein becomes pathological when the polyglutamine (polyQ) repeat sequence at its N-terminus expands beyond a normal length. This expansion is encoded by a corresponding increase in the CAG repeats in the HTT gene. When the polyQ tract exceeds approximately 35 to 40 repeats, the protein adopts a toxic configuration. The longer the polyQ tract, the greater the toxicity, and the earlier the protein begins to cause cellular damage.
The expanded polyQ tract promotes a change in the protein’s shape, causing it to misfold and become structurally unstable. The misfolded protein, known as mutant huntingtin (mHTT), gains a harmful property that disrupts normal cellular processes. This “gain of toxic function” is the primary mechanism of cellular damage, causing the mutant protein to self-associate and aggregate into insoluble clumps. These aggregates, sometimes called inclusion bodies, are found in the cytoplasm and within the neuron’s nucleus.
The presence of mHTT also actively interferes with the function of the healthy wild-type HTT. The mutant protein sequesters many of the crucial binding partners that the normal protein needs to carry out its protective and transport roles. For example, mHTT disrupts axonal transport by competing for binding sites on motor proteins. Furthermore, mHTT is susceptible to abnormal proteolytic cleavage by enzymes like calpain, which generates highly toxic N-terminal fragments. These fragments readily translocate into the nucleus, where they interfere with gene transcription and ultimately lead to cellular dysfunction.
Therapeutic Strategies Targeting HTT
Therapies are being developed to address the presence of the mutant huntingtin protein. The most advanced strategy involves “huntingtin lowering,” which aims to reduce the production of the toxic protein within the cells. This approach leverages gene-silencing techniques to interfere with protein synthesis. One prominent method uses antisense oligonucleotides (ASOs), which are short, synthetic strands of DNA or RNA.
ASOs are designed to be complementary to the messenger RNA (mRNA) transcript of the HTT gene. Once an ASO binds to the HTT mRNA, it flags the transcript for degradation by cellular enzymes. These compounds are typically administered via intrathecal injection into the fluid surrounding the spinal cord to reach the brain. While initial ASO compounds have been non-selective, lowering both the normal and mutant forms of HTT, there is a strong focus on developing allele-specific ASOs that target only the mutant transcript.
Small molecule drugs and gene therapy approaches are also being investigated. Small molecules are being explored to enhance the cell’s clearance mechanisms, such as promoting autophagy to accelerate the degradation of already-formed mHTT aggregates. Gene therapy techniques use viral vectors to deliver genetic material, introducing instructions for silencing the HTT gene. These diverse strategies share the goal of mitigating the toxic gain-of-function caused by the expanded polyglutamine tract, offering a path toward disease modification.