Cerebrovascular and Blood-Brain Barrier Impairments in Huntington’s Disease: Potential Implications for Its Pathophysiology

Background
The brain is a complex and energy demanding organ. In order to prosper it needs a constant flow of nutrients, oxygen, and stimulation. In the developing brain for example, neuronal connections, called synapses, that are not used will eventually be pruned away, resulting in highly specific neural networks. Nutrients and oxygen are delivered to the brain the same way they are delivered to other organs — in the blood stream. However, there is an important difference in how blood flows through the brain in comparison to the rest of the body. Instead of oxygen and nutrients easily crossing from the blood vessels and into the cells of the brain as is done in other organs, brain vasculature has an extra layer of protection known as the blood-brain barrier (BBB). The blood-brain barrier is made up of brain endothelial cells held together by tight junctions (TJs) that form a highly selective barrier between the blood supply and the cerebral spinal fluid (CSF). The BBB regulates the passage of certain molecules necessary for brain function while keeping out potential toxins(1). This is a very important job because substances that can easily enter the peripheral bloodstream (such as botulin toxin) would wreak havoc if they easily diffused or were transported into the CSF.

Figure One (2)

The root cause of Huntington’s disease (HD) is a genetic mutation resulting in an extended CAG repeat in the Huntingtin gene (HTT). The mutation that results in HD was isolated in 1993 and research since then has worked to further understand the inheritance patterns and implications of this genetic disorder. However, an aspect of HD that has been less well researched is HD pathophysiology, essentially the way in which HD interrupts normal brain function. In other neurodegenerative diseases, such as Alzheimer’s disease, there have been reports of impairments in cerebral vasculature as well as the BBB (3). Knowing the role that cerebral blood flow deficiency can play in neurodegenerative diseases, researchers from the University of Cambridge, the Hospital Center University of Quebec, the University of Nottingham, and the University Laval, collaborated on a project investigating potential vascular impairments in HD.

Research
Researchers used magnetic resonance imaging (MRI) as well as post-mortem tissue analysis in order to explore any potential vascular or BBB impairments in HD patients. Researchers also used the R6/2 mouse model, a mouse line that has been genetically engineered to represent human Huntington’s disease. Using a mouse model allows the researchers to use a wider range of experimental techniques in order to glean further details of HD pathophysiology.

Several differences were identified between cerebral blood vessels and the BBB of healthy controls in comparison to HD patients and R6/2 mice. Researchers found mHTT aggregates, the mutant form of the Huntingtin protein, in the cerebral blood vessels as well as the BBB of HD patients. These mHTT aggregates were also found in the blood vessels and BBB of R6/2 mice. Image 2 shows where these aggregates were seen as well as how they are able to move between cells. The effect of mHTT aggregates remains unclear —previous studies linked them to oxidative stress and impairment of brain cells’ ability to produce energy; however, recent research suggests that mHTT inclusions in neuronal cells may be protective. Regardless, mHTT aggregates are a deviation from the norm and this study found their presence in the cells making up the brain blood vessels and BBB, a fact of unknown consequence (4,5,6).

Figure Two. Used With Original Author’s Permission (5)

Researchers also identified changes in the structure of brain blood vessels. Both post-mortem tissue analysis of HD patients and R6/2 mice showed an increased density of blood vessels — so there were more of them in the same space. However, this increased number of blood vessels was accompanied by a decrease in the average size of the vessel. In addition, the increased density of blood vessels was negatively correlated with the amount of putaminal atrophy— the more brain cells that died the more of these smaller blood vessels there appeared to be.

This change in number and size of blood vessels does not sound too disastrous until you consider the massive amount of oxygen and nutrients that needs to be carried through the cerebral vasculature — about 750mL of blood per minute (3). With such a high demand for blood flow, small changes in blood vessels could affect the ability of those vessels to deliver the needed oxygen and nutrients.

In addition, researchers found significant changes in the BBB structure. There was a decrease in the expression of two of the proteins that form the tight junctions between the endothelial cells that make up the BBB. Decreased expression of these two proteins, occludin and claudin-5, has been found to be associated with increased permeability (6). Image 1 shows the locations of the tight junctions between the endothelial cells that surround blood vessels. Image 2(upper insets) shows how claudin-5 and occludin help hold together these tight junctions of the BBB. Finally, to check the effects of these structural changes, researchers looked for any leakage across the BBB by measuring the amount of fibrin, a protein in the blood that aids in clotting, outside of the blood vessels. These analyses showed an increased amount of extravascular fibrin. Fibrin’s role in clotting means that the majority of it is usually located within the blood vessels, higher levels outside of the blood vessels indicates leakage through the BBB. Furthermore, the amount of leakage through the BBB in the right-side caudate region of the brain was correlated with disease burden; meaning that the further progressed a patient was, the “leakier” their BBB appeared to be.

Impact
This research is significant because it identifies clear structural and functional changes in the cerebral vasculature and BBB in Huntington’s disease patients. Although it remains unclear exactly how these various changes affect or are affected by brain cell survival and disease progression, there is evidence that significant changes are happening in the HD brain’s circulatory system. In addition, this information about HD is valuable to researchers as they continue to work towards development of treatments because in order to fix something, you first need to know what’s going wrong.

Citations
1) https://en.wikipedia.org/wiki/Blood%E2%80%93brain_barrier#Function
2) http://neuroscience.uth.tmc.edu/s4/chapter11.html (Image 1)
3) Roher, Alex E., et al. “Cerebral blood flow in Alzheimer’s disease.” Vascular health and risk management 8 (2012): 599.
4) Xu J, Chen S, Ku G, et al. Amyloid beta peptide-induced cerebral endothelial cell death involves mitochondrial dysfunction        and caspase activation. J Cer eb Blood Flow Metab 2001;21:702–710
5) Rizzo MT, Leaver HA. Brain endothelial cell death: modes, signaling pathways, and relevance to neural development,  homeostasis, and disease. Mol Neurobiol 2010;42:52–63.
4) Jiao H, Wang Z, Liu Y, et al. Specific role of tight junction proteins claudin-5, occludin, and ZO-1 of the blood-brain barrier in  a focal cerebral ischemic insult. J Mol Neurosci 2011;44:130–139.
5) Drouin‐Ouellet, Janelle, et al. “Cerebrovascular and blood–brain barrier impairments in Huntington’s disease: Potential  implications for its pathophysiology.” Annals of neurology (2015).
6) Slow, Elizabeth J., Rona K. Graham, and Michael R. Hayden. “To be or not to be toxic: aggregations in Huntington and  Alzheimer disease.” Trends in Genetics 22.8 (2006): 408-411.

CJE 09/15