Tagged: BIIB092

The Tau of LRRK2

 

Dense spherical clusters of a protein – called Lewy bodies – are one of the classical hallmarks of the Parkinsonian brain. They are a common pathological feature, but curiously they are not present in all cases of Parkinson’s.

For example, some individuals with certain forms of Parkinson’s associated with specific genetic mutations do not exhibit any Lewy bodies. Variations in a region of DNA called LRRK2 will increase one’s risk of developing Parkinson’s, but many of those who go on to develop LRRK2-associated Parkinson’s will have a complete absence of Lewy bodies in their brains. These cases have represented an enigma for the Parkinson’s research community and have been difficult to reconcile.

Recently, however, researchers from the University of Pennsylvania have reported a different kind of protein clustering in these LRRK2-associated cases with “no Lewy bodies”. The accumulating protein is called Tau.

In today’s post, we will look at what Tau is, review what the new research report found, and discuss what this discovery could potentially mean for the future treatment of Parkinson’s.

 


Neuropathologists conducting a gross examination of a brain. Source: NBC

At present, a definitive diagnosis of Parkinson’s can only be made at the postmortem stage with an examination of the brain.

Until that moment, all cases of Parkinson’s are ‘suspected’. When a neuropathologist makes an examination of the brain of a person who passed away with the clinical features of Parkinson’s, there are two characteristic hallmarks that they will be looking for in order to provide a final diagnosis of the condition:

1.  The loss of specific populations of cells in the brain, such as the dopamine producing neurons in a region called the substantia nigra, which lies in an area called the midbrain (at the base of the brain/top of the brain stem). As the name suggests, the substantia nigra region is visible due to the production of a ‘substance dark’ molecule called neuromelanin in the dopamine neurons. And as you can see in the image below, the Parkinsonian brain has less dark pigmented cells in the substantia nigra region of the midbrain.

The dark pigmented dopamine neurons in the substantia nigra are reduced in the Parkinsonian brain (right). Source:Memorangapp

2.  Dense, circular clusters (or aggregates) of protein within cells, which are called Lewy bodies.

shutterstock_227273575A cartoon of a neuron, with the Lewy body indicated within the cell body. Source: Alzheimer’s news

A Lewy body is referred to as a cellular inclusion, as they are almost always found inside the cell body. They generally measure between 5–25 microns in diameter (5 microns is 0.005 mm) and thus they are tiny. But when compared to the neuron within which they reside they are rather large (neurons usually measures 40-100 microns in diameter).

A photo of a Lewy body inside of a neuron. Source: Neuropathology-web

Do all Parkinson’s brains have Lewy bodies?

Funnily enough, no.

And this is where the wheels fall off the wagon in our understanding (and ‘definitive’ definition) of Parkinson’s.

What do you mean?

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