Shining light on Aβ and ApoE – partners in crime in Alzheimer’s disease

Rohini Subrahmanyam

A collaborative study led by Sudipta Maiti, Tata Institute of Fundamental Research (TIFR), Mumbai, sheds light on the deadly partnership between ApoE (Apolipoprotein E) and Aβ (amyloid-beta) — two major protein players in Alzheimer’s disease. Their innovative FLIM (fluorescence lifetime microscopy) imaging approach offers potential value for drug development, promising better disease prediction and treatment strategies, and holding implications for neurological health and therapeutics.

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Image for representation only. Credits: Pixabay

A healthy brain is usually buzzing with activity. The neurons — a major brain cell — are rapidly relaying messages to one another across the synapses that connect them. In patients with Alzheimer’s disease, however, this carefully coordinated system begins to collapse.

The neurons start dying and the astrocytes — another major brain cell — start malfunctioning. Beginning with a loss of stored memories and eventually leading to losing control over the simplest of actions — ultimately, Alzheimer’s disease can end up being fatal. 

One of the main culprits behind the devastating mental decline associated with the disease is a seemingly innocuous protein called amyloid-beta (Aβ). It insidiously starts clumping together to form plaques, which — once they accumulate — begin a deadly rampage, destroying synapses and killing cells. Undoubtedly Aβ is toxic, but the question perturbing most scientists is — what does Aβ do to the cells that triggers their eventual demise? Sudipta Maiti, Professor, Tata Institute of Fundamental Research (TIFR), Mumbai, decided to turn this question on its head. 

Instead, he wondered, what does the cell do to Aβ, such that it transforms into a killer? 

The fat-carrying protein Apolipoprotein E (ApoE) is one clue. Most patients with Alzheimer’s disease have a particular variant of this gene that in some manner increases their risk of developing the disease later in life. Maiti hypothesised that ApoE’s ominous presence may not be just a coincidence. And indeed, in a paper published recently in ACS Chemical Neuroscience, Maiti and his colleagues report that ApoE and Aβ interact in a way that increases the toxicity of Aβ in cells.

To study Aβ, Arpan Dey, a graduate student in Maiti’s lab, turned towards fluorescence lifetime microscopy (FLIM). Using a synthetic Aβ protein that is tagged with a fluorescent molecule, he looked at how the fluorescence lifetime, or just lifetime” of Aβ — which is how long it remains fluorescent — changes when it is added to the cells. He found that the Aβ protein lifetime decreases in neurons and astrocytes — cells with an inherently higher amount of ApoE.

So clearly ApoE was affecting Aβ, but Maiti was interested in the molecules in this interplay. The Aβ protein exists in an oligomeric” form, where multiple molecules of Aβ cling loosely together — when two cling together it’s called a dimer, when three do, it’s a trimer and so on. Dey figured out that ApoE was not only affecting the Aβ protein lifetime, but it was also causing it to form more dimers – thus hinting that the dimers could be the dangerous version of Aβ. When he prevented the ApoE and Aβ from interacting, the lifetime went back to being longer, and the toxic effects of Aβ also reduced.

For the final clincher, Maiti teamed up with Odity Mukherjee at Institute for Stem Cell and Regenerative Medicine (InStem), Bangalore. Mukherjee’s group had access to induced pluripotent stem cells (iPSCs) — cells which have the power to become any cell in the body — from patients with Alzheimer’s disease. They turned these iPSCs into neurons, mashed the neurons up, and gave the extracts to Dey and Maiti. When Dey studied the fluorescently tagged Aβ proteins in these extracts, he found that as before, the Aβ lifetime decreased and its propensity to make dimers increased.

Maiti believes that measuring the highly quantifiable lifetime of Aβ would be a good way to test its toxicity, especially while screening potential drugs. 

This is something that has been missing from the field forever, a molecular parameter related to Aβ, which you can measure and tell whether a drug is having some effect.

He now plans to use iPSCs from patients with different variants of ApoE, to check if the riskiest variant, ApoE4, affects Aβ lifetime and toxicity.

Deepak Nair, Associate Professor, Indian Institute of Science (IISc), Bangalore, is convinced of the diagnostic potential of the study. Nair said, 

What is interesting is that this group has used FLIM and single-molecule strategies, that means they wanted a classical way to image something and see how the disease progression can be predicted. 

He added, The earlier you can understand these changes the better you can understand the progression.”