This study tries to suggest that depression may be a symptom of Alzheimer's.

In my personal observation, "depression" is one of the "precursors" of Alzheimer's.

It seems to me that a person who was depressed for some personal tragedy seems to 

get into Alzheimer's later. My theory is that depression comes before Alzheimer's.

There, Alzheimer's is one of mental defense mechanism to the depressed and stressed out brains.

It is just like a heart attack where a person gets a heart attack after a major depressive or stressful

events in his life, such as losing his job, or some personal failure or family tragedies.

What I am trying to say, here, is "Do not get into anything that can bring depression.

In our old age, try to live happily and stay away from any possibility that might cause depression to yourself.

Avoid those kinds of things by all means. Whether I am right or wrong, it won't hurt to stay away from those.

Practical examples:

1. Don't try to be a single handicapper in golf. You may surely end up depressed.

2. Don't try to be rich. You may end up to be a failure and get depressed.

3. Don't try to be famous or follow glamors. You may end up in a garbage can.

4. Don't get too ambitious or jealous. Know the limit of your capability.

Conclusion:

Don't overestimate yourself while everybody else thinks you are an idiot.

A good example is Donald Trump.

When Donald gets through the presidency, he will surely get into Alzheimer's.

There's a good possibility that he gets into a fatal heart attack even before he finishes the presidency.

I'm inclined to agree with your observation, i.e. depressive symptoms

may precede Alzheimer's as this article suggests.

Your advices are wise and medically appropriate as well.

Thanks for your well thought out comment as usual.

Is there any biological marker of a depression as clinical entity?

If not, it may be a collective symptoms of many disease entity that have 

distinctive pathological changes. Sometimes, I wonder about many 

psychiatric diagnoses as biological markers for some of them are revealed by 

neuroscience. You do not say that you have a “cough”. Patient should be told 

you have a bronchitis or lung cancer or tuberculosis or common cold. Yet you 

treat the cough as well as bronchitis.