You might be very familiar with the Batman movies. Batman is the hero and Joker is the villain. Batman

works together with other superheroes to save the world, whereas Joker, either works alone or with

other supervillains, trying to destroy Gotham City. At the end of the movies, Batman and Joker usually

have a decisive battle. Batman always smiles to the end, whereas Joker goes to jail and plots for his next

revenge. Funny and exciting to watch? Absolutely! Now, it’s time for us to read a medical paper. You

might start to yawn, but come on! It’s not that boring, it is like a Batman movie!

In this movie, a mouse model was used to study Alzheimer’s disease and the mouse brain was Gotham

City. Cortex was the city center. Billions of neurons were the main citizens. Astrocytes worked as the city

maintenance crew and microglia responded for security and risk management. There were millions of

guidebooks (genes) stored in the city for the citizens and workers to properly carry out their duties.

Guidebooks are kind of difficult to open for reading, so people usually use messages (RNA) to direct

what they should do, instead of using the guidebooks directly. Because the city was quite old and lacked

maintenance, β-amyloid, a kind of garbage difficult to decompose and remove, started to pile

everywhere. Another kind of dangerous material, P-Tau, also appeared in the transportation system,

making communication between different teams difficult. Consequently, the city gradually lost its

citizens (brain cells die), and the city size was shrinking significantly (cerebral atrophy).

The microglia team worked tirelessly in these challenging conditions. Their primary responsibility was to

monitor potential threats, like viruses or bacteria circulating through the city, as well as damage and

injuries to both citizens and the city’s infrastructure. They also helped to maintain a harmonious

situation and repair damage. An important guidebook for the team is Phospholipase C-g2 gene (PLCG2).

Using the guidebook, they could make the protein PLCγ2, which plays an indispensable role in

kickstarting various downstream processes (signaling pathways).

Batman oversaw the production of PLCG2. However, as nobody is perfect, a minor error occurred during

production, resulting in a variant known as PLCG2 P522R . Interestingly, PLCG2 P522R proved to be even more

effective than the original version in preventing the deterioration of Gotham City. In contrast, Joker,

while overseeing the production, a serious mistake was created, which was accompanied with a

malevolent guidebook called PLCG2 M28L .

As β-amyloid continued to accumulate in Gotham City, the microglia team received the order to take out

the trash. Regrettably, the team relied on the faulty guidebook PLCG2 M28L . Messages that appeared

normal were sent to the team members, but they struggled to make functional PLCγ2 proteins based on

the messages. Without the jump-starter, the maintenance and cleanup job cannot be initiated, and β-

amyloid plaques formed even more in the city. Upon recognizing this ominous development, the

microglia team decided to initiate crisis management by turning on proinflammatory cytokines

expression. This was done to galvanize the team’s efforts and accomplish the task more effectively.

Given the intensity of the process, it sometimes could be risky for the team members themselves.

Unfortunately, the jump-starter PLCγ2 was also required by the crisis management, and without it, the

action also failed (proinflammatory cytokines including IL1B, TNF-α, and IL4 expression down-regulated

in comparison to control brains). The poor Gotham City, burdened with heaps of β-amyloid at the city

center, was facing the fate of collapse and the working memories of the PLCG2 M28L mice were severely

impaired. Now, let’s talk about Batman’s guidebook. It won’t let you down. By using PLCG2 P522R , the

microglia team made sufficient PLCγ2, successfully fulfilled the maintenance and the crisis management

task, and prevented the β-amyloid building-up in the city center.

So far, it is common knowledge that Joker’s guidebook PLCG2 M28L was the project’s downfall. But how? A

thorough investigation was conducted throughout the city center and all the transmitted messages were

scrutinized by using bulk RNA sequencing. It became evident that Joker’s guidebook severely disrupted

the communication pathways of the city center (synaptic function) and led to the abnormal

interpretation of at least 593 other guidebooks, encompassing critical subjects like crisis management

(inflammatory response), cleanup and recycling (phagocytosis/endocytosis), personnel recruit and

training (glial cell development), city maintenance and reconstruction (fatty acid biosynthesis).

To prevent inaccuracies from large-scale investigations, a more targeted investigation was conducted on

individual microglia. Personnel from other functional teams, such as neurons, astrocytes, endothelia,

were used as controls. Investigators analyzed the original going-out messages from the computers

(nucleus) of the investigated subjects using Single-nuclei RNA-sequencing. This was a huge amount of

work, in total, 62,588 computers were collected. Investigators further confirmed that only microglia’s

team was under the bad influence of Joker’s PLCG2 M28L . After comparing messages from 7909 typical

computers collected from individual microglial guided by original PLCG2, PLCG2 M28L , or PLCG2 P522R ,

investigators identified 5 action clusters based on their varying interpretations of PLCG2 M28L and their

behaviors during the crisis. All five clusters faltered in their duties, resulting in abnormal message-

sending, ineffective field operation (cell migration), impaired harm clearance (endocytosis), and

compromised crisis management (inflammatory responses and apoptotic process). Following this,

historical files were created for cluster members based on their responses to PLCG2, PLCG2 M28L , or

PLCG2 P522R . Additionally, guidebooks they utilized during the crisis were archived to facilitate future

investigations.

The above story was published in the renowned medical journal Immunity on September 12, 2023,

under the title “Genetic variants of phospholipase C-g2 alter the phenotype and function of microglia

and confer differential risk for Alzheimer’s disease”. (https://www.cell.com/action/showPdf?pii=S1074-

7613%2823%2900364-3). You might be wondering, where’s the epic showdown between Batman and

Joker? Regrettably, in this tale, Batman and Joker never crossed paths as they operated in different

Gotham Cities in the parallel universe. In the upcoming story, they might find themselves in the same

city. How? You may ask. Well, Batman will be a male mouse, and Joker a female mouse. After they tie

the knot, the decisive battle could unfold within their offspring’s brains. What a twisted plot! Hard to

imagine.