A postdoctoral scientist working in the lab of neurobiologist Kate O’Connor-Giles has identified important properties of a gene called PDZD8 and how, in a mutated form, the gene causes a rare intellectual disability. The lab published their findings in Cell Reports, and highlighted the significance of the work in Autophagy.

Working in fruit flies, Rajan Thakur found that PDZD8 is critical during early development of the nervous system to help neurons form the trillions of synapses that let the cells talk to each other. PDZD8 facilitates this process by speeding up a form of cellular housekeeping known as autophagy, where proteins, organelles and other cell components are broken down in order to be reused. 

“Because we were able to get at this detailed cell biology in neurons in the intact nervous system for the first time, we now know PDZD8 is essential for healthy neurodevelopment,” said Thakur. “When neuronal activity is high during early development, PDZD8 speeds up autophagy, allowing the cell to make more new synapses.”

The lab’s work may provide an explanation for a 2022 discovery by scientists in the UK of a rare intellectual disability, where patients have a PDZD8 mutation that renders it inactive.

“Rajan’s work tells us that in the absence of PDZD8, developing neurons cannot increase autophagy to help make more synapses in active neurons. While more work in laboratory models is necessary, this suggests that drugs that increase autophagy might mitigate the neuronal effects of loss of PDZD8, ” said O’Connor-Giles, Provost’s Professor of Brain Science at Brown University and associate director of the Carney Institute's Center for the Neurobiology of Cells and Circuits. 

PDZD8 in synapse formation

The O’Connor-Giles lab focuses on understanding brain biology in health and disease. They conduct their research using fruit flies because these organisms are easy to genetically alter and share with humans many of the same genes responsible for neuronal development and function. First, they screen large, pre-existing fruitfly datasets to identify all of the genes turned on in neurons just before the brain starts building lots of synapses. They then narrow their focus to genes shared with humans that haven’t yet been studied in the brain. Once they have a target gene, they use CRISPR, the gene modification and editing technology, to label gene products in living fruit fly cells so they can observe them under a microscope. They also use CRISPR to remove or “knock out” the gene to see how the nervous system and brain behaves in that gene’s absence.

Thakur, said O’Connor-Giles, joined the lab at a perfect time. PDZD8 had shown up in a screen, and was thought to be connected to lipids: molecules that play important roles in building membranes of cell components and in sending messages within a cell. Lipids were a class of molecules the lab hadn’t previously focused on. Thakur happened to be an expert on lipids in the nervous system.

Through detailed studies of developing neurons in larval fruit flies, Thakur learned that PDZD8 was crucial to synapse formation. In one notable phase of the research, he created fly lines with PDZD8 knocked out and used multiple approaches to increase neuronal activity. He found the knockout larvae were not able to add more synapses, whereas larvae with normal versions of the gene were. 

By studying healthy neurons and mutant neurons with different parts of the PDZD8 protein–the material the PDZD8 gene creates–missing under a super-resolution microscope, Thakur ascertained that PDZD8 was transferring lipids from one cellular structure to another. One type of structure the lipids passed into is called a lysosome and its role is to break down cellular material. Thakur and O’Connor-Giles suspect the lipids are sending a signal to the lysosome to tell it to begin to do its job.

Thakur also established that PDZD8 carries out the same function in humans by putting human PDZD8 in flies.

Next steps

Thakur is continuing to to study PDZD8 and other genes related to the transfer of lipids between cellular compartments to learn more about this process in nervous system development. He also led a collaboration with Sarah Neuman and Arash Bashirullahat at UW-Madison, finding a synaptic role for a second lipid-transfer protein linked to intellectual disability. 

“Dr. Thakur’s studies highlight the importance of the dynamic exchange of lipids between cellular compartments in regulating nervous system development and are revealing key cellular processes controlled by lipid-transfer proteins. He has really driven an entirely new research area for the lab,” said O’Connor-Giles.