Neurodegenerative Disease Treatment Strategy Sparked by Novel DLK Inhibitor

Neurodegenerative conditions such as Parkinson’s disease and Alzheimer’s disease involve progressive neuronal loss due to disease-induced damage. An enzyme known as dual leucine-zipper kinase (DLK) plays a key role in this process, telling neurons that are damaged or unhealthy when they should cut their losses and self-destruct. While sparing neurons from DLK represents a potential therapeutic strategy for slowing disease progression, a clinical trial testing a DLK inhibitor in human patients resulted in unexpected side effects affecting the nervous system, suggesting that blocking DLK indiscriminately is harmful.

A group of scientists led by Gareth Thomas, PhD, associate professor of neural sciences at the Center for Neural Development and Repair at the Lewis Katz School of Medicine at Temple University, has now reported on what they suggest is a more precise way to block DLK in damaged neurons, while preserving its function in healthy neurons.

The team suggests that their preclinical research, through which they identified a new type of DLK inhibitor, could reignite interest in DLK inhibition as a treatment strategy for neurodegenerative diseases. Thomas and colleagues published their findings in Nature Communications (“Inhibiting acute, axonal DLK palmitoylation is neuroprotective and avoids deleterious effects of cell-wide DLK inhibition”) concluding, “These new classes of inhibitors thus reduce DLK-dependent pro-degenerative retrograde signaling without causing side effects associated with global DLK inhibition.”

Dual-leucine zipper kinase is an upstream activator (a “MAP3K”) of mitogen-activated protein kinase (MAPK) pathways that is highly expressed in neurons, the authors explained. Axons are the long, thin projections on neurons that convey impulses within the brain and spinal cord and to other regions of the body. When axons are damaged, DLK sends signals from the site of injury in the axon back to the neuron’s nucleus, which triggers the self-destruct process. “DLK functions as an evolutionarily conserved stress sensor, and one major role of DLK is to convey retrograde signals from sites of axonal damage or disruption back to neuronal nuclei,” the scientists continued.

Research results have previously indicated that inhibiting DLK’s kinase activity could be a promising therapeutic strategy to prevent different types of neurodegeneration, and this encouraged the development of DLK kinase inhibitors, one of which progressed into a Phase I clinical trial. However, as the authors noted, “… numerous patients in this trial developed symptoms indicative of sensory neuropathy which, along with other adverse events, led a significant proportion of those enrolled to reduce or cease dosage.” Further analysis showed that DLK inhibitor therapy increased plasma levels of neurofilament, “suggestive of axonal cytoskeletal disruption.” The findings, the team further noted, “… suggested that broadly inhibiting DLK’s kinase activity causes deleterious side effects, cautioning against such a therapeutic approach.”

Thomas further noted, “This clinical finding suggested that the conventional DLK inhibitor might be disrupting the normal structure and function of axons.” Confirming this idea, when the scientists treated cultured neurons with an existing DLK inhibitor, they saw that axonal structure was rapidly disrupted. This inspired them to seek an alternative approach to more selectively block the enzyme.

For their newly reported study, Thomas and colleagues considered the way damage affects axons. “From some of our previous research, we knew that DLK initiates the self-destruction signals from very specific locations in neurons,” Thomas further commented. “We thought that if we could stop DLK getting to those locations, it wouldn’t be able to initiate the self-destruction process.” The team’s previous work had shown that DLK is modified with the lipid palmitate. “This modification, palmitoylation, targets DLK to lipid membranes and in neurons is critical for DLK to hitchhike on axonal trafficking vesicles,” they wrote.

Working with Wayne Childers, PhD, at the Moulder Center for Drug Discovery in Temple’s School of Pharmacy and with Margret Einarson, PhD, at Fox Chase Cancer Center, Thomas’s team sought to identify compounds that alter the location of DLK in cells. “We screened more than 28,000 compounds and eventually hit on two in particular that protect neurons from DLK-driven damage,” Thomas said. The two compounds not only protected cultured neurons from degeneration but also reduced DLK signaling in animal models.

Importantly, the compounds did not cause the axonal disruption observed with the conventional DLK inhibitor. “Mechanistically, the two most neuroprotective compounds selectively prevent DLK’s stimulus-dependent palmitoylation and subsequent recruitment to axonal vesicles, but do not affect palmitoylation of other axonal proteins assessed and avoid the cytoskeletal disruption associated with direct DLK inhibition,” the investigators explained.

“Our findings reveal an exciting, novel way to block DLK-dependent signals,” Thomas added. The next steps involve working with medicinal chemists to make the compounds more potent and even more specific to minimize off-target effects. “The current compounds also need to be made more stable if we want to move forward and develop them as drugs,” Thomas noted. “We hope that moving this class of compounds toward the clinic may yield a valuable therapy for patients in the future.”

This study exemplifies the innovative spirit and collaborative strength of our research community at the Katz School of Medicine,” said Amy J. Goldberg, MD, FACS, the Marjorie Joy Katz Dean of the Lewis Katz School of Medicine. “By uncovering a more precise way to protect neurons, Dr. Thomas and his team are paving the way for treatments that could truly change the trajectory of neurodegenerative diseases.”