Restraint of presynaptic protein levels by Wnd/DLK signaling mediates synaptic defects associated with the kinesin-3 motor Unc-104

Each nerve cell, or neuron, has a long nerve fiber – called an axon – that forms specialized sites for information exchange – called synapses – with other cells. Many molecules work at synapses to coordinate the exchange of information. These molecules are largely made in the central part of the neuron – known as the cell body – and are then transported along the axon to the synapses. The transport of these molecules is carried out by proteins known as molecular motors. One molecular motor, called KIF1A in humans and Unc-104 in fruit flies, is thought to be a major transporter of synaptic molecules. Mutations that hinder this molecular motor result in neurons failing to form synapses and, instead, synaptic components accumulate in the cell body. However, it was not clearif Unc-104 does actually carry all of the components needed to assemble synapses along axons, or if it influences synapse formation in another way. Now, Li, Zhang et al. report new evidence that supports the second of these two hypotheses. The experiments made use of fruit flies in which the gene for Unc-104 had been deleted, and revealed that inhibiting enzymes in a specific signaling pathway could reverse the synaptic problems caused by the loss of Unc-104. The signaling pathway, which is conserved between flies and humans, involves an enzyme that is called Wnd in flies and DLK in humans. The Wnd/DLK signaling pathway was previously known to regulate how neurons respond when their axons are damaged (either by growing new axons or dying, depending on the context). Further investigation by Li, Zhang et al. revealed that signaling via the Wnd enzyme becomes triggered whenever the Unc-104 molecular motor is impaired. This activation correlates with the build-up of synaptic proteins in the cell body. Once activated, the pathway then reduces the total amount of synaptic proteins that the cell makes. This reduction matches the neuron’s reduced ability to transport them along the axon, and may help the neuron to adapt when axonal transport is impaired. However, the reduction in synaptic proteins also impaired the exchange of information at the synapses. These findings suggest how DLK could be behind problems with synapses in diseases in which transport along axons is impaired. These diseases include hereditary spastic paraplegia, which has been linked to mutations in human KIF1A, and may also include ALS and Alzheimer’s disease, which have recently been linked to DLK. DLK has received recent attention as a candidate drug target because it contributes to the deterioration of damaged neurons. These new findings further expand that interest by suggesting that inhibiting DLK may help neurons to maintain working synapses, which is more useful than simply preventing damaged neurons from dying.