The peripheral nervous system is a vast network of nerves that exists primarily outside of brain and spinal cord and connects to the far reaches of the body. The very expanse of peripheral nerves makes them highly vulnerable to injuries such as blunt-force blows, cuts, and leg and arm fractures, as well as diseases that attack peripheral nerves such as diabetes, Charcot-Marie-Tooth, and Guillain-Barre syndrome. Unlike the central nervous system, however, the peripheral nerves do have the capacity to regenerate, with inflammatory immune responses playing a key role in regeneration.
Now, a study from researchers at Case Western Reserve University has demonstrated in an animal model the regenerative dynamics of a specific signaling protein, C-C class chemokine 2 (CCL2). The team state that CCL2 sends inflammatory immune cells, called macrophages, to peripheral nerve cell clusters to promote repair and to trigger gene expression that leads to new growth in nerve cells. The study is published in the journal Experimental Neurology.
Previous studies show that post-injury, CCL2 affects the workings of peripheral nerve cell clusters, known as ganglia, and the nerve fibers distal to the site of injury. Each peripheral nerve cell has a main body and a tail-like extension, known as an axon. For sensory nerve cells, the axon splits after leaving the cell body, with one part projecting from the nerve cell body to capture sensations, while the other forwards the information to the spinal cord and brain. After an injury to a region of peripheral nerves, CCL2 signals macrophages to move to the damaged areas of axons and remove cellular debris, clearing the way for new axon growth. Earlier studies from the team showed that CCL2 also signals macrophages to enter into the injured ganglia regions that house individual nerve cells to promote nerve regeneration as well. At least in part, regeneration also occurs because macrophages trigger genes that promote new axon growth. The current study demonstrated the workings of this CCL2 mechanism in lab animals.
The current study first utilised uninjured wild-type mice injected with a virus designed to trigger CCL2 expression. Results show that the boost in CCL2 expression in these mice led to greater accumulation of macrophages three weeks later in dorsal root ganglia, a cluster of sensory nerve cells that project both to peripheral areas and to the central nervous system. Data findings show that more macrophage build-up, in turn, produced greater neuron sprouting, the beginnings of nerve regeneration.
Results show the experience was entirely different in another set of mice lacking a receptor called CCR2 which enables CCL2 to act. Without the receptor, the lab observed the same spike in macrophage accumulation simply did not occur in CCR2-deficient mice. Data findings show that the animals only had scant nerve growth that did not nearly match the nerve regeneration observed in wild-type mice. The team explain that they did the same experiments in another type of mouse and found the same correlation and found that if macrophages don’t come into the ganglia, then regeneration is substantially impeded. They go on to state that they found this true of sensory and sympathetic neurons and conclude that there was a correlation between macrophage entry into ganglia and nerve regeneration.
The researchers also tested for changes in the expression of certain genes by screening mRNA molecules associated with nerve regeneration in the animals where CCL2 overexpression prompted neuron outgrowth. Changes in one mRNA and one protein emerged, leukemia-inhibitory factor (LIF) mRNA and neuronal pSTAT3 (signal transducers and activators of transcription 3). The lab explain that these special molecules act on cells by expressing genes important to instructing neurons to grow.
The group state by causing macrophage accumulation, CCL2 increases levels of LIF mRNA and pSTAT3, which leads to an increased regenerative capacity of dorsal root ganglia neurons. To test their hypothesis, the lab blocked the activation of LIF signaling using inhibitors of STAT3 activation. Results show that inhibiting STAT3 activation did not result in an increase in neurite growth, despite an increase in CCL2 that would normally increase growth of dorsal root ganglia neurons.
The team surmise that their findings shed a new light on inflammation and suggest that rather than fight inflammation at the very outset of a peripheral nerve injury, perhaps allowing limited inflammation post-injury may be therapeutic in stimulating neuron regeneration. For the future, the researchers state that their findings could also have implications for illnesses that affect peripheral nerves. They go on to conclude that the immune system and the nervous system are interacting in a beneficial way to create macrophage-induced inflammation and promote nerve regeneration.
Source: Case Western Reserve University