Macrophage roles in peripheral nervous system injury and pathology: Allies in neuromuscular junction recovery

Highlights

• Macrophages are essential to nerve regeneration after injury or in denervating disease.

• Different macrophage phenotypes contribute to nerve degeneration and regeneration, distinguished by differential growth factor expression.

• Macrophages likely play a significant role at the NMJ, and therefore are good candidates for future research.

• A more complete understanding of the interplay between the immune system and nerve repair may allow new treatments and enhance muscle recovery.

Abstract

Peripheral nerve injuries remain challenging to treat despite extensive research on reparative processes at the injury site. Recent studies have emphasized the importance of immune cells, particularly macrophages, in recovery from nerve injury. Macrophage plasticity enables numerous functions at the injury site. At early time points, macrophages perform inflammatory functions, but at later time points, they adopt pro-regenerative phenotypes to support nerve regeneration. Research has largely been limited, however, to the injury site. The neuromuscular junction (NMJ), the synapse between the nerve terminal and end target muscle, has received comparatively less attention, despite the importance of NMJ reinnervation for motor recovery. Macrophages are present at the NMJ following nerve injury. Moreover, in denervating diseases, such as amyotrophic lateral sclerosis (ALS), macrophages may also play beneficial roles at the NMJ. Evidence of positive macrophages roles at the injury site after peripheral nerve injury and at the NMJ in denervating pathologies suggest that macrophages may promote NMJ reinnervation. In this review, we discuss the intersection of nerve injury and immunity, with a focus on macrophages.

Introduction

Insults to the peripheral nervous system (PNS) are common and can be debilitating. Over 2% of patients with limb trauma suffer peripheral nerve injuries (Padovano et al., 2020). These injuries often have long term detrimental effects on quality of life, with 30% of patients with work-related nerve injuries suffering permanent disabilities (Akel et al., 2013; Bergmeister et al., 2020). Nerve-related injuries also have a high financial burden. Traumatic injuries to the brachial plexus, for example, are estimated to incur over $1.1 million in indirect costs per patient (Hong et al., 2019). Degenerative neuromuscular diseases, while rarer, have devastating effects on the PNS. Neuromuscular diseases can cause temporary or permanent paralysis, which can be life-threatening. Amyotrophic lateral sclerosis (ALS), for example, affects approximately 5 in 100,000 people in the United States (Mehta et al., 2014) and is characterized by an ultimately fatal progressive paralysis, with only 7% of patients surviving 5 years after diagnosis (del Aguila et al., 2003). Understanding the processes underlying PNS damage—from either traumatic or neurodegenerative etiologies—and optimizing repair is critical to reducing the morbidity and mortality associated with peripheral nerve injuries and neuromuscular diseases.

The body's response to PNS nerve damage involves multiple cell types, including axonal Schwann cells (SCs), terminal (perisynaptic) Schwann cells (tSCs), endothelial cells, and immune cells such as macrophages, neutrophils, and T-cells. tSCs are non-myelinating glial cells that reside at the neuromuscular junction (NMJ) and perform a wide variety of roles, including maintaining NMJ structure (Reddy et al., 2003), phagocytosing nerve debris after injury (Duregotti et al., 2015), and facilitating endplate reinnervation (Son and Thompson, 1995; Kang et al., 2003). Endothelial cells associated with blood vessels increase in number with angiogenesis and guide myelinating SCs, which then guide regenerating axons, across a nerve injury site (Cattin et al., 2015). Immune cells mount inflammatory responses to PNS insults and perform a variety of other roles, including clearing cellular debris and secreting factors integral to regeneration, including vascular endothelial growth factor (VEGF), a key regulator of angiogenesis (Lu et al., 2020).

Peripheral nerve damage ultimately disrupts the NMJ, causing functional muscle denervation. The NMJ is where an axon terminal contacts a motor endplate (Fig. 1) and is critical for muscle function. The axon terminal releases acetylcholine, which binds to receptors on the motor endplate to cause muscle contraction (Dale et al., 1936; Fertuck and Salpeter, 1974). Mechanisms of denervation and reinnervation at the level of the synapse have been understudied; however, they are critically important. When a nerve is damaged, the distal segment of the axon beyond the injury degenerates, and the nerve endings that once innervated NMJs are lost. If axons grow across the injury site but do not properly reinnervate NMJs, motor improvement will be limited. NMJ reinnervation is vital for regaining muscle function. Additionally, functional recovery is positively correlated with completely reinnervated NMJs (Vannucci et al., 2019). NMJ disturbances are central to the development of neuromuscular diseases as well. ALS (Fischer et al., 2004), spinal muscular atrophy (SMA) (Swoboda et al., 2005), and Guillain-Barré syndrome (GBS) (Ho et al., 1997) can all result in denervation at the level of the NMJ. In ALS rodent models, therapies that rescue lower motor neurons but fail to protect NMJs do not improve lifespan (Suzuki et al., 2007); reviewed in (Murray et al., 2010). It is clear that treatment and research strategies focused only at the motor neuron or axonal regeneration are incomplete; understanding degenerative and regenerative processes at the NMJ is vital to improving recovery after injury and disease.

The immune response is important for axonal regeneration at the nerve injury site and also occurs downstream in the muscle, where is integral to NMJ reinnervation. Changes in monocyte/macrophage activity at the NMJ have been observed after PNS insults, including peripheral nerve injuries (Jablonka-Shariff et al., 2020), ALS (Van Dyke et al., 2016; Trias et al., 2017), and SMA (Dachs et al., 2011). Macrophages are an incredibly diverse cell type; they can both stimulate and control inflammation and they have the capacity to promote nerve repair. They are known to phagocytose debris of necrotic tissue fragments (Mueller et al., 2001), secrete growth factors (Tomlinson et al., 2018; Bombeiro et al., 2018), promote angiogenesis (Cattin et al., 2015), and interact with tSCs and immune cells (Lu et al., 2020; Savage et al., 2008). Unfortunately, their role at the NMJ in peripheral nerve injury has received less investigative focus. While nerve injuries are distinct from other PNS insults like ALS, SMA, and GBS, all of these pathologies ultimately converge on the functional denervation of the NMJ and consequent paralysis. Evaluating macrophage roles at the NMJ in multiple types of PNS damage provides a more complete picture of macrophage activity in the PNS and may yield insights as to their activity in nerve injuries. In this review, we discuss macrophage roles in PNS pathologies with a focus on the NMJ.