New perspectives on amyotrophic lateral sclerosis: the role of glial cells at the neuromuscular junction

Abstract

Amyotrophic lateral sclerosis (ALS) is a disease leading to the death of motor neurons (MNs). It is also recognized as a non-cell autonomous disease where glial cells in the CNS are involved in its pathogenesis and progression. However, although denervation of neuromuscular junctions (NMJs) represents an early and major event in ALS, the importance of glial cells at this synapse receives little attention. An interesting possibility is that altered relationships between glial cells and MNs in the spinal cord in ALS may also take place at the NMJ. Perisynaptic Schwann cells (PSCs), which are glial cells at the NMJ, show great morphological and functional adaptability to ensure NMJ stability, maintenance and repair. More specifically, PSCs change their properties according to the state of innervation. Hence, abnormal changes or lack of changes can have detrimental effects on NMJs in ALS. This review will provide an overview of known and hypothesized interactions between MN nerve terminals and PSCs at NMJs during development, aging and ALS-induced denervation. These neuron-PSC interactions may be crucial to the understanding of how degenerative changes begin and progress at NMJs in ALS, and represent a novel therapeutic target.

Keywords: SOD1; denervation; motor unit; non-cell autonomy; perisynaptic Schwann cells; re-innervation; remodeling; synaptic transmission.

Figure 1. Spatial organization of MNs and NMJs

A, ALS is characterized by MN loss, axonal degeneration and NMJ destruction. The MN cell body is located in the ventral horn of the spinal cord while the axon projects outside of the CNS to directly innervate multiple muscle fibres (motor unit). The MN cell body and its local environment (dotted area highlights the region enlarged in B) are important sites of mutation‐mediated toxicity. Indeed, the contribution of glial cells (B, microglia in green, astrocyte in red and oligodendrocyte in blue) in disease development and progression is well established in the CNS. However, the contribution of glial cells at the NMJ, the output of the MNs in the PNS, remains ill‐defined. C, the NMJ is composed of three synaptic elements: the presynaptic nerve terminal (green), the postsynaptic motor endplate enriched in nicotinic receptors (red stripes on the darker pink muscle fibre) and the perisynaptic Schwann cells (PSCs; blue). Note that these three synaptic elements are precisely aligned with each other. Unlike motor axons that are wrapped with myelinating Schwann cells, the nerve terminal is only covered by non‐myelinating PSCs that do not invade the synaptic cleft. A close view of the dotted area in C represents the sagittal plane of the NMJ shown in D. The different components and the potential interactions between them are described in Fig. 2.

Figure 2. Potential molecular interactions between PSCs, muscle fibres and motor nerve terminals at NMJs

In normal conditions (left panel), the presynaptic nerve terminal (green), the postsynaptic muscle fibre (pink) and the PSCs (blue) regulate synaptic functions in a coordinated fashion. Synaptic transmission is induced when an action potential reaches the presynaptic nerve terminal, activating voltage‐dependent calcium channels. This activation will trigger a rapid calcium entry into the nerve terminal, and induce synaptic vesicle exocytosis and subsequent neurotransmitter release into the synaptic cleft. ACh will be co‐released with ATP. ACh will bind to nAChRs on the muscle fibre and to mAChRs on PSCs. The binding of ACh to the nAChRs will depolarize the muscle fibre, which can result in the opening of voltage‐dependent sodium channels and subsequent muscle contraction (myosin and actin movement). The binding of ACh to mAChRs will trigger an increase in intracellular Ca²⁺ via the activation of the IP₃ receptors (IP₃Rs) of the ER. ATP released during synaptic activity will be detected by PSCs via P2Y G‐protein‐coupled receptors. They, too, will trigger an increase of intra‐PSC Ca²⁺ concentration by releasing Ca²⁺ from IP₃‐driven internal stores. In return, PSC detection of neurotransmission will regulate synaptic activity by acting on presynaptic adenosine receptors (A₁/A_2ARs). Interactions between MN nerve terminals and PSCs can also occur via different pathways that influence NMJ stability and repair. For example, PSCs express receptors such as ErbBs that, if activated, can influence NMJ structure. PSCs can also release TGF‐β1 and agrin, and synthesize MMPs. TGF‐β1 will promote NMJ formation and stability, while agrin, which can be cleaved by MMP, will act on the LRP4 (low density lipoprotein receptor‐related protein 4)–MuSK complex to influence NMJ stability. Also, note the presence of mitochondria and wt SOD1 in all three synaptic elements. In ALS pathological conditions (right panel), these different PSC signalling pathways can be altered to promote NMJ denervation (D). Over‐activation of PSC muscarinic pathway leads to greater intracellular Ca²⁺ responses, alters gene expression and, hence, influences NMJ repair. Furthermore, the activation of the ErbB pathway can be implicated in alterations in PSC position and morphology as well as synaptic loss. Finally, the PSC agrin/MuSK pathway can be altered such that MMP release by PSCs can be upregulated and released agrin can be reduced, leading to NMJ instability. Boxes: hypothesis and proposed mechanisms. Dotted lines: pathways that are yet to be confirmed. Line thickness: relative increase or decrease of the pathway in comparison to the normal condition. LRP4, low density lipoprotein receptor‐related protein 4; nAChR, nicotinic acetylcholine receptor; SOD1, superoxide dismutase 1.