Functional decline at the aging neuromuscular junction is associated with altered laminin-α4 expression

Abstract

Laminin-α4 is involved in the alignment of active zones to postjunctional folds at the neuromuscular junction (NMJ). Prior study has implicated laminin-α4 in NMJ maintenance, with altered NMJ morphology observed in adult laminin-α4 deficient mice (lama4−/−). The present study further investigated the role of laminin-α4 in NMJ maintenance by functional characterization of transmission properties, morphological investigation of synaptic proteins including synaptic laminin-α4, and neuromotor behavioral testing. Results showed maintained perturbed transmission properties at lama4−/− NMJs from adult (3 months) through to aged (18-22 months). Hind-limb grip force demonstrated similar trends as transmission properties, with maintained weaker grip force across age groups in lama4−/−. Interestingly, both transmission properties and hind-limb grip force in aged wild-types resembled those observed in adult lama4−/−. Most significantly, altered expression of laminin-α4 was noted at the wild-type NMJs prior to the observed decline in transmission properties, suggesting that altered laminin-α4 expression precedes the decline of neurotransmission in aging wild-types. These findings significantly support the role of laminin-α4 in maintenance of the NMJ during aging.

Defects in synaptic transmission at the neuromuscular junction precede motor deficits in a TDP‐43 Q331K transgenic mouse model of amyotrophic lateral sclerosis

“…We next followed‐up our functional work with a morphologic examination of the presynaptic nerve terminal of Q331K mutants to determine the volume of synaptic vesicle protein staining with respect to postsynaptic AChRs (SV2/AChR volume ratio) (37, 42). At age 3 mo, NTg, WT, and Q331K nerve terminals displayed dense SV2 staining that was aligned directly above postsynaptic AChRs ( Fig.…”

“…These stained NMJs were reconstructed into three dimensions (3D) utilizing a surface‐rendering algorithm that was based on local contrast in fluorescent z‐stacks as previously described (37, 40, 41). Using this approach, we were able to investigate changes in the expansion of SV2 volume—the size of the total vesicle pool—in relation to the postsynaptic AChRs, rather than the standard 2‐dimensional colocalization approach (37, 42). To solely quantify the volume of SV2 staining at these 3D‐rendered NMJs, neurofilament‐positive motor axons were carefully and accurately cut off from the reconstructed SV2‐labeled vesicles.…”

“…We subsequently undertook an examination of SV2 volume relative to the postsynaptic end plate (SV2/AChR volume ratio) by dividing the volume of 3D‐reconstructed, SV2‐stained synaptic vesicles over the volume of 3D‐reconstructed postsynaptic end plates. The postsynaptic NMJ (end plate) volume was defined as the entire 3D‐reconstructed endplate, including interstitial spaces, as we have previously reported (37, 42).…”

NMJ maintenance and repair in aging

“…Aging also affects the expression of genes known to have important functions at the NMJ, but are not critical for its formation or function. For example, levels of laminin-α4 [26] and of the fibroblast growth factor binding protein 1 (FGFBP1) [27] decrease in old skeletal muscles. The dysregulated expression of these synaptic molecules, in addition to altered cholinergic transmission, has been shown to precipitate age- and disease-related morphological and functional changes at NMJs [28,29].…”

“…Interestingly, Zainul Z and colleagues recently showed that the transmembrane domain of collagen XIII promotes regeneration of NMJs following injury to motor axons [71]. NMJs have also been found to progressively degenerate, characterized by increased fragmentation and denervation, in mice lacking laminin-α4 [26]. These findings suggest that augmenting levels and activity of collagen XIII and laminin-α4 may slow and reverse damages at NMJs that occur with advancing age.…”