The role of synaptic laminins-α4 and -β2 in maturation and maintenance of the neuromuscular synapse

Abstract

The proper development, maturation and maintenance of the neuromuscular junction (NMJ) are critical for ensuring efficient neurotransmission. Proteins found within the basal lamina of the synaptic cleft, namely the laminins family, are heavily involved in maintaining normal structure and function of the NMJ. In particular, individual laminin chains such as laminin-β2, -α4 and -α5 play essential roles in organisation and maintenance of the NMJ. These laminin chains interact together to form three synapse specific laminin heterotrimers; laminin-221 (α2β2γ1), laminin-421 (α4β2γ1) and laminin-521 (α5β2γ1).Laminin-β2 is the common laminin chain found in each of these synapse specific laminin heterotrimers. In-vitro it has been shown to play a key role in the organisation of the presynaptic elements at the NMJ via its interaction and clustering of the N-and P/Q-type voltage-gated calciums (VGCCs). During development of the NMJ, both N-and P/Q-type VGCCs are involved in mediating neurotransmitter release. As the NMJ matures, P/Q-type becomes the dominant channel involved in release while N-type takes on the role of fine-tuning calcium influx. Laminin-α4, on the other hand, ensures proper alignment of presynaptic active zones to postjunctional folds at the NMJ, with its loss resulting in misalignment of these specialisations. Furthermore, this laminin chain has been shown to be involved in the maintenance of the NMJ with the observation of premature ageing features at 6 months of age (6MO) in laminin-α4 deficient mice (lama4 NMJs.In conclusion, this thesis has identified the significant roles laminins-α4 and -β2 play at the NMJ during development, maturation and maintenance. Results suggest that laminin-β2 is not only an important regulator of the presynaptic maturation through the switching of VGCCs and clustering of P/Q-type VGCCs, but also equally important for maturation of the postsynaptic apparatus. Ipropose that laminin-β2 is important for the maturation of each component at the NMJ. The altered responses in transmission properties presented by aged wild-type NMJs which resembled adult lama4 -/-, preceded by altered expression of laminin-α4 chain, strongly suggest that laminin-α4 is not only important for maintaining proper alignment of pre-to postsynaptic NMJ, but also essentially important for maintaining a healthy adult NMJ. Finally, I observed early alterations in expression of laminin-α4 at the NMJs in diseased mouse model of ALS prior to any appearance of neuromotor impairment, suggesting a potential involvement of laminins in NMJ degeneration associated with neuromuscular disorders such as ALS.ii