LRP4 Serves as a Coreceptor of Agrin

Abstract

SUMMARY Formation of the neuromuscular junction (NMJ) requires agrin, a factor released from motoneurons, and MuSK, a transmembrane tyrosine kinase that is activated by agrin. However, how signal is transduced from agrin to MuSK remains unclear. Here we report that low-density lipoprotein receptor (LDLR)-related protein (LRP) 4 (LRP4) functions as a co-receptor of agrin. LRP4 is specifically expressed in myotubes and is concentrated at the NMJ. The extracellular domain of LRP4 interacts with neuronal, but not muscle, agrin. Expression of LRP4 enables agrin binding activity and MuSK signaling in cells that otherwise does not respond to agrin. Suppression of LRP4 expression attenuates agrin binding activity, agrin-induced MuSK tyrosine phosphorylation and AChR clustering in muscle cells. LRP4 also interacts with MuSK in a manner that is stimulated by agrin. Finally, we showed that LRP4 becomes tyrosine-phosphorylated in agrin-stimulated muscle cells. These observations identify LRP4 as a functional co-receptor of agrin that is necessary for agrin-induced MuSK signaling and AChR clustering.

Neuronal LRP4 regulates synapse formation in the developing CNS

“…Moreover, cumulative evidence indicates that LRP4 plays crucial roles in synaptic function in the CNS (Kim et al, 2008;Wu et al, 2012;Zhang et al, 2008). However, the molecular mechanisms regulating these functions are still mostly unknown.…”

“…This indicates that the LRP4 knockdown-mediated reduction of synapse-like specializations and spines is accompanied by a reduced number of synapses. Agrin is required for the LRP4-mediated effect on dendritic development and synapse formation At the neuromuscular junction, LRP4 is the receptor for the motoneuron-derived extracellular matrix proteoglycan agrin (Kim et al, 2008;Zhang et al, 2008). We hypothesized that agrin might also regulate the effect of LRP4 on dendritic morphology in cultured CNS neurons.…”

“…At the NMJ, LRP4 forms a complex with the muscle-specific tyrosine kinase MuSK in the muscle fiber plasma membrane; this complex serves as the receptor for the motoneuron-derived extracellular matrix protein agrin (Kim et al, 2008;Zhang et al, 2008). Binding of agrin to the tetrameric LRP4/MuSK complex activates an intracellular signaling cascade in the muscle fiber resulting in the formation of most, if not all, postsynaptic specializations (Tintignac et al, 2015).…”

MuSK IgG4 autoantibodies cause myasthenia gravis by inhibiting binding between MuSK and Lrp4

“…AChR clustering and synapse formation are orchestrated by neuronally released Agrin, which binds to low-density lipoprotein receptor-related protein 4 (Lrp4), a member of the lipoprotein receptor-related protein family, causing Lrp4 to bind and activate MuSK (18)(19)(20). Once tyrosine-phosphorylated, MuSK recruits Dok-7, an adaptor protein that becomes phosphorylated and recruits additional signaling molecules essential for synapse formation (21)(22)(23).…”

Disruption of Lrp4 function by genetic deletion or pharmacological blockade increases bone mass and serum sclerostin levels

“…LRP4 has a well-recognized role in its interplay with agrin and muscle-specific kinase (MuSK) in the formation and stabilization of the neuromuscular junction (NMJ) (17)(18)(19), a synaptic connection that is required for communication between motor neurons and muscle fibers. Accordingly, Lrp4 loss-of-function results in perinatal lethality in mice due to breathing failure (17).…”