The Interaction of TRAF6 With Neuroplastin Promotes Spinogenesis During Early Neuronal Development

Abstract

Correct brain wiring depends on reliable synapse formation. Nevertheless, signaling codes promoting synaptogenesis are not fully understood. Here, we report a spinogenic mechanism that operates during neuronal development and is based on the interaction of tumor necrosis factor receptor-associated factor 6 (TRAF6) with the synaptic cell adhesion molecule neuroplastin. The interaction between these proteins was predicted in silico and verified by co-immunoprecipitation in extracts from rat brain and co-transfected HEK cells. Binding assays show physical interaction between neuroplastin’s C-terminus and the TRAF-C domain of TRAF6 with a Kd value of 88 μM. As the two proteins co-localize in primordial dendritic protrusions, we used young cultures of rat and mouse as well as neuroplastin-deficient mouse neurons and showed with mutagenesis, knock-down, and pharmacological blockade that TRAF6 is required by neuroplastin to promote early spinogenesis during in vitro days 6-9, but not later. Time-framed TRAF6 blockade during days 6–9 reduced mEPSC amplitude, number of postsynaptic sites, synapse density and neuronal activity as neurons mature. Our data unravel a new molecular liaison that may emerge during a specific window of the neuronal development to determine excitatory synapse density in the rodent brain.

Neuroplastin expression is essential for hearing and hair cell PMCA expression

“…in glia (Langnaese et al 1997). Neuroplastin has been shown to interact as a ligand with the fibroblast growth factor (FGF) receptor (Owczarek et al 2010), gamma-aminobutyric acid type A (GABAA) receptors (Sarto-Jackson et al 2012), S100A8/A9, basigin (Sakaguchi et al 2016;Sumardika et al 2019), mesencephalic astrocyte-derived neurotrophic factor (MANF) (Yagi et al 2020), tumor necrosis factor receptor-associated factor 6 (TRAF6) (Vemula et al 2020), and with itself undergoing homophilic binding (Langnaese et al 1997;Owczarek et al 2010). Furthermore, plasma membrane trafficking of monocarboxylic acid transporter 2 (MCT-2, SLC16A7) and XK-related protein 8 (Xkr8) may require neuroplastin as chaperone (Wilson et al 2013;Suzuki et al 2016).…”

Neuroplastin in Neuropsychiatric Diseases

“…Furthermore, TRAF6 is proposed to play a role in Alzheimer's disease (AD) and neuroinflammation [86]. TRAF binding sites have been identified in neuroplastin [38,60] (see below, association of neuroplastin to cancer). In particular, we showed that the binding of TRAF6 to its binding motif (RKRPDEVPD) within the C-terminal domain of neuroplastin promotes spinogenesis [60].…”

“…TRAF binding sites have been identified in neuroplastin [38,60] (see below, association of neuroplastin to cancer). In particular, we showed that the binding of TRAF6 to its binding motif (RKRPDEVPD) within the C-terminal domain of neuroplastin promotes spinogenesis [60]. Genetic inactivation of Nptn or TRAF6-RNA interference strongly reduced the protrusion density of young hippocampal neurons, which could be rescued by the over-expression of Np55 or Np65.…”

“…Genetic inactivation of Nptn or TRAF6-RNA interference strongly reduced the protrusion density of young hippocampal neurons, which could be rescued by the over-expression of Np55 or Np65. In mature neurons, TRAF6 does not co-localize with neuroplastin and does not promote spinogenesis, thus limiting the function of neuroplastin-TRAF6 interactions to the early neuronal spinogenetic phase [60]. Synapse malformation and alterations in synaptic density occurring during early neuronal development have been associated with schizophrenia [88,89].…”

“…Furthermore, the paths to ASDs may involve unbalanced excitatory-inhibitory synaptic transmission and abnormal synaptogenesis [122]. Several studies have observed an imbalance of excitatory-inhibitory transmission and altered synaptogenesis in different Nptn-deficient mice [40,51,59,60,123]. In addition, neuroplastin-deficient mice displayed altered social interactions avoiding unfamiliar mice [50].…”