ErbB4-Neuregulin Signaling Modulates Synapse Development and Dendritic Arborization through Distinct Mechanisms

Abstract

Perturbations in neuregulin-1 (NRG1)/ErbB4 function have been associated with schizophrenia. Affected patients exhibit altered levels of these proteins and display hypofunction of glutamatergic synapses as well as altered neuronal circuitry. However, the role of NRG1/ErbB4 in regulating synapse maturation and neuronal process formation has not been extensively examined. Here we demonstrate that ErbB4 is expressed in inhibitory interneurons at both excitatory and inhibitory postsynaptic sites. Overexpression of ErbB4 postsynaptically enhances size but not number of presynaptic inputs. Conversely, knockdown of ErbB4 using shRNA decreases the size of presynaptic inputs, demonstrating a specific role for endogenous ErbB4 in synapse maturation. Using ErbB4 mutant constructs, we demonstrate that ErbB4-mediated synapse maturation requires its extracellular domain, whereas its tyrosine kinase activity is dispensable for this process. We also demonstrate that depletion of ErbB4 decreases the number of primary neurites and that stimulation of ErbB4 using a soluble form of NRG1 results in exuberant dendritic arborization through activation of the tyrosine kinase domain of ErbB4 and the phosphoinositide 3-kinase pathway. These findings demonstrate that NRG1/ErbB4 signaling differentially regulates synapse maturation and dendritic morphology via two distinct mechanisms involving trans-synaptic signaling and tyrosine kinase activity, respectively.Although central nervous system synapses utilize a variety of brain-specific molecules to mediate contact formation and maturation, some of the proteins implicated in this process are also major players in neuromuscular junction development. Among these shared molecules are NRG1 3 and its receptor, ErbB4, which are expressed in both the developing and adult brain. Neuregulins comprise a family of four related genes (nrg1-4), each producing a large number of isoforms via differential promoter usage and alternative splicing (1, 2). NRGs contain EGF-like repeats, which enable them to bind to and activate EGF family receptors (ErbB2-4). Previous studies showed that NRG1 is initially synthesized as a transmembrane protein, which then undergoes proteolytic processing, whereby the extracellular EGF-containing fragment is released into the extracellular environment. The remaining intracellular fragment has been shown to translocate into the nucleus, where it regulates neuronal survival and transcription of PSD-95 (3, 4). Proteolytic processing of NRG1 is also regulated by neuronal activity and by interaction with ErbB receptors (3, 5). NRG1 is widely expressed throughout development and adulthood, with the highest expression in nervous tissue (6) and is essential for survival. In the central nervous system NRG1 is also required for differentiation, migration, and development of neurons and glia as well as for axonal myelination and pathfinding, dendritic development, and neurotransmitter receptor maintenance. During development, NRG1-ErbB signaling mediates radial glia maintenance and elonga...

Neuregulin-1 Signals from the Periphery Regulate AMPA Receptor Sensitivity and Expression in GABAergic Interneurons in Developing Neocortex

“…These results suggest that NRG1 slowly promotes developmental maturation of and/or synapse formation at excitatory nerve terminals in this neuronal population. This explanation is in accordance with the reports that the activation of ErbB4 promotes presynaptic development of excitatory input synapses in hippocampal pyramidal cells Krivosheya et al, 2008) as well as in cultured cortical GABAergic neurons (Ting et al, 2011).…”

Neuregulin 1-ErbB4-PI3K signaling in schizophrenia and phosphoinositide 3-kinase-p110δ inhibition as a potential therapeutic strategy

“…Class IA PI3Ks are activated by receptor tyrosine kinases and exist as obligate heterodimers (25,26) consisting of a p110 catalytic subunit (PIK3CA, PIK3CB, and PIK3CD encode the isoforms p110α, p110β, and p110δ, respectively) and a p85 regulatory subunit (PIK3R1, PIK3R2, and PIK3R3, which encode the p85α, p85β, and p55γ isoforms, respectively) (25). PI3K signaling in turn mediates NRG1-induced cell survival and neuronal and synaptic development (21)(22)(23)(24). These findings implicate aberrant PI3K signaling as a potential pathogenic effect of schizophrenia-associated variation in ErbB4.…”

“…In particular, risk polymorphisms in ErbB4 (rs7598440, rs839523, and rs707284) predict increased expression of the ErbB4 CYT-1 receptor (5, 7), one of two biologically occurring ErbB4 isoforms, the other being ErbB4 CYT-2 (21). Unlike ErbB4 CYT-2, ErbB4 CYT-1 includes a phosphoinositide 3-kinase (PI3K)-binding site and is capable of activating the PI3K pathway (21)(22)(23)(24). Class IA PI3Ks are activated by receptor tyrosine kinases and exist as obligate heterodimers (25,26) consisting of a p110 catalytic subunit (PIK3CA, PIK3CB, and PIK3CD encode the isoforms p110α, p110β, and p110δ, respectively) and a p85 regulatory subunit (PIK3R1, PIK3R2, and PIK3R3, which encode the p85α, p85β, and p55γ isoforms, respectively) (25).…”

Are There Schizophrenia Genetic Markers and Mutations? A Systematic Review and Meta-Analyses

“…It is possible that elevated levels of cleaved ECTO-ERBB4 may neutralise the membrane-bound CRD-NRG1 (encoded by NRG1-Type III synthesized by pyramidal neurons) resulting in reduced cross-talk between NRG1 and ERBB4. Importantly, NRG1-ERBB4 signaling has been shown to be important for the development of inhibitory circuits, and a reduction in NRG1-ERBB4 cross-talk does result in reduced molecular markers of inhibitory synapses [81] [82].…”