Structural distinctions in BMPs underlie divergent signaling in spinal neurons

Abstract

BackgroundIn dorsal spinal neurons and monocytes, bone morphogenetic protein (BMP)7 activates distinct transduction pathways, one leading to inductive specification and the other to axon orientation and chemotaxis. BMP7-evoked induction, also stimulated by the closely related BMP6, acts through a Smad cascade, leading to nuclear signaling, and is not BMPR subunit selective. Orientation is evoked by BMP7, but not by BMP6, through PI3K-dependent cytoskeletal activation mediated by the type II BMPRs, ActRIIA and BMPRII and is independent of the Smad cascade. The responses can be stimulated concurrently and suggest that BMP7, but not BMP6, can selectively activate BMPR subunits that engage the divergent paths. Although structural and biochemical analyses of selected BMP/BMPR interfaces have identified key regions of interaction, how these translate into function by related BMPs is poorly understood. To determine the mechanisms underlying the distinct activities of BMP7 and the disparate properties of BMP7 and BMP6 in spinal cord development, we have performed a family-wide structure/function analysis of BMPs and used the information to predict and test sites within BMPs that may control agonist properties, in particular the ability of a BMP to orient axons, through interactions with BMPRs.ResultsWe demonstrate that whereas all BMPs can induce dorsal neurons, there is selectivity in the ability also to orient axons or evoke growth cone collapse. The degree to which a BMP orients is not predictable by overall protein similarity with other BMPs but comparison of sequences of potent and weakly orienting BMPs with that of the non-orienting BMP6 revealed three candidate positions within the BMPs at which the amino acid residues may confer or obstruct orienting ability. Residue swapping analysis has identified one residue, Gln48 in BMP6, that blocks axon orienting ability. Replacing Gln48 with any of the amino acids present at the equivalent residue position in the orienting subset of BMPs confers orienting activity on BMP6. Conversely, swapping Gln48 into BMP7 reduces orienting ability. The inductive capacity of the BMPs was unchanged by these residue swaps.ConclusionsThe results suggest that the presence of the Gln48 residue in BMP6 is structurally inhibitory for BMP/BMPR interactions that result in the activation of intracellular signaling, leading to axon orientation. Moreover, since residue 48 in BMP7 and the corresponding residue in BMP2 are important for type II BMPR binding, our results provide a basis for a mechanistic understanding of the diverse activities of BMPs in spinal cord development.

BMP signaling in telencephalic neural cell specification and maturation

“…However, combinatorial interactions of different type I and II receptors should allow selectivity and specificity of ligand binding as well as intracellular signaling in target cells. Numerous in vivo examples confirm the unique functions for an individual BMPs that are not shared even for closely related ligands with similar receptor binding affinities in vitro (Saremba et al, 2008; Meynard et al, 2009; Perron and Dodd, 2011, 2012). Further study into the molecular mechanisms that drive such functional specificity in living organisms is needed.…”

“…An intriguing functional and mechanistic question is that, whereas the closely related BMP-6 and -7 both induce the differentiation of commissural neurons, only BMP-7 is able to orients its axons in vitro and is required for appropriate axon projection in vivo (Perron and Dodd, 2011, 2012). Both ligands have been reported to bind hetero-dimers consisting of Acvr2A or Bmpr2 with Acvr1, Bmpr1A or Bmpr1B in numerous cellular models (Mueller and Nickel, 2012).…”

“…Both ligands have been reported to bind hetero-dimers consisting of Acvr2A or Bmpr2 with Acvr1, Bmpr1A or Bmpr1B in numerous cellular models (Mueller and Nickel, 2012). However, the facts that a single amino acid swapping allows BMP-6 to orient axons and vice versa and that binding of BMP-6 to type I receptor depends on N-glycosylation, suggest that distinct receptor recruitment is involved in these functional differences (Saremba et al, 2008; Perron and Dodd, 2012). Changes in expression of BMP receptor subunits at growth cones have been shown after motor neuronal differentiation (Benavente et al, 2012).…”

Bone morphogenetic protein signaling in vertebrate motor neurons and neuromuscular communication

“…In addition, BMPRI kinase activity that induces Smad-dependent signaling is required for cell specification but not for changes at the growth cone which require PI3K activity (Perron and Dodd, 2011 ). Other BMPs, including BMP-9, -4, -2, -5, 6, GDF-5 and -6 also have the ability to induce cell specification but only BMP-9, -4, -2 exhibit axon orientating activities (Perron and Dodd, 2012 ). Interestingly, immature mouse muscle fibers express BMP-6 whereas it is undetectable in muscle fibers or in post-synaptic domains (Wang et al, 2007a ).…”

The Role of Smad Proteins for Development, Differentiation and Dedifferentiation of Neurons

“…Both roof plate derived BMP6 and BMP7 are evoking Smad1,5,8 phosphorylation and subsequent induction of distinct dorsal interneurons (dI) via BMP receptor I activation [77][78][79]. While the BMP7/pSmad1,5 activated induction of dI1, dI3 and dI5 is independent of the patterned expression of progenitor proteins, e.g.…”