Novel procedure to investigate the effect of phosphorylation on protein complex formation in vitro and in cells

CRMP2 Participates in Regulating Mitochondrial Morphology and Motility in Alzheimer's Disease

Mitochondrial bioenergetics and dynamics (alterations in morphology and motility of mitochondria) play critical roles in neuronal reactions to varying energy requirements in health and disease. In Alzheimer's disease (AD), mitochondria undergo excessive fission and become less motile. The mechanisms leading to these alterations are not completely clear. Here, we show that collapsin response mediator protein 2 (CRMP2) is hyperphosphorylated in AD and that is accompanied by a decreased interaction of CRMP2 with Drp1, Miro 2, and Mitofusin 2, which are proteins involved in regulating mitochondrial morphology and motility. CRMP2 was hyperphosphorylated in postmortem brain tissues of AD patients, in brain lysates, and in cultured cortical neurons from the double transgenic APP/PS1 mice, an AD mouse model. CRMP2 hyperphosphorylation and dissociation from its binding partners correlated with increased Drp1 recruitment to mitochondria, augmented mitochondrial fragmentation, and reduced mitochondrial motility. (S)-lacosamide ((S)-LCM), a small molecule that binds to CRMP2, decreased its phosphorylation at Ser 522 and Thr 509/514, and restored CRMP2's interaction with Miro 2, Drp1, and Mitofusin 2. This was paralleled by decreased Drp1 recruitment to mitochondria, diminished mitochondrial fragmentation, and improved motility of the organelles. Additionally, (S)-LCM-protected cultured cortical AD neurons from cell death. Thus, our data suggest that CRMP2, in a phosphorylation-dependent manner, participates in the regulation of mitochondrial morphology and motility, and modulates neuronal survival in AD.

Facile synthesis of peanut-like Sn-doped silica nano-adsorbent for affinity separation of proteins

A peanut-like hollow silica (denoted as p-l-hSiO₂) adsorbent is prepared in a facile method, which is composed of several silica nanospheres and has an average diameter of 22 nm, with thickness of 5 nm. Its Brunauer–Emmett–Teller (BET) surface area, pore volume and pore size are 258.9 m² g⁻¹, 1.56 cm³ g⁻¹ and 3.9 nm, respectively. Then the afforded p-l-hSiO₂/GSH adsorbent is applied to purify glutathione S-transferases-tagged (denoted as GST-tagged) proteins. It is found that the p-l-hSiO₂ adsorbent exhibits a specific adsorption, a high binding capacity (6.80 mg g⁻¹), good recycling performance and high recovery (90.1%) to the target proteins, showing promising potential for the affinity separation of GST-tagged proteins.

Peanut-like Sn-doped hollow silica adsorbent is prepared in a facile method, which exhibits a specific adsorption, a high binding capacity , good recycle performance and high recovery to the GST-tagged proteins.

CRMP2 Is Involved in Regulation of Mitochondrial Morphology and Motility in Neurons

Regulation of mitochondrial morphology and motility is critical for neurons, but the exact mechanisms are unclear. Here, we demonstrate that these mechanisms may involve collapsin response mediator protein 2 (CRMP2). CRMP2 is attached to neuronal mitochondria and binds to dynamin-related protein 1 (Drp1), Miro 2, and Kinesin 1 light chain (KLC1). Treating neurons with okadaic acid (OA), an inhibitor of phosphatases PP1 and PP2A, resulted in increased CRMP2 phosphorylation at Thr509/514, Ser522, and Thr555, and augmented Drp1 phosphorylation at Ser616. The CRMP2-binding small molecule (S)-lacosamide ((S)-LCM) prevented an OA-induced increase in CRMP2 phosphorylation at Thr509/514 and Ser522 but not at Thr555, and also failed to alleviate Drp1 phosphorylation. The increased CRMP2 phosphorylation correlated with decreased CRMP2 binding to Drp1, Miro 2, and KLC1. (S)-LCM rescued CRMP2 binding to Drp1 and Miro 2 but not to KLC1. In parallel with CRMP2 hyperphosphorylation, OA increased mitochondrial fission and suppressed mitochondrial traffic. (S)-LCM prevented OA-induced alterations in mitochondrial morphology and motility. Deletion of CRMP2 with a small interfering RNA (siRNA) resulted in increased mitochondrial fission and diminished mitochondrial traffic. Overall, our data suggest that the CRMP2 expression level and phosphorylation state are involved in regulating mitochondrial morphology and motility in neurons.

NDRG2 regulates adherens junction integrity to restrict colitis and tumourigenesis

BACKGROUND

Paracellular barriers play an important role in the pathogenesis of Inflammatory bowel disease (IBD) and maintain gut homeostasis. N-myc downstream-regulated gene 2 (NDRG2) has been reported to be a tumour suppressor gene and to inhibit colorectal cancer metastasis. However, whether NDRG2 affects colitis initiation and colitis-associated colorectal cancer is unclear.

METHODS

Intestine-specific Ndrg2 deficiency mice (Ndrg2^ΔIEC) were subjected to DSS- or TNBS-induced colitis, and AOM-DSS-induced colitis-associated tumour. HT29 cells, Caco2 cells, primary intestinal epithelial cells (IECs) from Ndrg2^ΔIEC mice, mouse embryo fibroblasts (MEFs) from systemic Ndrg2 knockout mice, HEK293 cells and human UC and DC specimens were used to investigate NDRG2 function in colitis and colitis-associated tumour.

FINDINGS

Ndrg2 loss led to adherens junction (AJ) structure destruction via E-cadherin expression attenuation, resulting in diminished epithelial barrier function and increased intestinal epithelial permeability. Mechanistically, NDRG2 enhanced the interaction of E3 ligase FBXO11 with Snail, the repressor of E-cadherin, to promote Snail degradation by ubiquitination and maintained E-cadherin expression. In human ulcerative colitis patients, reduced NDRG2 expression is positively correlated with severe inflammation.

INTERPRETATION

These findings demonstrate that NDRG2 is an essential colonic epithelial barrier regulator and plays an important role in gut homeostasis maintenance and colitis-associated tumour development.

FUNDING

National Natural Science Foundation of China (No. 81770523, 31571437, 81672751), Creative Research Groups of China (No. 81421003), State Key Laboratory of Cancer Biology Project (CBSKL2019ZZ11, CBSKL201406, CBSKL2017Z08 and CBSKL2017Z11), Fund for Distinguished Young Scholars of ShaanXi province (2019JC-22).

GSK3-CRMP2 signaling mediates axonal regeneration induced by Pten knockout

Knockout of phosphatase and tensin homolog (PTEN-/-) is neuroprotective and promotes axon regeneration in mature neurons. Elevation of mTOR activity in injured neurons has been proposed as the primary underlying mechanism. Here we demonstrate that PTEN-/- also abrogates the inhibitory activity of GSK3 on collapsin response mediator protein 2 (CRMP2) in retinal ganglion cell (RGC) axons. Moreover, maintenance of GSK3 activity in Gsk3S/A knockin mice significantly compromised PTEN-/--mediated optic nerve regeneration as well as the activity of CRMP2, and to a lesser extent, mTOR. These GSK3S/A mediated negative effects on regeneration were rescued by viral expression of constitutively active CRMP2T/A, despite decreased mTOR activation. Gsk3S/A knockin or CRMP2 inhibition also decreased PTEN-/- mediated neurite growth of RGCs in culture and disinhibition towards CNS myelin. Thus, the GSK3/CRMP2 pathway is essential for PTEN-/- mediated axon regeneration. These new mechanistic insights may help to find novel strategies to promote axon regeneration.

Deletion of collapsin response mediator protein 4 results in abnormal layer thickness and elongation of mitral cell apical dendrites in the neonatal olfactory bulb

Collapsin response mediator protein 4 (CRMP4), a member of the CRMP family, is involved in the pathogenesis of neurodevelopmental disorders such as schizophrenia and autism. Here, we first compared layer thickness of the olfactory bulb between wild-type (WT) and CRMP4-knockout (KO) mice. The mitral cell layer (MCL) was significantly thinner, whereas the external plexiform layer (EPL) was significantly thicker in CRMP4-KO mice at postnatal day 0 (PD0) compared with WTs. However, differences in layer thickness disappeared by PD14. No apoptotic cells were found in the MCL, and the number of mitral cells (MCs) identified with a specific marker (i.e. Tbx21 antibody) did not change in CRMP4-KO neonates. However, DiI-tracing showed that the length of mitral cell apical dendrites was greater in CRMP4-KO neonates than in WTs. In addition, expression of CRMP4 mRNA in WT mice was most abundant in the MCL at PD0 and decreased afterward. These results suggest that CRMP4 contributes to dendritic elongation. Our in vitro studies showed that deletion or knockdown of CRMP4 resulted in enhanced growth of MAP2-positive neurites, whereas overexpression of CRMP4 reduced their growth, suggesting a new role for CRMP4 as a suppressor of dendritic elongation. Overall, our data suggest that disruption of CRMP4 produces a temporary alteration in EPL thickness, which is constituted mainly of mitral cell apical dendrites, through the enhanced growth of these dendrites.

Collapsin response mediator protein 4 regulates growth cone dynamics through the actin and microtubule cytoskeleton

Coordinated control of the growth cone cytoskeleton underlies axon extension and guidance. Members of the collapsin response mediator protein (CRMP) family of cytosolic phosphoproteins regulate the microtubule and actin cytoskeleton, but their roles in regulating growth cone dynamics remain largely unexplored. Here, we examine how CRMP4 regulates the growth cone cytoskeleton. Hippocampal neurons from CRMP4-/- mice exhibited a selective decrease in axon extension and reduced growth cone area, whereas overexpression of CRMP4 enhanced the formation and length of growth cone filopodia. Biochemically, CRMP4 can impact both microtubule assembly and F-actin bundling in vitro. Through a structure function analysis of CRMP4, we found that the effects of CRMP4 on axon growth and growth cone morphology were dependent on microtubule assembly, whereas filopodial extension relied on actin bundling. Intriguingly, anterograde movement of EB3 comets, which track microtubule protrusion, slowed significantly in neurons derived from CRMP4-/- mice, and rescue of microtubule dynamics required CRMP4 activity toward both the actin and microtubule cytoskeleton. Together, this study identified a dual role for CRMP4 in regulating the actin and microtubule growth cone cytoskeleton.

Collapsin response mediator protein 5 (CRMP5) induces mitophagy, thereby regulating mitochondrion numbers in dendrites

Degradation of damaged mitochondria by mitophagy is an essential process to ensure cell homeostasis. Because neurons, which have a high energy demand, are particularly dependent on the mitochondrial dynamics, mitophagy represents a key mechanism to ensure correct neuronal function. Collapsin response mediator proteins 5 (CRMP5) belongs to a family of cytosolic proteins involved in axon guidance and neurite outgrowth signaling during neural development. CRMP5, which is highly expressed during brain development, plays an important role in the regulation of neuronal polarity by inhibiting dendrite outgrowth at early developmental stages. Here, we demonstrated that CRMP5 was present in vivo in brain mitochondria and is targeted to the inner mitochondrial membrane. The mitochondrial localization of CRMP5 induced mitophagy. CRMP5 overexpression triggered a drastic change in mitochondrial morphology, increased the number of lysosomes and double membrane vesicles termed autophagosomes, and enhanced the occurrence of microtubule-associated protein 1 light chain 3 (LC3) at the mitochondrial level. Moreover, the lipidated form of LC3, LC3-II, which triggers autophagy by insertion into autophagosomes, enhanced mitophagy initiation. Lysosomal marker translocates at the mitochondrial level, suggesting autophagosome-lysosome fusion, and induced the reduction of mitochondrial content via lysosomal degradation. We show that during early developmental stages the strong expression of endogenous CRMP5, which inhibits dendrite growth, correlated with a decrease of mitochondrial content. In contrast, the knockdown or a decrease of CRMP5 expression at later stages enhanced mitochondrion numbers in cultured neurons, suggesting that CRMP5 modulated these numbers. Our study elucidates a novel regulatory mechanism that utilizes CRMP5-induced mitophagy to orchestrate proper dendrite outgrowth and neuronal function.

Identification of a proline-rich inositol polyphosphate 5-phosphatase (PIPP)•collapsin response mediator protein 2 (CRMP2) complex that regulates neurite elongation

Neuron polarization is essential for the formation of one axon and multiple dendrites, establishing the neuronal circuitry. Phosphoinositide 3-kinase (PI3K) signaling promotes axon selection and elongation. Here we report in hippocampal neurons siRNA knockdown of the proline-rich inositol polyphosphate 5-phosphatase (PIPP), which degrades PI3K-generated PtdIns(3,4,5)P(3), results in multiple hyperelongated axons consistent with a polarization defect. We identify collapsin response mediator protein 2 (CRMP2), which regulates axon selection by promoting WAVE1 delivery via Kinesin-1 motors to the axon growth cone, as a PIPP-interacting protein by Y2H screening, direct binding studies, and coimmunoprecipitation of an endogenous PIPP, CRMP2, and Kinesin-1 complex from brain lysates. The C-terminal growth cone-targeting domain of PIPP facilitates its interaction with CRMP2. PIPP growth cone localization is CRMP2-dependent. PIPP knockdown in PC12 cells promotes neurite elongation, WAVE1 and Kinesin-1 growth cone localization, whereas knockdown of CRMP2 exhibits the opposite phenotype, with shorter neurites and decreased WAVE1/Kinesin-1 at the growth cone. In contrast, CRMP2 overexpression promotes neurite elongation, a phenotype rescued by full-length PIPP, or expression of the CRMP2-binding PIPP domain. Therefore this study identifies PIPP and CRMP2 exert opposing roles in promoting axon selection and neurite elongation and the complex between these proteins serves to regulate the localization of effectors that promote neurite extension.

Brief alteration of NMDA or GABAA receptor-mediated neurotransmission has long term effects on the developing cerebral cortex

Neurotransmitter signaling is essential for physiologic brain development. Sedative and anticonvulsant agents that reduce neuronal excitability via antagonism at N-methyl-D-aspartate receptors (NMDARs) and/or agonism at gamma-aminobutyric acid subtype A receptors (GABA(A)Rs) are applied frequently in obstetric and pediatric medicine. We demonstrated that a 1-day treatment of infant mice at postnatal day 6 (P6) with the NMDAR antagonist dizocilpine or the GABA(A)R agonist phenobarbital not only has acute but also long term effects on the cerebral cortex. Changes of the cerebral cortex proteome 1 day (P7), 1 week (P14), and 4 weeks (P35) following treatment at P6 suggest that a suppression of synaptic neurotransmission during brain development dysregulates proteins associated with apoptosis, oxidative stress, inflammation, cell proliferation, and neuronal circuit formation. These effects appear to be age-dependent as most protein changes did not occur in mice subjected to such pharmacological treatment in adulthood. Previously performed histological evaluations of the brains revealed widespread apoptosis and decreased cell proliferation following such a drug treatment in infancy and are thus consistent with brain protein changes reported in this study. Our results point toward several pathways modulated by a reduction of neuronal excitability that might interfere with critical developmental events and thus affirm concerns about the impact of NMDAR- and/or GABA(A)R-modulating drugs on human brain development.

Relative resistance of Cdk5-phosphorylated CRMP2 to dephosphorylation

Collapsin response mediator protein 2 (CRMP2) binds to microtubules and regulates axon outgrowth in neurons. This action is regulated by sequential phosphorylation by the kinases cyclin-dependent kinase 5 (Cdk5) and glycogen synthase kinase 3 (GSK3) at sites that are hyperphosphorylated in Alzheimer disease. The increased phosphorylation in Alzheimer disease could be due to increases in Cdk5 and/or GSK3 activity or, alternatively, through decreased activity of a CRMP phosphatase. Here we establish that dephosphorylation of CRMP2 at the residues targeted by GSK3 (Ser-518/Thr-514/Thr-509) is carried out by a protein phosphatase 1 family member in vitro, in neuroblastoma cells, and primary cortical neurons. Inhibition of GSK3 activity using insulin-like growth factor-1 or the highly selective inhibitor CT99021 causes rapid dephosphorylation of CRMP2 at these sites. In contrast, pharmacological inhibition of Cdk5 using purvalanol results in only a gradual and incomplete dephosphorylation of CRMP2 at the site targeted by Cdk5 (Ser-522), suggesting a distinct phosphatase targets this residue. A direct comparison of dephosphorylation at the Cdk5 versus GSK3 sites in vitro shows that the Cdk5 site is comparatively resistant to phosphatase treatment. The presence of the peptidyl-prolyl isomerase enzyme, Pin1, does not affect dephosphorylation of Ser-522 in vitro, in cells, or in Pin1 transgenic mice. Instead, the relatively high resistance of this site to phosphatase treatment is at least in part due to the presence of basic residues located nearby. Similar sequences in Tau are also highly resistant to phosphatase treatment. We propose that relative resistance to phosphatases might be a common feature of Cdk5 substrates and could contribute to the hyperphosphorylation of CRMP2 and Tau observed in Alzheimer disease.