The majority of excitatory synapses in the mammalian CNS are formed on dendritic spines1, and spine morphology and distribution are critical for synaptic transmission2–6, synaptic integration and plasticity7. Here, we show that a secreted semaphorin, Sema3F, is a negative regulator of spine development and synaptic structure. Mice with null mutations in genes encoding Sema3F, and its holoreceptor components neuropilin-2 (Npn-2) and plexinA3 (PlexA3), exhibit increased dentate gyrus (DG) granule cell (GC) and cortical layer V pyramidal neuron spine number and size, and also aberrant spine distribution. Moreover, Sema3F promotes loss of spines and excitatory synapses in dissociated neurons in vitro, and in Npn-2−/− brain slices cortical layer V and DG GCs exhibit increased mEPSC frequency. In contrast, a distinct Sema3A–Npn-1/PlexA4 signaling cascade controls basal dendritic arborization in layer V cortical neurons but does not influence spine morphogenesis or distribution. These disparate effects of secreted semaphorins are reflected in the restricted dendritic localization of Npn-2 to apical dendrites and of Npn-1 to all dendrites of cortical pyramidal neurons. Therefore, Sema3F signaling controls spine distribution along select dendritic processes, and distinct secreted semaphorin signaling events orchestrate CNS connectivity through the differential control of spine morphogenesis, synapse formation, and the elaboration of dendritic morphology.

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Oligodendrocyte-myelin glycoprotein (OMgp) is a myelin component that has been shown in vitro to inhibit neurite outgrowth by binding to the Nogo-66 receptor (NgR1)/Lingo-1/Taj (TROY)/p75 receptor complex to activate the RhoA pathway. To investigate the effects of OMgp on axon regeneration in vivo, OMgp -/-mice on a mixed 129/Sv/C57BL/6 (129BL6) or a C57BL/6 (BL6) genetic background were tested in two spinal cord injury (SCI) models -a severe complete transection or a milder dorsal hemisection. OMgp -/-mice on the mixed 129BL6 genetic background showed greater functional improvement compared to OMgp +/+ littermates, with increased numbers of cholera toxin B-labeled ascending sensory axons and 5-HT + descending axons and less RhoA activation after spinal cord injury. Myelin isolated from OMgp -/-mice (129BL6) was significantly less inhibitory to neurite outgrowth than wild-type (wt) myelin in vitro. However, OMgp -/-mice on a BL/6 genetic background showed neither statistically significant functional recovery nor axonal sprouting following dorsal hemisection.

Abstract-Precise regulation of retinoid levels is critical for normal heart development. Retinol-binding protein (RBP), an extracellular retinol transporter, is strongly secreted by cardiogenic endoderm. This study addresses whether RBP gene ablation affects heart development. Despite exhibiting an Ͼ85% decrease in serum retinol, adult RBP-null mice are viable, breed, and have normal vision when maintained on a vitamin A-sufficient diet. Comparison of RBP-null with wild-type (WT) hearts from embryos at day 9.0 (E9.0) through E12.5 revealed an RBP-null phenotype similar to that of other retinoid-deficient models. At an early stage, RBP-null hearts display retarded trabecular development, which recovers by E9.5. This is accompanied at E9.5 and E10.5 by precocious differentiation of subepicardial cardiac myocytes. Most remarkably, RBP-null hearts display augmented deposition of fibronectin protein in the cardiac jelly at E9.0 through E10.5 and in the outflow tract at E12.5. This phenomenon, which was detected by immunohistochemistry and Western blotting without increased fibronectin transcript levels, is accompanied by increased numbers of mesenchymal cells in the outflow tract but not in the atrioventricular canal. RBP-null cardiac myocytes, especially in the subepicardial layer, display increased cell proliferation. This phenotype may present a model of subclinical retinoid insufficiency characterized by alteration of an extracellular matrix component and altered cellular differentiation and proliferation, changes that may have functional consequences for adult cardiac function. This murine model may have relevance to fetal development in human populations with inadequate retinoid intake. Key Words: retinol-binding protein knockout Ⅲ retinoic acid Ⅲ mouse heart development Ⅲ cardiac jelly Ⅲ fibronectin T he biologically active derivative of vitamin A, retinoic acid (RA), plays an essential role in regulating the homeostasis of adult organs as well as the development of numerous embryonic tissues. Precisely regulated retinoid levels are crucial for normal development of the cardiovascular system: too much or too little RA causes profound cardiac teratogenicity. 1,2 For example, excess RA causes effects ranging from diminished cardiac jelly and outflow tract (OFT) disruption 3,4 to the total and specific absence of the embryonic heart. 5 On the other hand, vitamin A deficiency (VAD) causes aortic arch anomalies, ventricular septal defects, and retarded myocardial development 6 reflecting cellular deficits, including hypoplastic myocardial walls that contain precociously differentiated cardiac myocytes. 7,8 Most deficits caused by VAD are recapitulated by ablation of the genes involved in retinoid synthesis, such as retinaldehydeoxidizing dehydrogenase (RALDH2), 9 or genes in the retinoid signaling pathway, including the RA receptor (RAR) and retinoid X receptor (RXR) genes. 7,8,10,11 These findings suggest that the ablation of genes that regulate retinoid delivery, such as the retinol-binding protein (RBP) gene, m...

SummaryThe very limited ability to regenerate axons after injury in the mature mammalian central nervous system (CNS) has been partly attributed to the growth restrictive nature of CNS myelin. Oligodendrocyte myelin glycoprotein (OMgp) was identified as a major myelin-derived inhibitor of axon growth. However, its role in axon regeneration in vivo is poorly understood. Here we describe the generation and molecular characterization of an OMgp allelic series. With a single gene targeting event and Cre/FLP mediated recombination, we generated an OMgp null allele with a LacZ reporter, one without a reporter gene, and an OMgp conditional allele. This allelic series will aid in the study of OMgp in adult CNS axon regeneration using mouse models of spinal cord injury. The conditional allele will overcome developmental compensation when employed with an inducible Cre, and allows for the study of temporal and tissue/cell type-specific roles of OMgp in CNS injury-induced axonal plasticity. KeywordsOMgp; conditional allele; axon regeneration; CNS repair; myelin inhibition Following development, axons in the mammalian central nervous system (CNS, including the brain and the spinal cord) have largely lost the ability to regenerate in response to injuries (Ramón y Cajal, 1928). Spinal cord injury patients suffer permanent functional deficits due to this inability for axon regeneration in adult CNS, which is in contrast with the robust regenerative ability of axons in the developing CNS or in the adult peripheral nervous system (PNS). The inhibitory action of myelin on axon growth has been hypothesized as one primary reason for this loss of regenerative ability (Berry, 1982;Schwab and Bartholdi, 1996). Several myelin-derived inhibitory proteins have been identified (Filbin, 2003;Yiu and He, 2006). Oligodendrocyte Myelin glycoprotein (OMgp) was identified as a major myelin-derived inhibitor that possesses potent inhibitory activity on neurite outgrowth in vitro (Kottis et al., 2002;Wang et al., 2002). Together with two other myelin inhibitors, Nogo and myelin-associated glycoprotein (MAG), it signals through at least two receptors, NgR1 and PirB, to effect the inhibitory action of myelin (Atwal et al., 2008;Yiu and He, 2006). However, the role of OMgp in axon regeneration in vivo is not well understood. In a published report, OMgp deficient mice were found to exhibit slightly enhanced axon regeneration in the ascending dorsal column sensory tract and the descending raphespinal serotonergic tract but not in the corticospinal tract (Ji et al., 2008 Zheng et al., 2005). A MAG mutant exhibits upregulated NgR1 expression (Pernet et al., 2008). Thus, mutations in either an inhibitory ligand or receptor in the myelin inhibition pathway can lead to changes in the expression of other genes in the pathway, which may mask the effect of gene deletion (in the case of the NgR1 or MAG mutants) or complicates the interpretation of the results (in the case of the OMgp mutant). This is particularly a challenge for genetic studies of axon regener...

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