Rapid nerve impulse conduction depends on specialized membrane domains in myelinated nerve, the node of Ranvier, the paranode, and the myelinated internodal region. We report that GPI-linked contactin enables the formation of the paranodal septate-like axo-glial junctions in myelinated peripheral nerve. Contactin clusters at the paranodal axolemma during Schwann cell myelination. Ablation of contactin in mutant mice disrupts junctional attachment at the paranode and reduces nerve conduction velocity 3-fold. The mutation impedes intracellular transport and surface expression of Caspr and leaves NF155 on apposing paranodal myelin disengaged. The contactin mutation does not affect sodium channel clustering at the nodes of Ranvier but alters the location of the Shaker-type Kv1.1 and Kv1.2 potassium channels. Thus, contactin is a crucial part in the machinery that controls junctional attachment at the paranode and ultimately the physiology of myelinated nerve.

We have generated a hybridoma cell line secreting a monoclonal antibody that specifically binds to the surfaces of oligodendrocytes and Schwann cells, the cells involved in myelin formation in the central and peripheral nervous systems, respectively. Binding studies using purified sphingolipids showed that this antibody reacts strongly with galactocerebroside (GalC), the major galactosphingolipid of myelin. The antibody was used in conjunction with rabbit antisera against sulfatide, the sulfated form of GalC, to examine the developmental appearance of these lipids on the surfaces of oligodendrocytes and Schwann cells. In addition, the loss of GalC and sulfatide from freshly dissociated Schwann cells was compared. These studies showed that GalC is expressed on the cell surface prior to sulfatide on both of these cell types in vivo and in vitro. Conversely, dissociated Schwann cells lose their cell surface sulfatide more rapidly than they lose their surface GalC under nonmyelinating conditions. Certain lipids and proteins are thought to be predominantly found in myelin (1). Among these are the galactosphingolipids, galactocerebroside (GalC) and sulfatide. Despite their myelin association, the relationship of these lipids to the development ofoligodendrocytes and Schwann cells (the myelin-forming cells ofthe central and peripheral nervous systems, respectively) has not been studied on a cellular level in detail. Previous work, using either chemical analysis of extracted lipid or radiotracer studies, has described the developmental appearance ofmyelinspecific components in the nervous system (2). However, the techniques used were unable to establish the sequence in which components appear in individual cells or to resolve when such lipids are first inserted into the plasma membrane. Given that these lipids may play a role in the neuronal-glial interaction that results in myelination (3), it is important to ascertain their exact sequence and onset of appearance.One method that allows developmental studies on a cellular level is immunohistochemical labeling of cells with specific, characterized antisera. It has been established that in culture, polyclonal antisera against GalC and sulfatide specifically stain oligodendrocytes and Schwann cells (4). However, such work has suffered from the difficulty of adequately characterizing such antisera, which are often of low titer and questionable specificity.In the present study, we describe a hybridoma cell line that produces a monoclonal antibody binding specifically to the surfaces of oligodendrocytes and Schwann cells. A modification to existing enzyme-linked immunosorbent assay (ELISA) techniques was developed that allows the characterization of antibodies to lipid determinants. With this assay, the monoclonal antibody was shown to have a high specificity for GalC and, consequently, was designated mGalC. Various polyclonal rabbit antisera were similarly characterized and shown to bind to either GaiC or sulfatide. We used mGalC and rabbit antisera in double-labeli...

Sorting nexin 27 (SNX27), a brain-enriched PDZ domain protein, regulates endocytic sorting and trafficking. Here, we show that Snx27−/− mice exhibit severe neuronal deficits in the hippocampus and cortex. While Snx27+/− mice exhibit grossly normal neuroanatomy, we find defects in synaptic function, learning and memory, and a reduction in ionotropic glutamate receptors (NMDARs and AMPARs). SNX27 interacts with these receptors through its PDZ domain, regulating their recycling to the plasma membrane. We demonstrate a concomitant reduction of SNX27 and C/EBPβ in Down syndrome brains and identify C/EBPβ as a transcription factor for SNX27. Down syndrome causes over-expression of miR-155, a chromosome 21-encoded microRNA that negatively regulates C/EBPβ, thereby reducing SNX27 and resulting in synaptic dysfunction. Up-regulating SNX27 in the hippocampus of Down syndrome mice rescues synaptic and cognitive deficits. Our identification of the role of SNX27 in synaptic function establishes a novel molecular mechanism of Down syndrome pathogenesis.

The circulating, adipocyte-secreted hormone adiponectin (APN) exerts protective effects on the heart under stress conditions. The receptors binding APN to cardiac tissue, however, have remained elusive. Here, we report that the glycosyl phosphatidylinositol-anchored cell surface glycoprotein T-cadherin (encoded by Cdh13) protects against cardiac stress through its association with APN in mice. We observed extensive colocalization of T-cadherin and APN on cardiomyocytes in vivo. In T-cadherin-deficient mice, APN failed to associate with cardiac tissue, and its levels dramatically increased in the circulation. Pressure overload stress resulted in exacerbated cardiac hypertrophy in T-cadherin-null mice and paralleled corresponding defects in mice lacking APN. During ischemia-reperfusion injury, the absence of T-cadherin increased infarct size similar to that in APN-null mice. Myocardial AMPK is a major downstream protective signaling target of APN. In both cardiac hypertrophy and ischemia-reperfusion models, T-cadherin was necessary for APN-dependent AMPK phosphorylation. In APN-null mice, recombinant adenovirus-expressed APN reduced exaggerated hypertrophy and infarct size and restored AMPK phosphorylation as previously reported. In contrast, rescue was ineffective in mice lacking T-cadherin in addition to APN. These data suggest that T-cadherin protects from stressinduced pathological cardiac remodeling by binding APN and activating its cardioprotective functions.