Peripheral nervous system myelin is an extension of the Schwann cell's plasma membrane that tightly enwraps axons in many layers and permits nerve impulses to be rapidly conducted. It is not known how these multiple membrane layers are held together in this compact form. Here we present evidence supporting the hypothesis that the extracellular leaflets of myelin are held together by the most abundant protein of myelin of the peripheral nervous system, P0, by homophilic interaction of its extracellular domains. Transfected Chinese hamster ovary cells expressing P0 protein adhere to each other in suspension, to form large aggregates, whereas cells that are identical but which do not express P0 do not. We also show that this aggregation is mediated by homophilic binding between P0-expressing cells and that the apposing plasma membranes of these cells specifically form desmosomes, whereas control transfected cells do not. As the only difference between the two cell populations is the expression of P0, this protein is apparently responsible for the changes in morphology and adhesion in the cells that express it. The idea that P0 is a homophilic adhesion molecule is supported by its inclusion in the immunoglobulin supergene family, all members of which are involved in recognition and/or adhesion.

SUMMARY1. Five major fibre types in chicken skeletal muscles are recognized, based upon their histochemical and morphological characteristics. A classification of these which is readily related to a commonly used classification of mammalian muscle fibre types is given.2. Seven muscles of the chicken were analysed in recognizing this range of fibre types. The proportions of the different types in each of these were determined. In some cases a gradient of fibre type composition exists across a single muscle.3. Measurements of muscle contraction were used in defining tonic muscles, which contain two fibre types. It was shown that in addition to the anterior latissimus dorsi (a.l.d.), previously well known to be a tonic muscle, two other muscles, the plantaris and the adductor profundus, are of the same class, but differ subtly from the ald. in certain features. Gross red colouration is not a useful diagnostic feature of slow muscles, since the tonic adductor profundus, for example, is white.4. Fibres similar histochemically to mammalian type I (slow-twitch) occur in some of the avian twitch muscles investigated. These are oxidative in character, and despite the fact that they are multiply innervated we suggest that these are avian slow-twitch fibres.5. The patterns of cholinesterases found in a skeletal muscle correspond to its fibre type composition, with regard to both the concentrations and the proportions of the multiple forms of enzyme present. The distinctive patterns of those forms of acetylcholinesterase in the different fibre types are described.6. The fibre type composition is changed by inherited muscular dystrophy in a characteristic manner. This change has so far been found (at the earlier stages of the disease) only in the muscles with a predominance of type II1B fibres in the normal chicken. Pathological changes within the fibres occur selectively in the type JIB fibres, but there are exceptions to this and the effect can be greatly modified by the type of neighbouring fibres.

The subpopulation of dorsal root ganglion (DRG) neurons recognized by Griffonia simplicifolia isolectin B4 (IB4) differ from other neurons by expressing receptors for glial cell line-derived neurotrophic factor (GDNF) rather than neurotrophins. Additionally, IB4-labeled neurons do not express the laminin receptor, ␣7-integrin (Gardiner et al., 2005), necessary for optimal axonal regeneration in the peripheral nervous system. In cultures of dissociated DRG neurons of adult mice on laminin, robust spontaneous neurite outgrowth from IB4-negative neurons occurs and is strongly enhanced by previous axotomy. In contrast, IB4-labeled neurons show little neurite outgrowth and do not express GAP 43, even after axotomy or culture with GDNF. Moreover, growth of their axons through collagen gels is impaired compared with other DRG neurons. To determine whether the sparse neurite outgrowth of IB4-labeled neurons is attributable to lack of integrin expression, DRG cultures were infected with a herpes simplex 1 vector encoding ␣7-integrin, but its forced expression failed to promote neurite outgrowth in either IB4-labeled or other DRG neurons or in cultured adult retinal ganglion cells. Forced coexpression of both ␣7-integrin and GAP 43 also failed to promote neurite outgrowth in IB4-labeled neurons. In addition, cultured sciatic nerve segments were found to release much lower levels of GDNF, demonstrated by ELISA, than nerve growth factor. These findings together with their impaired intrinsic axonal regeneration capacity may contribute to the known vulnerability of the IB4-labeled population of DRG neurons to peripheral nerve injury.

GATA-6 has been implicated in the regulation of myocardial differentiation during cardiogenesis. To determine how its expression is controlled, we have characterized the human and mouse genes. We have mapped their transcriptional start sites and demonstrate that two alternative promoters and 5 noncoding exons are utilized. Both transcript isoforms are expressed in the same tissue-specific and developmental stage-specific pattern, and their ratio appears similar wherever examined. The more upstream noncoding exon showed a substantial degree of homology between the two mammalian species, suggesting a conserved regulatory function. Moreover, in transfection assays we show that elements within this exon act to promote its transcription. Positive regulatory elements that effect transcription from the more downstream exon were not apparent in this assay, revealing a regulatory distinction between the two promoters. We also demonstrate alternative initiator codon usage in both the human and mouse GATA-6 genes. Both isoforms of the protein are synthesized in vitro regardless of which 5 noncoding exon is present in the RNA, although the larger protein has greater transcriptional activation potential in transfection assays. Thus, GATA-6 function in the cell is controlled by a complex interplay of transcriptional and translational regulation.

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