Hypokalaemic periodic paralysis (HypoPP) is an autosomal dominant muscle disease which has been linked to point mutations in the skeletal muscle L-type calcium channel cq subunit (~ls). Here, we have introduced one of the point mutations causing HypoPP (R528H) into cDNA of the rabbit Cqs. Expression of either the wild-type ~tsor the mutant R528H Cqs (~ls-Rszsn) subunits was obtained in mouse Ltk cells using a selectable expression vector. The Cqs-Rs28n subunit led to the expression of functional L-type Ca z+ channels. Corresponding whole-cell Ba 2+ currents exhibit very slow activation and inactivation kinetics, typical for recombinant skeletal Ca 2+ channel currents. Voltage-dependent activation and inactivation properties were similar for ~ts-and ~IS-RS2Sn, as well as their sensitivity to the dihydropyridine agonist Bay K 8644. Differences in Cqs-and ~ls-Rs28n-directed channels reside in the Ba 2+ current density, which was significantly reduced 3.2 fold in cells expressing ~lS-RS2Sn It was concluded that the R528H mutation of Cqs results in minor differences in the electrophysiological properties but significantly reduces the whole-cell Ca z÷ channel current in its amplitude.

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Cross-transplantations of neural tissue have been performed between jimpy (jp), shiverer (shi), and normal mice. Taking advantage of the absence of immunodetectable myelin basic protein (MBP) in the shi brain, jp myelin has been identified in the shi recipient by using an anti-MBP antiserum. By contrast, shi as well as normal myelin have been identified in the jp brain by using an anti-C-terminal hexapeptide of the proteolipid protein (PLP) (this PLP hexapeptide being absent in the jp PLP). When transplanted under homochronic conditions (newborn into newborn), jp oligodendrocytes (ODC) express their usual phenotype in a normal or a shi environment, suggesting that at birth the jp ODCs phenotype is strictly established and cannot be modified by environmental conditions. The reverse transplantations (newborn shi or normal into newborn jp brain) demonstrate that the jp environment does not modify the phenotype of normal or shi ODCs. Finally, these experiments demonstrated a normal timing of differentiation of jp axons and of jp ODCs.

Purified rat Schwann cells labeled with Hoechst 33342 fluorescent fluorochrome were transplanted into the brain of the newborn shiverer mouse. The grafted cells survived and were able to migrate through the host parenchyme. However, Schwann cell migration was restricted to the grafted hemisphere and to structures adjacent to the graft. With time, Hoechst labeled cells, present at the site of implantation or dispersed in the host parenchyme, decreased progressively in number. Instead, they concentrated along the blood vessels, meninges and ventricles. Despite the presence of Hoechst labeled Schwann cells in white matter tracks during the process of central myelination, Schwann cell myelination could not be evidenced by immunodetection of the peripheral myelin protein or by ultrastructural observation of the typical Schwann cell basement membrane surrounding peripheral myelin. A series of additional transplantations involving Schwann cells of mouse or rat origin, grafted either as cell suspensions or as nerve fragments, demonstrated that transplanted Schwann cells formed myelin around developing host axons only when included in a nerve fragment. Immunodetection of GFAP in astrocytes and type IV collagen in basement membranes as well as electron microscopy showed that reactive astrocytes invaded the grafted area after the first week of transplantation and sometimes formed basement membranes isolating partially the graft from the host parenchyme. During host myelination, astrocytes, which were present in most white matter structures, surrounded grafted cells. Occasionally, they enclosed Schwann cells in basement membranes or encircled host axons. Later, reactive astrocytes were associated with Schwann cells restricted to blood vessel and ventricular walls, and meninges. Our results suggest that in the presence of competitive developing Oligodendrocytes, astrocytes are able to limit migration and prevent myelination of Schwann cells transplanted in the newborn shiverer brain. In addition, astrocytes seem to be able to expell the grafted cells and finally exclude them from the host parenchyme.

Hypokalemic periodic paralysis (hypoPP) is an autosomal dominant disorder belonging to a group of muscle diseases known to involve an abnormal function of ion channels. The latter includes hypokalemic and hyperkalemic periodic paralyses, and non-dystrophic myotonias. We recently showed genetic linkage of hypoPP to loci on chromosome 1q31-32, co-localized with the DHP-sensitive calcium channel CACNL1A3. We propose to term this locus hypoPP-1. Using extended haplotypes with new markers located on chromosome 1q31-32, we now report the detailed mapping of hypoPP-1 within a 7 cM interval. Two recombinants between hypoPP-1 and the flanking markers D1S413 and D1S510 should help to reduce further the hypoPP-1 interval. We used this new information to demonstrate that a large family of French origin displaying hypoPP is not genetically linked to hypoPP-1. We excluded genetic linkage over the entire hypoPP-1 interval showing for the first time genetic heterogeneity in hypoPP.

Cross-transplantations of neural tissue between jimpy (jp) shiverer (shi) and normal mice have been performed under heterochronic conditions. In all series, fragments of E14-E15 embryonic neural tissue from the different donors have been transplanted into newborn host brain in order to study environmental influences by differentiated tissue on transplanted embryonic cell lines. Large patches of proteolipid protein (PLP)-positive myelin have been observed in the jp brain after transplantation of shi or normal embryonic tissue into the newborn jp brain, suggesting that the jp parenchyma did not inhibit the differentiation of other oligodendrocytes (ODCs). Jp embryonic tissue had the same mitotic potential as normal tissue, as demonstrated by the larger size of myelin patches observed when jp embryonic tissue was used instead of newborn jp tissue. By contrast, whatever the conditions, jp myelin patches were always obviously smaller than normal or shi myelin patches, suggesting that the myelinating capacity of jp ODCs was not enhanced by environmental factors. Finally, comparison of the ratio of successful outcomes observed following embryonic vs. newborn jp donor tissue, strongly suggests a partial or total normalization of jp embryonic ODCs survival by a more mature shi environment.

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The jimpy (jp) mutation of the mouse leads to a dramatic decrease of myelination in the hemizygous mutant central nervous system (CNS). Several descriptions based on classical histology, immunohistochemistry, and electron microscopy (EM) have demonstrated the scarcity of myelin formation in the different parts of the CNS. The immunohistochemical study presented here showed a very singular patchy pattern of myelin distribution in the different areas of the whole mutant brain. The myelin patches are randomly dispersed without bilateral symmetry, and their density and location vary from one animal to another. No reproducible pattern of myelination could be found among the population observed. This distribution has been compared with observations on young heterozygotes and wild-type homozygotes from the same strain. A similar patchy and random distribution of myelin could be observed in heterozygotes, which present an intermediate level of myelination. This strongly suggests that a migration of precursors or immature oligodendrocytes (ODCs) from the periventricular zone followed by local multiplication of colonies of ODCs before myelination is a general feature in normal as well as pathological conditions.