Neuronal activity disrupts myelinated axon integrity in the absence of NKCC1b

Through a genetic screen in zebrafish, we identified a mutant with disruption to myelin in both the CNS and PNS caused by a mutation in a previously uncharacterized gene, slc12a2b, predicted to encode a Na+, K+, and Cl− (NKCC) cotransporter, NKCC1b. slc12a2b/NKCC1b mutants exhibited a severe and progressive pathology in the PNS, characterized by dysmyelination and swelling of the periaxonal space at the axon–myelin interface. Cell-type–specific loss of slc12a2b/NKCC1b in either neurons or myelinating Schwann cells recapitulated these pathologies. Given that NKCC1 is critical for ion homeostasis, we asked whether the disruption to myelinated axons in slc12a2b/NKCC1b mutants is affected by neuronal activity. Strikingly, we found that blocking neuronal activity completely prevented and could even rescue the pathology in slc12a2b/NKCC1b mutants. Together, our data indicate that NKCC1b is required to maintain neuronal activity–related solute homeostasis at the axon–myelin interface, and the integrity of myelinated axons.

Periaxonal and nodal plasticity modulate action potential conduction in the adult mouse brain

SummaryCentral nervous system myelination increases action potential conduction velocity, however, it is unclear how myelination is coordinated to ensure the temporally precise arrival of action potentials, and facilitate information processing within cortical and associative circuits. Here, we show that mature myelin remains plastic in the adult mouse brain and can undergo subtle structural modifications to influence action potential arrival times. Repetitive transcranial magnetic stimulation and spatial learning, two stimuli that modify neuronal activity, alter the length of the nodes of Ranvier and the size of the periaxonal space within active brain regions. This change in the axon-glial configuration is independent of oligodendrogenesis and tunes conduction velocity to increase the synchronicity of action potential transit.

Computer Model for Action Potential Propagation Through Branch Point in Myelinated Nerves

A mathematical model is developed for simulation of action potential propagation through a single branch point of a myelinated nerve fiber with a parent branch bifurcating into two identical daughter branches. This model is based on a previously published multi-layer compartmental model for single unbranched myelinated nerve fibers. Essential modifications were made to couple both daughter branches to the parent branch. There are two major features in this model. First, the model could incorporate detailed geometrical parameters for the myelin sheath and the axon, accomplished by dividing both structures into many segments. Second, each segment has two layers, the myelin sheath and the axonal membrane, allowing voltages of intra-axonal space and periaxonal space to be calculated separately. In this model, K ion concentration in the periaxonal space is dynamically linked to the activity of axonal fast K channels underneath the myelin in the paranodal region. Our model demonstrates that the branch point acts like a low-pass filter, blocking high-frequency transmission from the parent to the daughter branches. Theoretical analysis showed that the cutoff frequency for transmission through the branch point is determined by temperature, local K ion accumulation, width of the periaxonal space, and internodal lengths at the vicinity of the branch point. Our result is consistent with empirical findings of irregular spacing of nodes of Ranvier at axon abors, suggesting that branch points of myelinated axons play important roles in signal integration in an axonal tree.

Cholinesterase activity of lamellated sensory corpuscles in the rat lip

Non-specific cholinesterase (ChE) activity was demonstrated in lamellated sensory corpuscles of the rat lip by light and electron microscopy using Karnovsky and Root's method. ChE activity was present in the interlamellar spaces between neighbouring lamellae as well as in the periaxonal space between axon terminals and their adjacent lamellae. Reaction products of ChE activity were also deposited in some caveolae of the lamellar cell plasma membrane, and in the cisternae of the rough endoplasmic reticulum as well as in the nuclear envelope of lamellar cell bodies. No definite reaction products were detected within the axon terminals. These findings show that the lamellated corpuscles in the rat lip, like other mechanoreceptors, have an intense ChE activity which is mainly associated with lamellar cells. It can be said that ChE histochemistry is useful to detect mechanoreceptors. The functional significance of ChE in mechanoreceptors is discussed.

Immunocytochemical studies of quaking mice support a role for the myelin-associated glycoprotein in forming and maintaining the periaxonal space and periaxonal cytoplasmic collar of myelinating Schwann cells.

The myelin-associated glycoprotein (MAG) is an integral membrane glycoprotein that is located in the periaxonal membrane of myelin-forming Schwann cells. On the basis of this localization, it has been hypothesized that MAG plays a structural role in (a) forming and maintaining contact between myelinating Schwann cells and the axon (the 12-14-nm periaxonal space) and (b) maintaining the Schwann cell periaxonal cytoplasmic collar of myelinated fibers. To test this hypothesis, we have determined the immunocytochemical localization of MAG in the L4 ventral roots from 11-mo-old quaking mice. These roots display various stages in the association of remyelinating Schwann cells with axons, and abnormalities including loss of the Schwann cell periaxonal cytoplasmic collar and dilation of the periaxonal space of myelinated fibers. Therefore, this mutant provides distinct opportunities to observe the relationships between MAG and (a) the formation of the periaxonal space during remyelination and (b) the maintenance of the periaxonal space and Schwann cell periaxonal cytoplasmic collar in myelinated fibers. During association of remyelinating Schwann cells and axons, MAG was detected in Schwann cell adaxonal membranes that apposed the axolemma by 12-14 nm. Schwann cell plasma membranes separated from the axolemma by distances greater than 12-14 nm did not react with MAG antiserum. MAG was present in adaxonal Schwann cell membranes that apposed the axolemma by 12-14 nm but only partially surrounded the axon and, therefore, may be actively involved in the ensheathment of axons by remyelinating Schwann cells. To test the dual role of MAG in maintaining the periaxonal space and Schwann cell periaxonal cytoplasmic collar of myelinated fibers, we determined the immunocytochemical localization of MAG in myelinated quaking fibers that displayed pathological alterations of these structures. Where Schwann cell periaxonal membranes were not stained by MAG antiserum, the cytoplasmic side of the periaxonal membrane was "fused" with the cytoplasmic side of the inner compact myelin lamella and formed a major dense line. This loss of MAG and the Schwann cell periaxonal cytoplasmic collar usually resulted in enlargement of the 12-14-nm periaxonal space and ruffling of the apposing axolemma. In myelinated fibers, there was a strict correlation between the presence of MAG in the Schwann cell periaxonal membrane and (a) maintenance of the 12-14-nm periaxonal space, and (b) presence of the Schwann cell periaxonal cytoplasmic collar.(ABSTRACT TRUNCATED AT 400 WORDS)

Myelin-associated glycoprotein and myelinating Schwann cell-axon interaction in chronic B,B'-iminodipropionitrile neuropathy.

The myelin-associated glycoprotein (MAG) is a heavily glycosylated integral membrane glycoprotein which is a minor component of isolated rat peripheral nervous system (PNS) myelin. Immunocytochemically MAG has been localized in the periaxonal region of PNS myelin sheaths. The periaxonal localization and biochemical features of MAG are consistent with the hypothesis that MAG plays a role in maintaining the periaxonal space of myelinated fibers. To test this hypothesis, MAG was localized immunocytochemically in 1-micron sections of the L5 ventral root from rats exposed to B,B'-iminodipropionitrile. In chronic states of B,B'-iminodipropionitrile intoxication, Schwann cell periaxonal membranes and the axolemma invaginate into giant axonal swellings and separate a central zone of normally oriented axoplasm from an outer zone of maloriented neurofilaments. Ultrastructurally, the width of the periaxonal space (12-14 nm) in the ingrowths is identical to that found in normally myelinated fibers. These Schwann cell ingrowths which are separated from compact myelin by several micra are stained intensely by MAG antiserum. Antiserum directed against Po protein, the major structural protein of compact PNS myelin, does not stain the ingrowths unless compact myelin is present. These results demonstrate the periaxonal localization of MAG and support a functional role for MAG in maintaining the periaxonal space of PNS myelinated fibers.

The periaxonal space of crayfish giant axons.

The influence of the glial cell layer on effective external ion concentrations has been studied in crayfish giant axons. Excess K ions accumulate in the periaxonal space during outward K+ current flow, but at a rate far below that expected from the total ionic flux and the measured thickness of the space. At the conclusion of outward current flow, the external K+ concentration returns to normal in an exponential fashion, with a time constant of approximately 2 ms. This process is about 25 times faster than is the case in squid axons. K+ repolarization (tail) currents are generally biphasic at potentials below about -40 mV and pass through a maximum before approaching a final asymptotic level. The initial rapid phase may in part reflect depletion of excess K+. After block of inactivation and reversal of the Na+ concentration gradient, we could demonstrate accumulation and washout of excess Na ions in the periaxonal space. Characteristics of these processes appeared similar to those of K+. Crayfish glial cell ultrastructure has been examined both in thin sections and after freeze fracture. Layers of connective tissue and extracellular fluid alternate with thin layers of glial cytoplasm. A membranous tubular lattice, spanning the innermost glial layers, may provide a pathway allowing rapid diffusion of excess ions from the axon surface.

Presence of the myelin-associated glycoprotein correlates with alterations in the periodicity of peripheral myelin.

The myelin-associated glycoprotein (MAG) is an integral membrane protein (congruent to 100,000 mol wt) which is a minor component of purified peripheral nervus system (PNS) myelin. In the present study, MAG was localized immunocytochemically in 1-micrometer thick Epon sections of 7-d and adult rat peripheral nerves, and its localization was compared to that of the major structural protein (Po) of PNS myelin. To determine more precisely the localization of MAG, immunostained areas in 1 micrometer sections were traced on electron micrographs of identical areas from adjacently cut thin sections.l MAG was localized in periaxonal membranes. Schmidt-Lantermann incisures, paranodal membranes, and the outer mesaxon of PNS myelin sheaths. Compact regions of PNS myelin did not react with MAG antiserum. The results demonstrate MAG's presence in "'semi-compact" Schwann cell or myelin membranes that have a gap of 12-14 nm between extracellular leaflets and a spacing of 5 nm or more between cytoplasmic leaflets. In compact regions of the myelin sheath which do not contain MAG, the cytoplasmic leaflets are "fused" and form the major dense line, whereas the extracellular leaflets are separated by a 2.0 nm gap appearing as paired minor dense lines. Thus, it is proposed that MAG plays a role in maintaining the periaxonal space, Schmidt-Lantermann incisures, paranodal myelin loops, and outer mesaxon by preventing "complete" compaction of Schwann cell and myelin membranes. The presence of MAG in these locations also suggests that MAG may serve a function in regulating myelination in the PNS.