Formation of synapses and myelin sheaths in cultures of dissociated chick embryonic spinal cord

Novel in vitro Experimental Approaches to Study Myelination and Remyelination in the Central Nervous System

Myelin is the lipidic insulating structure enwrapping axons and allowing fast saltatory nerve conduction. In the central nervous system, myelin sheath is the result of the complex packaging of multilamellar extensions of oligodendrocyte (OL) membranes. Before reaching myelinating capabilities, OLs undergo a very precise program of differentiation and maturation that starts from OL precursor cells (OPCs). In the last 20 years, the biology of OPCs and their behavior under pathological conditions have been studied through several experimental models. When co-cultured with neurons, OPCs undergo terminal maturation and produce myelin tracts around axons, allowing to investigate myelination in response to exogenous stimuli in a very simple in vitro system. On the other hand, in vivo models more closely reproducing some of the features of human pathophysiology enabled to assess the consequences of demyelination and the molecular mechanisms of remyelination, and they are often used to validate the effect of pharmacological agents. However, they are very complex, and not suitable for large scale drug discovery screening. Recent advances in cell reprogramming, biophysics and bioengineering have allowed impressive improvements in the methodological approaches to study brain physiology and myelination. Rat and mouse OPCs can be replaced by human OPCs obtained by induced pluripotent stem cells (iPSCs) derived from healthy or diseased individuals, thus offering unprecedented possibilities for personalized disease modeling and treatment. OPCs and neural cells can be also artificially assembled, using 3D-printed culture chambers and biomaterial scaffolds, which allow modeling cell-to-cell interactions in a highly controlled manner. Interestingly, scaffold stiffness can be adopted to reproduce the mechanosensory properties assumed by tissues in physiological or pathological conditions. Moreover, the recent development of iPSC-derived 3D brain cultures, called organoids, has made it possible to study key aspects of embryonic brain development, such as neuronal differentiation, maturation and network formation in temporal dynamics that are inaccessible to traditional in vitro cultures. Despite the huge potential of organoids, their application to myelination studies is still in its infancy. In this review, we shall summarize the novel most relevant experimental approaches and their implications for the identification of remyelinating agents for human diseases such as multiple sclerosis.

In vitro modeling of central nervous system myelination and remyelination

This review aims to summarize the current techniques to study myelination and remyelination in culture systems. We attempt to put these into historical context, and to identify the strengths and weaknesses of each approach, which vary depending on the experimental question to be tested. We discuss the difficulty and importance of quantification of myelination and in particular remyelination. Finally, we provide our predictions of how these techniques will and should develop in the future.

Primary dissociated cell culture of fetal rat central nervous tissue. I. Immunocytochemical and ultrastructural studies of cell development and synaptogenesis

We have tried to establish a method of primary dissociated cell culture of the central nervous system (CNS) for successful development of large numbers of synapses and myelinated axons. Cerebra from 18-day-old fetal rats were enzymatically dissociated into single cells and plated onto poly-D-lysine-precoated coverslips at high cell density. With the progress of cell maturation, mixed neuronal and non-neuronal cell processes grew heavily and piled up on each other, making three-dimensional structures which corresponded to 'neuropil' in vivo. Within these structures we could observe not only many mature neurons and remarkable synaptogenesis but also many myelinated axons. The synapses were mainly axo-dendritic but axo-somatic synapses were also occasionally observed. Although most of the axon terminals contained many round clear vesicles which were about 30 nm in diameter, some of them contained both round clear vesicles and 50 nm in diameter vesicles with electron-dense cores. Also a small number of large electron-dense core vesicles (about 130 nm in diameter) were found in the perikarya of mature neurons. The numerous synapse formations observed in 3-dimensional structures suggest that neurons can remain in a stable state and carry out an active metabolism through neurotransmitters. So these structures are considered to provide a favorable microenvironment for both synaptogenesis and myelinogenesis.

Primary dissociated cell culture of fetal rat central nervous tissue. II. Immunocytochemical and ultrastructural studies of myelinogenesis

It is generally considered that the dissociated cell culture is a type of monolayer culture, and when this method is used with the central nervous system (CNS) cells, synapses may be formed but myelin is either not formed at all or formed in very small quantities. So we tried to establish a dissociated cell culture of the CNS which contains many myelinated axons. Cerebra from 18-day-old fetal rat were dissociated into single cells and plated onto poly-D-lysine-precoated coverslips at high cell density. With the progress of cell maturation, neuronal and non-neuronal cell processes intermingled and piled up on each other, forming 3-dimensional structures. We were able to observe not only many mature neurons and remarkable synapses but also many myelinated axons. Moreover direct connections of oligodendrocyte (ODC) somas to myelin sheaths were demonstrated either immunocytochemically or ultrastructurally. The formation of the 3-dimensional structure is considered to provide a favorable microenvironment for the intimate interaction of neurons and ODCs leading to the formation of many myelinated axons. So this culture system may provide a useful model for the investigation of the details in myelinogenesis under physiological conditions and demyelination or remyelination under pathological conditions such as multiple sclerosis and allied diseases.

A new double labelling immunofluorescence technique for the determination of proliferation of human astrocytes in culture

We describe a method that can positively identify proliferating human astrocytes in culture. The procedure is an indirect double immunofluorescence staining technique that uses bromodeoxyuridine together with specific antibodies directed against it and glial fibrillary acidic protein. The method is simple, rapid, reproducible, and promises to decrease many of the limitations inherent in other previously used methods. Using this technique, we observed that mitosis occurred in 57 and 11% of human astrocytes that have been in culture for 11 days and 16 weeks, respectively.

Culture of purified rat astrocytes in serum-free medium supplemented with mitogen

We have obtained a highly purified astrocyte population in cultures originating from neonatal (2-5 days) rat cerebrum by use of the selection process provided by a serum-free chemically defined medium (DM). The addition of a glial growth factor isolated from bovine pituitary glands to DM induced in these astrocyte cultures both a stimulation of astrocytic proliferation and a morphological transformation of the astrocytes from flat fibroblastic form to multipolar stellate form.

Investigations on myelination in vitro: IV. "Myelin-like" or premyelin structures in cultures of dissociated brain cells from 14--15-day-old embryonic mice

The present study reports ultrastructural and biochemical data characteristic of myelin-related structures in 30- to 41-day-old cultures of dissociated brain cells from 14- to 15-day-old embryonic mice. Multilayered membranous material was identified and displayed an alternation of electron-lucent and electron-dense lamellae with a periodicity of 102 A. In these membranes, typical myelin constituents like basic protein, cerebrosides, sulfatides, and CNPase could be identified. Although we are still unable to distinguish if these membranes are premyelin or compact myelin, which could be partly degraded, these results indicate that cultured mouse brain cells retain, to a certain extent, potential to produce myelin-related membranes.

Spinal cord neuronotrophic factors (SCNTFs): I. Bioassay of schwannoma and other conditioned media

We present a procedure for the dissociation and growth in serum-free defined culture medium of 4-day chick embryo lumbar spinal cord (LC4) neurons. LC4 neurons will not survive for even 24 h without the addition of trophic supplements (putative spinal cord neuronotrophic factors, SCNTFs). Serum-free medium conditioned over chick embryo heart and skeletal muscle, mouse Schwann and rat RN22 Schwannoma cell cultures were found to contain SCNTF activity which could be quantitated using a convenient neuronal survival bioassay. RN22 conditioned medium also contains polyornithine-binding neurite promoting factors (PNPFs) which can be physically separated from SCNTF. When SCNTF and PNPF were presented to LC4 neurons individually or in combination (i) SCNTF, but not PNPF, supported neuronal survival whereas (ii) PNPF, but not SCNTF, induced neurite production. When LC4 neurons were grown in SCNTF alone, nearly all of them exhibited a flattened, circular, 'fried-egg' morphology. The subsequent addition of PNPF caused these cells to extend long neurites with characteristic terminal growth-cone-like structures.

Noncoordinate regulation of myelinogenic parameters in primary cultures of dissociated fetal rat brain

The developmental regulation of sulfogalactosylceramide (sulfatide) synthesis, 2', 3'-cyclic nucleotide-3'-phosphohydrolase activity (CNP), and myelin basic protein (MBP) accumulation, three markers characteristic of myelinogenesis, were observed in dispersed cultures of fetal rat brain in spite of the absence of the formation of compact, multilamellar myelin. Sulfatide synthetic rate and CNP activity began to increase by 8 days in vitro (DIV) (approximately comparable to days post-natal) and reached their maxima by 20 DIV. MBP began to accumulate after 14 DEV and reached a maximum by 38 DIV. Thus, the temporal regulation of the onset of expression of these parameters, which is coordinate in vivo, has been dissociated into two sequential periods in vitro. Similarly, the regulatory mechanisms controlling the subsequent decline of the net expression of these three parameters were dissociated. Whereas, the increase in the net CNP activity ceased on schedule, the decline of sulfatide synthetic rate was delayed 20 days, and the accumulation of MBP underwent a net loss. These data suggest that there are multiple parallel but separate mechanisms of temporal regulation controlling myelinogenic gene expression, and that, among them, those factors that control MBP accumulation are more sensitive to disruption by the rigors of dissociated culture.

Release of acetylcholinesterase by cultured spinal cord cells

The release of acetylcholinesterase from neurons was studied using cultured chick-embryo spinal-cord cells. Cells dissociated from 12-day-old chick-embryo spinal cords were grown in culture for 10-12 days. Numerous well differentiated spinal neurons were found after 7-10 days in culture. Acetylcholinesterase activity per dish increased by 60-fold from days 2-12. Acetylcholinesterase was released into the surrounding media by the cells when they were incubated either in the standard culture medium or the serum-free medium. Acetylcholinesterase release was significantly reduced when protein synthesis and microtubules were disrupted by cycloheximide and colchicine, respectively. Histochemical localization of acetylcholinesterase indicated that the synthesis and relase of acetylcholinesterase are attributable to neurons. Cultured chick-embryo brain and neuroblastoma cells also released acetylcholinesterase into the media. These results are discussed with regard to possible physiological roles for acetylcholinesterase secretion from neurons.

Myelinated granule cell bodies in the cerebellum of the monkey (Saimiri sciureus)

Electronmicrographs of both the vermis and hemisphere of the cerebellum of the squirrel monkey (Saimiri sciureus) show numerous granule cell bodies partially or completely surrounded by myelin. The myelin is of the compact type and consists of 1 to 13 lamellae. In several cases of partially ensheathed cells the myelin is clearly derived from extensions of myelin sheaths that surround small caliber axons. Either all or only the outer lamellae surrounding the axon contribute to the extensions. In the first instance the myelin buckles at its mesaxon pole and the resulting doubled flap extends for a variable distance along the cell surface.

Myelinated nerve cell bodies in the dorsal horn of the monkey (Saimiri sciureus)

While observing electronmicroscopic preparations of laminae I-III of Rexed ('52), taken from the lumbosacral region of squirrel monkey spinal cord, several small neuronal cell bodies were found which were partially or completely encircled by compact myelin sheaths of varying thickness. Though found in all three laminae, the occurrence of these perikaryal sheaths was less frequent in the "inner zone" of lamina II where there were few myelinated fibers. Perikaryal profiles which were completely surrounded by myelin exhibited meither internal mesaxons nor external tongue processes and the origin of their myelin is obscure. In cases of partially enveloped cells the myelin was often clearly derived from extensions of myelin sheaths surrounding small-diameter axons. These overgrowths of myelin extended away from their axons at a pole near their internal mesaxon and spread out across the surface of neighboring nerve cells. In some cases the extensions were derived from the entire axonal sheath while in others only the external lamellae were included. The external tongue process, when observed, was located at the distal end of the axonal myelin extension. Overgrowths of axonal myelin which were unrelated to neuronal cell bodies were also found but these formations were less extensive.

Myelin formation in cultures of previously dissociated mouse spinal cord

Myelin formation in cultures of previously dissociated spinal cord from foetal mice is described. In addition to the expected pattern of myelination, in which axons are closely wrapped by myelin lamellae, redundant folds of myelin have been found, as have double sheaths surrounding a single axon. Hypotheses concerning the generation of these appearances are discussed. It is suggested that certain intracytoplasmic laminar bodies found in oligodendrocytes in vitro may be of mitochondrial origin.