Expression of molecular chaperones and vesicle transport proteins in differentiating oligodendrocytes

Differential activation of ER stress pathways in myelinating cerebellar tracts

Myelin production during brain development requires an increase in membrane protein and lipid production in oligodendrocytes and this primarily occurs in the endoplasmic reticulum (ER), an organelle which initiates the Unfolded Protein Response (UPR) when under stress. We hypothesise that the UPR is activated in white matter tracts during myelination in order to expand the ER capacity of oligodendrocytes. Using early and late stage markers, critical myelination time points were identified by immunohistochemistry in developing rat cerebellum. These were correlated to peaks in ER stress signalling by staining for activated UPR transducers (pIRE1, ATF6 and pPERK) and associated downstream molecules (peIF2α, PDI, GRP78, GRP94, CHOP and calreticulin) in cerebellar tracts III and IV. Gene expression in developing cerebellum was assessed by qPCR. Actively myelinating tracts were shown to have differential expression of pIRE1, PERK and ATF6 as well as UPR targets GRP94, GRP78 and PDI. Activated pIRE1-positive cells were widespread at P14 and P17 and at significantly higher numbers during myelination than at other stages. Nuclear-localised ATF6 (indicative of the active transcription factor) peaked at P10, concurrent with the initial phase of myelination. The percentage of cells positive for pPERK was less than 1% at postnatal ages but increased significantly in adult tissue. The downstream targets GRP78, GRP94 and PDI were significantly up-regulated at P17 compared to P7 and remained significantly elevated in adults. The majority of cells positive for these markers and ATF6 were oligodendrocytes as confirmed by dual-labelling. Although gene expression in the cerebellum for GRP78, GRP94 and PDI did not change significantly over time, ATF6 and XBP1s both showed significant fold changes between early and late timepoints. This data helps promote understanding of events occurring during developmental myelination and may have implications for the development of reparative treatments in diseases such as multiple sclerosis.

Alpha-synuclein transgenic mice: Relevance to multiple system atrophy

(Oligodendro)glial cytoplasmic inclusions composed of the protein alpha-synuclein (alphaSYN) are the neuropathological hallmark lesions of multiple system atrophy (MSA). The recent generation of transgenic mouse models of oligodendroglial alpha-synucleinopathy has enabled studies to investigate how alphaSYN causally contributes to MSA neuropathology. Moreover, human disease-specific pathological modifications of alphaSYN were recapitulated in transgenic mice, including insolubility, phosphorylation at serine-129, and ubiquitination. Thus, the transgenic mice will be useful tools to assess cellular risk factors, such as protein folding stress, protein kinase hyperactivity, and failure of the ubiquitin-proteasome system. Moreover, transgenic mice expressing a hyperactive alpha(1B)-adrenergic receptor mutant showed evidence of alphaSYN pathology in oligodendrocytes, adding dysregulated adrenergic neurotransmission to the list of potential risk factors of MSA. Finally, a double-transgenic mouse model expressing both alphaSYN and tau revealed synergistic fibrillization of these two proteins, providing an animal model for the not uncommon neuropathological finding of concomitant alpha-synucleinopathy and tauopathy within oligodendrocytes. Despite the progress made modeling MSA neuropathology in the transgenic mouse models, the molecular mechanism of how alphaSYN aggregation in oligodendrocytes causes neurodegeneration remains to be established. Moreover, it will be important to understand what defines the predilection sites most severely affected by striatonigral degeneration (MSA-P) and olivopontocerebellar atrophy (MSA-C), respectively.

Myelin biogenesis: vesicle transport in oligodendrocytes

Intracellular trafficking of membranes plays an essential role in the biogenesis and maintenance of myelin. The requisite proteins and lipids are transported from their sites of synthesis to myelin via vesicles. Vesicle transport is tightly coordinated with synthesis of lipids and proteins. To maintain the structural and functional organization of oligodendrocytes it is essential synchronize the various pathways of vesicle transport and to coordinate vesicle transport with reorganization of cytoskeleton. The systems that regulate the targeting of protein to myelin by vesicle transport are now being described. Here we review the current knowledge of these systems including those involved in (a) protein folding, (b) protein sorting and formation of carrier vesicles, (c) vesicle transport along elements of the cytoskeleton, and (d) vesicle targeting/fusion.

Classic and soma-restricted proteolipids are targeted to different subcellular compartments in oligodendrocytes

The myelin proteolipid (PLP) gene is very active in oligodendrocytes (OLs) and generates at least four proteins: the classic PLP and DM20 proteolipids, which are associated with compact myelin and the srPLP and srDM20, which are associated with the cell soma. These proteins are extremely hydrophobic and appear to follow the biosynthetic route used by secretory proteins. In this study, we have analyzed the subcellular distribution of the newly described sr-proteolipids and compared it to that of the classic proteolipids. Immunocytochemical analysis indicates that the sr-proteolipids and classic proteolipids are found in association with the endoplasmic reticulum (ER) and Golgi apparatus of mature OLs in vitro. Whereas the classic proteolipids become associated with the myelin-like sheets elaborated by OLs, the sr-proteolipids are not targeted to the myelin leaflets. The sr-proteolipids were associated with endosomes and with recycling vesicles as determined by double immunocytochemistry with markers such as syntaxin 6 and clathrin. In vivo, immunohistochemical analysis showed a distribution of the sr-proteolipids that was similar to that obtained in vitro, with a total absence of incorporation of sr-proteolipids into compact myelin. This differential subcellular localization is further evidence for a biological role for these products of the PLP/DM20 gene, which is different from that of the classic proteolipids.

Membrane traffic in myelinating oligodendrocytes

In the central nervous system (CNS), the myelin sheath is synthesised by oligodendrocytes as a specialised subdomain of an extended plasma membrane, reminiscent of the segregated membrane domains of polarised cells. Myelination takes place within a relatively short period of time and oligodendrocytes must have adapted membrane sorting and transport mechanisms to achieve such a high rate of myelin synthesis and to maintain the unique organisation of the myelin membrane. In adult life, maintenance of the functional myelin sheath requires a carefully orchestrated balance of myelin synthesis and turnover. Imbalance in these processes may cause dys- or demyelination and disease. This review summarises what is currently known about myelin protein trafficking and mistrafficking in oligodendrocytes. We also present data demonstrating distinct transport pathways for myelin structural proteins and the expression of SNARE proteins in differentiating oligodendrocytes. Myelinating glial cells may well serve as a model system for studying general aspects of membrane trafficking and organisation of membrane domains.

Cultured glial cells express the SNAP-25 analogue SNAP-23

Astrocytes release glutamate and aspartate in response to elevated intracellular calcium levels, and it has been proposed that this occurs by a vesicular release mechanism, in which SNARE proteins are implicated. Although syntaxin, synaptobrevin, and cellubrevin have been shown to be expressed by cultured astrocytes, SNAP-25 has not been detected. By using immunocytochemical, immunoblotting, and polymerase chain reaction techniques, the present study demonstrates that SNAP-23, an analogue of SNAP-25, is expressed by astrocytes both in culture and in rat cerebellum. These findings provide additional evidence that astrocytes release excitatory amino acids by a vesicular mechanism involving SNARE proteins. SNAP-23 and also syntaxin 1 and cellubrevin were found to be expressed in glial precursor cells, oligodendrocytes, and microglia. These data suggest that the t-SNAREs SNAP-23 and syntaxin 1 and the v-SNARE cellubrevin participate in general membrane insertion mechanisms involved in diverse glial cell functions such as secretion, phagocytosis, and myelinogenesis.

SNARE complex proteins, including the cognate pair VAMP-2 and syntaxin-4, are expressed in cultured oligodendrocytes

Myelin membrane synthesis in the CNS by oligodendrocytes (OLs) involves directed intracellular transport and targeting of copious amounts of specialized lipids and proteins over a relatively short time span. As in other plasma membrane-directed fusion, this process is expected to use specific trafficking and vesicle fusion proteins characteristic of the SNARE model. We have investigated the developmental expression of SNARE proteins in highly enriched primary cultures of OLs at discrete stages of differentiation. VAMP-2/synaptobrevin-2, syntaxin-2 and -4, nsec-1/munc-18-1, Rab3a, synaptophysin, and synapsin were expressed. During differentiation, expression of the vesicular SNARE VAMP-2, the small GTP-binding protein Rab3a, and the target SNARE syntaxin-4 were up-regulated. VAMP-2 and Rab3 proteins detected immunocytochemically in cultured OLs were localized within the developing process network; in situ anti-VAMP-2 antibody stained the perikarya of rows of cells with the distribution and appearance of OLs. We discuss the potential involvement of SNARE complex proteins in a plasma membrane-directed transport mechanism targeting nascent myelin vesicles to the forming myelin sheath.

Many facets of the peripheral myelin protein PMP22 in myelination and disease

Peripheral myelin protein 22 (PMP22) is a small, hydrophobic glycoprotein, which is most prominently expressed by Schwann cells as a component of compact myelin of the peripheral nervous system (PNS). Recent progress in molecular genetics revealed that mutations affecting the PMP22 gene including duplications, deletions, and point mutations are responsible for the most common forms of hereditary peripheral neuropathies including Charcot-Marie-Tooth disease type 1A (CMT1A), hereditary neuropathy with liability to pressure palsies (HNPP), and a subtype of Dejerine-Sottas Syndrome (DSS). Functionally, PMP22 is involved in correct myelination during development of peripheral nerves, the stability of myelin, and the maintenance of axons. While most of these functions relate to a role of PMP22 as a structural component of myelin, PMP22 has also been proposed as a regulator of Schwann cell proliferation and differentiation. In this review, we will discuss our current knowledge of PMP22 and its related proteins in the normal organism as well as in disease. In particular, we will focus on how the function of PMP22 and its regulation may be relevant to particular disease mechanisms.