Patient iPSC models reveal glia-intrinsic phenotypes in multiple sclerosis
B. L. L. Clayton, L. Barbar, M. Sapar, K. Kalpana, C. Rao, B. Migliori, et al.
Cell Stem Cell 2024; Nov 7;31(11):1701-1713
Introduction:
Multiple sclerosis (MS) is usually characterized as an autoimmune disease, putting all the blame on an aberrant immune system that attacks a normal central nervous system. There is no doubt that the immune system plays a major role in both the pathogenesis of MS and as a target for treatment of the relapsing forms of multiple sclerosis, but increasing data challenges that hypothesis.
There are two opposing theories regarding the cause of MS. The most popular is the “outside-in” theory. It states that the immune systems of persons with MS are abnormal as a result of losing “immune tolerance” to central nervous system tissues. This in turn results in a destructive attack on the central nervous system.
The other theory, relatively neglected until now, is the “inside-out” theory. This hypothesizes that the central nervous system of persons with MS is not normal, resulting in a loss and death of cells. This in turn releases proteins of the central nervous system into the circulation, proteins that are usually hidden from the immune system. These released proteins stimulate an immune system in genetically predisposed individuals that leads to an “overresponse” to these proteins.
The paper by Clayton and fellow researchers provides new insights into changes in the central nervous system of persons with MS, changes that could lead to a loss of myelin and the death of brain cells, all in the absence of immune cells. These data challenge the concept that MS is “purely” an autoimmune disease.
Key Points:
1. Several MS research groups have described abnormalities in the central nervous systems of persons with MS unrelated to the effects of the immune system. A limitation of all these studies is their use of autopsy tissues.
2. The work described in the above paper by Clayton and co-scientists involved studies done with stem cells from persons with MS at different stages of their illness and with different patterns of disease.
3. Skin biopsies from persons with MS and normal controls (6 persons with relapsing forms of multiple sclerosis, 6 with progressive multiple sclerosis, 5 with primary progressive multiple sclerosis and 5 normal controls). The tissues were then reprogrammed such that skin cells converted to become undifferentiated or “basic” cells called stem cells. Stem cells have the capacity to become differentiated or tissue specialized depending on their exposure to different growth factors in tissue culture.
4. Clayton and colleagues differentiated the stem cells from persons with MS and controls and grew lines of brain cells consisting predominantly of astrocytes and oligodendrocytes with small numbers of nerve cells (neurons). Astrocytes are the binding cells of the brain, nurturing neurons and oligodendrocytes, and helping to maintain metabolic balance. They also have the capacity to stimulate the immune system by “presenting” antigens to immune cells. Oligodendrocytes are the cells that produce myelin, the fatty insulation encasing nerve cell fibers (axons) and are a major target of destruction in MS.
5. The scientists took individual cells from the cell lines and isolated RNA from the cells. RNA is responsible for generating the proteins a cell makes. Doing so enabled the researchers to identify the cell types (astrocytes, immature or oligodendrocyte precursor cells, mature oligodendrocytes, neurons and stressed cells). They found multiple changes in cells from persons with MS that were different from control cells.
6. Genes associated with increased risk for susceptibility to MS were more frequent in lines from persons with MS, showing that these MS cells lines were similar in genetic composition to other studies of genes from persons with MS.
7. Next, they noted that cells lines from persons with primary progressive MS had significantly fewer mature oligodendrocytes, even though numbers of immature oligodendrocytes or oligodendrocyte precursor cells, were either more frequent or the same as in controls and lines from persons with other forms of MS. These cells thus appeared to be “stalled” in a dysfunctional state. The investigators felt this was due to the death of mature oligodendrocytes via a process called “ferroptosis,” a cell-death pathway also noted in MS central nervous systems.
8. Oligodendrocyte precursor cells from persons with MS and controls responded equally to two small molecules (ketoconazole and TASIN-1) that increased maturation of precursors to mature oligodendrocytes. These drugs work by inhibiting pathways of cholesterol synthesis. (spoiler alert: my next blog will describe changes in cholesterol metabolism that are toxic to brain cells). The scientists concluded that oligodendrocyte precursor cells in primary progressive MS had the capacity to mature into myelin-producing oligodendrocytes but did not do so possibly due to metabolic changes.
9. The researchers looked for evidence of inflammatory markers in the cell lines. In particular they studied genes required for the production of proteins of the major histocompatibility complex (MHC). These proteins are needed to stimulate immune cells, especially T cells. They found that astrocytes from persons with MS had upregulated or activated genes involved in the production of MHC proteins, as well as activated genes involved in antigen presentation, genes involved in the production of soluble inflammatory substances such as interferons, and signs of infection with Epstein-Barr virus. Genes needed for maintenance of nerve cell health were reduced in MS-associated astrocytes.
10. The shapes of astrocytes from persons with MS were different than control astrocytes, with multiple nuclei in the cells indicating that cell division in astrocytes was impaired.
11. Previous studies demonstrated that MS brain oligodendrocytes and astrocytes showed signs of inflammation believed to result from exposure to inflammatory substances (cytokines) made by immune cells. When Clayton and colleagues looked at astrocytes from their stem cell lines, cultivated in the total absence of immune cells, they found signs of inflammatory activation identical to those noted in MS brains. As stated in the paper, these observations show that stem-cell derived astrocytes “from people with MS generate pathological astrocyte subtypes that mirror those in MS brains, even without inflammatory stimuli or peripheral immune activation”.
Discussion:
These exciting data support the “inside-out” hypothesis, that there are intrinsic changes in the central nervous systems of persons with MS that have the potential to cause a loss of cells, releasing proteins that stimulate a genetically predisposed immune system that hyper-responds to these tissues.
Given the results of the Clayton et al paper, and the evidence suggesting that persons with MS have an immune system predisposed to overreacting to central nervous system proteins, it would be reasonable to propose a hybrid hypothesis of MS causation that combines the “inside-out” theory with the “outside-in” theory. I will call this the “revolving door hypothesis”, or as stated by Sty and colleagues, that MS results from an “immunologic convolution”, namely a combination of a genetically modified central nervous system leading to tissue degeneration, and a genetically, “hypersensitive” immune system that contributes to continued tissue damage and continued immune stimulation. Additional data to support this hypothesis are the observations that genes associated with the severity of disease are different from genes associated with susceptibility of disease , and these genes reside in the central nervous system.
These data have profound implications for finding a true cure for MS. It’s clear that just treating the immune system in MS is not sufficient to prevent disease progression. To truly prevent central nervous system tissue destruction it will be necessary to not only reduce immune system activity but to also correct the intrinsic genetically and environmentally (e.g. Epstein-Barr virus infection) induced changes that appear to trigger the onset of the illness. The challenges are multiple, but the rapidity of advances allows me to feel cautiously optimistic.
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