The etiology of multiple sclerosis: a new and extended vascular-ischemic model

Vascular multiple sclerosis: addressing the pathogenesis, genetics, pro-angiogenic factors, and vascular abnormalities, along with the role of vascular intervention

Dysfunction in the epithelium, breakdown of the blood-brain barrier, and consequent leukocyte and T-cell infiltration into the central nervous system define Vascular Multiple Sclerosis. Multiple sclerosis (MS) affects around 2.5 million individuals worldwide, is the leading cause of neurological impairment in young adults, and can have a variety of progressions and consequences. Despite significant discoveries in immunology and molecular biology, the root cause of MS is still not fully understood, as do the immunological triggers and causative pathways. Recent research into vascular anomalies associated with MS suggests that a vascular component may be pivotal to the etiology of MS, and there can be actually a completely new entity in the already available classification of MS, which can be called 'vascular multiple sclerosis'. Unlike the usual other causes of MS, vascular MS is not dependent on autoimmune pathophysiologic mechanisms, instead, it is caused due to the blood vessels pathology. This review aims to thoroughly analyze existing information and updates about the scattered available findings of genetics, pro-angiogenetic factors, and vascular abnormalities in this important spectrum, the vascular facets of MS.

Hypoxia: molecular pathophysiological mechanisms in human diseases

Hypoxia, a low O2 tension, is a fundamental feature that occurs in physiological events as well as pathophysiological conditions, especially mentioned for its role in the mechanism of angiogenesis, glucose metabolism, and cell proliferation/survival. The hypoxic state through the activation of specific mechanisms is an aggravating circumstance commonly noticed in multiple sclerosis, cancer, heart disease, kidney disease, liver disease, lung disease, and in inflammatory bowel disease. On the other hand, hypoxia could play a key role in tissue regeneration and repair of damaged tissues, especially by acting on specific tissue stem cells, but their features may result as a disadvantage when it is concerned for neoplastic stem cells. Furthermore, hypoxia could also have a potential role in tissue engineering and regenerative medicine due to its capacity to improve the performance of biomaterials. The current review aims to highlight the hypoxic molecular mechanisms reported in different pathological conditions to provide an overview of hypoxia as a therapeutic agent in regenerative and molecular therapy.

Graphical abstract

Hypoxia in multiple sclerosis; is it the chicken or the egg?

Over the past 50 years, intense research effort has taught us a great deal about multiple sclerosis. We know that it is the most common neurological disease affecting the young-middle aged, that it affects two to three times more females than males, and that it is characterized as an autoimmune disease, in which autoreactive T lymphocytes cross the blood-brain barrier, resulting in demyelinating lesions. But despite all the knowledge gained, a key question still remains; what is the initial event that triggers the inflammatory demyelinating process? While most research effort to date has focused on the immune system, more recently, another potential candidate has emerged: hypoxia. Specifically, a growing number of studies have described the presence of hypoxia (both 'virtual' and real) at an early stage of demyelinating lesions, and several groups, including our own, have begun to investigate how manipulation of inspired oxygen levels impacts disease progression. In this review we summarize the findings of these hypoxia studies, and in particular, address three main questions: (i) is the hypoxia found in demyelinating lesions 'virtual' or real; (ii) what causes this hypoxia; and (iii) how does manipulation of inspired oxygen impact disease progression?

Multiple sclerosis disease progression: Contributions from a hypoxia-inflammation cycle

Hypoxia has been associated with multiple sclerosis (MS) and is an important area of research. Hypoxia can exacerbate inflammation via the prolylhydroxylase pathway. Inflammation can also trigger hypoxia by damaging mitochondria and endothelial cells to impair blood flow regulation. We hypothesize that there is a “hypoxia–inflammation cycle” in MS which plays an important role in MS disease progression. Therapies that break this cycle may be an interesting area of exploration for treatment of MS.

Hypoperfusion of the cerebral white matter in multiple sclerosis: possible mechanisms and pathophysiological significance

Multiple sclerosis (MS) is a disease of the central nervous system characterized by patchy areas of demyelination, inflammation, axonal loss and gliosis, and a diffuse axonal degeneration throughout the so-called normal-appearing white matter (NAWM). A number of recent studies using perfusion magnetic resonance imaging in both relapsing and progressive forms of MS have shown a decreased perfusion of the NAWM, which does not appear to be secondary to axonal loss. The reduced perfusion of the NAWM in MS might be caused by a widespread astrocyte dysfunction, possibly related to a deficiency in astrocytic beta(2)-adrenergic receptors and a reduced formation of cAMP, resulting in a reduced uptake of K(+) at the nodes of Ranvier and a reduced release of K(+) in the perivascular spaces. Pathologic and imaging studies suggest that ischemic changes might be involved in the development of a subtype of focal demyelinating lesions (type III lesions), and there appears to exist a relationship between decreased white matter perfusion and cognitive dysfunction in patients with MS.

The multiple sclerosis lesion: initiated by a localized hypoperfusion in a central nervous system where mechanisms allowing leukocyte infiltration are readily upregulated?

A mechanism is proposed that may explain the factors that initiate a multiple sclerosis (MS) lesion. It is based upon the following two hypotheses: (i) there is a lower stimulus threshold for upregulating the mechanisms that result in leukocyte infiltration in individuals predisposed to developing MS; (ii) the MS lesion is initiated as a reduction in blood flow to a localized region of white matter. This reduction in blood flow leads to: (a) degenerative white matter changes affecting oligodendrocytes; (b) upregulation of chemokines in the endothelial cells and/or glial cells; and (c) upregulation of cell adhesion molecules on endothelial cells. Signals from the hypoxemic and hypoglycemic glial cells, likely involving myelin molecules and cytokines, result in an inflammatory immune response that results in rampant demyelination. Evidence supporting the proposed mechanism is presented, as well as suggestions on how to test the validity of the proposal.

Increased oxygen tensions influence subset composition of the cellular immune system in aged mice

In acute and chronic experiments, each of eight groups of aged mice were assigned separately to different pressures of oxygen to which it was to be exposed. Lymphocytes from spleen, thymus, and peripheral blood were analyzed following oxygen exposure. Subset populations changed depending on the oxygen tension. Variable changes were observed in total numbers of lymphocytes, lymphocyte subsets, B cells, and macrophages depending on the organ studied and the oxygen pressure to which the mice were exposed. There were differences between acute and chronic exposure suggestive of adaptation to environmental stressors. The suggestion is made that the immune system has a reserve capacity that can be influenced by oxygen and, thereby, theoretically capable of being pharmacologically manipulated to assist patients with altered immune systems to promote defense mechanisms or, under certain circumstances, reduce autoimmunity. It is hypothesized that an underlying hypoxia may be involved in the age-associated decline in the immune system.