Multiple sclerosis (MS) is the leading cause of nontraumatic neurological disability in young adults in North America, yet there are no uniformly effective therapies and none yet proven effective at blocking long-term progression. The difficulty in predicting disease course and developing effective treatments is complicated by the heterogeneity of the disease, which can manifest with a benign, relapsing-remitting or progressive disease course.
Since effective therapeutic options are very limited for MS patients, particularly when the disease is in its more-progressive stages, it becomes essential to define both pathophysiological mediators of each stage and markers that would permit individualized therapeutic intervention. The clinical manifestations of MS, whether relapsing or progressive, are believed to relate in part to the degree of inflammation relative to neurodegeneration, which characterizes the individual disease course.
Advances in genetics, protein chemistry and bioinformatics and, most importantly, a skilled community of research and clinical experts create a fertile environment for Mayo Clinic's MS research team. Recently, Mayo researchers have discovered that select members of a family of secreted serine proteases known as tissue kallikreins play key roles in both the inflammatory and neurodegenerative processes that drive MS and may serve as important biomarkers of disease stage and future disability.
Tissue kallikreins comprise a family of 15 mainly newly identified serine proteases. These proteases comprise the largest continuous cluster of serine proteases in the human genome, although they have as yet poorly defined roles in disease.
Mayo Clinic's MS researchers are focusing on two members of the kallikrein protease family that are elevated in the blood and brain lesions of MS patients and which show strong evidence of contributing to pathology. In a study published in 2008, Mayo researchers showed that kallikrein 1 and kallikrein 6 are selectively elevated in the serum of MS patients experiencing a progressive disease course (when compared with patients with relapsing-remitting disease or to non-MS controls). Importantly, in cell culture studies each of these was shown to exert neurotoxic effects toward central nervous system (CNS) neurons. Kallikrein 6 was also particularly toxic to the myelin-producing oligodendrocytes.
Interestingly, kallikrein 1 levels were found to correlate with Expanded Disability Status Scale (EDSS) scores at the time of serum draw, while kallikrein 6 serum levels were predictive of future worsening of EDSS scores.
Additional research published in 2011 has shown that each kallikrein drives unique aspects of the inflammatory processes that underpin MS pathogenesis, with kallikrein 1 promoting lymphocyte proliferation while kallikrein 6 increased the resistance of lymphocytes to apoptosis. These findings predict a clinical model in which elevated levels of kallikrein 1 and kallikrein 6 at sites of CNS inflammation likely contribute directly to sustained chronic inflammation and neurodegeneration. These enzymes thus become important therapeutic targets in treating both the neurodegenerative and inflammatory events that so often characterize lesions in patients experiencing a progressive disease course.
A major advance in understanding the mechanism of action of kallikreins in MS and other neurological disorders and how these enzymes might be targeted therapeutically came with the discovery that kallikreins can activate protease-activated receptors to trigger discrete intracellular signaling cascades involved in inflammation and CNS injury.
Current research is therefore focused on determining the effects of genetic and pharmacologic modulation of these receptors in the CNS or in the immune compartments in animal models of MS. Mayo researchers are hopeful that the insights provided will point directly to new therapies for MS patients, particularly those at the more progressive stages of disease.
In a study published this year in Brain Pathology, Mayo researchers found that an antibody that neutralizes kallikrein 6 is capable of staving off MS in mice. "We were able to slow the course of disease through early chronic stages, both in the brain and spinal cord," says lead author Isobel A. Scarisbrick, Ph.D., of the Department of Physical Medicine and Rehabilitation at Mayo Clinic in Rochester, Minn.
Dr. Scarisbrick and colleagues studied mice infected with Theiler's murine encephalomyelitis virus (TMEV), a viral model of MS, and assessed the effects of kallikrein 6-neutralizing antibodies on disease progression. RNA expression of kallikrein 6 was elevated in the brain and spinal cord by seven days postinfection (dpi), and expression persisted primarily in the spinal cord, reaching a peak of fivefold over controls at midchronic stages (60-120 dpi). Significant elevations in kallikrein 6 RNA were also induced in lymphocytes stimulated with viral capsid proteins in vitro and in activated human acute monocytic leukemia cells.
Kallikrein 6-neutralizing antibodies reduced TMEV-driven brain and spinal cord pathology and delayed-type hypersensitivity responses when examined at early chronic time points (40 dpi).
The effects of kallikrein 6 on spinal cord pathology included a decrease in activated monocytes-microglia (inflammatory white blood cells) and a reduction in the loss of myelin basic protein, a key component of the myelin sheath. By 180 dpi, pathology scores no longer differed between groups. The kallikrein 6-neutralizing antibody had reduced inflammation and demyelination through early chronic time points, and the investigative team hopes to develop more powerful enzyme-inhibiting techniques to determine if more long-term effects can be achieved.
"These findings suggest kallikrein 6 plays a role in the inflammatory and demyelinating processes that can accompany many types of neurologic conditions," says Dr. Scarisbrick. "In the early chronic stages of some neurologic diseases, therefore, kallikrein 6 may represent an excellent drug target to coordinately reduce inflammation and myelin loss."