A therapeutic strategy for neurodegenerative diseases

Currently, NDD is becoming a growing problem as the risk of being susceptible to a certain neurodegenerative disease increases dramatically with age. Although treatments available may help relieve some of the physical or mental symptoms associated with NDD, the scope of effective treatment options remains limited. With this, the critical need to address this problem gave rise to study the possibilities of using mesenchymal stem cells (MSCs) for this group of disorders.

MSCs in Alzheimer’s Disease

Alzheimer’s disease (AD) is a neurological condition that is regarded as the main cause of dementia among old people. AD starts with mild memory loss, but as the cognitive impairment further progresses over time, it results in the loss of the ability to live independently. Pathologic features of this condition include chronic accumulation of plaques enriched in β-amyloid (Aβ) in the brain and intracellular tau-enriched neurofibrillary tangles. These Aβ plaques are the cause of the deregulation of kinases and phosphatases and, activation of brain specialized immune cells called microglia.

Microglia comprise 5% to 10% of total brain cells and carry out macrophage-like and other specialized functions. These cells function with minimal contribution from immune cells outside of the central nervous system (CNS) and provide rapid responses to damage or infection. However, recent studies have shown that microglia are important for synaptic modulation and promote neuroinflammation in many neurodegenerative diseases and major depressive disorder (MDD). Microglial inflammatory cytokines have neurotoxicity and lead to neuronal loss and cognitive decline, and brain atrophy. The determining role of microglial inflammatory cytokines has been complicated by their binary roles in neuroprotection and neurodegeneration. Thus, therapies aimed only at combating the accumulation of Aβ plaques are likely to fail due to the complex pathology of AD.

To date, the treatment of AD is only palliative, and involves mainly drugs to increase cerebral acetylcholine levels. But recent studies show that MSCs act through paracrine mechanisms, rather than through direct engraftment, showing both the neuroprotective and anti-inflammatory effects of MSCs secretome after its administration. MSCs are found to have the capacity to reduce the inflammatory effect because of its ability to switch the activated microglia from M1 proinflammatory phenotype to M2 phenotype, which have an anti- inflammatory activity, thereby improving neuron survival. Likewise, MSCs could promote the reduction of Aβ plaques through inducing the expression of the Aβ- degrading enzyme by microglial cells. In the AD mice model, it was shown that MSCs overexpress vascular endothelial growth factor into the hippocampus, promoting neovascularization, which was accompanied by behavioral benefits and alleviation of cognitive dysfunctions. On this account, MSCs may treat AD by reducing the Aβ deposition, immunomodulation, angiogenesis, and decreasing neuronal loss.

MSCs in Parkinson's Disease

Parkinson's disease (PD) is one of the most common progressive neurodegenerative disorders, characterized by slowness of movement, rigidity, tremor, and postural instability, and bradykinesia. The pathological features of PD consist in the loss of dopaminergic neurons in the substantia nigra (SN), leading to decreased dopamine levels, which is the primary cause of the motor symptoms of PD.

Current treatments for PD are mainly symptomatic, including drug therapy and auxiliary surgical treatment that can improve motor fluctuation in early stages, but become less effective as the disease progresses. However, recent therapeutic approaches such as stem cell therapy, and immunotherapy using antibodies and gene therapy have shown promising results in decreasing neuronal loss and slowing progression of the disease. MSCs-mediated therapy are found effective on PD patients based on multiple mechanisms. MSCs could induce migration of neuroblasts to lesioned brain areas and enhance neurogenesis, and led to the differentiation of neural precursor cells into dopaminergic neurons in the substantia nigra. MSCs exert their neuroprotective efficacy on dopaminergic neurons through anti-inflammatory effects by inhibiting microglial activation as was described above in AD. The infusion of MSCs significantly decreased neuronal loss, iNOS, and TNF-α secretion. In particular, the autologous MSCs injected into the subventricular zone of seven patients with PD appeared to be safe and well-tolerated, with long-lasting motor function improvement observed in some patients. On the grounds of this, MSCs-derived exosomes could be also effective to treat patients with PD.

MSCs in Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by a mutation in the huntingtin (HTT) gene. The neuropathology of HD consists in the progressive degeneration of striatal GABAergic neurons. Currently, there is no effective therapy for disease prevention or slowing down disease progression. Special medications could only alleviate the symptoms of HD to improve the quality of life of the patients. However, it cannot extend the patient’s life span.

That said, recent investigations of the effects of MSCs transplanted into the ipsilateral striatal border of a mice with HD showed increased survival, attenuated the loss of striatal neurons, and reduced htt aggregates. It was suggested that the therapeutic effects of MSCs are associated with their neuroprotective/immunomodulatory capacity. The trophic and growth factors released by MSCs could induce tissue repair and angiogenesis. HD patients are known to have low levels of BDNF needed for cortical neurons' survival and function. Restoration of BDNF level in rodent models with HD increases neuronal survival and improves HD symptoms. But the direct injection of BDNF appeared ineffective in HD disorders because of the short half-life of protein. As consequence, recent clinical trial transplantation of genetically modified MSCs overexpressing BDNF were performed.

MSCs in Amyotrophic Lateral Sclerosis

Amyotrophic lateral sclerosis (ALS) usually sets up spontaneously without obvious causes. To date, the ethology of this disease remains undefined. ALS is a rare neurological disease that develops rapidly wherein death occurs three to five years after the onset of symptoms. The progressive degeneration of the upper and lower motor neurons in the brain and spinal cord results in muscle weakness and respiratory failure.

Recent findings in animal experimental models of ALS and clinical trials showed that the administration of MSCs could alleviate disease symptoms and slowdown the disease progression. The observed clinical improvement in ALS patients might be related to MSCs' immunomodulatory and anti-inflammatory effects. MSCs reduce astrogliosis and microgliosis, and decrease peripheral levels of proinflammatory cytokines such as TNF-α, IL-1, IL-6. Transplanted MSCs could also provide neuroprotection, and stimulation of nerve tissue regeneration because of the secreted neurotrophic factors distributed through the cerebrospinal fluid from the motor cortex to the spinal cord. It was shown that infusion of genetically modified MSCs simultaneously expressing GDNF, IGF-1, and HGF leads to neurotrophic effect of donor cells and delays the development of the disease in a mouse model of ALS.

MSCs in Multiple Sclerosis

Multiple sclerosis (MS) is a chronic inflammatory autoimmune disease characterized by lesion of axons and demyelination. The primary cause of MS as in the case of ALS remains uncertain. It was suggested that a combination of some genetic and environmental factors, as well as infections may result in the formation of autoreactive lymphocytes and specific antigen-presenting cells in the body, which lead to an inflammation of the CNS. A neuroinflammation causes neurodegeneration, thereby destroying the myelin sheaths and axon neurons.

Currently, available medications help control disease progression by decreasing immune-mediated inflammation. They are more effective if given to patients before a severe widespread damage has occurred, because they neither cure the disease nor reverse the damage of the neurons and demyelination. Despite this, MSCs are known to have immunomodulatory activity, which could diminish neuroinflammatory process of MS by reducing activation of astrocytes and microglia, shifting proinflammatory phenotype of microglia (M1) to anti- inflammatory microglia (M2). MSCs can also improve the clinical severity of MS by secretion of anti-inflammatory cytokines and neurotrophic factors, thereby maintaining a favorable microenvironment for neuroprotection and regeneration. In some progressive MS patients, infusion of autologous MSCs resulted in a mild improvement of neurological disability. It was shown that MSCs were effective when administered at the onset of the disease and peak, but not after disease stabilization

Depression and Mood Diseases

Major depressive disorders (MDD) such as mania, bipolar disorder, and depression are heterogeneous conditions characterized by complex genetic predispositions with unclear pathophysiology. Low effectiveness of available antidepressant treatments confirm that MDD are clinically and etiologically heterogeneous disorders. Many clinical investigations have shown that bone metabolism abnormalities, which can lead to low bone mass, are correlated with mood diseases. This could be further expounded by the importance of bone- derived hormones to brain development, function, behavior, and regulation of neurotransmitter synthesis.

It was shown that depression is associated with the increased risk of developing cardiovascular disorders by 1.5-2 fold; stroke by 1.8 fold; Alzheimer’s disease by 2.1 fold; epilepsy by 4-6 fold; diabetes by 60%; and cancer by 1.3-1.8 fold. Although there are treatments available for depression, conventional treatment with antidepressant medications lead to resistance in up to 40% of patients. Likewise, antidepressant therapy has a variety of undesirable side effects such as sedation, decrease of blood pressure, increase of weight, sexual dysfunction, blurred vision, and dry mouth. These side effects often result in a break-up of medication with recurrence of depressive symptoms. To lessen the severe side effects, there is a reduction of drug dosage in some cases. However, the problem with this strategy is that it leads to insufficient treatment for an adequate period of time, which is why mood disorders of patients remain in an “undertreated” state.

There are many hypotheses about the causes that promote the development of MDD. One of the most plausible hypotheses is the impairment of neurotrophic growth factors expression and its associated signaling pathways. Brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), nerve growth factor (NGF), vascular endothelial growth factor (VEGF), insulin- like growth factor (IGF), glial cell-derived neurotrophic factor (GDNF), and fibroblast growth factor (FGF) are showed to be connected with MDD pathophysiology. That said, MSC therapy has shown great potential in regenerative medicine. In spite of the limited studies evaluating the effects of MSCs in psychiatric disorders, it is known that MSCs have the ability to express neurotrophic factors, such as VEGF, BDNF, NGF, and IGF. These neurotrophic factors promote neurogenesis, and the survival and differentiation of neural cells, which provide the therapeutic effect of MSCs on MDD. Moreover, it was shown that MDD are associated with a chronic inflammatory process, and are accompanied by increased oxidative and nitrosative stress. Nevertheless, the many investigations that showed the immunomodulatory properties of MSCs and their ability to decrease inflammation, give new hope for alleviation the course of MDD.

Stroke

Strokes could be divided into ischemic stroke and hemorrhagic stroke. Ischemic stroke is the most common type of stroke and is caused by a blood clot that blocks a blood vessel in the brain. Hemorrhagic stroke is caused by the rupture of cerebral arteries. Overall, stroke is accompanied by neuronal death and inflammation processes, which destroy hypoxic tissue in the insult region and enlarge the damaged area

Despite this, it was found that the transplanted MSCs could home to the infarcted area and act though paracrine mechanisms. MSC therapy appeared to be promising for stroke due to its specific activities. First, MSCs were found to decrease inflammation, promote neurogenesis, inhibit neuronal apoptosis, and improve behavioral and motor function in the ischemic stroke. Brain ischemia affects not only neurons but also vascular cells. As such, MSCs have been shown to promote angiogenesis and vasculogenesis by releasing different growth factors for increasing blood vessel density. Mitochondrial transfer between exogenous MSCs and damaged endothelial cells is another mechanism for amelioration of the patients' state with stroke. MSCs could also stabilize the blood-brain barrier (BBB). The decrease of BBB permeability was observed after MSC infusion in different animal models of stroke. Lastly, the administration of MSCs in the chronic phase of stroke has been shown to activate regenerative mechanisms important for brain function restoration.

Methods of MSCs administration for neurological disorders

Treatment with MSCs for different neurological disorders has been widely studied both in preclinical studies on different animal models and in clinical trials. Based on the studies performed, a high potential for this therapy were demonstrated. Researchers tried several ways of MSCs transplantation, including intravenous, intra-arterial, intrathecal, intranasal, intraspinal, intracerebroventricular, and intracerebral.

Even so, there is still no consensus on which method could give the best results with minimal invasiveness. The Blood-brain barrier (BBB) is an obstacle for delivery of many drugs to the CNS via intravenous injection. Likewise, increasing the survival rate of the transplanted cells still remains a challenge. Systemic transplantation by intravenous cell injection is characterized by a minimal surgical invasion level, but transplanted cells often appear in the damaged area of the brain only a few days after injection and in rather small quantities.

By contrast, the arterial route seems to be a much more attractive way of systemic administration. Intra-arterial administration of MSCshelps to deliver most of the cells to the brain's damaged area and their deposition in the lungs is much smaller after injection. In the animal model studies, intra-arterially delivered MSCs were found in the motor and sensory cortex, hippocampus, striatum, thalamus, and hypothalamus.

Intranasal delivery of MSCs to the brain is another noninvasive method. This type of delivery resulted in the appearance of cells in the olfactory bulb, cortex, hippocampus, striatum, cerebellum, brainstem, and spinal cord. Intranasal delivery of MSCs has shown beneficial effects in rodent models of HD and PD. The use of a conditioned medium of MSC in intranasal delivery as an option for chronic and safe application of its bioactive components is actively investigated. It is suggested that it could be a way to prepare ready-to-buy biologic products for routine use by patients to maintain a stable state of the nervous system from degradation for many NDD.

Many studies show that invasive methods of transplantation of MSCs via direct infusion of cells into the injured brain or cerebrospinal fluid could be even more effective. Intrathecal injection is usually made in the lower part of the spinal cord in the space between the arachnoid mater and pia mater. Intrathecally administered cells are distributed with the circulation of cerebrospinal fluid to fluid spaces within the brain. In particular, intrathecal injections of MSCs are currently being used in many clinical trials and show good results. However, these invasive methods may induce some adverse reactions. Patients could experience backache, as well as the risk of an abnormal cerebrospinal fluid flow, caused by cell-induced obstruction, and can result in hydrocephalus.

Given the growing number of basic research and clinical trials evaluating the therapeutic effects of MSCs in neurological diseases have shown its great potential to improve patient symptoms and quality of life, it has been identified that inflammation is a key factor in the pathophysiology of NDD, MDD, and stroke, affecting the central nervous system (CNS) functionality. In this case, the immunomodulatory activity of MSCs may become a good therapeutic strategy in these cases. MSCs have the potential to paracrine activity even without direct cell contact. MSCs also provide a neuroprotective effect through secretion of neurotrophic factors even at a distance from the damaged organ. For these reasons, the creation of genetically modified MSC lines with increased secretion of neurotrophic factors is very important for the development of drugs based on a conditioned media after MSC growth. Intranasal administration of these drugs will be absolutely safe, which could allow patients or their relatives to routinely use these medications to maintain CNS in NDD and delay their progression.

As mentioned earlier, most of these discoveries are limited. To be specific, these findings are limited to preclinical studies only for bone marrow derived MSCs. Kintaro cells are BMSCs and could be used for treatments of NDD, MMD, and stroke as this approach has been proven to be safe and has great potential to improve the symptoms of neurological disorders.

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