The Effects of Stem Cells on Epilepsy

Table 2: Classification of Antiepileptic Medications as Broad or Narrow Spectrum [14].

Treatment of epilepsy with AEDs may be the most common method of treatment, but under certain circumstances an alternate approach must be taken. It is estimated that one third of epilepsy patients experience recurrent seizures that are unable to be treated with AEDs alone[17].  A case of epilepsy is deemed drug resistant once two different tolerated AEDs are unsuccessful in controlling seizures [17]. The movement to surgery as an epilepsy treatment was due in large part to Victor Horsley; in the late 19^th century, he removed scar tissue from the frontal lobe of a patient experiencing chronic seizures [18]. This resulted in the eradication of the patient’s seizures and paved the path for future surgical practices [18]. Temporal lobe epilepsy (TLE) is the most common form of epilepsy that is drug resistant by a wide margin [18]. There are multiple surgical options available to those with drug resistant TLE. Two common operations performed under these circumstances are anterior temporal lobectomy and selective amygdalohippocampectomy [18]. Corpus colostomy is an effective operation in epileptic individuals with generalized seizures [19]. Severing the corpus callosum prevents the two hemispheres of the brain from communicating with one another, making epileptic attacks less substantial [19]. While 62% of patient’s families, of those receiving a corpus colostomy, report improvement in daily functioning, the surgery completely inhibits seizures in less than 10% of patients [20]. Approximately one third of all individuals who opt for a surgical treatment of epilepsy see no improvements in controlling seizures, while another third experienced significant improvements but still required AEDs to further inhibit epileptic activity [21]. The last third of patients receiving surgery reported to be completely seizure free [21].

Another modality of epilepsy treatment is neurostimulation. Recent technological developments allow for treatment by delivering stimuli to a specific target site [22]. One method of neurostimulation is repetitive transcranial magnetic stimulation (rTMS) [22]. This delivers a magnetic wave up to 2 cm deep from the surface of the skull; this is deep enough for the magnetic wave to reach the cortex, providing treatment in this region of the brain [22]. Studies have illustrated that rTMS reduced seizure occurrence by 72% for periods greater than 2 months [22]. Another common neurostimulation technique is vagus nerve stimulation (VNS) [22]. VNS requires the implantation of a neurocybernetic prosthesis in the chest of an epileptic patient [22]. The device delivers electrical currents in the vagus nerve, triggering organelles in the brainstem and reducing seizure activity by 50% in the limbic system [22]. This deep brain stimulation is ideal when epileptic patients are classified as drug resistant, especially in those that are not surgical candidates [23].

Though various viable treatments exist, proper treatment techniques are not available in many regions of the world [24]. An estimated 4 in every 5 individuals that suffer from epilepsy live in middle or low income countries [24]. Analyzing the treatment gap, the number of individuals that need treatment but do not receive it, there are many more untreated epilepsy cases in countries with a low average income, compared to countries with a high average income [15]. The sizable treatment gap in low income countries is due to various factors, such as lack of knowledge, poverty, deficient health delivery infrastructure, and limited of professional health care workers [24]. It is estimated that the mortality rate of people with epilepsy is two to three times higher in low income areas than that of the entire population [24]. Advancements have been made in the last several years in every aspect of epilepsy treatment; with no treatment having been found to be completely successful thus far, it is important to look to stem cell therapy as a plausible treatment of epilepsy.

Stem Cells Therapy

Stem cells are those that possess the potential to differentiate into various different specialized cells [25]. These cells are found in organisms in the embryonic, fetal and adult stages of life [25]. Stem cells are classified in two ways, their ability to differentiate and their origins [25]. Classification based upon differentiation consists of totipotent, pluripotent, multipotent, oligopotent and unipotent cells [26]. Totipotent cells can differentiate to become any cell type and create an entire organism [26]. Pluripotent cells can form nearly all cells, differentiating into cells originating from the endoderm, mesoderm, and ectoderm, the three germ layers of the embryo [25]. Multipotent cells differentiate more narrowly into closely related cells [27]. Lastly, oligopotent and unipotent cells specialize into an even narrower selection of cells, with unipotent cells only producing cells of their own kind [26].

In addition to potency and differentiation, stem cells are categorized based on their origins. Embryonic stem cells (ESCs) are obtained from the pre-implantation period of the fertilized ovum in humans, commonly referred to as a blastocyst [28]. The blastocyst contains an inner-cell and outer-cell mass. The inner-cell mass is responsible for embryo formation; therefore, ESCs are harvested here [25]. ESCs are pluripotent and unique in the aspect that they can remain in their undifferentiated state for extended periods of time [25]. Although ESCs have numerous qualities that make them advantageous for potential clinical applications, obtaining the cells requires the destruction of a blastocyst, creating ethical roadblocks [28].

Adult stem cells (ASCs), also termed somatic stem cells, are totipotent and multipoint cells located in numerous regions of the body in the postnatal stage of life [27]. In recent years, findings regarding the plasticity of ASCs have been encouraging [29]. Tissue-resident stem cells have the ability to provide regenerative repair; the ectoderm is believed to give rise to skin and neural entities [29]. Mesenchymal stem cells (MSCs) are a customary ASC, which can be isolated from bone marrow, adipose tissue, umbilical cord blood, and amniotic fluid [25]. ASCs are vastly important to methods of stem cell therapy, as they can differentiate into many types of specialized cells, without creating ethical controversy [25].

A newer stem cell origin is known as induced pluripotent stem cells (iPSCs) [25]. These are ASCs that have been genetically modified to exhibit characteristics of ESCs [25]. This development has potential groundbreaking implications, considering cells could be harvested from human somatic cells, avoiding the ethical concerns that coincides with ESCs [30]. The genetic modification introduces the ability to differentiate in a pluripotent fashion, without causing any harm to a human blastocyst [30]. Technological developments are currently underway in order to make iPSCs a viable treatment in humans, but there are still many obstacles to overcome before it can become a regular clinical practice [30].

Embryonic Stem Cells

ESCs have many differentiation pathways, making them, ethics aside, an intriguing option for epilepsy treatment [31]. Despite this intrigue, several aspects of ESC therapy need to be improved before they can be applied clinically to neurological disorders [32]. Potential problems include the lack of control in proliferation; this has been best accomplished thus far by combining the methods of using feeder cells, supplementing growth factors and practicing genetic engineering [32]. Another potential problem in hESC involves the survival of stem cell derived neurons after transplantation and the adverse effects post-grafting [32]. Although it is relatively rare, a big concern is the formation of tumors at the site of grafting; tumors can be residual proliferating ESCs or precursor cells [32]. A study observed the tumor incidence in rats was reduced when the hESC were co-cultured for a period of 20 days [32]. Another recommendation for reducing the risk factor of tumors is to undergo drug-induced apoptosis of undifferentiated hESCs [32]. When health risks are successfully averted, post-grafting survival requires the avoidance of inflammation and graft rejection; these instances can be evaded with immunosuppression, the induction of immunotolerance, and somatic cell nuclear transfer [32].

Despite many risk factors associated with the direct use of ESCs, ESC-derived neural precursor cells have experienced promising preclinical animal trials [33]. In one study, ESC-derived neural precursors were obtained from the fetal human brain (ventricle zone) and delivered to epileptic mice; rats received the progenitor cells via injection in the tail [33]. The delivered cells were able to migrate to areas of the brain exhibiting seizure activity, including the hippocampus, piriform cortex, and the amygdala [33]. Results showed that nearly 26% of transplanted cells were GABA positive in the piriform cortex, and 31% were positive for Parvalbumin [33]. This provides hope for this method as a treatment of epilepsy, given that fully differentiated neurons lose their ability to divide and multiply [33]. Although encouraging, this study does provide difficult interpretations, due to no measurement of the number of excitatory neurons compared to GABAergic cells [33].

In a study conducted in 2009, researchers conducted a bilateral transplantation of precursor cells originating in the embryonic medial ganglionic eminence (MGE) to postnatal neocortex of mice [34]. This study allowed for additional evidence regarding the transplantation of cells as a treatment of epilepsy [34]. Significant results were observed in epileptic mice lacking a Shaker-like potassium channel (Kv1.1) [34]. The channel mentioned mimicked channel activity associated with human epilepsy disorders [34]. The Kv1.1 mice received either MGE grafts, or no treatment (control group) [34]. Results showed that mice receiving grafts developed GABAergic neurons, yielding significant reductions in both the length and regularity of spontaneous electrographic seizures [34] (Figure 2).

Figure 2: Seizure suppression in the Kv1.1 mouse model of Epilepsy [34].

A grade IV electrographic seizure was deemed to have frequency, synchronized high voltage, and polyspike or paroxysmal shape waves with amplitude >2-fold background, lasting more than 6 seconds [34]. (A) An EEG from a controlled Kv1.1 mouse during a grade IV epileptic seizure, and (B) the same seizure in higher resolution split into four different stages of the seizure progression, have a noticeably longer progression than that of (C) the EEG from the Kv1.1 mouse grafted with MGE [34]. The shorter progression of the seizure in the Kv1.1 mouse grafted with MGE is even more apparent with (D) the higher resolution EEG, split into four progression stages [34]. (E, F) The duration and recurrence of seizures was significantly greater in mice that were not grafted with MGE [34]. Lastly, the results indicate (G) the suppression of seizure activity in the grafted mice was less significant compared to the control Kv1.1 mouse [34]. This was seen, despite the probable overestimate in the number of seizures occurring in mice grafted with MGE, due to a longer period of monitoring for seizures [34]. The findings of this study advocate for the possibility of MGE interneuron precursor transplantation as a treatment for epilepsy in those lacking completely functional potassium channels [34].  Overall, the use of ESCs as treatment for epilepsy has too many risks to be accepted for clinical applications immediately; however, there are many promising preclinical findings associated with ESC derived neural precursor cells [32]. With continued research, the risk factors and ethical concerns involving ESCs may be reduced. But for now, and the immediate future, other stem cell therapies should be investigated for potential groundbreaking discoveries.

Mesenchymal Stem Cells

MSCs are multipotent and can be found in several regions of the body, including bone marrow, adipose tissue, umbilical cord blood, and amniotic fluid [25]. With even more areas of MSC isolation, many consider MSCs to be the most practical potential clinical application of stem cell therapy [25]. In a successful animal preclinical trial, conducted in 2017 by Salem et al., MSCs were isolated from bone marrow and delivered to male rats; this allowed for the application of MSC use as a treatment of TLE to be seen [35]. The MSCs were isolated from the bone marrow of three male rat’s femurs and tibiae; they were then suspended in 1% penicillin-streptomycin medium and incubated at 37° C for 7 days [35]. 4 x 10³cells/cm² were obtained and used for experimentation [35]. MSCs were labeled with PKH-26, a pink fluorescent die, allowing for easier discrimination among cells after transplantation [35]. With the use of a Hamilton syringe, 3mL of suspension was injected into both sides of the hippocampus (~100,000 cells on each side) [35]. In addition to this hippocampal bilateral injection, rodents were also injected intravenously, allowing for the two modes of stem cell delivery to be compared [35].

Results from this study provide evidence of the significant reduction of inflammatory cytokines, TNF-a and IL-1b [35]. Additionally, an improved oxidative state in the hippocampus was observed, due to antioxidant defense markers, GHS and PON1 [35]. Although it was thought that the intravenous method of injection would be more successful due to the direct access of lesioned areas, the bilateral hippocampal injection yielded double the cell count of the intravenous method [35] (Figure 3).