Combining Senolytics and Cell Therapy for the Treatment of Parkinson’s Disease

Introduction

It is estimated that over 1.2 million people will suffer from Parkinson’s Disease (PD) in the United States alone by 2030, a nearly 1.8-fold increase from 2010 [1]. This growing disease significantly lowers patients’ quality of life due to its common signs and symptoms (e.g., impaired motor movement, rigidity, tremors, weakened cognitive abilities, and emotional changes) [2]. PD is caused by the loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) [2]. This region is highly involved in movement, resulting in the most recognized sign of PD, motor movement difficulties [3].

Currently, Levodopa (L-Dopa), the precursor to dopamine, is considered the most effective, commonly used PD treatment [4]. However, it comes with a host of common side effects such as somnolence, headaches, dizziness, and more severe side effects like involuntary movements [4, 5]. Furthermore, there are “off” times associated with L-Dopa treatment, the time during which the signs and symptoms of PD return [5]. While there are treatments that attempt to tackle “off” times, they often result in a worsening of motor complications [6]. The only medication proven to combat these complications, amantadine, can also cause side effects like hallucinations, confusion, and nausea [6].

Apart from these conventional therapies, newer ones such as BlueRock Therapeutics’ DA01 have entered the scene [7]. This treatment involves differentiating stem cells into midbrain DA neurons and inserting those into the brain. However, it only addresses one aspect of the PD problem: the death of DA neurons [8]. While it does replenish those dead cells, it does not necessarily stop the progression of cell death [8]. Therefore, the ultimate treatment for PD is combining cell therapy, such as DA01, with a method that rids the brain of diseased cells that cause DA neuron death. One such treatment is senolytics; these small molecules cause apoptosis in senescent cells selectively [9].

Senescence and Senolytics

For a cell to be considered senescent, it must be no longer proliferating, but also not undergoing apoptosis [10]. Some causes of senescence include severe DNA damage, telomere dysfunction, and mitochondrial deterioration [10]. If senescence is induced by severe DNA damage, the senescence-associated secretory phenotype (SASP) — containing interleukins, growth factors, extracellular matrix components, etc. — will develop in the cell [10]. The SASP can change the microenvironment of the tissue which can result in abnormal proliferation of epithelial cells, chronic inflammation, and spreading senescence to neighbouring cells [10, 11]. In turn, senescent cells can cause major tissue dysfunction [2].

While the negative impacts of senescent cells have been known for some time, there was not a strong focus on their role in PD until a study in 2018 by Chinta et al. showed robust evidence of senescence in astrocytes in PD caused by paraquat (PQ) [12]. PQ is a chemical used in herbicides, which has been linked to sporadic PD [12]. Chinta et al. examined human samples of SNpc tissues and noted an increased expression of p16INK4a (a senescence marker) alongside other SASP factors [12]. The researchers also found reduced lamin B1 (another senescence marker) in PD astrocytes from the SNpc region [12]. Moreover, in vivo tests supported the role of senescence in PD. In a senescent mouse model, Chinta et al. noted a reduction in neuronal lamin B1, increased SA-β-gal in the astrocytes (which also signifies senescence), as well as increased expression of p16INK4a [12]. When ganciclovir — a drug targeting senescent cells in these particular mouse models — was administered systemically, all senescent phenotypes found in the SNpc were eliminated and the DA neuron count returned to normal [12].

Currently, it is widely accepted that senescence contributes to PD, but one topic that remains controversial is whether it is only astrocytes that can be senescent or if neurons have this characteristic as well. Chinta et al. propose that while the presence of senescent astrocytes and their role in PD is confirmed in their study, this does not rule out the possibility of other senescent cells within the brain contributing [12]. Later, in a 2019 study by Riessland et al., it was discovered that post-mitotic neurons do indeed senesce [13]. In fact, using brain slices from sporadic PD patients, they found that DA neurons can easily become senescent when the protein, Special AT-Rich Sequence-Binding Protein 1 (SATB1), is knocked out [13]. Meanwhile, SATB1 knockout (KO) on cortical neurons did not result in senescent neurons [13].

Significantly, senolytics, including azithromycin, fisetin, and dasatinib plus quercetin treatment, were able to reduce the viability of senescent cells [13]. Azithromycin was particularly effective compared to the others [13]. Interestingly, DA neurons with a SATB1 KO were less viable after treatment with these established senolytics (Figure 1) [13]. As a result, Riessland et al. suggest that SATB1 itself could be a good option to explore as a treatment [13].

Figure 1. Cell viability of both wild-type and SATB1 KO cells after treatment with five compounds including DMSO (control), azithromycin (Azi), fisetin, dasatinib (D) plus quercetin (Q), and ABT-737 (another senolytic). Figure from [13].

In a 2020 commentary by Riessland, he further explained his hypothesis regarding the role of senescence in PD. The senescence of the particularly vulnerable DA neurons leads to SASP secretion, causing local inflammation in the midbrain [13, 14]. The body responds with a strong immune attack resulting in the death of DA neurons [14]. If SASP continues to inflame the brain, the immune attacks will persist, leading to a large loss of DA neurons and eventually PD [14]. Thus, using senolytics, like the previously proposed SATB1 plus another compound such as azithromycin, could be a great combatant [13, 14].

Cell Therapies: MSK-DA01

Alongside senolytics, cell therapies remain promising tools in the fight against PD. MSK-DA01 (DA01) is a group of midbrain DA (mDA) neurons intended for injection into the striatum which is currently being tested in a phase 1 clinical trial in the U.S. and Canada [7]. Before the clinical trial, DA01 was tested in 44 Parkinsonian rats which were split into a control group and a group receiving cell injections [15]. To insert the cells into the brain, a stereotactic injection, involving the precise use of 3D scanning devices, was given [15, 16]. Specifically, in the phase 1 trial, the ClearPoint Neuro Navigation System is being used. This system delivers accurate yet minimally invasive injections into the putamen (a part of the striatum that is connected to the SNpc through the nigrostriatal pathway) (Figure 2) [16, 17]. Eight months after the injection, both rat groups were tested [15]. The DA01 group had a statistically significant decline in PD symptoms, while the group that received the vehicle saw an increase in these symptoms (p < 0.0001) [15].

Figure 2. Schematics of the nigrostriatal pathway, both normal and from a PD patient.
In
(a) a normal individual, DA neurons from the SNpc travel to the putamen and caudate nucleus (both parts of the striatum) where they synapse. However, in (b) a PD patient, the neurons projecting to the putamen are significantly reduced, as demonstrated by the dashed red line. As well, projections to the caudate see a loss, albeit a smaller one, as indicated by the thinner red line. Figure from [17].

To further prepare for a human trial, BlueRock Therapeutics tested the safety of the mDA neurons and found them to be extremely safe [15]. For instance, the transplanted cells rarely travelled outside the brain and those that did could be explained by improper injection techniques [15]. Moreover, DA01 has great potential to become clinically available and accessible for several reasons. When creating four lots of DA01 cells, three of them yielded at least 2 billion cells [15]. As well, it only took 16 days to differentiate the cells, and they remained viable after thawing from being cryopreserved [15]. It is notable that drawbacks of the DA01 treatment do exist, including the mandatory use of immunosuppressive agents (i.e., prednisone and tacrolimus) [15]. Nevertheless, the use of immunosuppressants is a common practice in different fields of medicine such as organ transplantation [18].

Combining Two Approaches: Senolytics & Cell Therapies

Combining DA01 and senolytics is a valid approach for PD treatment as each has the potential to compensate for the shortcomings of the other [8]. Notably, this combinatory hypothesis is supported by J. Takahashi, an expert in the field of cell therapies for PD [19]. By first administering the senolytics, such as azithromycin along with SATB1 KO, the senescent cells that are responsible for the inflammation and ultimately the death of DA neurons undergo apoptosis [14]. This prevents the SASP from spreading to other cells and continuing the vicious cycle of DA neuron death [14]. The next step is to restore the neurons. DA01 achieves this goal by introducing new mDA neurons to restore motor functions [15]. Put together, they can prevent the deterioration and possibly reverse the course of PD (Figure 3).

Figure 3. Combining MSK-DA01 (putamen shown in dark green) and senolytics to eliminate senescent DA neurons in the SNpc (senescent neurons shown in blue) for a complete PD treatment. Created with BioRender.com

Conclusion

In summary, the most robust and complete treatment for PD patients necessitates a multi-faceted approach. Senolytics, small molecules that target and induce apoptosis in senescent cells, provide the first facet [9]. By selectively removing the senescent cells that cause the death of DA neurons, the spread of the disease can be controlled [14]. As the next step, cell therapies, such as MSK-DA01, can be used [15]. They improve the symptoms of patients who have already lost DA neurons [15]. Together, these methods can slow or stop the progression of the disease and possibly reverse its symptoms. However, this remains a new area of research and there is much work to be done before it can replace conventional treatments.

Acknowledgements

I would like to thank Cherisse Tan for her help with editing this manuscript.

Reference List

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Hi again, I’m Parmin, a 15 y/o student researcher studying stem cells 🧪 Everyday, I aspire to uncover the secrets of biology and learn something new! Make sure to follow me on Medium to hear about every new article I post, connect with me on LinkedIn, or contact me at parminsedigh@gmail.com! Also subscribe to my monthly newsletter to learn about every cool, new thing I’m working on ✍️

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Parmin Sedigh

Parmin Sedigh

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Science communicator trying to learn something new everyday | Published in Start It Up, Predict & The Writing Cooperative