Therapeutic potential of the MSC secretome

.


Introduction
Mesenchymal stem cells (MSCs) are adult stem cells that can be found and isolated from various sources in the human body.These cells were discovered by Friedenstein et al in 1970 in the bone marrow of guinea pigs [1].Some of the other locations where MSCs can be found are muscle, dermis, trabecular bone, adipose tissue, periosteum, pericyte, blood, synovial membrane, placenta, umbilical cord and other tissues [2].Due to the heterogeneous nature MSCs, International Society for Cellular Therapy (ISCT) has defined certain minimum criteria that a cell type must reach to classify as MSCs: 1) plastic-adherence by the cells, 2) ability to differentiate into osteocytes, chondrocytes & adipocytes & 3) presence of certain cell markers, CD73, CD90 and CD105, and the lack of expression of CD45, CD34, CD14 or CD11b, CD79α or CD19 and HLA-DR [3].
MSCs have great potential in regenerative medicine as they can differentiate into several cell types of mesodermal origin as well as transdifferentiate into neural cell types [4], [5].MSCs also have the capacity to modulate the immune system & migrate to sites of injury.Another major reason that makes mesenchymal stem cells advantageous for regenerative medicine is that the cells can be acquired from patients using non-invasive techniques thus, facilitating autologous transplants.Also, since MSCs do not possess HLA-DR on the cell surface, an allogenic transplant will result in a limited immune reaction thus allowing for a successful transplant [6].
However, this is not to say that MSC therapy has no disadvantages.Immune rejections of MSCs still occurs, as well as ectopic tissue formation and the possibility of tumour formation.To remedy these drawbacks there is increasing interest in the secretome of MSCs [7].The MSC secretome contains several molecular compounds such as cytokines, chemokines, immunomodulatory molecules & growth factors.Also, extracellular vesicles such as exosomes, microvesicles & apoptotic bodies tend to contain paracrine factors secreted by MSCs.Secretions and exosomes of MSCs can be used as cell-free therapeutic products for the treatment of various disorders and injuries [8].Various studies have established that the secretome of MSCs has a similar therapeutic effect in comparison to transplanted MSCs [9].

Bone Regeneration
Bone regeneration after injury is difficult in several cases such as patients of advanced age.Improper or failed healing could significantly lower the quality of life of an individual.The MSC secretome shows promising results in aiding the regenerative process [28].In 2012, a study established the therapeutic effect of Conditioned Media (CM) on bone regeneration in a rat model.They also observed a greater homing of rat MSCs in the treatment group in comparison to the control group [29].Similarly, a 2016 study observed that CM containing MSC-derived exosomes showed greater bone regeneration in rat models when compared to CM alone [30].
MSC secretome contains several growth factors, one of which is VEGF.A 2017 study established the importance of VEGF in the CM for the angiogenesis of newly formed bone tissue [31].Also, the MSC secretome has been found to promote osteoclast differentiation even in the presence of osteoclast differentiation inhibitors by regulating the expression of certain master transcription factors [32].The culture conditions for the production of CM from MSCs also plays a major role in the composition of the secretome.In 2015, a study found that CM from hypoxic MSC culture contained significantly higher concentrations of angiogenic factors in comparison to normoxic conditions [33].The safety of CM was used for the first time in 2016 when alveolar bone regeneration was carried out using CM on 8 patients.No complications were observed in any of the individuals [34].
In 2017, a scaffold functionalized with MSC-EVs have been shown to promote better bone regeneration as well as vascularization.This could be a novel technique to aid bone healing [35].However, it is important to note that the microenvironment has a major impact on the outcome of the treatment.The regenerative properties of exosomes were found to be greatly impaired in the case of the exosomes derived from diabetes mellitus type 1 affected MSCs [36].

Muscular Regeneration
A 2014 study found that treating a myectomy model with CM from MSCs resulted in less evident scar tissue and better performance in histological scoring [10].This could be a result of expression of VEGF-α, IGF-1, EGF, keratinocyte growth factor, angiopoietin-1, stromal-derived factor-1, macrophage inflammatory protein-1α and -β and erythropoietin [11].Mesenchymal stem cell therapy has also been proven to mediate inflammation thus resulting in reduced scar formation [12].CM has also been known to enhance the proliferation and migration of cells as well as modulate myoblast apoptosis [13].
A study in 2016 discovered that treating atrophied muscles with CM significantly helped in their recovery.This was a result of the activation of the PI3K/AKT pathway which caused the suppression of the atrophy-related ubiquitin E3ligases, muscle ring finger 1 and muscle atrophy F-box and the elevated expression of desmin and skeletal muscle actin [14], [15].CM can also help heal muscle injury caused as a result of vaginal birth [16].Similarly, CM has been proven to heal eccentric contraction muscle injury by increasing the expression of the PI3K/AKT pathway in a 2021 study [17].Extracellular Vesicles (EVs) of MSCs have also been established for their therapeutic potential to regenerate skeletal muscle in case of acute injury.This mechanism seems to work differently but synergistically with the soluble secretome of MSCs [18].

Neural Regeneration
Neurodegenerative diseases are a major class of diseases that affect the life expectancy and the quality of life of a significant portion of the population.The therapeutic effect of MSC secretome & exosomes for neurodegenerative diseases has been established across multiple studies [19], [20].A study carried out in 2015 proved that the secretome of MSCs resulted in neuronal differentiation both in vitro & in vivo [21].Exogenous CX3CL1 released from MSCs can switch microglia from a detrimental phenotype to a neuroprotective phenotype which can be useful in the treatment of Alzheimer's Disease [22].Endogenous IL-6 derived from MSCs can cause a significant reduction in autophagy in damaged hippocampal neurons.It does so by inhibiting the AMPK/mTOR pathway [23].Also, MSC-derived IL-6 activated the AMPK/mTOR pathway in astrocytes to inhibit their proliferation in the case of hypoxic-ischemic brain damage [24].
Injecting the MSC secretome directly into a Parkinson's Disease rat model has yielded positive results albeit with decreasing effect with passing time.The treatment resulted in an increase in dopaminergic neurons & neuronal terminals thus facilitating improvement in motor functions.Upon analysing the secretome, several neuroregulatory compounds such as cystatin C, glia-derived nexin, galectin-1, pigment epithelium-derived factor, vascular endothelial growth factor, brain-derived neurotrophic factor, interleukin-6, and glial cell line-derived neurotrophic factor were identified in the milieu [25].The MSC secretome in combination with brain-derived neurotrophic factor (BDNF) has been proven to give significantly better neuroprotection in case of intracerebral haemorrhage than BDNF alone [26].Also, the secretome can be used for oligodendrogenesis from adult neural stem cells as a result of multiple differentiation proteins present in the secretome including TIMP-1 [27].

Conclusion
MSC secretome & extracellular vesicles have been proven, using various studies, to have tremendous therapeutic potential in the case of various diseases and injuries spanning several different biological systems.With clinical trials underway, apart from establishing its efficacy and route of administration, it is imperative to conduct various safety assessments, to ensure no ill-effects occur as a result of its use.With the momentum of the current research, MSC secretome-or extracellular vesicle-based therapy might be a reality in the near future.

Disclosure of conflict of interest
No conflict of interest to be disclosed.