Sex, the Underestimated Potential Determining Factor in Brain Tissue Repair Strategy

Use of Brain-Derived Stem/Progenitor Cells and Derived Extracellular Vesicles to Repair Damaged Neural Tissues: Lessons Learned from Connective Tissue Repair Regarding Variables Limiting Progress and Approaches to Overcome Limitations

Pluripotent neural stem or progenitor cells (NSC/NPC) have been reported in the brains of adult preclinical models for decades, as have mesenchymal stem/stromal cells (MSC) been reported in a variety of tissues from adults. Based on their in vitro capabilities, these cell types have been used extensively in attempts to repair/regenerate brain and connective tissues, respectively. In addition, MSC have also been used in attempts to repair compromised brain centres. However, success in treating chronic neural degenerative conditions such as Alzheimer's disease, Parkinson's disease, and others with NSC/NPC has been limited, as have the use of MSC in the treatment of chronic osteoarthritis, a condition affecting millions of individuals. However, connective tissues are likely less complex than neural tissues regarding cell organization and regulatory integration, but some insights have been gleaned from the studies regarding connective tissue healing with MSC that may inform studies attempting to initiate repair and regeneration of neural tissues compromised acutely or chronically by trauma or disease. This review will discuss the similarities and differences in the applications of NSC/NPC and MSC, where some lessons have been learned, and potential approaches that could be used going forward to enhance progress in the application of cellular therapy to facilitate repair and regeneration of complex structures in the brain. In particular, variables that may need to be controlled to enhance success are discussed, as are different approaches such as the use of extracellular vesicles from stem/progenitor cells that could be used to stimulate endogenous cells to repair the tissues rather than consider cell replacement as the primary option. Caveats to all these efforts relate to whether cellular repair initiatives will have long-term success if the initiators for neural diseases are not controlled, and whether such cellular initiatives will have long-term success in a subset of patients if the neural diseases are heterogeneous and have multiple etiologies.

Age- and sex-dependent effects of metformin on neural precursor cells and cognitive recovery in a model of neonatal stroke

Resident neural stem and progenitor cells, collectively termed neural precursor cells (NPCs), reside in a well-defined neurogenic niche in the subventricular zone (SVZ) and contribute to ongoing postnatal neurogenesis. It is well established that the NPC niche can alter the behavior of NPCs. NPC activation is a promising therapeutic strategy for brain repair. The drug metformin has been shown to activate neural stem cells, promote differentiation, and lead to functional motor recovery in a neonatal stroke model. We demonstrate that metformin-induced NPC expansion and functional recovery is sex hormone dependent. Metformin increases the size of the NPC pool in adult females, but not males, and promotes cognitive recovery in a model of brain injury in females, but not males. Our data demonstrate that metformin has age- and sex-dependent effects on NPCs that correlate with functional recovery, which has important implications for neural repair.

Home sweet home: the neural stem cell niche throughout development and after injury

Neural stem cells and their progeny reside in two distinct neurogenic niches within the mammalian brain: the subventricular zone and the dentate gyrus. The interplay between the neural stem cells and the niche in which they reside can have significant effects on cell kinetics and neurogenesis. A comprehensive understanding of the changes to the niche that occur through postnatal development and aging, as well as following injury, is relevant for developing therapeutics and interventions to promote neural repair. We discuss changes that occur within the neural stem and progenitor cell populations, the vasculature, extracellular matrix, microglia, and secreted proteins through aging which impact cell behavior within the neurogenic niches. We examine neural precursor cell and niche responses to injury in neonatal hypoxia-ischemia, juvenile cranial irradiation, and adult stroke. This review examines the interplay between the niche and stem cell behavior through aging and following injury as a means to understand intrinsic and extrinsic factors that regulate neurogenesis in vivo.

Systemic injection of CD34(+)-enriched human cord blood cells modulates poststroke neural and glial response in a sex-dependent manner in CD1 mice

Stroke in the developing brain is an important cause of neurological morbidity. We determined the impact of human cord blood-derived CD34(+)-enriched mononuclear cells (CBSC) intraperitoneally injected 48 h after an ischemic stroke at postnatal day 12 by evaluating poststroke neurogenic niche proliferation, glial response, and recovery in CD1 mice. Percent brain atrophy was quantified from Nissl-stained sections. Density of BrdU, Iba-1, and GFAP staining were quantified in the dentate gyrus and the subventricular zone (SVZ). Immunohistochemistry for human nuclear antibody, human mitochondrial antibody, and human CD34(+) cells was done on injured and uninjured brains from CBSC- and vehicle-treated mice. Developmental neurobehavioral milestones were evaluated pre- and post-treatment. No significant differences in stroke severity were noted between CBSC and vehicle-treated injured animals. With a 1×10(5) CBSC dose, there was a significant increase in subgranular zone (SGZ) proliferation in the CBSC-versus vehicle-treated stroke-injured male mice. SVZ glial fibrillary acidic protein (GFAP) expression was increased contralaterally in injured females treated with CBSC but suppressed in injured males. Significant negative correlations between severity of the stroke-injury and spleen weights, and between spleen weights and SGZ proliferation, and a positive correlation between GFAP expression and severity of brain injury were noted in the vehicle-treated injured mice but not in the CBSC-treated mice. GFAP expression and SVZ proliferation were positively correlated. In conclusion, neurogenic niche proliferation and glial brain responses to CBSC after neonatal stroke may involve interactions with the spleen and are sex dependent.

Adult stem cell transplantation: is gender a factor in stemness?

Cell therapy now constitutes an important area of regenerative medicine. The aging of the population has mandated the discovery and development of new and innovative therapeutic modalities to combat devastating disorders such as stroke. Menstrual blood and Sertoli cells represent two sources of viable transplantable cells that are gender-specific, both of which appear to have potential as donor cells for transplantation in stroke. During the subacute phase of stroke, the use of autologous cells offers effective and practical clinical application and is suggestive of the many benefits of using the aforementioned gender-specific cells. For example, in addition to being exceptionally immunosuppressive, testis-derived Sertoli cells secrete many growth and trophic factors and have been shown to aid in the functional recovery of animals transplanted with fetal dopaminergic cells. Correspondingly, menstrual blood cells are easily obtainable and exhibit angiogenic characteristics, proliferative capability, and pluripotency. Of further interest is the ability of menstrual blood cells, following transplantation in stroke models, to migrate to the infarct site, secrete neurotrophic factors, regulate the inflammatory response, and be steered towards neural differentiation. From cell isolation to transplantation, we emphasize in this review paper the practicality and relevance of the experimental and clinical use of gender-specific stem cells, such as Sertoli cells and menstrual blood cells, in the treatment of stroke.

The battle of the sexes for stroke therapy: female- versus male-derived stem cells

Cell therapy is a major discipline of regenerative medicine that has been continually growing over the last two decades. The aging of the population necessitates discovery of therapeutic innovations to combat debilitating disorders, such as stroke. Menstrual blood and Sertoli cells are two gender-specific sources of viable transplantable cells for stroke therapy. The use of autologous cells for the subacute phase of stroke offers practical clinical application. Menstrual blood cells are readily available, display proliferative capacity, pluripotency and angiogenic features, and, following transplantation in stroke models, have the ability to migrate to the infarct site, regulate the inflammatory response, secrete neurotrophic factors, and have the possibility to differentiate into neural lineage. Similarly, the testis-derived Sertoli cells secrete many growth and trophic factors, are highly immunosuppressive, and exert neuroprotective effects in animal models of neurological disorders. We highlight the practicality of experimental and clinical application of menstrual blood cells and Sertoli cells to treat stroke, from cell isolation and cryopreservation to administration.

Reparative effects of neural stem cells in neonatal rats with hypoxic-ischemic injury are not influenced by host sex

BACKGROUND

Gender is increasingly recognized as an important influence on brain development, disease susceptibility, and response to pharmacologic/rehabilitative treatments. In regenerative medicine, it remains entirely unknown whether there is an interaction between transplanted stem cells and host gender that might bias efficacy and safety in some patients but not others.

METHODS

We examined the role of recipient gender in a neonatal rat hypoxic-ischemic injury (HII) model, treated with female human neuronal stem cells (hNSCs), labeled with superparamagnetic iron oxide particles implanted into the contralateral cerebral ventricle. We monitored HII evolution (by magnetic resonance imaging, histopathology, behavioral testing) and hNSC fate (migration, replication, viability).

RESULTS

Recipient gender after implantation did not influence the volume or location of ischemic injury (1, 30, or 90 d) or behavior (90 d). Superparamagnetic iron oxide labeling did not influence HII evolution. Implantation had its greatest benefit on mild/moderate injuries, which remained stable rather than increasing as in severe HII as is the natural history for such lesions.

CONCLUSION

Our results suggest that hNSC treatment (including using hNSCs that are prelabeled with iron to allow tracking in real time by magnetic resonance imaging) would be equally safe and effective for male and female human newborns with mild-to-moderate HII.

Pluripotent possibilities: human umbilical cord blood cell treatment after neonatal brain injury

Perinatal hypoxic-ischemic brain injury and stroke in the developing brain remain important causes of chronic neurologic morbidity. Emerging data suggest that transplantation of umbilical cord blood-derived stem cells may have therapeutic potential for neuroregeneration and improved functional outcome. The pluripotent capacity of stem cells from the human umbilical cord blood provides simultaneous targeting of multiple neuropathologic events initiated by a hypoxic-ischemic insult. Their high regenerative potential and naïve immunologic phenotype makes them a preferable choice for transplantation. A multiplicity of transplantation protocols have been studied with a variety of brain injury models; however, only a few have been conducted on immature animals. Biological recipient characteristics, such as age and sex, appear to differentially modulate responses of the animals to the transplanted cord blood stem cells. Survival, migration, and function of the transplanted cells have also been studied and reveal insights into the mechanisms of cord blood stem cell effects. Data from preclinical studies have informed current clinical safety trials of human cord blood in neonates, and further work is needed to continue progress in this field.

Stress and stem cells

The unique properties and functions of stem cells make them particularly susceptible to stresses and also lead to their regulation by stress. Stem cell division must respond to the demand to replenish cells during normal tissue turnover as well as in response to damage. Oxidative stress, mechanical stress, growth factors, and cytokines signal stem cell division and differentiation. Many of the conserved pathways regulating stem cell self-renewal and differentiation are also stress-response pathways. The long life span and division potential of stem cells create a propensity for transformation (cancer) and specific stress responses such as apoptosis and senescence act as antitumor mechanisms. Quiescence regulated by CDK inhibitors and a hypoxic niche regulated by FOXO transcription factor function to reduce stress for several types of stem cells to facilitate long-term maintenance. Aging is a particularly relevant stress for stem cells, because repeated demands on stem cell function over the life span can have cumulative cell-autonomous effects including epigenetic dysregulation, mutations, and telomere erosion. In addition, aging of the organism impairs function of the stem cell niche and systemic signals, including chronic inflammation and oxidative stress.