Noninvasive pulsed focused ultrasound allows spatiotemporal control of targeted homing for multiple stem cell types in murine skeletal muscle and the magnitude of cell homing can be increased through repeated applications

Ultrasound Pressure-Dependent Cytokine and Immune Cell Response Lost in Aged Muscle

OBJECTIVE

Therapeutic ultrasound remains a highly discussed topic in physical therapy due to uncertainty between treatment regimens and biological benefits. Its impact on aged populations, who are vulnerable to insufficient healing after muscle injury because of sarcopenia, is understudied. Despite the coupling between muscle inflammation and regeneration, research on the immune response after therapeutic ultrasound is limited. The objective of this study was to evaluate structure, inflammatory cytokine signaling and immune cell infiltration after therapeutic ultrasound in young and aging murine muscle.

METHODS

Young (6-week-old) and Adult (52-week-old) male and female mouse non-injured gastrocnemii were treated with either low-intensity pulsed ultrasound at 2 W/cm2 (∼0.243 MPa) or high-intensity pulsed focused ultrasound at 554 W/cm2 (∼5.96 MPa). Cytokine expression was evaluated at 1, 8 and 24 hours, cell infiltration was measured via flow cytometry at 1 and 24 hours and immunofluorescence assessed muscle fiber area, fibrosis and satellite cells at 24 hours after sonication.

RESULTS

Low-intensity pulsed ultrasound induced an early, transient inflammatory response where interleukin (IL)-15 and macrophages (M2 > M1) were increased 1 hour post-sonication. High-intensity pulsed focused ultrasound caused a late, extended immune response where monocyte chemoattractant protein 1 (MCP-1), neutrophils, monocytes and macrophages (M1 > M2) were increased 24 hours post-sonication. Notably, these changes manifested solely in Young gastrocnemius. The Adult gastrocnemius exhibited decreased cytokine expression (IL-1α, IL-6, IL-15, macrophage colony-stimulating factor [M-CSF]) and no alteration in immune cell recruitment post-sonication. There was no damage to muscle structure.

CONCLUSION

Therapeutic ultrasound induced a pressure-dependent inflammatory response that can augment or mitigate intrinsic muscle cytokine signaling and cell recruitment in adolescent or aged muscle, respectively.

Endogenous Neural Stem Cell Activation after Low-Intensity Focused Ultrasound-Induced Blood–Brain Barrier Modulation

Endogenous neural stem cells (eNSCs) in the adult brain, which have the potential to self-renew and differentiate into functional, tissue-appropriate cell types, have raised new expectations for neurological disease therapy. Low-intensity focused ultrasound (LIFUS)-induced blood–brain barrier modulation has been reported to promote neurogenesis. Although these studies have reported improved behavioral performance and enhanced expression of brain biomarkers after LIFUS, indicating increased neurogenesis, the precise mechanism remains unclear. In this study, we evaluated eNSC activation as a mechanism for neurogenesis after LIFUS-induced blood–brain barrier modulation. We evaluated the specific eNSC markers, Sox-2 and nestin, to confirm the activation of eNSCs. We also performed 3′-deoxy-3′[18F] fluoro-L-thymidine positron emission tomography ([18F] FLT-PET) to evaluate the activation of eNSCs. The expression of Sox-2 and nestin was significantly upregulated 1 week after LIFUS. After 1 week, the upregulated expression decreased sequentially; after 4 weeks, the upregulated expression returned to that of the control group. [18F] FLT-PET images also showed higher stem cell activity after 1 week. The results of this study indicated that LIFUS could activate eNSCs and induce adult neurogenesis. These results show that LIFUS may be useful as an effective treatment for patients with neurological damage or neurological disorders in clinical settings.

MRI Guided Focused Ultrasound-Mediated Delivery of Therapeutic Cells to the Brain: A Review of the State-of-the-Art Methodology and Future Applications

Stem cell and immune cell therapies are being investigated as a potential therapeutic modality for CNS disorders, performing functions such as targeted drug or growth factor delivery, tumor cell destruction, or inflammatory regulation. Despite promising preclinical studies, delivery routes for maximizing cell engraftment, such as stereotactic or intrathecal injection, are invasive and carry risks of hemorrhage and infection. Recent developments in MRI-guided focused ultrasound (MRgFUS) technology have significant implications for treating focal CNS pathologies including neurodegenerative, vascular and malignant processes. MRgFUS is currently employed in the clinic for treating essential tremor and Parkinson's Disease by producing precise, incisionless, transcranial lesions. This non-invasive technology can also be modified for non-destructive applications to safely and transiently open the blood-brain barrier (BBB) to deliver a range of therapeutics, including cells. This review is meant to familiarize the neuro-interventionalist with this topic and discusses the use of MRgFUS for facilitating cellular delivery to the brain. A detailed and comprehensive description is provided on routes of cell administration, imaging strategies for targeting and tracking cellular delivery and engraftment, biophysical mechanisms of BBB enhanced permeability, supportive proof-of-concept studies, and potential for clinical translation.

Pulsed-Focused Ultrasound Slows B16 Melanoma and 4T1 Breast Tumor Growth through Differential Tumor Microenvironmental Changes

Focused ultrasound (FUS) has shown promise as a non-invasive treatment modality for solid malignancies. FUS targeting to tumors has been shown to initiate pro-inflammatory immune responses within the tumor microenvironment. Pulsed FUS (pFUS) can alter the expression of cytokines, chemokines, trophic factors, cell adhesion molecules, and immune cell phenotypes within tissues. Here, we investigated the molecular and immune cell effects of pFUS on murine B16 melanoma and 4T1 breast cancer flank tumors. Temporal changes following sonication were evaluated by proteomics, RNA-seq, flow-cytometry, and histological analyses. Proteomic profiling revealed molecular changes occurring over 24 h post-pFUS that were consistent with a shift toward inflamed tumor microenvironment. Over 5 days post-pFUS, tumor growth rates were significantly decreased while flow cytometric analysis revealed differences in the temporal migration of immune cells. Transcriptomic analyses following sonication identified differences in gene expression patterns between the two tumor types. Histological analyses further demonstrated reduction of proliferation marker, Ki-67 in 4T1, but not in B16 tumors, and activated cleaved-caspase 3 for apoptosis remained elevated up to 3 days post-pFUS in both tumor types. This study revealed diverse biological mechanisms following pFUS treatment and supports its use as a possible adjuvant to ablative tumor treatment to elicit enhanced anti-tumor responses and slow tumor growth.

Cytosolic Ca2+ transients during pulsed focused ultrasound generate reactive oxygen species and cause DNA damage in tumor cells

Mechanical forces from non-ablative pulsed focused ultrasound (pFUS) generate pro-inflammatory tumor microenvironments (TME), marked by increased cytokines, chemokines, and trophic factors, as well as immune cell infiltration and reduced tumor growth. pFUS also causes DNA damage within tumors, which is a potent activator of immunity and could contribute to changes in the TME. This study investigated mechanisms behind the mechanotransductive effects of pFUS causing DNA damage in several tumor cell types. Methods: 4T1 (murine breast tumor), B16 (murine melanoma), C6 (rat glioma), or MDA-MB-231 (human breast tumor) cells were sonicated in vitro (1.1MHz; 6MPa PNP; 10ms pulses; 10% duty cycle; 300 pulses). DNA damage was detected by TUNEL, apoptosis was measured by immunocytochemistry for cleaved caspase-3. Calcium, superoxide, and H₂O₂ were detected by fluorescent indicators and modulated by BAPTA-AM, mtTEMPOL, or Trolox, respectively. Results: pFUS increased TUNEL reactivity (range = 1.6-2.7-fold) in all cell types except C6 and did not induce apoptosis in any cell line. All lines displayed cytosolic Ca²⁺ transients during sonication. pFUS increased superoxide (range = 1.6-2.0-fold) and H₂O₂ (range = 2.3-2.8-fold) in all cell types except C6. BAPTA-AM blocked increased TUNEL reactivity, superoxide and H₂O₂ formation, while Trolox also blocked increased TUNEL reactivity increased after pFUS. mtTEMPOL allowed H₂O₂ formation and did not block increased TUNEL reactivity after pFUS. Unsonicated C6 cells had higher baseline concentrations of cytosolic Ca²⁺, superoxide, and H₂O₂, which were not associated with greater baseline TUNEL reactivity than the other cell lines. Conclusions: Mechanotransduction of pFUS directly induces DNA damage in tumor cells by cytosolic Ca²⁺ transients causing formation of superoxide and subsequently, H₂O₂. These results further suggest potential clinical utility for pFUS. However, the lack of pFUS-induced DNA damage in C6 cells demonstrates a range of potential tumor responses that may arise from physiological differences such as Ca²⁺ or redox homeostasis.

MR-guided pulsed focused ultrasound improves mesenchymal stromal cell homing to the myocardium

Image-guided pulsed focused ultrasound (pFUS) is a non-invasive technique that can increase tropism of intravenously (IV)-infused mesenchymal stromal cells (MSC) to sonicated tissues. MSC have shown promise for cardiac regenerative medicine strategies but can be hampered by inefficient homing to the myocardium. This study sonicated the left ventricles (LV) in rats with magnetic resonance imaging (MRI)-guided pFUS and examined both proteomic responses and subsequent MSC tropism to treated myocardium. T2-weighted MRI was used for pFUS targeting of the entire LV. pFUS increased numerous pro- and anti-inflammatory cytokines, chemokines, and trophic factors and cell adhesion molecules in the myocardial microenvironment for up to 48 hours post-sonication. Cardiac troponin I and N-terminal pro-B-type natriuretic peptide were elevated in the serum and myocardium. Immunohistochemistry revealed transient hypoxia and immune cell infiltration in pFUS-targeted regions. Myocardial tropism of IV-infused human MSC following pFUS increased twofold-threefold compared with controls. Proteomic and histological changes in myocardium following pFUS suggested a reversible inflammatory and hypoxic response leading to increased tropism of MSC. MR-guided pFUS could represent a non-invasive modality to improve MSC therapies for cardiac regenerative medicine approaches.

Pulsed focused ultrasound enhances the therapeutic effect of mesenchymal stromal cell-derived extracellular vesicles in acute kidney injury

BACKGROUND

Acute kidney injury (AKI) is characterized by rapid failure of renal function and has no curative therapies. Mesenchymal stromal cell (MSC)-derived extracellular vesicles (EVs) are known to carry therapeutic factors, which have shown promise in regenerative medicine applications, including AKI. However, there remains an unmet need to optimize their therapeutic effect. One potential avenue of optimization lies in pulsed focused ultrasound (pFUS), where tissues-of-interest are treated with sound waves. pFUS has been shown to enhance MSC therapy via increased cell homing, but its effects on cell-free EV therapy remain largely unexplored.

METHODS

We combine pFUS pretreatment of the kidney with MSC-derived EV therapy in a mouse model of cisplatin-induced AKI.

RESULTS

EVs significantly improved kidney function, reduced injury markers, mediated increased proliferation, and reduced inflammation and apoptosis. While pFUS did not enhance EV homing to the kidney, the combined treatment resulted in a superior therapeutic effect compared to either treatment alone. We identified several molecular mechanisms underlying this synergistic therapeutic effect, including upregulation of proliferative signaling (MAPK/ERK, PI3K/Akt) and regenerative pathways (eNOS, SIRT3), as well as suppression of inflammation.

CONCLUSION

Taken together, pFUS may be a strategy for enhancing the therapeutic efficacy of MSC-derived EV treatment for the treatment of AKI.

Acoustic Radiation or Cavitation Forces From Therapeutic Ultrasound Generate Prostaglandins and Increase Mesenchymal Stromal Cell Homing to Murine Muscle

Non-ablative ultrasound (US)-based techniques to improve targeted tropism of systemically infused cell therapies, particularly mesenchymal stromal cell (MSC), have gained attention in recent years. Mechanotransduction following targeted US sonications have been shown to modulate tissue microenvironments by upregulating cytokines, chemokines, and trophic factors in addition to vascular cell adhesion molecules (CAM) that are necessary to promote tropism of MSC. While numerous US treatment parameters have demonstrated increased MSC homing, it remains unclear how the different mechanical US forces [i.e., acoustic radiation forces (ARF) or cavitation forces] influence tissue microenvironments. This study sonicated murine muscle tissue with pulsed focused ultrasound (pFUS) at 0.5 or 1.15 MHz each over a range of US intensities. Following sonication, tissue was assayed for the prostaglandins (PG) PGH₂ and PGE₂ as indicators of microenvironmental changes that would support MSC tropism. PGH₂ and PGE₂ levels were correlated to physical pFUS parameters and acoustic emissions measured by hydrophone. While ARF (pFUS with absence of cavitation signatures) was sufficient to increase PGH₂ and PGE₂, non-linear curve fitting revealed a frequency-independent relationship between prostaglandin production and mechanical index (MI), which accounts for increased cavitation probabilities of lower frequencies. The prostaglandin data suggested molecular changes in muscle would be particularly sensitive to cavitation. Therefore, low-intensity pulsed ultrasound (LIPUS) at 1 MHz was administered with low ARF (MI = 0.2) in combination with intravenous (IV) infusions of microbubble (MB) contrast agents. This combination upregulated prostaglandins and CAM without ultrasound-mediated microbubble destruction and ultimately promoted tropism of IV-infused MSC. This study revealed that accentuating non-destructive MB cavitation by US using parameters similar to diagnostic US contrast imaging increased MSC homing. Such approaches are particularly attractive to overcome clinical translation barriers of many still-experimental US parameters used in previous stem cell tropism studies.

Reversing Acute Kidney Injury Using Pulsed Focused Ultrasound and MSC Therapy: A Role for HSP-Mediated PI3K/AKT Signaling

Acute kidney injury (AKI) is characterized by a sudden failure of renal function, but despite increasing worldwide prevalence, current treatments are largely supportive, with no curative therapies. Mesenchymal stromal cell (MSC) therapy has been shown to have a promising regenerative effect in AKI but is limited by the ability of cells to home to damaged tissue. Pulsed focused ultrasound (pFUS), wherein target tissues are sonicated by short bursts of sound waves, has been reported to enhance MSC homing by upregulating local homing signals. However, the exact mechanism by which pFUS enhances MSC therapy remains insufficiently explored. In this study, we studied the effect of bone marrow-derived MSCs (BM-MSCs), in conjunction with pFUS, in a mouse model of cisplatin-induced AKI. Here, BM-MSCs improved kidney function, reduced histological markers of kidney injury, decreased inflammation and apoptosis, and promoted cellular proliferation. Surprisingly, whereas pFUS did not upregulate local cytokine expression or improve BM-MSC homing, it did potentiate the effect of MSC treatment in AKI. Further analysis linked this effect to the upregulation of heat shock protein (HSP)20/HSP40 and subsequent phosphatidylinositol 3-kinase (PI3K)/Akt signaling. In summary, our results suggest that pFUS and BM-MSCs have independent as well as synergistic therapeutic effects in the context of AKI.

The role of ultrasound in enhancing mesenchymal stromal cell-based therapies

Mesenchymal stromal cells (MSCs) have been a popular platform for cell‐based therapy in regenerative medicine due to their propensity to home to damaged tissue and act as a repository of regenerative molecules that can promote tissue repair and exert immunomodulatory effects. Accordingly, a great deal of research has gone into optimizing MSC homing and increasing their secretion of therapeutic molecules. A variety of methods have been used to these ends, but one emerging technique gaining significant interest is the use of ultrasound. Sound waves exert mechanical pressure on cells, activating mechano‐transduction pathways and altering gene expression. Ultrasound has been applied both to cultured MSCs to modulate self‐renewal and differentiation, and to tissues‐of‐interest to make them a more attractive target for MSC homing. Here, we review the various applications of ultrasound to MSC‐based therapies, including low‐intensity pulsed ultrasound, pulsed focused ultrasound, and extracorporeal shockwave therapy, as well as the use of adjunctive therapies such as microbubbles. At a molecular level, it seems that ultrasound transiently generates a local gradient of cytokines, growth factors, and adhesion molecules that facilitate MSC homing. However, the molecular mechanisms underlying these methods are far from fully elucidated and may differ depending on the ultrasound parameters. We thus put forth minimal criteria for ultrasound parameter reporting, in order to ensure reproducibility of studies in the field. A deeper understanding of these mechanisms will enhance our ability to optimize this promising therapy to assist MSC‐based approaches in regenerative medicine.

Effect of Pulsed Focused Ultrasound on the Native Pancreas

Pulsed focused ultrasound (pFUS) utilizes short cycles of sound waves to mechanically shake cells within tissues which, in turn, causes transient local increases in cytokines, growth factors and cell adhesion molecules. Although the effect of pFUS has been investigated in several different organs including the kidney, muscle and heart, its effect on the pancreas has not been investigated. In the present work, we applied pFUS to the rodent pancreas with the following parameters: 1.1-MHz frequency, 5-Hz pulse repetition frequency, 5% duty cycle, 10-ms pulse length, 160-s duration. Low-intensity pFUS had a spatial average temporal average intensity of 11.5 W/cm² and a negative peak pressure of 3 MPa; high-intensity pFUS had a spatial average temporal average intensity of 18.5 W/cm² and negative peak pressure of 4 MPa. Here we found that pFUS changed the expression of several cytokines while having no effect on the underlying tissue histology or health of pancreatic cells (as reflected by no significant change in plasma levels of amylase and lipase). Furthermore, we found that this effect on cytokine expression in the pancreas was acoustic intensity dependent; while pFUS at low intensities turned off the expression of several cytokines, at high intensities it had the opposite effect and turned on the expression of these cytokines. The ability to non-invasively manipulate the microenvironment of the pancreas using sound waves could have profound implications for priming and modulating this organ for the application of cellular therapies in the context of both regenerative medicine (i.e., diabetes and pancreatitis) and oncology (i.e., pancreatic cancer).

Non-invasively enhanced intracranial transplantation of mesenchymal stem cells using focused ultrasound mediated by overexpression of cell-adhesion molecules

Although there have been reports of promising results regarding the transplantation of mesenchymal stem cells (MSCs) for neurodegenerative diseases through the use of neuronal differentiation or control of the microenvironment, traditional surgical transplantation methods like parenchymal or intravenous injection have limitations such as secondary injuries in the brain, infection, and low survival rate of stem cells in the target site. Focused ultrasound (FUS) treatment is an emerging modality for the treatment of brain diseases, including neurodegenerative disorders. The various biological effects of FUS treatment have been investigated; therefore, the goal is now to improve the delivery efficiency and function of MSCs by capitalizing on the advantages of FUS. In this study, we demonstrated that FUS increases MSC transplantation into brain tissue by >2-fold, and that this finding might be related to the activation of intercellular adhesion molecule-1 in endothelial and subendothelial cells and vascular adhesion molecule-1 in endothelial cells.

Focused ultrasound activates voltage-gated calcium channels through depolarizing TRPC1 sodium currents in kidney and skeletal muscle

Pulsed focused ultrasound (pFUS) technology is being developed for clinical neuro/immune modulation and regenerative medicine. Biological signal transduction of pFUS forces can require mechanosensitive or voltage-gated plasma membrane ion channels. Previous studies suggested pFUS is capable of activating either channel type, but their mechanistic relationship remains ambiguous. We demonstrated pFUS bioeffects increased mesenchymal stem cell tropism (MSC) by altering molecular microenvironments through cyclooxygenase-2 (COX2)-dependent pathways. This study explored specific relationships between mechanosensitive and voltage-gated Ca²⁺ channels (VGCC) to initiate pFUS bioeffects that increase stem cell tropism.

Methods: Murine kidneys and hamstring were given pFUS (1.15 or 1.125 MHz; 4MPa peak rarefactional pressure) under ultrasound or magnetic resonance imaging guidance. Cavitation and tissue displacement were measure by hydrophone and ultrasound radiofrequency data, respectively. Elastic modeling was performed from displacement measurements. COX2 expression and MSC tropism were evaluated in the presence of pharmacological ion channel inhibitors or in transient-receptor-potential-channel-1 (TRPC1)-deficient mice. Immunohistochemistry and co-immunoprecipitation examined physical channel relationships. Fluorescent ionophore imaging of cultured C2C12 muscle cells or TCMK1 kidney cells probed physiological interactions.

Results: pFUS induced tissue deformations resulting in kPa-scale forces suggesting mechanical activation of pFUS-induced bioeffects. Inhibiting VGCC or TRPC1 in vivo blocked pFUS-induced COX2 upregulation and MSC tropism to kidneys and muscle. A TRPC1/VGCC complex was observed in plasma membranes. VGCC or TRPC1 suppression blocked pFUS-induced Ca²⁺ transients in TCMK1 and C2C12 cells. Additionally, Ca²⁺ transients were blocked by reducing transmembrane Na⁺ potentials and observed Na⁺ transients were diminished by genetic TRPC1 suppression.

Conclusion: This study suggests that pFUS acoustic radiation forces mechanically activate a Na⁺-containing TRPC1 current upstream of VGCC rather than directly opening VGCC. The electrogenic function of TRPC1 provides potential mechanistic insight into other pFUS techniques for physiological modulation and optimization strategies for clinical implementation.

Mesenchymal Stromal Cell Homing: Mechanisms and Strategies for Improvement

Mesenchymal stromal cells (MSCs) have been widely investigated for their therapeutic potential in regenerative medicine, owing to their ability to home damaged tissue and serve as a reservoir of growth factors and regenerative molecules. As such, clinical applications of MSCs are reliant on these cells successfully migrating to the desired tissue following their administration. Unfortunately, MSC homing is inefficient, with only a small percentage of cells reaching the target tissue following systemic administration. This attrition represents a major bottleneck in realizing the full therapeutic potential of MSC-based therapies. Accordingly, a variety of strategies have been employed in the hope of improving this process. Here, we review the molecular mechanisms underlying MSC homing, based on a multistep model involving (1) initial tethering by selectins, (2) activation by cytokines, (3) arrest by integrins, (4) diapedesis or transmigration using matrix remodelers, and (5) extravascular migration toward chemokine gradients. We then review the various strategies that have been investigated for improving MSC homing, including genetic modification, cell surface engineering, in vitro priming of MSCs, and in particular, ultrasound techniques, which have recently gained significant interest. Contextualizing these strategies within the multistep homing model emphasizes that our ability to optimize this process hinges on our understanding of its molecular mechanisms. Moving forward, it is only with a combined effort of basic biology and translational work that the potential of MSC-based therapies can be realized.

Ultrasound-targeted microbubble enhances migration and therapeutic efficacy of marrow mesenchymal stem cell on rat middle cerebral artery occlusion stroke model

To investigate the role of ultrasound-targeted microbubbles in the homing effect of bone marrow-derived mesenchymal stem cells (BMSCs) and in the therapeutic efficacy of BMSCs on the ischemic stroke. A middle cerebral artery occlusion (MCAO) model was induced by plug wire preparation. Seventy-two hours after MCAO, the treatment of BMSCs with ultrasound-targeted microbubble was assessed via modified neurological severity score (mNSS), infarct volumes, and cerebral edema. In addition, immunofluorescence was performed to analyze the homing effect of BMSCs with ultrasound-targeted microbubble. We find that BMSCs with ultrasound-targeted microbubble (BMMSCs with ultrasound-targeted microbubble [USMM] group) could significantly ameliorate mNSS, infarct volumes, and cerebral edema of MCAO compared with phosphate buffer saline group, BMSCs alone group (BMSC group), and BMSCs with Ultrasound group (Ultrasound group). Immunofluorescence analysis demonstrated that ultrasound-targeted microbubbles promoted the accumulation of BMSCs in rat MCAO brains. Our findings demonstrated that ultrasound-targeted microbubble could be an effective approach for the accumulation of BMSCs on ischemic stroke, and further improved the therapeutic efficacy of BMSCs on MCAO.

Stem Cell Homing: a Potential Therapeutic Strategy Unproven for Treatment of Myocardial Injury

Despite advances in the prevention and therapeutic modalities of ischemic heart disease, morbidity and mortality post-infarction heart failure remain big challenges in modern society. Stem cell therapy is emerging as a promising therapeutic strategy. Stem cell homing, the ability of stem cells to find their destination, is receiving more attention. Identification of specific cues and understanding the signaling pathways that direct stem cells to targeted destination will improve stem cell homing efficiency. This review discusses the cellular and molecular mechanism of stem cell homing at length in the light of literature and analyzes the problem and considerations of this approach as a treatment strategy for the treatment of ischemic heart disease clinically.

Mesenchymal stromal cell potency to treat acute kidney injury increased by ultrasound-activated interferon-γ/interleukin-10 axis

Mesenchymal stromal cell (MSC) therapies combined with renal pulsed focused ultrasound (pFUS) pretreatment increase MSC homing and improve cisplatin-induced acute kidney injury (AKI) better than MSC alone. However, mechanisms underlying improved outcomes remain unknown. We hypothesize pFUS up-regulates renal interferon-γ (IFNγ) and stimulates MSC to produce interleukin-10 (IL-10) after migrating to kidneys. To demonstrate initially, MSC cultured with IFNγ up-regulated IL-10. More MSC-derived IL-10 was detected in kidneys when IFNγ-stimulated MSC were infused and they improved AKI better than unstimulated MSC. Next, IFNγ-knockout mice with AKI received pFUS+MSC, but MSC-derived IL-10 expression and AKI were similar to using MSC alone. AKI in wild-type mice receiving pFUS and IL-10-deficient MSC was also unimproved compared to administering IL-10-deficient MSC alone. Indoleamine 2,3-dioxygenase (IDO), an anti-inflammatory enzyme up-regulated in MSC by IFNγ, was up-regulated during AKI, but was not further elevated in MSC from pFUS-treated kidneys, suggesting that IDO is not involved in improved AKI healing by pFUS+MSC. These data suggest IFNγ is up-regulated by pFUS and after i.v.-infused MSC home to pFUS-treated kidneys, IFNγ stimulates additional IL-10 production by MSC to improve AKI. Analogous mechanisms of ultrasound-treated tissue microenvironments stimulating therapeutic MSC may exist in other pathologies where adjuvant ultrasound techniques are successful.

Orthopaedic regenerative tissue engineering en route to the holy grail: disequilibrium between the demand and the supply in the operating room

Orthopaedic disorders are very frequent, globally found and often partially unresolved despite the substantial advances in science and medicine. Their surgical intervention is multifarious and the most favourable treatment is chosen by the orthopaedic surgeon on a case-by-case basis depending on a number of factors related with the patient and the lesion. Numerous regenerative tissue engineering strategies have been developed and studied extensively in laboratory through in vitro experiments and preclinical in vivo trials with various established animal models, while a small proportion of them reached the operating room. However, based on the available literature, the current strategies have not yet achieved to fully solve the clinical problems. Thus, the gold standards, if existing, remain unchanged in the clinics, notwithstanding the known limitations and drawbacks. Herein, the involvement of regenerative tissue engineering in the clinical orthopaedics is reviewed. The current challenges are indicated and discussed in order to describe the current disequilibrium between the needs and solutions made available in the operating room. Regenerative tissue engineering is a very dynamic field that has a high growth rate and a great openness and ability to incorporate new technologies with passion to edge towards the Holy Grail that is functional tissue regeneration. Thus, the future of clinical solutions making use of regenerative tissue engineering principles for the management of orthopaedic disorders is firmly supported by the clinical need.

Ultrasound Irradiation Combined with Hepatocyte Growth Factor Accelerate the Hepatic Differentiation of Human Bone Marrow Mesenchymal Stem Cells

This study investigated the impact of ultrasound (US) irradiation on the hepatic differentiation of human bone marrow mesenchymal stem cells (hBMSCs) induced by hepatocyte growth factor (HGF) and the possible mechanisms. We treated hBMSCs, using HGF with and without US irradiation. Cell viability and stem cell surface markers were analyzed. Hepatocyte-like cell markers and functional markers including α-fetoprotein (αFP/AFP), cytokeratin 18 (CK18), albumin (ALB) and glycogen content were analyzed at the time point of day 1, 3 and 5 after treatment. The involvement of Wnt/β-catenin signaling pathway was evaluated as well. The results showed that the US treatment at 1.0 W/cm² or 1.5 W/cm² for 30 s or 60 s conditions yielded favorable cell viability and engendered stem cell differentiation. At day 5, the expressions of AFP, CK18, ALB and the glycogen content were significantly elevated in the US-treated group at both messenger ribonucleic acid and protein levels (all p <0.05), in comparison with HGF and control groups. Among all the US treated groups, the expression levels of specific hepatic markers in the (1.5 W/cm² for 60 s) group were the highest. Furthermore, Wnt1, β-Catenin, c-Myc and Cyclin D1 were significantly increased after US irradiation (all p <0.05), and the enhancements of c-Myc and Cyclin D1 could be obviously impaired by the inhibitor ICG-001 (p <0.05, p <0.05), in accordance with decreased ALB and CK18 expression and glycogen content (all p <0.05). In conclusion, US irradiation was able to promote the hBMSCs' differentiation mediated by HGF in vitro safely, easily and controllably. The activation of Wnt/β-catenin signaling pathway was involved in this process. US irradiation could serve as a potentially beneficial tool for the research and application of stem cell differentiation.

Mesenchymal stem cells: potential application for the treatment of hepatic cirrhosis

Nowadays, orthotopic liver transplantation is considered the most efficient approach to the end stage of chronic hepatic cirrhosis. Because of the limitations of orthotopic liver transplantation, stem cells are an attractive therapeutic option. Mesenchymal stem cells (MSCs) especially show promise as an alternative treatment for hepatic cirrhosis in animal models and during clinical trials. Nevertheless, the homing of transplanted MSCs to the liver occurs in limited numbers. Therefore, we review the strategies for enhancing the homing of MSCs, mainly via the delivery routes, optimizing cell culture conditions, stimulating the target sites, and genetic modification.

Molecular and histological effects of MR-guided pulsed focused ultrasound to the rat heart

BACKGROUND

Image-guided high intensity focused ultrasound has been used as an extracorporeal cardiac pacing tool and to enhance homing of stem cells to targeted tissues. However, molecular changes in the myocardium after sonication have not been widely investigated. Magnetic-resonance (MR)-guided pulsed focused ultrasound (pFUS) was targeted to the rat myocardium over a range of pressures and the microenvironmental and histological effects were evaluated over time.

METHODS

Eight-to-ten-week-old Sprague-Dawley rats received T2-weighted MR images to target pFUS to the left ventricular and septum without cardiac or respiratory gating. Rats were sonicated through the thoracic wall at peak negative pressures (PNP) from 1 to 8 MPa at a center frequency of 1 MHz, 10 ms pulse duration and 1 Hz pulse repetition frequency for 100 pulses per focal target. Following pFUS, myocardium was harvested over 24 h and subjected to imaging, proteomic, and histological measurements.

RESULTS

pFUS to the myocardium increased expression of cytokines, chemokines, and trophic factors characterized by an initial increase in tumor necrosis factor (TNF)-α followed by increases in pro- and anti-inflammatory factors that returned to baseline by 24 h. Immediately after pFUS, there was a transient (< 1 h) increase in N-terminal pro b-type natriuretic peptide (NT-proBNP) without elevation of other cardiac injury markers. A relationship between PNP and expression of TNF-α and NT-proBNP was observed with significant changes (p < 0.05 ANOVA) ≥ 4 MPa compared to untreated controls. Contrast-enhanced ex vivo T1-weighted MRI revealed vascular leakage in sonicated myocardium that was accompanied by the presence of albumin upon immunohistochemistry. Histology revealed infiltration of neutrophils and macrophages without morphological myofibril changes in sonicated tissue accompanied by pulmonary hemorrhage at PNP > 4 MPa.

CONCLUSIONS

MR-guided pFUS to myocardium induced transient proteomic and histological changes. The temporal proteomic changes in the myocardium indicate a short-lived sterile inflammatory response consistent with ischemia or contusion. Further study of myocardial function and strain is needed to determine if pFUS could be developed as an experimental model of cardiac injury and chest trauma.

Focused ultrasound for immuno-adjuvant treatment of pancreatic cancer: An emerging clinical paradigm in the era of personalized oncotherapy

Current clinical treatment regimens, including many emergent immune strategies (e.g., checkpoint inhibitors) have done little to affect the devastating course of pancreatic ductal adenocarcinoma (PDA). Clinical trials for PDA often employ multi-modal treatment, and have started to incorporate stromal-targeted therapies, which have shown promising results in early reports. Focused ultrasound (FUS) is one such therapy that is uniquely equipped to address local and systemic limitations of conventional cancer therapies as well as emergent immune therapies for PDA. FUS methods can non-invasively generate mechanical and/or thermal effects that capitalize on the unique oncogenomic/proteomic signature of a tumor. Potential benefits of FUS therapy for PDA include: (1) emulsification of targeted tumor into undenatured antigens in situ, increasing dendritic cell maturation, and increasing intra-tumoral CD8+/ T regulatory cell ratio and CD8+ T cell activity; (2) reduction in intra-tumoral hypoxic stress; (3) modulation of tumor cell membrane protein localization to enhance immunogenicity; (4) modulation of the local cytokine milieu toward a Th1-type inflammatory profile; (5) up-regulation of local chemoattractants; (6) remodeling the tumor stroma; (7) localized delivery of exogenously packaged immune-stimulating antigens, genes and therapeutic drugs. While not all of these results have been studied in experimental PDA models to date, the principles garnered from other solid tumor and disease models have direct relevance to the design of optimal FUS protocols for PDA. In this review, we address the pertinent limitations in current and emergent immune therapies that can be improved with FUS therapy for PDA.

Anti-inflammatory drugs suppress ultrasound-mediated mesenchymal stromal cell tropism to kidneys

Mesenchymal stromal cells (MSC) are potential renal therapeutics. Clinically, results are mixed partly because MSC tropism to kidneys is minimal following infusion. Ultrasound augmentation of the renal microenvironment is becoming increasingly-important in renal MSC therapies. We demonstrated pulsed-focused-ultrasound (pFUS) increases enhanced homing permeability and retention of MSC in mouse kidneys. Here, we characterized the temporal proteomic response to pFUS in mouse kidneys and its association with MSC tropism. pFUS induced molecular cascades of initial increases in tumor necrosis factor-α (TNFα) and interleukin (IL)-1α, that activated nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) and cyclooxygenase-2 (COX2) pathways without cell death. This was followed by a 24–48 hour-long response of increased cell adhesion molecules (CAM), trophic and anti-inflammatory factors. Pretreating animals with anti-inflammatory drugs etanercept (TNFα inhibitor), anakinra (IL-1 receptor antagonist), prednisone (NFκB translocation inhibitor), or ibuprofen (COX inhibitor) suppressed molecular changes and inhibited renal MSC tropism. We further examined the role of COX2 using a COX2-knock-out mouse where pFUS was unable to increase MSC tropism. These results demonstrate that renal micro-environmental changes induce MSC tropism and could influence the therapeutic efficacy of MSC. Optimizing the microenvironment and understanding drug effects will enable improvements in MSC therapies for renal disease.

Improving the therapeutic efficacy of mesenchymal stromal cells to restore perfusion in critical limb ischemia through pulsed focused ultrasound

Mesenchymal stem cells (MSC) are promising therapeutics for critical limb ischemia (CLI). Mechanotransduction from pulsed focused ultrasound (pFUS) upregulates local chemoattractants to enhance homing of intravenously (IV)-infused MSC and improve outcomes. This study investigated whether pFUS exposures to skeletal muscle would improve local homing of iv-infused MSCs and their therapeutic efficacy compared to iv-infused MSCs alone. CLI was induced by external iliac arterial cauterization in 10–12-month-old mice. pFUS/MSC treatments were delayed 14 days, when surgical inflammation subsided. Mice were treated with iv-saline, pFUS alone, IV-MSC, or pFUS and IV-MSC. Proteomic analyses revealed pFUS upregulated local chemoattractants and increased MSC tropism to CLI muscle. By 7 weeks post-treatment, pFUS + MSC significantly increased perfusion and CD31 expression, while reducing fibrosis compared to saline. pFUS or MSC alone reduced fibrosis, but did not increase perfusion or CD31. Furthermore, MSCs homing to pFUS-treated CLI muscle expressed more vascular endothelial growth factor (VEGF) and interleukin-10 (IL-10) than MSCs homing to non-pFUS-treated muscle. pFUS + MSC improved perfusion and vascular density in this clinically-relevant CLI model. The molecular effects of pFUS increased both MSC homing and MSC production of VEGF and IL-10, suggesting microenvironmental changes from pFUS also increased potency of MSCs in situ to further enhance their efficacy.

Focused ultrasound treatment for central nervous system disease: neurosurgeon's perspectives

The concept of focused ultrasound (FUS) and its application in the field of medicine have been suggested since the mid-20th century. However, the clinical applications of this technique in central nervous system (CNS) diseases have been extremely limited because the skull inhibits efficient energy transmission. Therefore, early application of FUS treatment was only performed in patients who had already undergone invasive procedures including craniectomy and burr hole trephination. In the 1990s, the phased array technique was developed and this enabled the focus of ultrasonic energy through the skull, and in conjunction with another technique, magnetic resonance thermal monitoring, the possibility of applying FUS in the CNS was further strengthened. The first clinical trial using FUS treatment for CNS diseases was performed in the early 21^st century in patients with glioblastoma, which consists of highly malignant primary brain tumors. However, this trial resulted in a failure to make lesions in the tumors. Various causes were suggested for this outcome including different acoustic impedances across heterogeneous intracranial tissue (not only brain tissue, but also fibrous or tumor tissue). To avoid the influence of this factor, the targets for FUS treatment were shifted to functional diseases such as essential tremor, Parkinson's disease, and psychiatric disease, which usually occur in normal brain structures. The first trial for functional diseases was started in 2010, and the results were successful as accurate lesions were made in the target area. Nowadays, the indication of FUS treatment for functional CNS diseases is gradually widening, and many trials using the FUS technique are reporting good results. In addition to the lesioning technique using high intensity FUS treatment, the possibility of clinical application of low intensity FUS to CNS disease treatment has been investigated at a pre-clinical level, and it is expected that FUS treatment will become one of the most important novel techniques for the treatment of CNS diseases in the near future.

Disrupting the blood-brain barrier by focused ultrasound induces sterile inflammation

MRI-guided pulsed focused ultrasound (pFUS) combined with systemic infusion of ultrasound contrast agent microbubbles (MB) causes localized blood-brain barrier (BBB) disruption that is currently being advocated for increasing drug or gene delivery in neurological diseases. The mechanical acoustic cavitation effects of opening the BBB by low-intensity pFUS+MB, as evidenced by contrast-enhanced MRI, resulted in an immediate damage-associated molecular pattern (DAMP) response including elevations in heat-shock protein 70, IL-1, IL-18, and TNFα indicative of a sterile inflammatory response (SIR) in the parenchyma. Concurrent with DAMP presentation, significant elevations in proinflammatory, antiinflammatory, and trophic factors along with neurotrophic and neurogenesis factors were detected; these elevations lasted 24 h. Transcriptomic analysis of sonicated brain supported the proteomic findings and indicated that the SIR was facilitated through the induction of the NFκB pathway. Histological evaluation demonstrated increased albumin in the parenchyma that cleared by 24 h along with TUNEL+ neurons, activated astrocytes, microglia, and increased cell adhesion molecules in the vasculature. Infusion of fluorescent beads 3 d before pFUS+MB revealed the infiltration of CD68+ macrophages at 6 d postsonication, as is consistent with an innate immune response. pFUS+MB is being considered as part of a noninvasive adjuvant treatment for malignancy or neurodegenerative diseases. These results demonstrate that pFUS+MB induces an SIR compatible with ischemia or mild traumatic brain injury. Further investigation will be required before this approach can be widely implemented in clinical trials.

Enhanced Homing of CXCR-4 Modified Bone Marrow-Derived Mesenchymal Stem Cells to Acute Kidney Injury Tissues by Micro-Bubble-Mediated Ultrasound Exposure

Although the curative effects of bone marrow stromal cells (BMSCs) for acute kidney injury (AKI) have been recognized, their in vivo reparative capability is limited by the low levels of targeted homing and retention of intravenous injected cells. Stromal cell-derived factor-1 (SDF-1) plays an important role in stem cell homing and retention through interaction with its specific functional receptor, CXCR4, which is presumably related to the poor homing in AKI therapy. However, most of the functional CXCR4 chemokine receptors are lost upon in vitro culturing. Ultrasound-targeted micro-bubble destruction (UTMD) has become one of the most promising strategies for the targeted delivery of drugs and genes. To improve BMSC homing to AKI kidneys, we isolated and cultured rat BMSCs to third passage and enhanced CXCR-4 transfection efficiency in vitro by applying UTMD and polyethylenimine. Transwell migration assay showed that the migration ability of CXCR4-modified BMSCs was nine-fold higher than controls. Then, mercuric chloride-induced AKI rats were injected with transfected BMSCs through their tail veins. We showed that enhanced homing and retention of BMSCs were observed in the CXCR-4 modified group compared with other groups at 1, 2 and 3 d post-treatment. Collectively, our data indicated that UTMD was an effective method to increase BMSCs' engraftment to AKI kidney tissues by increasing CXCR-4 expression.

Cyclooxygenase-2 or tumor necrosis factor-α inhibitors attenuate the mechanotransductive effects of pulsed focused ultrasound to suppress mesenchymal stromal cell homing to healthy and dystrophic muscle

Maximal homing of infused stem cells to diseased tissue is critical for regenerative medicine. Pulsed focused ultrasound (pFUS) is a clinically relevant platform to direct stem cell migration. Through mechanotransduction, pFUS establishes local gradients of cytokines, chemokines, trophic factors (CCTF) and cell adhesion molecules (CAM) in treated skeletal muscle that subsequently infused mesenchymal stromal cells (MSC) can capitalize to migrate into the parenchyma. Characterizing molecular responses to mechanical pFUS effects revealed tumor necrosis factor-alpha (TNFα) drives cyclooxygenase-2 (COX2) signaling to locally increase CCTF/CAM that are necessary for MSC homing. pFUS failed to increase chemoattractants and induce MSC homing to treated muscle in mice pretreated with ibuprofen (nonspecific COX inhibitor) or etanercept (TNFα inhibitor). pFUS-induced MSC homing was also suppressed in COX2-knockout mice, demonstrating ibuprofen blocked the mechanically induced CCTF/CAM by acting on COX2. Anti-inflammatory drugs, including ibuprofen, are administered to muscular dystrophy (MD) patients, and ibuprofen also suppressed pFUS-induced homing to muscle in a mouse model of MD. Drug interactions with cell therapies remain unexplored and are not controlled for during clinical cell therapy trials. This study highlights potentially negative drug-host interactions that suppress stem cell homing and could undermine cell-based approaches for regenerative medicine.

Pulsed focused ultrasound pretreatment improves mesenchymal stromal cell efficacy in preventing and rescuing established acute kidney injury in mice

Animal studies have shown that mesenchymal stromal cell (MSC) infusions improve acute kidney injury (AKI) outcomes when administered early after ischemic/reperfusion injury or within 24 hours after cisplatin administration. These findings have spurred several human clinical trials to prevent AKI. However, no specific therapy effectively treats clinically obvious AKI or rescues renal function once advanced injury is established. We investigated if noninvasive image-guided pulsed focused ultrasound (pFUS) could alter the kidney microenvironment to enhance homing of subsequently infused MSC. To examine the efficacy of pFUS-enhanced cell homing in disease, we targeted pFUS to kidneys to enhance MSC homing after cisplatin-induced AKI. We found that pFUS enhanced MSC homing at 1 day post-cisplatin, prior to renal functional deficits, and that enhanced homing improved outcomes of renal function, tubular cell death, and regeneration at 5 days post-cisplatin compared to MSC alone. We then investigated whether pFUS+MSC therapy could rescue established AKI. MSC alone at 3 days post-cisplatin, after renal functional deficits were obvious, significantly improved 7-day survival of animals. Survival was further improved by pFUS and MSC. pFUS prior to MSC injections increased IL-10 production by MSC that homed to kidneys and generated an anti-inflammatory immune cell profile in treated kidneys. This study shows pFUS is a neoadjuvant approach to improve MSC homing to diseased organs. pFUS with MSC better prevents AKI than MSC alone and allows rescue therapy in established AKI, which currently has no meaningful therapeutic options.

Enhanced Homing Ability and Retention of Bone Marrow Stromal Cells to Diabetic Nephropathy by Microbubble-Mediated Diagnostic Ultrasound Irradiation

Bone marrow stromal cell (BMSC) transplantation can successfully treat diabetic nephropathy (DN), but the lack of a specific homing place for intravenously injected cells limits the effective implementation of stem cell therapies. The migration and survival of transplanted BMSCs are determined by inflammatory reactions in the local kidney micro-environment. We tested the hypothesis that microbubble-mediated diagnostic ultrasound irradiation could provide a suitable micro-environment for BMSC delivery and retention in DN therapy. In this study, red fluorescent protein-labeled BMSCs were administered combined with microbubbles to streptozotocin-induced DN rats 4 wk after diabetes onset. We observed enhanced BMSC homing and retention in microbubble-mediated diagnostic ultrasound-irradiated kidneys compared with the contralateral kidneys on days 1 and 3 post-treatment. The results from immunohistochemical analysis, Western blot and enzyme-linked immunosorbent assay indicated that the local and transient expression of various chemo-attractants (i.e., cytokines, integrins and trophic factors) found to promote BMSC homing was much higher than observed in non-treated kidneys. The local capillary endothelium rupture observed by transmission electron microscopy may account for local micro-environment changes. Histopathologic analysis revealed no signs of kidney damage. These results confirmed that renal micro-environment changes caused by appropriate microbubble-mediated diagnostic ultrasound irradiation may promote BMSC homing ability to the diabetic kidney without renal toxicity and cell damage. This non-invasive and effective technique may be a promising method for BMSC transplantation therapy.

Ultrasound in Radiology: From Anatomic, Functional, Molecular Imaging to Drug Delivery and Image-Guided Therapy

During the past decade, ultrasound has expanded medical imaging well beyond the "traditional" radiology setting: a combination of portability, low cost, and ease of use makes ultrasound imaging an indispensable tool for radiologists as well as for other medical professionals who need to obtain imaging diagnosis or guide a therapeutic intervention quickly and efficiently. Ultrasound combines excellent ability for deep penetration into soft tissues with very good spatial resolution, with only a few exceptions (ie, those involving overlying bone or gas). Real-time imaging (up to hundreds and thousands of frames per second) enables guidance of therapeutic procedures and biopsies; characterization of the mechanical properties of the tissues greatly aids with the accuracy of the procedures. The ability of ultrasound to deposit energy locally brings about the potential for localized intervention encompassing the following: tissue ablation, enhancing penetration through the natural barriers to drug delivery in the body and triggering drug release from carrier microparticles and nanoparticles. The use of microbubble contrast agents brings the ability to monitor and quantify tissue perfusion, and microbubble targeting with ligand-decorated microbubbles brings the ability to obtain molecular biomarker information, that is, ultrasound molecular imaging. Overall, ultrasound has become the most widely used imaging modality in modern medicine; it will continue to grow and expand.

Advancing stem cell therapy from bench to bedside: lessons from drug therapies

The inadequacy of existing therapeutic tools together with the paucity of organ donors have always led medical researchers to innovate the current treatment methods or to discover new ways to cure disease. Emergence of cell-based therapies has provided a new framework through which it has given the human world a new hope. Though relatively a new concept, the pace of advancement clearly reveals the significant role that stem cells will ultimately play in the near future. However, there are numerous uncertainties that are prevailing against the present setting of clinical trials related to stem cells: like the best route of cell administration, appropriate dosage, duration and several other applications. A better knowledge of these factors can substantially improve the effectiveness of disease cure or organ repair using this latest therapeutic tool. From a certain perspective, it could be argued that by considering certain proven clinical concepts and experience from synthetic drug system, we could improve the overall efficacy of cell-based therapies. In the past, studies on synthetic drug therapies and their clinical trials have shown that all the aforementioned factors have critical ascendancy over its therapeutic outcomes. Therefore, based on the knowledge gained from synthetic drug delivery systems, we hypothesize that by employing many of the clinical approaches from synthetic drug therapies to this new regenerative therapeutic tool, the efficacy of stem cell-based therapies can also be improved.

Enhancing the migration ability of mesenchymal stromal cells by targeting the SDF-1/CXCR4 axis

Mesenchymal stromal cells (MSCs) are currently being investigated in numerous clinical trials of tissue repair and various immunological disorders based on their ability to secrete trophic factors and to modulate inflammatory responses. MSCs have been shown to migrate to sites of injury and inflammation in response to soluble mediators including the chemokine stromal cell-derived factor-(SDF-)1, but during in vitro culture expansion MSCs lose surface expression of key homing receptors particularly of the SDF-1 receptor, CXCR4. Here we review studies on enhancement of SDF-1-directed migration of MSCs with the premise that their improved recruitment could translate to therapeutic benefits. We describe our studies on approaches to increase the CXCR4 expression in in vitro-expanded cord blood-derived MSCs, namely, transfection, using the commercial liposomal reagent IBAfect, chemical treatment with the histone deacetylase inhibitor valproic acid, and exposure to recombinant complement component C1q. These methodologies will be presented in the context of other cell targeting and delivery strategies that exploit pathways involved in MSC migration. Taken together, these findings indicate that MSCs can be manipulated in vitro to enhance their in vivo recruitment and efficacy for tissue repair.