Characterization of Pdgfrb-Cre transgenic mice reveals reduction of ROSA26 reporter activity in remodeling arteries

Long-wavelength traveling waves of vasomotion modulate the perfusion of cortex

Brain arterioles are active, multicellular complexes whose diameters oscillate at ∼ 0.1 Hz. We assess the physiological impact and spatiotemporal dynamics of vaso-oscillations in the awake mouse. First, vaso-oscillations in penetrating arterioles, which source blood from pial arterioles to the capillary bed, profoundly impact perfusion throughout neocortex. The modulation in flux during resting-state activity exceeds that of stimulus-induced activity. Second, the change in perfusion through arterioles relative to the change in their diameter is weak. This implies that the capillary bed dominates the hydrodynamic resistance of brain vasculature. Lastly, the phase of vaso-oscillations evolves slowly along arterioles, with a wavelength that exceeds the span of the cortical mantle and sufficient variability to establish functional cortical areas as parcels of uniform phase. The phase-gradient supports traveling waves in either direction along both pial and penetrating arterioles. This implies that waves along penetrating arterioles can mix, but not directionally transport, interstitial fluids.

Astrocytes modulate brain phosphate homeostasis via polarized distribution of phosphate uptake transporter PiT2 and exporter XPR1

Aberrant inorganic phosphate (Pi) homeostasis causes brain calcification and aggravates neurodegeneration, but the underlying mechanism remains unclear. Here, we found that primary familial brain calcification (PFBC)-associated Pi transporter genes Pit2 and Xpr1 were highly expressed in astrocytes, with importer PiT2 distributed over the entire astrocyte processes and exporter XPR1 localized to astrocyte end-feet on blood vessels. This polarized PiT2 and XPR1 distribution endowed astrocyte with Pi transport capacity competent for brain Pi homeostasis, which was disrupted in mice with astrocyte-specific knockout (KO) of either Pit2 or Xpr1. Moreover, we found that Pi uptake by PiT2, and its facilitation by PFBC-associated galactosidase MYORG, were required for the high Pi transport capacity of astrocytes. Finally, brain calcification was suppressed by astrocyte-specific PiT2 re-expression in Pit2-KO mice. Thus, astrocyte-mediated Pi transport is pivotal for brain Pi homeostasis, and elevating astrocytic Pi transporter function represents a potential therapeutic strategy for reducing brain calcification.

Cytoplasmic retention of the DNA/RNA-binding protein FUS ameliorates organ fibrosis in mice

Uncontrolled accumulation of extracellular matrix leads to tissue fibrosis and loss of organ function. We previously demonstrated in vitro that the DNA/RNA-binding protein fused in sarcoma (FUS) promotes fibrotic responses by translocating to the nucleus, where it initiates collagen gene transcription. However, it is still not known whether FUS is profibrotic in vivo and whether preventing its nuclear translocation might inhibit development of fibrosis following injury. We now demonstrate that levels of nuclear FUS are significantly increased in mouse models of kidney and liver fibrosis. To evaluate the direct role of FUS nuclear translocation in fibrosis, we used mice that carry a mutation in the FUS nuclear localization sequence (FUSR521G) and the cell-penetrating peptide CP-FUS-NLS that we previously showed inhibits FUS nuclear translocation in vitro. We provide evidence that FUSR521G mice or CP-FUS-NLS-treated mice showed reduced nuclear FUS and fibrosis following injury. Finally, differential gene expression analysis and immunohistochemistry of tissues from individuals with focal segmental glomerulosclerosis or nonalcoholic steatohepatitis revealed significant upregulation of FUS and/or collagen genes and FUS protein nuclear localization in diseased organs. These results demonstrate that injury-induced nuclear translocation of FUS contributes to fibrosis and highlight CP-FUS-NLS as a promising therapeutic option for organ fibrosis.

Endothelial and mural laminin-α5 contributes to neurovascular integrity maintenance

BACKGROUND

Laminin-α5, a major component of the basal lamina, is predominantly synthesized by endothelial and mural cells (pericytes and vascular smooth muscle cells) in the CNS. Loss of laminin-α5 in either population fails to induce any abnormalities due to functional redundancy. Thus, the functional significance of laminin-α5 in neurovascular integrity remains unknown. Here, we hypothesize that ablation of laminin-α5 in both endothelial and mural cells increases neurovascular permeability.

METHODS

The compound knockout mice were generated by crossing laminin-α5 floxed mice with Tie2-Cre and PDGFRβ-Cre, which target endothelial cells and mural cells, respectively. Neurovascular permeability in these mutants was determined with both exogenous and endogenous tracers. Endothelial paracellular and transcellular permeability was assessed by examining the expression of tight junction proteins and transcytosis-associated proteins. In addition, transmission electron microscopy (TEM) was used to visualize tight junction ultrastructure and endothelial caveolae vesicles. Defects in pericytes and astrocytes were investigated by examining pericyte coverage/contact and astrocyte polarity.

RESULTS

Elevated neurovascular permeability was observed in the mutants. Subsequent studies found increased Caveolin-1 and decreased major facilitator superfamily domain-containing protein 2a (MFSD2A) expression, but unaltered Claudin-5 or zonula occludens-1 (ZO-1) expression. Consistent with these results, mutant mice exhibited increased endothelial caveolae vesicle number with intact tight junction structure under TEM. Additionally, pericyte coverage and contact were also decreased in the mutant mice, while astrocyte polarity was unaffected.

CONCLUSIONS

These results strongly indicate that endothelial and mural cell-derived laminin-α5 actively maintains neurovascular integrity via the transcellular rather than paracellular mechanism.

Efficient Treatment of Pulpitis via Transplantation of Human Pluripotent Stem Cell-Derived Pericytes Partially through LTBP1-Mediated T Cell Suppression

Dental pulp pericytes are reported to have the capacity to generate odontoblasts and express multiple cytokines and chemokines that regulate the local immune microenvironment, thus participating in the repair of dental pulp injury in vivo. However, it has not yet been reported whether the transplantation of exogenous pericytes can effectively treat pulpitis, and the underlying molecular mechanism remains unknown. In this study, using a lineage-tracing mouse model, we showed that most dental pulp pericytes are derived from cranial neural crest. Then, we demonstrated that the ablation of pericytes could induce a pulpitis-like phenotype in uninfected dental pulp in mice, and we showed that the significant loss of pericytes occurs during pupal inflammation, implying that the transplantation of pericytes may help to restore dental pulp homeostasis during pulpitis. Subsequently, we successfully generated pericytes with immunomodulatory activity from human pluripotent stem cells through the intermediate stage of the cranial neural crest with a high level of efficiency. Most strikingly, for the first time we showed that, compared with the untreated pulpitis group, the transplantation of hPSC-derived pericytes could substantially inhibit vascular permeability (the extravascular deposition of fibrinogen, ** p < 0.01), alleviate pulpal inflammation (TCR+ cell infiltration, * p < 0.05), and promote the regeneration of dentin (** p < 0.01) in the mouse model of pulpitis. In addition, we discovered that the knockdown of latent transforming growth factor beta binding protein 1 (LTBP1) remarkably suppressed the immunoregulation ability of pericytes in vitro and compromised their in vivo regenerative potential in pulpitis. These results indicate that the transplantation of pericytes could efficiently rescue the aberrant phenotype of pulpal inflammation, which may be partially due to LTBP1-mediated T cell suppression.

Capillary regression leads to sustained local hypoperfusion by inducing constriction of upstream transitional vessels

In the brain, a microvascular sensory web coordinates oxygen delivery to regions of neuronal activity. This involves a dense network of capillaries that send conductive signals upstream to feeding arterioles to promote vasodilation and blood flow. Although this process is critical to the metabolic supply of healthy brain tissue, it may also be a point of vulnerability in disease. Deterioration of capillary networks is a hallmark of many neurological disorders and how this web is engaged during vascular damage remains unknown. We performed in vivo two-photon microscopy on young adult mural cell reporter mice and induced focal capillary injuries using precise two-photon laser irradiation of single capillaries. We found that ∼63% of the injuries resulted in regression of the capillary segment 7-14 days following injury, and the remaining repaired to re-establish blood flow within 7 days. Injuries that resulted in capillary regression induced sustained vasoconstriction in the upstream arteriole-capillary transition (ACT) zone at least 21 days post-injury in both awake and anesthetized mice. This abnormal vasoconstriction involved attenuation of vasomotor dynamics and uncoupling from mural cell calcium signaling following capillary regression. Consequently, blood flow was reduced in the ACT zone and in secondary, uninjured downstream capillaries. These findings demonstrate how capillary injury and regression, as often seen in age-related neurological disease, can impair the microvascular sensory web and contribute to cerebral hypoperfusion.

SIGNIFICANCE

Deterioration of the capillary network is a characteristic of many neurological diseases and can exacerbate neuronal dysfunction and degeneration due to poor blood perfusion. Here we show that focal capillary injuries can induce vessel regression and elicit sustained vasoconstriction in upstream transitional vessels that branch from cortical penetrating arterioles. This reduces blood flow to broader, uninjured regions of the same microvascular network. These findings suggest that widespread and cumulative damage to brain capillaries in neurological disease may broadly affect blood supply and contribute to hypoperfusion through their remote actions.

Spontaneous vasomotion propagates along pial arterioles in the awake mouse brain like stimulus-evoked vascular reactivity

Sensory stimulation evokes a local, vasodilation-mediated blood flow increase to the activated brain region, which is referred to as functional hyperemia. Spontaneous vasomotion is a change in arteriolar diameter that occurs without sensory stimulation, at low frequency (∼0.1 Hz). These vessel diameter changes are a driving force for perivascular soluble waste clearance, the failure of which has been implicated in neurodegenerative disease. Stimulus-evoked vascular reactivity is known to propagate along penetrating arterioles to pial arterioles, but it is unclear whether spontaneous vasomotion propagates similarly. We therefore imaged both stimulus-evoked and spontaneous changes in pial arteriole diameter in awake, head-fixed mice with 2-photon microscopy. By cross-correlating different regions of interest (ROIs) along the length of imaged arterioles, we assessed vasomotion propagation. We found that both during rest and during visual stimulation, one-third of the arterioles showed significant propagation (i.e., a wave), with a median (interquartile range) wave speed of 405 (323) µm/s at rest and 345 (177) µm/s during stimulation. In a second group of mice, with GCaMP expression in their vascular smooth muscle cells, we also found spontaneous propagation of calcium signaling along pial arterioles. In summary, we demonstrate that spontaneous vasomotion propagates along pial arterioles like stimulus-evoked vascular reactivity.

Somatic variants of MAP3K3 are sufficient to cause cerebral and spinal cord cavernous malformations

Cerebral cavernous malformations (CCMs) and spinal cord cavernous malformations (SCCMs) are common vascular abnormalities of the CNS that can lead to seizure, haemorrhage and other neurological deficits. Approximately 85% of patients present with sporadic (versus congenital) CCMs. Somatic mutations in MAP3K3 and PIK3CA were recently reported in patients with sporadic CCM, yet it remains unknown whether MAP3K3 mutation is sufficient to induce CCMs. Here we analysed whole-exome sequencing data for patients with CCM and found that ∼40% of them have a single, specific MAP3K3 mutation [c.1323C>G (p.Ile441Met)] but not any other known mutations in CCM-related genes. We developed a mouse model of CCM with MAP3K3I441M uniquely expressed in the endothelium of the CNS. We detected pathological phenotypes similar to those found in patients with MAP3K3I441M. The combination of in vivo imaging and genetic labelling revealed that CCMs were initiated with endothelial expansion followed by disruption of the blood-brain barrier. Experiments with our MAP3K3I441M mouse model demonstrated that CCM can be alleviated by treatment with rapamycin, the mTOR inhibitor. CCM pathogenesis has usually been attributed to acquisition of two or three distinct genetic mutations involving the genes CCM1/2/3 and/or PIK3CA. However, our results demonstrate that a single genetic hit is sufficient to cause CCMs.

Autophagy gene Atg7 regulates the development of radiation-induced skin injury and fibrosis of skin

BACKGROUND

Radiation-induced skin injury, which may progress to fibrosis, is a severe side effect of radiotherapy in patients with cancer. However, currently, there is a lack of preventive or curative treatments for this injury. Meanwhile, the mechanisms underlying this injury remain poorly understood. Here, we elucidated whether autophagy is essential for the development of radiation-induced skin injury and the potential molecular pathways and mechanisms involved.

METHODS AND RESULTS

We used the myofibroblast-specific Atg7 knockout (namely, conditional Atg7 knockout) mice irradiated with a single electron beam irradiation dose of 30 Gy. Vaseline-based 0.2% rapamycin ointment was topically applied once daily from the day of irradiation for 30 days. On day 30 post irradiation, skin tissues were harvested for further analysis. In vitro, human foreskin fibroblast cells were treated with rapamycin (100 nM) for 24 h and pretreated with 3-MA (5 mM) for 12 h. Macroscopic skin manifestations, histological changes, and fibrosis markers at the mRNA and protein expression levels were measured. Post irradiation, the myofibroblast-specific autophagy-deficient (Atg7Flox/Flox Cre+ ) mice had increased fibrosis marker (COL1A1, CTGF, TGF-β1, and α-SMA) levels in the irradiated area and had more severe macroscopic skin manifestations than the control group (Atg7Flox/Flox Cre- ) mice. Treatment with an autophagy agonist rapamycin attenuated macroscopic skin injury scores and skin fibrosis marker levels with decreased epidermal thickness and dermal collagen deposition in Atg7Flox/Flox Cre+ mice compared with the vehicle control. Moreover, in vitro experiment results were consistent with the in vivo results. Together with studies at the molecular level, we found that these changes involved the Akt/mTOR pathway. In addition, this phenomenon might also relate to Nrf2-autophagy signaling pathway under oxidative stress conditions.

CONCLUSION

In conclusion, Atg7 and autophagy-related mechanisms confer radioprotection, and reactivation of the autophagy process can be a novel therapeutic strategy to reduce and prevent the occurrence of radiodermatitis, particularly skin fibrosis, in patients with cancer.

Aging drives cerebrovascular network remodeling and functional changes in the mouse brain

Aging is the largest risk factor for neurodegenerative disorders, and commonly associated with compromised cerebrovasculature and pericytes. However, we do not know how normal aging differentially impacts the vascular structure and function in different brain areas. Here we utilize mesoscale microscopy methods (serial two-photon tomography and light sheet microscopy) and in vivo imaging (wide field optical spectroscopy and two-photon imaging) to determine detailed changes in aged cerebrovascular networks. Whole-brain vascular tracing showed an overall ~10% decrease in vascular length and branching density, and light sheet imaging with 3D immunolabeling revealed increased arteriole tortuosity in aged brains. Vasculature and pericyte densities showed significant reductions in the deep cortical layers, hippocampal network, and basal forebrain areas. Moreover, in vivo imaging in awake mice identified delays in neurovascular coupling and disrupted blood oxygenation. Collectively, we uncover regional vulnerabilities of cerebrovascular network and physiological changes that can mediate cognitive decline in normal aging.

Orai, RyR, and IP3R channels cooperatively regulate calcium signaling in brain mid-capillary pericytes

Pericytes are multifunctional cells of the vasculature that are vital to brain homeostasis, yet many of their fundamental physiological properties, such as Ca²⁺ signaling pathways, remain unexplored. We performed pharmacological and ion substitution experiments to investigate the mechanisms underlying pericyte Ca²⁺ signaling in acute cortical brain slices of PDGFRβ-Cre::GCaMP6f mice. We report that mid-capillary pericyte Ca²⁺ signalling differs from ensheathing type pericytes in that it is largely independent of L- and T-type voltage-gated calcium channels. Instead, Ca²⁺ signals in mid-capillary pericytes were inhibited by multiple Orai channel blockers, which also inhibited Ca²⁺ entry triggered by endoplasmic reticulum (ER) store depletion. An investigation into store release pathways indicated that Ca²⁺ transients in mid-capillary pericytes occur through a combination of IP₃R and RyR activation, and that Orai store-operated calcium entry (SOCE) is required to sustain and amplify intracellular Ca²⁺ increases evoked by the GqGPCR agonist endothelin-1. These results suggest that Ca²⁺ influx via Orai channels reciprocally regulates IP₃R and RyR release pathways in the ER, which together generate spontaneous Ca²⁺ transients and amplify Gq-coupled Ca²⁺ elevations in mid-capillary pericytes. Thus, SOCE is a major regulator of pericyte Ca²⁺ and a target for manipulating their function in health and disease.

Stretch regulates alveologenesis and homeostasis via mesenchymal Gαq/11-mediated TGFβ2 activation

Alveolar development and repair require tight spatiotemporal regulation of numerous signalling pathways that are influenced by chemical and mechanical stimuli. Mesenchymal cells play key roles in numerous developmental processes. Transforming growth factor-β (TGFβ) is essential for alveologenesis and lung repair, and the G protein α subunits Gαq and Gα11 (Gαq/11) transmit mechanical and chemical signals to activate TGFβ in epithelial cells. To understand the role of mesenchymal Gαq/11 in lung development, we generated constitutive (Pdgfrb-Cre+/-;Gnaqfl/fl;Gna11-/-) and inducible (Pdgfrb-Cre/ERT2+/-;Gnaqfl/fl;Gna11-/-) mesenchymal Gαq/11 deleted mice. Mice with constitutive Gαq/11 gene deletion exhibited abnormal alveolar development, with suppressed myofibroblast differentiation, altered mesenchymal cell synthetic function, and reduced lung TGFβ2 deposition, as well as kidney abnormalities. Tamoxifen-induced mesenchymal Gαq/11 gene deletion in adult mice resulted in emphysema associated with reduced TGFβ2 and elastin deposition. Cyclical mechanical stretch-induced TGFβ activation required Gαq/11 signalling and serine protease activity, but was independent of integrins, suggesting an isoform-specific role for TGFβ2 in this model. These data highlight a previously undescribed mechanism of cyclical stretch-induced Gαq/11-dependent TGFβ2 signalling in mesenchymal cells, which is imperative for normal alveologenesis and maintenance of lung homeostasis.

Dissecting the neurovascular unit in physiology and Alzheimer's disease: Functions, imaging tools and genetic mouse models

The neurovascular unit (NVU) plays an essential role in regulating neurovascular coupling, which refers to the communication between neurons, glia, and vascular cells to control the supply of oxygen and nutrients in response to neural activity. Cellular elements of the NVU coordinate to establish an anatomical barrier to separate the central nervous system from the milieu of the periphery system, restricting the free movement of substances from the blood to the brain parenchyma and maintaining central nervous system homeostasis. In Alzheimer's disease, amyloid-β deposition impairs the normal functions of NVU cellular elements, thus accelerating the disease progression. Here, we aim to describe the current knowledge of the NVU cellular elements, including endothelial cells, pericytes, astrocytes, and microglia, in regulating the blood-brain barrier integrity and functions in physiology as well as alterations encountered in Alzheimer's disease. Furthermore, the NVU functions as a whole, therefore specific labeling and targeting NVU components in vivo enable us to understand the mechanism mediating cellular communication. We review approaches including commonly used fluorescent dyes, genetic mouse models, and adeno-associated virus vectors for imaging and targeting NVU cellular elements in vivo.

Protocol for using serial two-photon tomography to map cell types and cerebrovasculature at single-cell resolution in the whole adult mouse brain

Here, we present a protocol using serial two-photon tomography (STPT) to quantitatively map genetically defined cell types and cerebrovasculature at single-cell resolution across the entire adult mouse brain. We describe the preparation of brain tissue and sample embedding for cell type and vascular STPT imaging and image processing using MATLAB codes. We detail the computational analyses for cell signal detection, vascular tracing, and three-dimensional image registration to anatomical atlases, which can be implemented for brain-wide mapping of different cell types. For complete details on the use and execution of this protocol, please refer to Wu et al. (2022),1 Son et al. (2022),2 Newmaster et al. (2020),3 Kim et al. (2017),4 and Ragan et al. (2012).5.

Activated fibroblasts in cancer: Perspectives and challenges

Activated fibroblasts in tumors, or cancer-associated fibroblasts (CAFs), have become a popular research area over the past decade. As important players in many aspects of tumor biology, with functions ranging from collagen deposition to immunosuppression, CAFs have been the target of clinical and pre-clinical studies that have revealed their potential pro- and anti-tumorigenic dichotomy. In this review, we describe the important role of CAFs in the tumor microenvironment and the technological advances that made these discoveries possible, and we detail the models that are currently available for CAF investigation. Additionally, we present evidence to support the value of encompassing CAF investigation as a future therapeutic avenue alongside immune and cancer cells while highlighting the challenges that must be addressed for successful clinical translation of new findings.

Separate optogenetic manipulation of Nerve/glial antigen 2 (NG2) glia and mural cells using the NG2 promoter

Nerve/glial antigen 2 (NG2) is a protein marker of NG2 glia and mural cells, and NG2 promoter activity is utilized to target these cells. However, the NG2 promoter cannot target NG2 glia and mural cells separately. This has been an obstacle for NG2 glia-specific manipulation. Here, we developed transgenic mice in which either cell type can be targeted using the NG2 promoter. We selected a tetracycline-controllable gene induction system for cell type-specific transgene expression, and generated NG2-tetracycline transactivator (tTA) transgenic lines. We crossed tTA lines with the tetO-ChR2 (channelrhodopsin-2)-EYFP line to characterize tTA-dependent transgene induction. We isolated two unique NG2-tTA mouse lines: one that induced ChR2-EYFP only in mural cells, likely due to the chromosomal position effect of NG2-tTA insertion, and the other that induced it in both cell types. We then applied a Cre-mediated set-subtraction strategy to the latter case and eliminated ChR2-EYFP from mural cells, resulting in NG2 glia-specific transgene induction. We further demonstrated that tTA-dependent ChR2 expression could manipulate cell function. Optogenetic mural cell activation decreased cerebral blood flow, as previously reported, indicating that tTA-mediated ChR2 expression was sufficient to impact cellular function. ChR2-mediated depolarization was observed in NG2 glia in acute hippocampal slices. In addition, ChR2-mediated depolarization of NG2 glia inhibited their proliferation but promoted their differentiation in juvenile mice. Since the tTA-tetO combination is expandable, the mural cell-specific NG2-tTA line and the NG2 glia-specific NG2-tTA line will permit us to conduct observational and manipulation studies to examine in vivo function of these cells separately.

Anti-diabetic effects of GLP1 analogs are mediated by thermogenic interleukin-6 signaling in adipocytes

Mechanisms underlying anti-diabetic effects of GLP1 analogs remain incompletely understood. We observed that in prediabetic humans exenatide treatment acutely induces interleukin-6 (IL-6) secretion by monocytes and IL-6 in systemic circulation. We hypothesized that GLP1 analogs signal through IL-6 in adipose tissue (AT) and used the mouse model to test if IL-6 receptor (IL-6R) signaling underlies the effects of the GLP1-IL-6 axis. We show that liraglutide transiently increases IL-6 in mouse circulation and IL-6R signaling in AT. Metronomic liraglutide treatment resulted in AT browning and thermogenesis linked with STAT3 activation. IL-6-blocking antibody treatment inhibited STAT3 activation in AT and suppressed liraglutide-induced increase in thermogenesis and glucose utilization. We show that adipose IL-6R knockout mice still display liraglutide-induced weight loss but lack thermogenic adipocyte browning and metabolism activation. We conclude that the anti-diabetic effects of GLP1 analogs are mediated by transient upregulation of IL-6, which activates canonical IL-6R signaling and thermogenesis.

Pericyte remodeling is deficient in the aged brain and contributes to impaired capillary flow and structure

Deterioration of brain capillary flow and architecture is a hallmark of aging and dementia. It remains unclear how loss of brain pericytes in these conditions contributes to capillary dysfunction. Here, we conduct cause-and-effect studies by optically ablating pericytes in adult and aged mice in vivo. Focal pericyte loss induces capillary dilation without blood-brain barrier disruption. These abnormal dilations are exacerbated in the aged brain, and result in increased flow heterogeneity in capillary networks. A subset of affected capillaries experience reduced perfusion due to flow steal. Some capillaries stall in flow and regress, leading to loss of capillary connectivity. Remodeling of neighboring pericytes restores endothelial coverage and vascular tone within days. Pericyte remodeling is slower in the aged brain, resulting in regions of persistent capillary dilation. These findings link pericyte loss to disruption of capillary flow and structure. They also identify pericyte remodeling as a therapeutic target to preserve capillary flow dynamics.

TWIST1, a gene associated with Saethre-Chotzen syndrome, regulates extraocular muscle organization in mouse

Heterozygous loss of function mutations in TWIST1 cause Saethre-Chotzen syndrome, which is characterized by craniosynostosis, facial asymmetry, ptosis, strabismus, and distinctive ear appearance. Individuals with syndromic craniosynostosis have high rates of strabismus and ptosis, but the underlying pathology is unknown. Some individuals with syndromic craniosynostosis have been noted to have absence of individual extraocular muscles or abnormal insertions of the extraocular muscles on the globe. Using conditional knock-out alleles for Twist1 in cranial mesenchyme, we test the hypothesis that Twist1 is required for extraocular muscle organization and position, attachment to the globe, and/or innervation by the cranial nerves. We examined the extraocular muscles in conditional Twist1 knock-out animals using Twist2-cre and Pdgfrb-cre drivers. Both are expressed in cranial mesoderm and neural crest. Conditional inactivation of Twist1 using these drivers leads to disorganized extraocular muscles that cannot be reliably identified as specific muscles. Tendons do not form normally at the insertion and origin of these dysplastic muscles. Knock-out of Twist1 expression in tendon precursors, using scleraxis-cre, however, does not alter EOM organization. Furthermore, developing motor neurons, which do not express Twist1, display abnormal axonal trajectories in the orbit in the presence of dysplastic extraocular muscles. Strabismus in individuals with TWIST1 mutations may therefore be caused by abnormalities in extraocular muscle development and secondary abnormalities in innervation and tendon formation.

In Vivo Targeting of the Neurovascular Unit: Challenges and Advancements

The blood-brain barrier (BBB) is essential for the homeostasis of the central nervous system (CNS). Functions of the BBB are performed by the neurovascular unit (NVU), which consists of endothelial cells, pericytes, astrocytes, microglia, basement membrane, and neurons. NVU cells interact closely and together are responsible for neurovascular coupling, BBB integrity, and transendothelial fluid transport. Studies have shown that NVU dysfunction is implicated in several acute and chronic neurological diseases, including Alzheimer's disease, multiple sclerosis, and stroke. The mechanisms of NVU disruption remain poorly understood, partially due to difficulties in selective targeting of NVU cells. In this review, we discuss the relative merits of available protein markers and drivers of the NVU along with recent advancements that have been made in the field to increase efficiency and specificity of NVU research.

Quantitative relationship between cerebrovascular network and neuronal cell types in mice

The cerebrovasculature and its mural cells must meet brain regional energy demands, but how their spatial relationship with different neuronal cell types varies across the brain remains largely unknown. Here we apply brain-wide mapping methods to comprehensively define the quantitative relationships between the cerebrovasculature, capillary pericytes, and glutamatergic and GABAergic neurons, including neuronal nitric oxide synthase-positive (nNOS+) neurons and their subtypes in adult mice. Our results show high densities of vasculature with high fluid conductance and capillary pericytes in primary motor sensory cortices compared with association cortices that show significant positive and negative correlations with energy-demanding parvalbumin+ and vasomotor nNOS+ neurons, respectively. Thalamo-striatal areas that are connected to primary motor sensory cortices also show high densities of vasculature and pericytes, suggesting dense energy support for motor sensory processing areas. Our cellular-resolution resource offers opportunities to examine spatial relationships between the cerebrovascular network and neuronal cell composition in largely understudied subcortical areas.

In vivo Single Cell Optical Ablation of Brain Pericytes

Pericytes have myriad functions in cerebrovascular regulation but remain understudied in the living brain. To dissect pericyte functions in vivo, prior studies have used genetic approaches to induce global pericyte loss in the rodent brain. However, this leads to complex outcomes, making it challenging to disentangle the physiological roles of pericytes from the pathophysiological effects of their depletion. Here, we describe a protocol to optically ablate individual pericytes of the mouse cerebral cortex in vivo for fine-scale studies of pericyte function. The strategy relies on two-photon microscopy and cranial window-implanted transgenic mice with mural cell-specific expression of fluorescent proteins. Single pericyte somata are precisely targeted with pulsed infrared laser light to induce selective pericyte death, but without overt blood-brain barrier leakage. Following pericyte ablation, the changes to the local capillary network and remaining pericytes can be examined longitudinally. The approach has been used to study pericyte roles in capillary flow regulation, and the structural remodeling of pericytes involved in restoration of endothelial coverage after pericyte loss.

Bim Expression Promotes the Clearance of Mononuclear Phagocytes during Choroidal Neovascularization, Mitigating Scar Formation in Mice

Inflammation is increasingly recognized as an important modulator in the pathogenesis of neovascular age-related macular degeneration (nAMD). Although significant progress has been made in delineating the pathways that contribute to the recruitment of inflammatory cells and their contribution to nAMD, we know little about what drives the resolution of these inflammatory responses. Gaining a better understanding of how immune cells are cleared in the choroid will give a novel insight into how sustained inflammation could influence the pathogenesis of nAMD. The pro-apoptotic Bcl-2 family member Bim is a master regulator of immune cell homeostasis. In its absence, immune cell lifespan and numbers increase. Most therapeutic regimes that squelch inflammation do so by enhancing immune cell apoptosis through enhanced Bim expression and activity. To test the hypothesis that Bim expression tempers inflammation during the pathogenesis of nAMD, we used the mouse laser-induced choroidal neovascularization (CNV) model in which inflammation acts as a facilitator of CNV. Here, we showed minimal to no change in the recruitment of F4/80-, CD80-, CD11b-, and Iba1-positive myeloid-derived mononuclear phagocytes to the site of laser photocoagulation in the absence of Bim expression. However, the resolution of these cells from the choroid of Bim-deficient (Bim -/-) mice was significantly diminished following laser photocoagulation. With time, we noted increased scar formation, demonstrated by collagen I staining, in Bim -/- mice with no change in the resolution of neovascularization compared to wild-type littermates. We also noted that mice lacking Bim expression in mononuclear phagocytes (Bim^Flox/Flox; Lyz2-Cre (Bim^MP) mice) had delayed resolution of F4/80-, CD80-, CD11b-, and Iba1-positive cells, while those lacking Bim expression in endothelial cells (Bim^Flox/Flox; Cad5-Cre (Bim^EC) mice) had delayed resolution of only CD11b- and Iba1-positive cells. Both Bim^MP and Bim^EC mice demonstrated increased scar formation, albeit to differing degrees. Thus, our studies show that resolving inflammation plays an important role in moderating scar formation in nAMD, and it is impacted by Bim expression in both the endothelium and mononuclear phagocyte lineages.

Divergent effects of distinct perivascular cell subsets for intra-articular cell therapy in posttraumatic osteoarthritis

Intra-articular injection of mesenchymal stem cells has shown benefit for the treatment of osteoarthritis (OA). However, mesenchymal stem/stromal cells at the origin of these clinical results are heterogenous cell populations with limited cellular characterization. Here, two transgenic reporter mice were used to examine the differential effects of two precisely defined perivascular cell populations (Pdgfrα⁺  and Pdgfrβ⁺  cells) from white adipose tissue for alleviation of OA. Perivascular mesenchymal cells were isolated from transgenic Pdgfrα-and Pdgfrβ-CreER^T2 reporter animals and delivered as a one-time intra-articular dose to C57BL/6J mice after destabilization of the medial meniscus (DMM). Both Pdgfrα⁺  and Pdgfrβ⁺  cell preparations improved metrics of cartilage degradation and reduced markers of chondrocyte hypertrophy. While some similarities in cell distribution were identified within the synovial and perivascular spaces, injected Pdgfrα⁺  cells remained in the superficial layers of articular cartilage, while Pdgfrβ⁺  cells were more widely dispersed. Pdgfrβ⁺  cell therapy prevented subchondral sclerosis induced by DMM, while Pdgfrα⁺  cell therapy had no effect. In summary, while both cell therapies showed beneficial effects in the DMM model, important differences in cell incorporation, persistence, and subchondral sclerosis were identified.

Emerging roles for CNS fibroblasts in health, injury and disease

Recent transcriptomic, histological and functional studies have begun to shine light on the fibroblasts present in the meninges, choroid plexus and perivascular spaces of the brain and spinal cord. Although the origins and functions of CNS fibroblasts are still being described, it is clear that they represent a distinct cell population, or populations, that have likely been confused with other cell types on the basis of the expression of overlapping cellular markers. Recent work has revealed that fibroblasts play crucial roles in fibrotic scar formation in the CNS after injury and inflammation, which have also been attributed to other perivascular cell types such as pericytes and vascular smooth muscle cells. In this Review, we describe the current knowledge of the location and identity of CNS perivascular cell types, with a particular focus on CNS fibroblasts, including their origin, subtypes, roles in health and disease, and future areas for study.

Cardiac Fibrosis: Cellular Effectors, Molecular Pathways, and Exosomal Roles

Cardiac fibrosis, a common pathophysiologic process in most heart diseases, refers to an excess of extracellular matrix (ECM) deposition by cardiac fibroblasts (CFs), which can lead to cardiac dysfunction and heart failure subsequently. Not only CFs but also several other cell types including macrophages and endothelial cells participate in the process of cardiac fibrosis via different molecular pathways. Exosomes, ranging in 30-150 nm of size, have been confirmed to play an essential role in cellular communications by their bioactive contents, which are currently a hot area to explore pathobiology and therapeutic strategy in multiple pathophysiologic processes including cardiac fibrosis. Cardioprotective factors such as RNAs and proteins packaged in exosomes make them an excellent cell-free system to improve cardiac function without significant immune response. Emerging evidence indicates that targeting selective molecules in cell-derived exosomes could be appealing therapeutic treatments in cardiac fibrosis. In this review, we summarize the current understandings of cellular effectors, molecular pathways, and exosomal roles in cardiac fibrosis.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).

A Versatile Tiling Light Sheet Microscope for Imaging of Cleared Tissues

We present a tiling light sheet microscope compatible with all tissue clearing methods for rapid multicolor 3D imaging of cleared tissues with micron-scale (4 × 4 × 10 μm³) to submicron-scale (0.3 × 0.3 × 1 μm³) spatial resolution. The resolving ability is improved to sub-100 nm (70 × 70 × 200 nm³) via tissue expansion. The microscope uses tiling light sheets to achieve higher spatial resolution and better optical sectioning ability than conventional light sheet microscopes. The illumination light is phase modulated to adjust the position and intensity profile of the light sheet based on the desired spatial resolution and imaging speed and to keep the microscope aligned. The ability of the microscope to align via phase modulation alone also ensures its accuracy for multicolor 3D imaging and makes the microscope reliable and easy to operate. Here we describe the working principle and design of the microscope. We demonstrate its utility by imaging various cleared tissues.

Brain capillary pericytes exert a substantial but slow influence on blood flow

The majority of the brain's vasculature is composed of intricate capillary networks lined by capillary pericytes. However, it remains unclear whether capillary pericytes influence blood flow. Using two-photon microscopy to observe and manipulate brain capillary pericytes in vivo, we find that their optogenetic stimulation decreases lumen diameter and blood flow, but with slower kinetics than similar stimulation of mural cells on upstream pial and precapillary arterioles. This slow vasoconstriction was inhibited by the clinically used vasodilator fasudil, a Rho-kinase inhibitor that blocks contractile machinery. Capillary pericytes were also slower to constrict back to baseline following hypercapnia-induced dilation, and slower to dilate towards baseline following optogenetically induced vasoconstriction. Optical ablation of single capillary pericytes led to sustained local dilation and a doubling of blood cell flux selectively in capillaries lacking pericyte contact. These data indicate that capillary pericytes contribute to basal blood flow resistance and slow modulation of blood flow throughout the brain.

Targeted Thrombospondin-1 Expression in Ocular Vascular Development and Neovascularization

Tight regulation of positive and negative regulators of angiogenesis is essential, particularly in the eye where their dysregulation can lead to vision loss. Thrombospondin-1 (TSP1) is a matricellular protein that negatively regulates angiogenesis and inflammation in the eye. It aids ocular vascular homeostasis such that its loss contributes to increased retinal vascular density and pathologic ocular neovascularization. Our previous studies demonstrated that mice globally lacking TSP1 expression had increased retinal vascular density, decreased hyperoxia-induced retinal vessel loss, and increased choroidal neovascularization. Here we determined the impact to the ocular vasculature of endothelial cell, pericyte, or astrocyte loss of TSP1 expression. Only lack of TSP1 expression in endothelial cells was sufficient to increase choroidal neovascularization with mice lacking expression in pericytes or astrocytes not demonstrating a significant impact. Although the global TSP1 knockout mice demonstrated increased retinal vascular density, individual cell type loss of TSP1 resulted in decreased retinal endothelial cell numbers before and/or after vascular maturation in a cell type specific fashion. Retinas from mice lacking TSP1 expression in endothelial cells, pericytes or astrocytes were not protected from retinal vessel regression in response to hyperoxia as we previously observed in the global knockout. Thus, modulation of TSP1 expression in individual cell types demonstrates a response that is unique to the role TSP1 plays in that cell type of interest, and their coordinated activity is critical for vision.

Progression of prostate carcinoma is promoted by adipose stromal cell-secreted CXCL12 signaling in prostate epithelium

Aggressiveness of carcinomas is linked with tumor recruitment of adipose stromal cells (ASC), which is increased in obesity. ASC promote cancer through molecular pathways not fully understood. Here, we demonstrate that epithelial-mesenchymal transition (EMT) in prostate tumors is promoted by obesity and suppressed upon pharmacological ASC depletion in HiMyc mice, a spontaneous genetic model of prostate cancer. CXCL12 expression in tumors was associated with ASC recruitment and localized to stromal cells expressing platelet-derived growth factor receptors Pdgfra and Pdgfrb. The role of this chemokine secreted by stromal cells in cancer progression was further investigated by using tissue-specific knockout models. ASC deletion of CXCL12 gene in the Pdgfr + lineages suppressed tumor growth and EMT, indicating stroma as the key source of CXCL12. Clinical sample analysis revealed that CXCL12 expression by peritumoral adipose stroma is increased in obesity, and that the correlating increase in Pdgfr/CXCL12 expression in the tumor is linked with decreased survival of patients with prostate carcinoma. Our study establishes ASC as the source of CXCL12 driving tumor aggressiveness and outlines an approach to treatment of carcinoma progression.

VasoMetrics: unbiased spatiotemporal analysis of microvascular diameter in multi-photon imaging applications

BACKGROUND

Multi-photon imaging of the cerebrovasculature provides rich data on the dynamics of cortical arterioles, capillaries, and venules. Vascular diameter is the major determinant of blood flow resistance, and is the most commonly quantified metric in studies of the cerebrovasculature. However, there is a lack of accessible and easy-to-use methods to quantify vascular diameter in imaging data.

METHODS

We created VasoMetrics, a macro written in ImageJ/Fiji for spatiotemporal analysis of microvascular diameter. The key feature of VasoMetrics is rapid analysis of many evenly spaced cross-sectional lines along the vessel of interest, permitting the extraction of numerous diameter measurements from individual vessels. Here we demonstrated the utility of VasoMetrics by analyzing in vivo multi-photon imaging stacks and movies collected from lightly sedated mice, as well as data from optical coherence tomography angiography (OCTA) of human retina.

RESULTS

Compared to the standard approach, which is to measure cross-sectional diameters at arbitrary points along a vessel, VasoMetrics accurately reported spatiotemporal features of vessel diameter, reduced measurement bias and time spent analyzing data, and improved the reproducibility of diameter measurements between users. VasoMetrics revealed the dynamics in pial arteriole diameters during vasomotion at rest, as well as changes in capillary diameter before and after pericyte ablation. Retinal arteriole diameter was quantified from a human retinal angiogram, providing proof-of-principle that VasoMetrics can be applied to contrast-enhanced clinical imaging of microvasculature.

CONCLUSIONS

VasoMetrics is a robust macro for spatiotemporal analysis of microvascular diameter in imaging applications.

Rapid, Nitric Oxide Synthesis-Dependent Activation of MMP-9 at Pericyte Somata During Capillary Ischemia in vivo

Nitric oxide serves essential roles in normal vascular physiology, but paradoxically contributes to vascular pathology in disease. During brain ischemia, aberrant nitric oxide levels can cause cellular injury through induction of nitrosative/oxidative stress and post-translational activation of matrix-metalloproteinase-9 (MMP-9). We recently demonstrated that brain pericyte somata were associated with very early and localized MMP-9 activation along capillaries during cerebral ischemia, leading to focal blood-brain barrier disruption. Here, we tested whether this effect was dependent upon nitric oxide production. In vivo two-photon imaging was used to directly visualize MMP9 activity using a FITC-gelatin probe and leakage of intravenous dye during photothrombotically induced capillary ischemia. Results showed that the NOS inhibitor, L-NIL, at concentrations affecting both iNOS and constitutive NOS isoforms, attenuated capillary leakage at pericyte soma-specific locations and substantially reduced FITC-gelatin cleavage. We also found that combined administration of L-NIL and anisomycin, an inhibitor of protein synthesis, led to near complete elimination of FITC-gelatin cleavage and vascular leakage. These results indicate that both nitric oxide synthase and new protein synthesis are involved in the rapid activation of MMP-9 at somata of capillary pericytes during ischemia.

Glycosaminoglycan Modification of Decorin Depends on MMP14 Activity and Regulates Collagen Assembly

Proper processing of collagens COL1 and COL6 is required for normal function of adipose tissue and skeletal muscle. Proteoglycan decorin (DCN) regulates collagen fiber formation. The amino-terminus of DCN is modified with an O-linked glycosaminoglycan (GAG), the function of which has remained unclear. Previously, non-glycanated DCN (ngDCN) was identified as a marker of adipose stromal cells. Here, we identify MMP14 as the metalloprotease that cleaves DCN to generate ngDCN. We demonstrate that mice ubiquitously lacking DCN GAG (ngDCN mice) have reduced matrix rigidity, enlarged adipocytes, fragile skin, as well as skeletal muscle hypotrophy, fibrosis, and dysfunction. Our results indicate that DCN deglycanation results in reduced intracellular DCN—collagen binding and increased production of truncated COL6 chains, leading to aberrant procollagen processing and extracellular localization. This study reveals that the GAG of DCN functions to regulate collagen assembly in adipose tissue and skeletal muscle and uncovers a new mechanism of matrix dysfunction in obesity and aging.

Imaging and optogenetic modulation of vascular mural cells in the live brain

Mural cells (smooth muscle cells and pericytes) are integral components of brain blood vessels that play important roles in vascular formation, blood-brain barrier maintenance, and regulation of regional cerebral blood flow (rCBF). These cells are implicated in conditions ranging from developmental vascular disorders to age-related neurodegenerative diseases. Here we present complementary tools for cell labeling with transgenic mice and organic dyes that allow high-resolution intravital imaging of the different mural cell subtypes. We also provide detailed methodologies for imaging of spontaneous and neural activity-evoked calcium transients in mural cells. In addition, we describe strategies for single- and two-photon optogenetics that allow manipulation of the activity of individual and small clusters of mural cells. Together with measurements of diameter and flow in individual brain microvessels, calcium imaging and optogenetics allow the investigation of pericyte and smooth muscle cell physiology and their role in regulating rCBF. We also demonstrate the utility of these tools to investigate mural cells in the context of Alzheimer's disease and cerebral ischemia mouse models. Thus, these methods can be used to reveal the functional and structural heterogeneity of mural cells in vivo, and allow detailed cellular studies of the normal function and pathophysiology of mural cells in a variety of disease models. The implementation of this protocol can take from several hours to days depending on the intended applications.

Diverse origins and activation of fibroblasts in cardiac fibrosis

Cardiac fibroblasts (cFBs) have emerged as a heterogenous cell population. Fibroblasts are considered the main cell source for synthesis of the extracellular matrix (ECM) and as such a dysregulation in cFB function, activity, or viability can lead to disrupted ECM structure or fibrosis. Fibrosis can be initiated in response to different injuries and stimuli, and can be reparative (beneficial) or reactive (damaging). FBs need to be activated to myofibroblasts (MyoFBs) which have augmented capacity in synthesizing ECM proteins, causing fibrosis. In addition to the resident FBs in the myocardium, a number of other cells (pericytes, fibrocytes, mesenchymal, and hematopoietic cells) can transform into MyoFBs, further driving the fibrotic response. Multiple molecules including hormones, cytokines, and growth factors stimulate this process leading to generation of activated MyoFBs. Contribution of different cell types to cFBs and MyoFBs can result in an exponential increase in the number of MyoFBs and an accelerated pro-fibrotic response. Given the diversity of the cell sources, and the array of interconnected signalling pathways that lead to formation of MyoFBs and subsequently fibrosis, identifying a single target to limit the fibrotic response in the myocardium has been challenging. This review article will delineate the importance and relevance of fibroblast heterogeneity in mediating fibrosis in different models of heart failure and will highlight important signalling pathways implicated in myofibroblast activation.

Age-associated telomere attrition in adipocyte progenitors predisposes to metabolic disease

White and beige adipocytes in subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) are maintained by proliferation and differentiation of adipose progenitor cells (APCs). Here we use mice with tissue-specific telomerase reverse transcriptase (TERT) gene knockout (KO), which undergo premature telomere shortening and proliferative senescence in APCs, to investigate the effect of over-nutrition on APC exhaustion and metabolic dysfunction. We find that TERT KO in the Pdgfra⁺ cell lineage results in adipocyte hypertrophy, inflammation and fibrosis in SAT, while TERT KO in the Pdgfrb⁺ lineage leads to adipocyte hypertrophy in both SAT and VAT. Systemic insulin resistance is observed in both KO models and is aggravated by a high-fat diet. Analysis of human biopsies demonstrates that telomere shortening in SAT is associated with metabolic disease progression after bariatric surgery. Our data indicate that over-nutrition can promote APC senescence and provide a mechanistic link between ageing, obesity and diabetes.

Angiogenesis and Blood-Brain Barrier Permeability in Vascular Remodeling after Stroke

Angiogenesis, the growth of new blood vessels, is a natural defense mechanism helping to restore oxygen and nutrient supply to the affected brain tissue following an ischemic stroke. By stimulating vessel growth, angiogenesis may stabilize brain perfusion, thereby promoting neuronal survival, brain plasticity, and neurologic recovery. However, therapeutic angiogenesis after stroke faces challenges: new angiogenesis-induced vessels have a higher than normal permeability, and treatment to promote angiogenesis may exacerbate outcomes in stroke patients. The development of therapies requires elucidation of the precise cellular and molecular basis of the disease. Microenvironment homeostasis of the central nervous system is essential for its normal function and is maintained by the blood-brain barrier (BBB). Tight junction proteins (TJP) form the tight junction (TJ) between vascular endothelial cells (ECs) and play a key role in regulating the BBB permeability. We demonstrated that after stroke, new angiogenesis-induced vessels in peri-infarct areas have abnormally high BBB permeability due to a lack of major TJPs in ECs. Therefore, promoting TJ formation and BBB integrity in the new vessels coupled with speedy angiogenesis will provide a promising and safer treatment strategy for improving recovery from stroke. Pericyte is a central neurovascular unite component in vascular barriergenesis and are vital to BBB integrity. We found that pericytes also play a key role in stroke-induced angiogenesis and TJ formation in the newly formed vessels. Based on these findings, in this article, we focus on regulation aspects of the BBB functions and describe cellular and molecular special features of TJ formation with an emphasis on role of pericytes in BBB integrity during angiogenesis after stroke.

Seizures elevate gliovascular unit Ca2+ and cause sustained vasoconstriction

Seizures can result in a severe hypoperfusion/hypoxic attack that causes postictal memory and behavioral impairments. However, neither postictal changes to microvasculature nor Ca²⁺ changes in key cell types controlling blood perfusion have been visualized in vivo, leaving essential components of the underlying cellular mechanisms unclear. Here, we use 2-photon microvascular and Ca²⁺ imaging in awake mice to show that seizures result in a robust vasoconstriction of cortical penetrating arterioles, which temporally mirrors the prolonged postictal hypoxia. The vascular effect was dependent on cyclooxygenase 2, as pretreatment with ibuprofen prevented postictal vasoconstriction. Moreover, seizures caused a rapid elevation in astrocyte endfoot Ca²⁺ that was confined to the seizure period, and vascular smooth muscle cells displayed a significant increase in Ca²⁺ both during and following seizures, lasting up to 75 minutes. Our data show enduring postictal vasoconstriction and temporal activities of 2 cell types within the neurovascular unit that are associated with seizure-induced hypoperfusion/hypoxia. These findings support prevention of this event may be a novel and tractable treatment strategy in patients with epilepsy who experience extended postseizure impairments.

Seizures cause enduring microvascular constriction via long lasting calcium elevations in vascular smooth muscle cells.

The vascular endothelial cell-expressed prion protein doppel promotes angiogenesis and blood-brain barrier development

The central nervous system (CNS) contains a complex network of blood vessels that promote normal tissue development and physiology. Abnormal control of blood vessel morphogenesis and maturation is linked to the pathogenesis of various neurodevelopmental diseases. The CNS-specific genes that regulate blood vessel morphogenesis in development and disease remain largely unknown. Here, we have characterized functions for the gene encoding prion protein 2 (Prnd) in CNS blood vessel development and physiology. Prnd encodes the glycosylphosphatidylinositol (GPI)-linked protein doppel, which is expressed on the surface of angiogenic vascular endothelial cells, but is absent in quiescent endothelial cells of the adult CNS. During CNS vascular development, doppel interacts with receptor tyrosine kinases and activates cytoplasmic signaling pathways involved in endothelial cell survival, metabolism and migration. Analysis of mice genetically null for Prnd revealed impaired CNS blood vessel morphogenesis and associated endothelial cell sprouting defects. Prnd^-/- mice also displayed defects in endothelial barrier integrity. Collectively, these data reveal novel mechanisms underlying doppel control of angiogenesis in the developing CNS, and may provide new insights about dysfunctional pathways that cause vascular-related CNS disorders.

Generation of a novel mouse strain with fibroblast-specific expression of Cre recombinase

Cell-specific expression of genes offers the possibility to use their promoters to drive expression of Cre-recombinase, thereby allowing for detailed expression analysis using reporter gene systems, cell lineage tracing, conditional gene deletion, and cell ablation. In this context, current data suggest that the integrin α11 subunit has the potential to serve as a fibroblast biomarker in tissue regeneration and pathology, in particular in wound healing and in tissue- and tumor fibrosis. The mesenchyme-restricted expression pattern of integrin α11 thus prompted us to generate a novel ITGA11-driver Cre mouse strain using a ϕC31 integrase-mediated knock-in approach. In this transgenic mouse, the Cre recombinase is driven by regulatory promoter elements within the 3 kb segment of the human ITGA11 gene. β-Galactosidase staining of embryonic tissues obtained from a transgenic ITGA11-Cre mouse line crossed with Rosa 26R reporter mice (ITGA11-Cre;R26R) revealed ITGA11-driven Cre expression and activity in mesenchymal cells in a variety of mesenchymal tissues in a pattern reminiscent of endogenous α11 protein expression in mouse embryos. Interestingly, X-gal staining of mouse embryonic fibroblasts (MEFs) isolated from the ITGA11-Cre;R26R mice indicated heterogeneity in the MEF population. ITGA11-driven Cre activity was shown in approximately 60% of the MEFs, suggesting that the expression of integrin α11 could be exploited for isolation of different fibroblast populations. ITGA11-driven Cre expression was found to be low in adult mouse tissues but was induced in granulation tissue of excisional wounds and in fibrotic hearts following aortic banding. We predict that the ITGA11-Cre transgenic mouse strain described in this report will be a useful tool in matrix research for the deletion of genes in subsets of fibroblasts in the developing mouse and for determining the function of subsets of pro-fibrotic fibroblasts in tissue fibrosis and in different subsets of cancer-associated fibroblasts in the tumor microenvironment.

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A mouse strain with Cre-recombinase driven by the human integrin α11 promoter has been generated.

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Cre-recombinase expression in this strain has been characterized using the Rosa26R reporter mouse.

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ITGA11-Cre is restricted to fibroblast subsets in mouse embryos, skin wounds and fibrotic hearts.

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This Cre-driver strain will be a useful tool to study role fibroblasts in fibrosis and tumors.

Acute Ablation of Cortical Pericytes Leads to Rapid Neurovascular Uncoupling

Pericytes are perivascular mural cells that enwrap brain capillaries and maintain blood-brain barrier (BBB) integrity. Most studies suggest that pericytes regulate cerebral blood flow (CBF) and oxygen delivery to activated brain structures, known as neurovascular coupling. While we have previously shown that congenital loss of pericytes leads over time to aberrant hemodynamic responses, the effects of acute global pericyte loss on neurovascular coupling have not been studied. To address this, we used our recently reported inducible pericyte-specific Cre mouse line crossed to iDTR mice carrying Cre-dependent human diphtheria toxin (DT) receptor, which upon DT treatment leads to acute pericyte ablation. As expected, DT led to rapid progressive loss of pericyte coverage of cortical capillaries up to 50% at 3 days post-DT, which correlated with approximately 50% reductions in stimulus-induced CBF responses measured with laser doppler flowmetry (LDF) and/or intrinsic optical signal (IOS) imaging. Endothelial response to acetylcholine, microvascular density, and neuronal evoked membrane potential responses remained, however, unchanged, as well as arteriolar smooth muscle cell (SMC) coverage and functional responses to adenosine, as we previously reported. Together, these data suggest that neurovascular uncoupling in this model is driven by pericyte loss, but not other vascular deficits or neuronal dysfunction. These results further support the role of pericytes in CBF regulation and may have implications for neurological conditions associated with rapid pericyte loss such as hypoperfusion and stroke, as well as conditions where the exact time course of global regional pericyte loss is less clear, such as Alzheimer's disease (AD) and other neurogenerative disorders.

Structure and Junctional Complexes of Endothelial, Epithelial and Glial Brain Barriers

The homeostasis of the central nervous system (CNS) is ensured by the endothelial, epithelial, mesothelial and glial brain barriers, which strictly control the passage of molecules, solutes and immune cells. While the endothelial blood-brain barrier (BBB) and the epithelial blood-cerebrospinal fluid barrier (BCSFB) have been extensively investigated, less is known about the epithelial and mesothelial arachnoid barrier and the glia limitans. Here, we summarize current knowledge of the cellular composition of the brain barriers with a specific focus on describing the molecular constituents of their junctional complexes. We propose that the brain barriers maintain CNS immune privilege by dividing the CNS into compartments that differ with regard to their role in immune surveillance of the CNS. We close by providing a brief overview on experimental tools allowing for reliable in vivo visualization of the brain barriers and their junctional complexes and thus the respective CNS compartments.

Activation of Skeletal Stem and Progenitor Cells for Bone Regeneration Is Driven by PDGFRβ Signaling

Bone repair and regeneration critically depend on the activation and recruitment of osteogenesis-competent skeletal stem and progenitor cells (SSPCs). Yet, the origin and triggering cues for SSPC propagation and migration remain largely elusive. Through bulk and single-cell transcriptome profiling of fetal osterix (Osx)-expressing cells, followed by lineage mapping, cell tracing, and conditional mouse mutagenesis, we here identified PDGF-PDGFRβ signaling as critical functional mediator of SSPC expansion, migration, and angiotropism during bone repair. Our data show that cells marked by a history of Osx expression, including those arising in fetal or early postnatal periods, represent or include SSPCs capable of delivering all the necessary differentiated progeny to repair acute skeletal injuries later in life, provided that they express functional PDGFRβ. Mechanistically, MMP-9 and VCAM-1 appear to be involved downstream of PDGF-PDGFRβ. Our results reveal considerable cellular dynamism in the skeletal system and show that activation and recruitment of SSPCs for bone repair require functional PDGFRβ signaling.

Bcl-2 Expression in Pericytes and Astrocytes Impacts Vascular Development and Homeostasis

B-cell lymphoma 2 (Bcl-2) protein is the founding member of a group of proteins known to modulate apoptosis. Its discovery set the stage for identification of family members with either pro- or anti-apoptotic properties. Expression of Bcl-2 plays an important role during angiogenesis by influencing not only vascular cell survival, but also migration and adhesion. Although apoptosis and migration are postulated to have roles during vascular remodeling and regression, the contribution of Bcl-2 continues to emerge. We previously noted that the impaired retinal vascularization and an inability to undergo pathologic neovascularization observed in mice globally lacking Bcl-2 did not occur when mice lacked the expression of Bcl-2 only in endothelial cells. To further examine the effect of Bcl-2 expression during vascularization of the retina, we assessed its contribution in pericytes or astrocytes by generating mice with a conditional Bcl-2 allele (Bcl-2Flox/Flox) and Pdgfrb-cre (Bcl-2PC mice) or Gfap-cre (Bcl-2AC mice). Bcl-2PC and Bcl-2AC mice demonstrated increased retinal vascular cell apoptosis, reduced numbers of pericytes and endothelial cells and fewer arteries and veins in the retina. Bcl-2PC mice also demonstrated delayed advancement of the superficial retinal vascular layer and aberrant vascularization of the deep vascular plexus and central retina. Although pathologic neovascularization in oxygen-induced ischemic retinopathy (OIR) was not affected by lack of expression of Bcl-2 in either pericytes or astrocytes, laser-induced choroidal neovascularization (CNV) was significantly reduced in Bcl-2PC mice compared to littermate controls. Together these studies begin to reveal how cell autonomous modulation of apoptosis in vascular cells impacts development and homeostasis.

Pericyte loss leads to circulatory failure and pleiotrophin depletion causing neuron loss

Pericytes are positioned between brain capillary endothelial cells, astrocytes and neurons. They degenerate in multiple neurological disorders. However, their role in the pathogenesis of these disorders remains debatable. Here we generate an inducible pericyte-specific Cre line and cross pericyte-specific Cre mice with iDTR mice carrying Cre-dependent human diphtheria toxin receptor. After pericyte ablation with diphtheria toxin, mice showed acute blood-brain barrier breakdown, severe loss of blood flow, and a rapid neuron loss that was associated with loss of pericyte-derived pleiotrophin (PTN), a neurotrophic growth factor. Intracerebroventricular PTN infusions prevented neuron loss in pericyte-ablated mice despite persistent circulatory changes. Silencing of pericyte-derived Ptn rendered neurons vulnerable to ischemic and excitotoxic injury. Our data demonstrate a rapid neurodegeneration cascade that links pericyte loss to acute circulatory collapse and loss of PTN neurotrophic support. These findings may have implications for the pathogenesis and treatment of neurological disorders that are associated with pericyte loss and/or neurovascular dysfunction.

Dynamic Remodeling of Pericytes In Vivo Maintains Capillary Coverage in the Adult Mouse Brain

Direct contact and communication between pericytes and endothelial cells is critical for maintenance of cerebrovascular stability and blood-brain barrier function. Capillary pericytes have thin processes that reach hundreds of micrometers along the capillary bed. The processes of adjacent pericytes come in close proximity but do not overlap, yielding a cellular chain with discrete territories occupied by individual pericytes. Little is known about whether this pericyte chain is structurally dynamic in the adult brain. Using in vivo two-photon imaging in adult mouse cortex, we show that while pericyte somata were immobile, the tips of their processes underwent extensions and/or retractions over days. The selective ablation of single pericytes provoked exuberant extension of processes from neighboring pericytes to contact uncovered regions of the endothelium. Uncovered capillary regions had normal barrier function but were dilated until pericyte contact was regained. Pericyte structural plasticity may be critical for cerebrovascular health and warrants detailed investigation.

Organizational hierarchy and structural diversity of microvascular pericytes in adult mouse cortex

Smooth muscle cells and pericytes, together called mural cells, coordinate many distinct vascular functions. Canonically, smooth muscle cells are ring-shaped and cover arterioles with circumferential processes, whereas pericytes extend thin processes that run longitudinally along capillaries. In between these canonical mural cell types are cells with features of both smooth muscle cells and pericytes. Recent studies suggest that these transitional cells are critical for controlling blood flow to the capillary bed during health and disease, but there remains confusion on how to identify them and where they are located in the brain microvasculature. To address this issue, we measured the morphology, vascular territory, and α-smooth muscle actin content of structurally diverse mural cells in adult mouse cortex. We first imaged intact 3D vascular networks to establish the locations of major gradations in mural cell appearance as arterioles branched into capillaries. We then imaged individual mural cells occupying the regions within these gradations. This revealed two transitional cells that were often similar in appearance, but with sharply contrasting levels of α-smooth muscle actin. Our findings highlight the diversity of mural cell morphologies in brain microvasculature, and provide guidance for identification and categorization of mural cell types.

Targeted deletion of Cyp1b1 in pericytes results in attenuation of retinal neovascularization and trabecular meshwork dysgenesis

Mutations in cytochrome P450 1B1 (CYP1B1) gene are reported in patients with primary congenital glaucoma. Cyp1b1-deficient (Cyp1b1-/-) mice show dysgenesis of the trabecular meshwork (TM) tissue and attenuation of retinal neovascularization during oxygen-induced ischemic retinopathy (OIR). Although retinal vascular cells, including endothelial cells (EC), pericytes (PC), astrocytes (AC), and TM endothelial cells express CYP1B1, the cell autonomous contribution of CYP1B1 to attenuation of retinal neovascularization and TM tissue dysgenesis remains unknown. Here we determined the impact lack of CYP1B1 expression in EC, PC or AC has on retinal neovascularization and TM tissue integrity. We generated Cyp1b1-transgenic mice with vascular cell-specific targeted Cre⁺-deletion in EC (Cyp1b1 ^EC), in PC (Cyp1b1 ^PC) and in AC (Cyp1b1 ^AC). Pathologic retinal neovascularization during OIR was evaluated by collagen IV staining of retinal wholemounts. Structural morphology of TM tissue was examined by transmission electron microscopy (TEM). The assessment of retinal neovascularization indicated a significant decrease in retinal neovascular tufts only in Cyp1b1 ^PC mice compared with control mice. TEM evaluation demonstrated Cyp1b1 ^PC mice also exhibited a defect in TM tissue morphology and integrity similar to that reported in Cyp1b1-/- mice. Thus, Cyp1b1 expression in PC plays a significant role in retinal neovascularization and the integrity of TM tissue.