A new function for Prokineticin 2: Recruitment of SVZ-derived neuroblasts to the injured cortex in a mouse model of traumatic brain injury

Olfactory Development and Dysfunction: Involvement of Microglia

Olfactory deficits are increasingly recognized in a variety of neurological, neurodevelopmental, psychiatric, and viral diseases. While the pathology underlying olfactory loss is likely to differ across diseases, one shared feature may be an immune response mediated by microglia. Microglia orchestrate the brain's response to environmental insults and maintain neurodevelopmental homeostasis. Here, we explore the potential involvement of microglia in olfactory development and loss in disease. The effects of microglia-mediated immune response during development may be of special relevance to the olfactory system, which is unique in both its vulnerability to environmental insults as well as its extended period of neurogenesis and neuronal migration.

Therapeutic Potential of Targeting Prokineticin Receptors in Diseases

The prokineticins (PKs) were discovered approximately 20 years ago as small peptides inducing gut contractility. Today, they are established as angiogenic, anorectic, and proinflammatory cytokines, chemokines, hormones, and neuropeptides involved in variety of physiologic and pathophysiological pathways. Their altered expression or mutations implicated in several diseases make them a potential biomarker. Their G-protein coupled receptors, PKR1 and PKR2, have divergent roles that can be therapeutic target for treatment of cardiovascular, metabolic, and neural diseases as well as pain and cancer. This article reviews and summarizes our current knowledge of PK family functions from development of heart and brain to regulation of homeostasis in health and diseases. Finally, the review summarizes the established roles of the endogenous peptides, synthetic peptides and the selective ligands of PKR1 and PKR2, and nonpeptide orthostatic and allosteric modulator of the receptors in preclinical disease models. The present review emphasizes the ambiguous aspects and gaps in our knowledge of functions of PKR ligands and elucidates future perspectives for PK research. SIGNIFICANCE STATEMENT: This review provides an in-depth view of the prokineticin family and PK receptors that can be active without their endogenous ligand and exhibits "constitutive" activity in diseases. Their non- peptide ligands display promising effects in several preclinical disease models. PKs can be the diagnostic biomarker of several diseases. A thorough understanding of the role of prokineticin family and their receptor types in health and diseases is critical to develop novel therapeutic strategies with safety concerns.

Effects of Prokineticins on Cerebral Cell Function and Blood-Brain Barrier Permeability

Prokineticins are a family of small proteins with diverse roles in various tissues, including the brain. However, their specific effects on different cerebral cell types and blood-brain barrier (BBB) function remain unclear. The aim of this study was to investigate the effects of PROK1 and PROK2 on murine cerebral cell lines, bEnd.3, C8.D30, and N2a, corresponding to microvascular endothelial cells, astrocytes and neurons, respectively, and on an established BBB co-culture model. Western blot analysis showed that prokineticin receptors (PROKR1 and PROKR2) were differentially expressed in the considered cell lines. The effect of PROK1 and PROK2 on cell proliferation and migration were assessed using time-lapse microscopy. PROK1 decreased neural cells' proliferation, while it had no effect on the proliferation of endothelial cells and astrocytes. In contrast, PROK2 reduced the proliferation of all cell lines tested. Both PROK1 and PROK2 increased the migration of all cell lines. Blocking PROKRs with the PROKR1 antagonist (PC7) and the PROKR2 antagonist (PKR-A) inhibited astrocyte PROK2-mediated migration. Using the insert co-culture model of BBB, we demonstrated that PROKs increased BBB permeability, which could be prevented by PROKRs' antagonists.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

Astrocyte and Oligodendrocyte Responses From the Subventricular Zone After Injury

Under normal conditions, neural stem cells (NSCs or B cells) in the adult subventricular zone (SVZ) give rise to amplifying neural progenitor cells (NPCs or C cells), which can produce neuroblasts (or A cells) that migrate to the olfactory bulb and differentiate into new neurons. However, following brain injury, these cells migrate toward the injury site where they differentiate into astrocytes and oligodendrocytes. In this review, we will focus on recent findings that chronicle how astrocytes and oligodendrocytes derived from SVZ-NSCs respond to different types of injury. We will also discuss molecular regulators of SVZ-NSC proliferation and their differentiation into astrocytes and oligodendrocytes. Overall, the goal of this review is to highlight how SVZ-NSCs respond to injury and to summarize the regulatory mechanisms that oversee their glial response. These molecular and cellular processes will provide critical insights needed to develop strategies to promote brain repair following injury using SVZ-NSCs.

Prokineticin-2 prevents neuronal cell deaths in a model of traumatic brain injury

Prokineticin-2 (Prok2) is an important secreted protein likely involved in the pathogenesis of several acute and chronic neurological diseases through currently unidentified regulatory mechanisms. The initial mechanical injury of neurons by traumatic brain injury triggers multiple secondary responses including various cell death programs. One of these is ferroptosis, which is associated with dysregulation of iron and thiols and culminates in fatal lipid peroxidation. Here, we explore the regulatory role of Prok2 in neuronal ferroptosis in vitro and in vivo. We show that Prok2 prevents neuronal cell death by suppressing the biosynthesis of lipid peroxidation substrates, arachidonic acid-phospholipids, via accelerated F-box only protein 10 (Fbxo10)-driven ubiquitination, degradation of long-chain-fatty-acid-CoA ligase 4 (Acsl4), and inhibition of lipid peroxidation. Mice injected with adeno-associated virus-Prok2 before controlled cortical impact injury show reduced neuronal degeneration and improved motor and cognitive functions, which could be inhibited by Fbxo10 knockdown. Our study shows that Prok2 mediates neuronal cell deaths in traumatic brain injury via ferroptosis.

Leukemia Inhibitory Factor Is Required for Subventricular Zone Astrocyte Progenitor Proliferation and for Prokineticin-2 Production after a Closed Head Injury in Mice

Astrogliosis is one of the hallmarks of brain injury, and after a mild injury activated astrocytes subserve neuroprotective and pro-regenerative functions. We previously found that the astroglial response to closed head injury (CHI) was blunted in mice that were haplodeficient in leukemia inhibitory factor (LIF); therefore, the goal of these studies was to determine if the delayed astrogliosis was due to decreased recruitment of subventricular zone (SVZ) progenitors. CHI's were performed on post-natal day 20 on LIF heterozygous (Het) and wild-type (WT) mice. At 48 h post-CHI, astrocyte progenitor proliferation within the SVZ increased ∼250% in WT mice but was reduced by ∼200% in LIF Het mice compared with sham controls. Using neurospheres to model the SVZ, LIF increased the percentage of proliferating astrocyte progenitors by 2-fold compared with controls but had no effect on neural stem cell proliferation. To rule out the involvement of other cytokines, 105 cytokines were analyzed using a multi-plex array and with targeted validation on injured LIF Het versus WT neocortex. Of the cytokines analyzed, only prokineticin-2 (ProK2) required LIF signaling. Correspondingly, LIF-treated neurospheres expressed higher levels of ProK2, the ProK1 and ProK2 receptors (ProKR1 and ProKR2). Using in situ hybridization, ProK2 messenger RNA (mRNA) was most abundant in neocortical neurons and highly expressed within the SVZ. However, in contrast to LIF, ProK2 decreased astrocyte progenitor proliferation 2-fold. Altogether, these data demonstrate that LIF is necessary for astrocyte progenitor proliferation after injury and reveal a new role for LIF as an essential regulator of the neurotrophic factor ProK2.

Notch1 and Galectin-3 Modulate Cortical Reactive Astrocyte Response After Brain Injury

After a brain lesion, highly specialized cortical astrocytes react, supporting the closure or replacement of the damaged tissue, but fail to regulate neural plasticity. Growing evidence indicates that repair response leads astrocytes to reprogram, acquiring a partially restricted regenerative phenotype in vivo and neural stem cells (NSC) hallmarks in vitro. However, the molecular factors involved in astrocyte reactivity, the reparative response, and their relation to adult neurogenesis are poorly understood and remain an area of intense investigation in regenerative medicine. In this context, we addressed the role of Notch1 signaling and the effect of Galectin-3 (Gal3) as underlying molecular candidates involved in cortical astrocyte response to injury. Notch signaling is part of a specific neurogenic microenvironment that maintains NSC and neural progenitors, and Gal3 has a preferential spatial distribution across the cortex and has a central role in the proliferative capacity of reactive astrocytes. We report that in vitro scratch-reactivated cortical astrocytes from C57Bl/6J neonatal mice present nuclear Notch1 intracellular domain (NICD1), indicating Notch1 activation. Colocalization analysis revealed a subpopulation of reactive astrocytes at the lesion border with colocalized NICD1/Jagged1 complexes compared with astrocytes located far from the border. Moreover, we found that Gal3 increased intracellularly, in contrast to its extracellular localization in non-reactive astrocytes, and NICD1/Gal3 pattern distribution shifted from diffuse to vesicular upon astrocyte reactivation. In vitro, Gal3^–/– reactive astrocytes showed abolished Notch1 signaling at the lesion core. Notch1 receptor, its ligands (Jagged1 and Delta-like1), and Hes5 target gene were upregulated in C57Bl/6J reactive astrocytes, but not in Gal3^–/– reactive astrocytes. Finally, we report that Gal3^–/– mice submitted to a traumatic brain injury model in the somatosensory cortex presented a disrupted response characterized by the reduced number of GFAP reactive astrocytes, with smaller cell body perimeter and decreased NICD1 presence at the lesion core. These results suggest that Gal3 might be essential to the proper activation of Notch signaling, facilitating the cleavage of Notch1 and nuclear translocation of NICD1 into the nucleus of reactive cortical astrocytes. Additionally, we hypothesize that reactive astrocyte response could be dependent on Notch1/Jagged1-Hes5 signaling activation following brain injury.

A1/A2 astrocytes in central nervous system injuries and diseases: Angels or devils?

Astrocytes play a pivotal role in maintaining the central nervous system (CNS) homeostasis and function. In response to CNS injuries and diseases, reactive astrocytes are triggered. By purifying and genetically profiling reactive astrocytes, it has been now found that astrocytes can be activated into two polarization states: the neurotoxic or pro-inflammatory phenotype (A1) and the neuroprotective or anti-inflammatory phenotype (A2). Although the simple dichotomy of the A1/A2 phenotypes does not reflect the wide range of astrocytic phenotypes, it facilitates our understanding of the reactive state of astrocytes in various CNS disorders. This article reviews the recent evidences regarding A1/A2 astrocytes, including (a) the specific markers and morphological characteristics, (b) the effects of A1/A2 astrocytes on the neurovascular unit, and (c) the molecular mechanisms involved in the phenotypic switch of astrocytes. Although many questions remain, a deeper understanding of different phenotypic astrocytes will eventually help us to explore effective strategies for neurological disorders by targeting astrocytes.

Neurorepair and Regeneration of the Brain: A Decade of Bioscaffolds and Engineered Microtissue

Repairing the human brain remains a challenge, despite the advances in the knowledge of inflammatory response to injuries and the discovery of adult neurogenesis. After brain injury, the hostile microenvironment and the lack of structural support for neural cell repopulation, anchoring, and synapse formation reduce successful repair chances. In the past decade, we witnessed the rise of studies regarding bioscaffolds’ use as support for neuro repair. A variety of natural and synthetic materials is available and have been used to replace damaged tissue. Bioscaffolds can assume different shapes and may or may not carry a diversity of content, such as stem cells, growth factors, exosomes, and si/miRNA that promote specific therapeutic effects and stimulate brain repair. The use of these external bioscaffolds and the creation of cell platforms provide the basis for tissue engineering. More recently, researchers were able to engineer brain organoids, neural networks, and even 3D printed neural tissue. The challenge in neural tissue engineering remains in the fabrication of scaffolds with precisely controlled topography and biochemical cues capable of directing and controlling neuronal cell fate. The purpose of this review is to highlight the existing research in the growing field of bioscaffolds’ development and neural tissue engineering. Moreover, this review also draws attention to emerging possibilities and prospects in this field.

Microglia induce neurogenic protein expression in primary cortical cells by stimulating PI3K/AKT intracellular signaling in vitro

Emerging evidence suggests that microglia can support neurogenesis. Little is known about the mechanisms by which microglia regulate the cortical environment and stimulate cortical neurogenesis. We used an in vitro co-culture model system to investigate the hypothesis that microglia respond to soluble signals from cortical cells, particularly following mechanical injury, to alter the cortical environment and promote cortical cell proliferation, differentiation, and survival. Analyses of cortical cell proliferation, cell death, neurogenic protein expression, and intracellular signaling were performed on uninjured and injured cortical cells in co-culture with microglial cell lines. Microglia soluble cues enhanced cortical cell viability and proliferation cortical cells. Co-culture of injured cortical cells with microglia significantly reduced cell death of cortical cells. Microglial co-culture significantly increased Nestin + and α-internexin + cortical cells. Multiplex ELISA and RT-PCR showed decreased pro-inflammatory cytokine production by microglia co-cultured with injured cortical cells. Inhibition of AKT phosphorylation in cortical cells blocked microglial-enhanced cortical cell viability and expression of neurogenic markers in vitro. This in vitro model system allows for assessment of the effect of microglial-derived soluble signals on cortical cell viability, proliferation, and stages of differentiation during homeostasis or following mechanical injury. These data suggest that microglia cells can downregulate inflammatory cytokine production following activation by mechanical injury to enhance proliferation of new cells capable of neurogenesis via activation of AKT intracellular signaling. Increasing our understanding of the mechanisms that drive microglial-enhanced cortical neurogenesis during homeostasis and following injury in vitro will provide useful information for future primary cell and in vivo studies.

Leptin enhances adult neurogenesis and reduces pathological features in a transgenic mouse model of Alzheimer's disease

Alzheimer's disease (AD) is the most common dementia worldwide and is characterized by the presence of senile plaques by amyloid-beta (Aβ) and neurofibrillary tangles of hyperphosphorylated Tau protein. These changes lead to progressive neuronal degeneration and dysfunction, resulting in severe brain atrophy and cognitive deficits. With the discovery that neurogenesis persists in the adult mammalian brain, including brain regions affected by AD, studies of the use of neural stem cells (NSCs) for the treatment of neurodegenerative diseases to repair or prevent neuronal cell loss have increased. Here we demonstrate that leptin administration increases the neurogenic process in the dentate gyrus of the hippocampus as well as in the subventricular zone of lateral ventricles of adult and aged mice. Chronic treatment with leptin increased NSCs proliferation with significant effects on proliferation and differentiation of newborn cells. The expression of the long form of the leptin receptor, LepRb, was detected in the neurogenic niches by reverse qPCR and immunohistochemistry. Moreover, leptin modulated astrogliosis, microglial cell number and the formation of senile plaques. Additionally, leptin led to attenuation of Aβ-induced neurodegeneration and superoxide anion production as revealed by Fluoro-Jade B and dihydroethidium staining. Our study contributes to the understanding of the effects of leptin in the brain that may lead to the development of new therapies to treat Alzheimer's disease.

Prokineticin signaling in heart-brain developmental axis: Therapeutic options for heart and brain injuries

Heart and brain development occur simultaneously during the embryogenesis, and both organ development and injuries are interconnected. Early neuronal and cardiac injuries share mutual cellular events, such as angiogenesis and plasticity that could either delay disease progression or, in the long run, result in detrimental health effects. For this reason, the common mechanisms provide a new and previously undervalued window of opportunity for intervention. Because angiogenesis, cardiogenesis and neurogenesis are essential for the development and regeneration of the heart and brain, we discuss therein the role of prokineticin as an angiogenic neuropeptide in heart-brain development and injuries. We focus on the role of prokineticin signaling and the effect of drugs targeting prokineticin receptors in neuroprotection and cardioprotection, with a special emphasis on heart failure, neurodegenerativParkinson's disease and ischemic heart and brain injuries. Indeed, prokineticin triggers common pro-survival signaling pathway in heart and brain. Our review aims at stimulating researchers and clinicians in neurocardiology to focus on the role of prokineticin signaling in the reciprocal interaction between heart and brain. We hope to facilitate the discovery of new treatment strategies, acting in both heart and brain degenerative diseases.

Roles of Prokineticin 2 in Subarachnoid Hemorrhage-Induced Early Brain Injury via Regulation of Phenotype Polarization in Astrocytes

Previous studies have postulated that neuroinflammation can induce two different types of reactive astrocytes, A1 and A2. A1 astrocytes may be harmful, whereas A2 astrocytes may be protective. Specifically, prokineticin 2 (PK2) has been shown to regulate neuron-astrocyte signaling mechanism by promoting an alternative A2-protective phenotype in astrocytes. This study aimed to examine the role of PK2 in early brain injury (EBI) caused by subarachnoid hemorrhage (SAH). SAH-induced astrocytic activation was confirmed by Western blotting. We confirmed C3 and PTX3 as appropriate reactivity markers for discriminating A1 and A2 astrocytes, respectively. We also observed SAH-induced astrocytic activation in SAH patients. The increase of PK2 in neurons after SAH in both humans and rats suggested a possible relationship between PK2 and SAH pathology. PK2 knockdown promoted an A1 astrocytic phenotype with upregulation of neurodegenerative indicators, while intravascular injection of recombinant PK2 (rPK2) promoted A2 astrocytic phenotype and reduced SAH-induced neuronal injury and behavioral dysfunction. Finally, we identified that tumor necrosis factor alpha (TNF-α) was sufficient to elevate the protein level of PK2 in neurons and enhance astrocytic activation in vitro. Moreover, rPK2 selectively promoted astrocytic polarization to an A2 phenotype under a TNF-α stimulus and induced phosphorylation of signal transducer and activator of transcription 3 (STAT3), suggesting that SAH-induced increases in PK2 may function as an endogenous mechanism for self-repair. Collectively, our findings support that enhancing PK2 expression or administration of rPK2 may induce a selective modulation of astrocytic polarization to a protective phenotype following SAH-like stimuli.

Postnatal neuronal migration in health and disease

Postnatal neuronal migration modulates neuronal circuit formation and function throughout life and is conserved among species. Pathological conditions activate the generation of neuroblasts in the ventricular-subventricular zone (V-SVZ) and promote their migration towards a lesion. However, the neuroblasts generally terminate their migration before reaching the lesion site unless their intrinsic capacity is modified or the environment is improved. It is important to understand which factors impede neuronal migration for functional recovery of the brain. We highlight similarities and differences in the mechanisms of neuroblast migration under physiological and pathological conditions to provide novel insights into endogenous neuronal regeneration.

Tissue Engineering and Biomaterial Strategies to Elicit Endogenous Neuronal Replacement in the Brain

Neurogenesis in the postnatal mammalian brain is known to occur in the dentate gyrus of the hippocampus and the subventricular zone. These neurogenic niches serve as endogenous sources of neural precursor cells that could potentially replace neurons that have been lost or damaged throughout the brain. As an example, manipulation of the subventricular zone to augment neurogenesis has become a popular strategy for attempting to replace neurons that have been lost due to acute brain injury or neurodegenerative disease. In this review article, we describe current experimental strategies to enhance the regenerative potential of endogenous neural precursor cell sources by enhancing cell proliferation in neurogenic regions and/or redirecting migration, including pharmacological, biomaterial, and tissue engineering strategies. In particular, we discuss a novel replacement strategy based on exogenously biofabricated "living scaffolds" that could enhance and redirect endogenous neuroblast migration from the subventricular zone to specified regions throughout the brain. This approach utilizes the first implantable, biomimetic tissue-engineered rostral migratory stream, thereby leveraging the brain's natural mechanism for sustained neuronal replacement by replicating the structure and function of the native rostral migratory stream. Across all these strategies, we discuss several challenges that need to be overcome to successfully harness endogenous neural precursor cells to promote nervous system repair and functional restoration. With further development, the diverse and innovative tissue engineering and biomaterial strategies explored in this review have the potential to facilitate functional neuronal replacement to mitigate neurological and psychiatric symptoms caused by injury, developmental disorders, or neurodegenerative disease.

Neuroregenerative and protective functions of Leukemia Inhibitory Factor in perinatal hypoxic-ischemic brain injury

Neonatal hypoxic-ischemic encephalopathy remains the most important neurological problem of the newborn. Delays in diagnosing perinatal brain injuries are common, preventing access to acute therapies. Therefore, there is a critical need for therapeutic strategies that are beneficial when delivered beyond 24 h after birth. Here we show that Leukemia Inhibitory Factor (LIF) functions as an essential injury-induced neurotrophic cytokine in the CNS and that non-invasively administering LIF as late as 3 days after a hypoxic-ischemic insult improves neurological function. Using a mouse model of late preterm brain injury we show that astroglial and microglial/macrophage reactivity to hypoxia-ischemia was diminished at 3 days of recovery, but then exacerbated at 2 weeks of recovery in LIF haplodeficient mice. There also were significantly more CD68+/Iba-1+ cells in the ipsilateral striatum in LIF-Het mice compared to WT mice at 2 weeks of recovery. This desynchronized glial response was accompanied by increased neuronal cell death in the striatum and neocortex (Fluorojade C), hypomyelination (reduced MBP staining and thinner external capsule), increased extent of brain damage (Nissl) and diminished neurological function on sensorimotor tests. To our surprise, injured LIF-Het mice had ~7-fold higher IGF-1 levels than injured WT mice at 3 days after HI injury. Intranasally administered LIF activated the Jak-Stat-3 pathway both within the subventricular zone and the neocortex at 30 min after administration. When delivered with a delay of 3 days after the insult, intranasal LIF reduced the extent of brain injury by ~60%, attenuated astrogliosis and microgliosis in striatum, improved subcortical white matter thickness, increased numbers of Olig2+ cells in corpus callosum and improved performance on sensorimotor tests at 2 weeks of recovery. These studies provide key pre-clinical data recommending LIF administration as a neuroprotectant and regenerative cytokine and they highlight the feasibility of pursuing new therapeutics targeting the tertiary phase of neurodegeneration for hypoxic-ischemic encephalopathies.

Tracking Neural Progenitor Cell Migration in the Rodent Brain Using Magnetic Resonance Imaging

The study of neurogenesis and neural progenitor cells (NPCs) is important across the biomedical spectrum, from learning about normal brain development and studying disease to engineering new strategies in regenerative medicine. In adult mammals, NPCs proliferate in two main areas of the brain, the subventricular zone (SVZ) and the subgranular zone, and continue to migrate even after neurogenesis has ceased within the rest of the brain. In healthy animals, NPCs migrate along the rostral migratory stream (RMS) from the SVZ to the olfactory bulb, and in diseased animals, NPCs migrate toward lesions such as stroke and tumors. Here we review how MRI-based cell tracking using iron oxide particles can be used to monitor and quantify NPC migration in the intact rodent brain, in a serial and relatively non-invasive fashion. NPCs can either be labeled directly in situ by injecting particles into the lateral ventricle or RMS, where NPCs can take up particles, or cells can be harvested and labeled in vitro, then injected into the brain. For in situ labeling experiments, the particle type, injection site, and image analysis methods have been optimized and cell migration toward stroke and multiple sclerosis lesions has been investigated. Delivery of labeled exogenous NPCs has allowed imaging of cell migration toward more sites of neuropathology, which may enable new diagnostic and therapeutic opportunities for as-of-yet untreatable neurological diseases.