Connecting Metainflammation and Neuroinflammation Through the PTN-MK-RPTPβ/ζ Axis: Relevance in Therapeutic Development

Inflammation is a common factor of pathologies such as obesity, type 2 diabetes or neurodegenerative diseases. Chronic inflammation is considered part of the pathogenic mechanisms of different disorders associated with aging. Interestingly, peripheral inflammation and the associated metabolic alterations not only facilitate insulin resistance and diabetes but also neurodegenerative disorders. Therefore, the identification of novel pathways, common to the development of these diseases, which modulate the immune response and signaling is key. It will provide highly relevant information to advance our knowledge of the multifactorial process of aging, and to establish new biomarkers and/or therapeutic targets to counteract the underlying chronic inflammatory processes. One novel pathway that regulates peripheral and central immune responses is triggered by the cytokines pleiotrophin (PTN) and midkine (MK), which bind its receptor, Receptor Protein Tyrosine Phosphatase (RPTP) β/ζ, and inactivate its phosphatase activity. In this review, we compile a growing body of knowledge suggesting that PTN and MK modulate the immune response and/or inflammation in different pathologies characterized by peripheral inflammation associated with insulin resistance, such as aging, and in central disorders characterized by overt neuroinflammation, such as neurodegenerative diseases and endotoxemia. Evidence strongly suggests that regulation of the PTN and MK signaling pathways may provide new therapeutic opportunities particularly in those neurological disorders characterized by increased PTN and/or MK cerebral levels and neuroinflammation. Importantly, we discuss existing therapeutics, and others being developed, that modulate these signaling pathways, and their potential use in pathologies characterized by overt neuroinflammation

Pleiotrophin overexpression regulates amphetamine-induced reward and striatal dopaminergic denervation without changing the expression of dopamine D1 and D2 receptors: Implications for neuroinflammation.

It was previously shown that mice with genetic deletion of the neurotrophic factor pleiotrophin (PTN-/-) show enhanced amphetamine neurotoxicity and impair extinction of amphetamine conditioned place preference (CPP), suggesting a modulatory role of PTN in amphetamine neurotoxicity and reward. We have now studied the effects of amphetamine (10mg/kg, 4 times, every 2h) in the striatum of mice with transgenic PTN overexpression (PTN-Tg) in the brain and in wild type (WT) mice. Amphetamine caused an enhanced loss of striatal dopaminergic terminals, together with a highly significant aggravation of amphetamine-induced increase in the number of GFAP-positive astrocytes, in the striatum of PTN-Tg mice compared to WT mice. Given the known contribution of D1 and D2 dopamine receptors to the neurotoxic effects of amphetamine, we also performed quantitative receptor autoradiography of both receptors in the brains of PTN-Tg and WT mice. D1 and D2 receptors binding in the striatum and other regions of interest was not altered by genotype or treatment. Finally, we found that amphetamine CPP was significantly reduced in PTN-Tg mice. The data demonstrate that PTN overexpression in the brain blocks the conditioning effects of amphetamine and enhances the characteristic striatal dopaminergic denervation caused by this drug. These results indicate for the first time deleterious effects of PTN in vivo by mechanisms that are probably independent of changes in the expression of D1 and D2 dopamine receptors. The data also suggest that PTN-induced neuroinflammation could be involved in the enhanced neurotoxic effects of amphetamine in the striatum of PTN-Tg mice

Metabolomics and biochemical alterations caused by pleiotrophin in the 6‑hydroxydopamine mouse model of Parkinson’s disease.

Pleiotrophin (PTN) is a cytokine involved in nerve tissue repair processes, neuroinflammation and neuronal survival. PTN expression levels are upregulated in the nigrostriatal pathway of Parkinson’s Disease (PD) patients. We aimed to characterize the dopaminergic injury and glial responses in the nigrostriatal pathway of mice with transgenic Ptn overexpression in the brain (Ptn-Tg) after intrastriatal injection of the catecholaminergic toxic 6-hydroxydopamine (6-OHDA) at a low dose (5 µg). Ten days after surgery, the injection of 6-OHDA induced a significant decrease of the number of tyrosine hydroxylase (TH)-positive neurons in the substantia nigra and of the striatal TH contents in Wild type (Wt) mice. In contrast, these effects of 6-OHDA were absent in Ptn-Tg mice. When the striatal Iba1 and GFAP immunoreactivity was studied, no statistical differences were found between vehicle-injected Wt and Ptn-Tg mice. Furthermore, 6-OHDA did not cause robust glial responses neither on Wt or Ptn-Tg mice 10 days after injections. In metabolomics studies, we detected interesting metabolites that significantly discriminate the more injured 6-OHDA-injected Wt striatum and the more protected 6-OHDA-injected Ptn-Tg striatum. Particularly, we detected groups of metabolites, mostly corresponding to phospholipids, whose trends were opposite in both groups. In summary, the data confirm lower 6-OHDA-induced decreases of TH contents in the nigrostriatal pathway of Ptn-Tg mice, suggesting a neuroprotective effect of brain PTN overexpression in this mouse model of PD. New lipid-related PD drug candidates emerge from this study and the data presented here support the increasingly recognized “lipid cascade” in PD.post-print2513 K

La Midkina es un nuevo regulador de los efectos analgésicos de la morfina a nivel espinal: Evidencias sobre la regulación transcripcional del receptor opioide kappa en ratones knockout de midkina.

Midkine (MK) is a growth factor that exhibits neurotrophic actions and is upregulated at sites of nerve injury. It has been shown that morphine administration significantly regulates MK levels within the brain, suggesting MK could play a role in morphine-induced pharmacological effects. To test this hypothesis, we have now studied morphine-induced antinociceptive effects in MK genetically deficient (MK-/-) and wild type (WT+/+) mice. We found that basal pain responses do not differ between MK-/- and WT+/+ mice in the hot-plate and tail immersion tests, suggesting MK is not involved in the regulation of nociceptive transmission at the supraspinal and spinal levels. We did not find differences among genotypes using different doses of morphine (2.5, 5 and 10 mg/kg) in the hot-plate test. In contrast, we found that morphine (5 mg/kg) significantly delayed pain responses in MK-/- mice compared to WT+/+ mice in the tail-immersion test, an effect that greatly correlates with a significant increase in the levels of expression of the κ-opioid receptor in the dorsal root ganglia (DRG) of MK-/- mice compared to WT+/+ mice in normal condition. The data strongly suggest that MK is an endogenous modulator of morphine antinociceptive effects at the spinal, but not at the supraspinal, level. The data support the hypothesis that concomitant administration of known midkine inhibitors and morphine could result in potentiation of the opioid spinal antinoceptive effects which may be of critical importance in patients that are unresponsive to opioid analgesia in palliative careLa Midkina (MK) es un factor de crecimiento conocido por sus acciones neurotróficas y que se encuentra sobre-expresado en los lugares donde acontece daño neuronal. La administración de morfina regula los niveles de expresión cerebrales de MK lo que sugiere que la MK podría modular los efectos farmacológicos de la morfina. Para probar esta hipótesis, hemos estudiado los efectos antinociceptivos de la morfina en ratones knockout de MK (MK-/-) y en ratones salvajes (WT+/+). Hemos comprobado que las respuestas basales al dolor en ambos genotipos son semejantes tanto en el test de la placa caliente como en el test de la retirada de la cola, lo que sugiere que la MK no es un factor fundamental para la regulación de la transmisión dolorosa a nivel supraespinal, ni a nivel espinal. Tampoco encontramos diferencias en los efectos analgésicos de la morfina en el test de la placa caliente con ninguna de las dosis que utilizamos (2.5, 5 and 10 mg/kg). Sin embargo, la morfina a la dosis de 5 mg/Kg provocó un efecto analgésico significativamente mayor en ratones MK-/- en el test de la retirada de la cola. Este mayor efecto de la morfina podría estar relacionado con el aumento significativo de los niveles de expresión del receptor opioide κ en los ganglios dorsales de ratones MK-/- comparado con los ratones WT+/+ en condiciones normales. Los datos sugieren que la MK es un nuevo regulador endógeno de los efectos antinociceptivos de la morfina a nivel espinal, pero no a nivel supraespinal. Además, los resultados apoyan la hipótesis de que la administración conjunta de inhibidores conocidos de la MK y morfina podría resultar en una potenciación de los efectos analgésicos del opiáceo lo cual podría resultar importante para el tratamiento de pacientes no respondedores a los opiáceo

Midkine-a Protein Localization in the Developing and Adult Retina of the Zebrafish and Its Function During Photoreceptor Regeneration

<div><p>Midkine is a heparin binding growth factor with important functions in neuronal development and survival, but little is known about its function in the retina. Previous studies show that in the developing zebrafish, Midkine-a (Mdka) regulates cell cycle kinetics in retinal progenitors, and following injury to the adult zebrafish retina, <i>mdka</i> is strongly upregulated in Müller glia and the injury-induced photoreceptor progenitors. Here we provide the first data describing Mdka protein localization during different stages of retinal development and during the regeneration of photoreceptors in adults. We also experimentally test the role of Mdka during photoreceptor regeneration. The immuno-localization of Mdka reflects the complex spatiotemporal pattern of gene expression and also reveals the apparent secretion and extracellular trafficking of this protein. During embryonic retinal development the Mdka antibodies label all mitotically active cells, but at the onset of neuronal differentiation, immunostaining is also localized to the nascent inner plexiform layer. Starting at five days post fertilization through the juvenile stage, Mdka immunostaining labels the cytoplasm of horizontal cells and the overlying somata of rod photoreceptors. Double immunolabeling shows that in adult horizontal cells, Mdka co-localizes with markers of the Golgi complex. Together, these data are interpreted to show that Mdka is synthesized in horizontal cells and secreted into the outer nuclear layer. In adults, Mdka is also present in the end feet of Müller glia. Similar to <i>mdka</i> gene expression, Mdka in horizontal cells is regulated by circadian rhythms. After the light-induced death of photoreceptors, Mdka immuonolabeling is localized to Müller glia, the intrinsic stem cells of the zebrafish retina, and proliferating photoreceptor progenitors. Knockdown of Mdka during photoreceptor regeneration results in less proliferation and diminished regeneration of rod photoreceptors. These data suggest that during photoreceptor regeneration Mdka regulates aspects of injury-induced cell proliferation.</p></div

Midkine Is a Novel Regulator of Amphetamine-Induced Striatal Gliosis and Cognitive Impairment: Evidence for a Stimulus-Dependent Regulation of Neuroinflammation by Midkine

Midkine (MK) is a cytokine that modulates amphetamine-induced striatal astrogliosis, suggesting a possible role of MK in neuroinflammation induced by amphetamine. To test this hypothesis, we studied astrogliosis and microglial response induced by amphetamine (10 mg/kg i.p. four times, every 2 h) in different brain areas of MK−/− mice and wild type (WT) mice. We found that amphetamine-induced microgliosis and astrocytosis are enhanced in the striatum of MK−/− mice in a region-specific manner. Surprisingly, LPS-induced astrogliosis in the striatum was blocked in MK−/− mice. Since striatal neuroinflammation induced by amphetamine-type stimulants correlates with the cognitive deficits induced by these drugs, we also tested the long-term effects of periadolescent amphetamine treatment (3 mg/kg i.p. daily for 10 days) in a memory task in MK−/− and WT mice. Significant deficits in the Y-maze test were only observed in amphetamine-pretreated MK−/− mice. The data demonstrate for the first time that MK is a novel modulator of neuroinflammation depending on the inflammatory stimulus and the brain area considered. The data indicate that MK limits amphetamine-induced striatal neuroinflammation. In addition, our data demonstrate that periadolescent amphetamine treatment in mice results in transient disruption of learning and memory processes in absence of endogenous MK

Mdka knockdown reduces the number of injury-induced progenitors.

<p>Progenitors were labeled with BrdU in the retinas of control (panel A) and experimental fish (panel B) at 3 dpl. Note the radial clusters of progenitors in the control retinas (arrows in A) and their relative absence in experimental retinas (panel B). BrdU-positive cells were counted, and this is graphically represented in panel C. INL: inner nuclear layer; GCL: ganglion cell layer; BrdU: Bromodeoxyuridine. Scale bar = 25 μm. ** p<0.01.</p

Mdka protein localization following photoreceptor ablation.

<p>In unlesioned retinas, Mdka immunostaining is localized to the horizontal cells and endfeet of glutamine synthetase (GS)-positive Müller glia (row A). At 4 dpl, Mdka antibodies label the radial processes of Müller glia (row B, arrows). Note the increased Mdka immunostaining in the endfeet of the Müller glia in lesioned retinas (cf. rows A and B). Also at 4 dpl, Mdka immunostaining is localized to each of the EdU-positive nuclei in both the INL and ONL (row C, arrowheads). ONL: outer nuclear layer; INL: inner nuclear layer; dpl: days post lesion. Scale bars = 25 μm.</p