Phuong B. Tran

Chemokines Regulate the Migration of Neural Progenitors to Sites of Neuroinflammation

Many studies have shown that transplanted or endogenous neural progenitor cells will migrate toward damaged areas of the brain. However, the mechanism underlying this effect is not clear. Here we report that, using hippocampal slice cultures, grafted neural progenitor cells (NPs) migrate toward areas of neuroinflammation and that chemokines are a major regulator of this process. Migration of NPs was observed after injecting an inflammatory stimulus into the area of the fimbria and transplanting enhanced green fluorescent protein (EGFP)-labeled NPs into the dentate gyrus of cultured hippocampal slices. Three to 7 d after transplantation, EGFP–NPs in control slices showed little tendency to migrate and had differentiated into neurons and glia. In contrast, in slices injected with inflammatory stimuli, EGFP–NPs migrated toward the site of the injection. NPs in these slices also survived less well. The inflammatory stimuli used were a combination of the cytokines tumor necrosis factor-α and interferon-γ, the bacterial toxin lipopolysaccharide, the human immunodeficiency virus-1 coat protein glycoprotein 120, or a β-amyloid-expressing adenovirus. We showed that these inflammatory stimuli increased the synthesis of numerous chemokines and cytokines by hippocampal slices. When EGFP–NPs from CC chemokine receptor CCR2 knock-out mice were transplanted into slices, they exhibited little migration toward sites of inflammation. Similarly, wild-type EGFP–NPs exhibited little migration toward inflammatory sites when transplanted into slices prepared from monocyte chemoattractant protein-1 (MCP-1) knock-out mice. These data indicate that factors secreted by sites of neuroinflammation are attractive to neural progenitors and suggest that chemokines such as MCP-1 play an important role in this process.

Chemokine receptors: signposts to brain development and disease

During the development of the nervous system, populations of progenitor cells that eventually become neurons and glia face the complex task of finding their way from their place of birth to their final destinations. What are the molecular processes that provide the information for guiding progenitor cells along their way? In this article, we discuss recent information indicating that chemokines and their receptors are of great importance in directing the proliferation and migration of immature neurons, glia and their precursors. Furthermore, chemokine receptor function in the nervous system continues to be important throughout adult life, and chemokines participate in various brain disorders, including AIDS dementia, neuroinflammatory disease and neuroplasia, making them important potential therapeutic targets in these cases.

Chemokine receptors are expressed widely by embryonic and adult neural progenitor cells.

We investigated the expression and functions of chemokine receptors in neural progenitor cells isolated from embryonic and adult mice. Reverse transcriptase‐polymerase chain reaction (RT‐PCR) analysis demonstrated mRNA expression for most known chemokine receptors in neural progenitor cells grown as neurospheres from embryonic (E17) and adult (4‐week‐old) mice. The expression of CXCR4 receptors was demonstrated further in E17 neurospheres using immunohistochemistry, in situ hybridization, Northern blot analysis and fura‐2‐based Ca2+ imaging. Most neurospheres grown from E17 mice responded to stromal cell‐derived factor‐1 (SDF‐1/CXCL12) in Ca2+ imaging studies. In addition, immunohistochemical studies demonstrated that these neurospheres consisted of dividing cells that uniformly colocalized nestin and CXCR4 receptors. Differentiation of E17 neurospheres yielded astrocytes and neurons exhibiting several different phenotypes, including expression of calbindin, calretinin, gamma‐aminobutyric acid (GABA), and glutamate, and many also coexpressed CXCR4 receptors. In addition, neurospheres grown from the subventricular zone (SVZ) of 4‐week‐old mice exhibited large increases in Ca2+ in response to CXCL12 and several other chemokines. In comparison, neurospheres prepared from olfactory bulb of adult mice exhibited only small Ca2+ responses to CXCL12, whereas neurospheres prepared from hippocampus were insensitive to CXCL12, although they did respond to other chemokines. Investigations designed to investigate whether CXCL12 can act as a chemoattractant demonstrated that cells dissociated from E17 or adult SVZ neurospheres migrated toward an CXCL12 gradient and this was blocked by the CXCR4 antagonist AMD3100. These results illustrate widespread chemokine sensitivity of embryonic and adult neural progenitor cells and support the view that chemokines may be of general importance in control of progenitor cell migration in embryonic and adult brain.

Chemokine receptor expression by neural progenitor cells in neurogenic regions of mouse brain

We previously demonstrated that chemokine receptors are expressed by neural progenitors grown as cultured neurospheres. To examine the significance of these findings for neural progenitor function in vivo, we investigated whether chemokine receptors were expressed by cells having the characteristics of neural progenitors in neurogenic regions of the postnatal brain. Using in situ hybridization we demonstrated the expression of CCR1, CCR2, CCR5, CXCR3, and CXCR4 chemokine receptors by cells in the dentate gyrus (DG), subventricular zone of the lateral ventricle, and olfactory bulb. The pattern of expression for all of these receptors was similar, including regions where neural progenitors normally reside. In addition, we attempted to colocalize chemokine receptors with markers for neural progenitors. In order to do this we used nestin‐EGFP and TLX‐LacZ transgenic mice, as well as labeling for Ki67, a marker for dividing cells. In all three areas of the brain we demonstrated colocalization of chemokine receptors with these three markers in populations of cells. Expression of chemokine receptors by neural progenitors was further confirmed using CXCR4‐EGFP BAC transgenic mice. Expression of CXCR4 in the DG included cells that expressed nestin and GFAP as well as cells that appeared to be immature granule neurons expressing PSA‐NCAM, calretinin, and Prox‐1. CXCR4‐expressing cells in the DG were found in close proximity to immature granule neurons that expressed the chemokine SDF‐1/CXCL12. Cells expressing CXCR4 frequently coexpressed CCR2 receptors. These data support the hypothesis that chemokine receptors are important in regulating the migration of progenitor cells in postnatal brain. J. Comp. Neurol. 500:1007–1033, 2007.

The Chemokine Stromal Cell-Derived Factor-1 Regulates the Migration of Sensory Neuron Progenitors

Chemokines and their receptors are essential for the development and organization of the hematopoietic/lymphopoietic system and have now been shown to be expressed by different types of cells in the nervous system. In mouse embryos, we observed expression of the chemokine (CXC motif) receptor 4 (CXCR4) by neural crest cells migrating from the dorsal neural tube and in the dorsal root ganglia (DRGs). Stromal cell-derived factor-1 (SDF-1), the unique agonist for CXCR4, was expressed along the path taken by crest cells to the DRGs, suggesting that SDF-1/CXCR4 signaling is needed for their migration. CXCR4 null mice exhibited small and malformed DRGs. Delayed migration to the DRGs was suggested by ectopic cells expressing tyrosine receptor kinase A (TrkA) and TrkC, neurotrophin receptors required by DRG sensory neuron development. In vitro, the CXCR4 chemokine receptor was upregulated by migratory progenitor cells just as they exited mouse neural tube explants, and SDF-1 acted as a chemoattractant for these cells. Most CXCR4-expressing progenitors differentiated to form sensory neurons with the properties of polymodal nociceptors. Furthermore, DRGs contained a population of progenitor cells that expressed CXCR4 receptors in vitro and differentiated into neurons with a similar phenotype. Our findings indicate an important role for SDF-1/CXCR4 signaling in directing the migration of sensory neuron progenitors to the DRG and potentially in other aspects of development once the DRGs have coalesced.

CCR2 chemokine receptor signaling mediates pain in experimental osteoarthritis

Osteoarthritis is one of the leading causes of chronic pain, but almost nothing is known about the mechanisms and molecules that mediate osteoarthritis-associated joint pain. Consequently, treatment options remain inadequate and joint replacement is often inevitable. Here, we use a surgical mouse model that captures the long-term progression of knee osteoarthritis to longitudinally assess pain-related behaviors and concomitant changes in the innervating dorsal root ganglia (DRG). We demonstrate that monocyte chemoattractant protein (MCP)-1 (CCL2) and its high-affinity receptor, chemokine (C-C motif) receptor 2 (CCR2), are central to the development of pain associated with knee osteoarthritis. After destabilization of the medial meniscus, mice developed early-onset secondary mechanical allodynia that was maintained for 16 wk. MCP-1 and CCR2 mRNA, protein, and signaling activity were temporarily up-regulated in the innervating DRG at 8 wk after surgery. This result correlated with the presentation of movement-provoked pain behaviors, which were maintained up to 16 wk. Mice that lack Ccr2 also developed mechanical allodynia, but this started to resolve from 8 wk onwards. Despite severe allodynia and structural knee joint damage equal to wild-type mice, Ccr2-null mice did not develop movement-provoked pain behaviors at 8 wk. In wild-type mice, macrophages infiltrated the DRG by 8 wk and this was maintained through 16 wk after surgery. In contrast, macrophage infiltration was not observed in Ccr2-null mice. These observations suggest a key role for the MCP-1/CCR2 pathway in establishing osteoarthritis pain.

Aggregates in neurodegenerative disease: crowds and power?

Parkinsons disease is the second most common neurodegenerative disorder after Alzheimers disease34xRiess, O., Jakes, R., and Kruger, R. Mol. Med. Today. 1998; 4: 438–444Abstract | Full Text | Full Text PDF | PubMed | Scopus (27)See all References34. The disease is characterized by tremor, bradykinesia, rigidity and postural instability that results primarily from a loss of dopaminergic neurons of the nigro–striatal pathway. In addition to the loss of nigral neurons, PD is also characterized by the widespread distribution of Lewy bodies, which are intracytoplasmic aggregates 5–25 μm in diameter with a dense eosinophilic core and a pale surrounding ‘halo’. As with other types of aggregates discussed here, it has been suggested that Lewy bodies might have a causative role in neurodegeneration. It is worth noting that they are not completely specific for PD and are also found in association with several other types of neurodegenerative disorders, which include some cases of Alzheimers disease and dementia with Lewy bodies (DLB)34xRiess, O., Jakes, R., and Kruger, R. Mol. Med. Today. 1998; 4: 438–444Abstract | Full Text | Full Text PDF | PubMed | Scopus (27)See all References, 35xMezey, E. et al. Nat. Med. 1998; 4: 755–757Crossref | PubMed | Scopus (163)See all References. Patients with PD also exhibit Lewy neurites, which appear bloated with an unidentified proteinaceous material.It has been of great interest to establish the composition of Lewy bodies and neurites. Until recently, Lewy bodies were known to contain ubiquitin, neurofilaments and several other proteins35xMezey, E. et al. Nat. Med. 1998; 4: 755–757Crossref | PubMed | Scopus (163)See all References35. However, recent work on inherited forms of PD has added significantly to our understanding of their composition. As in the case of ALS, most cases of PD occur spontaneously. However, in a small percentage of cases, PD can be inherited in an autosomally dominant fashion34xRiess, O., Jakes, R., and Kruger, R. Mol. Med. Today. 1998; 4: 438–444Abstract | Full Text | Full Text PDF | PubMed | Scopus (27)See all References34. Polymeropoulos and colleagues36xPolymeropoulos, M.H. et al. Science. 1997; 276: 2045–2047Crossref | PubMed | Scopus (4389)See all References36 demonstrated that one form of familial PD was associated with a missense mutation in a protein called α-synuclein. Another mutation in the same protein has also subsequently been linked to familial PD (Ref. 34xRiess, O., Jakes, R., and Kruger, R. Mol. Med. Today. 1998; 4: 438–444Abstract | Full Text | Full Text PDF | PubMed | Scopus (27)See all ReferencesRef. 34). α-Synuclein is a 140 kDa protein that is homologous to two other gene products: β- and γ-synuclein37xClayton, D.F. and George, J.M. Trends Neurosci. 1998; 21: 249–254Abstract | Full Text | Full Text PDF | PubMed | Scopus (530)See all References37. The functions of all of these proteins are unknown. α-Synuclein is found in high concentrations in the nervous system, where it is primarily localized to nerve terminals, as well as other tissues. Curiously, one of the mutations in α-synuclein that has been linked to inherited PD (a Thr to Ala substitution at residue 53) occurs normally in certain species, for example, rats and some birds, that show no evidence of developing PD (Ref. 37xClayton, D.F. and George, J.M. Trends Neurosci. 1998; 21: 249–254Abstract | Full Text | Full Text PDF | PubMed | Scopus (530)See all ReferencesRef. 37). It has been suggested that these species do not live long enough to develop the disease, although other explanations are also possible. It was subsequently shown that α-synuclein was a major component of the Lewy bodies and Lewy neurites in PD as well as in DLB (38xSpillantini, M.G. et al. Nature. 1997; 388: 839–840Crossref | PubMed | Scopus (3326)See all References, 39xSpillantini, M.G. et al. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 6469–6473Crossref | PubMed | Scopus (1358)See all References). Other members of the synuclein family were absent. Significantly, α-synuclein has also been found in aggregates in other types of neurodegenerative disease. For example, a portion of the α-synuclein molecule known as the ‘non-amyloid component’ (NAC) has been detected in the neuritic plaques of patients with Alzheimers disease34xRiess, O., Jakes, R., and Kruger, R. Mol. Med. Today. 1998; 4: 438–444Abstract | Full Text | Full Text PDF | PubMed | Scopus (27)See all References, 35xMezey, E. et al. Nat. Med. 1998; 4: 755–757Crossref | PubMed | Scopus (163)See all References, 36xPolymeropoulos, M.H. et al. Science. 1997; 276: 2045–2047Crossref | PubMed | Scopus (4389)See all References. Interestingly, NAC has been shown to self aggregate in vitro37xClayton, D.F. and George, J.M. Trends Neurosci. 1998; 21: 249–254Abstract | Full Text | Full Text PDF | PubMed | Scopus (530)See all References37. In addition, α-synuclein-containing aggregates are found in glial cells in ‘multiple system atrophy’ and even in sporadic ALS (Ref. 35xMezey, E. et al. Nat. Med. 1998; 4: 755–757Crossref | PubMed | Scopus (163)See all ReferencesRef. 35). The latter observation seems of great interest in view of current speculation as to the role of glial dysfunction in ALS (Ref. 11xRothstein, J.D., Martin, L.J., and Kuncl, R.W. New Engl. J. Med. 1992; 326: 1464–1468Crossref | PubMedSee all ReferencesRef. 11). It has been suggested that the protein mutations observed in familial PD allow α-synuclein to aggregate more easily. In idiopathic PD, however, α-synuclein aggregation could be triggered by damage to the normal protein, through free-radical-mediated oxidation or some other mechanism. The widespread detection of α-synuclein in many types of aggregates in different neurodegenerative diseases has lead to the suggestion that it could be a common ‘seeding’ factor in initiating their formation.Recently, another type of gene mutation that is linked to familial PD has been discovered. Leroy et al.40xLeroy, E. et al. Nature. 1998; 395: 451–452Crossref | PubMed | Scopus (1149)See all References40 have reported that familial PD in a German family is not linked to a mutation in α-synuclein but in the enzyme ubiquitin carboxy-terminal hydrolase L1 (UCH-L1). Immunoreactivity for this protein has been previously associated with Lewy bodies, together with ubiquitin40xLeroy, E. et al. Nature. 1998; 395: 451–452Crossref | PubMed | Scopus (1149)See all References40. The enzyme, which is very abundant in the brain, normally hydrolyses the bonds between ubiquitin molecules or between ubiquitin and other molecules such as glutathione. The mutation detected by Leroy et al. leads to a decrease in the enzyme activity of UCH-L1. However, exactly how this produces PD is currently unclear, although a change in the state of ubiquitination of a key protein might be important. These results are analogous to those obtained from experiments on HD by Saudou et al. discussed above. However, it is also possible that the mutated UCH-L1 itself is prone to aggregate.The widespread detection of α-synuclein-related peptides in aggregates and the ability to manipulate aggregate formation via changes in ubiquitination are certainly important. However, the precise role of aggregates in each of the diseases discussed still remains to be defined. Answers will certainly be forthcoming from current studies on the emerging sociology of macromolecules.

Dopamine stimulation of postnatal murine subventricular zone neurogenesis via the D3 receptor

J. Neurochem. (2010) 114, 750–760.

Chemokine receptors in the brain: a developing story.

In this issue of the Journal of Comparative Neurology, Cowell and Silverstein describe the localization of CCR1 chemokine receptors in the cerebellum of neonatal rats. They show in detail that these receptors are expressed by a variety of cell types with an expression pattern that changes dramatically during the period of perinatal cerebellar development. They also show that the chemokine macrophage inflammatory protein-1 (MIP-1 ; also known as CCL3), a major agonist at CCR1 receptors, is similarly expressed in the cerebellum. The authors suggest that CCR1 receptors and MIP-1 may play an important role in cerebellar development. Although the data are extremely interesting and timely, understanding why this is so requires an explanation of why the emerging neurobiology of chemokines and their receptors is such an exciting field.

HIV-1, Chemokines and Neurogenesis

HIV-1 infection of the brain results in a large number of behavioural defecits accompanied by diverse neuropathological signs. However,it is not clear how the virus produces these effects or exactly how the neuropathology and behavioural defecits are related. In this article we discuss the possibility that HIV-1 infection may negatively impact the process of neurogenesis in the adult brain and that this may contribute to HIV-1 related effects on the nervous system. We have previously demonstrated that the development of the dentate gyrus during embryogenesis requires signaling by the chemokine SDF-1 via its receptor CXCR4. We demonstrated that neural progenitor cells that give rise to dentate granule neurons express CXCR4 and other chemokine receptors and migrate into the nascent dentate gyrus along SDF-1 gradients. Animals deficient in CXCR4 receptors exhibit a malformed dentate gyrus in which the migration of neural progenitors is stalled. In the adult, neurogenesis continues in the dentate gyrus. Adult neural progenitor cells existing in the subgranlar zone, that produce granule neurons, express CXCR4 and other chemokine receptors, and granule neurons express SDF-1 suggesting that SDF-1/CXCR4 signaling is also important in adult neurogenesis. Because the cellular receptors for HIV-1 include chemokine receptors such as CXCR4 and CCR5 it is possible that the virus may interfere with SDF-1/CXCR4 signaling in the brain including disruption of the formation of new granule neurons in the adult brain.