Comment by Daniel Stewart and Kenneth Davis
The Corfas results are intriguing. Their findings confirm much of what we have either found or suspect in schizophrenia relating to white matter involvement. Demonstrations of OLIG2 interactions with ErbB4 in the cortex and with CNP in the striatum in schizophrenia from our team (Georgieva et al., 2006) fit well with this investigation in providing evidence for a link between a variety of potential etiologic oligodendrocyte-related mechanisms in schizophrenia. While in our study, we did not find interaction with NRG1 and OLIG2, it is important to note that differential expression of NRG1 might be found only at certain points in the timeline of disease development. Other recent support from our team for white matter involvement in schizophrenia comes from an investigation in which an SNP associated with CNP was found to be significantly correlated with schizophrenia (Peirce et al., 2006). Interestingly, Corfas’s group reports that when ErbB signaling is abolished in oligodendrocytes, myelin structure appears normal, but the myelin sheath is significantly thinner. This is in line with some of the ultrastructural findings of Uranova’s group and in rodent studies looking at MAG-deficient mice (both reviewed in Davis et al., 2003)—another downregulated myelin-related gene found in brains of schizophrenia patients.

Reductions in oligodendrocyte number on the order of 20 percent have been demonstrated in the brains of schizophrenia patients (Hof et al., 2002). Although this finding does not precisely parallel the findings in this investigation, the authors’ adroitly point out that this may be because the abnormalities they induced were during early oligodendrocyte and myelin expression, while it is possible that the abnormalities seen in the brains of schizophrenia patients occur relatively later in development, more likely during the second large wave of cortical myelination at the end of the second decade of life. The authors also point out that “defects in ErbB signaling in different cell types may contribute to different aspects of psychiatric symptoms.” This might also be the case in schizophrenia, giving rise to the myriad presentations of the disease, as might the fact that expression of both NRG1 and ErbB4 are susceptible to environmental insult.

Other important similarities between the authors’ findings and schizophrenia include that, even in light of these myelin abnormalities, gross brain volumes, as well as several other measures, remained normal. This buttresses the idea that in schizophrenia, myelin abnormalities might be at the root of the often unimpressive brain changes noted in schizophrenia on gross structural imaging. And finally, although speculative, the authors do note an intriguing set of behavioral abnormalities, some of which could mimic the social isolation and poor relatedness of schizophrenia, which is particularly remarkable given the increased susceptibility to amphetamines and the trends seen in DAT, D1, and D2 expression in this investigation.

Corfas’s findings are indeed exciting, and we commend his team on an eloquently designed and implemented investigation.

View all comments by Daniel Stewart
View all comments by Kenneth Davis

Neuregulin1 (NRG1) is the most promising risk factor for schizophrenia, and the study of the signaling of NRG1 and its receptor ErbB4 is very important in understanding the pathophysiology of the disease. Like other promising risk factors for schizophrenia, NRG1/ErbB4 is multifunctional with many molecular isoforms. NRG1/ErbB signaling plays a role both before and after birth. Furthermore, ErbB4 is expressed not only in neurons but also in other types of cells, such as oligodendrocytes.

To address context-dependent functions one by one, dominant-negative transgenic mice can be very useful. The advantage of dominant-negative transgenics is that we can knock down the endogenous function of our target molecules (in this work, ErbB4) in a temporally and spatially specific manner by utilizing a well-characterized promoter. In this outstanding study by Corfas and colleagues, they used the CNP promoter that confirms dominant-negative ErbB4 selectively in oligodendrocytes (but not in astrocytes and neurons) only after birth. This approach will be very useful in schizophrenia research.

The remarkable finding is that they observed alterations in dopamine-mediated neuronal networks and associated behaviors by disturbing NRG1/ErbB4 selectively in cells of oligodendrocyte lineage. Three important paradigms for schizophrenia (white matter pathology, dopamine, and a susceptibility gene) converge in this paper, and in this sense, I find it very exciting.

View all comments by Akira Sawa

Does the effect of NRG1/ErbB4 signaling on myelination occur downstream of purinergic signaling? Fields suggests that adenosine is of primary importance in regulating early development of OPCs, where it stimulates differentiation and myelination (Fields, 2006). It's of interest that cAMP stimulates expression of neuregulin and cAMP levels in the lung are decreased in A2A adenosine receptor (22q11.2)-deficient mice (Tokita et al., 2001; Nadeem et al., 2007). Do you see reduced neuregulin levels in 22q11 deletion syndrome? Of particular interest is the study by Desai and colleagues reporting that signaling via the adenosine A2A receptor downregulates thrombospondin 1 (Desai et al., 2005). Perhaps overexpression of thrombospondin 1 may help explain the occular abnormalities in this syndrome (Wu et al., 2006; Forbes et al., 2007; Stalmans, 2005). Thrombospondins are also involved in synaptogenesis (Christopherson et al., 2005).

References:

Fields RD. Nerve impulses regulate myelination through purinergic signalling. Novartis Found Symp. 2006;276:148-58; discussion 158-61, 233-7, 275-81.

Tokita Y, Keino H, Matsui F, Aono S, Ishiguro H, Higashiyama S, Oohira A. Regulation of neuregulin expression in the injured rat brain and cultured astrocytes. J Neurosci. 2001 Feb 15;21(4):1257-64.

Nadeem A, Fan M, Ansari HR, Ledent C, Mustafa SJ. Enhanced airway reactivity and inflammation in A2A adenosine receptor deficient allergic mice. Am J Physiol Lung Cell Mol Physiol. 2007 Feb 9; [Epub ahead of print]

Desai A, Victor-Vega C, Gadangi S, Montesinos MC, Chu CC, Cronstein BN. Adenosine A2A receptor stimulation increases angiogenesis by down-regulating production of the antiangiogenic matrix protein thrombospondin 1. Mol Pharmacol. 2005 May;67(5):1406-13. Epub 2005 Jan 26. Comment in: Mol Pharmacol. 2005 May;67(5):1385-7.

Wu Z, Wang S, Sorenson CM, Sheibani N. Attenuation of retinal vascular development and neovascularization in transgenic mice over-expressing thrombospondin-1 in the lens. Dev Dyn. 2006 Jul;235(7):1908-20.

Forbes BJ, Binenbaum G, Edmond JC, Delarato N, McDonald-McGinn DM, Zackai EH. Ocular findings in the chromosome 22q11.2 deletion syndrome. J AAPOS. 2007 Apr;11(2):179-182. Epub 2006 Nov 30.

Stalmans I. Role of the vascular endothelial growth factor isoforms in retinal angiogenesis and DiGeorge syndrome. Verh K Acad Geneeskd Belg. 2005;67(4):229-76.

Christopherson KS, Ullian EM, Stokes CC, Mullowney CE, Hell JW, Agah A, Lawler J, Mosher DF, Bornstein P, Barres BA. Thrombospondins are astrocyte-secreted proteins that promote CNS synaptogenesis. Cell. 2005 Feb 11;120(3):421-33. Comment in: Cell. 2005 Feb 11;120(3):292-3.

View all comments by Mary Reid

  I recommend the Primary Papers