Cyclin-dependent kinase 5 (Cdk5) plays key roles in normal brain development and function. Dysregulation of Cdk5 may cause neurodegeneration and cognitive impairment. Besides the well demonstrated role of Cdk5 in neurons, emerging evidence suggests the functional requirement of Cdk5 in oligodendroglia (OL) and CNS myelin development. However, whether neurons and OLs employ similar or distinct mechanisms to regulate Cdk5 activity remains elusive. We report here that in contrast to neurons that harbor high levels of two Cdk5 activators, p35 and p39, OLs express abundant p39 but negligible p35. In addition, p39 is selectively up-regulated in OLs during differentiation along with elevated Cdk5 activity, whereas p35 expression remains unaltered. Specific knockdown of p39 by siRNA significantly attenuates Cdk5 activity and OL differentiation without affecting p35. Finally, expression of p39, but not p35, is increased during myelin repair, and remyelination is impaired in p39−/− mice. Together, these results reveal that neurons and OLs harbor distinct preference of Cdk5 activators and demonstrate important functions of p39-dependent Cdk5 activation in OL differentiation during de novo myelin development and myelin repair.
Background:
Cyclin-dependent kinase 5 (Cdk5) is crucial for brain development.
Results:
In contrast to neurons that utilize p35 as the primary Cdk5 activator, oligodendroglia employ p39-dependent Cdk5 activation to advance differentiation and myelin repair.
Conclusion:
p39 is the primary Cdk5 activator in oligodendroglia, essential for oligodendroglia development.
Significance:
Our study revealed distinct mechanisms controlling Cdk5 activity in neurons and oligodendroglia.
Introduction
Normal brain function relies upon proper development and integrity of neurons and oligodendroglia (OLs),
the myelinating glia that ensheath neuronal axons and enable rapid information flow in the CNS. OL impairment has become an increasingly recognized pathophysiological feature of numerous neurological disorders originally believed to result from neuronal defects alone, including Alzheimer disease and schizophrenia (
1
, 2
, 3
, 4
). Moreover, key components of cell signaling pathways traditionally thought to control neuronal development and function have now been found expressed and playing critical roles in OLs as well. Thus, whether neuronal and OL lineages share common molecular pathways that govern their development and cooperation is an intriguing question and a growing area of research in neuroscience.One such signaling molecule is Cyclin-dependent kinase 5 (Cdk5), an unconventional Cdk member that does not control cell cycle progression but primarily functions in post-mitotic cells (
5
, 6
, 7
, 8
, 9
). Despite the ubiquitous expression and increasingly recognized broad function of Cdk5 in numerous cell types, the highest activity of this kinase is detected in the brain (5
, 10
). Earlier studies demonstrated essential functions of Cdk5 in neuronal migration, neural network formation, and synaptic plasticity in mammalian brains (5
). Besides the traditional view of neuronal centric roles of Cdk5, emerging evidence suggests that Cdk5 also governs the development of oligodendroglia progenitor cells (OPCs) (11
, 12
, 13
). Similar to brain neurons, OPCs migrate to distant brain areas and become mature OLs to myelinate neuronal axons (14
). However, unlike neuronal development in the embryonic brains, the most vigorous OPC development occurs postnatally (14
). Loss of Cdk5 function affects OPC migration and differentiation in culture and results in CNS hypomyelination (12
, 13
, 15
, 16
). However, molecular mechanisms that regulate Cdk5 function in neurons and OLs remain elusive.The activity of Cdk5 is controlled by the available amounts of two activator homologs, p35 and p39 (
9
, 17
, 18
). Mice lacking both p35 and p39 displayed nearly identical defects in embryonic brain development and perinatal lethality as those found in the Cdk5-null mice (19
, 20
), indicating that p35 and p39 together are solely responsible for Cdk5 activity and function in the brain. However, the Cdk5 activators display distinct temporal expression profiles, suggesting non-redundant roles for each activator over the course of brain development that have yet to be identified (9
, 21
, 22
, 23
, 24
). p35 is abundantly expressed in embryonic neurons and is essential for Cdk5 activation in these cells, which govern corticogenesis (17
, 20
). Conventional p35 knock-out mice display impaired neuronal migration and cortical organization during embryonic development, similar to but less severe than that in Cdk5 null mice (20
, 25
). In contrast to the early function of p35 in the embryonic brain, p39 levels are elevated and most prominently expressed in the postnatal brain (26
). Nonetheless, mice lacking p39 alone showed no overt neuronal abnormalities (19
). Thus, p35 is considered the most important Cdk5 activator in neurons, whereas the functional importance of p39 remains undetermined.We report here that neurons and OLs display distinct profiles of Cdk5 activator expression and regulation. In contrast to the major role of p35 in activating Cdk5 in neurons, p39 is the primary Cdk5 activator in OLs, where p35 expression is negligible. In addition, selective up-regulation of p39, but not p35, accompanies the increased Cdk5 activity during normal OPC differentiation in culture and CNS myelin development in vivo. Up-regulation of p39 is also detected in OLs during myelin repair upon induced demyelination. Finally, we show that p39 plays essential roles in advancing OPC differentiation, and the loss of p39 leads to impairment in myelin repair in the brain. Together, these studies suggest that p39 is the primary activator responsible for Cdk5 function in CNS myelinogenesis in normal and diseased brains.
DISCUSSION
In this study we demonstrated for the first time that neurons and glia employ distinct molecular mechanisms to control Cdk5 activity by differential regulation of Cdk5 activator expression. Moreover, using multiple experimental paradigms, our studies clearly established that in contrast to the predominant function of p35 in Cdk5 activation in neurons and many other peripheral cell types (
40
, 41
, 42
, 43
, 44
, 45
, 46
, 47
), p39 is the primary Cdk5 activator in OLs and plays essential roles to advance normal OL differentiation as well as repair of myelin lesion in the brain.The critical roles of Cdk5 in controlling normal neuronal development and function have been well established (
5
, 20
). More recent studies have also established a parallel role of Cdk5 in OL development (12
, 13
, 15
, 16
). However, molecular mechanisms that regulate Cdk5 function in different neural cell lineages still remain vastly elusive. In particular, because of the lack of overt detrimental phenotypes in adult p39−/− mice (19
), the functional importance of p39 is not understood. We show for the first time that OLs primarily express p39 for Cdk5 activation, with scarce levels of p35. Moreover, the selective up-regulation of p39, but not p35, during OL differentiation in culture and in vivo clearly demonstrated that p39 is responsible for the increased Cdk5 activity during OL differentiation (11
, 12
). Given the fact that the most rigorous OL development occurs in the first few postnatal weeks (14
), OL-produced p39 must be an important contributor to the previously reported preferential increase of p39 during neonatal brain development (26
). Importantly, selective up-regulation of p39 is also observed during myelin repair in adults, suggesting that similar mechanisms control p39 expression in neonatal and adult OPCs.We have provided in vitro and in vivo evidence that p39-dependent Cdk5 activation is crucial for OPC differentiation, which expands the importance of p39-dependent Cdk5 function from the previous report that shRNA knockdown of p39 impairs OPC migration in culture (
13
). In fact, the reduced numbers of MBP+ cells in the corpus callosum of p39−/− mice likely originate from defects in both migration and differentiation, two concurrent events tightly coupled during in vivo myelinogenesis. Although the lack of p39 in neurons may also contribute to the delayed OL development in vivo in the p39−/− mice, impairment of morphological differentiation of CG4 cells by siRNA-mediated knockdown of p39 and Cdk5 clearly demonstrated the essential role of cell autonomous p39-dependent Cdk5 function in OPCs.Unlike the conditional Cdk5 knock-out mice that exhibit CNS hypomyelination (
15
, 16
), we found that p39−/− mice only exhibit a delay in early OL development without overt hypomyelination. One likely explanation is the compensatory increase of p35 in OLs due to the absence of p39, which may permit delayed myelination in p39−/− adult. In addition, given the facts that many Cdk5 target proteins can be phosphorylated by other kinases (48
, 49
, 50
, 51
, 52
), compensatory cross-talk by other signaling mechanisms may also contribute to myelinogenesis in the absence of p39-dependent Cdk5 function. Nonetheless, the defects in early OPC development in p39−/− mice argue that the function of p39 could not be completely spared despite compensation by various possible mechanisms.Importantly, in contrast to the largely intact de novo myelination in the absence of p39, myelin repair is severely impaired in the p39−/− mice after lysolecithin-induced acute demyelination. On one hand, the much more rapid lesion development may not leave sufficient time to induce compensatory changes. On the other hand, the more challenging environment of lysolecithin-induced lesion may exacerbate the functional defects of lacking p39-dependent Cdk5 signaling, which lead to failures of myelin repair. Notably, increased p39 expression can also be observed in cells beyond the OL lineage during myelin repair. Thus, whether increased p39 expression from neurons and other types of glia cells together with OL-specific p39 up-regulation are synergistically required for myelin repair is an interesting unanswered question that could be addressed once neural-lineage specific, inducible knock-out of p39 can be achieved.
The low levels of p39 and p35 mRNAs in OLs as compared with that in neurons suggest the existence of neural lineage-specific mechanisms for differential mRNA biogenesis of these Cdk5 activators. We showed that p39 mRNA is active for translation in OLs. Thus, p39 protein levels are largely regulated by the abundance of p39 mRNA, likely involving unidentified OL-specific transcription factors as well as posttranscriptional mechanisms that control the stability of the p39 mRNA. Noticeably, up-regulation of p39 mRNA is observed in de novo myelin development as well as myelin repair. Thus, molecular mechanisms that regulate p39 in neonatal OPCs are likely applicable to myelinating OLs during lesion repair in the adults and warrant rigorous investigation. In contrast, p35 mRNA is translationally repressed in OLs, which is an apparent mechanism that contributes to the scarce expression of p35 protein in OLs. The transacting factors that suppress p35 translation in OLs still remain unknown. Nonetheless, it is worth mentioning that a number of microRNAs are predicted to target the lengthy 3′-UTR of p35 mRNA specifically but not the p39 mRNA (
53
). Which of these microRNAs are expressed in OLs and responsible for translational suppression of p35 is a challenging question to be addressed by future studies.In conclusion, our studies have demonstrated differential regulation of Cdk5 activator expression in neurons and OLs. Further delineating molecular mechanisms that control p35 and p39 expression may allow differential and independent manipulation of Cdk5 function in neuronal and glial lineages during normal development as well as in the pathogenesis of brain disorders involving cell type-specific Cdk5 dysregulation, such as neuronal degeneration in Alzheimer disease and glioma tumorigenesis (
38
, 54
, 55
). In particular, identification of OL-specific mechanisms that control p39 expression may ultimately help to develop novel strategies to advance CNS myelinogenesis in numerous disorders for which the pathogenic impacts of myelin impairment have become increasingly recognized, represented by multiple sclerosis and schizophrenia (1
).Acknowledgments
We thank Dr. Zixu Mao for the p39 construct, Dr. Nancy Ip for the anti-p39 antibody, and Gabriel Mettlach for technical assistance. We also thank Jennifer Strange for excellent technical assistance with the FACS isolation of OL. The FACS sorting facility at the University of Kentucky is supported in part by the National Institutes of Health Shared Instrument Program (S10 RR026827-01A1).
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Article Info
Publication History
Published online: May 03, 2013
Received in revised form:
May 2,
2013
Received:
January 15,
2013
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© 2013 ASBMB. Currently published by Elsevier Inc; originally published by American Society for Biochemistry and Molecular Biology.
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