In the present study we demonstrate that induction of FAK knock-out just prior to and during active stages of myelination results in hypomyelination in the optic nerve at a time-point when normally myelinated fibers can be found extended throughout the whole length of the nerve, i.e. at P14. Furthermore, our data suggest that this effect of FAK knock-out induction is due, at least in part, to a reduced outgrowth and/or impaired remodeling of primary oligodendrocyte processes. However, myelination appears to have reached normal levels by P28. Thus, our data suggest that in vivo in the optic nerve FAK promotes efficient and properly timed myelination during the active phases of myelin sheath formation.
In light of the known role of FAK in integrating ECM-integrin signaling events, our data are in support of the previously suggested, but somewhat controversial, regulatory role of integrin signaling during CNS myelination (Benninger et al., 2006
; Colognato et al., 2007
; Lee et al., 2006
; Relvas et al., 2001
). Furthermore, they are in good agreement with findings that demonstrate developmental myelination to be controlled by signals up- and downstream of the integrin-FAK axis, such as laminin2/merosin and fyn, respectively (Biffiger et al., 2000
; Chun et al., 2003
; Sperber et al., 2001
). Interestingly, expression of dominant-negative β1 integrin, laminin2/merosin deficiency and fyn knock-out all result in hypomyelination in a region-specific manner with only the optic nerve affected to similar extents in these mutant mice. Thus, the roles of ECM proteins, integrins and FAK may differ in different CNS areas. Those areas that are potentially not controlled by FAK may depend on the FAK-related kinase proline-rich tyrosine kinase 2 (Pyk2). While Pyk2 expression in oligodendrocytes has not been well defined, it represents a good candidate for functionally substituting FAK (Avraham et al., 2000
; Klingbeil et al., 2001
; Nakamura et al., 2007
; Orr and Murphy-Ullrich, 2004
). However, future studies will be necessary to dissect the exact roles of FAK and Pyk2 in the regulation of CNS myelination.
FAK has been characterized as a regulator of morphological remodeling, and in particular it has been implicated in the regulation of process outgrowth from both neurons and oligodendrocytes (Beggs et al., 2003
; Falk et al., 2005
; Hoshina et al., 2007
; Robles and Gomez, 2006
). Our data demonstrating a reduction in the number of primary processes upon induction of FAK knock-out are consistent with these previous findings. A pivotal role of FAK in the regulation of morphological oligodendrocyte differentiation is further supported by the fact that fyn, a known FAK effector, has been found to be important for the development of the extensive oligodendrocyte process network (Klein et al., 2002
; Liang et al., 2004
; Osterhout et al., 1999
). In the case of fyn, this morphological maturation can be regulated independent of changes in gene expression typically associated with oligodendrocyte differentiation (Buttery and ffrench-Constant, 2001
; Osterhout et al., 1999
). The extent to which FAK is involved in the control of gene expression in differentiating oligodendrocytes, however, has not yet been characterized. Nevertheless, the above data support the idea that impaired process outgrowth and/or remodeling may at least in part be responsible for the hypomyelination seen in the optic nerve of tamoxifen-treated Fakflox/flox:PLP/CreERT
In our studies, the effect of FAK knock-out induction on myelination was found to be transient with normal levels of myelination detectable at P28. Thus, FAK’s role appears to mainly affect the efficiency and timing of myelination. However, as discussed above, Pyk2 may be able to substitute FAK’s function not only in a region-specific manner but also in case of a loss of FAK. Future studies will be necessary to assess such a potentially important role of Pyk2 in myelination.
Taken together, our data suggest that FAK promotes efficient and properly timed myelination in the optic nerve, where it likely acts as an effector of integrin signaling activated by oligodendrocyte-ECM interactions. Our data further suggest that this signaling event promotes process outgrowth and potentially remodeling during the initial stages of myelination. Impairment of these steps of oligodendrocyte maturation appears at least in part responsible for the limited repair of the myelin sheath seen in lesions of patients suffering from the major demyelinating disease in human, Multiple Sclerosis (Chang et al., 2002
; Franklin and ffrench-Constant 2008
; Kuhlmann et al. 2008
). Thus, further understanding of the role of FAK for CNS myelination does not only further our understanding of normal CNS development but may also reveal novel targets suitable to stimulate remyelination under pathological demyelinating conditions.