This is the first study to our knowledge to demonstrate that a molecular intervention can reduce the size and functional effects of a pre-existing disruption in neuronal migration. In general, our study raises the possibility that in some contexts neuronal migration is a form of neuronal plasticity that may be engaged to induce neural repair. Previous examples of continued neuronal migration in the brain include neuronal migration of endogenous neurons to the olfactory bulb and dentate gyrus throughout life29
, as well as migration of transplanted neuronal progenitors into neocortex and other regions of CNS30
. Our study extends this phenomenon of continued migration to include induced migration of mis-positioned neurons in neocortex. Our study also suggests that reversing a migration disruption in development can reduce aberrant excitability levels in more mature brain.
In this study we demonstrate that there are significant developmental constraints on stimulating migration in the postnatal neocortex by re-expression of Dcx. Whereas Dcx re-expression at P0 induced a migration that significantly regressed heterotopia and restored neocortical lamination, re-expression at P5 led to partial restoration of neuronal position with heterotopia regression, and re-expression at P10 led to partial recovery of position without significant heterotopia reduction. These ages in rats correspond roughly in humans to preterm gestational weeks, 25–35 for P0 to several months after birth for P10. We do not currently know the reason for absence of regression at P10 or the reduced migration following P5 intervention. It is likely, however, that developmental changes in neocortical tissue and within differentiating neurons limit continued neuronal migration into the postnatal period. Migration of pyramidal neurons during normal neocortical development is guided by radial glial processes31
, and it is likely that developmental loss of radial glia, which normally occurs by the middle of the second postnatal week in the rat, reduces the potential for continued migration32,33
. In addition, the developmental window for migration out of SBH may close because of neuronal maturation. Neurons within SBH form synapses and send axonal projections to subcortical targets26,27
, and such neuronal differentiation may preclude induced migration by re-expression of Dcx. It is important to note, however, that even as late as P10 our results show that there is still a capacity for plasticity, and that mis-positioned neurons can be shifted from deeper to more superficial positions by expression of Dcx. In the future it will be important to determine which specific cellular changes are responsible for developmental restriction of induced re-migration, and whether these can be manipulated to allow for heterotopia regression in even more mature animals.
Virtually all neuronal migration disorders including lissencephaly, periventricular nodular heterotopias, and subcortical band heterotopia are risk factors for seizures34,35,36
. In addition, focal cortical dysplasias (FCDs) are estimated to be associated with 30% of intractable temporal lobe epilepsies37,38
. The specific mechanisms that cause dysplastic cortices to cause epileptiform activity remain unclear. It has remained unknown for example when in development the presence of a neocortical malformation first disposes cortical tissue to overexcitability39,40,1,35,41
. Our results suggest that an underlying cause of seizure susceptibility to reduced PTZ dose is directly related to aberrant neuronal positioning and to the malformation. We can not yet distinguish from our results at what point in development migration impairment causes increased excitability, but regression initiated following birth is sufficient to restore PTZ thresholds to control levels. Future experiments can now be directed at determining the time course of changes in synaptic and cellular physiology that correlate with reduced seizure susceptibility.
The specific experimental intervention we used to restore migration patterns, electroporation of an inducible construct, is clearly not applicable to human patients. There are at least two possible directions indicated by our findings for interventions that may one day be therapeutically viable. First, Dcx expression may be induced by viral transduction of cells within SBH malformations. Our results currently suggest that such gene therapy would probably need to be applied perinatally, but could be restricted to mis-positioned neurons within SBH. An alternative to gene therapy could be to enhance the function of other members of the DCX
superfamily that may have redundant function with DCX. For example, in mouse knockouts of Dcx
there are no cortical dysplasias42
, and only when Dcx and Dclk are both disrupted do migration disorders similar to human lissencephaly appear43
. Thus Dclk appears in the mouse to be capable of compensating for Dcx function. There are now multiple defined members of the DCX
super-family with similar cellular and biochemical functions44
, and perhaps pharmacological enhancement of the function of one or several of these may facilitate regression of SBH in humans. Finally, the present study serves as a proof of concept that mis-migrated cells can be manipulated to restore a normal morphological pattern and level of neuronal excitability in the cerebral cortex.