MACKLIS LAB

Bradley Molyneaux
Paola Arlotta
Noriyuki Kishi
Jason Emsley
Pembe Hande Ozdinler
Joao Meneze
Shivraj Sohur

Denis Jabaudon
Tina Lai
Arthur Czupryn

Liyun Li
Bartley Mitchell
Sanjay Magavi

Justine M. Nagurney

Kyle MacQuarrie
Alexander Eswar

MGH-HMS Center for Nervous System Repair
Macklis Lab

Dr Macklis | Recent Publications | Links

Bartley Mitchell

Overview:

In order to develop cell replacement strategies that could lead to therapies for degenerative, developmental, or acquired diseases of the neocortex, it is critical to understand the precise anatomical connectivity of neuronal populations as well as the ability of the adult neocortex to support the integration of new neurons. My work in the Macklis Lab focuses on the development and reconstruction of complex neocortical circuitry in the mammalian brain. I have concentrated on the two populations of cortico-cortical projection neurons: callosal projection neurons (CPN), and frontal association projection neurons (FPN). CPN connect homotopic areas of neocortex in opposite hemispheres, and are a particularly vulnerable population in Alzheimer's Disease and autism. FPN send local or long-distance projections from the sensory-motor cortex to frontal or premotor cortex. These projection neuron populations are involved in the integration of sensory-motor information in the premotor neocortex of rodents, and have been implicated in tasks such as long-term memory retrieval as well. Using precise stereotaxic placement of multiple neuronal tracers across development, as well as transplanting genetically-marked immature neurons into specific neocortical locations of adult mice, we have been able to: 1) define the development of CPN and FPN in the mouse sensory-motor cortex; and 2) describe the ability of transplanted neurons to integrate into the adult neocortex, sending area-specific and trans-callosal projections.

Development of Cortical Projections:

Although most anatomical studies in rodents have been performed in rats, genetic models of mammalian research have focused on C57BL/6 mice, due to successful manipulation at the genetic and cell biological levels. In order to examine the anatomical development and inter-relations of CPN and FPN, we have injected the retrograde tracers FluoroGold and DiI into sensory-motor and premotor cortex (respectively) of C57BL/6 mice at different developmental times (P2, P8, P21, and adult). We found that, in contrast to primate and cat, there is widespread overlap in populations of long-distance projection neurons in mice, such that many projection neurons have simultaneous projections to both contralateral somatosensory cortex and ipsilateral frontal cortex, and a considerable number of these dual-projections persist into adulthood. In addition, there are significant laminar differences in the percentage of neurons with simultaneous callosal and frontal projections, and an isolated population of layer V FPN has bilateral projections to both premotor cortical hemispheres. Taken together, our results indicate that the callosal and frontal association projection systems are highly integrated in C57BL/6 mice at the level of individual neurons.

Axonal outgrowth in the adult brain from transplanted neurons:

A critical factor in developing regenerative strategies in the adult mammalian CNS is whether the adult brain is capable of supporting and/or directing the outgrowth and maintenance of new axonal projections to appropriate target areas. It is therefore essential to know the precise anatomical pattern of projections from transplanted or endogenously derived neurons, especially across mature white matter tracts such as the corpus callosum, which is known to contain potent axonal growth inhibitory factors. To investigate this, we have transplanted immature cortical neurons (isolated from eGFP+ E17-19 embryos) into medial or lateral areas of the sensory-motor cortex in unmanipulated adult C57Bl/6 mice. We then determined the long-term patterns of axon-outgrowth on three levels: 1) local outgrowth in specific cortical laminae; 2) long-distance intra-hemispheric projections; and 3) long-distance trans-callosal projections. We found that the undamaged adult mouse brain has the remarkable ability to support the outgrowth of large numbers of axons from transplanted neurons with a high degree of laminar, areal, and trans-callosal specificity. Local fiber outgrowth was specifically limited to layers I, II/III, and V, especially in lateral cortical transplants within the barrel cortex, where fibers grew exclusively in non-barrel and inter-barrel regions. Intra-hemispheric projections were observed traversing long distances within and immediately above white matter tracts, terminating in regionally appropriate secondary somatosensory and premotor cortical regions. Axons traversing the corpus callosum were commonly observed, terminating in contralateral cortical areas homotopic to the transplantation site. Interestingly, although animals were only included in this study if there was no white-matter involvement from the transplantation injection tract, when separate transplant injection sites were made to directly implant some neurons into the white matter of the corpus callosum, axonal outgrowth was greatly enhanced, although the specificity of these projections was markedly reduced. Taken together, these results indicate that the adult mammalian brain may not be as inhibitory to new axonal outgrowth, even through white matter tracts, as has been previously thought. Rather, the adult CNS appears to retain a great deal of permissiveness for axonal outgrowth and substantial ability to direct the formation of region-specific axonal projections from newly incorporated immature neurons.

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