NEUROGENESIS and EPILEPSY

1. Mossy Fiber Sprouting
   1. Functional implications
      1. microscope- light/ electron

         Nota:

         • Integration of light microscopic and EM data suggests that the majority of synapses formed by mossy fibers in the granule cell and molecular layers are with other granule cells, leading to recurrent excitation
         1. mossy sprouting- synpasing with other granule cells>> excitation
            1. Buckmaster et al (2002)
      2. decreased GABA inhibition

         Nota:

         • Additionally, evidence suggests that normal GABA inhibition is diminished by mossy fiber terminals, further contributing to hyperexcitability
         1. Buhl et al (1996)
      3. inconsistency

         Nota:

         • These data suggest that other mechanisms in addition to mossy fiber sprouting might contribute to enhanced hippocampal excitability during epileptogenesis
         1. sprouting contributing to inhibitory interneurons
            1. Okazaki et al (1995)

               Nota:

               • Anatomical analysis of epileptic rat hippocampi reveals that some aberrant granule cell axons densely innervate inhibitory neurons
         2. no correlation with total seizures

            Nota:

            • the density of mossy fiber sprouting may not be associated with the total number of lifetime seizures or the seizure frequency in experimental or human TLE
   2. development of numerous mossy fiber-granule cell synapses
   3. Timm staining, dynorphin immunoreactivity, and biocytin fills
      1. dentate inner molecular layer

         Nota:

         • sprouting into
      2. rat pilocarpine model + Timm

         Nota:

         • used to visualize the zinc present in mossy fiber axons, also reveals significant amounts of mossy fiber sprouting (MFS) in the dentate inner molecular layer
         1. Mello et al (1993)
      3. Electron microscopy studies

         Nota:

         • demonstrate that mossy fibers synapse onto neighboring DGCs, creating recurrent excitatory synapses
         1. Okazaki et al (1995)
   4. Kron and Parent (2010)

      Nota:

      • Over a dozen years ago, we first hypothesized that mossy fiber sprouting in experimental mTLE arises from adult-born, rather than pre-existing, DGCs.27 When we used irradiation to kill adult-born cells in the setting of SE, however, we did not block inner molecular layer mossy fiber sprouting 4 weeks later, suggesting that DGCs generated after SE do not send axons aberrantly into the dentate inner molecular layer
      1. retroviral reporter labeling and low-dose irradiation

         Nota:

         • retroviral reporter labeling to birthdate DGCs in combination with low-dose irradiation to transiently suppress DGC neurogenesis
      2. 2-4 weeks old DGC contributed to sprouting

         Nota:

         • At 4 weeks after pilocarpine-induced SE in adult animals, we found that neither neonatally generated DGCs nor those born after SE contributed to sprouting, but instead only DGCs that were 2–4 weeks old at the time of SE showed aberrant axonal reorganization
      3. Adult DGC following SE also contributed, but only 10 weeks after

         Nota:

         • When we labeled adult-born DGCs 4 days after SE but allowed the animals to survive 10 (instead of 4) weeks, in contrast, we found that the adult-born DGCs contributed robustly to mossy fiber sprouting 
      4. >> mossy fiber remodeling mainly by developing or newborn DGC

         Nota:

         • The finding of mossy fiber remodeling only by developing or newborn, and not mature, DGCs has key mechanistic implications for understanding seizure-induced DGC plasticity. Rather than recapitulating development, mossy fiber sprouting after SE appears to involve an alteration of ongoing development. This sprouting is thought to be progressive in nature, beginning 2 weeks after SE and peaking around 100 days post-SE.11, 48 The idea that successive generations of adult-born DGCs sprout aberrantly as they develop is consistent with this progression of mossy fiber reorganization for several months after SE.11 In fact, the timing of transiently increased neurogenesis for 2–3 weeks after the initial seizures27 followed by a potential suppression of neurogenesis chronically,34along with the delay in adult-born neurons manifesting aberrant axonal outgrowth, fits well with a model in which most or all of the newborn DGCs eventually sprout. Such a time course would lead to a peak in MFS at about 2–3 months after SE.
2. Hilar Ectopic Granule Cells

   Nota:

   • The vast majority of neurons born in the subgranular zone during adult life migrate into the granule cell layer. After SE, many DGCs migrate instead into the dentate hilus or through the granular layer into the molecular layer- these are extopic granule cells
   1. Functional implications
      1. hyperexcitable

         Nota:

         • They have been shown to be postsynaptic to mossy fibers and have less inhibitory input on their somata and proximal dendrites than DGCs in the granule cell layer
      2. ablating neurogenesis after SE

         Nota:

         • ablating neurogenesis after SE, the time when ectopic cells form,46 attenuates subsequent epileptogenesis, with a reduction of the frequency and severity of spontaneous recurrent seizures
         1. Jung et al (2004)
      3. inconsistency
         1. Jakubs et al (2009)
            1. electrical stimulation and GFP

               Nota:

               • induced SE with electrical stimulation and then labeled adult-born neurons with retrovirus expressing green fluorescent protein (GFP).
            2. increased inhibitory, decreased excitatory

               Nota:

               • They recorded from the GFP-labeled cells in acute hippocampal slices and found that they showed increased inhibitory drive and decreased excitatory input compared to GFP-labeled cells in controls and mature DGCs in epileptic rats
            3. (-) studied only adult-born DGC

               Nota:

               • However, they only studied adult-born neurons that integrated normally. These findings suggest that increased DGC neurogenesis after SE leads to heterogeneous populations of adult-born DGCs, some of which integrate aberrantly and may become hyperexcitable, while others integrate normally and may restore inhibition
   2. Rodent models
      1. pilocarpine
         1. Prox1 immunolabeling of DGCs

            Nota:

            • increase in ectopic DGC following an mTLE model
   3. only DGCs generated after SE migrate ectopically

      Nota:

      • Hilar ectopic granule cells may result from abnormal migratory behavior of DGC progenitors after epileptogenic insults, as SE appears to cause aberrant chain migration of DGC progenitors to the hilus and molecular layer.51 Some propose a critical period after the birth of adult-generated neurons during which they are vulnerable to being recruited into epileptogenic neuronal circuits,53and indeed, we find that only DGCs generated after SE migrate ectopically.46 One proposed cause of the aberrant migration is loss of the migration guidance cue Reelin, which is expressed in the adult rodent hippocampus.54
      1. Kron and Parent (2010)
3. Hilar Basal Dendrites (HBD)
   1. Functional implications
   2. mainly in immature DGC

      Nota:

      • The persistence of hilar basal dendrites (HBDs), normally a feature of only immature DGCs, may be a mechanism contributing to hyperexcitability of adult-born DGCs in epilepsy
   3. % is higher in mTPE
   4. animal models

      Nota:

      • prolonged seizures induce an increased percentage of granule cells with basal dendrites located at the hilar border and extending into the granule cell layer
      1. Walter et al (2007)
         1. Thy1-GFP mice
         2. 50% HBD

            Nota:

            • 50% of immature granule cells in mice exposed to pilocarpine-induced SE exhibited HBDs
      2. Kron and Parent (2010)

         Nota:

         • in the rat pilocarpine model, about a third of adult-born DGCs that are 2 weeks old at the time of SE, or born 4 days after SE, develop HBDs
         1. adult-born DGC develop HBD

Nota:

• contributes to abnormalities- see below

1. restore inhibition?

   Nota:

   • compensatory role
2. The rostal subventricular zone (SVZ)

   Nota:

   • exhibits increased neurogenesis within weeks following prolonged seizure activity
   • Neuroblasts generated in the SVZ migrate more rapidly to the olfactory bulb, and some exit the migratory stream prematurely
3. DGC proliferation increases 5–10 fold

   Nota:

   • After either pilocarpine- or kainic acid-induced SE >> dentate gyrus is proliferation is measured using assessed by the expression of Ki-67, an endogenous cell proliferation marker, or short-pulse bromodeoxyuridine (BrdU) mitotic labeling
   1. mediated by radial glial-like neural progenitor cells
   2. accelerated maturation
   3. after 3-4 weeks proliferation rate returns to normal

      Nota:

      • Potential reasons for this decrease include exhaustion of the progenitor pool, loss of needed growth/trophic factors or altered cellular interactions

Nota:

• a prolonged seizure (termed status epilepticus or SE) is induced by either electrical stimulation or a chemoconvulsant, leading to injury as the “initial precipitating event.” 

1. kainic acid

   Nota:

   • chemconvulsant-induced SE models
2. pilocarpine

   Nota:

   • chemconvulsant-induced SE models

Nota:

• epilepticus