Summary Organoid techniques provide unique platforms to model brain development and neurological disorders. While several methods for recapitulating corticogenesis have been described, a system modeling human medial ganglionic eminence (MGE) development, a critical ventral brain domain producing cortical interneurons and related lineages, has been lacking until recently. Here, we describe the generation of MGE and cortex-specific organoids from human pluripotent stem cells that recapitulate the development of MGE and cortex domains respectively. Population and single-cell RNA-seq profiling combined with bulk ATAC-seq analyses revealed transcriptional and chromatin accessibility dynamics and lineage relationships during MGE and cortical organoid development. Furthermore, MGE and cortical organoids generated physiologically functional neurons and neuronal networks. Finally, fusing region-specific organoids followed by live-imaging enabled analysis of human interneuron migration and integration. Together, our study provides a platform for generating domain-specific brain organoids, for modeling human interneuron migration, and offers deeper insight into molecular dynamics during human brain development.

Regional variations in the expression of a striatal enriched protein tyrosine phosphatase called STEP were studied in the adult rat brain by a combination of immunocytochemistry, lesion studies, Western blotting, and in situ hybridization. Monoclonal antibodies generated against STEP identified multiple polypeptides of M(r) 46, 37, 33 and a doublet of M(r) 64–66 kDa on Western blots. Although the three STEP immunoreactive bands with lower molecular weights were enriched in cytosolic fractions, the 64–66 kDa doublet was enriched in membrane fractions. All of the immunoreactive forms were abundant in the caudate-putamen and were present in lower amounts or were undetectable in other brain regions. In substantia nigra, the M(r) 64–66 kDa doublet was not detected but bands with M(r) 46, 37, and 33 kDa were present. Immunocytochemical and lesion experiments demonstrated that the cytosolic STEP isoforms present in the substantia nigra are in presynaptic axons originating from the projection neurons of the caudate putamen, which innervate this structure. Additional in situ hybridization studies showed that STEP mRNA expression patterns correlate with the patterns of immunocytochemical staining. These findings indicate that there are multiple polypeptide isoforms of STEP enriched in the basal ganglia and related structures which differ in terms of their intracellular locations and functional roles.

Focal cortical dysplasia (FCD), a local malformation of cortical development, is the most common cause of pharmacoresistant epilepsy associated with life-long neurocognitive impairments. It remains unclear whether neuronal misplacement is required for seizure activity. Here we show that dyslamination and white matter heterotopia are not necessary for seizure generation in a murine model of type II FCDs. These experimental FCDs generated by increasing mTOR activity in layer 2/3 neurons of the medial prefrontal cortex are associated with tonic-clonic seizures and a normal survival rate. Preventing all FCD-related defects, including neuronal misplacement and dysmorphogenesis, with rapamycin treatments from birth eliminates seizures, but seizures recur after rapamycin withdrawal. In addition, bypassing neuronal misplacement and heterotopia using inducible vectors do not prevent seizure occurrence. Collectively, data obtained using our new experimental FCD-associated epilepsy suggest that life-long treatment to reduce neuronal dysmorphogenesis is required to suppress seizures in individuals with FCD.

One therapeutic approach to treating Parkinson's disease is to convert endogenous striatal cells into levo-3,4-dihydroxyphenylalanine (L-dopa)-producing cells. A defective herpes simplex virus type 1 vector expressing human tyrosine hydroxylase was delivered into the partially denervated striatum of 6-hydroxydopamine-lesioned rats, used as a model of Parkinson's disease. Efficient behavioral and biochemical recovery was maintained for 1 year after gene transfer. Biochemical recovery included increases in both striatal tyrosine hydroxylase enzyme activity and in extracellular dopamine concentrations. Persistence of human tyrosine hydroxylase was revealed by expression of RNA and immunoreactivity.Parkinson's disease (PD), a neurodegenerative disorder, is characterized by the progressive loss of the dopaminergic neurons in the substantia nigra pars compacta that project to the corpus striatum (1). The principal therapy for PD is the oral administration of L-dopa (2), which is converted to dopamine (DA) by endogenous striatal aromatic amino acid decarboxylase (AADC) (3). Although it is initially effective, L-dopa therapy loses efficacy over a period of several years (1). Transplantation of cells that produce L-dopa or DA into the striatum can correct animal models of PD (4) but has not been a viable therapy in most human trials (5). Peripheral cell types that are genetically modified to express tyrosine hydroxylase (TH) and produce L-dopa have supported only short-term improvement (less than 2 months) in animal models of PD (6,7). Genetically modified muscle cells support longer improvements (6 months) (8), but the viability of a muscle cell graft in the human striatum is not yet clear. An alternative therapeutic strategy is to convert a fraction of the striatal cells into L-dopa-producing cells by expression of TH in striatal cells (9) from a defective herpes simplex virus type 1 (HSV-1) vector (10). Potential advantages of this approach include production of L-dopa at the required site of action, so that diffusion over substantial distances is not necessary, and alleviation of potential problems caused by graft rejection or tumor formation. To test this strategy, a human TH complementary DNA (cDNA) (form II) (11, 12) was inserted into an HSV-1 vector (pHSVth). Infection of cultured striatal cells with pHSVth resulted in expression of human TH RNA, TH immunoreactivity, and the release of L-dopa into the culture medium (13). The amounts of L-dopa released per infected cell suggested that pHSVth might be evaluated in the 6-hydroxydopamine (6-OHDA)-lesioned rat, a model of PD.pHSVth virus or pHSVlac virus or vehicle alone [phosphate-buffered saline (PBS)], was delivered by stereotactic injection into the partially denervated striatum of unilaterally 6- OHDA-lesioned rats (14). The apomorphine-induced rotation rate was measured as an index of behavioral recovery. The average decrease in the rotation rate caused by pHSVth was 64 ± 6% at 2 weeks after gene transfer. This value remained relatively constant over a...

Studies in rodent epilepsy models suggest that GABAergic interneuron progenitor grafts can reduce hyperexcitability and seizures in temporal lobe epilepsy (TLE). Although integration of the transplanted cells has been proposed as the underlying mechanism for these disease-modifying effects, prior studies have not explicitly examined cell types and synaptic mechanisms for long-term seizure suppression. To address this gap, we transplanted medial ganglionic eminence (MGE) cells from embryonic day 13.5 VGAT-Venus or VGATChR2-EYFP transgenic embryos into the dentate gyrus (DG) of adult mice 2 weeks after induction of TLE with pilocarpine. Beginning 3-4 weeks after status epilepticus, we conducted continuous video-electroencephalographic recording until 90 -100 d. TLE mice with bilateral MGE cell grafts in the DG had significantly fewer and milder electrographic seizures, compared with TLE controls. Immunohistochemical studies showed that the transplants contained multiple neuropeptide or calcium-binding protein-expressing interneuron types and these cells established dense terminal arborizations onto the somas, apical dendrites, and axon initial segments of dentate granule cells (GCs). A majority of the synaptic terminals formed by the transplanted cells were apposed to large postsynaptic clusters of gephyrin, indicative of mature inhibitory synaptic complexes. Functionality of these new inhibitory synapses was demonstrated by optogenetically activating VGAT-ChR2-EYFP-expressing transplanted neurons, which generated robust hyperpolarizations in GCs. These findings suggest that fetal GABAergic interneuron grafts may suppress pharmacoresistant seizures by enhancing synaptic inhibition in DG neural circuits.

Fragile X syndrome (FXS), the most common inherited form of intellectual disability and prevailing known genetic basis of autism, is caused by an expansion in the Fmr1 gene that prevents transcription and translation of fragile X mental retardation protein (FMRP). FMRP binds to and controls translation of mRNAs downstream of metabotropic glutamate receptor (mGluR) activation. Recent work identified striatal-enriched protein tyrosine phosphatase (STEP) as an FMRP target mRNA. STEP opposes synaptic strengthening and promotes synaptic weakening by dephosphorylating its substrates, including ERK1/2, p38, Fyn, Pyk2, and subunits of NMDA and AMPA receptors. Here we demonstrate that STEP translation is dysregulated in Fmr1KO mice, resulting in elevated basal levels of STEP with a concomitant loss of mGluR-dependent STEP translation. We hypothesized that the weakened synaptic strength and behavioral abnormalities reported in FXS may be linked to excess levels of STEP. To test this hypothesis, we reduced or eliminated STEP genetically in Fmr1KO mice. In addition to attenuating audiogenic seizures and seizure-induced c-Fos activation in the periaqueductal gray, genetically reducing STEP in Fmr1KO mice reversed characteristic social abnormalities, including approach, investigation, novelty-induced hyperactivity and anxiety. Loss of STEP also corrected select non-social anxiety-related behaviors in Fmr1KO mice, such as open arm exploration in the elevated plus maze. Our findings indicate that genetically reducing STEP significantly diminishes seizures and restores social and non-social anxiety-related behaviors in Fmr1KO mice, suggesting that strategies to inhibit STEP activity may be effective for treating patients with FXS.

Immunocytochemical and biochemical studies were conducted to characterize a brain-specific protein tyrosine phosphatase, designated STEP for striatal enriched phosphatase. STEP immunoreactivity was most intense in select regions of the CNS receiving a dopaminergic input, and was localized to cell bodies, dendrites, and axonal processes. Western blot analyses of rat brain homogenates revealed a triplet of polypeptides with relative mobilities (M(r)) of 46 kDa, 37 kDa, and 33 kDa enriched within the striatum. Phase separation of protein homogenates by Triton X-114 extraction indicated that this triplet was enriched in soluble but not membrane fractions. Affinity-purified STEP fusion protein exhibited phosphatase activity while a mutated form of the STEP fusion protein (Cys300Ser) showed no demonstrable phosphatase activity.

The STEP family of protein tyrosine phosphatases is highly enriched within the CNS. Members of this family are alternatively spliced to produce both transmembrane and cytosolic variants. This manuscript describes the distinctive intracellular distribution and enzymatic activity of the membrane-associated isoform STEP 61 . Transfection experiments in fibroblasts, as well as subcellular fractionations, sucrose density gradients, immunocytochemical labeling, and electron microscopy in brain tissue, show that STEP 61 is an intrinsic membrane protein of striatal neurons and is associated with the endoplasmic reticulum. In addition, structural analysis of the novel N-terminal region of STEP 61 reveals several motifs not present in the cytosolic variant STEP 46 . These include two putative transmembrane domains, two sequences rich in Pro, Glu, Asp, Ser, and Thr (PEST sequences), and two polyproline-rich domains. Like STEP 46 , STEP 61 is enriched in the brain, but the recombinant protein has less enzymatic activity than STEP 46 . Because STEP 46 is contained in its entirety within STEP 61 and differs only in the extended N terminus of STEP 61 , this amino acid sequence is responsible for the association of STEP 61 with membrane compartments and may also regulate its enzymatic activity.