石川 充

The striatum plays a central role in motor control, cognition, reward processing, and habit formation, and its dysfunction is implicated in a broad spectrum of neurological and psychiatric disorders. Although animal models have provided important insights into striatal development and disease mechanisms, species-specific differences in cellular composition, developmental timing, and circuit organization limit their translational relevance to the human brain. In this context, human pluripotent stem cells (PSCs), including embryonic stem cells and induced pluripotent stem cells, have emerged as valuable platforms for modeling human striatal development and pathology in vitro. In this review, we summarize current approaches for generating striatal cell types from PSCs, with a particular focus on medium spiny neurons (MSNs), the principal projection neurons of the striatum. We discuss key developmental principles underlying dorsal and ventral striatal specification and highlight the protracted maturation of human MSNs, which may contribute to human-specific disease vulnerability. Advances in differentiation strategies, including small molecule-based patterning, transcription factor-driven induction, and three-dimensional organoid and assembloid systems, have progressively improved the efficiency, reproducibility, and cellular complexity of PSC-derived striatal models. We further review applications of PSC-derived striatal systems in disease modeling, noting that most studies to date have focused on Huntington's disease, where these models have revealed early developmental, transcriptional, synaptic, and network-level abnormalities. More recent studies have begun to extend these approaches to other neurological conditions and to incorporate circuit-level analyses using cortico-striatal assembloids. In parallel, the growing availability of single-cell and single-nucleus transcriptomic datasets from the human striatum provides powerful reference frameworks for benchmarking the identity and maturation state of PSC-derived striatal cells. Finally, we discuss current challenges and limitations of PSC-based striatal models, including incomplete maturation, limited representation of non-neuronal cell types, and restricted applicability to psychiatric disorders. We propose that continued integration of developmental biology, public multi-omics resources, and advanced in vitro modeling strategies will be essential for advancing human striatal models and expanding their utility in translational neuroscience.

Benign adult familial myoclonus epilepsy (BAFME) is caused by intronic TTTCA and TTTTA repeat expansions in SAMD12 and other genes; the neuronal basis of cortical hyperexcitability, however, remains unclear. We generated induced pluripotent stem cell (iPSC)-derived glutamatergic and GABAergic neurons from three BAFME1 patients and examined functional and transcriptomic phenotypes. Patient-derived neurons retained the pathogenic repeat expansions and showed a tendency toward upstream intronic RNA accumulation. Calcium imaging revealed increased spontaneous Ca2 + transient frequency in both neuronal subtypes, indicating heightened activity. Pharmacological profiling demonstrated attenuated responses to calcium-permeable α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptor (CP-AMPAR) blockade and GABAA receptor antagonism in GABAergic neurons, suggesting altered inhibitory signaling. RNA sequencing revealed transcriptomic alterations without differential expression of ion channels and neurotransmitter receptors. In glutamatergic neurons, ATF4-regulated genes, including SLC7A5 encoding LAT1, a Kv1.2 channel modulator, were downregulated. Reduced SLC7A5 expression was validated at both mRNA and protein levels. In GABAergic neurons, synapse-associated genes PTPRD and GPC6 were upregulated. TCERG1L and NLRP2 were suppressed across both neuronal subtypes. These findings suggest subtype-specific alterations may contribute to neuronal hyperexcitability in BAFME and provide a platform for mechanistic studies of repeat expansion-associated epilepsies.

Several protocols for generating oligodendrocytes (OLs) from human pluripotent stem cells have been reported. However, they are limited by long culture duration, intensive handling, and low yield of mature OLs. Transcription factor-based strategies have improved efficiency, but OLIG2 and SOX10, key regulators of oligodendrocyte precursor cells (OPCs), also promote alternative neural fates. Here, we developed a Tet-inducible system to control SOX10 and OLIG2 expression, including that of a phosphorylation-deficient OLIG2 mutant (S147A). Co-expression of SOX10 and OLIG2 enhanced OPC induction, confirmed by O4 positivity and transcriptomic profiling. Interestingly, only a brief induction of SOX10 + OLIG2(S147A) (2-5 days) efficiently yielded myelin basic protein positive OLs within 25 days, reaching approximately 20% of total cells. In contrast, sustained doxycycline-mediated expression of SOX10 and OLIG2(S147A) favored OPC proliferation and delayed OL maturation. These findings highlight the importance of temporal control of transcription factor activity in accelerating OL differentiation and provide a practical platform for disease modeling and regenerative applications.

The direct reprogramming of cells has tremendous potential in in vitro neurological studies. Previous attempts to convert blood cells into induced neurons have presented several challenges, necessitating a less invasive, efficient, rapid, and convenient approach. The current study introduces an optimized method for converting somatic cells into neurons using a nonsurgical approach that employs peripheral blood cells as an alternative source to fibroblasts. We have demonstrated the efficacy of a unique combination of transcription factors, including NEUROD1, and four Yamanaka reprogramming factors (OCT3/4, SOX2, KLF4, and c-MYC), in generating glutamatergic neurons within 3 wk. This approach, which requires only five pivotal factors (NEUROD1, OCT3/4, SOX2, KLF4, and c-MYC), has the potential to create functional neurons and circumvents the need for induced pluripotent stem cell (iPSC) intermediates, as evidenced by single-cell RNA sequencing and whole-genome bisulfite sequencing, along with lineage-tracing experiments using Cre-LoxP system. While fibroblasts have been widely used for neuronal reprogramming, our findings suggest that peripheral blood cells offer a potential alternative, particularly in contexts where minimally invasive sampling and procedures convenient for patients are emphasized. This method provides a rapid strategy for modeling neuronal diseases and contributes to advancements in drug discovery and personalized medicine.