SUMMARY The promise of using reprogrammed human neurons for disease modeling and regenerative medicine relies on the ability to induce patient-derived neurons with high efficiency and subtype-specificity. We have previously shown that ectopic expression of brain-enriched microRNAs (miRNA), miR-9/9* and miR-124 (miR-9/9*-124), promoted direct conversion of human fibroblasts into neurons. Here we show that co-expression of miR-9/9*-124 with transcription factors enriched in the developing striatum, BCL11B (also known as CTIP2), DLX1, DLX2, MYT1L, can guide the conversion of human postnatal and adult fibroblasts into an enriched population of neurons analogous to striatal medium spiny neurons (MSNs). When transplanted in the mouse brain, the reprogrammed human cells persisted in situ for over 6 months, exhibited membrane properties equivalent to native MSNs and extended projections to the anatomical targets of MSNs. These findings highlight the potential of exploiting the synergism between miR-9/9*-124 and transcription factors to generate specific neuronal subtypes.

In Huntington’s disease (HD), expansion of CAG codons within the huntingtin gene (HTT) leads to the aberrant formation of protein aggregates and the differential degeneration of striatal medium spiny neurons (MSNs). Modeling HD using patient-specific MSNs has been challenging, as neurons differentiated from induced pluripotent stem cells are free of aggregates and lack an overt cell death phenotype. Here we generated MSNs from HD patient fibroblasts through microRNA-based neuronal conversion, previously shown to bypass the induction of pluripotency and retain age signatures of original fibroblasts. We found that patient MSNs consistently exhibited mutant HTT (mHTT) aggregates, mHTT-dependent DNA damage, mitochondrial dysfunction, and spontaneous degeneration over time in culture. We further provide evidence that erasure of age stored in starting fibroblasts and neuronal conversion of pre-symptomatic HD patient fibroblasts resulted in differential manifestation of cellular phenotypes associated with HD, highlighting the importance of age in modeling late-onset neurological disorders.

In Alzheimer's disease (AD), amyloid deposits along the brain vasculature leading to a condition known as cerebral amyloid angiopathy (CAA), which impairs blood-brain barrier (BBB) function and accelerates cognitive degeneration. APOE4 is the strongest risk factor for CAA, yet the mechanisms underlying this genetic susceptibility are unknown. Here, we developed an iPSCbased 3D model that recapitulates anatomical and physiological properties of the human BBB in vitro. Similar to CAA, our in vitro BBB displayed significantly more amyloid accumulation in APOE4 compared to APOE3. Combinatorial experiments revealed that dysregulation of Calcineurin/NFAT-signaling and APOE in pericyte-like mural cells induces APOE4-associated CAA pathology. In the human brain, we identify APOE and NFAT are selectively dysregulated in pericytes of APOE4-carriers, and that inhibiting calcineurin/NFAT-signaling reduces APOE4associated CAA pathology in vitro and in vivo. Our study reveals the role of pericytes in APOE4mediated CAA and highlights calcineurin/NFAT-signaling as a therapeutic target in CAA and AD.The BBB is critical for proper neuronal function, protecting the brain from pathogens and tightly regulating the composition of brain fluids. Neuronal health is directly coupled to the *

Aging is a major risk factor in many forms of late-onset neurodegenerative disorders. The ability to recapitulate age-related characteristics of human neurons in culture will offer unprecedented opportunities to study the biological processes underlying neuronal aging. Here, we show that using a recently demonstrated microRNA-based cellular reprogramming approach, human fibroblasts from postnatal to near centenarian donors can be efficiently converted into neurons that maintain multiple age-associated signatures. Application of an epigenetic biomarker of aging (referred to as epigenetic clock) to DNA methylation data revealed that the epigenetic ages of fibroblasts were highly correlated with corresponding age estimates of reprogrammed neurons. Transcriptome and microRNA profiles reveal genes differentially expressed between young and old neurons. Further analyses of oxidative stress, DNA damage and telomere length exhibit the retention of age-associated cellular properties in converted neurons from corresponding fibroblasts. Our results collectively demonstrate the maintenance of age after neuronal conversion.DOI: http://dx.doi.org/10.7554/eLife.18648.001