Summary Sporadic Alzheimer’s disease (AD) exclusively affects elderly people. Using direct conversion of AD patient fibroblasts into induced neurons (iNs), we generated an age-equivalent neuronal model. AD patient-derived iNs exhibit strong neuronal transcriptome signatures characterized by downregulation of mature neuronal properties and upregulation of immature and progenitor-like signaling pathways. Mapping iNs to longitudinal neuronal differentiation trajectory data demonstrated that AD iNs reflect a hypo-mature neuronal identity characterized by markers of stress, cell cycle, and de-differentiation. Epigenetic landscape profiling revealed an underlying aberrant neuronal state that shares similarities with malignant transformation and age-dependent epigenetic erosion. To probe for the involvement of aging, we generated rejuvenated iPSC-derived neurons that showed no significant disease-related transcriptome signatures, a feature that is consistent with epigenetic clock and brain ontogenesis mapping, which indicate that fibroblast-derived iNs more closely reflect old adult brain stages. Our findings identify AD-related neuronal changes as age-dependent cellular programs that impair neuronal identity.

Trait-associated loci often map to genomic regions encoding long noncoding RNAs (lncRNAs), but the role of these lncRNAs in disease etiology is largely unexplored. We show that a pair of sense/antisense lncRNA (6p22lncRNAs) encoded by CASC15 and NBAT1 located at the neuroblastoma (NB) risk-associated 6p22.3 locus are tumor suppressors and show reduced expression in high-risk NBs. Loss of functional synergy between 6p22lncRNAs results in an undifferentiated state that is maintained by a gene-regulatory network, including SOX9 located on 17q, a region frequently gained in NB. 6p22lncRNAs regulate SOX9 expression by controlling CHD7 stability via modulating the cellular localization of USP36, encoded by another 17q gene. This regulatory nexus between 6p22.3 and 17q regions may lead to potential NB treatment strategies.

Mitochondria are a major target for aging and are instrumental in the age-dependent deterioration of the human brain, but studying mitochondria in aging human neurons has been challenging. Direct fibroblast-to-induced neuron (iN) conversion yields functional neurons that retain important signs of aging, in contrast to iPSC differentiation. Here, we analyzed mitochondrial features in iNs from individuals of different ages. iNs from old donors display decreased oxidative phosphorylation (OXPHOS)-related gene expression, impaired axonal mitochondrial morphologies, lower mitochondrial membrane potentials, reduced energy production, and increased oxidized proteins levels. In contrast, the fibroblasts from which iNs were generated show only mild age-dependent changes, consistent with a metabolic shift from glycolysis-dependent fibroblasts to OXPHOS-dependent iNs. Indeed, OXPHOS-induced old fibroblasts show increased mitochondrial aging features similar to iNs. Our data indicate that iNs are a valuable tool for studying mitochondrial aging and support a bioenergetic explanation for the high susceptibility of the brain to aging.

Presynaptic α 2 δ subunits of voltage-gated calcium channels regulate channel abundance and are involved in glutamatergic synapse formation. However, little is known about the specific functions of the individual α 2 δ isoforms and their role in GABAergic synapses. Using primary neuronal cultures of embryonic mice of both sexes, we here report that presynaptic overexpression of α 2 δ-2 in GABAergic synapses strongly increases clustering of postsynaptic GABA A Rs. Strikingly, presynaptic α 2 δ-2 exerts the same effect in glutamatergic synapses, leading to a mismatched localization of GABA A Rs. This mismatching is caused by an aberrant wiring of glutamatergic presynaptic boutons with GABAergic postsynaptic positions. The trans-synaptic effect of α 2 δ-2 is independent of the prototypical cell-adhesion molecules α-neurexins (α-Nrxns); however, α-Nrxns together with α 2 δ-2 can modulate postsynaptic GABA A R abundance. Finally, exclusion of the alternatively spliced exon 23 of α 2 δ-2 is essential for the trans-synaptic mechanism. The novel function of α 2 δ-2 identified here may explain how abnormal α 2 δ subunit expression can cause excitatory–inhibitory imbalance often associated with neuropsychiatric disorders. SIGNIFICANCE STATEMENT Voltage-gated calcium channels regulate important neuronal functions such as synaptic transmission. α 2 δ subunits modulate calcium channels and are emerging as regulators of brain connectivity. However, little is known about how individual α 2 δ subunits contribute to synapse specificity. Here, we show that presynaptic expression of a single α 2 δ variant can modulate synaptic connectivity and the localization of inhibitory postsynaptic receptors. Our findings provide basic insights into the development of specific synaptic connections between nerve cells and contribute to our understanding of normal nerve cell functions. Furthermore, the identified mechanism may explain how an altered expression of calcium channel subunits can result in aberrant neuronal wiring often associated with neuropsychiatric disorders such as autism or schizophrenia.