Intron retention (IR) by alternative splicing is a conserved regulatory mechanism that can affect gene expression and protein function during adult development and age‐onset diseases. However, it remains unclear whether IR undergoes spatial or temporal changes during different stages of aging or neurodegeneration like Alzheimer's disease (AD). By profiling the transcriptome of Drosophila head cells at different ages, we observed a significant increase in IR events for many genes during aging. Differential IR affects distinct biological functions at different ages and occurs at several AD‐associated genes in older adults. The increased nucleosome occupancy at the differentially retained introns in young animals suggests that it may regulate the level of IR during aging. Notably, an increase in the number of IR events was also observed in healthy older mouse and human brain tissues, as well as in the cerebellum and frontal cortex from independent AD cohorts. Genes with differential IR shared many common features, including shorter intron length, no perturbation in their mRNA level, and enrichment for biological functions that are associated with mRNA processing and proteostasis. The differentially retained introns identified in AD frontal cortex have higher GC content, with many of their mRNA transcripts showing an altered level of protein expression compared to control samples. Taken together, our results suggest that an increased IR is an conserved signature that is associated with aging. By affecting pathways involved in mRNA and protein homeostasis, changes of IR pattern during aging may regulate the transition from healthy to pathological state in late‐onset sporadic AD.

Abstract. A 40-kD protein kinase C (PKC)E related activity was found to associate with human epithelial specific cytokeratin (CK) polypeptides 8 and 18. The kinase activity coimmunoprecipitated with CK8 and 18 and phosphorylated immunoprecipitates of the CK. Immunoblot analysis of CK8/18 immunoprecipitates using an anti-PKCE specific antibody showed that the 40-kD species, and not native PKCE (90 kD) associated with the cytokeratins. Reconstitution experiments demonstrated that purified CK8 or CK18 associated with a 40-kD tryptic fragment of purified PKCE, or with a similar species obtained from cells that express the fragment constitutively but do not express CK8/18. C YTOKERATINS (CK)' are a group of intermediate filament (IF) proteins which are expressed primarily in epithelial tissues (Lazarides, 1982 ;Steinert and Roop, 1988 ;Franke et al ., 1981;Osborn and Weber, 1986) . The 30 or so polypeptides which make up the family ofcytokeratin proteins are divided into acidic (type I) and basic/ neutral (type II) keratins. In epithelial cells, CK are found as mosaic noncovalent polymers with an assembly consisting of at least one type I and one type II CK (Steinert and Roop, 1988). For example, "simple" single layer epithelial cells such as intestinal epithelia express CK8 (type II) and CK18 (type I), whereas esophageal epithelial cells express CK4 (type II) and CK13 (type I) predominantly. Cytokeratins are not only important as tissue-specific markers, they also form important markers of cell differentiation .IF proteins, including cytokeratins, undergo several posttranslational modifications such as N112-terminal acetylation (Steinert and Idler, 1975), glycosylation (King and Hounsell, 1989;Roberts and Brunt, 1986), and serine/threonine phosphorylation (Steinert, 1988;Gilmartin et al ., 1984 ;Yeagle et al., 1990;Baribault et al., 1989) . The functional role of IF protein phosphorylation is not well under- A peptide pseudosubstrate specific for PKCE inhibited phosphorylation of CK8/18 in intact cells or in a kinase assay with CK8/18 immunoprecipitates . Tryptic peptide map analysis -of the cytokeratins that were phosphorylated by purified rat brain PKCE or as immunoprecipitates by the associated kinase showed similar phosphopeptides . Furthermore, PKCE immunoreactive species and CK8/18 colocalized using immunofluorescent double staining . We propose that a kinase related to the catalytic fragment of PKCE physically associates with and phosphorylates cytokeratins 8 and 18.

Alzheimer’s disease (AD) is characterized by the accumulation of amyloid-β plaques and Tau tangles in brain tissues. Recent studies indicate that aberrant splicing and increased level of intron retention is linked to AD pathogenesis. Bioinformatic analysis revealed increased retention of intron 11 at the Tau gene in AD female dorsal lateral prefrontal cortex as compared to healthy controls, an observation validated by quantitative polymerase chain reaction using different brain tissues. Retention of intron 11 introduces a premature stop codon, resulting in the production of truncated Tau11i protein. Probing with customized antibodies designed against amino acids encoded by intron 11 showed that Tau11i protein is more enriched in AD hippocampus, amygdala, parietal, temporal, and frontal lobe than in healthy controls. This indicates that Tau messenger RNA with the retained intron is translated in vivo instead of being subjected to nonsense-mediated decay. Compared to full-length Tau441 isoform, ectopically expressed Tau11i forms higher molecular weight species, is enriched in Sarkosyl-insoluble fraction, and exhibits greater protein stability in cycloheximide assay. Stably expressed Tau11i also shows weaker colocalization with α-tubulin of microtubule network in human mature cortical neurons as compared to Tau441. Endogenous Tau11i is enriched in Sarkosyl-insoluble fraction in AD hippocampus and forms aggregates that colocalize weakly with Tau4R fibril-like structure in AD temporal lobe. The elevated level of Tau11i protein in AD brain tissues tested, coupled with biochemical properties resembling pathological Tau species suggest that retention of intron 11 of Tau gene might be an early biomarker of AD pathology.