We do not know the cause of Dicer loss but since Dicer is expressed in neurons the finding may simply reflect severe neuron loss in the tissue. Other biogenesis components, however, did not show reduced levels and astrocytes, which also express Dicer, were increased in HS tissue. Indeed, the selective deletion of Dicer from neurons only reduces hippocampal protein levels by,60%, presumably due to residual glial Dicer. Taken together, this favors a more specific cause of the down-regulation. In addition to possible transcriptional downregulation, Dicer is known to be a substrate for cleavagedependent inactivation by caspases and caspases are activated in HS tissue from TLE patients. Caspasemediated Dicer cleavage can also convert Dicer into a proapoptotic DNase and cells positive for DNA fragmentation are present in resected TLE material. We detected possible Dicer cleavage in both human and experimental epilepsy, although the lower-weight species was most similar to a reported calpain-generated fragment. Identifying ways to rescue Dicer or block its down-regulation could offer a means to reconstitute miRNA expression in HS tissue. The GDC-0941 temporal cortex samples from TLE patients displayed normal levels of both Dicer and Drosha.

It is notable that AGO2 is capable of processing certain miRNAs without Dicer, including miR-451, and miR-451 was among the few upregulated miRNAs in TLE-HS tissue. Thus, elevated AGO2 in human TLE may contribute to maintaining levels of this miRNA. Recent work suggests there are about 600 miRNAs expressed in the human brain, although a collection of,20 miRNAs account for nearly 90%. The present study included profiling of mature miRNA expression in normal human autopsy hippocampus, and nearly 200 were detected. Among those expressed, 15 have previously been reported as abundant in human dorsolateral prefrontal cortex, including miR-26a and miR-125b. Among other abundant miRNAs were known brain-enriched miRNAs, including miR-9, and miR-132 and astrocyte-expressed miRNAs such as miR-29a. The second major finding in the present study was that TLEHS tissue displayed lower expression of many mature miRNAs. Two-thirds of the detected miRNAs were present at lower levels than in controls, and nearly a quarter of the control-expressed miRNAs were not detected in TLE-HS samples. This indicates a failure or collapse of mature miRNA expression in human TLE-HS. Our data are in congruence, therefore, with the effect of experimental inactivation of Dicer which results in reduced levels of many, but not all, miRNAs.

The similar level of several pri-miRNAs and Drosha in TLE-HS is also supportive of the defect in mature miRNA production being at the level of Dicer. Notably, miR-486 levels do not change in the hippocampus after Dicer loss and this miRNA was not differently expressed in TLE-HS in our study. The persistence of some mature miRNA production in the sclerotic hippocampus may be due to residual Dicer activity, longevity of the mature miRNAs in hippocampus after Dicer loss, or compensation by other proteins within the miRNA biogenesis pathway. Although loss of Dicer is the most obvious explanation for the observed large-scale reduction in miRNA levels, differences in expression of other miRNA biogenesis components such as the Drosha partner DiGerorge syndrome chromosome 8 or Dicer partner transactivating response RNA-binding protein.