synaptojanin1 Is Required for Temporal Fidelity of Synaptic Transmission in Hair Cells

To faithfully encode mechanosensory information, auditory/vestibular hair cells utilize graded synaptic vesicle (SV) release at specialized ribbon synapses. The molecular basis of SV release and consequent recycling of membrane in hair cells has not been fully explored. Here, we report that comet, a gene identified in an ENU mutagenesis screen for zebrafish larvae with vestibular defects, encodes the lipid phosphatase Synaptojanin 1 (Synj1). Examination of mutant synj1 hair cells revealed basal blebbing near ribbons that was dependent on Cav1.3 calcium channel activity but not mechanotransduction. Synaptojanin has been previously implicated in SV recycling; therefore, we tested synaptic transmission at hair-cell synapses. Recordings of post-synaptic activity in synj1 mutants showed relatively normal spike rates when hair cells were mechanically stimulated for a short period of time at 20 Hz. In contrast, a sharp decline in the rate of firing occurred during prolonged stimulation at 20 Hz or stimulation at a higher frequency of 60 Hz. The decline in spike rate suggested that fewer vesicles were available for release. Consistent with this result, we observed that stimulated mutant hair cells had decreased numbers of tethered and reserve-pool vesicles in comparison to wild-type hair cells. Furthermore, stimulation at 60 Hz impaired phase locking of the postsynaptic activity to the mechanical stimulus. Following prolonged stimulation at 60 Hz, we also found that mutant synj1 hair cells displayed a striking delay in the recovery of spontaneous activity. Collectively, the data suggest that Synj1 is critical for retrieval of membrane in order to maintain the quantity, timing of fusion, and spontaneous release properties of SVs at hair-cell ribbon synapses

Novel regulators of megakaryopoiesis: The road less traveled by

Platelet transfusions are given to patients with low platelet counts due to, for example, chemotherapy. Currently, research is done to produce platelets in culture systems. Although researchers are successful in producing platelets, the yield of these systems is still inefficient due to our limited knowledge about the differentiation of hematopoietic stem cells into megakaryocytes, the platelet precursors. We identified the transcription factor MEIS1 to be specific for megakaryocytes when the transcriptomes of differentiated adult blood cells was compared. Given the exclusive expression in megakaryocytes, we therefore investigated the contribution of MEIS1 and several MEIS1 targets to megakaryopoiesis. We show that MEIS1 specifies hematopoietic progenitor cells to become megakaryocytes by enforcing expression of lineage specific genes and inhibiting granulocytic gene expression. Next, the role of two MEIS1 targets, ATXN2 and TPM1, was studied. Our results show that the RNA binding protein ATXN2 controls protein synthesis during early megakaryopoiesis which directly impacts on platelet reactivity. We thus provide first evidence for the importance of controlled protein synthesis in megakaryopoiesis. Furthermore, researchers have been debating the relevance of actin during megakaryopoiesis. When analyzing the role of the actin-binding protein TPM1, we could show that TPM1 controls proper actin localization and polymerization during early and late events of megakaryopoiesis, clearly underlining the importance of actin in these processes. In the context of current platelet culture systems, our resultsmay contribute to more efficient megakaryocytic lineage commitment and thus the theoretic yield of platelets, as well as to higher quality of the produced platelets