Flexible pivoting of dynamin pleckstrin homology domain catalyzes fission: insights into molecular degrees of freedom.

The neuronal dynamin1 functions in the release of synaptic vesicles by orchestrating the process of GTPase-dependent membrane fission. Dynamin1 associates with the plasma membrane-localized phosphatidylinositol-4,5-bisphosphate (PIP2) through the centrally located pleckstrin homology domain (PHD). The PHD is dispensable as fission (in model membranes) can be managed, even when the PHD-PIP2 interaction is replaced by a generic polyhistidine- or polylysine-lipid interaction. However, the absence of the PHD renders a dramatic dampening of the rate of fission. These observations suggest that the PHD-PIP2-containing membrane interaction could have evolved to expedite fission to fulfill the requirement of rapid kinetics of synaptic vesicle recycling. Here, we use a suite of multiscale modeling approaches to explore PHD-membrane interactions. Our results reveal that 1) the binding of PHD to PIP2-containing membranes modulates the lipids toward fission-favoring conformations and softens the membrane, and 2) PHD associates with membrane in multiple orientations using variable loops as pivots. We identify a new loop (VL4), which acts as an auxiliary pivot and modulates the orientation flexibility of PHD on the membrane-a mechanism that we believe may be important for high-fidelity dynamin collar assembly. Together, these insights provide a molecular-level understanding of the catalytic role of PHD in dynamin-mediated membrane fission.

Dynamin 2-dependent endocytosis is essential for mouse oocyte development and fertility.

During folliculogenesis, oocytes are dependent on metabolic and molecular support from surrounding somatic cells. Here, we examined the role of the dynamin (DNM) family of mechanoenzymes in mediating endocytotic uptake into growing follicular oocytes. We found DNM1 and DNM2 to be highly expressed in growing follicular oocytes as well as in mature germinal vesicle (GV) and metaphase II (MII) stage oocytes. Moreover, oocyte-specific conditional knockout (cKO) of DNM2 (DNM2Δ) led to complete sterility, with follicles arresting at the preantral stage of development. In addition, DNM2Δ ovaries were characterized by disrupted follicular growth as well as oocyte and follicle apoptosis. Further, the loss of DNM activity, either through DNM2 cKO or through pharmacological inhibition (Dyngo 6a) led to the impairment of endocytotic pathways in preantral oocytes as well as in mature GV and MII oocytes, respectively. Loss of DNM activity resulted in the redistribution of endosomes and the misslocalization of clathrin and actin, suggesting dysfunctional endocytosis. Notably, there was no observable effect on the fertility of DNM1Δ females. Our study has provided new insight into the complex and dynamic nature of oocyte growth during folliculogenesis, suggesting a role for DNM2 in mediating the endocytotic events that are essential for oocyte development.

Inhibition of Endocytosis of Clathrin-Mediated Angiotensin II Receptor Type 1 in Podocytes Augments Glomerular Injury.

BACKGROUND: Inhibition of the renin-angiotensin system remains a cornerstone in reducing proteinuria and progression of kidney failure, effects believed to be the result of reduction in BP and glomerular hyperfiltration. However, studies have yielded conflicting results on whether podocyte-specific angiotensin II (AngII) signaling directly induces podocyte injury. Previous research has found that after AngII stimulation, ß-arrestin-bound angiotensin II receptor type 1 (AT1R) is internalized in a clathrin- and dynamin-dependent manner, and that Dynamin1 and Dynamin2 double-knockout mice exhibit impaired clathrin-mediated endocytosis. METHODS: We used podocyte-specific Dyn double-knockout mice to examine AngII-stimulated AT1R internalization and signaling in primary podocytes and controls. We also examined the in vivo effect of AngII in these double-knockout mice through renin-angiotensin system blockers and through deletion of Agtr1a (which encodes the predominant AT1R isoform expressed in kidney, AT1aR). We tested calcium influx, Rac1 activation, and lamellipodial extension in control and primary podocytes of Dnm double-knockout mice treated with AngII. RESULTS: We confirmed augmented AngII-stimulated AT1R signaling in primary Dnm double-knockout podocytes resulting from arrest of clathrin-coated pit turnover. Genetic ablation of podocyte Agtr1a in Dnm double-knockout mice demonstrated improved albuminuria and kidney function compared with the double-knockout mice. Isolation of podocytes from Dnm double-knockout mice revealed abnormal membrane dynamics, with increased Rac1 activation and lamellipodial extension, which was attenuated in Dnm double-knockout podocytes lacking AT1aR. CONCLUSIONS: Our results indicate that inhibiting aberrant podocyte-associated AT1aR signaling pathways has a protective effect in maintaining the integrity of the glomerular filtration barrier.

Dynamin 1 Restrains Vesicular Release to a Subquantal Mode In Mammalian Adrenal Chromaffin Cells.

Dynamin 1 (dyn1) is required for clathrin-mediated endocytosis in most secretory (neuronal and neuroendocrine) cells. There are two modes of Ca2+-dependent catecholamine release from single dense-core vesicles: full-quantal (quantal) and subquantal in adrenal chromaffin cells, but their relative occurrences and impacts on total secretion remain unclear. To address this fundamental question in neurotransmission area using both sexes of animals, here we report the following: (1) dyn1-KO increased quantal size (QS, but not vesicle size/content) by ≥250% in dyn1-KO mice; (2) the KO-increased QS was rescued by dyn1 (but not its deficient mutant or dyn2); (3) the ratio of quantal versus subquantal events was increased by KO; (4) following a release event, more protein contents were retained in WT versus KO vesicles; and (5) the fusion pore size (dp) was increased from ≤9 to ≥9 nm by KO. Therefore, Ca2+-induced exocytosis is generally a subquantal release in sympathetic adrenal chromaffin cells, implying that neurotransmitter release is generally regulated by dynamin in neuronal cells.SIGNIFICANCE STATEMENT Ca2+-dependent neurotransmitter release from a single vesicle is the primary event in all neurotransmission, including synaptic/neuroendocrine forms. To determine whether Ca2+-dependent vesicular neurotransmitter release is "all-or-none" (quantal), we provide compelling evidence that most Ca2+-induced secretory events occur via the subquantal mode in native adrenal chromaffin cells. This subquantal release mode is promoted by dynamin 1, which is universally required for most secretory cells, including neurons and neuroendocrine cells. The present work with dyn1-KO mice further confirms that Ca2+-dependent transmitter release is mainly via subquantal mode, suggesting that subquantal release could be also important in other types of cells.

Mutations in the PH Domain of DNM1 are associated with a nonepileptic phenotype characterized by developmental delay and neurobehavioral abnormalities.

BACKGROUND: Dynamin 1 is a protein involved in the synaptic vesicle cycle, which facilitates the exocytosis of neurotransmitters necessary for normal signaling and development in the central nervous system. Pathogenic variants in DNM1 have been implicated in global developmental delay (DD), severe intellectual disability (ID), and notably, epileptic encephalopathy. All previously reported DNM1 pathogenic variants causing this severe phenotype occur in the GTPase and Middle domains of the dynamin 1 protein. METHODS: We used whole-exome sequencing to characterize the molecular basis of DD and autistic symptoms in two identical siblings. RESULTS: The twin siblings exhibit mild to moderate ID and autistic symptoms but no epileptic encephalopathy. Exome sequencing revealed a genetic variant, c.1603A>G (p.Lys535Glu), in the PH domain of dynamin 1. Previous in vitro studies showed that mutations at Lys535 inhibit endocytosis and impair PH loop binding to PIP2. CONCLUSIONS: Our data suggest a previously undescribed milder phenotype associated with a missense genetic variant in the PH domain of dynamin 1.

Dynamin-1 deletion enhances post-tetanic potentiation and quantal size after tetanic stimulation at the calyx of Held.

KEY POINTS: Post-tetanic potentiation (PTP) is attributed mainly to an increase in release probability (Pr ) and/or readily-releasable pool (RRP) in many synapses, but the role of endocytosis in PTP is unknown. Using the calyx of Held synapse from tissue-specific dynamin-1 knockout (cKO) mice (P16-20), we report that cKO synapses show enhanced PTP compared to control. We found significant increases in both spontaneous excitatory postsynaptic current (spEPSC) amplitude and RRP size (estimated by a train of 30 APs at 100 Hz) in cKO over control during PTP. Actin depolymerization blocks the increase in spEPSC amplitude in both control and cKO, and it abolishes the enhancement of PTP in cKO. PTP is sensitive to the PKC inhibitor GF109203X in both control and cKO. We conclude that an activity-dependent quantal size increase contributes to the enhancement of PTP in cKO over control and an altered endocytosis affects short-term plasticity through quantal size changes. ABSTRACT: High-frequency stimulation leads to post-tetanic potentiation (PTP) at many types of synapses. Previous studies suggest that PTP results primarily from a protein kinase C (PKC)-dependent increase in release probability (Pr ) and/or readily-releasable pool (RRP) of synaptic vesicles (SVs), but the role of SV endocytosis in PTP is unknown. Using the mature calyx of Held (P16-20), we report that tissue-specific ablation of dynamin-1 (cKO), an endocytic protein crucial for SV regeneration, enhances PTP in cKO over control. To explore the mechanism of this enhancement, we estimated the changes in paired-pulse ratios (PPRs) and RRP size during PTP. RRP was estimated by the back-extrapolation of cumulative EPSC amplitudes during a train of 30 action potentials at 100 Hz (termed RRPtrain ). We found an increase in RRPtrain during PTP in both control and cKO, but no significant changes in the PPR. Moreover, the amplitude and frequency of spontaneous excitatory postsynaptic currents (spEPSCs) increased during PTP in both control and cKO; however, the spEPSC amplitude in cKO during PTP was significantly larger than in control. Actin depolymerization reagent latrunculin-B (Lat-B) abolished the activity-dependent increase in spEPSC amplitude in both control and cKO, but selectively blocked the enhancement of PTP in cKO, without affecting PTP in control. PKC inhibitor GF109203X nearly abolished PTP in both control and cKO. These data suggest that the quantal size increase contributes to the enhancement of PTP in dynamin-1 cKO, and this change depends on strong synaptic activity and actin polymerization.

Tissue-specific dynamin-1 deletion at the calyx of Held decreases short-term depression through a mechanism distinct from vesicle resupply.

Dynamin is a large GTPase with a crucial role in synaptic vesicle regeneration. Acute dynamin inhibition impairs neurotransmitter release, in agreement with the protein's established role in vesicle resupply. Here, using tissue-specific dynamin-1 knockout [conditional knockout (cKO)] mice at a fast central synapse that releases neurotransmitter at high rates, we report that dynamin-1 deletion unexpectedly leads to enhanced steady-state neurotransmission and consequently less synaptic depression during brief periods of high-frequency stimulation. These changes are also accompanied by increased transmission failures. Interestingly, synaptic vesicle resupply and several other synaptic properties remain intact, including basal neurotransmission, presynaptic Ca(2+) influx, initial release probability, and postsynaptic receptor saturation and desensitization. However, acute application of Latrunculin B, a reagent known to induce actin depolymerization and impair bulk and ultrafast endocytosis, has a stronger effect on steady-state depression in cKO than in control and brings the depression down to a control level. The slow phase of presynaptic capacitance decay following strong stimulation is impaired in cKO; the rapid capacitance changes immediately after strong depolarization are also different between control and cKO and sensitive to Latrunculin B. These data raise the possibility that, in addition to its established function in regenerating synaptic vesicles, the endocytosis protein dynamin-1 may have an impact on short-term synaptic depression. This role comes into play primarily during brief high-frequency stimulation.

Modes and regulation of endocytic membrane retrieval in mouse auditory hair cells.

Synaptic vesicle recycling sustains high rates of neurotransmission at the ribbon-type active zones (AZs) of mouse auditory inner hair cells (IHCs), but its modes and molecular regulation are poorly understood. Electron microscopy indicated the presence of clathrin-mediated endocytosis (CME) and bulk endocytosis. The endocytic proteins dynamin, clathrin, and amphiphysin are expressed and broadly distributed in IHCs. We used confocal vglut1-pHluorin imaging and membrane capacitance (Cm) measurements to study the spatial organization and dynamics of IHC exocytosis and endocytosis. Viral gene transfer expressed vglut1-pHluorin in IHCs and targeted it to synaptic vesicles. The intravesicular pH was ∼6.5, supporting only a modest increase of vglut1-pHluorin fluorescence during exocytosis and pH neutralization. Ca(2+) influx triggered an exocytic increase of vglut1-pHluorin fluorescence at the AZs, around which it remained for several seconds. The endocytic Cm decline proceeded with constant rate (linear component) after exocytosis of the readily releasable pool (RRP). When exocytosis exceeded three to four RRP equivalents, IHCs additionally recruited a faster Cm decline (exponential component) that increased with the amount of preceding exocytosis and likely reflects bulk endocytosis. The dynamin inhibitor Dyngo-4a and the clathrin blocker pitstop 2 selectively impaired the linear component of endocytic Cm decline. A missense mutation of dynamin 1 (fitful) inhibited endocytosis to a similar extent as Dyngo-4a. We propose that IHCs use dynamin-dependent endocytosis via CME to support vesicle cycling during mild stimulation but recruit bulk endocytosis to balance massive exocytosis.

Protein partners of dynamin-1 in the retina.

Dynamin proteins are involved in vesicle generation, providing mechanical force to excise newly formed vesicles from membranes of cellular compartments. In the brain, dynamin-1, dynamin-2, and dynamin-3 have been well studied; however, their function in the retina remains elusive. A retina-specific splice variant of dynamin-1 interacts with the photoreceptor-specific protein Tubby-like protein 1 (Tulp1), which when mutated causes an early onset form of autosomal recessive retinitis pigmentosa. Here, we investigated the role of the dynamins in the retina, using immunohistochemistry to localize dynamin-1, dynamin-2, and dynamin-3 and immunoprecipitation followed by mass spectrometry to explore dynamin-1 interacting proteins in mouse retina. Dynamin-2 is primarily confined to the inner segment compartment of photoreceptors, suggesting a role in outer segment protein transport. Dynamin-3 is present in the terminals of photoreceptors and dendrites of second-order neurons but is most pronounced in the inner plexiform layer where second-order neurons relay signals from photoreceptors. Dynamin-1 appears to be the dominant isoform in the retina and is present throughout the retina and in multiple compartments of the photoreceptor cell. This suggests that it may function in multiple cellular pathways. Surprisingly, dynamin-1 expression and localization did not appear to be disrupted in tulp1−/− mice. Immunoprecipitation experiments reveal that dynamin-1 associates primarily with proteins involved in cytoskeletal-based membrane dynamics. This finding is confirmed by western blot analysis. Results further implicate dynamin-1 in vesicular protein transport processes relevant to synaptic and post-Golgi pathways and indicate a possible role in photoreceptor stability.

Phosphorylation and nitration of tyrosine residues affect functional properties of Synaptophysin and Dynamin I, two proteins involved in exo-endocytosis of synaptic vesicles.

Phosphorylation and nitration of protein tyrosine residues are thought to play a role in signaling pathways at the nerve terminal and to affect functional properties of proteins involved in the synaptic vesicle (SV) exo-endocytotic cycle. We previously demonstrated that the tyrosine residues in the C-terminal domain of the SV protein Synaptophysin (SYP) are targets of peroxynitrite (PN). Here, we have characterized the association between SYP and c-src tyrosine kinase demonstrating that phosphorylation of Tyr(273) in the C-terminal domain of SYP is crucial in mediating SYP binding to and activation of c-src. SYP forms a complex with Dynamin I (DynI), a GTPase required for SV endocytosis, which may be regulated by tyrosine phosphorylation of SYP. We here report that, in rat brain synaptosomes treated with PN, the formation of SYP/DynI complex was impaired. Noteworthy, we found that DynI was also modified by PN. DynI tyrosine phosphorylation was down-regulated in a dose-dependent manner, while DynI tyrosine nitration increased. Using mass spectrometry analysis, we identified Tyr(354) as one nitration site in DynI. In addition, we tested DynI self-assembly and GTPase activity, which are enhanced by c-src-dependent tyrosine phosphorylation of DynI, and found that both were inhibited by PN. Our results suggest that the site-specific tyrosine residue modifications may modulate the association properties of SV proteins and serve as a regulator of DynI function via control of self-assembly, thus influencing the physiology of the exo-endocytotic cycle.

Changed clathrin regulatory proteins in the brains of Alzheimer's disease patients and animal models.

In the study, the expression of clathrin regulatory proteins dynamin I, AP180, and synaptic vesicle protein synaptophysin in multiple brain regions of the patients with Alzheimer's disease (AD), the transgenic mice carrying the Swedish mutation of amyloid-ß protein precursor (AßPP) 670/671 (AßPPSWE), and the rats injected by bilateral hippocampus with amyloid-ß peptide (Aß)1-42 were examined by immunohistochemistry and Nissl staining, Western blotting, and Real-time PCR, respectively. Spatial learning and memory of the rats were evaluated by Morris Water Maze test, and the ability of endocytosis in the cultured rat hippocampal neurons was detected by FM1-43 fluorescence imaging. Significant decreases in protein levels of dynamin I, AP180, and synaptophysin were observed in both AD patients and mice with AßPPSWE as compared to controls. Obvious declines of dynamin I and synaptophysin at protein and mRNA levels and impaired learning and spatial memory ability were found in the rats injected with Aß1-42 as compared to controls. In addition, deposits of Aß localized in the hippocampus around the sites of Aß1-42 injection and the decreased numbers of Nissl bodies in neurons were found. Moreover, the disrupted synaptic vesicle endocytosis and decreased dynamin I protein were detected in stimulated hippocampal neurons treated with Aß1-42. These findings imply a malfunctioning clathrin-mediated endocytosis during AD pathological processes, which might be relevant to the mechanism underlying the cognitive deficit associated with AD.

Dynamin I plays dual roles in the activity-dependent shift in exocytic mode in mouse adrenal chromaffin cells.

Under low stimulation, adrenal chromaffin cells release freely soluble catecholamines through a restricted granule fusion pore while retaining the large neuropeptide-containing proteinacious granule core. Elevated activity causes dilation of the pore and release of all granule contents. Thus, physiological differential transmitter release is achieved through regulation of fusion pore dilation. We examined the mechanism for pore dilation utilizing a combined approach of peptide transfection, electrophysiology, electrochemistry and quantitative imaging techniques. We report that disruption of dynamin I function alters both fusion modes. Under low stimulation, interference with dynamin I does not affect granule fusion but blocks its re-internalization. In full collapse mode, disruption of dynamin I limits fusion pore dilation, but does not block membrane re-internalization. These data suggest that dynamin I is involved in both modes of exocytosis by regulating contraction or dilation of the fusion pore and thus contributes to activity-dependent differential transmitter release from the adrenal medulla.

Cell- and stimulus-dependent heterogeneity of synaptic vesicle endocytic recycling mechanisms revealed by studies of dynamin 1-null neurons.

Mice lacking expression of dynamin 1, a GTPase implicated in the fission reaction of synaptic vesicle endocytosis, fail to thrive and exhibit severe activity-dependent endocytic defects at their synapses. Here, we have used electron tomography to investigate the massive increase in clathrin-coated pit abundance that is selectively observed at a subset of synapses in dynamin 1 KO primary neuron cultures under conditions of spontaneous network activity. This increase, leading to branched tubular plasma membrane invaginations capped by clathrin-coated buds, occurs selectively at inhibitory synapses. A similar massive increase of clathrin-coated profiles (in this case, of clathrin-coated vesicles) is observed at inhibitory synapses of neurons that lack expression of synaptojanin 1, a phosphoinositide phosphatase involved in clathrin-coated vesicle uncoating. Thus, although excitatory synapses are largely spared under these conditions, inhibitory synapses are uniquely sensitive to perturbation of endocytic proteins, probably as a result of their higher levels of tonic activity leading to a buildup of clathrin-coated intermediates in these synapses. In contrast, the predominant endocytic structures observed at the majority of dynamin 1 KO synapses after acute stimulation are endosome-like intermediates that originate by a dynamin 1-independent form of endocytosis. These findings reveal a striking heterogeneity in the mode of synaptic vesicle recycling in different synapses and functional states.

Myristyl trimethyl ammonium bromide and octadecyl trimethyl ammonium bromide are surface-active small molecule dynamin inhibitors that block endocytosis mediated by dynamin I or dynamin II.

Dynamin is a GTPase enzyme involved in membrane constriction and fission during endocytosis. Phospholipid binding via its pleckstrin homology domain maximally stimulates dynamin activity. We developed a series of surface-active small-molecule inhibitors, such as myristyl trimethyl ammonium bromide (MiTMAB) and octadecyltrimethyl ammonium bromide (OcTMAB), and we now show MiTMAB targets the dynamin-phospholipid interaction. MiTMAB inhibited dynamin GTPase activity, with a Ki of 940 +/- 25 nM. It potently inhibited receptor-mediated endocytosis (RME) of transferrin or epidermal growth factor (EGF) in a range of cells without blocking EGF binding, receptor number, or autophosphorylation. RME inhibition was rapidly reversed after washout. The rank order of potency for a variety of MiTMAB analogs on RME matched the rank order for dynamin inhibition, suggesting dynamin recruitment to the membrane is a primary cellular target. MiTMAB also inhibited synaptic vesicle endocytosis in rat brain nerve terminals (synaptosomes) without inducing depolarization or morphological defects. Therefore, the drug rapidly and reversibly blocks multiple forms of endocytosis with no acute cellular damage. The unique mechanism of action of MiTMAB provides an important tool to better understand dynamin-mediated membrane trafficking events in a variety of cells.

A selective activity-dependent requirement for dynamin 1 in synaptic vesicle endocytosis.

Dynamin 1 is a neuron-specific guanosine triphosphatase thought to be critically required for the fission reaction of synaptic vesicle endocytosis. Unexpectedly, mice lacking dynamin 1 were able to form functional synapses, even though their postnatal viability was limited. However, during spontaneous network activity, branched, tubular plasma membrane invaginations accumulated, capped by clathrin-coated pits, in synapses of dynamin 1-knockout mice. Synaptic vesicle endocytosis was severely impaired during strong exogenous stimulation but resumed efficiently when the stimulus was terminated. Thus, dynamin 1-independent mechanisms can support limited synaptic vesicle endocytosis, but dynamin 1 is needed during high levels of neuronal activity.

Calcium inhibits renin gene expression by transcriptional and posttranscriptional mechanisms.

The aim of this study was to investigate the role of cytosolic calcium for renin gene expression in juxtaglomerular cells. For this purpose, we used the immortalized juxtaglomerular mouse cell line As4.1. To increase cytosolic calcium concentration, we treated the cells with thapsigargin and cyclopiazonic acid, inhibitors of the endoplasmatic reticulum Ca- ATPase. Thapsigargin and cyclopiazonic acid inhibited renin gene expression in a characteristic time and concentration-dependent manner. This effect was concentration-dependently blocked by BAPTA-AM, an intracellular Ca2+ chelator. Pharmacological blocking of protein kinase C activity by calphostin, Gö6976, and Gö6983 did not change the effect of thapsigargin on renin gene expression. Experiments with renin1C-promoter-reporter constructs revealed that thapsigargin inhibited renin gene transcription. Analysis of deletion constructs of the renin1C promoter indicated that regulatory elements involved in the calcium-mediated inhibition of renin gene transcription are located in the enhancer region of the renin gene and that > or =3 transcription factor-binding sites are involved in this process. In addition, thapsigargin reduced the renin mRNA half-life from 10 hours (control conditions) to 4 hours. Knockdown studies with small interfering RNA directed to dynamin-1 mRNA revealed that dynamin-1 is likely to be involved in the calcium-mediated destabilization of renin mRNA. These data suggest that calcium inhibits renin gene expression in juxtaglomerular cells via a concerted action of inhibition of renin gene transcription and destabilization of renin mRNA.

Inhibition of dynamin-dependent endocytosis increases shedding of the amyloid precursor protein ectodomain and reduces generation of amyloid beta protein.

BACKGROUND: The amyloid precursor protein (APP) is transported via the secretory pathway to the cell surface, where it may be cleaved within its ectodomain by alpha-secretase, or internalized within clathrin-coated vesicles. An alternative proteolytic pathway occurs within the endocytic compartment, where the sequential action of beta- and gamma-secretases generates the amyloid beta protein (Abeta). In this study, we investigated the effects of modulators of endocytosis on APP processing. RESULTS: Human embryonic kidney cells were transfected with a dominant negative mutant of dynamin I, an important mediator of clathrin-dependent endocytosis, and APP proteolysis was analyzed. Overexpression of the mutant dynamin (dyn I K44A) resulted in increased shedding of the APP ectodomain (sAPPalpha), accumulation of the C-terminal alpha-secretase product C83, and a reduction in the release of Abeta. Levels of mature APP on the cell surface were increased in cells expressing dyn I K44A, and internalization of surface-immunolabeled APP, assessed by fluorescence microscopy, was inhibited. Dynamin is a substrate for protein kinase C (PKC), and it was hypothesized that activators of PKC, which are known to stimulate alpha-secretase-mediated cleavage of APP, might exert their effects by inhibiting dynamin-dependent endocytosis. However, the internalization of surface-biotinylated APP was unaffected by treatment of cells with phorbol 12-myristate 13-acetate in the presence of the alpha-secretase inhibitor TAPI-1. CONCLUSION: The results indicate that APP is internalized by a dynamin-dependent process, and suggest that alterations in the activity of proteins that mediate endocytosis might lead to significant changes in Abeta production.