Contrary to expectations, PGRN proved to be dispensable during embryonic development of the mouse. This is consistent with the ability of another group to generate PGRN-deficient mice as well (Kayasuga et al., 2007
). That study described only one phenotype: altered male sexual behavior. The phenotypes observed in the present study reveal a profound role for endogenous PGRN in the balanced production of anti- and proinflammatory cytokines and chemokines. Although PGRN has been previously reported to be antiinflammatory (Zhu et al., 2002
; Kessenbrock et al., 2008
), this function was only shown with respect to inhibition of neutrophil activation, and the inference was based on the provision of exogenous, recombinant PGRN. The present work extends PGRN’s antiinflammatory actions to inhibiting macrophage activation in vitro and global innate responses to bacterial infection in vivo, inferred in both cases from the effects of a loss of function mutation. Absence of PGRN led to overproduction of TNF and MCP-1 yet to a decreased ability to clear Listeria
, with prolongation of inflammation in infected organs. In addition, we showed that PGRN exerted its antiinflammatory function in large part by synergizing with microbial products such as LPS to induce the transcription of IL-10, a major antiinflammatory cytokine. Further studies will address whether the immunodeficiency state we discovered in association with PGRN deficiency pertains to infectious agents other than L. monocytogenes
Excessive proinflammatory responses have been implicated in many neurodegenerative diseases (González-Scarano and Baltuch, 1999
; Minghetti, 2005
; Lobsiger and Cleveland, 2007
). In the brain, PGRN is widely expressed by different neuronal populations such as cortical and hippocampal pyramidal cells, as well as nonneuronal cells such as microglia. Trauma, malignancy, infection, and neurodegeneration are all associated with increased expression of PGRN in the brain (Liau et al., 2000
; Ahmed et al., 2007
; Pereson et al., 2009
). The ability of PGRN to support resolution of inflammation suggests that increased PGRN in these settings may reflect a compensatory, antiinflammatory response. This is supported by two findings. First, we observed augmented, spontaneous, age-dependent activation of astrocytes and microglia in PGRN-deficient mice. Second, peripheral infection by L. monocytogenes
resulted in much greater brain involvement in PGRN-deficient than in WT mice. This invites the speculation that a state of PGRN insufficiency in humans may be permissive for brain damage accumulating over decades from repeated episodes of inflammation.
Even under conditions of husbandry intended to avoid intercurrent infection, PGRN-deficient mice developed several neuropathological features as they aged for 18 mo, a short time in comparison to the age of human adults with FTD. For example, the widespread activation of microglia and astrocytes in aged PGRN-deficient mice was reminiscent of FTD. Moreover, cortical ubiquitin- and TDP-43–immunoreactive cytoplasmic or intranulear inclusions are common in FTD (Arai et al., 2006
; Cairns et al., 2007
; Josephs et al., 2007
). We detected enhanced ubiquitination in the hippocampal and thalamic regions of aged PGRN-deficient mice. In addition, we detected cytosolic phosphorylated TPD-43 in the hippocampus and thalamus of aged PGRN-deficient mice and not in WT mice. However, in contrast to FTD, we did not detect neuropathology in the prefrontal or cortical regions of PGRN-deficient mice, and their distribution of phosphorylated TDP-43 differed from that of TDP-43 inclusions in human FTD. Most strikingly, our mice showed no evidence of cerebral atrophy within the period of study.
Aside from neuropathology, the mouse model in this study appears to differ from FTD in that no clinically apparent infectious or inflammatory phenotype has been reported in FTD patients to our knowledge, apart from increased MCP-1 and IL-8 levels in their cerebrospinal fluid (Galimberti et al., 2006
). Perhaps the ~50% deficiency of PGRN in FTD patients produces a less conspicuous defect than the near-total PGRN deficiency in this study. Conversely, it is formally possible, though unlikely, that hemizygosity for PGRN would result in a more extreme neuropathologic and less marked immunoinflammatory phenotype in mice than the homozygous PGRN deficiency in this study. Additionally or alternatively, a phenotype of prolonged infectious or inflammatory states in FTD patients in the decades preceding the onset of neuropsychiatric signs may have escaped attention. Our PGRN-deficient mice display specific behavioral abnormalities (unpublished data). It will be of interest to learn if those abnormalities progress as the mice age further, particularly under conditions in which they may experience repetitive bouts of infection or inflammation. Such conditions are routine in the wild but not in our mouse colony (Perry et al., 2007
; Björkqvist et al., 2008
). In sum, the reasons for the differences between the mice described in this paper and human FTD deserve further study. However, it is worth noting that no mouse model has yet recapitulated all the features of a human neurodegenerative disease.
It is unknown why haploinsufficiency in PGRN, a widely expressed protein, manifests chiefly by behavioral abnormalities and frontotemporal cerebral atrophy. A neurotrophic function of PGRN was first suggested by the ability of PGRN to promote the growth of PC12 cells, a pheochromocytoma-derived neuronal cell line (Daniel et al., 2000
), and supported by the finding that granulin E improved survival and neurite growth of motor neurons in vitro (Van Damme et al., 2008
). In this study, we demonstrated that PGRN-deficient macrophages, when activated, were more potent than WT macrophages in killing hippocampal neurons in co-culture. Moreover, neurons from PGRN-deficient mice were more vulnerable to damage by activated microglia and by the depletion of oxygen and glucose. Whether neurons from other regions of the brain share this vulnerability remains to be tested. Thus, FTD may arise from the congruence of two independent phenotypes of PGRN insufficiency: dysregulated inflammation and increased neuronal vulnerability to damage.