The emergence of the calvarial hematopoietic niche in health and disease

The diploë region of skull has recently been discovered to act as a myeloid cell reservoir to the underlying meninges. The presence of ossified vascular channels traversing the inner skull of cortex provides a passageway for the cells to traffic from the niche, and CNS-derived antigens traveling through cerebrospinal fluid in a perivascular manner reaches the niche to signal myeloid cell egress. This review will highlight the recent findings establishing this burgeoning field along with the known role this niche plays in CNS aging and disease. It will further highlight the anatomical routes and physiological properties of the vascular structures these cells use for trafficking, spanning from skull to brain parenchyma.

Evidence for cellular and solute passage between the brain and skull bone marrow across meninges: A systematic review

A potential two-way passage of cells and substances between the brain and skull bone marrow may open for new insights into neurological disease. The arachnoid membrane was traditionally considered to restrict cells and larger molecules in CSF from entering the dura and bone marrow directly. However, new data on exchange between brain and skull bone marrow have recently emerged. Here, we conducted a systematic literature to answer the question: What is the current evidence regarding the movement of cells and molecules between the brain and skull bone marrow, spanning CSF and meninges? We excluded studies related to head or skull trauma, cranial fractures or defects, cancer invasion, CSF leakage, spontaneous intracranial hypotension, spinal dura mater, and studies solely focusing on meningeal lymphatic vessels or the passage of substances from CSF to meningeal lymphatic vessels. The review identified 16 studies that provide evidence of communication between the brain, meninges and skull bone marrow. Cells (such as B and T cells and neutrophils), bacteria, and substances (tracers, drug compounds) have been reported to pass between the brain and skull bone. However, most studies are performed in rodents, emphasizing the need for translation to humans.

Single-Cell Transcriptomes of Immune Cells from Multiple Compartments Redefine the Ontology of Myeloid Subtypes Post-Stroke

The activation and infiltration of immune cells are hallmarks of ischemic stroke. However, the precise origins and the molecular alterations of these infiltrating cells post-stroke remain poorly characterized. Here, a murine model of stroke (permanent middle cerebral artery occlusion [p-MCAO]) is utilized to profile single-cell transcriptomes of immune cells in the brain and their potential origins, including the calvarial bone marrow (CBM), femur bone marrow (FBM), and peripheral blood mononuclear cells (PBMCs). This analysis reveals transcriptomically distinct populations of cerebral myeloid cells and brain-resident immune cells after stroke. These include a novel CD14+ neutrophil subpopulation that transcriptomically resembles CBM neutrophils. Moreover, the sequential activation of transcription factor regulatory networks in neutrophils during stroke progression is delineated, many of which are unique to the CD14+ population and underlie their acquisition of chemotaxis and granule release capacities. Two distinct origins of post-stroke disease-related immune cell subtypes are also identified: disease inflammatory macrophages, likely deriving from circulating monocytes in the skull, and transcriptionally immature disease-associated microglia, possibly arising from pre-existing homeostatic microglia. Together, a comprehensive molecular survey of post-stroke immune responses is performed, encompassing both local and distant bone marrow sites and peripheral blood.

Osteocalcin-expressing neutrophils from skull bone marrow exert immunosuppressive and neuroprotective effects after TBI

Neutrophils from skull bone marrow (Nskull) are activated under some brain stresses, but their effects on traumatic brain injury (TBI) are lacking. Here, we find Nskull infiltrates brain tissue quickly and persistently after TBI, which is distinguished by highly and specifically expressed osteocalcin (OCN) from blood-derived neutrophils (Nblood). Reprogramming of glucose metabolism by reducing glycolysis-related enzyme glyceraldehyde 3-phosphate dehydrogenase expression is involved in the antiapoptotic and proliferative abilities of OCN-expressing Nskull. The transcription factor Fos-like 1 governs the specific gene profile of Nskull including C-C motif chemokine receptor-like 2 (CCRL2), arginase 1 (Arg1), and brain-derived neurotrophic factor (BDNF) in addition to OCN. Selective knockout of CCRL2 in Nskull demonstrates that CCRL2 mediates its recruitment, whereas high Arg1 expression is consistent with its immunosuppressive effects on Nblood, and the secretion of BDNF facilitating dendritic growth contributes to its neuroprotection. Thus, our findings provide insight into the roles of Nskull in TBI.